Epithelial Tissue

Epithelial Basics - Body's First Line

• Sheets of tightly packed cells; line body surfaces, cavities, and form glands.
• Key Characteristics:
  • Avascular (nourished by diffusion).
  • Polarity (apical, basal, lateral domains).
  • Supported by a basement membrane.
  • High regenerative capacity.
  • Cellularity (minimal extracellular matrix).
• Major Functions: Protection, secretion, absorption, excretion, filtration, diffusion, sensory reception.

⭐ All true glands (exocrine and endocrine) are derived from epithelial tissue during development.

Classy Cells - Shape Shifters

Epithelial cells classified by shape & layering, vital for function.

• Cell Shapes:
  • Squamous: Flat; diffusion (alveoli, endothelium).
  • Cuboidal: Cube-shaped; secretion/absorption (kidney tubules, glands).
  • Columnar: Tall; secretion/absorption (GI tract; microvilli/cilia).
• Layering:
  • Simple: Single layer, all cells on basement membrane.
  • Stratified: Multiple layers; protection (skin). Named by apical cell shape.
  • Pseudostratified: Appears layered, all cells on basement membrane (respiratory tract).
• Transitional (Urothelium): "Shape shifter". Lines urinary system. Adapts to distension: cuboidal (relaxed) ↔ squamous (stretched). 📌 Apical umbrella cells.

⭐ Found exclusively in the urinary system, urothelium's distensibility prevents urine leakage and protects underlying tissues.

Sticky Situations - Cellular Glue

• Epithelial cells connect via Cell Junctions for cohesion, barrier formation, and communication.
• Junction Types:
  • Zonula Occludens (Tight Junctions):
    • Apical; seals paracellular pathway.
    • Proteins: Claudins, Occludins.
    • 
  • Anchoring Junctions: Mechanical strength.
    • Zonula Adherens: E-cadherin/catenin complex; links actin filaments.
    • Macula Adherens (Desmosomes): Desmogleins/Desmocollins (cadherins); links cytokeratin (intermediate filaments).

      ⭐ Pemphigus vulgaris: Autoantibodies to Desmoglein 3 disrupt desmosomes, causing intraepidermal blisters.

    • Hemidesmosomes: Integrins; anchors cytokeratin to basal lamina.
  • Gap Junctions (Communicating):
    • Connexons (formed by Connexins) allow direct passage of ions & small molecules.

Gland Central - Secretion Masters

• Epithelial cells specialized for secretion. Classified by duct presence & secretion mode.
• Ducts:
  • Exocrine: Ducts present (e.g., sweat, salivary).
  • Endocrine: Ductless; hormones into blood (e.g., thyroid).
  • Heterocrine (Mixed): Both (e.g., pancreas).
• Secretion Mode (Exocrine):
  • Merocrine (Eccrine): Exocytosis; no cell loss. E.g., salivary, pancreatic acini, most sweat glands. 📌 Mero=Part.
  • Apocrine: Apical cytoplasm lost with secretion. E.g., mammary glands, axillary sweat glands. 📌 Apo=Apical.
  • Holocrine: Whole cell disintegrates to release product. E.g., sebaceous glands, tarsal glands. 📌 Holo=Whole.

⭐ Goblet cells are unicellular exocrine glands secreting mucin (merocrine).

Surface Stars - Apical Aces

• Microvilli: ↑ surface area for absorption (intestine, kidney). Actin core. Non-motile. Forms brush/striated border.
• Stereocilia: Long microvilli. Absorption (epididymis), sensory (inner ear). Actin core. Non-motile.
• Cilia (Motile): Propel mucus/ova. 9+2 microtubule axoneme; dynein arms. Respiratory, fallopian tubes.
  • Primary Cilia (Non-motile): 9+0 axoneme; sensory.
• Flagella: Cell movement (sperm). 9+2 axoneme.

⭐ Primary Ciliary Dyskinesia (PCD), e.g., Kartagener's syndrome: Dynein arm defect → immotile cilia. Sinusitis, bronchiectasis, situs inversus_._

High‑Yield Points - ⚡ Biggest Takeaways

• Epithelium is avascular, resting on a basement membrane; nourished by diffusion.
• Key cell junctions: Zonula Occludens (tight), Zonula Adherens, Desmosomes, Gap Junctions, Hemidesmosomes.
• Glandular secretion: Merocrine (exocytosis), Apocrine (apical loss), Holocrine (cell lysis).
• Pseudostratified columnar (respiratory tract) and Transitional/Urothelium (urinary tract) are specialized types.
• Metaplasia: Reversible change of one epithelial type to another (e.g., Barrett's esophagus).
• Surface specializations: Microvilli (absorption), Cilia (motility), Stereocilia (non-motile, epididymis).