Lecture 5: Glands
Glands: An Overview
Epithelial Cell Modifications
Epithelial cells lining tubes are specialized to perform various functions, including:
Protection: e.g., epidermis
Absorption: e.g., small intestine
Stretch: e.g., ureter and urinary bladder
Transport (Conveyance): e.g., small ducts
Allow Diffusion: e.g., alveoli of lung
Definition of a Glandular Epithelial Cell
A glandular epithelial cell is specialized in the synthesis (manufacture) and secretion (release) of a product. These cells arrange themselves to form glands. Some specialize to become the secretory cells, while others line the ducts that transport the secretion away.
Definition of Secretion and Excretion
Secretion: The process by which a glandular epithelial cell, or any other secretory cell, produces a specific product and discharges it into either a duct or a blood capillary. This release can be:
Exocrine/External: Product released into an epithelial duct system.
Endocrine/Internal: Product released directly into blood capillaries.
Excretion: The discharge (elimination) of waste matter from the body, such as urine, feces, and sweat.
Definition of a Duct
A duct is a passage or a canal with well-defined walls, specifically a tube for the passage of secretions or excretions.
Gland Classification based on Product Release
Exocrine Glands: Secrete products through epithelial ducts that connect to a surface. Examples include sweat glands, salivary glands, liver (bile), and pancreas (digestive enzymes).
These specialized epithelial cells elaborate and release a secretory product into a duct system. The point of origin of the gland from surface or tubular epithelium is where the secretion will ultimately enter.
Endocrine Glands: Lack a duct system. Products are secreted into connective tissue, from which they enter the bloodstream to reach target cells. Examples include hormones.
These cells are typically surrounded by blood capillaries, allowing direct secretion.
Gland Development: How Glands Arise
Glands originate from epithelial cells covering the body or lining tubes within the body.
From stratified squamous keratinized epithelial cells (epidermis): Give rise to skin glands such as mammary glands and sweat glands.
From epithelial cells lining tubes within the body: Give rise to glands like salivary glands, gastric glands, intestinal glands, the liver, and the exocrine pancreas.
Development Process (Formation of Duct and Secretory Units):
Initial Growth: A solid cord (clump) of surface or lining epithelial cells grows into the underlying vascular loose connective tissue.
Branching: These cords profusely branch, each ending with an enlarged (bulbous) end.
Canalization (Duct Formation): Nature deliberately causes the programmed death (apoptosis) of the innermost cells of the cord and its enlarged end. This results in the formation of a central canal (lumen).
Apoptosis: Programmed or regulated cell death.
Necrosis: Unwelcome cell death resulting from injury or lack of blood supply.
The cords become ducts, and their dilated ends become the secretory units.
Specialization: Epithelial cells lining the duct system primarily have a conveyance (transporting) function, not secretory. Epithelial cells in the dilated, blind end of each cord become columnar to pyramidal in shape and specialize to manufacture and secrete a product.
These clumps of secretory cells are called adenomeres (also known as secretory units, secretory acini, or the secretory part of the gland). The term "Acinus" is Latin for grape or berry, indicating a grape-like shape for the secretory unit.
Fate of Adenomeres without Ducts (Endocrine Gland Formation):
If the duct system of a gland disappears, the adenomere cells persist as clumps of epithelial cells. These cells then secrete their product directly into the surrounding blood capillaries within the vascular loose connective tissue, forming an endocrine gland.
Example: Pancreatic hepatocytes can act as both exocrine cells (secreting bile into bile ducts) and endocrine cells (secreting glucose, blood proteins into blood sinusoids).
Classifying Glands
1. By the Number of Secretory Cells
Unicellular Gland: A single secretory cell, e.g., a goblet cell in the small intestine or trachea, which synthesizes and secretes mucus.
Multicellular Gland: Composed of hundreds to millions of secretory cells. Most glands (salivary glands, liver, pancreas, mammary glands) are multicellular.
2. By the Presence or Absence of a Duct System
Exocrine Gland: Exhibits a duct system into which the secretory product passes from the glandular epithelial cells of adenomeres (e.g., salivary glands, liver, pancreas, mammary glands).
Endocrine Gland: Has no duct system; the secretory product passes into intercellular spaces and then into blood capillaries to be transported elsewhere.
Pancreas Example: of the pancreas is an exocrine gland, secreting pancreatic juice into the small intestine. However, small islands of adenomeres (Islets of Langerhans), which have lost their duct systems, function as endocrine glands. These islets secrete hormones like insulin (decreasing blood glucose) and glucagon (increasing blood glucose) directly into nearby blood capillaries.
