{"id":45,"date":"2023-12-04T10:20:55","date_gmt":"2023-12-04T15:20:55","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/chapter\/epithelial-tissue\/"},"modified":"2023-12-04T10:38:50","modified_gmt":"2023-12-04T15:38:50","slug":"epithelial-tissue","status":"web-only","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/chapter\/epithelial-tissue\/","title":{"raw":"Epithelial Tissue","rendered":"Epithelial Tissue"},"content":{"raw":"\n<div class=\"textbox textbox--learning-objectives\">\n<div class=\"textbox textbox--learning-objectives\"><header class=\"textbox__header\">\n<p class=\"textbox__title\">Learning Objectives<\/p>\n\n<\/header>\n<div class=\"textbox__content\">\n\nThis section gives the background of how many cells make up epithelial tissue and key features. This background will be instrumental for the subsequent videos of histological examination of normal and pathological tissue. By the end of this section, you will be able to:\n<ul>\n \t<li>Explain the structure and function of epithelial tissue.<\/li>\n \t<li>Distinguish between tight junctions, anchoring junctions, and gap junctions.<\/li>\n \t<li>Distinguish between simple epithelia and stratified epithelia, as well as between squamous, cuboidal, and columnar epithelia.<\/li>\n \t<li>Describe the structure and function of endocrine and exocrine glands and their respective secretions.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<\/div>\n<p id=\"fs-id1508253\">Most epithelial tissues are essentially large sheets of cells covering all the surfaces of the body exposed to the outside world and lining the outside of organs. Epithelium also forms much of the glandular tissue of the body. Skin is not the only area of the body exposed to the outside. Other areas include the airways, the digestive tract, as well as the urinary and reproductive systems, all of which are lined by an epithelium. Hollow organs and body cavities that do not connect to the exterior of the body, which includes, blood vessels and serous membranes, are lined by[pb_glossary id=\"794\"] endothelium[\/pb_glossary] (plural = endothelia), which is a type of epithelium.<\/p>\n&nbsp;\n<p id=\"fs-id1454989\">All epithelia share some important structural and functional features. This tissue is highly cellular, with little or no extracellular material present between cells. Adjoining cells form a specialized intercellular connection between their cell membranes called a [pb_glossary id=\"792\"] <span data-type=\"term\">cell junction<\/span>[\/pb_glossary]. The epithelial cells exhibit polarity with differences in structure and function between the exposed or <span data-type=\"term\">[pb_glossary id=\"788\"]apical[\/pb_glossary]<\/span> facing surface of the cell and the basal surface close to the underlying body structures. The <span data-type=\"term\">basal lamina<\/span>, a mixture of glycoproteins and collagen, provides an attachment site for the epithelium, separating it from underlying connective tissue. The basal lamina attaches to a <span data-type=\"term\">[pb_glossary id=\"803\"]reticular lamina[\/pb_glossary]<\/span>, which is secreted by the underlying connective tissue, forming a <span data-type=\"term\">[pb_glossary id=\"791\"]basement membrane[\/pb_glossary]<\/span> that helps hold it all together.<\/p>\n<p id=\"fs-id1508318\">Epithelial tissues are nearly completely avascular. For instance, no blood vessels cross the basement membrane to enter the tissue, and nutrients must come by diffusion or absorption from underlying tissues or the surface. Many epithelial tissues are capable of rapidly replacing damaged and dead cells. Sloughing off of damaged or dead cells is a characteristic of surface epithelium and allows our airways and digestive tracts to rapidly replace damaged cells with new cells.<\/p>\n\n<div id=\"fs-id1511818\" class=\"bc-section section\" data-depth=\"1\">\n<h2 data-type=\"title\">Generalized Functions of Epithelial Tissue<\/h2>\n<p id=\"fs-id1486029\">Epithelial tissues provide the body\u2019s first line of protection from physical, chemical, and biological wear and tear. The cells of an epithelium act as gatekeepers of the body controlling permeability and allowing selective transfer of materials across a physical barrier. All substances that enter the body must cross an epithelium. Some epithelia often include structural features that allow the selective transport of molecules and ions across their cell membranes.<\/p>\n<p id=\"fs-id1300309\">Many epithelial cells are capable of secretion and release mucous and specific chemical compounds onto their apical surfaces. The epithelium of the small intestine releases digestive enzymes, for example. Cells lining the respiratory tract secrete mucous that traps incoming microorganisms and particles. A glandular epithelium contains many secretory cells.<\/p>\n\n<\/div>\n<div id=\"fs-id1168010\" class=\"bc-section section\" data-depth=\"1\">\n<h2 data-type=\"title\">The Epithelial Cell<\/h2>\n<p id=\"fs-id1233802\">Epithelial cells are typically characterized by the polarized distribution of organelles and membrane-bound proteins between their basal and apical surfaces. Particular structures found in some epithelial cells are an adaptation to specific functions. Certain organelles are segregated to the basal sides, whereas other organelles and extensions, such as cilia, when present, are on the apical surface.<\/p>\n<p id=\"fs-id1508184\">Cilia are microscopic extensions of the apical cell membrane that are supported by microtubules. They beat in unison and move fluids as well as trapped particles. Ciliated epithelium lines the ventricles of the brain where it helps circulate the cerebrospinal fluid. The ciliated epithelium of your airway forms a mucociliary escalator that sweeps particles of dust and pathogens trapped in the secreted mucous toward the throat. It is called an escalator because it continuously pushes mucous with trapped particles upward. In contrast, nasal cilia sweep the mucous blanket down towards your throat. In both cases, the transported materials are usually swallowed, and end up in the acidic environment of your stomach.<\/p>\n\n<\/div>\n<div id=\"fs-id1501084\" class=\"bc-section section\" data-depth=\"1\">\n<h2 data-type=\"title\">Cell to Cell Junctions<\/h2>\n<p id=\"fs-id1304667\">Cells of epithelia are closely connected and are not separated by intracellular material. Three basic types of connections allow varying degrees of interaction between the cells: tight junctions, [pb_glossary id=\"787\"]anchoring junctions[\/pb_glossary], and gap junctions <a class=\"autogenerated-content\" href=\"#fig-ch04_02_01\">(Figure 1.5)<\/a>.<\/p>\n\n<div id=\"fig-ch04_02_01\" class=\"bc-figure figure\">\n<div class=\"bc-figcaption figcaption\"><\/div>\n\n[caption id=\"attachment_870\" align=\"aligncenter\" width=\"600\"]<a href=\"https:\/\/pressbooks.bccampus.ca\/pathology\/wp-content\/uploads\/sites\/1260\/2021\/08\/402_Types_of_Cell_Junctions_new-scaled.jpg\"><img class=\"wp-image-870\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/wp-content\/uploads\/sites\/2131\/2023\/12\/402_Types_of_Cell_Junctions_new-scaled-1.jpg\" alt=\"These three illustrations each show the edges of two vertical cell membranes. The cell membranes are viewed partially from the side so that the inside edge of the right cell membrane is visible. The upper left image shows a tight junction. The two cell membranes are bound by transmembrane protein strands. The proteins travel the inside edge of the right cell membrane and cross over to the left cell membrane, cinching the two membranes together. The cell membranes are still somewhat separated in between neighboring strands, creating intercellular spaces. The upper right diagram shows a gap junction. The gap junctions are composed of two interlocking connexins, which are round, hollow tubes that extend through the cell membranes. Two connexins, one from the left cell membrane and the other from the right cell membrane, meet between the two cells, forming a connexon. Even at the site of the connexon, there is a small gap between the cell membranes. On the inside edge of the right cell membrane, the gap junction appears as a depression. Three connexins are embedded into the membranes like buttons on a shirt. The bottom images show the three types of anchoring junctions. The left image shows a desmosome. Here, the inside edge of both the right and left cell membranes have brown, round plaques. Each plaque has tentacle-like intermediate filaments (keratin) that extend into each cell\u2019s cytoplasm. The two plaques are connected across the intercellular space by several interlocking transmembrane glycoproteins (cadherin). The connected glycoproteins look similar to a zipped-up zipper between the right and left cell membranes. The right image shows an adheren. These are similar to desmosomes, with two plaques on the inside edge of each cell membrane connected across the intercellular space by glycoproteins. However, the plaques do not contain the tentacle-like intermediate filaments branching into the cytoplasm. Instead, the plaques are ribbed with green actin filaments. The filaments are neatly arranged in parallel, horizontal strands on the surface of the plaque facing the cytoplasm. The bottom image shows a hemidesmosome. Rather than located between two neighboring cells, the hemidesmosome is located between the bottom of a cell and the basement membrane. A hemidesmosome contains a single plaque on the inside edge of the cell membrane. Like the desmosome, intermediate filaments project from the plaque into the cytoplasm. The opposite side of the plaque has purple, knob-shaped integrins extending out to the basal lamina of the basement membrane.