{"id":1626,"date":"2025-08-07T21:47:33","date_gmt":"2025-08-07T21:47:33","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/?post_type=chapter&#038;p=1626"},"modified":"2025-08-15T21:45:47","modified_gmt":"2025-08-15T21:45:47","slug":"aa","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/chapter\/aa\/","title":{"raw":"Unit 11 Reproduction and Development","rendered":"Unit 11 Reproduction and Development"},"content":{"raw":"<div class=\"unit-10:-reproduction-and-development\">\r\n<div class=\"textbox shaded\">\r\n\r\n<strong>Unit outline<\/strong>\r\n\r\n<a href=\"#11-1\"><strong>Part 1: <\/strong>Anatomy and Physiology of the Male Reproductive System<\/a>\r\n<ul>\r\n \t<li><a href=\"#11-1a\">Scrotum<\/a><\/li>\r\n \t<li><a href=\"#11-1b\">Testes<\/a><\/li>\r\n \t<li><a href=\"#11-1c\">Structure of formed sperm<\/a><\/li>\r\n \t<li><a href=\"#11-1d\">Sperm transport<\/a><\/li>\r\n \t<li><a href=\"#11-1e\">The penis<\/a><\/li>\r\n \t<li><a href=\"#11-1f\">Sperm release<\/a><\/li>\r\n \t<li><a href=\"#11-1g\">Testosterone<\/a><\/li>\r\n \t<li><a href=\"#11-1h\">Function of testosterone<\/a><\/li>\r\n<\/ul>\r\n<a href=\"#11-2\"><strong>Part 2:<\/strong> Anatomy and Physiology of the Female Reproductive System<\/a>\r\n<ul>\r\n \t<li><a href=\"#11-2a\">External female genitalia<\/a><\/li>\r\n \t<li><a href=\"#11-2b\">Vagina<\/a><\/li>\r\n \t<li><a href=\"#11-2c\">Ovaries<\/a><\/li>\r\n \t<li><a href=\"#11-2d\">The ovarian cycle<\/a><\/li>\r\n \t<li><a href=\"#11-2e\">Hormonal control of the ovarian cycle<\/a><\/li>\r\n \t<li><a href=\"#11-2f\">The uterine tubes<\/a><\/li>\r\n \t<li><a href=\"#11-2g\">The uterus and cervix<\/a><\/li>\r\n \t<li><a href=\"#11-2h\">The menstrual (uterine) cycle<\/a><\/li>\r\n \t<li><a href=\"#11-2i\">The breasts<\/a><\/li>\r\n<\/ul>\r\n<a href=\"#11-3\"><strong>Part 3:<\/strong> Fertilization<\/a>\r\n<ul>\r\n \t<li><a href=\"#11-3a\">Transit of sperm<\/a><\/li>\r\n \t<li><a href=\"#11-3b\">Contact between sperm and oocyte<\/a><\/li>\r\n \t<li><a href=\"#11-3c\">The zygote<\/a><\/li>\r\n<\/ul>\r\n<a href=\"#11-4\"><strong>Part 4:<\/strong> Embryonic Development<\/a>\r\n<ul>\r\n \t<li><a href=\"#11-4a\">Pre-implantation embryonic development<\/a><\/li>\r\n \t<li><a href=\"#11-4b\">Implantation<\/a><\/li>\r\n \t<li><a href=\"#11-4c\">Embryonic membranes and Germ Layers<\/a><\/li>\r\n \t<li><a href=\"#11-4e\"><span style=\"text-align: initial;font-size: 1em\">D<\/span>evelopment of the placenta<\/a><\/li>\r\n<\/ul>\r\n<a href=\"#11-5a\"><strong>Part 5:<\/strong> Fetal Development<\/a>\r\n<ul>\r\n \t<li><a href=\"#11-5a\">The fetal circulatory system<\/a><\/li>\r\n<\/ul>\r\n<a href=\"#11-6\"><strong>Part 6:<\/strong> Maternal Changes During Pregnancy, Labour, and Birth<\/a>\r\n<ul>\r\n \t<li><a href=\"#11-6a\">Effects of hormones<\/a><\/li>\r\n \t<li><a href=\"#11-6b\">Physiology of labour<\/a><\/li>\r\n \t<li><a href=\"#11-6c\">Stages of childbirth<\/a><\/li>\r\n<\/ul>\r\n<a href=\"#11-7\"><strong>Part 7:<\/strong> Adjustments of the Infant at Birth and Postnatal Stages<\/a>\r\n<ul>\r\n \t<li><a href=\"#11-7a\">Reproductive adjustments<\/a><\/li>\r\n \t<li><a href=\"#11-7b\">Circulatory adjustments<\/a><\/li>\r\n<\/ul>\r\n<a href=\"#11-8\"><strong>Part 8:<\/strong> Lactation<\/a>\r\n<ul>\r\n \t<li><a href=\"#11-8a\">Structure of lactating breast<\/a><\/li>\r\n \t<li><a href=\"#11-8b\">The process of lactation<\/a><\/li>\r\n<\/ul>\r\n<h2><a href=\"#P\">Practice Questions<\/a><\/h2>\r\n<\/div>\r\n<div class=\"textbox textbox--learning-objectives\"><header class=\"textbox__header\">\r\n<p class=\"textbox__title\"><strong>Learning Objectives<\/strong><\/p>\r\n\r\n<\/header>\r\n<div class=\"textbox__content\">\r\n\r\nAt the end of this unit, you should be able to:\r\n<p class=\"hanging-indent\"><strong>I. <\/strong>Describe the location, structure and function(s) of the components of the male reproductive system.<\/p>\r\n<p class=\"hanging-indent\"><strong>II. <\/strong>Describe the male reproductive cycle, including both spermatogenesis and sperm transport.<\/p>\r\n<p class=\"hanging-indent\"><strong>III.<\/strong> Describe the location, structure and function(s) of each of the following components of the female reproductive system.<\/p>\r\n<strong>IV.<\/strong> Describe the female reproductive cycle, including both oogenesis and preparation of the uterus for pregnancy.\r\n<p class=\"hanging-indent\"><strong>V. <\/strong>Describe the process of fertilization.<\/p>\r\n<p class=\"hanging-indent\"><strong>VI. <\/strong>Describe the early stages of embryonic development.<\/p>\r\n<strong>VII.<\/strong> Describe the hormonal control of pregnancy, labour and lactation.\r\n\r\n<strong>VIII. <\/strong><span style=\"font-size: 1em;text-indent: 0px\">Describe changes in neonatal circulation and respiration after birth.<\/span>\r\n\r\n<\/div>\r\n<\/div>\r\n<div class=\"textbox textbox--learning-objectives\"><header class=\"textbox__header\">\r\n<p class=\"textbox__title\"><strong>Learning Objectives and Guiding Questions<\/strong><\/p>\r\n\r\n<\/header>\r\n<div class=\"textbox__content\">\r\n\r\nAt the end of this unit, you should be able to complete all the following tasks, including answering the guiding questions associated with each task.\r\n<p class=\"hanging-indent\"><strong>I. <\/strong>Describe the location, structure and function(s) of each of the following components of the male reproductive system:<\/p>\r\n\r\n<ol>\r\n \t<li class=\"hanging-indent\">Testes<\/li>\r\n \t<li class=\"hanging-indent\">Epididymis<\/li>\r\n \t<li class=\"hanging-indent\">Vas deferens<\/li>\r\n \t<li class=\"hanging-indent\">Ejaculatory duct<\/li>\r\n \t<li class=\"hanging-indent\">Urethra<\/li>\r\n \t<li class=\"hanging-indent\">Seminal vesicles (seminal glands)<\/li>\r\n \t<li class=\"hanging-indent\">Prostate gland<\/li>\r\n \t<li class=\"hanging-indent\">Penis<\/li>\r\n \t<li class=\"hanging-indent\">Bulbourethral gland<\/li>\r\n \t<li class=\"hanging-indent\">Scrotum<\/li>\r\n \t<li class=\"hanging-indent\">Seminiferous tubules<\/li>\r\n<\/ol>\r\n<p class=\"hanging-indent\"><strong>II. <\/strong>Describe the male reproductive cycle, including both spermatogenesis and sperm transport.<\/p>\r\n\r\n<ol>\r\n \t<li>Using direct reference to the structures identified in objective I, describe the process of spermatogenesis, including the timing and hormonal control of spermatogenesis (via FSH, LH and testosterone; refer back to <a href=\"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/chapter\/unit-1-the-endocrine-system\/\">Unit 1<\/a> for the roles of FSH and LH), and volume of sperm produced.<\/li>\r\n \t<li>Using direct reference to the structures identified in objective I, sketch the pathway of sperm movement through the male reproductive tract, noting the addition of substances by the accessory glands.<\/li>\r\n \t<li>Describe how the parasympathetic and sympathetic nervous systems are required for sperm release.<\/li>\r\n<\/ol>\r\n<p class=\"hanging-indent\"><strong>III.<\/strong> Describe the location, structure and function(s) of each of the following components of the female reproductive system:<\/p>\r\n\r\n<ol>\r\n \t<li class=\"hanging-indent\">Ovaries<\/li>\r\n \t<li class=\"hanging-indent\">Uterine tubes (oviducts)<\/li>\r\n \t<li class=\"hanging-indent\">Uterus<\/li>\r\n \t<li class=\"hanging-indent\">Cervix<\/li>\r\n \t<li class=\"hanging-indent\">Clitoris<\/li>\r\n \t<li class=\"hanging-indent\">Vagina<\/li>\r\n \t<li class=\"hanging-indent\">Endometrium<\/li>\r\n \t<li class=\"hanging-indent\">Myometrium<\/li>\r\n \t<li class=\"hanging-indent\">Perimetrium<\/li>\r\n \t<li class=\"hanging-indent\">Bartholin\u2019s glands<\/li>\r\n<\/ol>\r\n<strong>IV.<\/strong> Describe the two major female reproductive cycles, involved in oogenesis and the preparation of the uterus for pregnancy.\r\n<ol>\r\n \t<li>What is the ultimate goal of the ovarian cycle?<\/li>\r\n \t<li>Identify the two main phases of the ovarian cycle and the ovulation event happening at the transition point between the two phases. Identify the ovarian structures and locations involved in each phase, and in ovulation.<\/li>\r\n \t<li>Draw a time scale of days 1-28 (leave ample space beneath your drawing for the uterine cycle). Label the timescale with the two phases of the ovarian cycle and ovulation, and annotate your time scale with the main events occurring along the way.<\/li>\r\n \t<li>Consider the hormones FSH and LH. For each hormone, identify the organ which secretes the hormone, the conditions that result in increased secretion of that hormone (be sure to address the roles of both the hypothalamus and pituitary gland) and the effect of high concentrations of each hormone on the ovarian cycle.<\/li>\r\n \t<li>Label the time scale showing when each hormone is in highest concentration.<\/li>\r\n \t<li>What is the ultimate goal of the uterine (menstrual) cycle?<\/li>\r\n \t<li>Identify the three main phases of the uterine cycle. Identify the <span style=\"color: #ff00ff\">female reproductive structures and locations involved.<\/span><\/li>\r\n \t<li>Below your ovarian cycle timescale, draw another timescale of days 1-28. Label the time scale with the three phases of the uterine cycle and annotate your timescale with the main events that occur in each phase.<\/li>\r\n \t<li>Consider the hormones estrogens and progesterone. For each hormone, identify the structure(s) that secrete the hormone, the conditions that result in increased secretion of that hormone and the effect of high concentrations of each hormone on the uterine cycle.<\/li>\r\n \t<li>Label the uterine cycle time scale showing when each hormone is in highest concentration.<\/li>\r\n \t<li>Correlate the ovarian and uterine (menstrual) cycles and explain their integrated hormonal regulation by annotating your diagram to show the hormonal interactions between the ovarian and uterine cycles. Make sure to differentiate between the effects of low vs. high concentrations of estrogens.<\/li>\r\n<\/ol>\r\n<p class=\"hanging-indent\"><strong>V. <\/strong>Describe the process of fertilization.<\/p>\r\n\r\n<ol>\r\n \t<li>Compare and contrast the sperm and the oocyte with regards to size, main constituents, role in reproduction and longevity.<\/li>\r\n \t<li>Identify the secretions from the seminal vesicles, bulbourethral gland and prostate gland, and briefly describe the role of each in sperm movement and survival within the female reproductive tract.<\/li>\r\n \t<li>Identify the secretions from the Bartholin\u2019s glands and the vagina and briefly describe their role in reproduction.<\/li>\r\n \t<li>Trace the pathway of sperm from when it is deposited in the female reproductive tract until it meets the oocyte. Consider the structures through which the <span style=\"color: #ff00ff\">oocyte\/early embryo<\/span> moves, the mechanisms of movement and how long the sperm survives.<\/li>\r\n \t<li>Trace the pathway of the ovum after it is expelled from the ovary until it meets the sperm. Consider the structures through which the ovum moves, the mechanisms of movement and the timing of movement.<\/li>\r\n \t<li>Describe the processes by which sperm penetrates the corona radiata and then the zona pellucida.<\/li>\r\n \t<li>What is the event that specifically identifies fertilization?<\/li>\r\n \t<li>Explain why polyspermy is undesirable. Describe the mechanisms in the oocyte which protect against polyspermy.<\/li>\r\n \t<li>Why are hundreds of sperm required for successful fertilization if only one succeeds in the end?<\/li>\r\n<\/ol>\r\n<p class=\"hanging-indent\"><strong>VI. <\/strong>Describe the early stages of embryonic development.<\/p>\r\n\r\n<ol>\r\n \t<li>Describe the overall developmental changes occurring within the zygote, and the role of cell division in creating a conceptus of increasing size and complexity. Name the main pre-implantation stages of the conceptus.<\/li>\r\n \t<li>Distinguish between morula and blastocyst. Be sure to address the morphological differences between these structures, the timing of the development and the location within the female reproductive tract.<\/li>\r\n \t<li>Trace the pathway of the conceptus after fertilization until it implants in the uterus. Consider the structures through which the conceptus moves, the mechanisms of movement and the timing of movement.<\/li>\r\n \t<li>Describe the hormonal changes that occur at implantation of the blastocyst. Which structure secretes human chorionic gonadotropin (hCG)?\u00a0 What is the effect of increased levels of hCG on the ovarian cycle and the uterine cycle?\u00a0 Why is this important for a successful pregnancy?\u00a0 What other structure fulfills this role later in pregnancy and how do the hormonal mechanisms differ?<\/li>\r\n \t<li>Explain where gastrulation occurs, what it achieves in and around the future embryo, and how it sets the stage for organogenesis.<\/li>\r\n \t<li>Describe the formation, structure, and functions of the placenta and umbilical cord.<\/li>\r\n<\/ol>\r\n<strong>VII.<\/strong> Describe the hormonal control of pregnancy, labour and lactation.\r\n<ol>\r\n \t<li>Sketch a diagram showing changing levels of hCG, estrogens and progesterone throughout gestation. Annotate your sketch indicating the source for each hormone and describing the role of each hormone in maintaining pregnancy.<\/li>\r\n \t<li>Describe the hormonal changes that occur at the end of pregnancy, and describe the role of oxytocin, estrogens and prostaglandins in the initiation of labour.<\/li>\r\n \t<li>Consider the hormones oxytocin and prolactin and their role in lactation.<\/li>\r\n<\/ol>\r\n<strong style=\"text-align: initial;text-indent: -1em;font-size: 1em\">IX. <\/strong><span style=\"font-size: 1em;text-indent: 0px\">Describe changes in neonatal circulation and respiration after birth.<\/span>\r\n<ol>\r\n \t<li>Sketch fetal circulation and label all the blood vessels or structures found in fetal circulation but not in the adult circulation. For each structure, provide a functional explanation for the difference in both the fetus and the adult.<\/li>\r\n \t<li>Which organ is responsible for fetal gas exchange? Why does this change at birth?\u00a0 Describe the four possible stimuli for the initiation of ventilation.<\/li>\r\n \t<li>At what stage during development is surfactant produced and what is the function of surfactant in the neonatal lungs?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n&nbsp;\r\n<p style=\"text-align: justify\">Small, uncoordinated, and slick with amniotic fluid, a newborn encounters the world outside of her mother\u2019s womb. We do not often consider that a child\u2019s birth is proof of the healthy functioning of both her mother\u2019s and father\u2019s reproductive systems. Moreover, her parents\u2019 endocrine systems had to secrete the appropriate regulating hormones to induce the production and release of unique male and female gametes, reproductive cells containing the parents\u2019 genetic material (one set of 23 chromosomes). Her parent\u2019s reproductive behaviour had to facilitate the transfer of male gametes\u2014the sperm\u2014to the female reproductive tract at just the right time to encounter the female gamete, an oocyte (egg). Finally, combination of the gametes (fertilization) had to occur, followed by implantation and development. In this unit, you will explore the male and female reproductive systems, whose healthy functioning can culminate in the powerful sound of a newborn\u2019s first cry.<\/p>\r\n&nbsp;\r\n<h2 style=\"text-align: justify\"><strong><a id=\"11-1\"><\/a>Part 1: Anatomy and Physiology of the Male Reproductive System<\/strong><\/h2>\r\n<p style=\"text-align: justify\">Unique for its role in human reproduction, a <strong>[pb_glossary id=\"465\"]gamete[\/pb_glossary] <\/strong>is a specialized sex cell usually carrying 23 [pb_glossary id=\"1185\"]chromosomes[\/pb_glossary]\u2014one half the number in [pb_glossary id=\"1458\"]somatic[\/pb_glossary] cells. At fertilization, the chromosomes in one male gamete, called a <strong>sperm<\/strong> (or spermatozoon), combine with the chromosomes in one female gamete, called an oocyte. The function of the male reproductive system (Figure 1) is to produce sperm and transfer them to the female reproductive tract. The paired testes are a crucial component in this process, as they produce both sperm and [pb_glossary id=\"1186\"]androgens[\/pb_glossary], the hormones that support male reproductive physiology. In humans, the most important male androgen is testosterone. Several accessory organs and ducts aid the process of sperm maturation and transport the sperm and other seminal components to the penis, which delivers sperm to the female reproductive tract. In this section, we examine each of these different structures, and discuss the process of sperm production and transport.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"150\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2019\/06\/image1-8.png\" alt=\"image\" width=\"150\" height=\"150\" \/> Watch <a href=\"https:\/\/youtu.be\/-XQcnO4iX_U\">this CrashCourse video<\/a> for an overview of the male reproductive system! Direct link: <a href=\"https:\/\/youtu.be\/-XQcnO4iX_U\">https:\/\/youtu.be\/-XQcnO4iX_U<\/a>[\/caption]\r\n\r\n[caption id=\"\" align=\"alignleft\" width=\"805\"]<img style=\"font-weight: bold;font-size: 14pt\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image2-8.png\" alt=\"image\" width=\"805\" height=\"876\" \/> <strong>Figure 1. Male Reproductive System.<\/strong> The structures of the male reproductive system include the testes, the epididymides, the penis, and the ducts and glands that produce and carry semen. Sperm exit the scrotum through the ductus deferens, which is bundled in the spermatic cord. The seminal vesicles and prostate gland add fluids to the sperm to create semen.[\/caption]\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n<h5 style=\"text-align: justify\"><strong><a id=\"11-1a\"><\/a>Scrotum<\/strong><\/h5>\r\n<p style=\"text-align: justify\">The [pb_glossary id=\"484\"]testes[\/pb_glossary] are located in a highly pigmented, muscular sac called the <strong>[pb_glossary id=\"1187\"]scrotum[\/pb_glossary] <\/strong>that extends from the body behind the penis (Figure 1). This location is important in sperm production, which occurs within the testes, and proceeds more efficiently when the testes are kept 2 to 4\u00b0C below core body temperature.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"800\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image3-8.png\" alt=\"image\" width=\"800\" height=\"352\" \/> <strong>Figure 2. The Scrotum and Testes.<\/strong> This anterior view shows the structures of the scrotum and testes.[\/caption]\r\n<h5 style=\"text-align: justify\"><strong><a id=\"11-1b\"><\/a>Testes<\/strong><\/h5>\r\n<p style=\"text-align: justify\">The <strong>testes <\/strong>(singular = testis) are the male <strong>gonads<\/strong>. They produce both sperm and androgens, such as testosterone, and are active throughout the reproductive lifespan of the male.<\/p>\r\n<p style=\"text-align: justify\">As paired oval organs, the testes are each approximately 4 to 5 cm in length and are housed within the scrotum (Figure 2). Within the testes, sperm develop in structures called seminiferous tubules. During the seventh month of the developmental period of a male fetus, each testis moves through the abdominal musculature to descend into the scrotal cavity. This is called the \u201cdescent of the testis.\u201d<\/p>\r\n<p style=\"text-align: justify\">The tightly coiled <strong>[pb_glossary id=\"1188\"]seminiferous tubules[\/pb_glossary]<\/strong> form the bulk of each testis. They are composed of developing sperm cells surrounding a lumen, the hollow centre of the tubule, where formed sperm are released into the duct system of the testis (Figure 3). Specifically, from the lumens of the seminiferous tubules, sperm move into the straight tubules then into a fine meshwork of tubules and then leave the testis itself.<\/p>\r\n<p style=\"text-align: justify\">Inside the seminiferous tubules are supporting cells and developing sperm cells called [pb_glossary id=\"1190\"]germ cells[\/pb_glossary]. Germ cell development progresses from the basement membrane\u2014at the perimeter of the tubule\u2014toward the lumen.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"743\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image4-8.png\" alt=\"image\" width=\"743\" height=\"686\" \/> <strong>Figure 3. Anatomy of the Testis.<\/strong> This sagittal view shows the seminiferous tubules, the site of sperm production. Formed sperm are transferred to the epididymis, where they mature. They leave the epididymis during an ejaculation via the ductus deferens.[\/caption]\r\n<p style=\"text-align: justify\"><strong>1. Sertoli Cells:<\/strong> Surrounding all stages of the developing sperm cells are elongate, branching [pb_glossary id=\"1189\"]Sertoli cells[\/pb_glossary]. Sertoli cells secrete signaling molecules that promote sperm production and can control whether germ cells live or die.<\/p>\r\n<p style=\"text-align: justify\"><strong>2. Germ Cells:<\/strong> The least mature cells line the basement membrane inside the tubule. These divide to produce primary and secondary spermatocytes, then spermatids, which finally produce formed sperm. This process is called <strong>[pb_glossary id=\"1200\"]spermatogenesis[\/pb_glossary]<\/strong>. The process begins at puberty, after which time sperm are produced constantly throughout a man\u2019s life. Eventually, the sperm are released into the lumen and are moved along a series of ducts in the testis toward a structure called the epididymis for the next step of sperm maturation.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"11-1c\"><\/a>Structure of Formed Sperm<\/strong><\/h5>\r\n<p style=\"text-align: justify\">Sperm are smaller than most cells in the body; in fact, the volume of a sperm cell is 85,000 times less than that of the female gamete. Approximately 100 to 300 million sperm are produced each day, whereas typically only one oocyte per month matures fully and is then ovulated. As is true for most cells in the body, the structure of sperm cells speaks to their function. Sperm have a distinctive head, mid-piece, and tail region (Figure 4). The head of the sperm contains the extremely compact [pb_glossary id=\"1197\"]haploid[\/pb_glossary] (half the genetic content of a diploid somatic cell) nucleus with very little cytoplasm. These qualities contribute to the overall small size of the sperm (the head is only 5 \u03bcm long). A structure called the [pb_glossary id=\"1198\"]acrosome[\/pb_glossary] covers most of the head of the sperm cell as a \u201ccap\u201d that is filled with lysosomal enzymes important for preparing sperm to participate in fertilization. Tightly packed [pb_glossary id=\"1094\"]mitochondria[\/pb_glossary] fill the midpiece of the sperm. ATP produced by these mitochondria will power the [pb_glossary id=\"1199\"]flagellum[\/pb_glossary], which extends from the neck and the mid-piece through the tail of the sperm, enabling it to move the entire sperm cell.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"727\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image5-8.png\" alt=\"image\" width=\"727\" height=\"198\" \/> <strong>Figure 4. Structure of Sperm.<\/strong> Sperm cells are divided into a head, containing DNA; a mid-piece, containing mitochondria; and a tail, providing motility. The acrosome is oval and somewhat flattened.[\/caption]\r\n<h5 style=\"text-align: justify\"><strong><a id=\"11-1d\"><\/a>Sperm Transport<\/strong><\/h5>\r\n<p style=\"text-align: justify\">To fertilize an [pb_glossary id=\"1232\"]oocyte[\/pb_glossary], sperm must be moved from the [pb_glossary id=\"1188\"]seminiferous tubules[\/pb_glossary] in the testes, through the epididymis, and\u2014later during ejaculation\u2014along the length of the penis and out into the female reproductive tract.<\/p>\r\n<p style=\"text-align: justify\"><strong>1. Role of the Epididymis:<\/strong> From the lumen of the seminiferous tubules, the immotile sperm are surrounded by testicular fluid and moved to the <strong>[pb_glossary id=\"1201\"]epididymis[\/pb_glossary] <\/strong>(plural = epididymides), a coiled tube attached to the testis where newly formed sperm continue to mature (Figure 3). Though the epididymis does not take up much room in its tightly coiled state, it would be approximately 6 m (20 feet) long if straightened. It takes an average of 12 days for sperm to move through the coils of the epididymis. Sperm enter the head of the epididymis and are moved along predominantly by the contraction of smooth muscles lining the epididymal tubes. As they are moved along the length of the epididymis, the sperm further mature and acquire the ability to move under their own power. Once inside the female reproductive tract, they will use this ability to move independently toward the unfertilized egg. The more mature sperm are then stored in the tail of the epididymis (the final section) until ejaculation occurs.<\/p>\r\n<p style=\"text-align: justify\"><strong>2. Duct System:<\/strong> During ejaculation, sperm exit the tail of the epididymis and are pushed by smooth muscle contraction to the [pb_glossary id=\"1202\"]ductus deferens[\/pb_glossary] (also called the vas deferens). The ductus deferens is a thick, muscular tube that is bundled together inside the [pb_glossary id=\"1187\"]scrotum[\/pb_glossary] with connective tissue, blood vessels, and nerves into a structure called the <strong>spermatic cord<\/strong> (Figure 1 and Figure 2). Surgical sterilization to interrupt sperm delivery can be performed by cutting and sealing a small section of the ductus (vas) deferens. This procedure is called a vasectomy, and it is an effective and usually permanent form of male birth control.<\/p>\r\n<p style=\"text-align: justify\">From each epididymis, each ductus deferens extends superiorly into the abdominal cavity through the <strong>[pb_glossary id=\"1203\"]inguinal canal[\/pb_glossary]<\/strong> in the abdominal wall. From here, the ductus deferens continues posteriorly to the pelvic cavity, ending posterior to the bladder where it dilates in a region called the ampulla (meaning \u201cflask\u201d).<\/p>\r\n<p style=\"text-align: justify\">Sperm make up only 5 percent of the final volume of <strong>[pb_glossary id=\"1204\"]semen[\/pb_glossary]<\/strong>, the viscous, whitish-gray fluid that the male ejaculates. The bulk of semen is produced by three critical accessory glands of the male reproductive system: the [pb_glossary id=\"1205\"]seminal vesicles[\/pb_glossary], the [pb_glossary id=\"1206\"]prostate[\/pb_glossary], and the [pb_glossary id=\"1207\"]bulbourethral glands[\/pb_glossary].<\/p>\r\n<p style=\"text-align: justify\"><strong>3. Seminal Vesicles:<\/strong> As sperm pass through the [pb_glossary id=\"1208\"]ampulla[\/pb_glossary] of the ductus deferens at ejaculation, they mix with fluid from the associated <strong>[pb_glossary id=\"1205\"]seminal vesicle[\/pb_glossary]<\/strong> (Figure 1). The paired seminal vesicles are glands that contribute approximately 60 percent of the semen volume. Seminal vesicle fluid contains large amounts of fructose, which is used by the sperm mitochondria to generate [pb_glossary id=\"967\"]ATP[\/pb_glossary] to allow movement through the female reproductive tract.<\/p>\r\n<p style=\"text-align: justify\">The fluid, now containing both sperm and seminal vesicle secretions, next moves into the associated paired <strong>[pb_glossary id=\"1209\"]ejaculatory ducts[\/pb_glossary]<\/strong>, which transport the seminal fluid into the next structure, the prostate gland.<\/p>\r\n<p style=\"text-align: justify\"><strong>4. Prostate Gland:<\/strong> The centrally located <strong>[pb_glossary id=\"1206\"]prostate[\/pb_glossary] gland<\/strong> sits anterior to the rectum at the base of the bladder surrounding the prostatic [pb_glossary id=\"1156\"]urethra[\/pb_glossary], the portion of the urethra that runs within the prostate (Figure 1). About the size of a walnut, the prostate is formed of both muscular and glandular tissues. It excretes a fluid enriched with [pb_glossary id=\"464\"]enzymes[\/pb_glossary] and citric acid to the passing seminal fluid\u2014now called semen\u2014that is critical to first coagulate and then decoagulate the semen following ejaculation. The temporary thickening of semen helps retain it within the female reproductive tract, providing time for sperm to utilize the fructose provided by seminal vesicle secretions. When the semen regains its fluid state, sperm can then pass farther into the female reproductive tract.<\/p>\r\n<p style=\"text-align: justify\"><strong>5. Bulbourethral Glands:<\/strong> The final addition to semen is made by two [pb_glossary id=\"1207\"]bulbourethral glands[\/pb_glossary] (or Cowper\u2019s glands) that release a thick, salty fluid that lubricates the end of the urethra and the [pb_glossary id=\"1222\"]vagina[\/pb_glossary] and helps to clean urine residues from the penile urethra. The fluid from these accessory glands is released after the male becomes sexually aroused, and shortly before the release of the semen. It is therefore sometimes called pre-ejaculate. It is important to note that, in addition to the lubricating proteins, it is possible for bulbourethral fluid to pick up sperm already present in the urethra, and therefore it may be able to cause pregnancy.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"11-1e\"><\/a>The Penis<\/strong><\/h5>\r\n<p style=\"text-align: justify\">The penis is the male organ of copulation (sexual intercourse). When erect, the stiffness of the organ allows it to penetrate into the vagina and deposit semen into the female reproductive tract.<\/p>\r\n<p style=\"text-align: justify\">The shaft of the penis surrounds the urethra (Figure 1). The end of the penis, called the <strong>[pb_glossary id=\"1211\"]glans penis[\/pb_glossary]<\/strong>, has a high concentration of nerve endings, resulting in very sensitive skin that influences the likelihood of ejaculation (see Figure 1). The skin from the shaft extends down over the glans and forms a collar called the <strong>[pb_glossary id=\"1210\"]prepuce[\/pb_glossary]<\/strong> (or foreskin). The foreskin also contains a dense concentration of nerve endings, and both lubricate and protect the sensitive skin of the glans penis. A surgical procedure called circumcision, often performed for religious or social reasons, removes the prepuce, typically within days of birth.<\/p>\r\n\r\n<h5><strong><a id=\"11-1f\"><\/a>Sperm release<\/strong><\/h5>\r\nSperm release involves two stages: erection and ejaculation, each of which is controlled by a separate branch of the [pb_glossary id=\"1212\"]autonomic nervous system[\/pb_glossary]. Erection involves [pb_glossary id=\"536\"]parasympathetic[\/pb_glossary] activation causing the [pb_glossary id=\"1213\"]corpora cavernosa[\/pb_glossary] (sinuses that run the length of the penis) to fill with venous blood.\u00a0 Erection allows the penis to be inserted into the vagina.\u00a0 Ejaculation involves [pb_glossary id=\"535\"]sympathetic[\/pb_glossary] activation of muscles in the penis and in the pelvic floor.\u00a0 The contraction of these muscles forces sperm out of the urethra.\r\n<h5 style=\"text-align: justify\"><strong><a id=\"11-1g\"><\/a>Testosterone<\/strong><\/h5>\r\n<p style=\"text-align: justify\">[pb_glossary id=\"485\"]Testosterone[\/pb_glossary], an androgen, is a steroid hormone produced by <strong>[pb_glossary id=\"1214\"]Leydig cells[\/pb_glossary]<\/strong>, or interstitial cells, reflects their location between the seminiferous tubules in the testes. In male embryos, testosterone is secreted by Leydig cells by the seventh week of development, with peak concentrations reached in the second trimester. This early release of testosterone results in the anatomical differentiation of the male sexual organs. In childhood, testosterone concentrations are low. They increase during puberty, activating characteristic physical changes and initiating [pb_glossary id=\"1200\"]spermatogenesis[\/pb_glossary].<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"11-1h\"><\/a>Functions of Testosterone<\/strong><\/h5>\r\n<p style=\"text-align: justify\">The continued presence of testosterone is necessary to keep the male reproductive system working properly, and Leydig cells produce approximately 6 to 7 mg of testosterone per day. Testosterone concentrations can be 100 times higher in the testes than in the circulation. Maintaining these normal concentrations of testosterone promotes spermatogenesis, whereas low levels of testosterone can lead to infertility. Testosterone is also released into the systemic circulation and plays an important role in muscle development, bone growth, the development of secondary sex characteristics, and maintaining libido (sex drive) in both males and females. In females, the [pb_glossary id=\"483\"]ovaries[\/pb_glossary] secrete small amounts of testosterone, although most is converted to [pb_glossary id=\"1216\"]estradiol[\/pb_glossary]. A small amount of testosterone is also secreted by the [pb_glossary id=\"446\"]adrenal glands[\/pb_glossary] in both sexes.<\/p>\r\n\r\n<h2 style=\"text-align: justify\"><strong><a id=\"11-2\"><\/a>Part 2: Anatomy and Physiology of the Female Reproductive System<\/strong><\/h2>\r\n<p style=\"text-align: justify\">The female reproductive system functions to produce gametes and reproductive hormones; however, it also has the additional task of supporting the developing fetus and delivering it to the outside world. Unlike its male counterpart, the female reproductive system is located primarily inside the pelvic cavity (Figure 5). Recall that the ovaries are the female gonads. The gamete they produce is called an oocyte. We will discuss the production of oocytes in detail shortly. First, let\u2019s look at some of the structures of the female reproductive system.