Epithelial Tissue

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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. Epithelial tissue structure

⭐ 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.

Types of Epithelium

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.
      • Ultrastructure of Tight Junction in Epithelial Cells
    • 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. Glandular Secretion Mechanisms

⭐ 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. Apical Specializations of Epithelial Cells

⭐ 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).

Practice Questions: Epithelial Tissue

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Microscopically, epimyoepithelial islands are typically seen in:

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Flashcards: Epithelial Tissue

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What type of glands are salivary glands, glands of resp. passages and pancreas (based on duct morphology)?_____

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What type of glands are salivary glands, glands of resp. passages and pancreas (based on duct morphology)?_____

compund tubuloalveolar

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