\" width=\"600\" height=\"700\"><\/a> <strong>Figure 1.5<\/strong> -<em> Types of Cell Junctions - The three basic types of cell-to-cell junctions are tight junctions, gap junctions, and anchoring junctions.<\/em>[\/caption]\n\n<\/div>\n<p id=\"fs-id1211959\">At one end of the spectrum is the <span data-type=\"term\">tight junction<\/span>, which separates the cells into apical and basal compartments. When two adjacent epithelial cells form a tight junction, there is no extracellular space between them and the movement of substances through the extracellular space between the cells is blocked. This enables the epithelia to act as selective barriers. An <span data-type=\"term\">anchoring junction<\/span> includes several types of cell junctions that help stabilize epithelial tissues. Anchoring junctions are common on the lateral and basal surfaces of cells where they provide strong and flexible connections.&nbsp; These junctions influence the shape and folding of the epithelial tissue.<\/p>\n<p id=\"fs-id1535229\">In contrast with the tight and anchoring junctions, a <span data-type=\"term\">gap junction<\/span> forms an intercellular passageway between the membranes of adjacent cells to facilitate the movement of small molecules and ions between the cytoplasm of adjacent cells. These junctions allow electrical and metabolic coupling of adjacent cells, which coordinates function in large groups of cells.<\/p>\n\n<\/div>\n<div id=\"fs-id1490019\" class=\"bc-section section\" data-depth=\"1\">\n<h2 data-type=\"title\">Classification of Epithelial Tissues<\/h2>\n<p id=\"fs-id1513993\">Epithelial tissues are classified according to the shape of the cells and number of the cell layers formed <a class=\"autogenerated-content\" href=\"#fig-ch04_02_02\">(Figure 1.6)<\/a>. Cell shapes can be squamous (flattened and thin), cuboidal (boxy, as wide as it is tall), or columnar (rectangular, taller than it is wide). Similarly, the number of cell layers in the tissue can be one\u2014where every cell rests on the basal lamina\u2014which is a simple epithelium, or more than one, which is a stratified epithelium and only the basal layer of cells rests on the basal lamina. Pseudostratified (pseudo- = \u201cfalse\u201d) describes tissue with a single layer of irregularly shaped cells that give the appearance of more than one layer. Transitional describes a form of specialized stratified epithelium in which the shape of the cells can vary.<\/p>\n\n<div id=\"fig-ch04_02_02\" class=\"span-all\">\n<div class=\"bc-figcaption figcaption\"><\/div>\n<div class=\"mceTemp\"><\/div>\n\n[caption id=\"attachment_871\" align=\"aligncenter\" width=\"674\"]<a href=\"https:\/\/pressbooks.bccampus.ca\/pathology\/wp-content\/uploads\/sites\/1260\/2021\/08\/403_Epithelial_Tissue.jpg\"><img class=\"wp-image-41\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/wp-content\/uploads\/sites\/2131\/2023\/12\/403_Epithelial_Tissue-1024x822.jpg\" alt=\"This figure is a table showing the appearance of squamous, cuboidal and columnar epithelial tissues. Simple and compound forms are shown for each tissue type. In a simple squamous epithelium, the cells are flattened and single layered. In a simple cuboidal epithelium, the cells are cube shaped and single layered. In a simple columnar epithelium, the cells are rectangular and are attached to the basement membrane on one of their narrow sides, so that each cell is standing up like a column. There is only one layer of cells. In a pseudostratified columnar epithelium, the cells are column-like in appearance, but they vary in height. The taller cells bend over the tops of the shorter cells so that the top of the epithelial tissue is continuous. There is only one layer of cells. A stratified squamous epithelium contains many layers of flattened cells. Stratified cuboidal epithelium contains many layers of cube-shaped cells. Stratified columnar epithelium contains many layers of rectangular, column-shaped cells\" width=\"674\" height=\"541\"><\/a> <strong>Figure 1.6 <\/strong>- <em>Cells of Epithelial Tissue&nbsp; - Simple epithelial tissue is organized as a single layer of cells and stratified epithelial tissue is formed by several layers of cells.<\/em>[\/caption]\n\n<\/div>\n<div id=\"fs-id1431970\" class=\"bc-section section\" data-depth=\"2\">\n<h3 data-type=\"title\">Simple Epithelium<\/h3>\n<p id=\"fs-id1321611\">The shape of the cells in the single cell layer of simple epithelium reflects the functioning of those cells. The cells in <span data-type=\"term\">simple squamous epithelium<\/span> have the appearance of thin scales. Squamous cell nuclei tend to be flat, horizontal, and elliptical, mirroring the form of the cell. The <span data-type=\"term\">endothelium<\/span> is the epithelial tissue that lines vessels of the lymphatic and cardiovascular system, and it is made up of a single layer of squamous cells. Simple squamous epithelium, because of the thinness of the cell, is present where rapid passage of chemical compounds is observed. The alveoli of lungs where gases diffuse, segments of kidney tubules, and the lining of capillaries are also made of simple squamous epithelial tissue. The <span data-type=\"term\">[pb_glossary id=\"800\"]mesothelium[\/pb_glossary]<\/span> is a[pb_glossary id=\"807\"] simple squamous epithelium[\/pb_glossary] that forms the surface layer of the serous membrane that lines body cavities and internal organs. Its primary function is to provide a smooth and protective surface.<\/p>\n<p id=\"fs-id1511558\">In[pb_glossary id=\"806\"] <span data-type=\"term\">simple cuboidal epithelium<\/span>[\/pb_glossary], the nucleus of the box-like cells appears round and is generally located near the center of the cell. These epithelia are active in the secretion and absorptions of molecules. Simple cuboidal epithelia are observed in the lining of the kidney tubules and in the ducts of glands.<\/p>\n<p id=\"fs-id1319983\">In <span data-type=\"term\">[pb_glossary id=\"805\"]simple columnar epithelium[\/pb_glossary]<\/span>, the nucleus of the tall column-like cells tends to be elongated and located in the basal end of the cells. Like the cuboidal epithelia, this epithelium is active in the absorption and secretion of molecules. Simple columnar epithelium forms the lining of some sections of the digestive system and parts of the female reproductive tract. Ciliated columnar epithelium is composed of simple columnar epithelial cells with cilia on their apical surfaces. These epithelial cells are found in the lining of the fallopian tubes and parts of the respiratory system, where the beating of the cilia helps remove particulate matter.<\/p>\n<p id=\"fs-id1492560\"><span data-type=\"term\">[pb_glossary id=\"802\"]Pseudostratified columnar epithelium[\/pb_glossary]<\/span> is a type of epithelium that appears to be stratified but instead consists of a single layer of irregularly shaped and differently sized columnar cells. In pseudostratified epithelium, nuclei of neighboring cells appear at different levels rather than clustered in the basal end. The arrangement gives the appearance of stratification; but in fact all the cells are in contact with the basal lamina, although some do not reach the apical surface. Pseudostratified columnar epithelium is found in the respiratory tract, where some of these cells have cilia.<\/p>\n<p id=\"fs-id1510537\">Both simple and pseudostratified columnar epithelia are heterogeneous epithelia because they include additional types of cells interspersed among the epithelial cells. For example, a <span data-type=\"term\">[pb_glossary id=\"797\"]goblet cell[\/pb_glossary]<\/span> is a mucous-secreting unicellular \u201cgland\u201d interspersed between the columnar epithelial cells of mucous membranes <a class=\"autogenerated-content\" href=\"#fig-ch04_02_03\">(Figure 1.7)<\/a>.