PPT Slide
Epithelia serve a variety of functions and are classified according to (1) number of cell layers, (2) shape of superficial cells and (3) modifications at their free surface. Hence, there are "simple" (monolayer of cells) and "stratified" (at least 2 cell layers) epithelia; additionally there are "pseudostratified" and "transitional" epithelia. Epithelia are named by the shape of their most superficial cells. All epithelia sit upon a basement membrane below which is vascularized connective tissue. With one exception, blood vessels do not cross the basement membranes of epithelia.