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Appendices: Introduction to Histology for first-time learners

A New Learner’s Guide to Epithelial Tissue: Glandular Epithelia

Willie Wu and Athena Li

Learning Objectives

By the end of this section, you will be able to:

  • Define what a gland is and distinguish between endocrine and exocrine glands based on the presence or absence of a duct.

  • Compare and contrast the function and secretion method of endocrine and exocrine glands.
  • Classify exocrine glands based on their duct structure and the shape of their secretory units.
  • Classify endocrine glands based on their four primary morphological patterns.
  • Identify the key histological features of one major example for each exocrine and endocrine structural class.

Introduction to Glands

In the previous section, we learned about the lining epithelia and their classification. Now, let’s talk about glands. If you think about your body as a busy city, then glands are the factories, water treatment plants, and postal services all organized into one. These specialized structures, which mostly develop from your sheets of epithelial tissue, have one main role of making and secreting products. This could be anything from sweat and saliva to hormones that control and affect your metabolism. The field of studying these important structures is called adenology.

The first and most important thing to understand is that all glands fall into one of two big categories. It all comes down to how they deliver their products!

  1. Endocrine Glands are the body’s broadcast system. They are “ductless,” meaning they don’t have pipes to carry their secretions. Instead, they release their chemical messages, called hormones, directly into your bloodstream. From there, the hormones can travel anywhere and affect distant target organs. Think of it like a radio station broadcasting a signal! Anyone with a receiver can pick it up.
  2. Exocrine Glands are the local delivery services. They always have a duct (a little pipe) that carries their product to a specific location, like your skin surface or the inside of your stomach. Their effects are local, not systemic.

This difference in delivery method also means they look completely different under a microscope. Endocrine glands often seem simpler, like clusters of cells surrounded by a huge network of tiny blood vessels (capillaries), ready to soak up those hormones. Exocrine glands are all about the ducts, which makes their structure more complex and varied.

The major difference between the endocrine and exocrine gland is that, an endocrine gland is missing ducts and stays as blocks of tissue. Endocrine gland secretes chemical substances directly to the blood stream, while exocrine gland secrets its product into a duct. -Endocrine gland secretes chemical substances directly to the blood stream, while exocrine gland secrets its product into a duct. -The hormones produced by endocrine glands circulate through the blood stream and over the body and act on the target, while products of exocrine gland do not circulate all over the body. - These endocrine glands have relatively large blood supply than exocrine glands. -Exocrine glands have more complex structure than endocrine glands.
Endocrine and Exocrine Glands – An endocrine gland is missing ducts and stays as clusters of epithelial tissue (usually cuboidal) that secrete substances directly to the blood stream. Exocrine gland secrets its product into a duct on to the surface of the epithelia. Exocrine glands have more complex structure as ducts can vary in length, shape, and distribution.
Features Endocrine Glands Exocrine Glands
Ducts No ducts (“ductless” glands) Have ducts that carry secretions to a surface
Method of Secretion Secrete hormones directly into the bloodstream or surrounding tissue fluid Secrete products (enzymes, mucus, sweat, etc.) through a duct to a surface
Structure Often appear as cords or clusters (islets) of secretory cells surrounded by a rich network of capillaries Structurally more complex, consisting of a secretory unit and a branching duct system
Examples Thyroid gland, pituitary gland, Islets of Langerhans in the pancreas Salivary glands, sweat glands, mammary glands, pancreas (acinar cells)

For the purpose of this appendix, we will focus on exocrine glands and their ducts as they are a tricky structure to identify, as a first time learner.

Exocrine Glands

Since exocrine glands have such interesting structures, we classify them based on their architecture, shown in the diagram below. We look at two things:

  • The duct: Does it branch like a tree, or is it a single, simple tube?
  • The secretory portion: What is the shape of the factory where the product is actually made?
This image shows some of the various possible glandular arrangements. These are the simple tubular, simple branched tubular, simple coiled tubular, simple acinar, and simple branched acinar glands.
Glandular Arrangements – The green cells are the multicellular glandular exocrine cells whereas the purple is the ductal cells. The blue cells are the exposed epithelial surface. These are (from L to R) the simple tubular, simple branched tubular, simple coiled tubular, simple acinar, and simple branched acinar glands. Note that tubular glands have tube-shaped invaginations whereas acinar are more spherical.