3. By the Complexity of the Duct System
Simple: Duct system is unbranched (e.g., sweat glands).
Compound: Duct system is branched (e.g., salivary glands, liver, pancreas, mammary glands).
4. By the Shape of the Secretory Units (Adenomeres)
Tubular Gland: All adenomeres have a tubular shape.
Alveolar (Acinar) Gland: All adenomeres have an alveolar or grape-like shape.
Tubulo-alveolar Gland: Adenomeres exhibit both tubular and alveolar shapes.
The exact shape can vary widely in densely packed glands, making precise identification challenging.
5. By the Method of Secretion of Glandular Epithelial Cells
Merocrine (Eccrine) Secretion: The most common mechanism (exocytosis). The secretory product is synthesized, packaged into vesicles by the Golgi apparatus, and released from the cell without any loss of cytoplasm. The cell structure remains intact.
Mechanism: Secretory vesicles fuse with the apical cell membrane, releasing contents. The vesicle membrane is re-incorporated into the cell membrane. "The secretory product is thrown out – the dustbin is brought back into the house."
Examples: Saliva, pancreatic juice, and the protein component of milk.
Apocrine Secretion: A small portion of the apical cytoplasm, along with the secretory product, is pinched off and released. The cell loses some cytoplasm but does not die and recovers to repeat the process.
Mechanism: Lipid droplets coalesce and move to the apical region. They evaginate the cell membrane and a thin layer of cytoplasm, forming a bulge that is then pinched off.
Examples: The lipid (fat) component of milk (mammary gland).
Holocrine Secretion: The entire secretory cell becomes the secretory product and is discharged, leading to the death of the cell. The lost cells are replaced by mitotic activity of basal cells.
Mechanism: Basal (germinal) cells divide, and daughter cells move inwards. They synthesize and accumulate a lipid material (e.g., sebum) within their cytoplasm. This accumulation fills and kills the cells, which then constitute the oily secretion.
Examples: Sebaceous (oil) glands of the skin (secreting sebum into hair follicles).
6. By the Type of Secretion
Mucous Gland: Secretes mucus, a slimy, semi-liquid, sticky saliva for lubrication and protection.
Serous Gland: Secretes watery saliva resembling serum (clear, protein-rich fluid).
Mixed Gland: Secretes a saliva containing both serous and mucous elements.
Histological Structure of Salivary Glands
Salivary glands are compound, tubulo-alveolar, exocrine glands that secrete saliva into the oral cavity.
General Structure
Lobation: Glands are anatomically (macroscopically) divided into lobes by obvious macroscopic spaces or fissures.
Lobules: Microscopically, lobes are further divided into lobules by connective tissue septae (partitions) derived from the gland's outer capsule.
During histological processing, lobules may appear separated by clear, obvious spaces, which is a processing artifact useful for identification.
Connective Tissue Capsule: A dense connective tissue on the outside of the gland provides protection.
Connective Tissue Septa (Partitions): Extend inwards from the capsule, dividing the gland into lobes and lobules. These septa also facilitate the passage of ducts and blood vessels.
The bulk of a salivary gland is made of parenchyma (functional, secretory tissue: adenomeres and ducts), supported by stroma (connective tissue, blood vessels, nerves).
Duct System Hierarchy (from Adenomere to Oral Cavity)
The duct system typically exhibits dichotomous branching, becoming wider as it progresses towards the main duct.
Intercalated Ducts:
Location: Initial, tiniest, most narrow ducts emerging directly from adenomeres, within the lobule (intralobular).
Epithelium: Simple cuboidal epithelium.
Staining: Often difficult to identify due to small size and lack of distinctive features.
Function: Drains secretion from adenomeres.
Striated Ducts:
Location: Wider continuations of intercalated ducts, found within the lobules (intralobular).
Epithelium: Simple columnar epithelium.
Appearance: "Doughnut" shaped in cross-section, reddish (eosinophilic) due to high mitochondrial content. Exhibit radial striations at the basal part of the cells.
Striations Explained: These are infoldings of the basal cell membrane, increasing the surface area for active transport of ions. Numerous mitochondria are present between these infoldings to provide energy for transport.
Function: Actively modify saliva by reabsorbing sodium ions () and secreting potassium ions ().
Interlobular Ducts:
Location: Run between lobules, embedded in the connective tissue septa.
Epithelium: Generally stratified cuboidal or stratified columnar epithelium, sometimes simple columnar. They increase in diameter closer to the main ducts.