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"150\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image6-9.png\" alt=\"image\" width=\"150\" height=\"150\" \/> Watch <a href=\"https:\/\/youtu.be\/RFDatCchpus\">this Crash Course video<\/a> for an overview of the female reproductive system! Direct link: <a href=\"https:\/\/youtu.be\/RFDatCchpus\">https:\/\/youtu.be\/RFDatCchpus<\/a>[\/caption]\r\n<h5 style=\"text-align: justify\"><strong><a id=\"11-2a\"><\/a>External Female Genitals<\/strong><\/h5>\r\n<p style=\"text-align: justify\">The external female reproductive structures are referred to collectively as the vulva. The mons pubis is a pad of fat found in both males and females that is located at the anterior, over the pubic bone. After puberty, it becomes covered in pubic hair. In females, the <strong>[pb_glossary id=\"1217\"]labia majora[\/pb_glossary]<\/strong> (labia = \u201clips\u201d; majora = \u201clarger\u201d) are folds of hair-covered skin that begin just posterior to the mons pubis. The thinner and more pigmented <strong>[pb_glossary id=\"1218\"]labia minora[\/pb_glossary]<\/strong> (labia = \u201clips\u201d; minora = \u201csmaller\u201d) extend medial to the labia majora. The labia minora serve to protect the female urethra and the entrance to the female reproductive tract.<\/p>\r\n<p style=\"text-align: justify\">The superior, anterior portions of the labia minora come together to encircle the <strong>[pb_glossary id=\"1219\"]clitoris[\/pb_glossary]<\/strong> (or glans clitoris), an organ that has abundant nerves that make it important in sexual sensation and orgasm. The [pb_glossary id=\"1220\"]hymen[\/pb_glossary] is a thin membrane that sometimes partially covers the entrance to the [pb_glossary id=\"1222\"]vagina[\/pb_glossary]. The vaginal opening is located between the opening of the urethra and the anus. It is flanked by outlets to the <strong>[pb_glossary id=\"1221\"]Bartholin\u2019s glands[\/pb_glossary]<\/strong> (or greater vestibular glands).<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"555\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image7-8.png\" alt=\"image\" width=\"555\" height=\"789\" \/> <strong>Figure 5. Female Reproductive System.<\/strong> The major organs of the female reproductive system are located inside the pelvic cavity.[\/caption]\r\n<h5 style=\"text-align: justify\"><strong><a id=\"11-2b\"><\/a>Vagina<\/strong><\/h5>\r\n<p style=\"text-align: justify\">The <strong>vagina<\/strong> is a muscular canal (approximately 10 cm long) that serves as the entrance to the reproductive tract (Figure 5). It also serves as the exit from the uterus during menses and childbirth. The walls of the vagina are lined with an outer fibrous adventitia, a middle layer of smooth muscle, and an inner mucous membrane with transverse folds called rugae. Together, the middle and inner layers allow the expansion of the vagina to accommodate intercourse and childbirth. The Bartholin\u2019s glands and the lesser vestibular glands (located near the clitoris) secrete mucus, which keeps the vestibular area moist.<\/p>\r\n<p style=\"text-align: justify\">The vagina is home to a normal population of microorganisms that help to protect against infection by pathogenic bacteria, yeast, or other organisms that can enter the vagina. Beneficial bacteria secrete lactic acid, which protects the vagina by maintaining an acidic pH (below 4.5). Lactic acid, in combination with other vaginal secretions, makes the vagina a self-cleansing organ.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"11-2c\"><\/a>Ovaries<\/strong><\/h5>\r\n<p style=\"text-align: justify\">The <strong>[pb_glossary id=\"483\"]ovaries[\/pb_glossary]<\/strong> are the female gonads (Figure 5). These paired oval organs are each about 2 to 3 cm in length, about the size of an almond. The ovaries are located within the pelvic cavity, and are supported by [pb_glossary id=\"1224\"]ligaments[\/pb_glossary].<\/p>\r\nEach ovary contains an outer cortex, housing developing oocytes, and an inner medulla, housing large blood vessels and nerves. Within the ovarian cortex, oocytes develop surrounded by supporting cells. forming an ovarian\u00a0<strong>[pb_glossary id=\"1225\"]follicle[\/pb_glossary]<\/strong>. The growth and development of ovarian follicles will be described shortly.\r\n<h5 style=\"text-align: justify\"><strong><a id=\"11-2d\"><\/a>The Ovarian Cycle<\/strong><\/h5>\r\n<p style=\"text-align: justify\">The <strong>[pb_glossary id=\"1226\"]ovarian cycle[\/pb_glossary]<\/strong> is a set of predictable changes in a female\u2019s oocytes and ovarian follicles. During a woman\u2019s reproductive years, it is a roughly 28-day cycle that can be correlated with, but is not the same as, the [pb_glossary id=\"1227\"]menstrual cycle[\/pb_glossary] (discussed shortly). The cycle includes two interrelated processes: [pb_glossary id=\"1228\"]oogenesis[\/pb_glossary] (the production of female gametes) and [pb_glossary id=\"1229\"]folliculogenesis[\/pb_glossary] (the growth and development of ovarian follicles).<\/p>\r\n<p style=\"text-align: justify\"><strong>1. Oogenesis:<\/strong> Gametogenesis in females is called <strong>oogenesis<\/strong>. The process begins with the ovarian stem cells, or <strong>[pb_glossary id=\"1230\"]oogonia[\/pb_glossary]<\/strong>. Oogonia form primary oocytes in the fetal ovary prior to birth. These primary oocytes are then arrested in this stage, only to resume it years later, beginning at puberty and continuing until the woman is near menopause (the cessation of a woman\u2019s reproductive functions). The number of primary oocytes present in the ovaries declines from one to two million in an infant, to approximately 400,000 at puberty, to zero by the end of menopause.<\/p>\r\n<p style=\"text-align: justify\">The initiation of <strong>[pb_glossary id=\"1196\"]ovulation[\/pb_glossary]<\/strong>\u2014the release of an oocyte from the ovary\u2014marks the transition from puberty into reproductive maturity for women. From then on, throughout a woman\u2019s reproductive years, ovulation occurs approximately once every 28 days, triggered by a surge of [pb_glossary id=\"479\"]LH[\/pb_glossary] just prior to ovulation.<\/p>\r\n<p style=\"text-align: justify\">The larger amount of cytoplasm contained in the female gamete is used to supply the developing zygote with nutrients during the period between fertilization and implantation into the [pb_glossary id=\"1244\"]uterus[\/pb_glossary]. Interestingly, sperm contribute only DNA at fertilization, not [pb_glossary id=\"1243\"]cytoplasm[\/pb_glossary]. Therefore, the cytoplasm and all the cytoplasmic organelles in the developing embryo are of maternal origin. This includes mitochondria, which contain their own DNA.<\/p>\r\n<p style=\"text-align: justify\"><strong>2. Folliculogenesis:<\/strong><\/p>\r\nOvarian follicles are oocytes surrounded by supporting cells. They grow and develop in a process called\u00a0<strong>[pb_glossary id=\"1229\"]folliculogenesis[\/pb_glossary]<\/strong>, which typically leads to ovulation of one follicle approximately every 28 days, along with death to multiple other follicles. The death of ovarian follicles is called\u00a0[pb_glossary id=\"1240\"]atresia\u00a0[\/pb_glossary]and can occur at any point during follicular development. Recall that, a female infant at birth will have one to two million\u00a0[pb_glossary id=\"1232\"]oocytes\u00a0[\/pb_glossary]within her ovarian follicles, and that this number declines throughout life until menopause, when no follicles remain.\r\n<p style=\"text-align: justify\">Folliculogenesis begins with follicles in a resting state. These small <strong>[pb_glossary id=\"1233\"]primordial follicles[\/pb_glossary]<\/strong> are present in newborn females and are the prevailing follicle type in the adult ovary (Figure 6). Primordial follicles have only a single flat layer of supporting cells, called <strong>[pb_glossary id=\"1234\"]granulosa cells[\/pb_glossary]<\/strong>, that surround the oocyte, and they can stay in this resting state for years\u2014some until right before menopause.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"725\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image8-8.png\" alt=\"image\" width=\"725\" height=\"954\" \/> <strong>Figure 6. Folliculogenesis.<\/strong> (a) The maturation of a follicle is shown in a clockwise direction proceeding from the primordial follicles. FSH stimulates the growth of a tertiary follicle, and LH stimulates the production of estrogens by granulosa and theca cells. Once the follicle is mature, it ruptures and releases the oocyte. Cells remaining in the follicle then develop into the corpus luteum. (b) In this electron micrograph of a secondary follicle, the oocyte, theca cells (thecae folliculi), and developing antrum are clearly visible. EM \u00d7 1100. (Micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012)[\/caption]\r\n<p style=\"text-align: justify\">After puberty, a few primordial follicles will respond to a recruitment signal each day and will join a pool of immature growing follicles called primary follicles. Primary follicles start with a single layer of granulosa cells, but the granulosa cells then become active and transition from a flat or squamous shape to a rounded cuboidal shape as they increase in size and proliferate. As the granulosa cells divide, the follicles\u2014now called <strong>[pb_glossary id=\"1236\"]secondary follicles[\/pb_glossary]<\/strong> (Figure 6)\u2014increase in diameter, adding a new outer layer of connective tissue, blood vessels, and <strong>[pb_glossary id=\"1245\"]theca cells[\/pb_glossary]<\/strong>\u2014cells that work with the granulosa cells to produce estrogens.<\/p>\r\n<p style=\"text-align: justify\">Within the growing secondary follicle, the primary oocyte now secretes a thin acellular membrane called the zona pellucida that will play a critical role in fertilization. A thick fluid, called follicular fluid, that has formed between the granulosa cells also begins to collect into one large pool, or <strong>[pb_glossary id=\"1238\"]antrum[\/pb_glossary]<\/strong>. Follicles in which the antrum has become large and fully formed are considered <strong>[pb_glossary id=\"1239\"]tertiary follicles[\/pb_glossary]<\/strong> (or antral follicles). Several follicles reach the tertiary stage at the same time, and most of these will undergo [pb_glossary id=\"1240\"]atresia[\/pb_glossary]. The one that does not die will continue to grow and develop until [pb_glossary id=\"1196\"]ovulation[\/pb_glossary], when it will expel its secondary oocyte surrounded by several layers of granulosa cells from the ovary. Keep in mind that most follicles do not make it to this point. In fact, roughly 99 percent of the follicles in the ovary will undergo atresia..<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"11-2e\"><\/a>Hormonal Control of the Ovarian Cycle<\/strong><\/h5>\r\n<p style=\"text-align: justify\">The process of development that we have just described, from primordial follicle to early tertiary follicle, takes approximately two months in humans. The final stages of development of a small cohort of tertiary follicles, ending with ovulation of a secondary oocyte, occur over a course of approximately 28 days. These changes are mostly regulated by the hormones [pb_glossary id=\"1247\"]GnRH[\/pb_glossary], [pb_glossary id=\"479\"]LH[\/pb_glossary], and [pb_glossary id=\"478\"]FSH[\/pb_glossary]. For more about hormonal function, refer to the <a href=\"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/chapter\/unit-1-the-endocrine-system\/\">Endocrine System unit<\/a>.<\/p>\r\n<p style=\"text-align: justify\">As in men, the [pb_glossary id=\"392\"]hypothalamus[\/pb_glossary] produces GnRH, a hormone that signals the anterior [pb_glossary id=\"391\"]pituitary gland[\/pb_glossary] to produce the [pb_glossary id=\"394\"]gonadotropins[\/pb_glossary] FSH and LH (Figure 7). These gonadotropins leave the pituitary gland and travel through the bloodstream to the [pb_glossary id=\"483\"]ovaries[\/pb_glossary], where they bind to receptors on the [pb_glossary id=\"1234\"]granulosa[\/pb_glossary] and [pb_glossary id=\"1245\"]theca cells[\/pb_glossary] of the [pb_glossary id=\"1225\"]follicles[\/pb_glossary]. [pb_glossary id=\"478\"]FSH[\/pb_glossary] stimulates the follicles to grow (hence its name of follicle-stimulating hormone). The release of [pb_glossary id=\"479\"]LH[\/pb_glossary] stimulates the granulosa and theca cells to produce the sex steroid hormone [pb_glossary id=\"1216\"]estradiol[\/pb_glossary], a type of estrogen. This phase of the [pb_glossary id=\"1226\"]ovarian cycle,[\/pb_glossary] when the ovarian follicles are growing and secreting estrogens, is known as the [pb_glossary id=\"1405\"]follicular phase[\/pb_glossary].<\/p>\r\n<p style=\"text-align: justify\">The more granulosa and theca cells a follicle has (that is, the larger and more developed it is), the more [pb_glossary id=\"481\"]estrogens[\/pb_glossary] it will produce in response to LH stimulation. As a result of the large follicles producing large amounts of estrogens, systemic plasma estrogens concentrations increase. Following a classic negative feedback loop, these increasing concentrations of estrogens will inhibit the production of GnRH, LH, and FSH by the hypothalamus and pituitary gland. Because the large ovarian follicles require FSH to grow and survive at this point, this decline in FSH caused by negative feedback leads most of them to die ([pb_glossary id=\"1240\"]atresia[\/pb_glossary]). Typically, only one tertiary follicle from one ovary, now called the dominant follicle, will survive this reduction in FSH, and this follicle will be the one that releases an oocyte.<\/p>\r\n<p style=\"text-align: justify\">When only the one dominant follicle remains in the ovary, it again begins to secrete estrogens. It produces so much estrogen that the normal negative feedback does not occur. Instead, these extremely high concentrations of systemic plasma estrogens trigger a regulatory switch in the anterior pituitary that responds by secreting large amounts of LH and FSH into the bloodstream (Figure 7). The positive feedback loop by which more estrogens trigger release of more LH and FSH only occurs at this point in the cycle.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"829\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image9-8.png\" alt=\"image\" width=\"829\" height=\"972\" \/> <strong>Figure 7. Hormonal Regulation of Ovulation.<\/strong> The hypothalamus and pituitary gland regulate the ovarian cycle and ovulation. GnRH activates the anterior pituitary gland to produce LH and FSH, which stimulate the production of estrogens and progesterone by the ovaries.[\/caption]\r\n<p style=\"text-align: justify\">It is this large burst of LH (called the LH surge) which leads to ovulation of the dominant follicle. The LH surge induces many changes involving the degradation of the wall, and resulting in the expulsion of the [pb_glossary id=\"1232\"]oocyte[\/pb_glossary] surrounded by granulosa cells into the peritoneal cavity. This release is <strong>[pb_glossary id=\"1196\"]ovulation[\/pb_glossary]<\/strong>. In a typical 28 day cycle, ovulation occurs on day 14.<\/p>\r\n<p style=\"text-align: justify\">In the next section, you will follow the ovulated oocyte as it travels toward the\u00a0[pb_glossary id=\"1244\"]uterus[\/pb_glossary]; however, there is one more important event that occurs in the ovarian cycle. The surge of LH also stimulates a change in the granulosa and theca cells that remain in the ruptured follicle after the oocyte has been ovulated. This change is called [pb_glossary id=\"1254\"]luteinization[\/pb_glossary] (recall that the full name of LH is luteinizing hormone), and it transforms the ruptured follicle into a new endocrine structure called the <strong>[pb_glossary id=\"1255\"]corpus luteum[\/pb_glossary]<\/strong>, a term meaning \u201cyellowish body\u201d (Figure 8). The luteinized granulosa and theca cells of the corpus luteum begin to produce large amounts of the sex steroid hormone [pb_glossary id=\"482\"]progesterone[\/pb_glossary], a hormone that is critical for the establishment and maintenance of pregnancy. Progesterone triggers negative feedback on the hypothalamus and pituitary gland, which keeps GnRH, LH, and FSH secretions low, so no new dominant follicles develop at this time. The corpus luteum also secretes moderate amounts of estrogens.<\/p>\r\n<p style=\"text-align: justify\">The post-ovulatory phase is known as the [pb_glossary id=\"1406\"]luteal phase[\/pb_glossary] of the ovarian cycle. If pregnancy does not occur within 10 to 12 days, the corpus luteum will stop secreting progesterone and estrogens and degrade into the <strong>[pb_glossary id=\"1256\"]corpus albicans[\/pb_glossary]<\/strong>, a nonfunctional \u201cwhitish body\u201d that will disintegrate in the ovary over a period of several months. During this time of reduced steroid hormone secretion, FSH and LH are once again stimulated, and the follicular phase begins again with a new cohort of early ovarian follicles beginning to grow and secrete estrogens.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"11-2f\"><\/a>The Uterine Tubes<\/strong><\/h5>\r\n<p style=\"text-align: justify\">The <strong>[pb_glossary id=\"1257\"]uterine tubes[\/pb_glossary]<\/strong> (also called fallopian tubes or oviducts) serve as the conduit of the oocyte from the ovary to the uterus (Figure 8). Each of the two uterine tubes is close to, but not directly connected to, the ovary and divided into sections. The middle region of the tube, called the <strong>[pb_glossary id=\"1208\"]ampulla[\/pb_glossary]<\/strong>, is where fertilization often occurs. The uterine tubes also contain ciliated cells that beat in the direction of the uterus, producing a current that will be critical to move the oocyte.<\/p>\r\n<p style=\"text-align: justify\">Following ovulation, the secondary oocyte surrounded by a few granulosa cells is released into the peritoneal cavity. The nearby uterine tube, either left or right, receives the oocyte. High concentrations of estrogens that occur around the time of ovulation induce contractions of the smooth muscle along the length of the uterine tube and the result is a coordinated movement that sweeps the surface of the ovary and the pelvic cavity. Current flowing toward the uterus is generated by coordinated beating of the [pb_glossary id=\"1258\"]cilia[\/pb_glossary] that line the lumen of the length of the uterine tube. These cilia beat more strongly in response to the high estrogens concentrations that occur around the time of ovulation. As a result of these mechanisms, the oocyte\u2013granulosa cell complex is pulled into the interior of the tube. Once inside, the muscular contractions and beating cilia move the oocyte slowly toward the uterus. When fertilization does occur, sperm typically meet the egg while it is still moving through the [pb_glossary id=\"1208\"]ampulla[\/pb_glossary].<\/p>\r\n<p style=\"text-align: justify\">If the oocyte is successfully [pb_glossary id=\"1390\"]fertilized[\/pb_glossary], the resulting zygote will begin to divide into two cells, then four, and so on, as it makes its way through the uterine tube and toward the uterus. If the oocyte is not fertilized, it will simply degrade\u2014either in the uterine tube or in the uterus, where it may be shed with the next [pb_glossary id=\"1227\"]menstrual[\/pb_glossary] period.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"818\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image10-7.png\" alt=\"image\" width=\"818\" height=\"393\" \/> <strong>Figure 8. Ovaries, Uterine Tubes, and Uterus.<\/strong> This anterior view shows the relationship of the ovaries, uterine tubes (oviducts), and uterus. Sperm enter through the vagina, and fertilization of an ovulated oocyte usually occurs in the distal uterine tube. From left to right, LM \u00d7 400, LM \u00d7 20. (Micrographs provided by the Regents of University of Michigan Medical School \u00a9 2012)[\/caption]\r\n<h5 style=\"text-align: justify\"><strong><a id=\"11-2g\"><\/a>The Uterus<\/strong><strong> and Cervix<\/strong><\/h5>\r\n<p style=\"text-align: justify\">The uterus is the muscular organ that nourishes and supports the growing embryo (Figure 8). Its average size is approximately 5 cm wide by 7 cm long when a female is not pregnant. It has three sections. The portion of the uterus superior to the opening of the uterine tubes is called the <strong>[pb_glossary id=\"861\"]fundus[\/pb_glossary]<\/strong>. The middle section of the uterus is called the <strong>[pb_glossary id=\"1396\"]body of uterus[\/pb_glossary]<\/strong> (or corpus). The <strong>[pb_glossary id=\"1223\"]cervix[\/pb_glossary]<\/strong> is the narrow inferior portion of the uterus that projects into the vagina. The cervix produces mucus secretions that can facilitate sperm movement through the reproductive tract. Several ligaments maintain the position of the uterus within the abdominopelvic cavity.<\/p>\r\n<p style=\"text-align: justify\">The wall of the uterus is made up of three layers. The most superficial layer is the serous membrane, or <strong>[pb_glossary id=\"1393\"]perimetrium[\/pb_glossary]<\/strong>, which consists of epithelial tissue that covers the exterior portion of the uterus. The middle layer, or <strong>[pb_glossary id=\"1394\"]myometrium[\/pb_glossary]<\/strong>, is a thick layer of smooth muscle responsible for uterine contractions. Most of the uterus is myometrial tissue, which allows the powerful contractions that occur during labour and the less powerful contractions (or cramps) that help to expel menstrual blood during a woman\u2019s period.<\/p>\r\n<p style=\"text-align: justify\">The innermost layer of the uterus is called the endometrium. The <strong>[pb_glossary id=\"1395\"]endometrium[\/pb_glossary]<\/strong> contains a connective tissue lining, the lamina propria, which is covered by [pb_glossary id=\"654\"]epithelial tissue[\/pb_glossary] that lines the [pb_glossary id=\"777\"]lumen[\/pb_glossary]. Structurally, the endometrium consists of two layers: the [pb_glossary id=\"1397\"]stratum basalis[\/pb_glossary] and the [pb_glossary id=\"1399\"]stratum functionalis[\/pb_glossary] (the basal and functional layers). The stratum basalis layer is part of the [pb_glossary id=\"769\"]lamina propria[\/pb_glossary] and is adjacent to the myometrium; this layer does not shed during menses. In contrast, the thicker stratum functionalis layer contains the glandular portion of the lamina propria and the endothelial tissue that lines the uterine lumen. It is the stratum functionalis that grows and thickens in response to increased levels of estrogens and progesterone. In the [pb_glossary id=\"1406\"]luteal phase[\/pb_glossary] of the menstrual cycle, special branches off of the uterine artery called spiral arteries supply the thickened stratum functionalis. This inner functional layer provides the proper site of implantation for the fertilized oocyte, and\u2014should fertilization not occur\u2014it is only the stratum functionalis layer of the endometrium that sheds during menstruation.<\/p>\r\n<p style=\"text-align: justify\">Recall that during the [pb_glossary id=\"1405\"]follicular phase[\/pb_glossary] of the ovarian cycle, the ovarian follicles are growing and secreting [pb_glossary id=\"481\"]estrogens[\/pb_glossary]. At the same time, the stratum functionalis of the endometrium is thickening to prepare for a potential implantation. The post-ovulatory increase in progesterone, which characterizes the [pb_glossary id=\"1406\"]luteal phase[\/pb_glossary], is key for maintaining a thick stratum functionalis. As long as a functional [pb_glossary id=\"1255\"]corpus luteum[\/pb_glossary] is present in the ovary, the endometrial lining is prepared for implantation. Indeed, if a blastocyst implants, signals are sent to the [pb_glossary id=\"1255\"]corpus luteum[\/pb_glossary] to continue secreting progesterone to maintain the endometrium, and thus maintain the pregnancy. If a blastocyst does not implant, no signal is sent to the corpus luteum and it degrades, ceasing progesterone production and ending the luteal phase. Without [pb_glossary id=\"482\"]progesterone[\/pb_glossary], the endometrium becomes thinner and, under the influence of [pb_glossary id=\"1017\"]prostaglandins[\/pb_glossary], the spiral arteries of the endometrium constrict and rupture, preventing oxygenated blood from reaching the endometrial tissue. As a result, endometrial tissue dies and blood, pieces of the endometrial tissue, and white blood cells are shed through the vagina during menstruation, or the <strong>[pb_glossary id=\"1400\"]implantation[\/pb_glossary][pb_glossary id=\"1401\"]menses[\/pb_glossary]<\/strong>.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"11-2h\"><\/a>The Menstrual (Uterine) Cycle<\/strong><\/h5>\r\n<p style=\"text-align: justify\">Now that we have discussed the maturation of the cohort of ovarian follicles in the ovary, the build-up and then shedding of the endometrial lining in the uterus, and the function of the uterine tubes and vagina, we can put everything together to talk about the three phases of the <strong>menstrual (uterine) cycle<\/strong>\u2014the series of changes in which the uterine lining is shed, rebuilds, and prepares for implantation.<\/p>\r\n<p style=\"text-align: justify\">The timing of the menstrual cycle starts with the first day of menses, referred to as day one of a woman\u2019s period.\u00a0The average length of a woman\u2019s menstrual cycle is 28 days (typically varying from 21 to 32 days) and it is the time period used to identify the timing of events in the cycle.<\/p>\r\n<p style=\"text-align: justify\">Just as the hormones produced by the [pb_glossary id=\"1234\"]granulosa[\/pb_glossary] and [pb_glossary id=\"1245\"]theca cells[\/pb_glossary] of the ovary \u201cdrive\u201d the follicular and luteal phases of the [pb_glossary id=\"1226\"]ovarian cycle[\/pb_glossary], they also control the three distinct phases of the [pb_glossary id=\"1227\"]menstrual cycle[\/pb_glossary]. These are the [pb_glossary id=\"1404\"]menses phase[\/pb_glossary], the [pb_glossary id=\"1402\"]proliferative phase[\/pb_glossary], and the [pb_glossary id=\"1403\"]secretory phase[\/pb_glossary].<\/p>\r\n<p style=\"text-align: justify\"><strong>1. Menses Phase:<\/strong> The <strong>menses phase<\/strong> of the menstrual cycle is the phase during which the endometrial lining is shed; that is, the days that the woman menstruates. Although it averages approximately five days, the menses phase can last from 2 to 7 days, or longer. The menses phase occurs during the early days of the [pb_glossary id=\"1405\"]follicular phase[\/pb_glossary] of the ovarian cycle, when [pb_glossary id=\"482\"]progesterone[\/pb_glossary], [pb_glossary id=\"478\"]FSH[\/pb_glossary], and [pb_glossary id=\"479\"]LH[\/pb_glossary] levels are low (Figure 9). Recall that progesterone concentrations decline as a result of the degradation of the [pb_glossary id=\"1255\"]corpus luteum[\/pb_glossary], marking the end of the luteal phase. This decline in progesterone triggers the shedding of the stratum functionalis of the endometrium.<\/p>\r\n<p style=\"text-align: justify\"><strong>2. Proliferative Phase:<\/strong> Once the menstrual flow ceases, the [pb_glossary id=\"1395\"]endometrium[\/pb_glossary] begins to proliferate again, marking the beginning of the <strong>proliferative phase<\/strong> of the menstrual cycle (Figure 9). It occurs when the granulosa and theca cells of the [pb_glossary id=\"1239\"]tertiary follicles[\/pb_glossary] begin to produce increased amounts of [pb_glossary id=\"481\"]estrogens[\/pb_glossary]. These rising estrogen concentrations stimulate the endometrial lining to rebuild.<\/p>\r\n<p style=\"text-align: justify\">Recall that increasing estrogen concentrations leads to a decrease in [pb_glossary id=\"478\"]FSH[\/pb_glossary] as a result of negative feedback, resulting in atresia of all but one of the developing tertiary follicles. The switch to positive feedback\u2014which occurs with the elevated production of estrogens from the dominant follicle\u2014then stimulates the [pb_glossary id=\"479\"]LH[\/pb_glossary] surge that will trigger [pb_glossary id=\"1196\"]ovulation[\/pb_glossary]. Ovulation marks the end of the proliferative phase as well as the end of the follicular phase.<\/p>\r\n<p style=\"text-align: justify\"><strong>3. <\/strong><strong>Secretory Phase:<\/strong> In addition to prompting the LH surge, high estrogen levels increase the uterine tube contractions that facilitate the pick-up and transfer of the ovulated [pb_glossary id=\"1232\"]oocyte[\/pb_glossary]. High estrogen levels also slightly decrease the acidity of the vagina, making it more hospitable to sperm. In the ovary, the luteinization of the granulosa cells of the ruptured follicle forms the corpus luteum, marking the beginning of the luteal phase of the ovarian cycle. In the uterus, progesterone from the corpus luteum begins the <strong>secretory phase<\/strong> of the menstrual cycle, in which the endometrial lining prepares for implantation (Figure 9). Over the next 10 to 12 days, the endometrial glands secrete a fluid rich in glycogen. If fertilization has occurred, this fluid will nourish the ball of cells now developing from the zygote, known as the conceptus. At the same time, the spiral arteries develop to provide blood to the thickened [pb_glossary id=\"1399\"]stratum functionalis[\/pb_glossary].<\/p>\r\n<p style=\"text-align: justify\">If no pregnancy occurs within approximately 10 to 12 days, the corpus luteum will degrade into the corpus albicans. Levels of both estrogens and progesterone will fall, and the endometrium will grow thinner. [pb_glossary id=\"1017\"]Prostaglandins[\/pb_glossary] are then secreted which causes the constriction of the spiral arteries, reducing oxygen supply. The endometrial tissue will die, resulting in menses\u2014or the first day of the next cycle.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"619\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image11-7.png\" alt=\"image\" width=\"619\" height=\"1059\" \/> <strong>Figure 9. Hormone Levels in the Ovarian and Menstrual Cycles.<\/strong> The correlation of the hormone levels and their effects on the female reproductive system is shown in this timeline of the ovarian and menstrual cycles. The menstrual cycle begins at day one with the start of menses. Ovulation occurs around day 14 of a 28-day cycle, triggered by the LH surge.[\/caption]\r\n<h5 style=\"text-align: justify\"><strong><a id=\"11-2i\"><\/a>The Breasts<\/strong><\/h5>\r\n<p style=\"text-align: justify\">Although the breasts are located far from the other female reproductive organs, they are considered accessory organs of the female reproductive system. The function of the breasts is to supply milk to an infant in a process called lactation. The external features of the breast include a nipple surrounded by a pigmented <strong>[pb_glossary id=\"1407\"]areola[\/pb_glossary]<\/strong> (Figure 10). When a baby nurses, or draws milk from the breast, the entire areolar region is taken into the mouth.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"697\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image12-7.png\" alt=\"image\" width=\"697\" height=\"347\" \/> <strong>Figure 10. Anatomy of the Breast.<\/strong> During lactation, milk moves from the alveoli through the lactiferous ducts to the nipple.[\/caption]\r\n<p style=\"text-align: justify\">Breast milk is produced by the <strong>[pb_glossary id=\"1408\"]mammary glands[\/pb_glossary]<\/strong>, which are modified sweat glands. The milk itself exits the breast through the nipple via 15 to 20 <strong>lactiferous ducts<\/strong> that open on the surface of the nipple. These lactiferous ducts each extend to a <strong>lactiferous sinus<\/strong> that connects to a glandular lobe that contains groups of milk-secreting cells in clusters called <strong>alveoli<\/strong> (Figure 10). Once milk is made in the alveoli, stimulated myoepithelial cells that surround the alveoli contract to push the milk to the lactiferous sinuses. From here, the baby can draw milk through the lactiferous ducts by suckling. The non-pregnant and non-lactating female breast is composed primarily of adipose and collagenous tissue, with mammary glands making up a very minor proportion of breast volume.<\/p>\r\n\r\n<h2 style=\"text-align: justify\"><strong><a id=\"11-3\"><\/a>Part 3: Fertilization<\/strong><\/h2>\r\n<p style=\"text-align: justify\"><strong>[pb_glossary id=\"1390\"]Fertilization[\/pb_glossary]<\/strong> occurs when a sperm and an oocyte combine and their nuclei fuse. Because each of these reproductive cells is haploid, containing half of the genetic material needed to form a human being, their combination forms a diploid cell. The resulting diploid cell, called a<strong> [pb_glossary id=\"1409\"]zygote[\/pb_glossary]<\/strong>, contains all the genetic material needed to form a human\u2014half from the mother and half from the father.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"11-3a\"><\/a>Transit of Sperm<\/strong><\/h5>\r\n<p style=\"text-align: justify\">During ejaculation, hundreds of millions of sperm (spermatozoa) are released into the vagina. Almost immediately, millions of these sperm are overcome by the acidity of the [pb_glossary id=\"1222\"]vagina[\/pb_glossary] (approximately pH 3.8), and millions more may be blocked from entering the uterus by thick cervical mucus. Of those that do enter, thousands are destroyed by phagocytic uterine [pb_glossary id=\"544\"]leukocytes[\/pb_glossary]. Thus, the race into the uterine tubes, which is the most typical site for sperm to encounter the [pb_glossary id=\"1232\"]oocyte[\/pb_glossary], is reduced to a few thousand contenders. Their journey\u2014thought to be facilitated by uterine contractions\u2014usually takes from 30 minutes to 2 hours. If the sperm do not encounter an oocyte immediately, they can survive in the [pb_glossary id=\"1257\"]uterine tubes[\/pb_glossary] for another 3\u20135 days. Intercourse more than a day after ovulation is therefore unlikely to result in fertilization.<\/p>\r\n<p style=\"text-align: justify\">During the journey, fluids in the female reproductive tract prepare the sperm for fertilization through a process called <strong>[pb_glossary id=\"1410\"]capacitation[\/pb_glossary]<\/strong>, or priming. The fluids improve the motility of the spermatozoa and prepare them for contact with the oocyte. If sperm reach the oocyte before capacitation is complete, they will be unable to penetrate the oocyte\u2019s thick outer protective layers.