Simple Tubular Glands

Imagine a test tube that is sunk into the tissue. This is essentially a simple tubular gland. It’s a straight, unbranched tube. In your intestines, you’ll find these as Crypts of Lieberkühn. They’re simple little pockets that secrete mucus and house cells that are crucial for repair and absorption. What you see is what you get with these—no fancy twists or branches.

slide of duodenal tissue. Finger like projections are visible with deep folds which are highlighted in blue to note the simple tubular glands.
H&E Stained Simple Tubular Glands in Duodenum – Simple tubular glands (crypts) are marked out in blue in the duodenum. (Slide ID: Path 304 012a, Image ID: 1534 – Duodenum)

Simple Branched Tubular Glands

Now, imagine that same test tube, but at the bottom, it splits into several smaller tubes. This is the branched tubular design. They still have just one duct to the surface, but the “factory floor” has multiple chambers. This is a clever way to increase production space. Your stomach is lined with these glands, as shown below. They branch out to house different cells that produce acid, enzymes, and protective mucus, all funneling into the stomach to help digest your food.

A section of stomach tissue is shown with multiple invaginations visible on the 'free' surface on the bottom of the image. Blue and yellow marks the multiple branches of this simple branched gland.
H&E Stained Branched Tubular Glands in Stomach – Branched tubular glands marked out in yellow and blue (Slide ID: Path 304 011b, Image ID: 1533 – Stomach)

It is worthing mentioning that the yellow box shows an interesting sample which could be mistaken for two simple tubular glands, but this would be due to the cut of this stomach section.

The possible cut made is visualized below:

a hand drawn image of a branched tubular gland with a sectional plane bisecting it, revealing lumen sizes of different sizes and shapes
A 2-D visualization of a 3D branched tubular gland – A 2-D visualization of a 3D branched tubular gland with a sectional plane bisecting it, revealing lumen sizes of different sizes and shapes. Created by Athena Li under a CC-BY-NC-ND license.

It takes time and practice to recognize simple tubular branched glands compared to simple. When you have a section with lumens of different circumferences, around prominent tubular shaped lumens – yet the secretory cuboidal/columnar cell looks the same around each lumen space, this is highly suggestive of branched tubular gland (shown in the yellow box).

Simple Coiled Tubular Glands

What if you needed a very long tube but didn’t have much space? You would coil it up, right? That is exactly what your eccrine sweat glands do. Their secretory portion is wound into a tight little ball deep in your skin, connected to a straight duct that runs to the surface. This clever design packs a huge amount of tube into a tiny space, allowing you to produce a lot of sweat very efficiently to cool down.

A cross-section of thin skin from the scalp showing coiled tubular sweat glands in the deep dermis or hypodermis. The glands are characterized by a pale-staining secretory coil with a simple cuboidal epithelium and a darker, eosinophilic, layered myoepithelial cell layer. Adjacent adipose tissue and dermal collagen are visible. Stained with Hematoxylin and Eosin (H&E).
H&E Stained Coiled Tubular Sweat Glands in Scalp Skin – The glandular epithelium is typically simple cuboidal and stains faintly, giving it a pale appearance. A key identifying feature is the presence of a surrounding layer of contractile myoepithelial cells. (Slide ID: Path 304 003, Image ID: 1521 – Thin Skin From Scalp)

Here is a helpful analogy: imagine a garden hose coiled on the ground. If you look at the coil from above, it will look like a lot of circles which may (or may not) lie neatly on top of each other. Or, if the hose is in a tangled mass, the sectional plane would shows cut sections of small round to oval profiles of varying sizes. If you are lucky, the plane might go along the length of the hose allowing for a dominant tube to be seen with lots of coils nearby.