<\/p>\n\n\n[caption id=\"\" align=\"aligncenter\" width=\"300\"]<a href=\"https:\/\/pressbooks.bccampus.ca\/pathology\/wp-content\/uploads\/sites\/1260\/2021\/08\/404_Goblet_Cell_new-scaled.jpg\"><img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/wp-content\/uploads\/sites\/2131\/2023\/12\/404_Goblet_Cell_new-scaled-1.jpg\" alt=\"This illustration shows a diagram of a goblet cell. The goblet cell is shaped roughly like an upside down vase. The enlarged end at the top contains six finger like projections labeled microvilli. Between the microvilli, secretary vesicles containing mucin are moving from the upper half of the cell toward the microvilli. Below the secretory vesicles are several rough endoplasmic reticula and an irregularly shaped Golgi apparatus with secretory vesicles budding off of it. The narrow, lower half of the cell contains the oval-shaped nucleus as well as a few mitochondria and segments of the endoplasmic reticulum.\" width=\"300\" height=\"557\" data-media-type=\"image\/jpg\"><\/a> <strong>Figure 1.7a<\/strong> <em>- Goblet Cell - In the lining of the small intestine, columnar epithelium cells are interspersed with goblet cells.<\/em>[\/caption]\n\n&nbsp;\n\n[caption id=\"\" align=\"aligncenter\" width=\"563\"]<a href=\"https:\/\/pressbooks.bccampus.ca\/pathology\/wp-content\/uploads\/sites\/1260\/2021\/08\/404b_Goblet_Cell_new.jpg\"><img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/wp-content\/uploads\/sites\/2131\/2023\/12\/404b_Goblet_Cell_new.jpg\" alt=\"The second image is a micrograph of the innermost lining of the small intestine. This innermost lining is a simple columnar epithelium, with a single layer of rectangular cells oriented in a line. Occasionally, the line of epithelial cells is interrupted by a goblet cell. Goblet cells are thinner than the epithelial cells and appear roughly pill shaped. In this micrograph, the cells did not stain as darkly as the epithelial cells.\" width=\"563\" height=\"546\" data-media-type=\"image\/jpg\"><\/a> <strong>Figure 1.7b <\/strong>- <em>The arrows in this micrograph point to the mucous-secreting goblet cells. LM \u00d7 1600. (Micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012)<\/em>[\/caption]\n\n<div id=\"fig-ch04_02_03\" class=\"span-all\">\n<div id=\"fig-ch04_02_03b\" class=\"bc-figure figure\">\n<div class=\"mceTemp\"><\/div>\n<\/div>\n<\/div>\n<div class=\"anatomy interactive um\" data-type=\"note\" data-has-label=\"true\" data-label=\"\">\n\nView the <a href=\"http:\/\/openstax.org\/l\/goblet\">University of Michigan WebScope<\/a> to explore the tissue sample in greater detail.\n\n<\/div>\n<\/div>\n<div id=\"fs-id1243625\" class=\"bc-section section\" data-depth=\"2\">\n<h3 data-type=\"title\">Stratified Epithelium<\/h3>\n<p id=\"fs-id1178293\">A stratified epithelium consists of several stacked layers of cells. This epithelium protects against physical and chemical wear and tear. The stratified epithelium is named by the shape of the most apical layer of cells, closest to the free space. <span data-type=\"term\">Stratified squamous epithelium<\/span> is the most common type of stratified epithelium in the human body. The apical cells are squamous, whereas the basal layer contains either columnar or cuboidal cells. The top layer may be covered with dead cells filled with keratin. Mammalian skin is an example of this dry, keratinized, stratified squamous epithelium. The lining of the mouth cavity is an example of an unkeratinized, stratified squamous epithelium. <span data-type=\"term\">[pb_glossary id=\"809\"]Stratified cuboidal epithelium[\/pb_glossary]<\/span> and <span data-type=\"term\">[pb_glossary id=\"808\"]stratified columnar epithelium[\/pb_glossary]<\/span> can also be found in certain glands and ducts, but are uncommon in the human body.<\/p>\nAnother kind of stratified epithelium is <span data-type=\"term\">[pb_glossary id=\"812\"]transitional epithelium[\/pb_glossary]<\/span>, so-called because of the gradual changes in the shapes of the apical cells as the bladder fills with urine. It is found only in the urinary system, specifically the ureters and urinary bladder. When the bladder is empty, this epithelium is convoluted and has cuboidal apical cells with convex, umbrella shaped, apical surfaces. As the bladder fills with urine, this epithelium loses its convolutions and the apical cells transition from cuboidal to squamous. It appears thicker and more multi-layered when the bladder is empty, and more stretched out and less stratified when the bladder is full and distended. <a class=\"autogenerated-content\" href=\"#fig-ch04_02_04\">(Figure 1.8)<\/a> summarizes the different categories of epithelial cell tissue cells.\n<div id=\"fig-ch04_02_04\" class=\"span-all\">\n<div data-type=\"title\"><\/div>\n<div class=\"bc-figcaption figcaption\">\n\n[caption id=\"\" align=\"aligncenter\" width=\"594\"]<a href=\"https:\/\/pressbooks.bccampus.ca\/pathology\/wp-content\/uploads\/sites\/1260\/2021\/08\/423_Table_04_02_Summary_of_Epithelial_Tissue_CellsN.jpg\"><img class=\"https:\/\/pressbooks.bccampus.ca\/pathology\/wp-content\/uploads\/sites\/1260\/2021\/08\/423_Table_04_02_Summary_of_Epithelial_Tissue_CellsN.jpg\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/wp-content\/uploads\/sites\/2131\/2023\/12\/423_Table_04_02_Summary_of_Epithelial_Tissue_CellsN.jpg\" alt=\"This figure is a table with three columns and eight rows. The leftmost column is titled cells, and contains a drawing in each row showing how epithelial cells are arranged above a basement membrane. The middle column is titled location, while the rightmost column is titled function. In a simple squamous epithelium, the cells are flattened and single-layered. Simple squamous cells are found in the air sacs of the lungs, in the lining of the heart, blood vessels and lymphatic vessels. Their function is to allow materials to pass through by diffusion and filtration, as well as to secrete lubricating substances. In a simple cuboidal epithelium, the cells are cube shaped and single layered and located in ducts and secretory portions of small glands as well as in the kidney tubules. The function of simple cuboidal epithelium is to secrete and absorb. In a simple columnar epithelium, the cells are rectangular and are attached to the basement membrane on one of their narrow sides, so that each cell is standing up like a column. There is only one layer of cells. Simple columnar epithelium is found in ciliated tissues including the bronchi, uterine tubes, and uterus, as well as in smooth, nonciliated tissues such as the digestive tract bladder. The function of simple columnar epithelium is to absorb substances but also to secrete mucous and enzymes. In a pseudostratified columnar epithelium, the cells are column-like in appearance, but they vary in height. The taller cells bend over the tops of the shorter cells so that the top of the epithelial tissue is continuous. There is only one layer of cells. Pseudostratified columnar epithelium lines the trachea and much of the upper respiratory tract. The function of pseudostratified columnar epithelium is to secrete mucous and also move that mucus using the hair like cilia projecting from the top of each cell. A stratified squamous epithelium contains many layers of flattened cells. Stratified squamous epithelium lines the esophagus, mouth, and vagina. The function of stratified squamous epithelium is to protect against abrasion. Stratified cuboidal epithelium contains many layers of cube-shaped cells. Stratified cuboidal epithelium is found in the sweat glands, salivary glands, and mammary glands. The function of stratified cuboidal epithelium is to protect other tissues of the body. Stratified columnar epithelium contains many layers of rectangular, column-shaped cells. Stratified columnar epithelium is located in the male urethra and the ducts of some glands. The function of stratified columnar epithelium is to secrete and protect. Transitional epithelium consists of many layers of irregularly shaped cells with diverse sizes. Transitional epithelium is found lining the bladder, urethra and ureters. The function of transitional epithelium is to allow the urinary organs to expand and stretch.