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><span style=\"color: #000000\"><strong><a id=\"11-3b\" style=\"color: #000000\"><\/a>Contact Between Sperm and Oocyte<\/strong><\/span><\/h5>\r\n<p style=\"text-align: justify\">Upon ovulation, the oocyte released by the ovary is swept into\u2014and along\u2014the uterine tube. Fertilization should occur in the distal uterine tube because an unfertilized oocyte cannot survive the 72-hour journey to the uterus. This oocyte (specifically a [pb_glossary id=\"1242\"]secondary oocyte[\/pb_glossary]) is surrounded by two protective layers. The <strong>[pb_glossary id=\"1411\"]corona radiata[\/pb_glossary]<\/strong> is an outer layer of follicular (granulosa) cells that form around a developing oocyte in the ovary and remain with it upon ovulation. The underlying <strong>[pb_glossary id=\"1237\"]zona pellucida[\/pb_glossary]<\/strong> (pellucid = \u201ctransparent\u201d) is a transparent, but thick, [pb_glossary id=\"581\"]glycoprotein[\/pb_glossary] membrane that surrounds the cell\u2019s plasma membrane.<\/p>\r\n<p style=\"text-align: justify\">The oocyte encounters the surviving capacitated sperm, which stream toward it in response to chemical attractants released by the cells of the corona radiata. To reach the oocyte itself, the sperm must penetrate the two protective layers. The sperm first burrow through the cells of the corona radiata. Then, upon contact with the zona pellucida, the sperm bind to receptors in the zona pellucida. This initiates a process called the <strong>[pb_glossary id=\"1412\"]acrosomal reaction[\/pb_glossary]<\/strong> in which the enzyme-filled \u201ccap\u201d of the sperm, called the acrosome, releases its stored digestive enzymes. These enzymes clear a path through the zona pellucida that allows sperm to reach the oocyte. Finally, a single sperm makes contact with sperm-binding receptors on the oocyte\u2019s plasma membrane (Figure 11). The plasma membrane of that sperm then fuses with the oocyte\u2019s plasma membrane, and the head and mid-piece of this sperm enter the oocyte interior.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"1424\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image13-8.png\" alt=\"image\" width=\"1424\" height=\"983\" \/> <strong>Figure 11. Sperm and the Process of Fertilization.<\/strong> Before fertilization, hundreds of capacitated sperm must break through the surrounding corona radiata and zona pellucida so that one can contact and fuse with the oocyte plasma membrane.[\/caption]\r\n<p style=\"text-align: justify\">How do sperm penetrate the [pb_glossary id=\"1411\"]corona radiata[\/pb_glossary]? Some sperm undergo a spontaneous acrosomal reaction, which is an acrosomal reaction not triggered by contact with the zona pellucida. The digestive enzymes released by this reaction digest the extracellular matrix of the corona radiata. Thus, the first sperm to reach the oocyte is never the one to fertilize it. Rather, hundreds of sperm cells must undergo the [pb_glossary id=\"1412\"]acrosomal reaction[\/pb_glossary], each helping to degrade the corona radiata and zona pellucida until a path is created to allow one sperm to contact and fuse with the plasma membrane of the oocyte.<\/p>\r\n<p style=\"text-align: justify\">When the first sperm fuses with the oocyte, the oocyte deploys two mechanisms to prevent <strong>[pb_glossary id=\"1413\"]polyspermy[\/pb_glossary]<\/strong>, which is penetration by more than one sperm. This is critical because if more than one sperm were to fertilize the oocyte, the resulting zygote would be a triploid organism with three sets of chromosomes. This is incompatible with life.<\/p>\r\n<p style=\"text-align: justify\">The first mechanism is the fast block, which involves a near instantaneous change in sodium ion permeability upon binding of the first sperm, [pb_glossary id=\"521\"]depolarizing[\/pb_glossary] the oocyte plasma membrane and preventing the fusion of additional sperm cells. The fast block sets in almost immediately and lasts for about a minute, during which time an influx of calcium ions following sperm penetration triggers the second mechanism, the slow block. In this process, referred to as the <strong>[pb_glossary id=\"1415\"]cortical reaction[\/pb_glossary]<\/strong>, cortical granules sitting immediately below the oocyte plasma membrane fuse with the membrane and release certain proteins and polysaccharides into the space between the plasma membrane and the zona pellucida. The proteins cause the release of any other attached sperm and destroy the oocyte\u2019s sperm receptors, and the mucopolysaccharides then coat the nascent zygote in an impenetrable barrier, forming a <strong>[pb_glossary id=\"1414\"]fertilization membrane[\/pb_glossary]<\/strong>.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"11-3c\"><\/a>The Zygote<\/strong><\/h5>\r\n<p style=\"text-align: justify\">Upon fertilization the two haploid nuclei derived from the sperm and oocyte decondense, expand, and replicate their DNA. The pronuclei then migrate toward each other, their nuclear envelopes disintegrate, and the male- and female-derived genetic material intermingles. This step completes the process of fertilization and results in a single-celled [pb_glossary id=\"1416\"]diploid[\/pb_glossary] [pb_glossary id=\"1409\"]zygote[\/pb_glossary] with all the genetic instructions it needs to develop into a human.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"150\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image14-7.png\" alt=\"image\" width=\"150\" height=\"150\" \/> Watch <a href=\"https:\/\/youtu.be\/SUdAEGXLO-8\">this CrashCourse video<\/a> to learn more about fertilization! Direct link: <a href=\"https:\/\/youtu.be\/SUdAEGXLO-8\">https:\/\/youtu.be\/SUdAEGXLO-8<\/a>[\/caption]\r\n\r\n&nbsp;\r\n<h2 style=\"text-align: left\"><strong><a id=\"11-4\"><\/a>Part 4: Embryonic Development<\/strong><\/h2>\r\n<p style=\"text-align: justify\">Throughout this chapter, we will express embryonic and fetal ages in terms of weeks from [pb_glossary id=\"1390\"]fertilization[\/pb_glossary], commonly called conception. The period of time required for full development of a fetus in utero is referred to as <strong>[pb_glossary id=\"1421\"]gestation[\/pb_glossary]<\/strong> (gestare = \u201cto carry\u201d or \u201cto bear\u201d). It can be subdivided into distinct gestational periods. A developing human is referred to as an <strong>[pb_glossary id=\"1422\"]embryo[\/pb_glossary]<\/strong> during weeks 3\u20138, and a<strong> [pb_glossary id=\"1423\"]fetus[\/pb_glossary]<\/strong> from the ninth week of gestation until birth. In this section, we\u2019ll cover the embryonic stages of development, which are characterized by cell division, migration, and differentiation. By the end of the embryonic period, all the organ systems are structured in rudimentary form, although the organs themselves are either nonfunctional or only semi-functional.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"11-4a\"><\/a>Pre-implantation Embryonic Development<\/strong><\/h5>\r\n<p style=\"text-align: justify\">Following fertilization, the [pb_glossary id=\"1409\"]zygote[\/pb_glossary] and its associated membranes, together referred to as the <strong>[pb_glossary id=\"1424\"]conceptus[\/pb_glossary]<\/strong>, continue to be projected toward the uterus by [pb_glossary id=\"822\"]peristalsis[\/pb_glossary] and beating [pb_glossary id=\"1258\"]cilia[\/pb_glossary]. During its journey to the uterus, the zygote undergoes five or six rapid [pb_glossary id=\"1425\"]mitotic[\/pb_glossary] cell divisions. Although each <strong>[pb_glossary id=\"1426\"]cleavage[\/pb_glossary]<\/strong> results in more cells, it does not increase the total volume of the conceptus (Figure 12). Each daughter cell produced by cleavage is called a <strong>[pb_glossary id=\"1427\"]blastomere[\/pb_glossary]<\/strong> (blastos = \u201cgerm,\u201d in the sense of a seed or sprout).<\/p>\r\n<p style=\"text-align: justify\">Approximately 3 days after fertilization, a 16-cell conceptus reaches the uterus. The cells that had been loosely grouped are now compacted and look more like a solid mass. The name given to this structure is the <strong>[pb_glossary id=\"1428\"]morula[\/pb_glossary]<\/strong> (morula = \u201clittle mulberry\u201d). Once inside the uterus, the conceptus floats freely for several more days. It continues to divide, creating a ball of approximately 100 cells, and consuming nutritive endometrial secretions while the uterine lining thickens. The ball of now tightly bound cells starts to secrete fluid and organize themselves around a fluid-filled cavity, the<strong> blastocoel<\/strong>. At this developmental stage, the conceptus is referred to as a <strong>[pb_glossary id=\"1429\"]blastocyst[\/pb_glossary]<\/strong>. Within this structure, a group of cells forms into an <strong>[pb_glossary id=\"1431\"]inner cell mass[\/pb_glossary]<\/strong>, which is fated to become the embryo. Other cells form the outer shell called <strong>[pb_glossary id=\"1430\"]trophoblasts[\/pb_glossary]<\/strong> (trophe = \u201cto feed\u201d or \u201cto nourish\u201d). These cells will develop into the chorionic sac and the fetal portion of the <strong>[pb_glossary id=\"1417\"]placenta[\/pb_glossary] <\/strong>(the organ of nutrient, waste, and gas exchange between mother and the developing offspring).<\/p>\r\n<p style=\"text-align: justify\">The inner mass of embryonic cells is [pb_glossary id=\"1432\"]totipotent[\/pb_glossary] during this stage, meaning that each cell has the potential to differentiate into any cell type in the human body. Totipotency lasts for only a few days before the cells\u2019 fates are set as being the precursors to a specific lineage of cells.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"1262\"]<img class=\"\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image15-6.png\" alt=\"image\" width=\"1262\" height=\"1107\" \/> <strong>Figure 12. Early Embryonic Development.<\/strong> Cleavages make use of the abundant cytoplasm of the conceptus as the cells rapidly divide without changing the total volume.[\/caption]\r\n<h5 style=\"text-align: justify\"><strong><a id=\"11-4b\"><\/a>Implantation<\/strong><\/h5>\r\n<p style=\"text-align: justify\">At the end of the first week, the [pb_glossary id=\"1429\"]blastocyst[\/pb_glossary] comes in contact with the uterine wall and adheres to it, embedding itself in the uterine lining via the [pb_glossary id=\"1430\"]trophoblast[\/pb_glossary] cells. This process is called <strong>implantation<\/strong> (Figure 13), which can be accompanied by minor bleeding. The blastocyst typically implants in the [pb_glossary id=\"861\"]fundus[\/pb_glossary] of the uterus or on the posterior wall. However, if the [pb_glossary id=\"1395\"]endometrium[\/pb_glossary] is not fully developed and ready to receive the blastocyst, the blastocyst will detach and find a better spot. A significant percentage (50\u201375 percent) of blastocysts fail to implant; when this occurs, the blastocyst is shed with the endometrium during [pb_glossary id=\"1401\"]menses[\/pb_glossary]. The high rate of implantation failure is one reason why pregnancy typically requires several ovulation cycles to be achieved.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"1430\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image16-6.png\" alt=\"image\" width=\"1430\" height=\"1297\" \/> <strong>Figure 13. Early Embryonic Development.<\/strong> Ovulation, fertilization, cleavage embryonic development, and implantation occur at specific locations within the female reproductive system in a time span of approximately 1 week.[\/caption]\r\n<p style=\"text-align: justify\">When implantation succeeds and the blastocyst adheres to the endometrium, the [pb_glossary id=\"804\"]superficial[\/pb_glossary] cells of the trophoblast fuse with each other, forming the <strong>[pb_glossary id=\"1433\"]syncytiotrophoblast[\/pb_glossary]<\/strong>, a multinucleated body that digests endometrial cells to firmly secure the blastocyst to the uterine wall. In response, the uterine mucosa rebuilds itself and envelops the blastocyst (Figure 14). The trophoblast secretes <strong>[pb_glossary id=\"1434\"]human chorionic gonadotropin (hCG)[\/pb_glossary]<\/strong>, a hormone that directs the [pb_glossary id=\"1255\"]corpus luteum[\/pb_glossary] to survive, enlarge, and continue producing [pb_glossary id=\"482\"]progesterone[\/pb_glossary] and [pb_glossary id=\"481\"]estrogens[\/pb_glossary] to suppress menses. These functions of hCG are necessary for creating an environment suitable for the developing embryo. As a result of this increased production, hCG accumulates in the maternal bloodstream and is excreted in the urine. Implantation is complete by the middle of the second week. Just a few days after [pb_glossary id=\"1400\"]implantation[\/pb_glossary], the trophoblast has secreted enough hCG for an at-home urine pregnancy test to give a positive result.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"1430\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image17-6.png\" alt=\"image\" width=\"1430\" height=\"1783\" \/> <strong>Figure 14. Implantation.<\/strong> During implantation, the trophoblast cells of the blastocyst adhere to the endometrium and digest endometrial cells until it is attached securely.[\/caption]\r\n<h5 style=\"text-align: justify\"><strong><a id=\"11-4c\"><\/a>Embryonic Membranes and Germ Layers<\/strong><\/h5>\r\n<p style=\"text-align: justify\">During the second week of development, with the embryo implanted in the uterus, cells within the blastocyst start to organize into layers. Some grow to form the extra-embryonic membranes needed to support and protect the growing embryo: the [pb_glossary id=\"1435\"]amnion[\/pb_glossary], the [pb_glossary id=\"1438\"]yolk sac[\/pb_glossary], the [pb_glossary id=\"1436\"]allantois[\/pb_glossary], and the [pb_glossary id=\"1437\"]chorion[\/pb_glossary] (Figure 16).<\/p>\r\nAs the third week of development begins, the two-layered disc of cells becomes a three-layered disc through the process of\u00a0[pb_glossary id=\"1439\"]gastrulation[\/pb_glossary], during which the cells transition from an undifferentiated to a more differentiated state. Three groups of cells, called germ layers, are formed: the\u00a0<strong>[pb_glossary id=\"1440\"]endoderm[\/pb_glossary]<\/strong>,\u00a0[pb_glossary id=\"1442\"]<strong>mesoderm<\/strong>\u00a0[\/pb_glossary](middle), and\u00a0<strong>[pb_glossary id=\"1441\"]ectoderm[\/pb_glossary]<\/strong>. Each of these germ layers will develop into specific structures in the embryo, setting the stage for organogenesis.\r\n<h5 style=\"text-align: justify\"><strong><a id=\"11-4e\"><\/a>Development of the Placenta<\/strong><\/h5>\r\n<p style=\"text-align: justify\">During the first several weeks of development, the cells of the endometrium\u2014referred to as decidual cells\u2014nourish the [pb_glossary id=\"1443\"]nascent[\/pb_glossary] embryo. During prenatal weeks 4\u201312, the developing placenta gradually takes over the role of feeding the embryo, and the decidual cells are no longer needed. The mature placenta is composed of tissues derived from the embryo, as well as maternal tissues of the endometrium. The placenta connects to the conceptus via the <strong>[pb_glossary id=\"1444\"]umbilical cord[\/pb_glossary]<\/strong>, which carries deoxygenated blood and wastes from the fetus through two umbilical arteries; nutrients and oxygen are carried from the mother to the fetus through the single umbilical vein.<\/p>\r\n<p style=\"text-align: justify\">The maternal portion of the placenta develops from the deepest layer of the endometrium, the decidua basalis. To form the embryonic portion of the placenta, the [pb_glossary id=\"1433\"]syncytiotrophoblast[\/pb_glossary] and the underlying cells of the [pb_glossary id=\"1430\"]trophoblast[\/pb_glossary] (cytotrophoblast cells) begin to proliferate along with a layer of extraembryonic mesoderm cells. These form the <strong>[pb_glossary id=\"1446\"]chorionic membrane[\/pb_glossary]<\/strong>, which envelops the entire conceptus as the chorion. The chorionic membrane forms finger-like structures called <strong>[pb_glossary id=\"1445\"]chorionic villi[\/pb_glossary]<\/strong> that burrow into the endometrium like tree roots, making up the fetal portion of the placenta. The cytotrophoblast cells perforate the chorionic villi, burrow farther into the endometrium, and remodel maternal blood vessels to augment maternal blood flow surrounding the villi. Meanwhile, fetal [pb_glossary id=\"1447\"]mesenchymal[\/pb_glossary] cells derived from the mesoderm fill the villi and differentiate into blood vessels, including the three umbilical blood vessels that connect the embryo to the developing placenta (Figure 15).<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"1315\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image19-4.png\" alt=\"image\" width=\"1315\" height=\"682\" \/> <strong>Figure 15. Cross-Section of the Placenta.<\/strong> In the placenta, maternal and fetal blood components are conducted through the surface of the chorionic villi, but maternal and fetal bloodstreams never mix directly.[\/caption]\r\n<p style=\"text-align: justify\">The development of the placenta, or <strong>[pb_glossary id=\"1448\"]placentation[\/pb_glossary]<\/strong> is complete by weeks 14\u201316. As a fully developed organ, the placenta provides nutrition and excretion, respiration, and endocrine function (Table 1 and Figure 16). It receives blood from the fetus through the umbilical arteries. Capillaries in the chorionic villi filter fetal wastes out of the blood and return clean, oxygenated blood to the fetus through the umbilical vein. Nutrients and oxygen are transferred from maternal blood surrounding the villi through the capillaries and into the fetal bloodstream.<\/p>\r\n<p style=\"text-align: justify\">Maternal and fetal blood do not mingle because blood cells cannot move across the placenta. This separation prevents the mother\u2019s [pb_glossary id=\"641\"]cytotoxic T cells[\/pb_glossary] from reaching and subsequently destroying the fetus, which bears \u201cnon-self\u201d [pb_glossary id=\"623\"]antigens[\/pb_glossary].<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"854\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image20-4.png\" alt=\"image\" width=\"854\" height=\"644\" \/> <strong>Figure 16. Placenta.<\/strong> This post-expulsion placenta and umbilical cord (white) are viewed from the fetal side.[\/caption]\r\n<table style=\"border-collapse: collapse;width: 100%;height: 220px\" border=\"0\"><caption>Table 1: Functions of the Placenta<\/caption>\r\n<tbody>\r\n<tr style=\"height: 14px\">\r\n<th style=\"width: 33.3333%;height: 14px\" scope=\"col\"><strong>Nutrition &amp; digestion<\/strong><\/th>\r\n<th style=\"width: 33.3333%;height: 14px\" scope=\"col\"><strong>Respiration<\/strong><\/th>\r\n<th style=\"width: 33.3333%;height: 14px\" scope=\"col\"><strong>Endocrine function<\/strong><\/th>\r\n<\/tr>\r\n<tr style=\"height: 192px\">\r\n<td style=\"width: 33.3333%;height: 192px\">Mediates diffusion of maternal glucose, amino acids, fatty acids, vitamins, minerals\r\n\r\nStores nutrients during early pregnancy to accommodate increased fetal demand later in pregnancy\r\n\r\nExcretes &amp; filters fetal nitrogenous wastes into maternal blood<\/td>\r\n<td style=\"width: 33.3333%;height: 192px\">Mediates maternal-to-fetal oxygen transport\r\n\r\nMediates fetal-to-maternal carbon dioxide transport<\/td>\r\n<td style=\"width: 33.3333%;height: 192px\">Secretes hCG, estrogens, and progesterone to maintain the pregnancy and stimulate maternal &amp; fetal development\r\n\r\nMediates transmission of maternal hormones into fetal blood\r\n\r\nMediates transmission of fetal hormones into maternal blood<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n[caption id=\"\" align=\"alignnone\" width=\"829\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image22-5.png\" alt=\"image\" width=\"829\" height=\"885\" \/> <strong>Figure 17. Embryo at 7 Weeks.<\/strong> An embryo at the end of 7 weeks of development is only 10 mm in length, but its developing eyes, limb buds, and tail are already visible. (This embryo was derived from an ectopic pregnancy.) (credit: Ed Uthman)[\/caption]\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"150\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image23-5.png\" alt=\"image\" width=\"150\" height=\"150\" \/> Watch <a href=\"https:\/\/youtu.be\/BtsSbZ85yiQ\">this Crash Course video<\/a> to learn about the stages of embryonic development! Direct link: <a href=\"https:\/\/youtu.be\/BtsSbZ85yiQ\">https:\/\/youtu.be\/BtsSbZ85yiQ<\/a>[\/caption]\r\n<h2 style=\"text-align: left\"><strong><a id=\"11-5\"><\/a>Part 5: Fetal Development<\/strong><\/h2>\r\n<p style=\"text-align: justify\">A developing human is called a fetus from the ninth week of gestation until birth. This 30-week period of development is marked by continued cell growth and differentiation, which fully develop the structures and functions of the immature organ systems formed during the embryonic period. The completion of fetal development results in a newborn who, although still immature in many ways, is capable of survival outside the womb.<\/p>\r\n\r\n\r\n[caption id=\"attachment_340\" align=\"alignleft\" width=\"145\"]<a href=\"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2019\/08\/qrcode.jpeg\"><img class=\" wp-image-340\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2019\/08\/qrcode.jpeg\" alt=\"\" width=\"145\" height=\"145\" \/><\/a> Watch this National Geographic video for a summary of fetal development and maternal changes during pregnancy. <a href=\"https:\/\/www.youtube.com\/watch?v=XEfnq4Q4bfk\">https:\/\/www.youtube.com\/watch?v=XEfnq4Q4bfk<\/a>[\/caption]\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n\r\n&nbsp;\r\n<h5 style=\"text-align: justify\"><strong><a id=\"11-5a\"><\/a>The Fetal Circulatory System<\/strong><\/h5>\r\n<p style=\"text-align: justify\">During prenatal development, the fetal circulatory system is integrated with the placenta via the umbilical cord so that the fetus receives both oxygen and nutrients from the placenta. However, after childbirth, the umbilical cord is severed, and the newborn\u2019s circulatory system must be reconfigured. Unlike a mature cardiovascular system, however, the fetal cardiovascular system also includes circulatory shortcuts, or [pb_glossary id=\"1449\"]shunts[\/pb_glossary]. A <strong>shunt <\/strong>is an anatomical (or sometimes surgical) diversion that allows blood flow to bypass immature organs such as the lungs and liver until childbirth.<\/p>\r\n<p style=\"text-align: justify\">The placenta provides the fetus with necessary oxygen and nutrients via the umbilical vein. (Remember that veins carry blood toward the heart. In this case, the blood flowing to the fetal heart is oxygenated because it comes from the placenta. The respiratory system is immature and cannot yet oxygenate blood on its own.)<\/p>\r\n<p style=\"text-align: justify\">From the umbilical vein, the oxygenated blood flows toward the inferior vena cava, all but bypassing the immature liver, via the <strong>[pb_glossary id=\"1450\"]ductus venosus[\/pb_glossary]<\/strong> shunt (Figure 18). The liver receives just a trickle of blood, which is all that it needs in its immature, semifunctional state. Blood flows from the inferior vena cava to the right atrium, mixing with fetal venous blood along the way.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"1294\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image24-4.png\" alt=\"image\" width=\"1294\" height=\"1134\" \/> <strong>Figure 18. Fetal Circulatory System.<\/strong> The fetal circulatory system includes three shunts to divert blood from undeveloped and partially functioning organs, as well as blood supply to and from the placenta.[\/caption]\r\n<p style=\"text-align: justify\">Whereas the fetal liver is semi-functional, the fetal lungs are nonfunctional. The fetal circulation therefore bypasses the lungs by shifting some of the blood through the <strong>[pb_glossary id=\"501\"]foramen ovale[\/pb_glossary]<\/strong>, a shunt that directly connects the right and left atria and avoids the pulmonary trunk altogether. Most of the rest of the blood is pumped to the right ventricle, and from there, into the pulmonary trunk, which splits into pulmonary arteries. However, a shunt within the pulmonary artery, the <strong>[pb_glossary id=\"1451\"]ductus arteriosus[\/pb_glossary]<\/strong>, diverts a portion of this blood into the [pb_glossary id=\"1028\"]aorta[\/pb_glossary]. This ensures that only a small volume of oxygenated blood passes through the immature pulmonary circuit, which has only minor metabolic requirements. The oxygenated blood moves through the [pb_glossary id=\"501\"]foramen ovale[\/pb_glossary] into the left atrium, where it mixes with the now deoxygenated blood returning from the pulmonary circuit. This blood then moves into the left ventricle, where it is pumped into the aorta. Some of this blood moves through the coronary arteries into the myocardium, and some moves through the carotid arteries to the brain.<\/p>\r\n<p style=\"text-align: justify\">The descending aorta carries partially oxygenated and partially deoxygenated blood into the lower regions of the body. It eventually passes into the umbilical arteries through branches of the internal iliac arteries. The deoxygenated blood collects waste as it circulates through the fetal body and returns to the umbilical cord. Thus, the two umbilical arteries carry blood low in oxygen and high in carbon dioxide and fetal wastes. This blood is filtered through the placenta, where wastes diffuse into the maternal circulation. Oxygen and nutrients from the mother diffuse into the placenta and from there into the fetal blood, and the process repeats.<\/p>\r\n<p style=\"text-align: justify\">Prior to birth, the lungs are filled with amniotic fluid, mucus, and [pb_glossary id=\"1452\"]surfactant[\/pb_glossary]. As the fetus is squeezed through the birth canal, the fetal [pb_glossary id=\"664\"]thoracic cavity[\/pb_glossary] is compressed, expelling much of this fluid. The first inhalation occurs within 10 seconds after birth and not only serves as the first inspiration, but also acts to inflate the lungs. <strong>Pulmonary [pb_glossary id=\"1452\"]surfactant[\/pb_glossary]<\/strong> is critical for inflation to occur, as it reduces the surface tension of the alveoli. A baby born prior to 26 weeks has a low chance of survival, as it produces insufficient pulmonary surfactant, and the surfaces for gas exchange have not formed adequately.<\/p>\r\n\r\n<h2 style=\"text-align: left\"><strong><a id=\"11-6\"><\/a>Part 6: Maternal Changes During Pregnancy, Labour, and Birth<\/strong><\/h2>\r\n<p style=\"text-align: justify\">A full-term pregnancy lasts approximately 270 days (38.5 weeks) from conception to birth. Because it is easier to remember the first day of the last menstrual period (LMP) than to estimate the date of conception, obstetricians set the due date as 284 days (approximately 40.5 weeks) from the LMP. This assumes that conception occurred on day 14 of the woman\u2019s cycle, which is usually a good approximation. The 40 weeks of an average pregnancy are usually discussed in terms of three trimesters, each approximately 13 weeks. During the second and third trimesters, the pre-pregnancy uterus\u2014about the size of a fist\u2014grows dramatically to contain the fetus, causing a number of anatomical changes in the mother.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"11-6a\"><\/a>Effects of Hormones<\/strong><\/h5>\r\n<p style=\"text-align: justify\">Virtually all the effects of pregnancy can be attributed in some way to the influence of hormones\u2014particularly [pb_glossary id=\"481\"]estrogens[\/pb_glossary], [pb_glossary id=\"482\"]progesterone[\/pb_glossary], and [pb_glossary id=\"1434\"]hCG[\/pb_glossary]. During weeks 7\u201312 from the LMP, the pregnancy hormones are primarily generated by the [pb_glossary id=\"1255\"]corpus luteum[\/pb_glossary]. Progesterone secreted by the corpus luteum stimulates the production of decidual cells of the [pb_glossary id=\"1395\"]endometrium[\/pb_glossary] that nourish the blastocyst before placentation. As the [pb_glossary id=\"1417\"]placenta[\/pb_glossary] develops and the corpus luteum degenerates during weeks 12\u201317, the placenta gradually takes over as the endocrine organ of pregnancy.<\/p>\r\n<p style=\"text-align: justify\">The placenta converts weak androgens secreted by the maternal and fetal adrenal glands to estrogens, which are necessary for pregnancy to progress. Estrogen levels climb throughout the pregnancy, increasing 30-fold by childbirth. Estrogens have the following actions:<\/p>\r\n\r\n<ul style=\"text-align: left\">\r\n \t<li style=\"text-align: justify\">They suppress [pb_glossary id=\"478\"]FSH[\/pb_glossary] and [pb_glossary id=\"479\"]LH[\/pb_glossary] production, effectively preventing ovulation. (This function is the biological basis of hormonal birth control pills.)<\/li>\r\n \t<li style=\"text-align: justify\">They induce the growth of fetal tissues and are necessary for the maturation of the fetal lungs and liver.<\/li>\r\n \t<li style=\"text-align: justify\">They promote fetal viability by regulating progesterone production and triggering fetal synthesis of cortisol, which helps with the maturation of the lungs, liver, and endocrine organs such as the thyroid gland and adrenal gland.<\/li>\r\n \t<li style=\"text-align: justify\">They stimulate maternal tissue growth, leading to uterine enlargement and mammary duct expansion and branching.<\/li>\r\n<\/ul>\r\n<p style=\"text-align: justify\">The placenta takes over the synthesis and secretion of progesterone throughout pregnancy as the corpus luteum degenerates. Like estrogen, progesterone suppresses FSH and LH. It also inhibits uterine contractions, protecting the fetus from preterm birth. This hormone decreases in late gestation, allowing uterine contractions to intensify and eventually progress to true labour. The placenta also produces hCG. In addition to promoting survival of the corpus luteum, hCG stimulates the male fetal gonads to secrete testosterone, which is essential for the development of the male reproductive system.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"11-6b\"><\/a>Physiology of Labour<\/strong><\/h5>\r\n<p style=\"text-align: justify\">Childbirth, or [pb_glossary id=\"1453\"]parturition[\/pb_glossary], typically occurs within a week of a woman\u2019s due date. As a pregnancy progresses into its final weeks, several physiological changes occur in response to hormones that trigger labour.<\/p>\r\n<p style=\"text-align: justify\">First, recall that progesterone inhibits uterine contractions throughout the first several months of pregnancy. As the pregnancy enters its seventh month, progesterone levels plateau and then drop. Estrogen levels, however, continue to rise in the maternal circulation (Figure 19). The increasing ratio of estrogen to progesterone makes the myometrium (the uterine smooth muscle) more sensitive to stimuli that promote contractions (because progesterone no longer inhibits them). Moreover, in the eighth month of pregnancy, fetal [pb_glossary id=\"472\"]cortisol[\/pb_glossary] rises, which boosts estrogen secretion by the placenta and further overpowers the uterine-calming effects of progesterone.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"1429\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image26-3.png\" alt=\"image\" width=\"1429\" height=\"802\" \/> <strong>Figure 19. Hormones Initiating Labour.<\/strong> A positive feedback loop of hormones works to initiate labour.[\/caption]\r\n<p style=\"text-align: justify\">Meanwhile, the posterior [pb_glossary id=\"391\"]pituitary gland[\/pb_glossary] has been boosting its secretion of [pb_glossary id=\"471\"]oxytocin[\/pb_glossary], a hormone that stimulates the contractions of labour. At the same time, the [pb_glossary id=\"1394\"]myometrium[\/pb_glossary] increases its sensitivity to oxytocin by expressing more receptors for this hormone. As labour nears, oxytocin begins to stimulate stronger, more painful uterine contractions, which\u2014in a positive feedback loop\u2014stimulate the secretion of [pb_glossary id=\"1017\"]prostaglandins[\/pb_glossary] from fetal membranes. Like oxytocin, prostaglandins also enhance uterine contractile strength.<\/p>\r\n<p style=\"text-align: justify\">Finally, stretching of the myometrium and cervix by a full-term fetus in the vertex (head-down) position is regarded as a stimulant to uterine contractions. The sum of these changes initiates the regular contractions known as true labour, which become more powerful and more frequent with time. The pain of labour is attributed to myometrial hypoxia during uterine contractions.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"11-6c\"><\/a>Stages of Childbirth<\/strong><\/h5>\r\n<p style=\"text-align: justify\">The process of childbirth can be divided into three stages: cervical dilation, expulsion of the newborn ending with birth, and afterbirth (Figure 20).<\/p>\r\n<p style=\"text-align: justify\"><strong>1. Cervical Dilation:<\/strong> For vaginal birth to occur, the cervix must dilate fully to 10 cm in diameter\u2014wide enough to deliver the newborn\u2019s head. The dilation stage is the longest stage of labour and typically takes 6\u201312 hours.<\/p>\r\n<p style=\"text-align: justify\">True labour progresses in a positive feedback loop in which uterine contractions stretch the cervix, causing it to dilate and efface, or become thinner. Cervical stretching induces reflexive uterine contractions that dilate and efface the cervix further. In addition, cervical dilation boosts [pb_glossary id=\"471\"]oxytocin[\/pb_glossary] secretion from the [pb_glossary id=\"391\"]pituitary gland[\/pb_glossary], which in turn triggers more powerful uterine contractions. When labour begins, uterine contractions may occur only every 3\u201330 minutes and last only 20\u201340 seconds; however, by the end of this stage, contractions may occur as frequently as every 1.5\u20132 minutes and last for a full minute.