The superficial dead cells are continually being rubbed off with everyday activity. The cells that are lost are continually replaced by underlying cells that are continually dividing and being pushed upward. As the cells move upward, they are removed from their nutrient supply, die, and are lost due to friction. Epithelia are classified on the basis of cell shape and arrangement. Generally, the arrangements are simple and stratified.
Simple epithelia have one layer of cells attached to a basement membrane.
Stratified epithelia have two or more layers of cells attached to a basement membrane. So, if you see a tissue that consists of a free surface, a single layer of columnar cells, and a basement membrane, then you have a simple columnar epithelium. If you see a tissue with a free surface, three sheets of cuboidal cells, and a basement membrane, then you have a stratified squamous epithelium. It is important to note that stratified epithelia are named according to the cells at the free surface. Now, let's view some images of epithelial tissues .