\" width=\"594\" height=\"952\" data-media-type=\"image\/jpg\"><\/a> <strong>Figure 1.8<\/strong> - <em>Summary of Epithelial Tissue Cells<\/em>[\/caption]\n\n<\/div>\n<div id=\"fs-id921926\" class=\"anatomy interactive\" data-type=\"note\" data-has-label=\"true\" data-label=\"\">\n<p id=\"fs-id1137846\"><\/p>\n\n<\/div>\n<\/div>\n<\/div>\n<div id=\"fs-id1242715\" class=\"bc-section section\" data-depth=\"1\">\n<h2 data-type=\"title\">Glandular Epithelium<\/h2>\n<p id=\"fs-id1311216\">A gland is a structure made up of one or more cells modified to synthesize and secrete chemical substances. Most glands consist of groups of epithelial cells. A gland can be classified as an <span data-type=\"term\">[pb_glossary id=\"793\"]endocrine gland[\/pb_glossary]<\/span>, a ductless gland that releases secretions directly into surrounding tissues and fluids (endo- = \u201cinside\u201d), or an <span data-type=\"term\">[pb_glossary id=\"795\"]exocrine gland[\/pb_glossary]<\/span> whose secretions leave through a duct that opens directly, or indirectly, to the external environment (exo- = \u201coutside\u201d).<\/p>\n\n<div class=\"bc-section section\" data-depth=\"2\">\n<h3 data-type=\"title\">Endocrine Glands<\/h3>\nThe secretions of endocrine glands are called hormones. Hormones are released into the interstitial fluid, diffused into the bloodstream, and delivered to targets, in other words, cells that have receptors to bind the hormones. The endocrine system is part of a major regulatory system coordinating the regulation and integration of body responses. A few examples of endocrine glands include the anterior pituitary, thymus, adrenal cortex, and gonads.\n\n<\/div>\n<div class=\"bc-section section\" data-depth=\"2\">\n<h3 data-type=\"title\">Exocrine Glands<\/h3>\n<p id=\"fs-id1211565\">Exocrine glands release their contents through a duct that leads to the epithelial surface. Mucous, sweat, saliva, and breast milk are all examples of secretions from exocrine glands. They are all discharged through tubular ducts. Secretions into the lumen of the gastrointestinal tract, technically outside of the body, are of the exocrine category.<\/p>\n\n<\/div>\n<div id=\"fs-id1152860\" class=\"bc-section section\" data-depth=\"2\">\n<h3 data-type=\"title\">Glandular Structure<\/h3>\n<p id=\"fs-id1179972\">Exocrine glands are classified as either unicellular or multicellular. The unicellular glands are scattered single cells, such as goblet cells, found in the mucous membranes of the small and large intestine. The multicellular exocrine glands known as [pb_glossary id=\"804\"]serous glands[\/pb_glossary] develop from simple epithelium to form a secretory surface that secretes directly into an inner cavity. These glands line the internal cavities of the abdomen and chest and release their secretions directly into the cavities.<\/p>\n\n<\/div>\n<div class=\"bc-section section\" data-depth=\"2\">\n<h1>Section Review<\/h1>\nIn epithelial tissue, cells are closely packed with little or no extracellular matrix except for the [pb_glossary id=\"790\"]basal lamina[\/pb_glossary] that separates the epithelium from underlying tissue. The main functions of epithelia are protection from the environment, coverage, secretion and excretion, absorption, and filtration. Cells are bound together by [pb_glossary id=\"811\"]tight junctions[\/pb_glossary] that form an impermeable barrier. They can also be connected by [pb_glossary id=\"796\"]gap junctions[\/pb_glossary], which allow free exchange of soluble molecules between cells, and anchoring junctions, which attach cell to cell or cell to matrix. The different types of epithelial tissues are characterized by their cellular shapes and arrangements: squamous, cuboidal, or columnar epithelia. Single cell layers form simple epithelia, whereas stacked cells form stratified epithelia. Very few capillaries penetrate these tissues.\n\nGlands are secretory tissues and organs that are derived from epithelial tissues. Exocrine glands release their products through ducts. Endocrine glands secrete hormones directly into the interstitial fluid and blood stream.\n<h1>Review Questions<\/h1>\n[h5p id=\"12\"]\n\n[h5p id=\"13\"]\n<h1>Adaption<\/h1>\nThis chapter is adapted from the following text:\n\n<a href=\"https:\/\/opentextbc.ca\/anatomyandphysiologyopenstax\/chapter\/epithelial-tissue\/\">Epithelial Tissue<\/a> <strong>in <\/strong><a href=\"https:\/\/opentextbc.ca\/anatomyandphysiologyopenstax\">Anatomy and Physiology<\/a>&nbsp;by&nbsp;OSCRiceUniversity&nbsp;is licensed under a&nbsp;<a href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">Creative Commons Attribution 4.0 International License<\/a>\n\n<\/div>\n<\/div>\n<\/div>\n","rendered":"<div class=\"textbox textbox--learning-objectives\">\n<div class=\"textbox textbox--learning-objectives\">\n<header class=\"textbox__header\">\n<p class=\"textbox__title\">Learning Objectives<\/p>\n<\/header>\n<div class=\"textbox__content\">\n<p>This section gives the background of how many cells make up epithelial tissue and key features. This background will be instrumental for the subsequent videos of histological examination of normal and pathological tissue. By the end of this section, you will be able to:<\/p>\n<ul>\n<li>Explain the structure and function of epithelial tissue.<\/li>\n<li>Distinguish between tight junctions, anchoring junctions, and gap junctions.<\/li>\n<li>Distinguish between simple epithelia and stratified epithelia, as well as between squamous, cuboidal, and columnar epithelia.<\/li>\n<li>Describe the structure and function of endocrine and exocrine glands and their respective secretions.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<\/div>\n<p id=\"fs-id1508253\">Most epithelial tissues are essentially large sheets of cells covering all the surfaces of the body exposed to the outside world and lining the outside of organs. Epithelium also forms much of the glandular tissue of the body. Skin is not the only area of the body exposed to the outside. Other areas include the airways, the digestive tract, as well as the urinary and reproductive systems, all of which are lined by an epithelium. Hollow organs and body cavities that do not connect to the exterior of the body, which includes, blood vessels and serous membranes, are lined by<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_45_794\"> endothelium<\/a> (plural = endothelia), which is a type of epithelium.<\/p>\n<p>&nbsp;<\/p>\n<p id=\"fs-id1454989\">All epithelia share some important structural and functional features. This tissue is highly cellular, with little or no extracellular material present between cells. Adjoining cells form a specialized intercellular connection between their cell membranes called a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_45_792\"> <span data-type=\"term\">cell junction<\/span><\/a>. The epithelial cells exhibit polarity with differences in structure and function between the exposed or <span data-type=\"term\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_45_788\">apical<\/a><\/span> facing surface of the cell and the basal surface close to the underlying body structures. The <span data-type=\"term\">basal lamina<\/span>, a mixture of glycoproteins and collagen, provides an attachment site for the epithelium, separating it from underlying connective tissue. The basal lamina attaches to a <span data-type=\"term\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_45_803\">reticular lamina<\/a><\/span>, which is secreted by the underlying connective tissue, forming a <span data-type=\"term\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_45_791\">basement membrane<\/a><\/span> that helps hold it all together.<\/p>\n<p id=\"fs-id1508318\">Epithelial tissues are nearly completely avascular. For instance, no blood vessels cross the basement membrane to enter the tissue, and nutrients must come by diffusion or absorption from underlying tissues or the surface. Many epithelial tissues are capable of rapidly replacing damaged and dead cells. Sloughing off of damaged or dead cells is a characteristic of surface epithelium and allows our airways and digestive tracts to rapidly replace damaged cells with new cells.<\/p>\n<div id=\"fs-id1511818\" class=\"bc-section section\" data-depth=\"1\">\n<h2 data-type=\"title\">Generalized Functions of Epithelial Tissue<\/h2>\n<p id=\"fs-id1486029\">Epithelial tissues provide the body\u2019s first line of protection from physical, chemical, and biological wear and tear. The cells of an epithelium act as gatekeepers of the body controlling permeability and allowing selective transfer of materials across a physical barrier. All substances that enter the body must cross an epithelium. Some epithelia often include structural features that allow the selective transport of molecules and ions across their cell membranes.<\/p>\n<p id=\"fs-id1300309\">Many epithelial cells are capable of secretion and release mucous and specific chemical compounds onto their apical surfaces. The epithelium of the small intestine releases digestive enzymes, for example. Cells lining the respiratory tract secrete mucous that traps incoming microorganisms and particles. A glandular epithelium contains many secretory cells.