<\/p>\r\n<p style=\"text-align: justify\"><strong>2. Expulsion Stage:<\/strong> The expulsion stage begins when the fetal head enters the birth canal and ends with birth of the newborn. It typically takes up to 2 hours, but varies in length, depending in part on the orientation of the fetus. The most common presentation and associated with the greatest ease of vaginal birth is when the fetus faces the maternal spinal cord and the smallest part of the head (the posterior aspect called the occiput) exits the birth canal first.<\/p>\r\n<p style=\"text-align: justify\">Once the head is birthed, the rest of the body usually follows quickly. The umbilical cord is then double-clamped, and a cut is made between the clamps. This completes the second stage of childbirth.<\/p>\r\n<p style=\"text-align: justify\"><strong>3. Afterbirth:<\/strong> The delivery of the placenta and associated membranes, commonly referred to as the afterbirth, marks the final stage of childbirth. After expulsion of the newborn, the myometrium continues to contract. This movement shears the placenta from the back of the uterine wall. It is then easily delivered through the vagina. Continued uterine contractions then reduce blood loss from the site of the placenta.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"1031\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image27-2.png\" alt=\"image\" width=\"1031\" height=\"1612\" \/> <strong>Figure 20. Stages of Childbirth.<\/strong> The stages of childbirth begin with early cervical dilation, continue through full dilation and birth (expulsion of the newborn); and finally delivery of the placenta and associated fetal membranes. The position of the newborn\u2019s shoulder is described relative to the mother.[\/caption]\r\n\r\n&nbsp;\r\n<h2 style=\"text-align: justify\"><strong><a id=\"11-7\"><\/a>Part 7: Adjustments of the Infant at Birth and Postnatal Stages<\/strong><\/h2>\r\n<p style=\"text-align: justify\">From a fetal perspective, the process of birth is a crisis. In the womb, the fetus was snuggled in a soft, warm, dark, and quiet world. The placenta provided nutrition and oxygen continuously. Suddenly, the contractions of labour and vaginal childbirth forcibly squeeze the fetus through the birth canal, limiting oxygenated blood flow during contractions and shifting the skull bones to accommodate the small space. After birth, the newborn\u2019s system must make drastic adjustments to a world that is colder, brighter, and louder, and where he or she will experience hunger and thirst. The neonatal period (neo- = \u201cnew\u201d; -natal = \u201cbirth\u201d) spans the first to the thirtieth day of life outside of the uterus.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"11-7a\"><\/a>Respiratory Adjustments<\/strong><\/h5>\r\n<p style=\"text-align: justify\">Although the fetus \u201cpractices\u201d breathing by inhaling amniotic fluid in utero, there is no air in the uterus and thus no true opportunity to breathe. (There is also no need to breathe because the placenta supplies the fetus with all the oxygenated blood it needs.) During gestation, the partially collapsed lungs are filled with amniotic fluid and exhibit very little metabolic activity. Several factors stimulate newborns to take their first breath at birth. First, labour contractions temporarily constrict umbilical blood vessels, reducing oxygenated blood flow to the fetus and elevating carbon dioxide levels in the blood. High carbon dioxide levels cause acidosis and stimulate the respiratory centre in the brain, triggering the newborn to take a breath.<\/p>\r\n<p style=\"text-align: justify\">The first breaths inflate the lungs to nearly full capacity and dramatically decrease lung pressure and resistance to blood flow, causing a major circulatory reconfiguration. Pulmonary alveoli open, and alveolar capillaries fill with blood. Amniotic fluid in the lungs drains or is absorbed, and the lungs immediately take over the task of the placenta, exchanging carbon dioxide for oxygen by the process of respiration.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"11-7b\"><\/a>Circulatory Adjustments<\/strong><\/h5>\r\n<p style=\"text-align: justify\">The newborn\u2019s first breath is vital to initiate the transition from the fetal to the neonatal circulatory pattern. Inflation of the lungs decreases blood pressure throughout the pulmonary system, as well as in the right atrium and ventricle. In response to this pressure change, the flow of blood temporarily reverses direction through the foramen ovale, moving from the left to the right atrium, and blocking the shunt with two flaps of tissue. Within one year, the tissue flaps usually fuse over the shunt (Figure 21). The [pb_glossary id=\"1451\"]ductus arteriosus[\/pb_glossary] constricts as a result of increased oxygen concentration. Closing of the ductus arteriosus ensures that all blood pumped to the pulmonary circuit will be oxygenated by the newly functional neonatal lungs. Likewise, the [pb_glossary id=\"1450\"]ductus venosus[\/pb_glossary] degenerates.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"1430\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image28-2.png\" alt=\"image\" width=\"1430\" height=\"1211\" \/> <strong>Figure 21. Neonatal Circulatory System.<\/strong> A newborn\u2019s circulatory system reconfigures immediately after birth. The three fetal shunts have been closed permanently, facilitating blood flow to the liver and lungs.[\/caption]\r\n<h2 style=\"text-align: justify\"><strong><a id=\"11-8\"><\/a>Part 8: Lactation<\/strong><\/h2>\r\n<p style=\"text-align: justify\"><strong>Lactation<\/strong> is the process by which milk is synthesized and secreted from the mammary glands of the postpartum female breast. Breast milk provides ideal nutrition and passive immunity for the infant, encourages mild uterine contractions to return the uterus to its pre-pregnancy size (i.e., involution), and induces a substantial metabolic increase in the mother, consuming the fat reserves stored during pregnancy.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"11-8a\"><\/a>Structure of the Lactating Breast<\/strong><\/h5>\r\n<p style=\"text-align: justify\">The mammary gland is composed of milk-transporting lactiferous ducts, which expand and branch extensively during pregnancy in response to [pb_glossary id=\"481\"]estrogens[\/pb_glossary], [pb_glossary id=\"474\"]growth hormone[\/pb_glossary], [pb_glossary id=\"472\"]cortisol[\/pb_glossary], and [pb_glossary id=\"475\"]prolactin[\/pb_glossary]. Moreover, in response to [pb_glossary id=\"482\"]progesterone[\/pb_glossary], clusters of breast alveoli bud from the ducts and expand outward toward the chest wall.<\/p>\r\nBreast alveoli are balloon-like structures lined with milk-secreting cuboidal cells, or lactocytes, which are surrounded by a net of contractile myoepithelial cells. Milk secreted by lactocytes fills the alveoli, and is squeezed into the ducts. Clusters of alveoli that drain to a common duct are called lobules; the lactating female has 12\u201320 lobules organized radially around the nipple. Milk drains from lactiferous ducts into lactiferous sinuses that meet at 4 to 18 perforations in the nipple, called nipple pores.\r\n<h5 style=\"text-align: justify\"><strong><a id=\"11-8b\"><\/a>The Process of Lactation<\/strong><\/h5>\r\n<p style=\"text-align: justify\">The pituitary hormone <strong>prolactin<\/strong> is instrumental in the establishment and maintenance of breast milk supply.<\/p>\r\n<p style=\"text-align: justify\">Near the fifth week of pregnancy, the level of circulating prolactin begins to increase, eventually rising to approximately 10\u201320 times the pre-pregnancy concentration. During pregnancy, prolactin and other hormones prepare the breasts anatomically for the secretion of milk. The level of prolactin plateaus in late pregnancy, at a level high enough to initiate milk production. However, estrogen, progesterone, and other placental hormones inhibit prolactin-mediated milk synthesis during pregnancy. It is not until the placenta is expelled that this inhibition is lifted and milk production commences.<\/p>\r\n<p style=\"text-align: justify\">After childbirth, the baseline prolactin level drops sharply, but it is restored for a 1-hour spike during each feeding to stimulate the production of milk for the next feeding.<\/p>\r\n<p style=\"text-align: justify\">When the infant suckles, sensory nerve fibres in the [pb_glossary id=\"1407\"]areola[\/pb_glossary] trigger a neuroendocrine reflex that results in milk secretion fvia the let-down reflex (Figure 22). The posterior pituitary gland releases [pb_glossary id=\"471\"]oxytocin[\/pb_glossary], which stimulates myoepithelial cells to squeeze milk from the alveoli so it can eventually drain out through the nipples.<\/p>\r\n&nbsp;\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"1356\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image29-2.png\" alt=\"image\" width=\"1356\" height=\"1733\" \/> <strong>Figure 22. Let-Down Reflex.<\/strong> A positive feedback loop ensures continued milk production as long as the infant continues to breastfeed.[\/caption]\r\n\r\n<div class=\"textbox textbox--exercises\"><header class=\"textbox__header\">\r\n<p class=\"textbox__title\"><a id=\"P\"><\/a>Practice Questions<\/p>\r\n\r\n<\/header>\r\n<div class=\"textbox__content\">\r\n\r\n<strong>Part 1: <\/strong>Anatomy and Physiology of the Male Reproductive System\r\n\r\n[h5p id=\"42\"]\r\n\r\n[h5p id=\"43\"]\r\n\r\n[h5p id=\"44\"]\r\n\r\n<strong>Part 2:<\/strong> Anatomy and Physiology of the Female Reproductive System\r\n\r\n[h5p id=\"45\"]\r\n\r\n[h5p id=\"46\"]\r\n\r\n[h5p id=\"63\"]\r\n\r\n[h5p id=\"61\"]\r\n\r\n[h5p id=\"62\"]\r\n\r\n<strong>Part 3:<\/strong> Fertilization\r\n\r\n[h5p id=\"48\"]\r\n\r\n[h5p id=\"49\"]\r\n\r\n[h5p id=\"50\"]\r\n\r\n<strong>Part 4:<\/strong> Embryonic Development\r\n\r\n[h5p id=\"52\"]\r\n\r\n<strong>Part 5:<\/strong> Fetal Development\r\n\r\n<strong>Part 6:<\/strong> Maternal Changes During Pregnancy, Labour, and Birth\r\n\r\n[h5p id=\"59\"]\r\n\r\n<strong>Part 7:<\/strong> Adjustments of the Infant at Birth and Postnatal Stages\r\n\r\n[h5p id=\"56\"]\r\n\r\n[h5p id=\"57\"]\r\n\r\n[h5p id=\"58\"]\r\n\r\n<strong>Part 8:<\/strong> Lactation\r\n\r\n[h5p id=\"60\"]\r\n\r\n<\/div>\r\n<\/div>\r\n&nbsp;\r\n\r\n<\/div>\r\n<!--more-->","rendered":"<div class=\"unit-10:-reproduction-and-development\">\n<div class=\"textbox shaded\">\n<p><strong>Unit outline<\/strong><\/p>\n<p><a href=\"#11-1\"><strong>Part 1: <\/strong>Anatomy and Physiology of the Male Reproductive System<\/a><\/p>\n<ul>\n<li><a href=\"#11-1a\">Scrotum<\/a><\/li>\n<li><a href=\"#11-1b\">Testes<\/a><\/li>\n<li><a href=\"#11-1c\">Structure of formed sperm<\/a><\/li>\n<li><a href=\"#11-1d\">Sperm transport<\/a><\/li>\n<li><a href=\"#11-1e\">The penis<\/a><\/li>\n<li><a href=\"#11-1f\">Sperm release<\/a><\/li>\n<li><a href=\"#11-1g\">Testosterone<\/a><\/li>\n<li><a href=\"#11-1h\">Function of testosterone<\/a><\/li>\n<\/ul>\n<p><a href=\"#11-2\"><strong>Part 2:<\/strong> Anatomy and Physiology of the Female Reproductive System<\/a><\/p>\n<ul>\n<li><a href=\"#11-2a\">External female genitalia<\/a><\/li>\n<li><a href=\"#11-2b\">Vagina<\/a><\/li>\n<li><a href=\"#11-2c\">Ovaries<\/a><\/li>\n<li><a href=\"#11-2d\">The ovarian cycle<\/a><\/li>\n<li><a href=\"#11-2e\">Hormonal control of the ovarian cycle<\/a><\/li>\n<li><a href=\"#11-2f\">The uterine tubes<\/a><\/li>\n<li><a href=\"#11-2g\">The uterus and cervix<\/a><\/li>\n<li><a href=\"#11-2h\">The menstrual (uterine) cycle<\/a><\/li>\n<li><a href=\"#11-2i\">The breasts<\/a><\/li>\n<\/ul>\n<p><a href=\"#11-3\"><strong>Part 3:<\/strong> Fertilization<\/a><\/p>\n<ul>\n<li><a href=\"#11-3a\">Transit of sperm<\/a><\/li>\n<li><a href=\"#11-3b\">Contact between sperm and oocyte<\/a><\/li>\n<li><a href=\"#11-3c\">The zygote<\/a><\/li>\n<\/ul>\n<p><a href=\"#11-4\"><strong>Part 4:<\/strong> Embryonic Development<\/a><\/p>\n<ul>\n<li><a href=\"#11-4a\">Pre-implantation embryonic development<\/a><\/li>\n<li><a href=\"#11-4b\">Implantation<\/a><\/li>\n<li><a href=\"#11-4c\">Embryonic membranes and Germ Layers<\/a><\/li>\n<li><a href=\"#11-4e\"><span style=\"text-align: initial;font-size: 1em\">D<\/span>evelopment of the placenta<\/a><\/li>\n<\/ul>\n<p><a href=\"#11-5a\"><strong>Part 5:<\/strong> Fetal Development<\/a><\/p>\n<ul>\n<li><a href=\"#11-5a\">The fetal circulatory system<\/a><\/li>\n<\/ul>\n<p><a href=\"#11-6\"><strong>Part 6:<\/strong> Maternal Changes During Pregnancy, Labour, and Birth<\/a><\/p>\n<ul>\n<li><a href=\"#11-6a\">Effects of hormones<\/a><\/li>\n<li><a href=\"#11-6b\">Physiology of labour<\/a><\/li>\n<li><a href=\"#11-6c\">Stages of childbirth<\/a><\/li>\n<\/ul>\n<p><a href=\"#11-7\"><strong>Part 7:<\/strong> Adjustments of the Infant at Birth and Postnatal Stages<\/a><\/p>\n<ul>\n<li><a href=\"#11-7a\">Reproductive adjustments<\/a><\/li>\n<li><a href=\"#11-7b\">Circulatory adjustments<\/a><\/li>\n<\/ul>\n<p><a href=\"#11-8\"><strong>Part 8:<\/strong> Lactation<\/a><\/p>\n<ul>\n<li><a href=\"#11-8a\">Structure of lactating breast<\/a><\/li>\n<li><a href=\"#11-8b\">The process of lactation<\/a><\/li>\n<\/ul>\n<h2><a href=\"#P\">Practice Questions<\/a><\/h2>\n<\/div>\n<div class=\"textbox textbox--learning-objectives\">\n<header class=\"textbox__header\">\n<p class=\"textbox__title\"><strong>Learning Objectives<\/strong><\/p>\n<\/header>\n<div class=\"textbox__content\">\n<p>At the end of this unit, you should be able to:<\/p>\n<p class=\"hanging-indent\"><strong>I. <\/strong>Describe the location, structure and function(s) of the components of the male reproductive system.<\/p>\n<p class=\"hanging-indent\"><strong>II. <\/strong>Describe the male reproductive cycle, including both spermatogenesis and sperm transport.<\/p>\n<p class=\"hanging-indent\"><strong>III.<\/strong> Describe the location, structure and function(s) of each of the following components of the female reproductive system.<\/p>\n<p><strong>IV.<\/strong> Describe the female reproductive cycle, including both oogenesis and preparation of the uterus for pregnancy.<\/p>\n<p class=\"hanging-indent\"><strong>V. <\/strong>Describe the process of fertilization.<\/p>\n<p class=\"hanging-indent\"><strong>VI. <\/strong>Describe the early stages of embryonic development.<\/p>\n<p><strong>VII.<\/strong> Describe the hormonal control of pregnancy, labour and lactation.<\/p>\n<p><strong>VIII. <\/strong><span style=\"font-size: 1em;text-indent: 0px\">Describe changes in neonatal circulation and respiration after birth.<\/span><\/p>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--learning-objectives\">\n<header class=\"textbox__header\">\n<p class=\"textbox__title\"><strong>Learning Objectives and Guiding Questions<\/strong><\/p>\n<\/header>\n<div class=\"textbox__content\">\n<p>At the end of this unit, you should be able to complete all the following tasks, including answering the guiding questions associated with each task.<\/p>\n<p class=\"hanging-indent\"><strong>I. <\/strong>Describe the location, structure and function(s) of each of the following components of the male reproductive system:<\/p>\n<ol>\n<li class=\"hanging-indent\">Testes<\/li>\n<li class=\"hanging-indent\">Epididymis<\/li>\n<li class=\"hanging-indent\">Vas deferens<\/li>\n<li class=\"hanging-indent\">Ejaculatory duct<\/li>\n<li class=\"hanging-indent\">Urethra<\/li>\n<li class=\"hanging-indent\">Seminal vesicles (seminal glands)<\/li>\n<li class=\"hanging-indent\">Prostate gland<\/li>\n<li class=\"hanging-indent\">Penis<\/li>\n<li class=\"hanging-indent\">Bulbourethral gland<\/li>\n<li class=\"hanging-indent\">Scrotum<\/li>\n<li class=\"hanging-indent\">Seminiferous tubules<\/li>\n<\/ol>\n<p class=\"hanging-indent\"><strong>II. <\/strong>Describe the male reproductive cycle, including both spermatogenesis and sperm transport.<\/p>\n<ol>\n<li>Using direct reference to the structures identified in objective I, describe the process of spermatogenesis, including the timing and hormonal control of spermatogenesis (via FSH, LH and testosterone; refer back to <a href=\"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/chapter\/unit-1-the-endocrine-system\/\">Unit 1<\/a> for the roles of FSH and LH), and volume of sperm produced.<\/li>\n<li>Using direct reference to the structures identified in objective I, sketch the pathway of sperm movement through the male reproductive tract, noting the addition of substances by the accessory glands.<\/li>\n<li>Describe how the parasympathetic and sympathetic nervous systems are required for sperm release.<\/li>\n<\/ol>\n<p class=\"hanging-indent\"><strong>III.<\/strong> Describe the location, structure and function(s) of each of the following components of the female reproductive system:<\/p>\n<ol>\n<li class=\"hanging-indent\">Ovaries<\/li>\n<li class=\"hanging-indent\">Uterine tubes (oviducts)<\/li>\n<li class=\"hanging-indent\">Uterus<\/li>\n<li class=\"hanging-indent\">Cervix<\/li>\n<li class=\"hanging-indent\">Clitoris<\/li>\n<li class=\"hanging-indent\">Vagina<\/li>\n<li class=\"hanging-indent\">Endometrium<\/li>\n<li class=\"hanging-indent\">Myometrium<\/li>\n<li class=\"hanging-indent\">Perimetrium<\/li>\n<li class=\"hanging-indent\">Bartholin\u2019s glands<\/li>\n<\/ol>\n<p><strong>IV.<\/strong> Describe the two major female reproductive cycles, involved in oogenesis and the preparation of the uterus for pregnancy.<\/p>\n<ol>\n<li>What is the ultimate goal of the ovarian cycle?<\/li>\n<li>Identify the two main phases of the ovarian cycle and the ovulation event happening at the transition point between the two phases. Identify the ovarian structures and locations involved in each phase, and in ovulation.<\/li>\n<li>Draw a time scale of days 1-28 (leave ample space beneath your drawing for the uterine cycle). Label the timescale with the two phases of the ovarian cycle and ovulation, and annotate your time scale with the main events occurring along the way.<\/li>\n<li>Consider the hormones FSH and LH. For each hormone, identify the organ which secretes the hormone, the conditions that result in increased secretion of that hormone (be sure to address the roles of both the hypothalamus and pituitary gland) and the effect of high concentrations of each hormone on the ovarian cycle.<\/li>\n<li>Label the time scale showing when each hormone is in highest concentration.<\/li>\n<li>What is the ultimate goal of the uterine (menstrual) cycle?<\/li>\n<li>Identify the three main phases of the uterine cycle. Identify the <span style=\"color: #ff00ff\">female reproductive structures and locations involved.<\/span><\/li>\n<li>Below your ovarian cycle timescale, draw another timescale of days 1-28. Label the time scale with the three phases of the uterine cycle and annotate your timescale with the main events that occur in each phase.<\/li>\n<li>Consider the hormones estrogens and progesterone. For each hormone, identify the structure(s) that secrete the hormone, the conditions that result in increased secretion of that hormone and the effect of high concentrations of each hormone on the uterine cycle.<\/li>\n<li>Label the uterine cycle time scale showing when each hormone is in highest concentration.<\/li>\n<li>Correlate the ovarian and uterine (menstrual) cycles and explain their integrated hormonal regulation by annotating your diagram to show the hormonal interactions between the ovarian and uterine cycles. Make sure to differentiate between the effects of low vs. high concentrations of estrogens.<\/li>\n<\/ol>\n<p class=\"hanging-indent\"><strong>V. <\/strong>Describe the process of fertilization.<\/p>\n<ol>\n<li>Compare and contrast the sperm and the oocyte with regards to size, main constituents, role in reproduction and longevity.<\/li>\n<li>Identify the secretions from the seminal vesicles, bulbourethral gland and prostate gland, and briefly describe the role of each in sperm movement and survival within the female reproductive tract.<\/li>\n<li>Identify the secretions from the Bartholin\u2019s glands and the vagina and briefly describe their role in reproduction.<\/li>\n<li>Trace the pathway of sperm from when it is deposited in the female reproductive tract until it meets the oocyte. Consider the structures through which the <span style=\"color: #ff00ff\">oocyte\/early embryo<\/span> moves, the mechanisms of movement and how long the sperm survives.<\/li>\n<li>Trace the pathway of the ovum after it is expelled from the ovary until it meets the sperm. Consider the structures through which the ovum moves, the mechanisms of movement and the timing of movement.<\/li>\n<li>Describe the processes by which sperm penetrates the corona radiata and then the zona pellucida.<\/li>\n<li>What is the event that specifically identifies fertilization?<\/li>\n<li>Explain why polyspermy is undesirable. Describe the mechanisms in the oocyte which protect against polyspermy.<\/li>\n<li>Why are hundreds of sperm required for successful fertilization if only one succeeds in the end?<\/li>\n<\/ol>\n<p class=\"hanging-indent\"><strong>VI. <\/strong>Describe the early stages of embryonic development.<\/p>\n<ol>\n<li>Describe the overall developmental changes occurring within the zygote, and the role of cell division in creating a conceptus of increasing size and complexity. Name the main pre-implantation stages of the conceptus.<\/li>\n<li>Distinguish between morula and blastocyst. Be sure to address the morphological differences between these structures, the timing of the development and the location within the female reproductive tract.<\/li>\n<li>Trace the pathway of the conceptus after fertilization until it implants in the uterus. Consider the structures through which the conceptus moves, the mechanisms of movement and the timing of movement.<\/li>\n<li>Describe the hormonal changes that occur at implantation of the blastocyst. Which structure secretes human chorionic gonadotropin (hCG)?\u00a0 What is the effect of increased levels of hCG on the ovarian cycle and the uterine cycle?\u00a0 Why is this important for a successful pregnancy?\u00a0 What other structure fulfills this role later in pregnancy and how do the hormonal mechanisms differ?<\/li>\n<li>Explain where gastrulation occurs, what it achieves in and around the future embryo, and how it sets the stage for organogenesis.<\/li>\n<li>Describe the formation, structure, and functions of the placenta and umbilical cord.<\/li>\n<\/ol>\n<p><strong>VII.<\/strong> Describe the hormonal control of pregnancy, labour and lactation.<\/p>\n<ol>\n<li>Sketch a diagram showing changing levels of hCG, estrogens and progesterone throughout gestation. Annotate your sketch indicating the source for each hormone and describing the role of each hormone in maintaining pregnancy.<\/li>\n<li>Describe the hormonal changes that occur at the end of pregnancy, and describe the role of oxytocin, estrogens and prostaglandins in the initiation of labour.<\/li>\n<li>Consider the hormones oxytocin and prolactin and their role in lactation.<\/li>\n<\/ol>\n<p><strong style=\"text-align: initial;text-indent: -1em;font-size: 1em\">IX. <\/strong><span style=\"font-size: 1em;text-indent: 0px\">Describe changes in neonatal circulation and respiration after birth.<\/span><\/p>\n<ol>\n<li>Sketch fetal circulation and label all the blood vessels or structures found in fetal circulation but not in the adult circulation. For each structure, provide a functional explanation for the difference in both the fetus and the adult.<\/li>\n<li>Which organ is responsible for fetal gas exchange? Why does this change at birth?\u00a0 Describe the four possible stimuli for the initiation of ventilation.<\/li>\n<li>At what stage during development is surfactant produced and what is the function of surfactant in the neonatal lungs?<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\">Small, uncoordinated, and slick with amniotic fluid, a newborn encounters the world outside of her mother\u2019s womb. We do not often consider that a child\u2019s birth is proof of the healthy functioning of both her mother\u2019s and father\u2019s reproductive systems. Moreover, her parents\u2019 endocrine systems had to secrete the appropriate regulating hormones to induce the production and release of unique male and female gametes, reproductive cells containing the parents\u2019 genetic material (one set of 23 chromosomes). Her parent\u2019s reproductive behaviour had to facilitate the transfer of male gametes\u2014the sperm\u2014to the female reproductive tract at just the right time to encounter the female gamete, an oocyte (egg). Finally, combination of the gametes (fertilization) had to occur, followed by implantation and development. In this unit, you will explore the male and female reproductive systems, whose healthy functioning can culminate in the powerful sound of a newborn\u2019s first cry.<\/p>\n<p>&nbsp;<\/p>\n<h2 style=\"text-align: justify\"><strong><a id=\"11-1\"><\/a>Part 1: Anatomy and Physiology of the Male Reproductive System<\/strong><\/h2>\n<p style=\"text-align: justify\">Unique for its role in human reproduction, a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_465\">gamete<\/a> <\/strong>is a specialized sex cell usually carrying 23 <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1185\">chromosomes<\/a>\u2014one half the number in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1458\">somatic<\/a> cells. At fertilization, the chromosomes in one male gamete, called a <strong>sperm<\/strong> (or spermatozoon), combine with the chromosomes in one female gamete, called an oocyte. The function of the male reproductive system (Figure 1) is to produce sperm and transfer them to the female reproductive tract. The paired testes are a crucial component in this process, as they produce both sperm and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1186\">androgens<\/a>, the hormones that support male reproductive physiology. In humans, the most important male androgen is testosterone. Several accessory organs and ducts aid the process of sperm maturation and transport the sperm and other seminal components to the penis, which delivers sperm to the female reproductive tract. In this section, we examine each of these different structures, and discuss the process of sperm production and transport.<\/p>\n<figure style=\"width: 150px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2019\/06\/image1-8.png\" alt=\"image\" width=\"150\" height=\"150\" \/><figcaption class=\"wp-caption-text\">Watch <a href=\"https:\/\/youtu.be\/-XQcnO4iX_U\">this CrashCourse video<\/a> for an overview of the male reproductive system! Direct link: <a href=\"https:\/\/youtu.be\/-XQcnO4iX_U\">https:\/\/youtu.be\/-XQcnO4iX_U<\/a><\/figcaption><\/figure>\n<figure style=\"width: 805px\" class=\"wp-caption alignleft\"><img loading=\"lazy\" decoding=\"async\" style=\"font-weight: bold;font-size: 14pt\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image2-8.png\" alt=\"image\" width=\"805\" height=\"876\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 1. Male Reproductive System.<\/strong> The structures of the male reproductive system include the testes, the epididymides, the penis, and the ducts and glands that produce and carry semen. Sperm exit the scrotum through the ductus deferens, which is bundled in the spermatic cord. The seminal vesicles and prostate gland add fluids to the sperm to create semen.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"11-1a\"><\/a>Scrotum<\/strong><\/h5>\n<p style=\"text-align: justify\">The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_484\">testes<\/a> are located in a highly pigmented, muscular sac called the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1187\">scrotum<\/a> <\/strong>that extends from the body behind the penis (Figure 1). This location is important in sperm production, which occurs within the testes, and proceeds more efficiently when the testes are kept 2 to 4\u00b0C below core body temperature.<\/p>\n<figure style=\"width: 800px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image3-8.png\" alt=\"image\" width=\"800\" height=\"352\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 2. The Scrotum and Testes.<\/strong> This anterior view shows the structures of the scrotum and testes.<\/figcaption><\/figure>\n<h5 style=\"text-align: justify\"><strong><a id=\"11-1b\"><\/a>Testes<\/strong><\/h5>\n<p style=\"text-align: justify\">The <strong>testes <\/strong>(singular = testis) are the male <strong>gonads<\/strong>. They produce both sperm and androgens, such as testosterone, and are active throughout the reproductive lifespan of the male.<\/p>\n<p style=\"text-align: justify\">As paired oval organs, the testes are each approximately 4 to 5 cm in length and are housed within the scrotum (Figure 2). Within the testes, sperm develop in structures called seminiferous tubules. During the seventh month of the developmental period of a male fetus, each testis moves through the abdominal musculature to descend into the scrotal cavity. This is called the \u201cdescent of the testis.\u201d<\/p>\n<p style=\"text-align: justify\">The tightly coiled <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1188\">seminiferous tubules<\/a><\/strong> form the bulk of each testis. They are composed of developing sperm cells surrounding a lumen, the hollow centre of the tubule, where formed sperm are released into the duct system of the testis (Figure 3). Specifically, from the lumens of the seminiferous tubules, sperm move into the straight tubules then into a fine meshwork of tubules and then leave the testis itself.<\/p>\n<p style=\"text-align: justify\">Inside the seminiferous tubules are supporting cells and developing sperm cells called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1190\">germ cells<\/a>. Germ cell development progresses from the basement membrane\u2014at the perimeter of the tubule\u2014toward the lumen.<\/p>\n<figure style=\"width: 743px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image4-8.png\" alt=\"image\" width=\"743\" height=\"686\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 3. Anatomy of the Testis.<\/strong> This sagittal view shows the seminiferous tubules, the site of sperm production. Formed sperm are transferred to the epididymis, where they mature. They leave the epididymis during an ejaculation via the ductus deferens.<\/figcaption><\/figure>\n<p style=\"text-align: justify\"><strong>1. Sertoli Cells:<\/strong> Surrounding all stages of the developing sperm cells are elongate, branching <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1189\">Sertoli cells<\/a>. Sertoli cells secrete signaling molecules that promote sperm production and can control whether germ cells live or die.<\/p>\n<p style=\"text-align: justify\"><strong>2. Germ Cells:<\/strong> The least mature cells line the basement membrane inside the tubule. These divide to produce primary and secondary spermatocytes, then spermatids, which finally produce formed sperm. This process is called <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1200\">spermatogenesis<\/a><\/strong>. The process begins at puberty, after which time sperm are produced constantly throughout a man\u2019s life. Eventually, the sperm are released into the lumen and are moved along a series of ducts in the testis toward a structure called the epididymis for the next step of sperm maturation.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"11-1c\"><\/a>Structure of Formed Sperm<\/strong><\/h5>\n<p style=\"text-align: justify\">Sperm are smaller than most cells in the body; in fact, the volume of a sperm cell is 85,000 times less than that of the female gamete. Approximately 100 to 300 million sperm are produced each day, whereas typically only one oocyte per month matures fully and is then ovulated. As is true for most cells in the body, the structure of sperm cells speaks to their function. Sperm have a distinctive head, mid-piece, and tail region (Figure 4). The head of the sperm contains the extremely compact <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1197\">haploid<\/a> (half the genetic content of a diploid somatic cell) nucleus with very little cytoplasm. These qualities contribute to the overall small size of the sperm (the head is only 5 \u03bcm long). A structure called the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1198\">acrosome<\/a> covers most of the head of the sperm cell as a \u201ccap\u201d that is filled with lysosomal enzymes important for preparing sperm to participate in fertilization. Tightly packed <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1094\">mitochondria<\/a> fill the midpiece of the sperm. ATP produced by these mitochondria will power the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1199\">flagellum<\/a>, which extends from the neck and the mid-piece through the tail of the sperm, enabling it to move the entire sperm cell.<\/p>\n<figure style=\"width: 727px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image5-8.png\" alt=\"image\" width=\"727\" height=\"198\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 4. Structure of Sperm.<\/strong> Sperm cells are divided into a head, containing DNA; a mid-piece, containing mitochondria; and a tail, providing motility. The acrosome is oval and somewhat flattened.<\/figcaption><\/figure>\n<h5 style=\"text-align: justify\"><strong><a id=\"11-1d\"><\/a>Sperm Transport<\/strong><\/h5>\n<p style=\"text-align: justify\">To fertilize an <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1232\">oocyte<\/a>, sperm must be moved from the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1188\">seminiferous tubules<\/a> in the testes, through the epididymis, and\u2014later during ejaculation\u2014along the length of the penis and out into the female reproductive tract.<\/p>\n<p style=\"text-align: justify\"><strong>1. Role of the Epididymis:<\/strong> From the lumen of the seminiferous tubules, the immotile sperm are surrounded by testicular fluid and moved to the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1201\">epididymis<\/a> <\/strong>(plural = epididymides), a coiled tube attached to the testis where newly formed sperm continue to mature (Figure 3). Though the epididymis does not take up much room in its tightly coiled state, it would be approximately 6 m (20 feet) long if straightened. It takes an average of 12 days for sperm to move through the coils of the epididymis. Sperm enter the head of the epididymis and are moved along predominantly by the contraction of smooth muscles lining the epididymal tubes. As they are moved along the length of the epididymis, the sperm further mature and acquire the ability to move under their own power. Once inside the female reproductive tract, they will use this ability to move independently toward the unfertilized egg. The more mature sperm are then stored in the tail of the epididymis (the final section) until ejaculation occurs.