Epithelium (3) and BM

📌 Glandular Epithelium

· Specialized type of epithelium modified to form glands and produce secretions.
· Formation: Down-growth of surface epithelium into underlying connective tissue, remaining separated by basal lamina.
· Development:

• Proliferation and downgrowth of epithelial cells.
• Exocrine glands retain duct.
• Endocrine glands lose duct and form secretory or follicular structures. (Exam Point)

🖇 Classification of Glands

📌 Based on Duct Presence (Exam Point)
· a. Endocrine: Ductless, release hormones directly into blood/tissue.

• Examples: Thyroid, Pituitary, Adrenal gland. (Exam Point)
  · b. Exocrine: With duct system carrying secretion to surface or lumen.
• Example: Salivary glands.
  · c. Mixed: Have both endocrine & exocrine functions.
• Examples: Pancreas, Ovary, Testis. (Exam Point)

📌 Detailed Classification of Exocrine Glands

🟣 1. According to Number of Cells
· Multicellular: Many cells.

• Examples: Pancreas, stomach, sweat, salivary, mammary, sebaceous glands. (Exam Point)
  · Unicellular: Single cell.
• Example: Goblet cell. (Exam Point)

🟠 2. According to Type of Secretion (Exam Point)
· Mucous: Secretes mucus → Goblet cell.
· Serous: Watery enzyme-rich secretion → Parotid gland.
· Mixed: Both mucous & serous → Mixed salivary gland.
· Fatty: Lipids/oils → Sebaceous glands.
· Watery: Watery fluid → Sweat glands.
· Cellular secretion: Entire cells released → Ovary, Testis.
· Milky secretion: Milk → Mammary gland.
· Wax secretion: Wax → External ear glands.

🟣 3. According to Mode of Secretion (Exam Point)
· Merocrine: By exocytosis, cell remains intact → Parotid gland.
· Holocrine: Whole cell disintegrates and releases → Sebum in sebaceous gland. (Exam Point)
· Apocrine: Apical cytoplasm pinched off → Mammary gland. (Exam Point)

🟠 4. According to Function
· Secretory: Useful substances → Salivary glands.
· Excretory: Waste products → Kidney, Sweat gland.

🟣 5. According to Branching of Duct
· Simple: Single non-branching duct.
· Simple branched: Single duct, branched secretory part.
· Compound: Branching duct system.

🟠 6. According to Shape of Secretory Part
· Tubular: Tube-shaped.
· Alveolar/Acinar: Rounded or sac-like.
· Tubulo-Alveolar: Flask-shaped (mix of both).

📌 Combined Structural Classification (Exam Point)

🖇 I. Tubular Glands
· Simple tubular: Intestinal gland.
· Simple coiled tubular: Sweat gland.
· Simple branched tubular: Fundic gland (stomach).
· Compound tubular: Liver, kidney. (Exam Point)

🖇 II. Alveolar Glands
· Simple alveolar: Sebaceous gland.
· Simple branched alveolar: Sebaceous gland.
· Compound alveolar: Mammary gland. (Exam Point)

🖇 III. Tubulo-Alveolar Glands
· Simple tubulo-alveolar: Not found in humans.
· Simple branched tubulo-alveolar: Minor salivary gland.
· Compound tubulo-alveolar: Major salivary gland (parotid), pancreas, prostate. (Exam Point)

📌 A. Neuro-Epithelium

· Cells act as sensory receptors for special stimuli.
· Composed of: Sensory, supporting, and basal cells.
· Structure: Free surface bears small hairs; base connected with sensory nerves. (Exam Point)

🖇 Examples: (Exam Point)

1. Lingual taste buds – taste.
2. Internal ear:
3. • Organ of Corti – hearing.
   • Cristae ampullares – equilibrium.
   • Macula utriculi & Macula sacculi – equilibrium.
3. Olfactory epithelium – smell.

📌 B. Myo-Epithelium

· Modified epithelial cells capable of contraction.
· Site: Between glandular cells & basal lamina.
· Function: Contraction helps expel secretions. (Exam Point)
· Found in: Sweat, mammary, and salivary glands. (Exam Point)

🖇 Basal Cell Surface of Epithelia

📌 1. Basement Membrane

· Definition: The basal surface of all epithelia rests on a thin, extracellular, felt-like sheet of macromolecules known as the basement membrane. (Exam Point)

🟠 Light Microscopy (LM) Appearance

· Basement membrane cannot be seen using the routine Hematoxylin and Eosin (H&E) stain. (Exam Point)
· It is best visualized using special stains:

• Periodic acid–Schiff (PAS) → stains basement membrane red.
• Silver stain → stains basement membrane brown. (Exam Point)

🟣 Electron Microscopy (EM) Ultrastructure

The basement membrane is composed of two distinct layers:

1. Basal Lamina → nearest to epithelial cells. (Exam Point)
2. • Subdivided into:
     · Lamina Densa: Electron-dense layer containing glycoproteins and Type IV collagen. (Exam Point)
     · Lamina Lucida: Electron-lucent (clear) layer adjacent to the epithelial cell membrane; also contains Type IV collagen, more densely packed here. (Exam Point)
2. Reticular Lamina (Fibroreticular Lamina) → thicker layer that merges into surrounding connective tissue. (Exam Point)
3. • Composed of Type III collagen and Type VII collagen (anchoring fibrils). (Exam Point)

📌 Functions of the Basement Membrane

1. Provides adhesion between epithelial cells (or parenchyma) and underlying connective tissue (via collagen fibers). (Exam Point)
2. Acts as a barrier to the diffusion of molecules. (Exam Point)
3. May influence nerve regeneration after injury and assist in the re-establishment of neuromuscular junctions. (Exam Point)

📌 2. Basal Infoldings (Basal Plications)

· Location: Found in ion-transporting epithelia, e.g.:

• Distal convoluted tubule of kidney
• Striated ducts of salivary glands (Exam Point)

· Structure: Deep invaginations of the basal plasma membrane that compartmentalize mitochondria. (Exam Point)

· Function:

• Increase surface area for transport.
• Bring ion pumps in the plasma membrane close to mitochondria, ensuring immediate ATP supply for active transport. (Exam Point)

📌 3. Hemidesmosomes

· Definition: Specialized anchoring junctions on the basal epithelial surface, attaching cells to the basal lamina. (Exam Point)
· Ultrastructure: Resemble half a desmosome; visible under Transmission Electron Microscopy (TEM). (Exam Point)
· Function: Provide strong adhesion of epithelial cells to the underlying extracellular matrix (basal lamina). (Exam Point)