4.2

Overview and Characteristics of Epithelial Tissue

• Definition: A sheet of cells covering body surfaces, lining cavities, and forming glands.

• Types:

  • Covering and lining epithelium: Skin outer layer, lines urogenital, digestive, and respiratory tracts, and covers ventral body cavity organs.

  • Glandular epithelium: Forms the glands of the body.

• Functions: Protection, absorption, filtration, excretion, secretion, and sensory reception.

• Key Characteristics:

  • Polarity: Has an apical surface (exposed to exterior/cavity, often with microvilli or cilia) and a basal surface (attached to basal lamina).

  • Specialized Contacts: Form continuous sheets joined by tight junctions and desmosomes.

  • Connective Tissue Support: Rests on a basement membrane formed by the basal lamina and reticular lamina.

  • Avascular but Innervated: Contains no blood vessels; nourished by diffusion from underlying connective tissue.

  • Regeneration: High capacity for rapid cell division and repair when adequately nourished.

Structural Classification

• By Cell Layers:

  • Simple Epithelia: Single cell layer; ideal for absorption, secretion, and filtration.

  • Stratified Epithelia: Multiple cell layers; provides protection in high-abrasion areas.

• By Cell Shape:

  • Squamous: Flattened, scale-like cells with flattened nuclei.

  • Cuboidal: Box-like cells with spherical nuclei.

  • Columnar: Tall, column-shaped cells with elongated nuclei.

Simple Epithelia Types

• Simple Squamous Epithelium:

  • Function: Diffusion and filtration.

  • Location: Kidney glomeruli, lung air sacs, lining of heart, blood vessels, and lymphatic vessels.

  • Special Types: Endothelium (lines lymphatic vessels and cardiovascular system) and Mesothelium (found in serous membranes).

• Simple Cuboidal Epithelium:

  • Function: Secretion and absorption.

  • Location: Kidney tubules, small gland ducts/secretory units, and ovary surface.

• Simple Columnar Epithelium:

  • Function: Absorption and secretion.

  • Location: Non-ciliated type lines digestive tract and gallbladder; ciliated type lines small bronchi and uterine tubes.

• Pseudostratified Columnar Epithelium:

  • Function: Secretion (particularly mucus) and mucus propulsion via cilia.

  • Location: Ciliated type in trachea and upper respiratory tract; non-ciliated in sperm-carrying ducts and large gland ducts.

Stratified Epithelia Types

• Stratified Squamous Epithelium:

  • Function: Protection against abrasion.

  • Location: Non-keratinized type in esophagus, mouth, and vagina; keratinized type forms epidermis of skin.

• Stratified Cuboidal Epithelium:

  • Function: Protection.

  • Location: Ducts of sweat and mammary glands.

• Stratified Columnar Epithelium:

  • Function: Protection and secretion.

  • Location: Pharynx, male urethra, and lining of some glandular ducts.

• Transitional Epithelium:

  • Function: Stretches readily to allow distension of urinary organs.

  • Location: Lines ureters, urinary bladder, and part of the urethra.

Glandular Epithelia

• Classification by Site of Release:

  • Endocrine Glands: Ductless; secrete hormones directly into extracellular space to enter blood or lymph.

  • Exocrine Glands: Secrete products onto body surfaces or into body cavities via ducts or exocytosis.

• Cellularity of Exocrine Glands:

  • Unicellular: Include mucous cells and goblet cells; produce mucin which dissolves to form mucus.

  • Multicellular: Composed of an epithelium-derived duct and a secretory unit (acinus) supported by connective tissue.

• Modes of Exocrine Secretion:

  • Merocrine: Products are secreted via exocytosis without cell damage (e.g., pancreas, sweat glands, salivary glands).

  • Holocrine: Secretory cells accumulate products until they rupture (e.g., sebaceous glands).

  • Apocrine: Products accumulate beneath apical surface and release by pinching off (e.g., lipid droplet release in lactating mammary glands).