<\/p>\n<p style=\"text-align: justify\"><strong>2. Duct System:<\/strong> During ejaculation, sperm exit the tail of the epididymis and are pushed by smooth muscle contraction to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1202\">ductus deferens<\/a> (also called the vas deferens). The ductus deferens is a thick, muscular tube that is bundled together inside the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1187\">scrotum<\/a> with connective tissue, blood vessels, and nerves into a structure called the <strong>spermatic cord<\/strong> (Figure 1 and Figure 2). Surgical sterilization to interrupt sperm delivery can be performed by cutting and sealing a small section of the ductus (vas) deferens. This procedure is called a vasectomy, and it is an effective and usually permanent form of male birth control.<\/p>\n<p style=\"text-align: justify\">From each epididymis, each ductus deferens extends superiorly into the abdominal cavity through the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1203\">inguinal canal<\/a><\/strong> in the abdominal wall. From here, the ductus deferens continues posteriorly to the pelvic cavity, ending posterior to the bladder where it dilates in a region called the ampulla (meaning \u201cflask\u201d).<\/p>\n<p style=\"text-align: justify\">Sperm make up only 5 percent of the final volume of <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1204\">semen<\/a><\/strong>, the viscous, whitish-gray fluid that the male ejaculates. The bulk of semen is produced by three critical accessory glands of the male reproductive system: the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1205\">seminal vesicles<\/a>, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1206\">prostate<\/a>, and the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1207\">bulbourethral glands<\/a>.<\/p>\n<p style=\"text-align: justify\"><strong>3. Seminal Vesicles:<\/strong> As sperm pass through the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1208\">ampulla<\/a> of the ductus deferens at ejaculation, they mix with fluid from the associated <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1205\">seminal vesicle<\/a><\/strong> (Figure 1). The paired seminal vesicles are glands that contribute approximately 60 percent of the semen volume. Seminal vesicle fluid contains large amounts of fructose, which is used by the sperm mitochondria to generate <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_967\">ATP<\/a> to allow movement through the female reproductive tract.<\/p>\n<p style=\"text-align: justify\">The fluid, now containing both sperm and seminal vesicle secretions, next moves into the associated paired <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1209\">ejaculatory ducts<\/a><\/strong>, which transport the seminal fluid into the next structure, the prostate gland.<\/p>\n<p style=\"text-align: justify\"><strong>4. Prostate Gland:<\/strong> The centrally located <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1206\">prostate<\/a> gland<\/strong> sits anterior to the rectum at the base of the bladder surrounding the prostatic <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1156\">urethra<\/a>, the portion of the urethra that runs within the prostate (Figure 1). About the size of a walnut, the prostate is formed of both muscular and glandular tissues. It excretes a fluid enriched with <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_464\">enzymes<\/a> and citric acid to the passing seminal fluid\u2014now called semen\u2014that is critical to first coagulate and then decoagulate the semen following ejaculation. The temporary thickening of semen helps retain it within the female reproductive tract, providing time for sperm to utilize the fructose provided by seminal vesicle secretions. When the semen regains its fluid state, sperm can then pass farther into the female reproductive tract.<\/p>\n<p style=\"text-align: justify\"><strong>5. Bulbourethral Glands:<\/strong> The final addition to semen is made by two <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1207\">bulbourethral glands<\/a> (or Cowper\u2019s glands) that release a thick, salty fluid that lubricates the end of the urethra and the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1222\">vagina<\/a> and helps to clean urine residues from the penile urethra. The fluid from these accessory glands is released after the male becomes sexually aroused, and shortly before the release of the semen. It is therefore sometimes called pre-ejaculate. It is important to note that, in addition to the lubricating proteins, it is possible for bulbourethral fluid to pick up sperm already present in the urethra, and therefore it may be able to cause pregnancy.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"11-1e\"><\/a>The Penis<\/strong><\/h5>\n<p style=\"text-align: justify\">The penis is the male organ of copulation (sexual intercourse). When erect, the stiffness of the organ allows it to penetrate into the vagina and deposit semen into the female reproductive tract.<\/p>\n<p style=\"text-align: justify\">The shaft of the penis surrounds the urethra (Figure 1). The end of the penis, called the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1211\">glans penis<\/a><\/strong>, has a high concentration of nerve endings, resulting in very sensitive skin that influences the likelihood of ejaculation (see Figure 1). The skin from the shaft extends down over the glans and forms a collar called the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1210\">prepuce<\/a><\/strong> (or foreskin). The foreskin also contains a dense concentration of nerve endings, and both lubricate and protect the sensitive skin of the glans penis. A surgical procedure called circumcision, often performed for religious or social reasons, removes the prepuce, typically within days of birth.<\/p>\n<h5><strong><a id=\"11-1f\"><\/a>Sperm release<\/strong><\/h5>\n<p>Sperm release involves two stages: erection and ejaculation, each of which is controlled by a separate branch of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1212\">autonomic nervous system<\/a>. Erection involves <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_536\">parasympathetic<\/a> activation causing the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1213\">corpora cavernosa<\/a> (sinuses that run the length of the penis) to fill with venous blood.\u00a0 Erection allows the penis to be inserted into the vagina.\u00a0 Ejaculation involves <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_535\">sympathetic<\/a> activation of muscles in the penis and in the pelvic floor.\u00a0 The contraction of these muscles forces sperm out of the urethra.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"11-1g\"><\/a>Testosterone<\/strong><\/h5>\n<p style=\"text-align: justify\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_485\">Testosterone<\/a>, an androgen, is a steroid hormone produced by <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1214\">Leydig cells<\/a><\/strong>, or interstitial cells, reflects their location between the seminiferous tubules in the testes. In male embryos, testosterone is secreted by Leydig cells by the seventh week of development, with peak concentrations reached in the second trimester. This early release of testosterone results in the anatomical differentiation of the male sexual organs. In childhood, testosterone concentrations are low. They increase during puberty, activating characteristic physical changes and initiating <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1200\">spermatogenesis<\/a>.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"11-1h\"><\/a>Functions of Testosterone<\/strong><\/h5>\n<p style=\"text-align: justify\">The continued presence of testosterone is necessary to keep the male reproductive system working properly, and Leydig cells produce approximately 6 to 7 mg of testosterone per day. Testosterone concentrations can be 100 times higher in the testes than in the circulation. Maintaining these normal concentrations of testosterone promotes spermatogenesis, whereas low levels of testosterone can lead to infertility. Testosterone is also released into the systemic circulation and plays an important role in muscle development, bone growth, the development of secondary sex characteristics, and maintaining libido (sex drive) in both males and females. In females, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_483\">ovaries<\/a> secrete small amounts of testosterone, although most is converted to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1216\">estradiol<\/a>. A small amount of testosterone is also secreted by the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_446\">adrenal glands<\/a> in both sexes.<\/p>\n<h2 style=\"text-align: justify\"><strong><a id=\"11-2\"><\/a>Part 2: Anatomy and Physiology of the Female Reproductive System<\/strong><\/h2>\n<p style=\"text-align: justify\">The female reproductive system functions to produce gametes and reproductive hormones; however, it also has the additional task of supporting the developing fetus and delivering it to the outside world. Unlike its male counterpart, the female reproductive system is located primarily inside the pelvic cavity (Figure 5). Recall that the ovaries are the female gonads. The gamete they produce is called an oocyte. We will discuss the production of oocytes in detail shortly. First, let\u2019s look at some of the structures of the female reproductive system.<\/p>\n<figure style=\"width: 150px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image6-9.png\" alt=\"image\" width=\"150\" height=\"150\" \/><figcaption class=\"wp-caption-text\">Watch <a href=\"https:\/\/youtu.be\/RFDatCchpus\">this Crash Course video<\/a> for an overview of the female reproductive system! Direct link: <a href=\"https:\/\/youtu.be\/RFDatCchpus\">https:\/\/youtu.be\/RFDatCchpus<\/a><\/figcaption><\/figure>\n<h5 style=\"text-align: justify\"><strong><a id=\"11-2a\"><\/a>External Female Genitals<\/strong><\/h5>\n<p style=\"text-align: justify\">The external female reproductive structures are referred to collectively as the vulva. The mons pubis is a pad of fat found in both males and females that is located at the anterior, over the pubic bone. After puberty, it becomes covered in pubic hair. In females, the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1217\">labia majora<\/a><\/strong> (labia = \u201clips\u201d; majora = \u201clarger\u201d) are folds of hair-covered skin that begin just posterior to the mons pubis. The thinner and more pigmented <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1218\">labia minora<\/a><\/strong> (labia = \u201clips\u201d; minora = \u201csmaller\u201d) extend medial to the labia majora. The labia minora serve to protect the female urethra and the entrance to the female reproductive tract.<\/p>\n<p style=\"text-align: justify\">The superior, anterior portions of the labia minora come together to encircle the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1219\">clitoris<\/a><\/strong> (or glans clitoris), an organ that has abundant nerves that make it important in sexual sensation and orgasm. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1220\">hymen<\/a> is a thin membrane that sometimes partially covers the entrance to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1222\">vagina<\/a>. The vaginal opening is located between the opening of the urethra and the anus. It is flanked by outlets to the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1221\">Bartholin\u2019s glands<\/a><\/strong> (or greater vestibular glands).<\/p>\n<figure style=\"width: 555px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image7-8.png\" alt=\"image\" width=\"555\" height=\"789\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 5. Female Reproductive System.<\/strong> The major organs of the female reproductive system are located inside the pelvic cavity.<\/figcaption><\/figure>\n<h5 style=\"text-align: justify\"><strong><a id=\"11-2b\"><\/a>Vagina<\/strong><\/h5>\n<p style=\"text-align: justify\">The <strong>vagina<\/strong> is a muscular canal (approximately 10 cm long) that serves as the entrance to the reproductive tract (Figure 5). It also serves as the exit from the uterus during menses and childbirth. The walls of the vagina are lined with an outer fibrous adventitia, a middle layer of smooth muscle, and an inner mucous membrane with transverse folds called rugae. Together, the middle and inner layers allow the expansion of the vagina to accommodate intercourse and childbirth. The Bartholin\u2019s glands and the lesser vestibular glands (located near the clitoris) secrete mucus, which keeps the vestibular area moist.<\/p>\n<p style=\"text-align: justify\">The vagina is home to a normal population of microorganisms that help to protect against infection by pathogenic bacteria, yeast, or other organisms that can enter the vagina. Beneficial bacteria secrete lactic acid, which protects the vagina by maintaining an acidic pH (below 4.5). Lactic acid, in combination with other vaginal secretions, makes the vagina a self-cleansing organ.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"11-2c\"><\/a>Ovaries<\/strong><\/h5>\n<p style=\"text-align: justify\">The <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_483\">ovaries<\/a><\/strong> are the female gonads (Figure 5). These paired oval organs are each about 2 to 3 cm in length, about the size of an almond. The ovaries are located within the pelvic cavity, and are supported by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1224\">ligaments<\/a>.<\/p>\n<p>Each ovary contains an outer cortex, housing developing oocytes, and an inner medulla, housing large blood vessels and nerves. Within the ovarian cortex, oocytes develop surrounded by supporting cells. forming an ovarian\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1225\">follicle<\/a><\/strong>. The growth and development of ovarian follicles will be described shortly.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"11-2d\"><\/a>The Ovarian Cycle<\/strong><\/h5>\n<p style=\"text-align: justify\">The <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1226\">ovarian cycle<\/a><\/strong> is a set of predictable changes in a female\u2019s oocytes and ovarian follicles. During a woman\u2019s reproductive years, it is a roughly 28-day cycle that can be correlated with, but is not the same as, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1227\">menstrual cycle<\/a> (discussed shortly). The cycle includes two interrelated processes: <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1228\">oogenesis<\/a> (the production of female gametes) and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1229\">folliculogenesis<\/a> (the growth and development of ovarian follicles).<\/p>\n<p style=\"text-align: justify\"><strong>1. Oogenesis:<\/strong> Gametogenesis in females is called <strong>oogenesis<\/strong>. The process begins with the ovarian stem cells, or <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1230\">oogonia<\/a><\/strong>. Oogonia form primary oocytes in the fetal ovary prior to birth. These primary oocytes are then arrested in this stage, only to resume it years later, beginning at puberty and continuing until the woman is near menopause (the cessation of a woman\u2019s reproductive functions). The number of primary oocytes present in the ovaries declines from one to two million in an infant, to approximately 400,000 at puberty, to zero by the end of menopause.<\/p>\n<p style=\"text-align: justify\">The initiation of <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1196\">ovulation<\/a><\/strong>\u2014the release of an oocyte from the ovary\u2014marks the transition from puberty into reproductive maturity for women. From then on, throughout a woman\u2019s reproductive years, ovulation occurs approximately once every 28 days, triggered by a surge of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_479\">LH<\/a> just prior to ovulation.<\/p>\n<p style=\"text-align: justify\">The larger amount of cytoplasm contained in the female gamete is used to supply the developing zygote with nutrients during the period between fertilization and implantation into the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1244\">uterus<\/a>. Interestingly, sperm contribute only DNA at fertilization, not <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1243\">cytoplasm<\/a>. Therefore, the cytoplasm and all the cytoplasmic organelles in the developing embryo are of maternal origin. This includes mitochondria, which contain their own DNA.<\/p>\n<p style=\"text-align: justify\"><strong>2. Folliculogenesis:<\/strong><\/p>\n<p>Ovarian follicles are oocytes surrounded by supporting cells. They grow and develop in a process called\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1229\">folliculogenesis<\/a><\/strong>, which typically leads to ovulation of one follicle approximately every 28 days, along with death to multiple other follicles. The death of ovarian follicles is called\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1240\">atresia\u00a0<\/a>and can occur at any point during follicular development. Recall that, a female infant at birth will have one to two million\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1232\">oocytes\u00a0<\/a>within her ovarian follicles, and that this number declines throughout life until menopause, when no follicles remain.<\/p>\n<p style=\"text-align: justify\">Folliculogenesis begins with follicles in a resting state. These small <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1233\">primordial follicles<\/a><\/strong> are present in newborn females and are the prevailing follicle type in the adult ovary (Figure 6). Primordial follicles have only a single flat layer of supporting cells, called <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1234\">granulosa cells<\/a><\/strong>, that surround the oocyte, and they can stay in this resting state for years\u2014some until right before menopause.<\/p>\n<figure style=\"width: 725px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image8-8.png\" alt=\"image\" width=\"725\" height=\"954\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 6. Folliculogenesis.<\/strong> (a) The maturation of a follicle is shown in a clockwise direction proceeding from the primordial follicles. FSH stimulates the growth of a tertiary follicle, and LH stimulates the production of estrogens by granulosa and theca cells. Once the follicle is mature, it ruptures and releases the oocyte. Cells remaining in the follicle then develop into the corpus luteum. (b) In this electron micrograph of a secondary follicle, the oocyte, theca cells (thecae folliculi), and developing antrum are clearly visible. EM \u00d7 1100. (Micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012)<\/figcaption><\/figure>\n<p style=\"text-align: justify\">After puberty, a few primordial follicles will respond to a recruitment signal each day and will join a pool of immature growing follicles called primary follicles. Primary follicles start with a single layer of granulosa cells, but the granulosa cells then become active and transition from a flat or squamous shape to a rounded cuboidal shape as they increase in size and proliferate. As the granulosa cells divide, the follicles\u2014now called <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1236\">secondary follicles<\/a><\/strong> (Figure 6)\u2014increase in diameter, adding a new outer layer of connective tissue, blood vessels, and <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1245\">theca cells<\/a><\/strong>\u2014cells that work with the granulosa cells to produce estrogens.<\/p>\n<p style=\"text-align: justify\">Within the growing secondary follicle, the primary oocyte now secretes a thin acellular membrane called the zona pellucida that will play a critical role in fertilization. A thick fluid, called follicular fluid, that has formed between the granulosa cells also begins to collect into one large pool, or <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1238\">antrum<\/a><\/strong>. Follicles in which the antrum has become large and fully formed are considered <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1239\">tertiary follicles<\/a><\/strong> (or antral follicles). Several follicles reach the tertiary stage at the same time, and most of these will undergo <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1240\">atresia<\/a>. The one that does not die will continue to grow and develop until <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1196\">ovulation<\/a>, when it will expel its secondary oocyte surrounded by several layers of granulosa cells from the ovary. Keep in mind that most follicles do not make it to this point. In fact, roughly 99 percent of the follicles in the ovary will undergo atresia..<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"11-2e\"><\/a>Hormonal Control of the Ovarian Cycle<\/strong><\/h5>\n<p style=\"text-align: justify\">The process of development that we have just described, from primordial follicle to early tertiary follicle, takes approximately two months in humans. The final stages of development of a small cohort of tertiary follicles, ending with ovulation of a secondary oocyte, occur over a course of approximately 28 days. These changes are mostly regulated by the hormones <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1247\">GnRH<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_479\">LH<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_478\">FSH<\/a>. For more about hormonal function, refer to the <a href=\"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/chapter\/unit-1-the-endocrine-system\/\">Endocrine System unit<\/a>.<\/p>\n<p style=\"text-align: justify\">As in men, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_392\">hypothalamus<\/a> produces GnRH, a hormone that signals the anterior <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_391\">pituitary gland<\/a> to produce the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_394\">gonadotropins<\/a> FSH and LH (Figure 7). These gonadotropins leave the pituitary gland and travel through the bloodstream to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_483\">ovaries<\/a>, where they bind to receptors on the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1234\">granulosa<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1245\">theca cells<\/a> of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1225\">follicles<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_478\">FSH<\/a> stimulates the follicles to grow (hence its name of follicle-stimulating hormone). The release of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_479\">LH<\/a> stimulates the granulosa and theca cells to produce the sex steroid hormone <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1216\">estradiol<\/a>, a type of estrogen. This phase of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1226\">ovarian cycle,<\/a> when the ovarian follicles are growing and secreting estrogens, is known as the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1405\">follicular phase<\/a>.<\/p>\n<p style=\"text-align: justify\">The more granulosa and theca cells a follicle has (that is, the larger and more developed it is), the more <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_481\">estrogens<\/a> it will produce in response to LH stimulation. As a result of the large follicles producing large amounts of estrogens, systemic plasma estrogens concentrations increase. Following a classic negative feedback loop, these increasing concentrations of estrogens will inhibit the production of GnRH, LH, and FSH by the hypothalamus and pituitary gland. Because the large ovarian follicles require FSH to grow and survive at this point, this decline in FSH caused by negative feedback leads most of them to die (<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1240\">atresia<\/a>). Typically, only one tertiary follicle from one ovary, now called the dominant follicle, will survive this reduction in FSH, and this follicle will be the one that releases an oocyte.<\/p>\n<p style=\"text-align: justify\">When only the one dominant follicle remains in the ovary, it again begins to secrete estrogens. It produces so much estrogen that the normal negative feedback does not occur. Instead, these extremely high concentrations of systemic plasma estrogens trigger a regulatory switch in the anterior pituitary that responds by secreting large amounts of LH and FSH into the bloodstream (Figure 7). The positive feedback loop by which more estrogens trigger release of more LH and FSH only occurs at this point in the cycle.<\/p>\n<figure style=\"width: 829px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image9-8.png\" alt=\"image\" width=\"829\" height=\"972\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 7. Hormonal Regulation of Ovulation.<\/strong> The hypothalamus and pituitary gland regulate the ovarian cycle and ovulation. GnRH activates the anterior pituitary gland to produce LH and FSH, which stimulate the production of estrogens and progesterone by the ovaries.<\/figcaption><\/figure>\n<p style=\"text-align: justify\">It is this large burst of LH (called the LH surge) which leads to ovulation of the dominant follicle. The LH surge induces many changes involving the degradation of the wall, and resulting in the expulsion of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1232\">oocyte<\/a> surrounded by granulosa cells into the peritoneal cavity. This release is <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1196\">ovulation<\/a><\/strong>. In a typical 28 day cycle, ovulation occurs on day 14.<\/p>\n<p style=\"text-align: justify\">In the next section, you will follow the ovulated oocyte as it travels toward the\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1244\">uterus<\/a>; however, there is one more important event that occurs in the ovarian cycle. The surge of LH also stimulates a change in the granulosa and theca cells that remain in the ruptured follicle after the oocyte has been ovulated. This change is called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1254\">luteinization<\/a> (recall that the full name of LH is luteinizing hormone), and it transforms the ruptured follicle into a new endocrine structure called the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1255\">corpus luteum<\/a><\/strong>, a term meaning \u201cyellowish body\u201d (Figure 8). The luteinized granulosa and theca cells of the corpus luteum begin to produce large amounts of the sex steroid hormone <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_482\">progesterone<\/a>, a hormone that is critical for the establishment and maintenance of pregnancy. Progesterone triggers negative feedback on the hypothalamus and pituitary gland, which keeps GnRH, LH, and FSH secretions low, so no new dominant follicles develop at this time. The corpus luteum also secretes moderate amounts of estrogens.<\/p>\n<p style=\"text-align: justify\">The post-ovulatory phase is known as the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1406\">luteal phase<\/a> of the ovarian cycle. If pregnancy does not occur within 10 to 12 days, the corpus luteum will stop secreting progesterone and estrogens and degrade into the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1256\">corpus albicans<\/a><\/strong>, a nonfunctional \u201cwhitish body\u201d that will disintegrate in the ovary over a period of several months. During this time of reduced steroid hormone secretion, FSH and LH are once again stimulated, and the follicular phase begins again with a new cohort of early ovarian follicles beginning to grow and secrete estrogens.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"11-2f\"><\/a>The Uterine Tubes<\/strong><\/h5>\n<p style=\"text-align: justify\">The <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1257\">uterine tubes<\/a><\/strong> (also called fallopian tubes or oviducts) serve as the conduit of the oocyte from the ovary to the uterus (Figure 8). Each of the two uterine tubes is close to, but not directly connected to, the ovary and divided into sections. The middle region of the tube, called the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1208\">ampulla<\/a><\/strong>, is where fertilization often occurs. The uterine tubes also contain ciliated cells that beat in the direction of the uterus, producing a current that will be critical to move the oocyte.<\/p>\n<p style=\"text-align: justify\">Following ovulation, the secondary oocyte surrounded by a few granulosa cells is released into the peritoneal cavity. The nearby uterine tube, either left or right, receives the oocyte. High concentrations of estrogens that occur around the time of ovulation induce contractions of the smooth muscle along the length of the uterine tube and the result is a coordinated movement that sweeps the surface of the ovary and the pelvic cavity. Current flowing toward the uterus is generated by coordinated beating of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1258\">cilia<\/a> that line the lumen of the length of the uterine tube. These cilia beat more strongly in response to the high estrogens concentrations that occur around the time of ovulation. As a result of these mechanisms, the oocyte\u2013granulosa cell complex is pulled into the interior of the tube. Once inside, the muscular contractions and beating cilia move the oocyte slowly toward the uterus. When fertilization does occur, sperm typically meet the egg while it is still moving through the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1208\">ampulla<\/a>.<\/p>\n<p style=\"text-align: justify\">If the oocyte is successfully <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1390\">fertilized<\/a>, the resulting zygote will begin to divide into two cells, then four, and so on, as it makes its way through the uterine tube and toward the uterus. If the oocyte is not fertilized, it will simply degrade\u2014either in the uterine tube or in the uterus, where it may be shed with the next <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1227\">menstrual<\/a> period.<\/p>\n<figure style=\"width: 818px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image10-7.png\" alt=\"image\" width=\"818\" height=\"393\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 8. Ovaries, Uterine Tubes, and Uterus.<\/strong> This anterior view shows the relationship of the ovaries, uterine tubes (oviducts), and uterus. Sperm enter through the vagina, and fertilization of an ovulated oocyte usually occurs in the distal uterine tube. From left to right, LM \u00d7 400, LM \u00d7 20. (Micrographs provided by the Regents of University of Michigan Medical School \u00a9 2012)<\/figcaption><\/figure>\n<h5 style=\"text-align: justify\"><strong><a id=\"11-2g\"><\/a>The Uterus<\/strong><strong> and Cervix<\/strong><\/h5>\n<p style=\"text-align: justify\">The uterus is the muscular organ that nourishes and supports the growing embryo (Figure 8). Its average size is approximately 5 cm wide by 7 cm long when a female is not pregnant. It has three sections. The portion of the uterus superior to the opening of the uterine tubes is called the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_861\">fundus<\/a><\/strong>. The middle section of the uterus is called the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1396\">body of uterus<\/a><\/strong> (or corpus). The <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1223\">cervix<\/a><\/strong> is the narrow inferior portion of the uterus that projects into the vagina. The cervix produces mucus secretions that can facilitate sperm movement through the reproductive tract. Several ligaments maintain the position of the uterus within the abdominopelvic cavity.<\/p>\n<p style=\"text-align: justify\">The wall of the uterus is made up of three layers. The most superficial layer is the serous membrane, or <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1393\">perimetrium<\/a><\/strong>, which consists of epithelial tissue that covers the exterior portion of the uterus. The middle layer, or <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1394\">myometrium<\/a><\/strong>, is a thick layer of smooth muscle responsible for uterine contractions. Most of the uterus is myometrial tissue, which allows the powerful contractions that occur during labour and the less powerful contractions (or cramps) that help to expel menstrual blood during a woman\u2019s period.<\/p>\n<p style=\"text-align: justify\">The innermost layer of the uterus is called the endometrium. The <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1395\">endometrium<\/a><\/strong> contains a connective tissue lining, the lamina propria, which is covered by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_654\">epithelial tissue<\/a> that lines the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_777\">lumen<\/a>. Structurally, the endometrium consists of two layers: the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1397\">stratum basalis<\/a> and the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1399\">stratum functionalis<\/a> (the basal and functional layers). The stratum basalis layer is part of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_769\">lamina propria<\/a> and is adjacent to the myometrium; this layer does not shed during menses. In contrast, the thicker stratum functionalis layer contains the glandular portion of the lamina propria and the endothelial tissue that lines the uterine lumen. It is the stratum functionalis that grows and thickens in response to increased levels of estrogens and progesterone. In the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1406\">luteal phase<\/a> of the menstrual cycle, special branches off of the uterine artery called spiral arteries supply the thickened stratum functionalis. This inner functional layer provides the proper site of implantation for the fertilized oocyte, and\u2014should fertilization not occur\u2014it is only the stratum functionalis layer of the endometrium that sheds during menstruation.