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

Learn about the characteristics and functions of epithelial tissue, including its structure, junctions, and polarity. Understand the role of epithelium in covering body surfaces, lining cavities, and forming glands.