<\/p>\n<\/div>\n<div id=\"fs-id1168010\" class=\"bc-section section\" data-depth=\"1\">\n<h2 data-type=\"title\">The Epithelial Cell<\/h2>\n<p id=\"fs-id1233802\">Epithelial cells are typically characterized by the polarized distribution of organelles and membrane-bound proteins between their basal and apical surfaces. Particular structures found in some epithelial cells are an adaptation to specific functions. Certain organelles are segregated to the basal sides, whereas other organelles and extensions, such as cilia, when present, are on the apical surface.<\/p>\n<p id=\"fs-id1508184\">Cilia are microscopic extensions of the apical cell membrane that are supported by microtubules. They beat in unison and move fluids as well as trapped particles. Ciliated epithelium lines the ventricles of the brain where it helps circulate the cerebrospinal fluid. The ciliated epithelium of your airway forms a mucociliary escalator that sweeps particles of dust and pathogens trapped in the secreted mucous toward the throat. It is called an escalator because it continuously pushes mucous with trapped particles upward. In contrast, nasal cilia sweep the mucous blanket down towards your throat. In both cases, the transported materials are usually swallowed, and end up in the acidic environment of your stomach.<\/p>\n<\/div>\n<div id=\"fs-id1501084\" class=\"bc-section section\" data-depth=\"1\">\n<h2 data-type=\"title\">Cell to Cell Junctions<\/h2>\n<p id=\"fs-id1304667\">Cells of epithelia are closely connected and are not separated by intracellular material. Three basic types of connections allow varying degrees of interaction between the cells: tight junctions, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_45_787\">anchoring junctions<\/a>, and gap junctions <a class=\"autogenerated-content\" href=\"#fig-ch04_02_01\">(Figure 1.5)<\/a>.<\/p>\n<div id=\"fig-ch04_02_01\" class=\"bc-figure figure\">\n<div class=\"bc-figcaption figcaption\"><\/div>\n<figure id=\"attachment_870\" aria-describedby=\"caption-attachment-870\" style=\"width: 600px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/pressbooks.bccampus.ca\/pathology\/wp-content\/uploads\/sites\/1260\/2021\/08\/402_Types_of_Cell_Junctions_new-scaled.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-870\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/wp-content\/uploads\/sites\/2131\/2023\/12\/402_Types_of_Cell_Junctions_new-scaled-1.jpg\" alt=\"These three illustrations each show the edges of two vertical cell membranes. The cell membranes are viewed partially from the side so that the inside edge of the right cell membrane is visible. The upper left image shows a tight junction. The two cell membranes are bound by transmembrane protein strands. The proteins travel the inside edge of the right cell membrane and cross over to the left cell membrane, cinching the two membranes together. The cell membranes are still somewhat separated in between neighboring strands, creating intercellular spaces. The upper right diagram shows a gap junction. The gap junctions are composed of two interlocking connexins, which are round, hollow tubes that extend through the cell membranes. Two connexins, one from the left cell membrane and the other from the right cell membrane, meet between the two cells, forming a connexon. Even at the site of the connexon, there is a small gap between the cell membranes. On the inside edge of the right cell membrane, the gap junction appears as a depression. Three connexins are embedded into the membranes like buttons on a shirt. The bottom images show the three types of anchoring junctions. The left image shows a desmosome. Here, the inside edge of both the right and left cell membranes have brown, round plaques. Each plaque has tentacle-like intermediate filaments (keratin) that extend into each cell\u2019s cytoplasm. The two plaques are connected across the intercellular space by several interlocking transmembrane glycoproteins (cadherin). The connected glycoproteins look similar to a zipped-up zipper between the right and left cell membranes. The right image shows an adheren. These are similar to desmosomes, with two plaques on the inside edge of each cell membrane connected across the intercellular space by glycoproteins. However, the plaques do not contain the tentacle-like intermediate filaments branching into the cytoplasm. Instead, the plaques are ribbed with green actin filaments. The filaments are neatly arranged in parallel, horizontal strands on the surface of the plaque facing the cytoplasm. The bottom image shows a hemidesmosome. Rather than located between two neighboring cells, the hemidesmosome is located between the bottom of a cell and the basement membrane. A hemidesmosome contains a single plaque on the inside edge of the cell membrane. Like the desmosome, intermediate filaments project from the plaque into the cytoplasm. The opposite side of the plaque has purple, knob-shaped integrins extending out to the basal lamina of the basement membrane.\" width=\"600\" height=\"700\" \/><\/a><figcaption id=\"caption-attachment-870\" class=\"wp-caption-text\"><strong>Figure 1.5<\/strong> &#8211;<em> Types of Cell Junctions &#8211; The three basic types of cell-to-cell junctions are tight junctions, gap junctions, and anchoring junctions.<\/em><\/figcaption><\/figure>\n<\/div>\n<p id=\"fs-id1211959\">At one end of the spectrum is the <span data-type=\"term\">tight junction<\/span>, which separates the cells into apical and basal compartments. When two adjacent epithelial cells form a tight junction, there is no extracellular space between them and the movement of substances through the extracellular space between the cells is blocked. This enables the epithelia to act as selective barriers. An <span data-type=\"term\">anchoring junction<\/span> includes several types of cell junctions that help stabilize epithelial tissues. Anchoring junctions are common on the lateral and basal surfaces of cells where they provide strong and flexible connections.&nbsp; These junctions influence the shape and folding of the epithelial tissue.<\/p>\n<p id=\"fs-id1535229\">In contrast with the tight and anchoring junctions, a <span data-type=\"term\">gap junction<\/span> forms an intercellular passageway between the membranes of adjacent cells to facilitate the movement of small molecules and ions between the cytoplasm of adjacent cells. These junctions allow electrical and metabolic coupling of adjacent cells, which coordinates function in large groups of cells.<\/p>\n<\/div>\n<div id=\"fs-id1490019\" class=\"bc-section section\" data-depth=\"1\">\n<h2 data-type=\"title\">Classification of Epithelial Tissues<\/h2>\n<p id=\"fs-id1513993\">Epithelial tissues are classified according to the shape of the cells and number of the cell layers formed <a class=\"autogenerated-content\" href=\"#fig-ch04_02_02\">(Figure 1.6)<\/a>. Cell shapes can be squamous (flattened and thin), cuboidal (boxy, as wide as it is tall), or columnar (rectangular, taller than it is wide). Similarly, the number of cell layers in the tissue can be one\u2014where every cell rests on the basal lamina\u2014which is a simple epithelium, or more than one, which is a stratified epithelium and only the basal layer of cells rests on the basal lamina. Pseudostratified (pseudo- = \u201cfalse\u201d) describes tissue with a single layer of irregularly shaped cells that give the appearance of more than one layer. Transitional describes a form of specialized stratified epithelium in which the shape of the cells can vary.<\/p>\n<div id=\"fig-ch04_02_02\" class=\"span-all\">\n<div class=\"bc-figcaption figcaption\"><\/div>\n<div class=\"mceTemp\"><\/div>\n<figure id=\"attachment_871\" aria-describedby=\"caption-attachment-871\" style=\"width: 674px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/pressbooks.bccampus.ca\/pathology\/wp-content\/uploads\/sites\/1260\/2021\/08\/403_Epithelial_Tissue.