<\/p>\n<p style=\"text-align: justify\">Recall that during the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1405\">follicular phase<\/a> of the ovarian cycle, the ovarian follicles are growing and secreting <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_481\">estrogens<\/a>. At the same time, the stratum functionalis of the endometrium is thickening to prepare for a potential implantation. The post-ovulatory increase in progesterone, which characterizes the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1406\">luteal phase<\/a>, is key for maintaining a thick stratum functionalis. As long as a functional <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1255\">corpus luteum<\/a> is present in the ovary, the endometrial lining is prepared for implantation. Indeed, if a blastocyst implants, signals are sent to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1255\">corpus luteum<\/a> to continue secreting progesterone to maintain the endometrium, and thus maintain the pregnancy. If a blastocyst does not implant, no signal is sent to the corpus luteum and it degrades, ceasing progesterone production and ending the luteal phase. Without <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_482\">progesterone<\/a>, the endometrium becomes thinner and, under the influence of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1017\">prostaglandins<\/a>, the spiral arteries of the endometrium constrict and rupture, preventing oxygenated blood from reaching the endometrial tissue. As a result, endometrial tissue dies and blood, pieces of the endometrial tissue, and white blood cells are shed through the vagina during menstruation, or the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1400\">implantation<\/a><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1401\">menses<\/a><\/strong>.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"11-2h\"><\/a>The Menstrual (Uterine) Cycle<\/strong><\/h5>\n<p style=\"text-align: justify\">Now that we have discussed the maturation of the cohort of ovarian follicles in the ovary, the build-up and then shedding of the endometrial lining in the uterus, and the function of the uterine tubes and vagina, we can put everything together to talk about the three phases of the <strong>menstrual (uterine) cycle<\/strong>\u2014the series of changes in which the uterine lining is shed, rebuilds, and prepares for implantation.<\/p>\n<p style=\"text-align: justify\">The timing of the menstrual cycle starts with the first day of menses, referred to as day one of a woman\u2019s period.\u00a0The average length of a woman\u2019s menstrual cycle is 28 days (typically varying from 21 to 32 days) and it is the time period used to identify the timing of events in the cycle.<\/p>\n<p style=\"text-align: justify\">Just as the hormones produced by the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1234\">granulosa<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1245\">theca cells<\/a> of the ovary \u201cdrive\u201d the follicular and luteal phases of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1226\">ovarian cycle<\/a>, they also control the three distinct phases of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1227\">menstrual cycle<\/a>. These are the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1404\">menses phase<\/a>, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1402\">proliferative phase<\/a>, and the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1403\">secretory phase<\/a>.<\/p>\n<p style=\"text-align: justify\"><strong>1. Menses Phase:<\/strong> The <strong>menses phase<\/strong> of the menstrual cycle is the phase during which the endometrial lining is shed; that is, the days that the woman menstruates. Although it averages approximately five days, the menses phase can last from 2 to 7 days, or longer. The menses phase occurs during the early days of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1405\">follicular phase<\/a> of the ovarian cycle, when <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_482\">progesterone<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_478\">FSH<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_479\">LH<\/a> levels are low (Figure 9). Recall that progesterone concentrations decline as a result of the degradation of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1255\">corpus luteum<\/a>, marking the end of the luteal phase. This decline in progesterone triggers the shedding of the stratum functionalis of the endometrium.<\/p>\n<p style=\"text-align: justify\"><strong>2. Proliferative Phase:<\/strong> Once the menstrual flow ceases, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1395\">endometrium<\/a> begins to proliferate again, marking the beginning of the <strong>proliferative phase<\/strong> of the menstrual cycle (Figure 9). It occurs when the granulosa and theca cells of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1239\">tertiary follicles<\/a> begin to produce increased amounts of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_481\">estrogens<\/a>. These rising estrogen concentrations stimulate the endometrial lining to rebuild.<\/p>\n<p style=\"text-align: justify\">Recall that increasing estrogen concentrations leads to a decrease in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_478\">FSH<\/a> as a result of negative feedback, resulting in atresia of all but one of the developing tertiary follicles. The switch to positive feedback\u2014which occurs with the elevated production of estrogens from the dominant follicle\u2014then stimulates the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_479\">LH<\/a> surge that will trigger <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1196\">ovulation<\/a>. Ovulation marks the end of the proliferative phase as well as the end of the follicular phase.<\/p>\n<p style=\"text-align: justify\"><strong>3. <\/strong><strong>Secretory Phase:<\/strong> In addition to prompting the LH surge, high estrogen levels increase the uterine tube contractions that facilitate the pick-up and transfer of the ovulated <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1232\">oocyte<\/a>. High estrogen levels also slightly decrease the acidity of the vagina, making it more hospitable to sperm. In the ovary, the luteinization of the granulosa cells of the ruptured follicle forms the corpus luteum, marking the beginning of the luteal phase of the ovarian cycle. In the uterus, progesterone from the corpus luteum begins the <strong>secretory phase<\/strong> of the menstrual cycle, in which the endometrial lining prepares for implantation (Figure 9). Over the next 10 to 12 days, the endometrial glands secrete a fluid rich in glycogen. If fertilization has occurred, this fluid will nourish the ball of cells now developing from the zygote, known as the conceptus. At the same time, the spiral arteries develop to provide blood to the thickened <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1399\">stratum functionalis<\/a>.<\/p>\n<p style=\"text-align: justify\">If no pregnancy occurs within approximately 10 to 12 days, the corpus luteum will degrade into the corpus albicans. Levels of both estrogens and progesterone will fall, and the endometrium will grow thinner. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1017\">Prostaglandins<\/a> are then secreted which causes the constriction of the spiral arteries, reducing oxygen supply. The endometrial tissue will die, resulting in menses\u2014or the first day of the next cycle.<\/p>\n<figure style=\"width: 619px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image11-7.png\" alt=\"image\" width=\"619\" height=\"1059\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 9. Hormone Levels in the Ovarian and Menstrual Cycles.<\/strong> The correlation of the hormone levels and their effects on the female reproductive system is shown in this timeline of the ovarian and menstrual cycles. The menstrual cycle begins at day one with the start of menses. Ovulation occurs around day 14 of a 28-day cycle, triggered by the LH surge.<\/figcaption><\/figure>\n<h5 style=\"text-align: justify\"><strong><a id=\"11-2i\"><\/a>The Breasts<\/strong><\/h5>\n<p style=\"text-align: justify\">Although the breasts are located far from the other female reproductive organs, they are considered accessory organs of the female reproductive system. The function of the breasts is to supply milk to an infant in a process called lactation. The external features of the breast include a nipple surrounded by a pigmented <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1407\">areola<\/a><\/strong> (Figure 10). When a baby nurses, or draws milk from the breast, the entire areolar region is taken into the mouth.<\/p>\n<figure style=\"width: 697px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image12-7.png\" alt=\"image\" width=\"697\" height=\"347\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 10. Anatomy of the Breast.<\/strong> During lactation, milk moves from the alveoli through the lactiferous ducts to the nipple.<\/figcaption><\/figure>\n<p style=\"text-align: justify\">Breast milk is produced by the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1408\">mammary glands<\/a><\/strong>, which are modified sweat glands. The milk itself exits the breast through the nipple via 15 to 20 <strong>lactiferous ducts<\/strong> that open on the surface of the nipple. These lactiferous ducts each extend to a <strong>lactiferous sinus<\/strong> that connects to a glandular lobe that contains groups of milk-secreting cells in clusters called <strong>alveoli<\/strong> (Figure 10). Once milk is made in the alveoli, stimulated myoepithelial cells that surround the alveoli contract to push the milk to the lactiferous sinuses. From here, the baby can draw milk through the lactiferous ducts by suckling. The non-pregnant and non-lactating female breast is composed primarily of adipose and collagenous tissue, with mammary glands making up a very minor proportion of breast volume.<\/p>\n<h2 style=\"text-align: justify\"><strong><a id=\"11-3\"><\/a>Part 3: Fertilization<\/strong><\/h2>\n<p style=\"text-align: justify\"><strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1390\">Fertilization<\/a><\/strong> occurs when a sperm and an oocyte combine and their nuclei fuse. Because each of these reproductive cells is haploid, containing half of the genetic material needed to form a human being, their combination forms a diploid cell. The resulting diploid cell, called a<strong> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1409\">zygote<\/a><\/strong>, contains all the genetic material needed to form a human\u2014half from the mother and half from the father.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"11-3a\"><\/a>Transit of Sperm<\/strong><\/h5>\n<p style=\"text-align: justify\">During ejaculation, hundreds of millions of sperm (spermatozoa) are released into the vagina. Almost immediately, millions of these sperm are overcome by the acidity of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1222\">vagina<\/a> (approximately pH 3.8), and millions more may be blocked from entering the uterus by thick cervical mucus. Of those that do enter, thousands are destroyed by phagocytic uterine <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_544\">leukocytes<\/a>. Thus, the race into the uterine tubes, which is the most typical site for sperm to encounter the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1232\">oocyte<\/a>, is reduced to a few thousand contenders. Their journey\u2014thought to be facilitated by uterine contractions\u2014usually takes from 30 minutes to 2 hours. If the sperm do not encounter an oocyte immediately, they can survive in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1257\">uterine tubes<\/a> for another 3\u20135 days. Intercourse more than a day after ovulation is therefore unlikely to result in fertilization.<\/p>\n<p style=\"text-align: justify\">During the journey, fluids in the female reproductive tract prepare the sperm for fertilization through a process called <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1410\">capacitation<\/a><\/strong>, or priming. The fluids improve the motility of the spermatozoa and prepare them for contact with the oocyte. If sperm reach the oocyte before capacitation is complete, they will be unable to penetrate the oocyte\u2019s thick outer protective layers.<\/p>\n<h5 style=\"text-align: justify\"><span style=\"color: #000000\"><strong><a id=\"11-3b\" style=\"color: #000000\"><\/a>Contact Between Sperm and Oocyte<\/strong><\/span><\/h5>\n<p style=\"text-align: justify\">Upon ovulation, the oocyte released by the ovary is swept into\u2014and along\u2014the uterine tube. Fertilization should occur in the distal uterine tube because an unfertilized oocyte cannot survive the 72-hour journey to the uterus. This oocyte (specifically a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1242\">secondary oocyte<\/a>) is surrounded by two protective layers. The <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1411\">corona radiata<\/a><\/strong> is an outer layer of follicular (granulosa) cells that form around a developing oocyte in the ovary and remain with it upon ovulation. The underlying <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1237\">zona pellucida<\/a><\/strong> (pellucid = \u201ctransparent\u201d) is a transparent, but thick, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_581\">glycoprotein<\/a> membrane that surrounds the cell\u2019s plasma membrane.<\/p>\n<p style=\"text-align: justify\">The oocyte encounters the surviving capacitated sperm, which stream toward it in response to chemical attractants released by the cells of the corona radiata. To reach the oocyte itself, the sperm must penetrate the two protective layers. The sperm first burrow through the cells of the corona radiata. Then, upon contact with the zona pellucida, the sperm bind to receptors in the zona pellucida. This initiates a process called the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1412\">acrosomal reaction<\/a><\/strong> in which the enzyme-filled \u201ccap\u201d of the sperm, called the acrosome, releases its stored digestive enzymes. These enzymes clear a path through the zona pellucida that allows sperm to reach the oocyte. Finally, a single sperm makes contact with sperm-binding receptors on the oocyte\u2019s plasma membrane (Figure 11). The plasma membrane of that sperm then fuses with the oocyte\u2019s plasma membrane, and the head and mid-piece of this sperm enter the oocyte interior.<\/p>\n<figure style=\"width: 1424px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image13-8.png\" alt=\"image\" width=\"1424\" height=\"983\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 11. Sperm and the Process of Fertilization.<\/strong> Before fertilization, hundreds of capacitated sperm must break through the surrounding corona radiata and zona pellucida so that one can contact and fuse with the oocyte plasma membrane.<\/figcaption><\/figure>\n<p style=\"text-align: justify\">How do sperm penetrate the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1411\">corona radiata<\/a>? Some sperm undergo a spontaneous acrosomal reaction, which is an acrosomal reaction not triggered by contact with the zona pellucida. The digestive enzymes released by this reaction digest the extracellular matrix of the corona radiata. Thus, the first sperm to reach the oocyte is never the one to fertilize it. Rather, hundreds of sperm cells must undergo the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1412\">acrosomal reaction<\/a>, each helping to degrade the corona radiata and zona pellucida until a path is created to allow one sperm to contact and fuse with the plasma membrane of the oocyte.<\/p>\n<p style=\"text-align: justify\">When the first sperm fuses with the oocyte, the oocyte deploys two mechanisms to prevent <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1413\">polyspermy<\/a><\/strong>, which is penetration by more than one sperm. This is critical because if more than one sperm were to fertilize the oocyte, the resulting zygote would be a triploid organism with three sets of chromosomes. This is incompatible with life.<\/p>\n<p style=\"text-align: justify\">The first mechanism is the fast block, which involves a near instantaneous change in sodium ion permeability upon binding of the first sperm, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_521\">depolarizing<\/a> the oocyte plasma membrane and preventing the fusion of additional sperm cells. The fast block sets in almost immediately and lasts for about a minute, during which time an influx of calcium ions following sperm penetration triggers the second mechanism, the slow block. In this process, referred to as the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1415\">cortical reaction<\/a><\/strong>, cortical granules sitting immediately below the oocyte plasma membrane fuse with the membrane and release certain proteins and polysaccharides into the space between the plasma membrane and the zona pellucida. The proteins cause the release of any other attached sperm and destroy the oocyte\u2019s sperm receptors, and the mucopolysaccharides then coat the nascent zygote in an impenetrable barrier, forming a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1414\">fertilization membrane<\/a><\/strong>.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"11-3c\"><\/a>The Zygote<\/strong><\/h5>\n<p style=\"text-align: justify\">Upon fertilization the two haploid nuclei derived from the sperm and oocyte decondense, expand, and replicate their DNA. The pronuclei then migrate toward each other, their nuclear envelopes disintegrate, and the male- and female-derived genetic material intermingles. This step completes the process of fertilization and results in a single-celled <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1416\">diploid<\/a> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1409\">zygote<\/a> with all the genetic instructions it needs to develop into a human.<\/p>\n<figure style=\"width: 150px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image14-7.png\" alt=\"image\" width=\"150\" height=\"150\" \/><figcaption class=\"wp-caption-text\">Watch <a href=\"https:\/\/youtu.be\/SUdAEGXLO-8\">this CrashCourse video<\/a> to learn more about fertilization! Direct link: <a href=\"https:\/\/youtu.be\/SUdAEGXLO-8\">https:\/\/youtu.be\/SUdAEGXLO-8<\/a><\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<h2 style=\"text-align: left\"><strong><a id=\"11-4\"><\/a>Part 4: Embryonic Development<\/strong><\/h2>\n<p style=\"text-align: justify\">Throughout this chapter, we will express embryonic and fetal ages in terms of weeks from <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1390\">fertilization<\/a>, commonly called conception. The period of time required for full development of a fetus in utero is referred to as <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1421\">gestation<\/a><\/strong> (gestare = \u201cto carry\u201d or \u201cto bear\u201d). It can be subdivided into distinct gestational periods. A developing human is referred to as an <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1422\">embryo<\/a><\/strong> during weeks 3\u20138, and a<strong> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1423\">fetus<\/a><\/strong> from the ninth week of gestation until birth. In this section, we\u2019ll cover the embryonic stages of development, which are characterized by cell division, migration, and differentiation. By the end of the embryonic period, all the organ systems are structured in rudimentary form, although the organs themselves are either nonfunctional or only semi-functional.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"11-4a\"><\/a>Pre-implantation Embryonic Development<\/strong><\/h5>\n<p style=\"text-align: justify\">Following fertilization, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1409\">zygote<\/a> and its associated membranes, together referred to as the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1424\">conceptus<\/a><\/strong>, continue to be projected toward the uterus by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_822\">peristalsis<\/a> and beating <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1258\">cilia<\/a>. During its journey to the uterus, the zygote undergoes five or six rapid <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1425\">mitotic<\/a> cell divisions. Although each <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1426\">cleavage<\/a><\/strong> results in more cells, it does not increase the total volume of the conceptus (Figure 12). Each daughter cell produced by cleavage is called a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1427\">blastomere<\/a><\/strong> (blastos = \u201cgerm,\u201d in the sense of a seed or sprout).<\/p>\n<p style=\"text-align: justify\">Approximately 3 days after fertilization, a 16-cell conceptus reaches the uterus. The cells that had been loosely grouped are now compacted and look more like a solid mass. The name given to this structure is the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1428\">morula<\/a><\/strong> (morula = \u201clittle mulberry\u201d). Once inside the uterus, the conceptus floats freely for several more days. It continues to divide, creating a ball of approximately 100 cells, and consuming nutritive endometrial secretions while the uterine lining thickens. The ball of now tightly bound cells starts to secrete fluid and organize themselves around a fluid-filled cavity, the<strong> blastocoel<\/strong>. At this developmental stage, the conceptus is referred to as a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1429\">blastocyst<\/a><\/strong>. Within this structure, a group of cells forms into an <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1431\">inner cell mass<\/a><\/strong>, which is fated to become the embryo. Other cells form the outer shell called <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1430\">trophoblasts<\/a><\/strong> (trophe = \u201cto feed\u201d or \u201cto nourish\u201d). These cells will develop into the chorionic sac and the fetal portion of the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1417\">placenta<\/a> <\/strong>(the organ of nutrient, waste, and gas exchange between mother and the developing offspring).<\/p>\n<p style=\"text-align: justify\">The inner mass of embryonic cells is <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1432\">totipotent<\/a> during this stage, meaning that each cell has the potential to differentiate into any cell type in the human body. Totipotency lasts for only a few days before the cells\u2019 fates are set as being the precursors to a specific lineage of cells.<\/p>\n<figure style=\"width: 1262px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" class=\"\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image15-6.png\" alt=\"image\" width=\"1262\" height=\"1107\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 12. Early Embryonic Development.<\/strong> Cleavages make use of the abundant cytoplasm of the conceptus as the cells rapidly divide without changing the total volume.<\/figcaption><\/figure>\n<h5 style=\"text-align: justify\"><strong><a id=\"11-4b\"><\/a>Implantation<\/strong><\/h5>\n<p style=\"text-align: justify\">At the end of the first week, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1429\">blastocyst<\/a> comes in contact with the uterine wall and adheres to it, embedding itself in the uterine lining via the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1430\">trophoblast<\/a> cells. This process is called <strong>implantation<\/strong> (Figure 13), which can be accompanied by minor bleeding. The blastocyst typically implants in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_861\">fundus<\/a> of the uterus or on the posterior wall. However, if the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1395\">endometrium<\/a> is not fully developed and ready to receive the blastocyst, the blastocyst will detach and find a better spot. A significant percentage (50\u201375 percent) of blastocysts fail to implant; when this occurs, the blastocyst is shed with the endometrium during <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1401\">menses<\/a>. The high rate of implantation failure is one reason why pregnancy typically requires several ovulation cycles to be achieved.<\/p>\n<figure style=\"width: 1430px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image16-6.png\" alt=\"image\" width=\"1430\" height=\"1297\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 13. Early Embryonic Development.<\/strong> Ovulation, fertilization, cleavage embryonic development, and implantation occur at specific locations within the female reproductive system in a time span of approximately 1 week.<\/figcaption><\/figure>\n<p style=\"text-align: justify\">When implantation succeeds and the blastocyst adheres to the endometrium, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_804\">superficial<\/a> cells of the trophoblast fuse with each other, forming the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1433\">syncytiotrophoblast<\/a><\/strong>, a multinucleated body that digests endometrial cells to firmly secure the blastocyst to the uterine wall. In response, the uterine mucosa rebuilds itself and envelops the blastocyst (Figure 14). The trophoblast secretes <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1434\">human chorionic gonadotropin (hCG)<\/a><\/strong>, a hormone that directs the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1255\">corpus luteum<\/a> to survive, enlarge, and continue producing <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_482\">progesterone<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_481\">estrogens<\/a> to suppress menses. These functions of hCG are necessary for creating an environment suitable for the developing embryo. As a result of this increased production, hCG accumulates in the maternal bloodstream and is excreted in the urine. Implantation is complete by the middle of the second week. Just a few days after <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1400\">implantation<\/a>, the trophoblast has secreted enough hCG for an at-home urine pregnancy test to give a positive result.<\/p>\n<figure style=\"width: 1430px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image17-6.png\" alt=\"image\" width=\"1430\" height=\"1783\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 14. Implantation.<\/strong> During implantation, the trophoblast cells of the blastocyst adhere to the endometrium and digest endometrial cells until it is attached securely.<\/figcaption><\/figure>\n<h5 style=\"text-align: justify\"><strong><a id=\"11-4c\"><\/a>Embryonic Membranes and Germ Layers<\/strong><\/h5>\n<p style=\"text-align: justify\">During the second week of development, with the embryo implanted in the uterus, cells within the blastocyst start to organize into layers. Some grow to form the extra-embryonic membranes needed to support and protect the growing embryo: the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1435\">amnion<\/a>, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1438\">yolk sac<\/a>, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1436\">allantois<\/a>, and the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1437\">chorion<\/a> (Figure 16).<\/p>\n<p>As the third week of development begins, the two-layered disc of cells becomes a three-layered disc through the process of\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1439\">gastrulation<\/a>, during which the cells transition from an undifferentiated to a more differentiated state. Three groups of cells, called germ layers, are formed: the\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1440\">endoderm<\/a><\/strong>,\u00a0<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1442\"><strong>mesoderm<\/strong>\u00a0<\/a>(middle), and\u00a0<strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1441\">ectoderm<\/a><\/strong>. Each of these germ layers will develop into specific structures in the embryo, setting the stage for organogenesis.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"11-4e\"><\/a>Development of the Placenta<\/strong><\/h5>\n<p style=\"text-align: justify\">During the first several weeks of development, the cells of the endometrium\u2014referred to as decidual cells\u2014nourish the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1443\">nascent<\/a> embryo. During prenatal weeks 4\u201312, the developing placenta gradually takes over the role of feeding the embryo, and the decidual cells are no longer needed. The mature placenta is composed of tissues derived from the embryo, as well as maternal tissues of the endometrium. The placenta connects to the conceptus via the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1444\">umbilical cord<\/a><\/strong>, which carries deoxygenated blood and wastes from the fetus through two umbilical arteries; nutrients and oxygen are carried from the mother to the fetus through the single umbilical vein.<\/p>\n<p style=\"text-align: justify\">The maternal portion of the placenta develops from the deepest layer of the endometrium, the decidua basalis. To form the embryonic portion of the placenta, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1433\">syncytiotrophoblast<\/a> and the underlying cells of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1430\">trophoblast<\/a> (cytotrophoblast cells) begin to proliferate along with a layer of extraembryonic mesoderm cells. These form the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1446\">chorionic membrane<\/a><\/strong>, which envelops the entire conceptus as the chorion. The chorionic membrane forms finger-like structures called <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1445\">chorionic villi<\/a><\/strong> that burrow into the endometrium like tree roots, making up the fetal portion of the placenta. The cytotrophoblast cells perforate the chorionic villi, burrow farther into the endometrium, and remodel maternal blood vessels to augment maternal blood flow surrounding the villi. Meanwhile, fetal <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1447\">mesenchymal<\/a> cells derived from the mesoderm fill the villi and differentiate into blood vessels, including the three umbilical blood vessels that connect the embryo to the developing placenta (Figure 15).<\/p>\n<figure style=\"width: 1315px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image19-4.png\" alt=\"image\" width=\"1315\" height=\"682\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 15. Cross-Section of the Placenta.<\/strong> In the placenta, maternal and fetal blood components are conducted through the surface of the chorionic villi, but maternal and fetal bloodstreams never mix directly.<\/figcaption><\/figure>\n<p style=\"text-align: justify\">The development of the placenta, or <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1448\">placentation<\/a><\/strong> is complete by weeks 14\u201316. As a fully developed organ, the placenta provides nutrition and excretion, respiration, and endocrine function (Table 1 and Figure 16). It receives blood from the fetus through the umbilical arteries. Capillaries in the chorionic villi filter fetal wastes out of the blood and return clean, oxygenated blood to the fetus through the umbilical vein. Nutrients and oxygen are transferred from maternal blood surrounding the villi through the capillaries and into the fetal bloodstream.<\/p>\n<p style=\"text-align: justify\">Maternal and fetal blood do not mingle because blood cells cannot move across the placenta. This separation prevents the mother\u2019s <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_641\">cytotoxic T cells<\/a> from reaching and subsequently destroying the fetus, which bears \u201cnon-self\u201d <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_623\">antigens<\/a>.<\/p>\n<figure style=\"width: 854px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image20-4.png\" alt=\"image\" width=\"854\" height=\"644\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 16. Placenta.<\/strong> This post-expulsion placenta and umbilical cord (white) are viewed from the fetal side.<\/figcaption><\/figure>\n<table style=\"border-collapse: collapse;width: 100%;height: 220px\">\n<caption>Table 1: Functions of the Placenta<\/caption>\n<tbody>\n<tr style=\"height: 14px\">\n<th style=\"width: 33.3333%;height: 14px\" scope=\"col\"><strong>Nutrition &amp; digestion<\/strong><\/th>\n<th style=\"width: 33.3333%;height: 14px\" scope=\"col\"><strong>Respiration<\/strong><\/th>\n<th style=\"width: 33.3333%;height: 14px\" scope=\"col\"><strong>Endocrine function<\/strong><\/th>\n<\/tr>\n<tr style=\"height: 192px\">\n<td style=\"width: 33.3333%;height: 192px\">Mediates diffusion of maternal glucose, amino acids, fatty acids, vitamins, minerals<\/p>\n<p>Stores nutrients during early pregnancy to accommodate increased fetal demand later in pregnancy<\/p>\n<p>Excretes &amp; filters fetal nitrogenous wastes into maternal blood<\/td>\n<td style=\"width: 33.3333%;height: 192px\">Mediates maternal-to-fetal oxygen transport<\/p>\n<p>Mediates fetal-to-maternal carbon dioxide transport<\/td>\n<td style=\"width: 33.3333%;height: 192px\">Secretes hCG, estrogens, and progesterone to maintain the pregnancy and stimulate maternal &amp; fetal development<\/p>\n<p>Mediates transmission of maternal hormones into fetal blood<\/p>\n<p>Mediates transmission of fetal hormones into maternal blood<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<figure style=\"width: 829px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image22-5.png\" alt=\"image\" width=\"829\" height=\"885\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 17. Embryo at 7 Weeks.