Histology of sweat gland showing stratified cuboidal epithelium
Coiled Tubular Glands – Histology of eccrine sweat gland of the scalp showing coiled tubular glands with stratified cuboidal epithelium

Simple Acinar & Branched Acinar Glands

Sometimes, a tube might not always be the best shape. If you need a small, round sac to produce and store a product, you form an acinus (which means “berry” in Latin).

  • A simple acinar gland is just a single, berry-like sac connected to a duct. You can find tiny versions of these in your urethra.
  • A simple branched acinar gland is a cluster of these berries (acini) all connected to the same single duct. The best example is your sebaceous (oil) glands. Multiple little sacs filled with oily cells all drain into one short duct that leads to a hair follicle, keeping your skin and hair lubricated.
A microscopic cross-section of thin skin from the scalp, stained with H&E. A large, lobulated sebaceous gland is prominent. The periphery of each lobule contains small, darkly stained basal cells. The center contains larger, mature sebocytes with abundant clear, vacuolated cytoplasm, indicating accumulated lipids. The gland empties into the outer root sheath of a hair follicle. The surrounding dermis is composed of dense, pink collagen fibers.
H&E Stained Sebaceous Gland in Scalp Skin – A microscopic cross-section of thin skin from the scalp, stained with H&E. A large, lobulated sebaceous gland is prominent. The periphery of each lobule contains small, darkly stained basal cells. The center contains larger, mature sebocytes with abundant clear, vacuolated cytoplasm, indicating accumulated lipids. The gland empties into the outer root sheath of a hair follicle. The surrounding dermis is composed of dense, pink collagen fibers. (Slide ID: Path 304 003, Image ID: 1521 – Thin Skin from Scalp)

Compound Tubular Glands

Imagine a tree with many small branches, and at the end of each tiny twig, there is not a leaf but a straight, narrow tube. This is essentially the structure of a compound tubular gland. These glands feature an elaborate, branching duct system where even the smallest ducts end in tubular secretory units. It is like having a sophisticated pipeline network where the product is both manufactured and transported through an increasingly complex system of tubes.

A perfect anatomical example of this can be found in the Brunner’s glands of the duodenum. These small glands are located in the submucosa of the first part of the small intestine and secrete an alkaline mucus rich in bicarbonate. This secretion helps neutralize the acidic chyme arriving from the stomach, protecting the intestinal lining and creating an optimal environment for enzymatic digestion. The compound tubular architecture allows numerous tubular secretory units to operate simultaneously, all feeding into a branched duct system that efficiently channels the mucus into the intestinal lumen.

Microscopic view of the duodenum showing Brunner's glands in the submucosa. The glands are composed of tightly packed, pale-staining acinar cells with a tubular structure. A duct is visible draining into the base of a crypt of Lieberkühn. Stained with Hematoxylin and Eosin (H&E).
H&E Stained Brunner’s Glands in the Duodenum – Microscopic view of the duodenum showing Brunner’s glands in the submucosa. The glands are composed of tightly packed, pale-staining acinar cells with a tubular structure, as indicated by the arrow. (Slide ID: Path 304 012a, Image ID: 1534 – Duodenum)

Compound Acinar Glands

Now picture that same branching tree, but instead of tubes at the end of the twigs, imagine clusters of berries. That’s the structure of compound acinar glands. These glands feature an extensive branching duct system that terminates in numerous spherical, berry-like sacs called acini. Each acinus is a tiny factory where secretions are synthesized and promptly released into the duct system for delivery.

The exocrine pancreas is a textbook example of a compound acinar gland. Each pancreatic acinus consists of pyramidal-shaped serous cells that produce digestive enzymes. These enzymes are secreted into an intricate duct system, beginning with intercalated ducts, moving into intralobular and interlobular ducts, and eventually merging into the main pancreatic duct. This complex arrangement allows the pancreas to safely and efficiently transport large quantities of digestive enzymes to the small intestine.

