Epithelium Tissue Structure and Function

Epithelium Characteristics

• Composed of closely aggregated polyhedral cells with little extracellular matrix
• Cells cover body surfaces, line body cavities, and constitute glands
• General features:
  • Contain more cells and less extracellular ground substance
  • Polarity: free surface faces air or other substances, basal surface faces underlying CT, and lateral surface faces adjacent cells
  • Avascularity, but innervation: no blood vessels, but rich in nerve terminals
  • Functions: protection, secretion, absorption, excretion, and sensory reception

Classification of Epithelium

• Covering epithelium: covers body surface or lines inner surface of body cavities, tubes, and sacs
• Glandular epithelium: main function is secretion
• Sensory epithelium: has special sensory function

Covering Epithelium Classification

• Simple epithelium:
  • Simple squamous epithelium: one layer of flattened cells, found in mesothelium, endothelium, alveoli, and parietal layers of renal capsule
  • Simple cuboidal epithelium: one layer of cells with same height and width, found in renal tubule, thyroid, and some ducts of many glands
  • Simple columnar epithelium: one layer of columnar cells, found in respiratory tract, digestive tract, and glands
  • Pseudostratified ciliated columnar epithelium: one layer of columnar cells with cilia, found in respiratory tract
• Stratified epithelium:
  • Stratified squamous epithelium: multiple layers of cells, found in skin, mouth, pharynx, esophagus, urethra, and vagina
  • Stratified cuboidal epithelium: multiple layers of cells, found in sweat glands and developing ovarian follicles
  • Stratified columnar epithelium: multiple layers of cells, found in conjunctiva

Epithelial Specializations

• Specialization of free surface:
  • Microvilli: increase cell surface area, found in intestinal epithelium and proximal renal tubule
  • Cilia: move substances or particles stuck to the epithelium, found in respiratory tract
  • Stereocilia: increase cell surface area, found in epididymis, ductus deferens, and sensory cells of the ear
• Specialization of lateral surface:
  • Intercellular junctions:
    • Tight junction (zonula occludens): seal the space between cells
    • Intermediate junction (zonula adherens): adhere and keep cell shape
    • Gap junction (communicating junction): provide a pathway between cells
    • Desmosome (macula adherens): firmly connect cells
• Specialization of basal surface:
  • Basement membrane: a sheet of amorphous material between epithelial cells and underlying CT
  • Plasma membrane infolding: increase basal surface area and facilitate passage of water and ions, found in proximal and distal renal tubule
  • Hemidesmosomes: bind epithelial cells to the subjacent basal lamina

Glandular Epithelium and Gland

• Glandular epithelium: specialized for secretion
• Gland: organs composed mainly of glandular epithelium
• Classification of glands:
  • Exocrine gland: maintain contact with the surface, discharge secretion through a duct system
  • Endocrine gland: do not maintain contact with the surface, release secretion directly into blood stream
• Formation of glands from covering epithelia: epithelial cells proliferate and penetrate connective tissue