Epithelial Tissue Summary

Types of Tissues

  • Epithelial: Lining and covering
  • Connective: Movement
  • Nervous: Control
  • Muscular: Movement

Epithelial Tissue

  • Lines all external/internal surfaces of the body
  • Functions: Protection, absorption, secretion, ion transport, filtration, forming slippery surfaces.

Characteristics of Epithelial Tissue

  1. Cellularity: Composed of closely packed cells.
  2. Specialized Contacts: Cell junctions holding cells together.
  3. Polarity: Distinct apical (top) and basal (bottom) surfaces.
  4. Support by Connective Tissue: Underlies and supports epithelial tissue.
  5. Avascular and Innervated: No blood vessels; has nerve endings.
  6. Regeneration: High capacity for renewal.

Cell Junctions

  • Tight Junctions: Prevent leakage (Zona Occludens).
  • Adherens Junctions: Anchors cells (Zonula Adherens).
  • Desmosomes: Provide tensile strength (Macula Adherens).
  • Gap Junctions: Allow communication (nexus).

Basement Membrane

  • Noncellular, supports epithelium.
  • Composed of glycoproteins, collagen, and proteoglycans.
  • Functions: Selective filter, scaffolding for regenerating cells.

Types of Epithelial Cells

  1. Simple Squamous: Thin, flat for diffusion (e.g., lining of blood vessels).
  2. Simple Cuboidal: Cube-shaped, for secretion (e.g., kidney tubules).
  3. Simple Columnar: Tall cells, for absorption (e.g., digestive tract).
  4. Stratified Squamous: Multiple layers, for protection (e.g., skin).
  5. Pseudostratified: Appears layered but is not (e.g., respiratory tract).
  6. Transitional: Stretches, found in urinary bladder.

Glandular Epithelium

  • Exocrine Glands: Secrete through ducts (e.g., sweat glands).
  • Endocrine Glands: Release hormones directly into blood (e.g., thyroid gland).
Types of Secretions:
  • Merocrine: Exocytosis (e.g., salivary glands).
  • Apocrine: Part of the cell is lost (e.g., mammary glands).
  • Holocrine: Whole cell is destroyed (e.g., sebaceous gland).