3.1 Epithelial Tissue

• What is the primary function of epithelial tissue?

  Epithelial tissue covers exposed surfaces and lines internal cavities and passageways.
• What are the two main components of epithelial tissue?

  Epithelial tissue includes epithelia and glands.
• Why is the surface of the skin considered a good example of epithelia?

  Because it covers an exposed surface of the body.
• Which tracts are lined by epithelia?

  Epithelia line the digestive, respiratory, reproductive, and urinary tracts.
• What other internal areas are lined by epithelia besides tracts?

  Internal epithelia line the chest cavity, fluid-filled chambers in the brain, eye, inner ear, and the inner surfaces of blood vessels and the heart.
• What are the important characteristics of epithelia?

  • Cellularity
  • Polarity
  • Attachment
  • Avascularity
  • Layers
  • Regeneration
• How does polarity affect the distribution of organelles in epithelial cells?

  Polarity causes organelles and other cytoplasmic structures to be distributed unevenly between the exposed and attached surfaces.
• In what special situations might epithelial cells lack a free surface?

  In special situations, epithelial cells may lack a free surface in most endocrine glands.
• Functions of epithelial tissue include the following

  • protecting surfaces
  • controlling permeability
  • providing sensations
  • producing secretions
• Epithelial tissue protects surfaces from abrasion, dehydration, and destruction
• Epithelial tissue can sometimes be relatively impermeable but can be modified in response to stimuli
• Neuroepithelia are specialized sensory epithelia found in special sense organs that provide our sensations of smell, taste, sight, balance, and hearing
• Gland cells are epithelial cells that produce secretions
• Unicellular glands are individual gland cells scattered among other cell types in an epithelium
• In glandular epithelia, most or all of the epithelial cells produce secretions
• Basolateral surfaces, where the epithelium contacts the basement membrane and neighboring epithelial cells
• Levels of organization for the body go in the following order, from smallest to largest: chemical or molecular level, cellular level, tissue level, organ level, and finally organ level system
• Most epithelial cells have microvilli, projections on exposed apical surfaces where absorption and secretion occur
• Stereocilia are very long microvilli, but cannot move
• A ciliated epithelium has cilia on its apical surface that usually beat together
• Three factors interact to keep epithelial tissues intact and healthy:

  • intercellular connections
  • attachment to the basement membrane
  • epithelial maintenance and renewal
• Cell junctions are the infolding of adjacent cell membranes interlocking the cells and increasing the surface area of the cell junctions
• Basement membranes are a complex structure produced by the epithelium and cells of the underlying connective tissue
• The superficial portion of the basement membrane consists of the basal lamina, which is secreted by epithelial cells
• The basal lamina restricts the movement of proteins and other molecules from underlying connective tissue into the epithelium
• Deep to the basal lamina is the reticular lamina, which is secreted by underlying connected tissue
• The reticular lamina contains bundles of coarse protein fibers that anchor the basement membrane to the underlying connective tissue
• What is the primary function of epithelial tissues?

  Epithelial tissues must continually repair and renew themselves.
• Why do epithelial cells need to be replaced quickly?

  Because exposure to enzymes, toxic chemicals, pathogens, and abrasion damages epithelial cells.
• How does the lifespan of epithelial cells in the small intestine compare to other areas?

  Epithelial cells in the small intestine may survive for only a day or two under severe conditions.
• Where are stem cells located in epithelial tissues?

  Stem cells are usually found close to the basal lamina.
• How is epithelial tissue classified?

  • According to the number of cell layers: simple or stratified
  • According to the shape of the cells: squamous, cuboidal, or columnar
• What is a simple epithelium?

  A simple epithelium has only one layer of cells covering its basement membrane.
• Why are simple epithelia considered thin and fragile?

  Because a single layer of cells cannot provide much protection.
• Where are simple epithelia typically found in the body?

  In protected areas such as the thoracic, abdominal, and pelvic cavities, chambers of the heart, and blood vessels.
• What advantage does a simple epithelium provide in areas of secretion, absorption, or filtration?

  It decreases the diffusion distance and the time needed for materials to pass through the epithelial barrier.
• What characterizes a stratified epithelium?

  A stratified epithelium has two or more layers of cells.
• How do the cells in a stratified epithelium differ from each other?

  The height and shape of the cells in stratified epithelium differ from layer to layer.
• What is the significance of the superficial cells in stratified epithelium classification?

  We use only the shape of the most superficial cells in epithelium classification.
• Why are stratified epithelia thicker and stronger than simple epithelia?

  Because they have multiple layers of cells.