<\/strong> An embryo at the end of 7 weeks of development is only 10 mm in length, but its developing eyes, limb buds, and tail are already visible. (This embryo was derived from an ectopic pregnancy.) (credit: Ed Uthman)<\/figcaption><\/figure>\n<figure style=\"width: 150px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image23-5.png\" alt=\"image\" width=\"150\" height=\"150\" \/><figcaption class=\"wp-caption-text\">Watch <a href=\"https:\/\/youtu.be\/BtsSbZ85yiQ\">this Crash Course video<\/a> to learn about the stages of embryonic development! Direct link: <a href=\"https:\/\/youtu.be\/BtsSbZ85yiQ\">https:\/\/youtu.be\/BtsSbZ85yiQ<\/a><\/figcaption><\/figure>\n<h2 style=\"text-align: left\"><strong><a id=\"11-5\"><\/a>Part 5: Fetal Development<\/strong><\/h2>\n<p style=\"text-align: justify\">A developing human is called a fetus from the ninth week of gestation until birth. This 30-week period of development is marked by continued cell growth and differentiation, which fully develop the structures and functions of the immature organ systems formed during the embryonic period. The completion of fetal development results in a newborn who, although still immature in many ways, is capable of survival outside the womb.<\/p>\n<figure id=\"attachment_340\" aria-describedby=\"caption-attachment-340\" style=\"width: 145px\" class=\"wp-caption alignleft\"><a href=\"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2019\/08\/qrcode.jpeg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-340\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2019\/08\/qrcode.jpeg\" alt=\"\" width=\"145\" height=\"145\" srcset=\"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2019\/08\/qrcode.jpeg 400w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2019\/08\/qrcode-300x300.jpeg 300w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2019\/08\/qrcode-150x150.jpeg 150w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2019\/08\/qrcode-65x65.jpeg 65w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2019\/08\/qrcode-225x225.jpeg 225w, https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-content\/uploads\/sites\/1536\/2019\/08\/qrcode-350x350.jpeg 350w\" sizes=\"auto, (max-width: 145px) 100vw, 145px\" \/><\/a><figcaption id=\"caption-attachment-340\" class=\"wp-caption-text\">Watch this National Geographic video for a summary of fetal development and maternal changes during pregnancy. <a href=\"https:\/\/www.youtube.com\/watch?v=XEfnq4Q4bfk\">https:\/\/www.youtube.com\/watch?v=XEfnq4Q4bfk<\/a><\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"11-5a\"><\/a>The Fetal Circulatory System<\/strong><\/h5>\n<p style=\"text-align: justify\">During prenatal development, the fetal circulatory system is integrated with the placenta via the umbilical cord so that the fetus receives both oxygen and nutrients from the placenta. However, after childbirth, the umbilical cord is severed, and the newborn\u2019s circulatory system must be reconfigured. Unlike a mature cardiovascular system, however, the fetal cardiovascular system also includes circulatory shortcuts, or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1449\">shunts<\/a>. A <strong>shunt <\/strong>is an anatomical (or sometimes surgical) diversion that allows blood flow to bypass immature organs such as the lungs and liver until childbirth.<\/p>\n<p style=\"text-align: justify\">The placenta provides the fetus with necessary oxygen and nutrients via the umbilical vein. (Remember that veins carry blood toward the heart. In this case, the blood flowing to the fetal heart is oxygenated because it comes from the placenta. The respiratory system is immature and cannot yet oxygenate blood on its own.)<\/p>\n<p style=\"text-align: justify\">From the umbilical vein, the oxygenated blood flows toward the inferior vena cava, all but bypassing the immature liver, via the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1450\">ductus venosus<\/a><\/strong> shunt (Figure 18). The liver receives just a trickle of blood, which is all that it needs in its immature, semifunctional state. Blood flows from the inferior vena cava to the right atrium, mixing with fetal venous blood along the way.<\/p>\n<figure style=\"width: 1294px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image24-4.png\" alt=\"image\" width=\"1294\" height=\"1134\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 18. Fetal Circulatory System.<\/strong> The fetal circulatory system includes three shunts to divert blood from undeveloped and partially functioning organs, as well as blood supply to and from the placenta.<\/figcaption><\/figure>\n<p style=\"text-align: justify\">Whereas the fetal liver is semi-functional, the fetal lungs are nonfunctional. The fetal circulation therefore bypasses the lungs by shifting some of the blood through the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_501\">foramen ovale<\/a><\/strong>, a shunt that directly connects the right and left atria and avoids the pulmonary trunk altogether. Most of the rest of the blood is pumped to the right ventricle, and from there, into the pulmonary trunk, which splits into pulmonary arteries. However, a shunt within the pulmonary artery, the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1451\">ductus arteriosus<\/a><\/strong>, diverts a portion of this blood into the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1028\">aorta<\/a>. This ensures that only a small volume of oxygenated blood passes through the immature pulmonary circuit, which has only minor metabolic requirements. The oxygenated blood moves through the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_501\">foramen ovale<\/a> into the left atrium, where it mixes with the now deoxygenated blood returning from the pulmonary circuit. This blood then moves into the left ventricle, where it is pumped into the aorta. Some of this blood moves through the coronary arteries into the myocardium, and some moves through the carotid arteries to the brain.<\/p>\n<p style=\"text-align: justify\">The descending aorta carries partially oxygenated and partially deoxygenated blood into the lower regions of the body. It eventually passes into the umbilical arteries through branches of the internal iliac arteries. The deoxygenated blood collects waste as it circulates through the fetal body and returns to the umbilical cord. Thus, the two umbilical arteries carry blood low in oxygen and high in carbon dioxide and fetal wastes. This blood is filtered through the placenta, where wastes diffuse into the maternal circulation. Oxygen and nutrients from the mother diffuse into the placenta and from there into the fetal blood, and the process repeats.<\/p>\n<p style=\"text-align: justify\">Prior to birth, the lungs are filled with amniotic fluid, mucus, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1452\">surfactant<\/a>. As the fetus is squeezed through the birth canal, the fetal <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_664\">thoracic cavity<\/a> is compressed, expelling much of this fluid. The first inhalation occurs within 10 seconds after birth and not only serves as the first inspiration, but also acts to inflate the lungs. <strong>Pulmonary <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1452\">surfactant<\/a><\/strong> is critical for inflation to occur, as it reduces the surface tension of the alveoli. A baby born prior to 26 weeks has a low chance of survival, as it produces insufficient pulmonary surfactant, and the surfaces for gas exchange have not formed adequately.<\/p>\n<h2 style=\"text-align: left\"><strong><a id=\"11-6\"><\/a>Part 6: Maternal Changes During Pregnancy, Labour, and Birth<\/strong><\/h2>\n<p style=\"text-align: justify\">A full-term pregnancy lasts approximately 270 days (38.5 weeks) from conception to birth. Because it is easier to remember the first day of the last menstrual period (LMP) than to estimate the date of conception, obstetricians set the due date as 284 days (approximately 40.5 weeks) from the LMP. This assumes that conception occurred on day 14 of the woman\u2019s cycle, which is usually a good approximation. The 40 weeks of an average pregnancy are usually discussed in terms of three trimesters, each approximately 13 weeks. During the second and third trimesters, the pre-pregnancy uterus\u2014about the size of a fist\u2014grows dramatically to contain the fetus, causing a number of anatomical changes in the mother.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"11-6a\"><\/a>Effects of Hormones<\/strong><\/h5>\n<p style=\"text-align: justify\">Virtually all the effects of pregnancy can be attributed in some way to the influence of hormones\u2014particularly <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_481\">estrogens<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_482\">progesterone<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1434\">hCG<\/a>. During weeks 7\u201312 from the LMP, the pregnancy hormones are primarily generated by the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1255\">corpus luteum<\/a>. Progesterone secreted by the corpus luteum stimulates the production of decidual cells of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1395\">endometrium<\/a> that nourish the blastocyst before placentation. As the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1417\">placenta<\/a> develops and the corpus luteum degenerates during weeks 12\u201317, the placenta gradually takes over as the endocrine organ of pregnancy.<\/p>\n<p style=\"text-align: justify\">The placenta converts weak androgens secreted by the maternal and fetal adrenal glands to estrogens, which are necessary for pregnancy to progress. Estrogen levels climb throughout the pregnancy, increasing 30-fold by childbirth. Estrogens have the following actions:<\/p>\n<ul style=\"text-align: left\">\n<li style=\"text-align: justify\">They suppress <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_478\">FSH<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_479\">LH<\/a> production, effectively preventing ovulation. (This function is the biological basis of hormonal birth control pills.)<\/li>\n<li style=\"text-align: justify\">They induce the growth of fetal tissues and are necessary for the maturation of the fetal lungs and liver.<\/li>\n<li style=\"text-align: justify\">They promote fetal viability by regulating progesterone production and triggering fetal synthesis of cortisol, which helps with the maturation of the lungs, liver, and endocrine organs such as the thyroid gland and adrenal gland.<\/li>\n<li style=\"text-align: justify\">They stimulate maternal tissue growth, leading to uterine enlargement and mammary duct expansion and branching.<\/li>\n<\/ul>\n<p style=\"text-align: justify\">The placenta takes over the synthesis and secretion of progesterone throughout pregnancy as the corpus luteum degenerates. Like estrogen, progesterone suppresses FSH and LH. It also inhibits uterine contractions, protecting the fetus from preterm birth. This hormone decreases in late gestation, allowing uterine contractions to intensify and eventually progress to true labour. The placenta also produces hCG. In addition to promoting survival of the corpus luteum, hCG stimulates the male fetal gonads to secrete testosterone, which is essential for the development of the male reproductive system.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"11-6b\"><\/a>Physiology of Labour<\/strong><\/h5>\n<p style=\"text-align: justify\">Childbirth, or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1453\">parturition<\/a>, typically occurs within a week of a woman\u2019s due date. As a pregnancy progresses into its final weeks, several physiological changes occur in response to hormones that trigger labour.<\/p>\n<p style=\"text-align: justify\">First, recall that progesterone inhibits uterine contractions throughout the first several months of pregnancy. As the pregnancy enters its seventh month, progesterone levels plateau and then drop. Estrogen levels, however, continue to rise in the maternal circulation (Figure 19). The increasing ratio of estrogen to progesterone makes the myometrium (the uterine smooth muscle) more sensitive to stimuli that promote contractions (because progesterone no longer inhibits them). Moreover, in the eighth month of pregnancy, fetal <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_472\">cortisol<\/a> rises, which boosts estrogen secretion by the placenta and further overpowers the uterine-calming effects of progesterone.<\/p>\n<figure style=\"width: 1429px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image26-3.png\" alt=\"image\" width=\"1429\" height=\"802\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 19. Hormones Initiating Labour.<\/strong> A positive feedback loop of hormones works to initiate labour.<\/figcaption><\/figure>\n<p style=\"text-align: justify\">Meanwhile, the posterior <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_391\">pituitary gland<\/a> has been boosting its secretion of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_471\">oxytocin<\/a>, a hormone that stimulates the contractions of labour. At the same time, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1394\">myometrium<\/a> increases its sensitivity to oxytocin by expressing more receptors for this hormone. As labour nears, oxytocin begins to stimulate stronger, more painful uterine contractions, which\u2014in a positive feedback loop\u2014stimulate the secretion of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1017\">prostaglandins<\/a> from fetal membranes. Like oxytocin, prostaglandins also enhance uterine contractile strength.<\/p>\n<p style=\"text-align: justify\">Finally, stretching of the myometrium and cervix by a full-term fetus in the vertex (head-down) position is regarded as a stimulant to uterine contractions. The sum of these changes initiates the regular contractions known as true labour, which become more powerful and more frequent with time. The pain of labour is attributed to myometrial hypoxia during uterine contractions.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"11-6c\"><\/a>Stages of Childbirth<\/strong><\/h5>\n<p style=\"text-align: justify\">The process of childbirth can be divided into three stages: cervical dilation, expulsion of the newborn ending with birth, and afterbirth (Figure 20).<\/p>\n<p style=\"text-align: justify\"><strong>1. Cervical Dilation:<\/strong> For vaginal birth to occur, the cervix must dilate fully to 10 cm in diameter\u2014wide enough to deliver the newborn\u2019s head. The dilation stage is the longest stage of labour and typically takes 6\u201312 hours.<\/p>\n<p style=\"text-align: justify\">True labour progresses in a positive feedback loop in which uterine contractions stretch the cervix, causing it to dilate and efface, or become thinner. Cervical stretching induces reflexive uterine contractions that dilate and efface the cervix further. In addition, cervical dilation boosts <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_471\">oxytocin<\/a> secretion from the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_391\">pituitary gland<\/a>, which in turn triggers more powerful uterine contractions. When labour begins, uterine contractions may occur only every 3\u201330 minutes and last only 20\u201340 seconds; however, by the end of this stage, contractions may occur as frequently as every 1.5\u20132 minutes and last for a full minute.<\/p>\n<p style=\"text-align: justify\"><strong>2. Expulsion Stage:<\/strong> The expulsion stage begins when the fetal head enters the birth canal and ends with birth of the newborn. It typically takes up to 2 hours, but varies in length, depending in part on the orientation of the fetus. The most common presentation and associated with the greatest ease of vaginal birth is when the fetus faces the maternal spinal cord and the smallest part of the head (the posterior aspect called the occiput) exits the birth canal first.<\/p>\n<p style=\"text-align: justify\">Once the head is birthed, the rest of the body usually follows quickly. The umbilical cord is then double-clamped, and a cut is made between the clamps. This completes the second stage of childbirth.<\/p>\n<p style=\"text-align: justify\"><strong>3. Afterbirth:<\/strong> The delivery of the placenta and associated membranes, commonly referred to as the afterbirth, marks the final stage of childbirth. After expulsion of the newborn, the myometrium continues to contract. This movement shears the placenta from the back of the uterine wall. It is then easily delivered through the vagina. Continued uterine contractions then reduce blood loss from the site of the placenta.<\/p>\n<figure style=\"width: 1031px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image27-2.png\" alt=\"image\" width=\"1031\" height=\"1612\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 20. Stages of Childbirth.<\/strong> The stages of childbirth begin with early cervical dilation, continue through full dilation and birth (expulsion of the newborn); and finally delivery of the placenta and associated fetal membranes. The position of the newborn\u2019s shoulder is described relative to the mother.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<h2 style=\"text-align: justify\"><strong><a id=\"11-7\"><\/a>Part 7: Adjustments of the Infant at Birth and Postnatal Stages<\/strong><\/h2>\n<p style=\"text-align: justify\">From a fetal perspective, the process of birth is a crisis. In the womb, the fetus was snuggled in a soft, warm, dark, and quiet world. The placenta provided nutrition and oxygen continuously. Suddenly, the contractions of labour and vaginal childbirth forcibly squeeze the fetus through the birth canal, limiting oxygenated blood flow during contractions and shifting the skull bones to accommodate the small space. After birth, the newborn\u2019s system must make drastic adjustments to a world that is colder, brighter, and louder, and where he or she will experience hunger and thirst. The neonatal period (neo- = \u201cnew\u201d; -natal = \u201cbirth\u201d) spans the first to the thirtieth day of life outside of the uterus.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"11-7a\"><\/a>Respiratory Adjustments<\/strong><\/h5>\n<p style=\"text-align: justify\">Although the fetus \u201cpractices\u201d breathing by inhaling amniotic fluid in utero, there is no air in the uterus and thus no true opportunity to breathe. (There is also no need to breathe because the placenta supplies the fetus with all the oxygenated blood it needs.) During gestation, the partially collapsed lungs are filled with amniotic fluid and exhibit very little metabolic activity. Several factors stimulate newborns to take their first breath at birth. First, labour contractions temporarily constrict umbilical blood vessels, reducing oxygenated blood flow to the fetus and elevating carbon dioxide levels in the blood. High carbon dioxide levels cause acidosis and stimulate the respiratory centre in the brain, triggering the newborn to take a breath.<\/p>\n<p style=\"text-align: justify\">The first breaths inflate the lungs to nearly full capacity and dramatically decrease lung pressure and resistance to blood flow, causing a major circulatory reconfiguration. Pulmonary alveoli open, and alveolar capillaries fill with blood. Amniotic fluid in the lungs drains or is absorbed, and the lungs immediately take over the task of the placenta, exchanging carbon dioxide for oxygen by the process of respiration.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"11-7b\"><\/a>Circulatory Adjustments<\/strong><\/h5>\n<p style=\"text-align: justify\">The newborn\u2019s first breath is vital to initiate the transition from the fetal to the neonatal circulatory pattern. Inflation of the lungs decreases blood pressure throughout the pulmonary system, as well as in the right atrium and ventricle. In response to this pressure change, the flow of blood temporarily reverses direction through the foramen ovale, moving from the left to the right atrium, and blocking the shunt with two flaps of tissue. Within one year, the tissue flaps usually fuse over the shunt (Figure 21). The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1451\">ductus arteriosus<\/a> constricts as a result of increased oxygen concentration. Closing of the ductus arteriosus ensures that all blood pumped to the pulmonary circuit will be oxygenated by the newly functional neonatal lungs. Likewise, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1450\">ductus venosus<\/a> degenerates.<\/p>\n<figure style=\"width: 1430px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image28-2.png\" alt=\"image\" width=\"1430\" height=\"1211\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 21. Neonatal Circulatory System.<\/strong> A newborn\u2019s circulatory system reconfigures immediately after birth. The three fetal shunts have been closed permanently, facilitating blood flow to the liver and lungs.<\/figcaption><\/figure>\n<h2 style=\"text-align: justify\"><strong><a id=\"11-8\"><\/a>Part 8: Lactation<\/strong><\/h2>\n<p style=\"text-align: justify\"><strong>Lactation<\/strong> is the process by which milk is synthesized and secreted from the mammary glands of the postpartum female breast. Breast milk provides ideal nutrition and passive immunity for the infant, encourages mild uterine contractions to return the uterus to its pre-pregnancy size (i.e., involution), and induces a substantial metabolic increase in the mother, consuming the fat reserves stored during pregnancy.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"11-8a\"><\/a>Structure of the Lactating Breast<\/strong><\/h5>\n<p style=\"text-align: justify\">The mammary gland is composed of milk-transporting lactiferous ducts, which expand and branch extensively during pregnancy in response to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_481\">estrogens<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_474\">growth hormone<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_472\">cortisol<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_475\">prolactin<\/a>. Moreover, in response to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_482\">progesterone<\/a>, clusters of breast alveoli bud from the ducts and expand outward toward the chest wall.<\/p>\n<p>Breast alveoli are balloon-like structures lined with milk-secreting cuboidal cells, or lactocytes, which are surrounded by a net of contractile myoepithelial cells. Milk secreted by lactocytes fills the alveoli, and is squeezed into the ducts. Clusters of alveoli that drain to a common duct are called lobules; the lactating female has 12\u201320 lobules organized radially around the nipple. Milk drains from lactiferous ducts into lactiferous sinuses that meet at 4 to 18 perforations in the nipple, called nipple pores.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"11-8b\"><\/a>The Process of Lactation<\/strong><\/h5>\n<p style=\"text-align: justify\">The pituitary hormone <strong>prolactin<\/strong> is instrumental in the establishment and maintenance of breast milk supply.<\/p>\n<p style=\"text-align: justify\">Near the fifth week of pregnancy, the level of circulating prolactin begins to increase, eventually rising to approximately 10\u201320 times the pre-pregnancy concentration. During pregnancy, prolactin and other hormones prepare the breasts anatomically for the secretion of milk. The level of prolactin plateaus in late pregnancy, at a level high enough to initiate milk production. However, estrogen, progesterone, and other placental hormones inhibit prolactin-mediated milk synthesis during pregnancy. It is not until the placenta is expelled that this inhibition is lifted and milk production commences.<\/p>\n<p style=\"text-align: justify\">After childbirth, the baseline prolactin level drops sharply, but it is restored for a 1-hour spike during each feeding to stimulate the production of milk for the next feeding.<\/p>\n<p style=\"text-align: justify\">When the infant suckles, sensory nerve fibres in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_1407\">areola<\/a> trigger a neuroendocrine reflex that results in milk secretion fvia the let-down reflex (Figure 22). The posterior pituitary gland releases <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1626_471\">oxytocin<\/a>, which stimulates myoepithelial cells to squeeze milk from the alveoli so it can eventually drain out through the nipples.<\/p>\n<p>&nbsp;<\/p>\n<figure style=\"width: 1356px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311094thed\/wp-content\/uploads\/sites\/1536\/2021\/10\/image29-2.png\" alt=\"image\" width=\"1356\" height=\"1733\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 22. Let-Down Reflex.<\/strong> A positive feedback loop ensures continued milk production as long as the infant continues to breastfeed.<\/figcaption><\/figure>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<p class=\"textbox__title\"><a id=\"P\"><\/a>Practice Questions<\/p>\n<\/header>\n<div class=\"textbox__content\">\n<p><strong>Part 1: <\/strong>Anatomy and Physiology of the Male Reproductive System<\/p>\n<div id=\"h5p-42\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-42\" class=\"h5p-iframe\" data-content-id=\"42\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"1203 11-1\"><\/iframe><\/div>\n<\/div>\n<div id=\"h5p-43\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-43\" class=\"h5p-iframe\" data-content-id=\"43\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"1203 11-2\"><\/iframe><\/div>\n<\/div>\n<div id=\"h5p-44\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-44\" class=\"h5p-iframe\" data-content-id=\"44\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"1203 11-3\"><\/iframe><\/div>\n<\/div>\n<p><strong>Part 2:<\/strong> Anatomy and Physiology of the Female Reproductive System<\/p>\n<div id=\"h5p-45\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-45\" class=\"h5p-iframe\" data-content-id=\"45\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"1203 11-4\"><\/iframe><\/div>\n<\/div>\n<div id=\"h5p-46\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-46\" class=\"h5p-iframe\" data-content-id=\"46\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"1203 11-5\"><\/iframe><\/div>\n<\/div>\n<div id=\"h5p-63\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-63\" class=\"h5p-iframe\" data-content-id=\"63\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"1203 11-6\"><\/iframe><\/div>\n<\/div>\n<div id=\"h5p-61\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-61\" class=\"h5p-iframe\" data-content-id=\"61\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"1203 11-7\"><\/iframe><\/div>\n<\/div>\n<div id=\"h5p-62\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-62\" class=\"h5p-iframe\" data-content-id=\"62\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"1203 11-8\"><\/iframe><\/div>\n<\/div>\n<p><strong>Part 3:<\/strong> Fertilization<\/p>\n<div id=\"h5p-48\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-48\" class=\"h5p-iframe\" data-content-id=\"48\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"1203 11-9\"><\/iframe><\/div>\n<\/div>\n<div id=\"h5p-49\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-49\" class=\"h5p-iframe\" data-content-id=\"49\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"1203 11-10\"><\/iframe><\/div>\n<\/div>\n<div id=\"h5p-50\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-50\" class=\"h5p-iframe\" data-content-id=\"50\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"1203 11-11\"><\/iframe><\/div>\n<\/div>\n<p><strong>Part 4:<\/strong> Embryonic Development<\/p>\n<div id=\"h5p-52\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-52\" class=\"h5p-iframe\" data-content-id=\"52\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"1203 11-13\"><\/iframe><\/div>\n<\/div>\n<p><strong>Part 5:<\/strong> Fetal Development<\/p>\n<p><strong>Part 6:<\/strong> Maternal Changes During Pregnancy, Labour, and Birth<\/p>\n<div id=\"h5p-59\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-59\" class=\"h5p-iframe\" data-content-id=\"59\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"1203 11-19\"><\/iframe><\/div>\n<\/div>\n<p><strong>Part 7:<\/strong> Adjustments of the Infant at Birth and Postnatal Stages<\/p>\n<div id=\"h5p-56\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-56\" class=\"h5p-iframe\" data-content-id=\"56\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"1203 11-16\"><\/iframe><\/div>\n<\/div>\n<div id=\"h5p-57\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-57\" class=\"h5p-iframe\" data-content-id=\"57\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"1203 11-17\"><\/iframe><\/div>\n<\/div>\n<div id=\"h5p-58\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-58\" class=\"h5p-iframe\" data-content-id=\"58\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"1203 11-18\"><\/iframe><\/div>\n<\/div>\n<p><strong>Part 8:<\/strong> Lactation<\/p>\n<div id=\"h5p-60\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-60\" class=\"h5p-iframe\" data-content-id=\"60\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"1203 11-21\"><\/iframe><\/div>\n<\/div>\n<\/div>\n<\/div>\n<p>&nbsp;<\/p>\n<\/div>\n<p><!--more --><\/p>\n<div class=\"glossary\"><span class=\"screen-reader-text\" id=\"definition\">definition<\/span><template id=\"term_1626_465\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_465\"><div tabindex=\"-1\"><p>Haploid reproductive cell (egg or sperm in humans) that contributes genetic material to form an offspring.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1185\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1185\"><div tabindex=\"-1\"><p>A long DNA molecule, combined with proteins that contains a number of genes. The normal chromosome compliment is 23 pairs of homologous chromosomes, one each from mother and father.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1458\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1458\"><div tabindex=\"-1\"><p>A body cell, excluding germ cells. Normally diploid, each cell containing a complete set of genes.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1186\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1186\"><div tabindex=\"-1\"><p>Steroid hormone (the most well-known of which is testosterone) that regulates development of male characteristics.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_484\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_484\"><div tabindex=\"-1\"><p>Male gonad (plural = testes).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1187\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1187\"><div tabindex=\"-1\"><p>Skin-covered muscular sac containing the testes.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1188\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1188\"><div tabindex=\"-1\"><p>Location of meiosis in the male, and production of sperm.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1190\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1190\"><div tabindex=\"-1\"><p>Cell that gives rise to a gamete.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1189\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1189\"><div tabindex=\"-1\"><p>Cells that support germ cells through the process of spermatogenesis.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1200\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1200\"><div tabindex=\"-1\"><p>Process of producing sperm.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1197\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1197\"><div tabindex=\"-1\"><p>One copy of each homologous chromosomes, (half the normal genetic complement), as in gametes.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1198\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1198\"><div tabindex=\"-1\"><p>A sac-like organelle forming a cap over the anterior portion of the head of a sperm cell, derived from Golgi.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1094\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1094\"><div tabindex=\"-1\"><p>One of the cellular organelles bound by a double lipid bilayer that function primarily in the production of cellular energy (ATP).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1199\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1199\"><div tabindex=\"-1\"><p>Appendage on certain cells formed by microtubules and modified for movement (plural = flagella)<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1232\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1232\"><div tabindex=\"-1\"><p>Cell that results from the division of the oogonium and undergoes meiosis I at the LH surge and meiosis II at fertilization to become a haploid ovum.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1201\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1201\"><div tabindex=\"-1\"><p>A coiled tube attached to the testis where newly formed sperm continue to mature.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1202\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1202\"><div tabindex=\"-1\"><p>(Also, vas deferens) duct that transports sperm from the epididymis through the spermatic cord and into the ejaculatory duct.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1203\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1203\"><div tabindex=\"-1\"><p>Opening in abdominal wall that connects the testes to the abdominal cavity.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1204\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1204\"><div tabindex=\"-1\"><p>Ejaculatory fluid composed of sperm and secretions from the seminal vesicles, prostate, and bulbourethral glands.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1205\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1205\"><div tabindex=\"-1\"><p>Gland that produces seminal fluid, which contributes to semen.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1206\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1206\"><div tabindex=\"-1\"><p>Doughnut-shaped gland at the base of the bladder surrounding the urethra and contributing fluid to semen during ejaculation.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1207\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1207\"><div tabindex=\"-1\"><p>(Also, Cowper\u2019s glands) glands that secrete a lubricating mucus that cleans and lubricates the urethra prior to and during ejaculation.