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-41\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/wp-content\/uploads\/sites\/2131\/2023\/12\/403_Epithelial_Tissue-1024x822.jpg\" alt=\"This figure is a table showing the appearance of squamous, cuboidal and columnar epithelial tissues. Simple and compound forms are shown for each tissue type. In a simple squamous epithelium, the cells are flattened and single layered. In a simple cuboidal epithelium, the cells are cube shaped and single layered. In a simple columnar epithelium, the cells are rectangular and are attached to the basement membrane on one of their narrow sides, so that each cell is standing up like a column. There is only one layer of cells. In a pseudostratified columnar epithelium, the cells are column-like in appearance, but they vary in height. The taller cells bend over the tops of the shorter cells so that the top of the epithelial tissue is continuous. There is only one layer of cells. A stratified squamous epithelium contains many layers of flattened cells. Stratified cuboidal epithelium contains many layers of cube-shaped cells. Stratified columnar epithelium contains many layers of rectangular, column-shaped cells\" width=\"674\" height=\"541\" srcset=\"https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/wp-content\/uploads\/sites\/2131\/2023\/12\/403_Epithelial_Tissue-1024x822.jpg 1024w, https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/wp-content\/uploads\/sites\/2131\/2023\/12\/403_Epithelial_Tissue-300x241.jpg 300w, https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/wp-content\/uploads\/sites\/2131\/2023\/12\/403_Epithelial_Tissue-768x616.jpg 768w, https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/wp-content\/uploads\/sites\/2131\/2023\/12\/403_Epithelial_Tissue-65x52.jpg 65w, https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/wp-content\/uploads\/sites\/2131\/2023\/12\/403_Epithelial_Tissue-225x181.jpg 225w, https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/wp-content\/uploads\/sites\/2131\/2023\/12\/403_Epithelial_Tissue-350x281.jpg 350w, https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/wp-content\/uploads\/sites\/2131\/2023\/12\/403_Epithelial_Tissue.jpg 1209w\" sizes=\"auto, (max-width: 674px) 100vw, 674px\" \/><\/a><figcaption id=\"caption-attachment-871\" class=\"wp-caption-text\"><strong>Figure 1.6 <\/strong>&#8211; <em>Cells of Epithelial Tissue&nbsp; &#8211; Simple epithelial tissue is organized as a single layer of cells and stratified epithelial tissue is formed by several layers of cells.<\/em><\/figcaption><\/figure>\n<\/div>\n<div id=\"fs-id1431970\" class=\"bc-section section\" data-depth=\"2\">\n<h3 data-type=\"title\">Simple Epithelium<\/h3>\n<p id=\"fs-id1321611\">The shape of the cells in the single cell layer of simple epithelium reflects the functioning of those cells. The cells in <span data-type=\"term\">simple squamous epithelium<\/span> have the appearance of thin scales. Squamous cell nuclei tend to be flat, horizontal, and elliptical, mirroring the form of the cell. The <span data-type=\"term\">endothelium<\/span> is the epithelial tissue that lines vessels of the lymphatic and cardiovascular system, and it is made up of a single layer of squamous cells. Simple squamous epithelium, because of the thinness of the cell, is present where rapid passage of chemical compounds is observed. The alveoli of lungs where gases diffuse, segments of kidney tubules, and the lining of capillaries are also made of simple squamous epithelial tissue. The <span data-type=\"term\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_45_800\">mesothelium<\/a><\/span> is a<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_45_807\"> simple squamous epithelium<\/a> that forms the surface layer of the serous membrane that lines body cavities and internal organs. Its primary function is to provide a smooth and protective surface.<\/p>\n<p id=\"fs-id1511558\">In<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_45_806\"> <span data-type=\"term\">simple cuboidal epithelium<\/span><\/a>, the nucleus of the box-like cells appears round and is generally located near the center of the cell. These epithelia are active in the secretion and absorptions of molecules. Simple cuboidal epithelia are observed in the lining of the kidney tubules and in the ducts of glands.<\/p>\n<p id=\"fs-id1319983\">In <span data-type=\"term\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_45_805\">simple columnar epithelium<\/a><\/span>, the nucleus of the tall column-like cells tends to be elongated and located in the basal end of the cells. Like the cuboidal epithelia, this epithelium is active in the absorption and secretion of molecules. Simple columnar epithelium forms the lining of some sections of the digestive system and parts of the female reproductive tract. Ciliated columnar epithelium is composed of simple columnar epithelial cells with cilia on their apical surfaces. These epithelial cells are found in the lining of the fallopian tubes and parts of the respiratory system, where the beating of the cilia helps remove particulate matter.<\/p>\n<p id=\"fs-id1492560\"><span data-type=\"term\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_45_802\">Pseudostratified columnar epithelium<\/a><\/span> is a type of epithelium that appears to be stratified but instead consists of a single layer of irregularly shaped and differently sized columnar cells. In pseudostratified epithelium, nuclei of neighboring cells appear at different levels rather than clustered in the basal end. The arrangement gives the appearance of stratification; but in fact all the cells are in contact with the basal lamina, although some do not reach the apical surface. Pseudostratified columnar epithelium is found in the respiratory tract, where some of these cells have cilia.<\/p>\n<p id=\"fs-id1510537\">Both simple and pseudostratified columnar epithelia are heterogeneous epithelia because they include additional types of cells interspersed among the epithelial cells. For example, a <span data-type=\"term\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_45_797\">goblet cell<\/a><\/span> is a mucous-secreting unicellular \u201cgland\u201d interspersed between the columnar epithelial cells of mucous membranes <a class=\"autogenerated-content\" href=\"#fig-ch04_02_03\">(Figure 1.7)<\/a>.<\/p>\n<figure style=\"width: 300px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/pressbooks.bccampus.ca\/pathology\/wp-content\/uploads\/sites\/1260\/2021\/08\/404_Goblet_Cell_new-scaled.jpg\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/wp-content\/uploads\/sites\/2131\/2023\/12\/404_Goblet_Cell_new-scaled-1.jpg\" alt=\"This illustration shows a diagram of a goblet cell. The goblet cell is shaped roughly like an upside down vase. The enlarged end at the top contains six finger like projections labeled microvilli. Between the microvilli, secretary vesicles containing mucin are moving from the upper half of the cell toward the microvilli. Below the secretory vesicles are several rough endoplasmic reticula and an irregularly shaped Golgi apparatus with secretory vesicles budding off of it. The narrow, lower half of the cell contains the oval-shaped nucleus as well as a few mitochondria and segments of the endoplasmic reticulum.\" width=\"300\" height=\"557\" data-media-type=\"image\/jpg\" \/><\/a><figcaption class=\"wp-caption-text\"><strong>Figure 1.7a<\/strong> <em>&#8211; Goblet Cell &#8211; In the lining of the small intestine, columnar epithelium cells are interspersed with goblet cells.<\/em><\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<figure style=\"width: 563px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/pressbooks.bccampus.ca\/pathology\/wp-content\/uploads\/sites\/1260\/2021\/08\/404b_Goblet_Cell_new.jpg\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/wp-content\/uploads\/sites\/2131\/2023\/12\/404b_Goblet_Cell_new.jpg\" alt=\"The second image is a micrograph of the innermost lining of the small intestine. This innermost lining is a simple columnar epithelium, with a single layer of rectangular cells oriented in a line. Occasionally, the line of epithelial cells is interrupted by a goblet cell. Goblet cells are thinner than the epithelial cells and appear roughly pill shaped. In this micrograph, the cells did not stain as darkly as the epithelial cells.\" width=\"563\" height=\"546\" data-media-type=\"image\/jpg\" \/><\/a><figcaption class=\"wp-caption-text\"><strong>Figure 1.7b <\/strong>&#8211; <em>The arrows in this micrograph point to the mucous-secreting goblet cells. LM \u00d7 1600. (Micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012)<\/em><\/figcaption><\/figure>\n<div id=\"fig-ch04_02_03\" class=\"span-all\">\n<div id=\"fig-ch04_02_03b\" class=\"bc-figure figure\">\n<div class=\"mceTemp\"><\/div>\n<\/div>\n<\/div>\n<div class=\"anatomy interactive um\" data-type=\"note\" data-has-label=\"true\" data-label=\"\">\n<p>View the <a href=\"http:\/\/openstax.org\/l\/goblet\">University of Michigan WebScope<\/a> to explore the tissue sample in greater detail.