A microscopic section of the pancreas stained with H&E. The predominant exocrine tissue consists of numerous, small, berry-like acini. Each acinus is made of pyramidal serous cells with intensely purple-stained (basophilic) basal regions due to abundant rough endoplasmic reticulum and pink-stained (eosinophilic) apical regions containing digestive enzyme granules. Several pale-staining, spherical clusters of endocrine cells (Islets of Langerhans) are scattered throughout. Small, thin-walled intercalated ducts are present within the acinar tissue.
H&E Stained Compound Acinar Glands of Exocrine Pancreas – The exocrine pancreas is organized into lobules of acini, which secrete digestive enzymes into a branched duct system. The cellular polarity of the acinar cells is a key feature: the strong basophilia of the basal cytoplasm reflects intense protein synthesis machinery, while the eosinophilic apical cytoplasm is filled with secretory (zymogen) granules. The presence of a large, pale islet of Langerhans highlights the dual exocrine and endocrine nature of the pancreas. (Slide ID: Path 304 016, Image ID: 1542 – Pancreas)

Compound Tubuloacinar Glands

Some glands need to produce different types of secretions, and for this purpose, evolution created the compound tubuloacinar design—one of the most sophisticated glandular architectures. These glands have a branching duct system that ends in a mix of tubular and acinar secretory units. Think of it as a hybrid production facility, with both tube-like factories and berry-shaped factories, all linked to a shared delivery network.

The parotid salivary gland, the largest salivary gland, located just in front of your ears, is a classic example of this structural design. Although it is composed almost entirely of serous acini, its secretory architecture still classifies it as tubuloacinar due to the tubular components involved in early duct formation and branching. The serous cells produce a watery, enzyme-rich saliva packed with amylase, crucial for breaking down starches in the mouth. This secretion drains through an intricately branched duct system, ensuring rapid delivery into the oral cavity, especially when eating or smelling food.

Histology of the parotid salivary gland, a pure serous, compound tubuloacinar gland. The tissue is densely packed with serous acini, composed of pyramidal cells with basal basophilia and apical zymogen granules. Prominent interlobular ducts with a stratified cuboidal epithelium and occasional adipose tissue cells are visible. Stained with Hematoxylin and Eosin (H&E).
H&E Stained Compound Tubuloacinar Serous Gland in Parotid Gland – The parotid gland is a classic example of a compound tubuloacinar gland with exclusively serous secretion. The secretory acini are composed of pyramidal cells with round, basal nuclei. The intense purple staining at the cell base indicates high protein synthesis activity for producing salivary amylase. The duct system is well-developed, with visible striated ducts (not easily resolved at this magnification) responsible for modifying the saliva. (Slide ID: Path 304 010, Image ID: 1531 – Parotid Gland)

Endocrine Glands

Now that we have solid understanding of exocrine glands, let’s switch gears and take a look at endocrine glands! There are three main structural classes based on the glandular morphology.

Insular Pattern

The insular pattern is characterized by discrete, rounded clusters or “islands” of endocrine cells embedded within a supporting framework of connective tissue. These cellular islands are not isolated, but are thoroughly infiltrated by an extensive and delicate network of fenestrated capillaries, which creates an immense surface area for the efficient transfer of hormones from the secretory cells into the circulatory system. This structure functions like a series of specialized manufacturing plants, each with direct access to a highway system for immediate product distribution. The prime example of this pattern is the pancreatic islets (islets of Langerhans), which appear as pale-staining, spherical clusters scattered like islands in a sea of darker exocrine tissue. Within each islet, different cell types, such as insulin-producing beta cells and glucagon-producing alpha cells, are intermingled, all contributing to the precise regulation of blood glucose.

