BIOL 220: Tissues -Epithelial Tissue2.2

BIOL 220 Lecture 2.2: Tissues - Epithelial Tissue

Overview of Tissue Types

  • Four main types of tissues in the human body:
    • Epithelial Tissue: Composed of tightly packed layers of cells.
    • Functions:
      • Lines outer surfaces and internal tracts (e.g., digestive and respiratory tracts).
      • Acts as a barrier, secretes substances, and forms glands.

Characteristics of Epithelial Tissues

  • Cell Arrangement
    • Tightly packed layers of cells.
  • Vascularity
    • Avascular: No blood vessels within epithelial tissue.
  • Surface Exposure
    • One surface exposed to the external environment or to internal bodily fluids.
Functions of Epithelial Tissues
  1. Protection: Protects underlying structures.
  2. Secretion: Secretes substances such as sweat, mucus, and enzymes.
  3. Absorption: Absorbs nutrients and other materials.
  4. Selective Barrier: Forms barriers and controls the passage of substances in and out of the body.
Major Types of Epithelial Tissues
  1. Covering and Lining Epithelium: Covers surfaces and lines cavities.
  2. Glandular Epithelium: Specialized for secretion.

Basement Membrane

  • Definition: A layer of connective tissue not part of epithelial tissue.
  • Function: Provides support and anchors epithelial cells to either the internal or external environment (e.g., the hollow lumen of digestive organs).

Directional Terms for Covering & Lining Epithelium

  • Apical: Toward the external environment.
  • Lateral: Sides of cells.
  • Basal: Toward the basement membrane.

Glands

  • Source: Can develop from covering & lining epithelium.
  • Types:
    • Exocrine Glands: Secrete substances into ducts.
    • Endocrine Glands: Secrete substances directly into capillaries (blood vessels).

Classifications of Covering & Lining Epithelium

  • Classes:
    • Simple: Single layer of cells.
    • Pseudostratified: Appears stratified but consists of a single layer of cells.
    • Stratified: Multiple layers of cells.
Cell Shapes
  • Squamous: Flat cells.
  • Cuboidal: Cube-shaped cells.
  • Columnar: Tall, column-like cells.

Simple Squamous Epithelium

  • Description: Composed of flat epithelial cells.
  • Examples:
    • Blood vessels
    • Alveoli in lungs (location of gas exchange)
    • Lining of heart
    • Bowman’s capsule in the kidneys
  • Function: Thin structure allows for quick exchange of materials.

Simple Cuboidal Epithelium

  • Description: Composed of cube-shaped epithelial cells.
  • Examples:
    • Kidney tubules
    • Surface of ovaries
    • Bronchioles in the lungs
    • Ducts and secretory portions of some glands.

Simple Columnar Epithelium

  • Description: Composed of tall, column-like cells, often containing goblet cells.
  • Goblet Cells: Secrete mucin, a component of mucus.
  • Examples:
    • Digestive tract
    • Uterus
  • Function: Provides protection from debris and infection. May be ciliated (having hair-like projections that move mucus).

Pseudostratified Epithelium

  • Description: Appears stratified; all cells touch the basement membrane, but not all reach the apical surface.
  • Characteristics: Similar to columnar epithelium with potential goblet cells and cilia.
  • Examples: Upper respiratory tract.

Stratified Squamous Epithelium

  • Description: Composed of multiple layers, providing durability.
  • Types:
    • Keratinized: Contains layers of dead, keratinized cells, found in the top layer of skin.
    • Non-keratinized: Composed of all living cells, found in:
    • Tongue
    • Mouth
    • Esophagus
    • Anal canal
    • Vagina
  • Function: Provides defense against pathogens and protection from abrasion.

Stratified Cuboidal Epithelium

  • Description: Composed of multiple layers of cube-shaped cells.
  • Examples:
    • Sweat gland ducts
    • Ovarian follicles
    • Seminiferous tubules (sperm-producing ducts of testes)
  • Characteristic: Typically only a few layers thick, unlike stratified squamous epithelium.

Transitional Epithelium

  • Description: Similar to stratified squamous epithelium but with rounded apical surfaces, allowing for distension (stretching).
  • Function: Resistant to osmotic damage while accommodating variable volumes of fluid.
  • Examples: Urinary tract (including part of the kidney, ureter, bladder, and part of the urethra).