<\/p>\n<\/div>\n<\/div>\n<div id=\"fs-id1243625\" class=\"bc-section section\" data-depth=\"2\">\n<h3 data-type=\"title\">Stratified Epithelium<\/h3>\n<p id=\"fs-id1178293\">A stratified epithelium consists of several stacked layers of cells. This epithelium protects against physical and chemical wear and tear. The stratified epithelium is named by the shape of the most apical layer of cells, closest to the free space. <span data-type=\"term\">Stratified squamous epithelium<\/span> is the most common type of stratified epithelium in the human body. The apical cells are squamous, whereas the basal layer contains either columnar or cuboidal cells. The top layer may be covered with dead cells filled with keratin. Mammalian skin is an example of this dry, keratinized, stratified squamous epithelium. The lining of the mouth cavity is an example of an unkeratinized, stratified squamous epithelium. <span data-type=\"term\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_45_809\">Stratified cuboidal epithelium<\/a><\/span> and <span data-type=\"term\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_45_808\">stratified columnar epithelium<\/a><\/span> can also be found in certain glands and ducts, but are uncommon in the human body.<\/p>\n<p>Another kind of stratified epithelium is <span data-type=\"term\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_45_812\">transitional epithelium<\/a><\/span>, so-called because of the gradual changes in the shapes of the apical cells as the bladder fills with urine. It is found only in the urinary system, specifically the ureters and urinary bladder. When the bladder is empty, this epithelium is convoluted and has cuboidal apical cells with convex, umbrella shaped, apical surfaces. As the bladder fills with urine, this epithelium loses its convolutions and the apical cells transition from cuboidal to squamous. It appears thicker and more multi-layered when the bladder is empty, and more stretched out and less stratified when the bladder is full and distended. <a class=\"autogenerated-content\" href=\"#fig-ch04_02_04\">(Figure 1.8)<\/a> summarizes the different categories of epithelial cell tissue cells.<\/p>\n<div id=\"fig-ch04_02_04\" class=\"span-all\">\n<div data-type=\"title\"><\/div>\n<div class=\"bc-figcaption figcaption\">\n<figure style=\"width: 594px\" class=\"wp-caption aligncenter\"><a href=\"https:\/\/pressbooks.bccampus.ca\/pathology\/wp-content\/uploads\/sites\/1260\/2021\/08\/423_Table_04_02_Summary_of_Epithelial_Tissue_CellsN.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"https:\/\/pressbooks.bccampus.ca\/pathology\/wp-content\/uploads\/sites\/1260\/2021\/08\/423_Table_04_02_Summary_of_Epithelial_Tissue_CellsN.jpg\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/wp-content\/uploads\/sites\/2131\/2023\/12\/423_Table_04_02_Summary_of_Epithelial_Tissue_CellsN.jpg\" alt=\"This figure is a table with three columns and eight rows. The leftmost column is titled cells, and contains a drawing in each row showing how epithelial cells are arranged above a basement membrane. The middle column is titled location, while the rightmost column is titled function. In a simple squamous epithelium, the cells are flattened and single-layered. Simple squamous cells are found in the air sacs of the lungs, in the lining of the heart, blood vessels and lymphatic vessels. Their function is to allow materials to pass through by diffusion and filtration, as well as to secrete lubricating substances. In a simple cuboidal epithelium, the cells are cube shaped and single layered and located in ducts and secretory portions of small glands as well as in the kidney tubules. The function of simple cuboidal epithelium is to secrete and absorb. In a simple columnar epithelium, the cells are rectangular and are attached to the basement membrane on one of their narrow sides, so that each cell is standing up like a column. There is only one layer of cells. Simple columnar epithelium is found in ciliated tissues including the bronchi, uterine tubes, and uterus, as well as in smooth, nonciliated tissues such as the digestive tract bladder. The function of simple columnar epithelium is to absorb substances but also to secrete mucous and enzymes. In a pseudostratified columnar epithelium, the cells are column-like in appearance, but they vary in height. The taller cells bend over the tops of the shorter cells so that the top of the epithelial tissue is continuous. There is only one layer of cells. Pseudostratified columnar epithelium lines the trachea and much of the upper respiratory tract. The function of pseudostratified columnar epithelium is to secrete mucous and also move that mucus using the hair like cilia projecting from the top of each cell. A stratified squamous epithelium contains many layers of flattened cells. Stratified squamous epithelium lines the esophagus, mouth, and vagina. The function of stratified squamous epithelium is to protect against abrasion. Stratified cuboidal epithelium contains many layers of cube-shaped cells. Stratified cuboidal epithelium is found in the sweat glands, salivary glands, and mammary glands. The function of stratified cuboidal epithelium is to protect other tissues of the body. Stratified columnar epithelium contains many layers of rectangular, column-shaped cells. Stratified columnar epithelium is located in the male urethra and the ducts of some glands. The function of stratified columnar epithelium is to secrete and protect. Transitional epithelium consists of many layers of irregularly shaped cells with diverse sizes. Transitional epithelium is found lining the bladder, urethra and ureters. The function of transitional epithelium is to allow the urinary organs to expand and stretch.\" width=\"594\" height=\"952\" data-media-type=\"image\/jpg\" \/><\/a><figcaption class=\"wp-caption-text\"><strong>Figure 1.8<\/strong> &#8211; <em>Summary of Epithelial Tissue Cells<\/em><\/figcaption><\/figure>\n<\/div>\n<div id=\"fs-id921926\" class=\"anatomy interactive\" data-type=\"note\" data-has-label=\"true\" data-label=\"\">\n<p id=\"fs-id1137846\">\n<\/div>\n<\/div>\n<\/div>\n<div id=\"fs-id1242715\" class=\"bc-section section\" data-depth=\"1\">\n<h2 data-type=\"title\">Glandular Epithelium<\/h2>\n<p id=\"fs-id1311216\">A gland is a structure made up of one or more cells modified to synthesize and secrete chemical substances. Most glands consist of groups of epithelial cells. A gland can be classified as an <span data-type=\"term\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_45_793\">endocrine gland<\/a><\/span>, a ductless gland that releases secretions directly into surrounding tissues and fluids (endo- = \u201cinside\u201d), or an <span data-type=\"term\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_45_795\">exocrine gland<\/a><\/span> whose secretions leave through a duct that opens directly, or indirectly, to the external environment (exo- = \u201coutside\u201d).<\/p>\n<div class=\"bc-section section\" data-depth=\"2\">\n<h3 data-type=\"title\">Endocrine Glands<\/h3>\n<p>The secretions of endocrine glands are called hormones. Hormones are released into the interstitial fluid, diffused into the bloodstream, and delivered to targets, in other words, cells that have receptors to bind the hormones. The endocrine system is part of a major regulatory system coordinating the regulation and integration of body responses. A few examples of endocrine glands include the anterior pituitary, thymus, adrenal cortex, and gonads.<\/p>\n<\/div>\n<div class=\"bc-section section\" data-depth=\"2\">\n<h3 data-type=\"title\">Exocrine Glands<\/h3>\n<p id=\"fs-id1211565\">Exocrine glands release their contents through a duct that leads to the epithelial surface. Mucous, sweat, saliva, and breast milk are all examples of secretions from exocrine glands. They are all discharged through tubular ducts. Secretions into the lumen of the gastrointestinal tract, technically outside of the body, are of the exocrine category.<\/p>\n<\/div>\n<div id=\"fs-id1152860\" class=\"bc-section section\" data-depth=\"2\">\n<h3 data-type=\"title\">Glandular Structure<\/h3>\n<p id=\"fs-id1179972\">Exocrine glands are classified as either unicellular or multicellular. The unicellular glands are scattered single cells, such as goblet cells, found in the mucous membranes of the small and large intestine. The multicellular exocrine glands known as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_45_804\">serous glands<\/a> develop from simple epithelium to form a secretory surface that secretes directly into an inner cavity. These glands line the internal cavities of the abdomen and chest and release their secretions directly into the cavities.