lesson 3

• Epithelial tissue is composed of closely aggregated polyhedral cells with very little extracellular substance
• These cells have strong adhesion and form cellular sheets that cover the surface of the body and line its cavities
• Principal functions of epithelial tissue include:

  • Covering, lining, protecting surfaces (e.g., skin)
  • Absorption (e.g., intestine)
  • Secretion (epithelial cells of glands/glandular epithelium)
  • Contractility (myoepithelial cells)
• Specific cells of certain epithelia are also highly specialized sensory cells, such as those of taste buds or the olfactory epithelium
• Epithelial cells line all external and internal surfaces of the body, so everything that enters or leaves the body must cross an epithelial sheet
• General characteristics of epithelial tissue:

  • Consists of closely packed cells with little extracellular material between adjacent cells
  • Arranged in continuous sheets, either single or multiple layers
  • Has an apical surface exposed to a body cavity, lining of an internal organ, and basal surface attached to the basement membrane
  • Plentiful cell junctions secure attachment among cells
  • Avascular
  • Attached firmly to adjacent connective tissue which holds it in position and prevents tearing
  • Has a nerve supply
  • High capacity for renewal
  • Diverse in origin
• • Cuboidal epithelium (cuboid/hexagon shaped)
  • Columnar epithelium (tall/cylindrical/rectangular)
  • Transitional epithelium (changes in shape from flat to columnar due to distention or stretching)
• Types of epithelia:

  • Covering or lining epithelia are tissues in which the cells are organized in layers that cover the external surface or line the cavities of the body
  • Classified according to the number of cell layers and the morphologic features of the cells in the surface layer
  • Classification based on arrangement of layers:
  • Simple epithelium (one layer)
  • Pseudostratified epithelium (has a multilayered appearance)
  • Stratified epithelium (2 or more layers)
  • Classification based on cell shapes:
  • Squamous epithelium (flat cells)
• Examples of distribution and main functions of different types of epithelia:

  • Simple squamous epithelium: lining of vessels (endothelium), serous lining of cavities (mesothelium), facilitates movement of viscera, active transport, secretion of biologically active molecules
  • Cuboidal epithelium: covering the ovary, thyroid, covering, secretion
  • Columnar epithelium: lining of intestine, gallbladder, protection, lubrication, absorption, secretion
• • Pseudostratified epithelium: lining of trachea, bronchi, nasal cavity, protection, secretion, cilia-mediated transport of particles trapped in mucus out of the air passages
  • Stratified squamous (keratinized): epidermis, protection, prevents water loss
  • Stratified squamous (nonkeratinized): mouth, esophagus, larynx, vagina, anal canal, protection, secretion, prevents water loss
  • Cuboidal epithelium: sweat glands, developing ovarian follicles, protection, secretion
  • Transitional epithelium: bladder, ureter, protection, distensibility
  • Columnar epithelium: conjunctiva, protection
• • Function: produce mucus, perspiration, oil, earwax, milk, or digestive enzymes
  • Endocrine glands: secretory products (hormones) diffuse into blood after passing through extracellular fluid without flowing through a duct
  • Location: pituitary glands, pineal glands, thyroid and parathyroid, adrenal glands, pancreas, ovaries, testes, thymus gland
  • Function: produce hormones
• Glandular epithelia are formed by cells specialized to secrete molecules stored in small membrane-bound vesicles called secretory granules

  • May synthesize, store, and secrete proteins, lipids, or complexes of carbohydrates and proteins
  • Classification of glandular epithelia:
  • Exocrine glands: secretory products released into ducts (e.g., mucus, perspiration, skin oil, earwax, digestive enzymes)
  • Location: sweat, oil, earwax, mammary glands of skin, digestive glands like salivary glands, pancreas