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1208\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1208\"><div tabindex=\"-1\"><p>General anatomical term for the dilated end part of certain ducts of canals.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_967\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_967\"><div tabindex=\"-1\"><p>Nucleotide containing ribose and an adenine base that is essential in energy transfer.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1209\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1209\"><div tabindex=\"-1\"><p>Duct that connects the ampulla of the ductus deferens with the duct of the seminal vesicle at the prostatic urethra.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1156\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1156\"><div tabindex=\"-1\"><p>Transports urine from the bladder to the outside environment.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_464\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_464\"><div tabindex=\"-1\"><p>Molecule (usually a protein) that catalyzes chemical reactions.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1222\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1222\"><div tabindex=\"-1\"><p>Tunnel-like organ that provides access to the uterus for the insertion of semen and from the uterus for the birth of a baby.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1211\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1211\"><div tabindex=\"-1\"><p>Bulbous end of the penis that contains a large number of nerve endings.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1210\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1210\"><div tabindex=\"-1\"><p>(Also, foreskin) flap of skin that forms a collar around, and thus protects and lubricates, the glans penis.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1212\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1212\"><div tabindex=\"-1\"><p>Branch of the efferent (impulses travelling away from the brain) peripheral nervous system dealing with involuntary actions; consists of the sympathetic and parasympathetic branches.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_536\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_536\"><div tabindex=\"-1\"><p>Branch of the autonomic nervous system associated with resting systems (\"rest and digest\").<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1213\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1213\"><div tabindex=\"-1\"><p>Either of two columns of erectile tissue in the penis that fill with blood during an erection (plural = corpora cavernosa).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_535\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_535\"><div tabindex=\"-1\"><p>Branch of the autonomic nervous system associated with emergency systems (\"fight of flight\").<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_485\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_485\"><div tabindex=\"-1\"><p>Steroid hormone secreted by the male testes and important in the maturation of sperm cells, growth and development of the male reproductive system, and the development of male secondary sex characteristics.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1214\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1214\"><div tabindex=\"-1\"><p>Cells between the seminiferous tubules of the testes that produce testosterone; a type of interstitial cell.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_483\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_483\"><div tabindex=\"-1\"><p>Female gonads that produce oocytes and sex steroid hormones (notably estrogen and progesterone).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1216\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1216\"><div tabindex=\"-1\"><p>One of the hormones collectively referred to as estrogens.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_446\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_446\"><div tabindex=\"-1\"><p>Endocrine gland located at the top of each kidney that is important for the regulation of the stress response, blood pressure and blood volume, water homeostasis, and electrolyte levels.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1217\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1217\"><div tabindex=\"-1\"><p>Hair-covered folds of skin located behind the mons pubis.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1218\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1218\"><div tabindex=\"-1\"><p>Thin, pigmented, hairless flaps of skin located medial and deep to the labia majora.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1219\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1219\"><div tabindex=\"-1\"><p>(Also, glans clitoris) nerve-rich area of the vulva that contributes to sexual sensation during intercourse.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1220\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1220\"><div tabindex=\"-1\"><p>Membrane that covers part of the opening of the vagina.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1221\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1221\"><div tabindex=\"-1\"><p>(Also, greater vestibular glands) glands that produce a thick mucus that maintains moisture in the vulva area.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1224\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1224\"><div tabindex=\"-1\"><p>Strong band of dense connective tissue spanning between bones.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1225\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1225\"><div tabindex=\"-1\"><p>Ovarian structure of one oocyte and surrounding granulosa (and later theca) cells.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1226\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1226\"><div tabindex=\"-1\"><p>Approximately 28-day cycle of changes in the ovary consisting of a follicular phase and a luteal phase.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1227\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1227\"><div tabindex=\"-1\"><p>Approximately 28-day cycle of changes in the uterus consisting of a menses phase, a proliferative phase, and a secretory phase.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1228\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1228\"><div tabindex=\"-1\"><p>Process by which oogonia divide by mitosis to primary oocytes, which undergo meiosis to produce the secondary oocyte and, upon fertilization, the ovum.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1229\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1229\"><div tabindex=\"-1\"><p>Development of ovarian follicles from primordial to tertiary under the stimulation of gonadotropins.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1230\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1230\"><div tabindex=\"-1\"><p>Ovarian stem cells that undergo mitosis during female fetal development to form primary oocytes.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1196\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1196\"><div tabindex=\"-1\"><p>Release of a secondary oocyte and associated granulosa cells from an ovary.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_479\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_479\"><div tabindex=\"-1\"><p>Anterior pituitary hormone that triggers ovulation and the production of ovarian hormones in females, and the production of testosterone in males.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1244\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1244\"><div tabindex=\"-1\"><p>Muscular hollow organ in which a fertilized egg develops into a fetus.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1243\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1243\"><div tabindex=\"-1\"><p>Internal material between the cell membrane and nucleus of a cell, mainly consisting of a water-based fluid called cytosol, within which are all the other organelles and cellular solute and suspended materials.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1240\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1240\"><div tabindex=\"-1\"><p>Death of ovarian follicles.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1233\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1233\"><div tabindex=\"-1\"><p>Least developed ovarian follicles that consist of a single oocyte and a single layer of flat (squamous) granulosa cells.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1234\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1234\"><div tabindex=\"-1\"><p>Supportive cells in the ovarian follicle that produce estrogens.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1236\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1236\"><div tabindex=\"-1\"><p>Ovarian follicle with a primary oocyte and multiple layers of granulosa cells.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1245\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1245\"><div tabindex=\"-1\"><p>Estrogen-producing cells in a maturing ovarian follicle.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1238\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1238\"><div tabindex=\"-1\"><p>Fluid-filled chamber that characterizes a mature tertiary (antral) follicle.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1239\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1239\"><div tabindex=\"-1\"><p>(Also, antral follicle) ovarian follicle with a primary or secondary oocyte, multiple layers of granulosa cells, and a fully formed antrum.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1247\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1247\"><div tabindex=\"-1\"><p>Hormone released by the hypothalamus that stimulates the release of\u00a0hormones known as gonadotropins from the anterior pituitary: follicle stimulating hormone (FSH) and luteinizing hormone (LH)<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_478\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_478\"><div tabindex=\"-1\"><p>Anterior pituitary hormone that stimulates the production and maturation of sex cells.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_392\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_392\"><div tabindex=\"-1\"><p>Region of the diencephalon inferior to the thalamus that functions in neural and endocrine signaling.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_391\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_391\"><div tabindex=\"-1\"><p>Bean-sized organ suspended from the hypothalamus that produces, stores, and secretes hormones in response to hypothalamic stimulation (also called hypophysis).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_394\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_394\"><div tabindex=\"-1\"><p>Hormones that regulate the function of the gonads.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1405\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1405\"><div tabindex=\"-1\"><p>Days 1-14 of the ovarian cycle, during which tertiary follicles grow and secrete estrogens.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_481\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_481\"><div tabindex=\"-1\"><p>Class of predominantly female sex hormones important for the development and growth of the female reproductive tract, secondary sex characteristics, the female reproductive cycle, and the maintenance of pregnancy. Estradiol is the most common active estrogen.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1254\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1254\"><div tabindex=\"-1\"><p>Process of transforming a follicle post-ovulation to the corpus luteum.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1255\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1255\"><div tabindex=\"-1\"><p>Transformed follicle after ovulation that secretes progesterone.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_482\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_482\"><div tabindex=\"-1\"><p>Predominantly female sex hormone important in regulating the female reproductive cycle and the maintenance of pregnancy.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1406\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1406\"><div tabindex=\"-1\"><p>The phase of the ovarian cycle that occurs after ovulation (days 14-28) during which the corpus luteum secretes high levels of progesterone, and new dominant follicles develop (in time for the next cycle). if pregnancy does not occur, the corpus luteum begins to disintegrate after 1-12 days (meaning that the luteal phase is relatively fixed in duration, whereas the follicular phase can vary). <\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1256\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1256\"><div tabindex=\"-1\"><p>Nonfunctional structure remaining in the ovarian stroma following structural and functional regression of the corpus luteum.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1257\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1257\"><div tabindex=\"-1\"><p>(Also, fallopian tube or oviduct) duct that facilitates transport of an ovulated oocyte to the uterus.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1258\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1258\"><div tabindex=\"-1\"><p>Small appendage on certain cells formed by microtubules and modified for movement of materials across the cellular surface (singular = cilium).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1390\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1390\"><div tabindex=\"-1\"><p>Unification of genetic material from male and female haploid gametes.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_861\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_861\"><div tabindex=\"-1\"><p>General anatomical term for the part of a hollow organ furthest from its mouth. In the stomach: the uppermost portion, above and to the left of the cardia. In the uterus: domed portion superior to the uterine tubes.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1396\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1396\"><div tabindex=\"-1\"><p>Middle section of the uterus.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1223\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1223\"><div tabindex=\"-1\"><p>Elongate inferior end of the uterus where it connects to the vagina.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1393\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1393\"><div tabindex=\"-1\"><p>Outer epithelial layer of uterine wall.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1394\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1394\"><div tabindex=\"-1\"><p>Smooth muscle layer of uterus that allows for uterine contractions during labor and expulsion of menstrual blood.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1395\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1395\"><div tabindex=\"-1\"><p>Inner lining of the uterus, part of which builds up during the secretory phase of the menstrual cycle and then sheds with menses.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_654\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_654\"><div tabindex=\"-1\"><p>Type of tissue that serves primarily as a covering or lining of body parts, protecting the body; it also functions in absorption, transport, and secretion.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_777\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_777\"><div tabindex=\"-1\"><p>Space inside of a tube, hollow organ or blood vessel.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1397\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1397\"><div tabindex=\"-1\"><p>Basal layer of the uterine epithelium that remains relatively unchanged during the menstrual cycle.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1399\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1399\"><div tabindex=\"-1\"><p>The thicker of the two uterine epithelial layers, that thickens and is shed during the menstrual cycle in response to estrogens and progesterone.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_769\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_769\"><div tabindex=\"-1\"><p>Part of a mucous membrane (composed of loose connective tissue) that supports an epithelium.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1017\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1017\"><div tabindex=\"-1\"><p>Similar to hormones, lipids produced by various cells (not glands), usually at the site of an injury or other issue, that are active over a short distance (targeting other cells in the same tissue).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1400\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1400\"><div tabindex=\"-1\"><p>Process by which a blastocyst embeds itself in the uterine endometrium.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1401\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1401\"><div tabindex=\"-1\"><p>Shedding of the inner portion of the endometrium out though the vagina; also referred to as menstruation.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1404\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1404\"><div tabindex=\"-1\"><p>Phase of the menstrual cycle in which the endometrial lining is shed.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1402\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1402\"><div tabindex=\"-1\"><p>Phase of the menstrual cycle in which the endometrium proliferates.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1403\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1403\"><div tabindex=\"-1\"><p>Phase of the menstrual cycle in which the endometrium secretes a nutrient-rich fluid in preparation for implantation of an embryo.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1407\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1407\"><div tabindex=\"-1\"><p>Highly pigmented, circular area surrounding the raised nipple and containing areolar glands that secrete fluid important for lubrication during suckling.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1408\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1408\"><div tabindex=\"-1\"><p>Glands in the breast that secrete milk.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1409\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1409\"><div tabindex=\"-1\"><p>Fertilized egg; a diploid cell resulting from the fertilization of haploid gametes from the male and female lines.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_544\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_544\"><div tabindex=\"-1\"><p>(also, white blood cell) colorless, nucleated blood cell, the chief function of which is to protect the body from disease.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1410\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1410\"><div tabindex=\"-1\"><p>Process that occurs in the female reproductive tract in which sperm are prepared for fertilization; leads to increased motility and changes in their outer membrane that improve their ability to release enzymes capable of digesting an oocyte\u2019s outer layers.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1242\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1242\"><div tabindex=\"-1\"><p>Immature oocyte after the first meiotic division (which occurs in the tertiary follicle); oocytes are ovulated at this stage.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1411\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1411\"><div tabindex=\"-1\"><p>In an oocyte, a layer of granulosa cells that surrounds the oocyte and that must be penetrated by sperm before fertilization can occur.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1237\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1237\"><div tabindex=\"-1\"><p>Glycoprotein layer surrounding the cell membrane of an oocyte.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_581\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_581\"><div tabindex=\"-1\"><p>Proteins with short polysaccharide chains attached.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1412\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1412\"><div tabindex=\"-1\"><p>Release of digestive enzymes by sperm that enables them to burrow through the corona radiata and penetrate the zona pellucida of an oocyte prior to fertilization.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1413\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1413\"><div tabindex=\"-1\"><p>Penetration of an oocyte by more than one sperm.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_521\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_521\"><div tabindex=\"-1\"><p>Change in a cell membrane potential from rest toward or above zero (as during an action potential).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1415\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1415\"><div tabindex=\"-1\"><p>Following fertilization, the release of cortical granules from the oocyte\u2019s plasma membrane into the zona pellucida creating a fertilization membrane that prevents any further attachment or penetration of sperm; part of the slow block to polyspermy.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1414\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1414\"><div tabindex=\"-1\"><p>Impenetrable barrier that coats a nascent zygote; part of the slow block to polyspermy.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1416\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1416\"><div tabindex=\"-1\"><p>Having two copies of genetic material.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1421\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1421\"><div tabindex=\"-1\"><p>In human development, the period required for embryonic and fetal development in utero; pregnancy.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1422\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1422\"><div tabindex=\"-1\"><p>Developing human during weeks 3\u20138.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1423\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1423\"><div tabindex=\"-1\"><p>Developing human during the time from the end of the embryonic period (week 9) to birth.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1424\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1424\"><div tabindex=\"-1\"><p>Pre-implantation stage of a fertilized egg and its associated membranes.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_822\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_822\"><div tabindex=\"-1\"><p>Muscular contractions and relaxations that propel food through the GI tract.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1425\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1425\"><div tabindex=\"-1\"><p>Referring to mitosis, often indicating cell division though mitosis and cell division (cytokinesis) are separate, usually linked, processes.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1426\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1426\"><div tabindex=\"-1\"><p>Form of mitotic cell division in which the cell divides but the total volume remains unchanged; this process serves to produce smaller and smaller cells.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1427\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1427\"><div tabindex=\"-1\"><p>Daughter cell of a cleavage.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1428\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1428\"><div tabindex=\"-1\"><p>Tightly packed sphere of blastomeres that has reached the uterus but has not yet implanted itself.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1429\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1429\"><div tabindex=\"-1\"><p>Term for the conceptus at the developmental stage that consists of about 100 cells shaped into an inner cell mass that is fated to become the embryo and an outer trophoblast that is fated to become the associated fetal membranes and placenta.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1431\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1431\"><div tabindex=\"-1\"><p>Cluster of cells within the blastocyst that is fated to become the embryo.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1430\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1430\"><div tabindex=\"-1\"><p>Fluid-filled shell of squamous cells destined to become the chorionic villi, placenta, and associated fetal membranes.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1417\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1417\"><div tabindex=\"-1\"><p>Organ that forms during pregnancy to nourish the developing fetus; also regulates waste and gas exchange between mother and fetus.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1432\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1432\"><div tabindex=\"-1\"><p>Embryonic cells that have the ability to differentiate into any type of cell and organ in the body.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_804\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_804\"><div tabindex=\"-1\"><p>Describes a position nearer to the surface of the body.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1433\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1433\"><div tabindex=\"-1\"><p>Superficial cells of the trophoblast that fuse to form a multinucleated body that digests endometrial cells to firmly secure the blastocyst to the uterine wall.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1434\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1434\"><div tabindex=\"-1\"><p>Hormone that directs the corpus luteum to survive, enlarge, and continue producing progesterone and estrogen to suppress menses and secure an environment suitable for the developing embryo.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1435\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1435\"><div tabindex=\"-1\"><p>Transparent membranous sac that encloses the developing fetus and fills with amniotic fluid.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1438\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1438\"><div tabindex=\"-1\"><p>Membrane associated with primitive circulation to the developing embryo; source of the first blood cells and germ cells and contributes to the umbilical cord structure.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1436\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1436\"><div tabindex=\"-1\"><p>Finger-like outpocketing of yolk sac forms the primitive excretory duct of the embryo; precursor to the urinary bladder.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1437\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1437\"><div tabindex=\"-1\"><p>Membrane that develops from the syncytiotrophoblast, cytotrophoblast, and mesoderm; surrounds the embryo and forms the fetal portion of the placenta through the chorionic villi.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1439\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1439\"><div tabindex=\"-1\"><p>Process of cell migration and differentiation into three primary germ layers following cleavage and implantation.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1440\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1440\"><div tabindex=\"-1\"><p>Primary germ layer that goes on to form the gastrointestinal tract, liver, pancreas, and lungs.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1442\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1442\"><div tabindex=\"-1\"><p>Primary germ layer that becomes the skeleton, muscles, connective tissue, heart, blood vessels, and kidneys.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1441\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1441\"><div tabindex=\"-1\"><p>Primary germ layer that develops into the central and peripheral nervous systems, sensory organs, epidermis, hair, and nails.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1443\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1443\"><div tabindex=\"-1\"><p>Beginning to grow and develop.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1444\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1444\"><div tabindex=\"-1\"><p>Connection between the developing conceptus and the placenta; carries deoxygenated blood and wastes from the fetus and returns nutrients and oxygen from the mother.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1446\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1446\"><div tabindex=\"-1\"><p>Precursor to the chorion; forms from extra-embryonic mesoderm cells.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1445\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1445\"><div tabindex=\"-1\"><p>Projections of the chorionic membrane that burrow into the endometrium and develop into the placenta.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1447\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1447\"><div tabindex=\"-1\"><p>Embryonic tissue from which connective tissue cells derive.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1448\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1448\"><div tabindex=\"-1\"><p>Formation of the placenta; complete by weeks 14\u201316 of pregnancy.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_641\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_641\"><div tabindex=\"-1\"><p>T lymphocyte with the ability to induce apoptosis in target cells.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_623\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_623\"><div tabindex=\"-1\"><p>Molecule recognized by the receptors of B and T lymphocytes.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1449\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1449\"><div tabindex=\"-1\"><p>Circulatory shortcut that diverts the flow of blood from one region to another.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1450\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1450\"><div tabindex=\"-1\"><p>Shunt that causes oxygenated blood to bypass the fetal liver on its way to the inferior vena cava.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_501\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_501\"><div tabindex=\"-1\"><p>Opening in the fetal heart that allows blood to flow directly from the right atrium to the left atrium, bypassing the fetal pulmonary circuit.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1451\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1451\"><div tabindex=\"-1\"><p>Shunt in the pulmonary trunk that diverts oxygenated blood back to the aorta.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1028\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1028\"><div tabindex=\"-1\"><p>Largest artery in the body, originating from the left ventricle and descending to the abdominal region where it bifurcates into the common iliac arteries at the level of the fourth lumbar vertebra; arteries originating from the aorta distribute blood to virtually all tissues of the body.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1452\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1452\"><div tabindex=\"-1\"><p>A substance that reduces surface tension of a liquid.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_664\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_664\"><div tabindex=\"-1\"><p>Division of the anterior (ventral) cavity that houses the heart, lungs, esophagus, and trachea.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_1453\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_1453\"><div tabindex=\"-1\"><p>Childbirth<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_472\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_472\"><div tabindex=\"-1\"><p>Hormone produced by the adrenal gland in response to stress, stimulates gluconeogenesis, the catabolism of glycogen, and downregulation of the immune system and glucose levels.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_471\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_471\"><div tabindex=\"-1\"><p>Hypothalamic hormone stored in the posterior pituitary gland and important in stimulating uterine contractions in labor, milk ejection during breastfeeding, and feelings of attachment (also produced in males).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_474\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_474\"><div tabindex=\"-1\"><p>(Also, somatotropin) anterior pituitary hormone that promotes tissue building and influences nutrient metabolism.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1626_475\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1626_475\"><div tabindex=\"-1\"><p>Anterior pituitary hormone that promotes development of the mammary glands and the production of breast milk. Often abbreviated PRL<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><\/div>","protected":false},"author":1440,"menu_order":1,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-1626","chapter","type-chapter","status-publish","hentry"],"part":227,"_links":{"self":[{"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-json\/pressbooks\/v2\/chapters\/1626","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-json\/wp\/v2\/users\/1440"}],"version-history":[{"count":7,"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-json\/pressbooks\/v2\/chapters\/1626\/revisions"}],"predecessor-version":[{"id":1635,"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-json\/pressbooks\/v2\/chapters\/1626\/revisions\/1635"}],"part":[{"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-json\/pressbooks\/v2\/parts\/227"}],"metadata":[{"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-json\/pressbooks\/v2\/chapters\/1626\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-json\/wp\/v2\/media?parent=1626"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-json\/pressbooks\/v2\/chapter-type?post=1626"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-json\/wp\/v2\/contributor?post=1626"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/wp-json\/wp\/v2\/license?post=1626"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}