Stratified epithelia consist of two or more layers of cells. They are usually classified according to the shape of the cells in their most superficial layer, that is, the layer farthest from the underlying connective tissue.
The stratified squamous epithelium shown in the image consists of several layers of cells. An important characteristic of this epithelium is that the shape of the cells changes from one layer to another.
The deepest layer consists of cuboidal or polyhedral basal cells. These cells rest on a basal lamina, as in other epithelia. Remember that, when viewed with a light microscope, the structure seen immediately beneath an epithelium is often called the basement membrane. It consists of the basal lamina together with components of the extracellular matrix of the underlying connective tissue.
The basal epithelial cells undergo mitosis, producing new cells that gradually migrate toward the epithelial surface (they are “pushed up” by the newly formed cells). As they move upward, they undergo progressive changes in their gene expression, protein production, and shape. They become progressively larger and flatter, changing from roughly cuboidal or polyhedral cells into elongated cells and, finally, into very flat cells.
The most superficial cells are thin and flattened, resembling plates or scales. They are therefore called squamous cells. Because the superficial cells are squamous, the entire epithelium is classified as stratified squamous epithelium.
Most stratified squamous epithelia continuously produce new cells by mitosis in the basal layer while older superficial cells are continuously shed from the epithelial surface. This ongoing process replaces cells that have been damaged or lost.
Stratified squamous epithelium is commonly found in areas exposed to friction and other mechanical stresses, such as the skin, oral cavity, esophagus, and vagina. The thickness of the epithelium, which reflects the number of cell layers, is often greater in regions exposed to greater mechanical stress.