<\/p>\n<\/div>\n<div class=\"bc-section section\" data-depth=\"2\">\n<h1>Section Review<\/h1>\n<p>In epithelial tissue, cells are closely packed with little or no extracellular matrix except for the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_45_790\">basal lamina<\/a> that separates the epithelium from underlying tissue. The main functions of epithelia are protection from the environment, coverage, secretion and excretion, absorption, and filtration. Cells are bound together by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_45_811\">tight junctions<\/a> that form an impermeable barrier. They can also be connected by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_45_796\">gap junctions<\/a>, which allow free exchange of soluble molecules between cells, and anchoring junctions, which attach cell to cell or cell to matrix. The different types of epithelial tissues are characterized by their cellular shapes and arrangements: squamous, cuboidal, or columnar epithelia. Single cell layers form simple epithelia, whereas stacked cells form stratified epithelia. Very few capillaries penetrate these tissues.<\/p>\n<p>Glands are secretory tissues and organs that are derived from epithelial tissues. Exocrine glands release their products through ducts. Endocrine glands secrete hormones directly into the interstitial fluid and blood stream.<\/p>\n<h1>Review Questions<\/h1>\n<h1>Adaption<\/h1>\n<p>This chapter is adapted from the following text:<\/p>\n<p><a href=\"https:\/\/opentextbc.ca\/anatomyandphysiologyopenstax\/chapter\/epithelial-tissue\/\">Epithelial Tissue<\/a> <strong>in <\/strong><a href=\"https:\/\/opentextbc.ca\/anatomyandphysiologyopenstax\">Anatomy and Physiology<\/a>&nbsp;by&nbsp;OSCRiceUniversity&nbsp;is licensed under a&nbsp;<a href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">Creative Commons Attribution 4.0 International License<\/a><\/p>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"glossary\"><span class=\"screen-reader-text\" id=\"definition\">definition<\/span><template id=\"term_45_794\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_45_794\"><div tabindex=\"-1\"><p>Tissue that lines vessels of the lymphatic and cardiovascular system, made up of a simple squamous 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_45_792\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_45_792\"><div tabindex=\"-1\"><p>Point of cell-to-cell contact that connects one cell to another in a 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_45_788\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_45_788\"><div tabindex=\"-1\"><p>The part of a cell or tissue which, in general, faces an open space.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_45_803\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_45_803\"><div tabindex=\"-1\"><p>A matrix containing collagen and elastin secreted by connective tissue; a component of the basement membrane.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_45_791\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_45_791\"><div tabindex=\"-1\"><p>In epithelial tissue, a thin layer of fibrous material that anchors the epithelial tissue to the underlying connective tissue; made up of the basal lamina and reticular lamina.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_45_787\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_45_787\"><div tabindex=\"-1\"><p>Mechanically attaches adjacent cells to each other or to the basement membrane.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_45_800\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_45_800\"><div tabindex=\"-1\"><p>Simple squamous epithelial tissue which covers the major body cavities and is the epithelial portion of serous 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_45_807\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_45_807\"><div tabindex=\"-1\"><p>Tissue that consists of a single layer of flat scale-like cells; promotes diffusion and filtration across surface.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_45_806\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_45_806\"><div tabindex=\"-1\"><p>Tissue that consists of a single layer of cube-shaped cells; promotes secretion and absorption in ducts and tubules.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_45_805\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_45_805\"><div tabindex=\"-1\"><p>Tissue that consists of a single layer of column-like cells; promotes secretion and absorption in tissues and organs.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_45_802\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_45_802\"><div tabindex=\"-1\"><p>Tissue that consists of a single layer of irregularly shaped and sized cells that give the appearance of multiple layers; found in ducts of certain glands and the upper respiratory 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_45_797\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_45_797\"><div tabindex=\"-1\"><p>Unicellular gland found in columnar epithelium that secretes mucous.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_45_809\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_45_809\"><div tabindex=\"-1\"><p>Tissue that consists of two or more layers of cube-shaped cells, found in some ducts.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_45_808\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_45_808\"><div tabindex=\"-1\"><p>Tissue that consists of two or more layers of column-like cells, contains glands and is found in some ducts.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_45_812\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_45_812\"><div tabindex=\"-1\"><p>Form of stratified epithelium found in the urinary tract, characterized by an apical layer of cells that change shape in response to the presence of urine.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_45_793\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_45_793\"><div tabindex=\"-1\"><p>Groups of cells that release chemical signals into the intercellular fluid to be picked up and transported to their target organs by 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_45_795\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_45_795\"><div tabindex=\"-1\"><p>Group of epithelial cells that secrete substances through ducts that open to the skin or to internal body surfaces that lead to the exterior 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_45_804\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_45_804\"><div tabindex=\"-1\"><p>A group of cells within the serous membrane that secrete a lubricating substance onto the surface.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_45_790\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_45_790\"><div tabindex=\"-1\"><p>Thin extracellular layer that lies underneath epithelial cells and separates them from other tissues.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_45_811\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_45_811\"><div tabindex=\"-1\"><p>Forms an impermeable barrier between 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_45_796\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_45_796\"><div tabindex=\"-1\"><p>Allows cytoplasmic communications to occur between cells.<\/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":103,"menu_order":6,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":["j-gordon-betts","kelly-a-young","james-a-wise","eddie-johnson","brandon-poe","dean-h-kruse","oksana-korol","jody-e-johnson","mark-womble","peter-desaix"],"pb_section_license":"cc-by"},"chapter-type":[],"contributor":[167],"license":[53],"class_list":["post-45","chapter","type-chapter","status-web-only","hentry","contributor-peter-desaix","license-cc-by"],"part":29,"_links":{"self":[{"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/wp-json\/pressbooks\/v2\/chapters\/45","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/wp-json\/wp\/v2\/users\/103"}],"version-history":[{"count":1,"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/wp-json\/pressbooks\/v2\/chapters\/45\/revisions"}],"predecessor-version":[{"id":1217,"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/wp-json\/pressbooks\/v2\/chapters\/45\/revisions\/1217"}],"part":[{"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/wp-json\/pressbooks\/v2\/parts\/29"}],"metadata":[{"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/wp-json\/pressbooks\/v2\/chapters\/45\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/wp-json\/wp\/v2\/media?parent=45"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/wp-json\/pressbooks\/v2\/chapter-type?post=45"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/wp-json\/wp\/v2\/contributor?post=45"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol2200\/wp-json\/wp\/v2\/license?post=45"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}