Histology of the pancreas showing a prominent pancreatic islet (Islet of Langerhans). The islet appears as a large, pale-staining, lightly basophilic cluster of cells, richly vascularized, and surrounded by darker, densely packed exocrine acinar tissue. Stained with Hematoxylin and Eosin (H&E).
H&E Stained Pancreatic Islet of Langerhans – A large islet is visible, characterized by its pale staining appearance compared to the surrounding exocrine acini. The islet contains polygonal endocrine cells with central nuclei, organized into cords and clusters surrounded by a dense network of capillaries. This vascular network allows for the direct secretion of hormones like insulin and glucagon into the bloodstream. (Slide ID: Path 304 016, Image ID: 1542 – Pancreas)

Trabecular Pattern

In the trabecular pattern, endocrine cells are organized into long, branching, and often interconnecting cords or ribbons that are typically only one or two cells thick. These linear cords are separated by parallel rows of fenestrated capillaries, ensuring that every single secretory cell is in close proximity to the blood supply. This architectural design prioritizes rapid and direct secretion, as hormones can be released and absorbed almost instantly without the need for a storage phase. The parathyroid glands are a classic example of this pattern, with dense, winding cords of chief cells responsible for producing parathyroid hormone. This structure is perfectly suited for the gland’s role in minute-to-minute calcium homeostasis that requires swift hormone release into the circulation. Similarly, the adrenal cortex also shows a cord-like arrangement in its zones, with long, straight columns of steroid-producing cells aligned alongside extensive capillary networks, which help facilitate the immediate release of hormones like cortisol and aldosterone.

Histology of the parathyroid gland showing characteristic cellular cords and nests. The cells are organized in trabeculae separated by thin capillaries and delicate fibrous septa. Dense adipose tissue surrounds the gland. Stained with Hematoxylin and Eosin (H&E).
H&E Stained Trabecular Pattern of Parathyroid Gland – The cells are organized in trabeculae separated by thin capillaries and delicate fibrous septa. Dense adipose tissue surrounds the gland. (Slide ID: Path 304 058a, Image ID: 7847 – Parathyroid Gland)

Follicular Pattern

The follicular pattern presents a unique and highly distinctive structure defined by spherical units called follicles. Each follicle consists of a single layer of secretory cells arranged in a circle, creating a central lumen that is filled with a stored precursor of the hormone known as colloid. This design is fundamentally different from other patterns, as it is built for storage rather than immediate release. The follicular cells synthesize a glycoprotein called thyroglobulin and secrete it into the colloid reservoir, where it is stored until needed. Upon stimulation, the cells reclaim the thyroglobulin, convert it into the active thyroid hormones (T3 and T4), and then secrete them into the capillaries on the follicle’s outer surface. The thyroid gland is a great example of this pattern, as it is composed entirely of countless follicles of varying sizes. The appearance of the follicles can even indicate the gland’s activity level: tall follicular cells and pale, scalloped colloid suggest active hormone production, while flat cells and dense, pink colloid indicate more of a resting state.

Histology of the thyroid gland showing spherical thyroid follicles of varying sizes. Each follicle is lined by a simple cuboidal epithelium and filled with a homogeneous, pale pink colloid. Parafollicular C-cells are present but not easily distinguishable with H&E staining. The tissue is highly vascularized with capillaries in the interfollicular stroma. Stained with Hematoxylin and Eosin (H&E).
H&E Stained Follicular Pattern of Thyroid Gland – Each follicle is lined by a simple cuboidal epithelium and filled with a homogeneous, pale pink colloid. Parafollicular C-cells are present but might not be easily distinguishable with H&E staining. The tissue is highly vascularized with capillaries in the interfollicular stroma. (Slide ID: Path 304 059, Image ID: 7848 – Thyroid Gland)

Key Takeaways

  • Exocrine glands use ducts to transport their secretions to an epithelial surface.
  • Endocrine glands are ductless and release their hormones directly into the bloodstream.
  • The structural classification of exocrine glands depends on their duct branching pattern and the shape of their secretory unit.
  • The structural classification of endocrine glands is based on their cellular arrangement, such as insular, follicular, or trabecular.
  • A key functional difference is that exocrine secretions, like enzymes, have a local effect, while endocrine hormones have a systemic effect.
  • The pancreas is a mixed gland, as it contains both exocrine acini that secrete digestive enzymes and endocrine islets that secrete hormones.

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Pathology Copyright © 2022 by Jennifer Kong, Zoe Soon, and Helen Dyck is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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