Each epithelium includes stem cells that divide to replace cells removed from the apical surface, and to allow for growth of the tissue. Epithelial cells may be exposed to enzymes or to abrasion. Under these circumstances, they do not last long, and must be replaced when they are destroyed or shed. Epithelia are replaced by the division of stem cells, or germinative cells, within the epithelium, located in the deepest layer closest to the basement lamina. Classification of Epithelial Cells.
Epithelial cells come in three shapes, squamous, cuboidal or columnar.
Squamous cells are flat, and shaped like irregular polygonal tiles. Cuboidal cells have more depth. In section they appear square, but in surface view they are polygonal. Columnar cells are also polygonal in cross section, but are long and slender in section. Classification of Epithelial Tissues.