Microscopic Anatomy of Epithelial Tissues

Epithelial Tissue Basics - Cellular Linings Unveiled

• Sheets of contiguous cells; cover body surfaces, line cavities, form glands.
• Key Characteristics:
  • Cellularity: Predominantly cells, minimal extracellular matrix.
  • Specialized Contacts: Intercellular junctions (e.g., desmosomes, tight junctions).
  • Polarity: Distinct apical (luminal) and basal (abluminal) surfaces.
  • Support: Rests on a basement membrane, anchored to connective tissue.
  • Avascular: Lacks blood vessels; innervated.
  • Regeneration: High mitotic activity for repair.
• Primary Functions: Protection, secretion, absorption, excretion, filtration, diffusion, sensory reception.

⭐ Epithelium is avascular and receives nutrients by diffusion from the underlying connective tissue, which is highly vascular.

Classifying Epithelia - Shape & Layer Showdown

• Categorized by:

  • Cell Shape:
    • Squamous (flattened)
    • Cuboidal (cube-shaped)
    • Columnar (tall, rectangular)
  • Number of Layers:
    • Simple (single layer)
    • Stratified (multiple layers)
    • Pseudostratified (single layer, nuclei at different levels, all cells on basement membrane)

• Common Examples:

  • Simple Squamous: Lining blood vessels (endothelium), body cavities (mesothelium), alveoli.
  • Simple Cuboidal: Kidney tubules, thyroid follicles, gland ducts.
  • Simple Columnar: Stomach, intestines (often with microvilli/cilia).
  • Pseudostratified Columnar: Trachea, bronchi (typically ciliated).
  • Stratified Squamous: Epidermis (keratinized), esophagus, vagina (non-keratinized).
  • Stratified Cuboidal/Columnar: Ducts of sweat, salivary glands (rare).

⭐ Transitional epithelium (urothelium), found lining the urinary bladder, ureters, and renal pelvis, is specialized for distension and is impermeable to urine.

Cell Junctions & Surfaces - Sticking & Specializing

• Zonula Occludens (Tight Junctions): Seals paracellular pathway.
  • Proteins: Claudins, Occludins.
  • Function: Regulates solute passage; barrier.
• Zonula Adherens (Adherens Junctions): Links actin cytoskeletons.
  • Proteins: Cadherins (E-cadherin), Catenins.
  • Function: Mechanical adhesion.
• Macula Adherens (Desmosomes): Links intermediate filaments.
  • Proteins: Desmogleins, Desmocollins (Cadherin family); Plakins.
  • Function: Strong spot adhesion; resists shearing forces.
• Gap Junctions: Direct cell-to-cell communication.
  • Proteins: Connexins (form connexons).
  • Function: Passage of ions, small molecules.

• Apical Surface Specializations:
  • Microvilli: ↑ Surface area for absorption (e.g., small intestine, kidney tubules). Core: Actin filaments.
  • Cilia: Motile; move substances (e.g., respiratory tract, fallopian tube). Core: Microtubules (9+2 axoneme).
  • Stereocilia: Long, non-motile microvilli; absorption/sensory (e.g., epididymis, inner ear).

⭐ Pemphigus vulgaris is an autoimmune disease targeting desmogleins (components of desmosomes), leading to acantholysis and intraepidermal blistering.

• Basal Surface Specializations:
  • Basal Lamina: Attachment, filtration. Components: Laminin, Type IV collagen.
  • Hemidesmosomes: Anchor basal cells to basal lamina. Proteins: Integrins, Plectin, BPAG1e (BP230).

Glandular Epithelium - Secretory Superstars

Specialized for secretion. Two main types: Exocrine (ducts) & Endocrine (ductless, hormones).

• Exocrine Glands:
  • Secretion Mechanisms:
    • Merocrine (Eccrine): Exocytosis (e.g., salivary, pancreas). Most common.
    • Apocrine: Apical cytoplasm lost (e.g., mammary glands).
    • Holocrine: Whole cell lysis (e.g., sebaceous glands). 📌 Holo = Whole.

      ⭐ Holocrine glands (e.g., sebaceous glands of the skin) release their secretions by complete cell lysis, where the entire cell disintegrates to release its product.

  • Structure: Classified by duct (simple/compound) & secretory unit (tubular/acinar).
• Cellularity:
  • Unicellular: e.g., Goblet cells (mucus).
  • Multicellular: e.g., Salivary glands.

High‑Yield Points - ⚡ Biggest Takeaways

• All epithelia rest on a basement membrane (basal & reticular laminae).
• Key junctions: zonula occludens (seals), desmosomes (strong adhesion), gap junctions (communication).
• Apical specializations: microvilli (absorption), cilia (motility), stereocilia (sensory).
• Endothelium (simple squamous) lines vessels; mesothelium lines serous cavities.
• Pseudostratified ciliated columnar epithelium is characteristic of the upper respiratory tract.
• Transitional epithelium (urothelium) lines the urinary bladder, allowing distension.
• Stratified squamous epithelium provides protection; keratinized (epidermis) or non-keratinized (oral cavity).