Appearance: Significantly larger in diameter than intralobular ducts. Easily identified in the connective tissue between lobules.
Lobar Ducts:
Location: Drain a gland lobe, found outside the lobules.
Epithelium: Similar to interlobular ducts (stratified cuboidal or columnar, or pseudostratified columnar).
Main (Primary/Excretory) Duct:
Location: Drains the entire gland, typically found outside the gland structure, within the connective tissue capsule.
Epithelium: May be simple columnar, pseudostratified columnar, or stratified cuboidal epithelium.
Function: Empties the final secretion (e.g., saliva) onto the epithelial surface of the oral cavity. In other glands (liver, pancreas), it empties into the small intestine.
Myoepithelial Cells (Basket Cells)
Location: Contractile epithelial cells surrounding the adenomeres (and sometimes intercalated ducts), typically found between the secretory cells' base and the basal lamina. Their cytoplasmic processes pass between the cells lining the adenomere.
Function: Resemble smooth muscle cells in their ability to contract. They assist in the rapid expulsion of glandular secretions (e.g., saliva from salivary glands, tears from lacrimal glands, milk from mammary glands, sweat from sweat glands).
Pancreas lacks myoepithelial cells, as rapid expulsion of pancreatic juice into the duodenum is not critical.
Types of Salivary Glands (Based on Adenomere Structure)
1. Mucous Glands (e.g., Sublingual Gland)
Characteristic Microscopic Features (Mucous Adenomeres):
Appearance: Majority of the lobule consists of mucous adenomeres. Due to processing and the nature of mucinogen, they appear pale-staining and "ghost-like."
Cytoplasm: Appears empty or lightly stained because mucinogen granules (precursors to mucus, rich in carbohydrates) dissolve during processing and do not readily take up H&E stain (light pink/pale).
Nuclei: Flattened and squashed against the basal part of the epithelial cells due to the accumulation of mucus within the cell (appear as black dots).
Shape: Often puffy or distended.
Lumen: A tiny, often indistinct, central circle.
Secretion Product: Mucus (slimy, sticky, for lubrication and protection).
Examples: Sublingual gland, also found in oesophagus, tonsil, duodenum, epiglottis, soft palate (minor mucous salivary glands).
2. Serous Glands (e.g., Pancreas, Parotid Gland)
Characteristic Microscopic Features (Serous Adenomeres):
Appearance: Take on an overall red/purple color because proteins (enzymes) readily take up H&E stain. No "ghost-like" appearance.
Cytoplasm: The basal part of the cells appears darkly stained (basophilic) due to an abundance of rough endoplasmic reticulum (RER) and ribosomes, which are involved in protein synthesis (affinity for hematoxylin).
The apical region contains red-stained (eosinophilic) zymogen granules (secretory granules containing proteins), giving it a pink color (affinity for eosin).
Nuclei: Round or circular, located close to the cell base, but not squashed or flattened.
Shape: Pyramidal in shape, with a wide base facing the basal lamina and a narrow apical surface facing the lumen. Arranged in an acinus.
Lumen: Often visible.
Secretion Product: Watery saliva containing proteins (enzymes).
Examples: Pancreas (exocrine portion secretes pancreatic juice with digestive enzymes), Parotid salivary gland.
3. Mixed Glands (e.g., Submandibular Gland)
Characteristic Microscopic Features (Mixed Adenomeres):
Each adenomere exhibits a central, pale-staining mucous secretory unit.
This is typically capped by a half-moon (ear) shaped serous secretory unit at the periphery, known as a serous demilune.
The serous secretion from the demilune must pass between the mucous cells to reach the lumen of the adenomere.
Secretion Product: Contains both mucous and serous elements.
Example: Submandibular gland.
Analogy: Salivary Gland and a Tree
Main Duct: Analogous to the single, wide trunk at the base of the tree.
Branching Duct System: Analogous to the network of tubular, wooden branches (secondary, tertiary, etc., getting smaller) that arise from the trunk. This represents the compound duct portion of the gland.
Intercalated Ducts: Analogous to the tiniest, terminal branches of the tree.
Adenomeres: Analogous to the leaves that sprout from the tiniest wooden tubes (intercalated ducts) in summer. Just as a tree is mainly composed of leaves, a salivary gland is mainly composed of adenomeres.
Different Adenomeres: Just as different trees have different leaves (e.g., chestnut vs. oak), different salivary glands have different adenomeres (mucous, serous, mixed).