Fig 008 Epithelium

Epithelia

Epithelia are classified by the shape of the surface cells and whether it has single or multiple layers of cells.

Shape of surface cells:

• Squamous - flattened cells whose width is greater than their height
  • Nuclei are often taller than the flattened cells and bulge into the lumen
  • Difficult to see their thin cytoplasm
• Cuboidal - cells appear as high as they are wide
  • Spherical nuclei are centrally positioned
• Columnar - cells are taller than they are wide
  • Oval nuclei located near the base of the cells

Number of layers:

• Simple - a single layer of cells
  • All cells contact the basement membrane (purple)
• Stratified - two or more layers of cell
  • Only the lowest layer of cells (yellow) contact the basement membrane (purple)

Specialized epithelia:

• Pseudostratified Columnar
  • Single layer of cells
  • Nuclei appear being to be different levels
  • Not all cells reach the surface
  • All cells contact the basement membrane (purple)
• Transitional - used where the epithelium is stretched
  • Multiple layer of cells
  • Only the lowest layer of cells (yellow) contact the basement membrane (purple)
  • Morphology varies depending on whether it is relaxed or stretched
    • Relaxed - surface layer of large, dome-shaped cells
    • Stretched - surface layer of flattened cells