In the varieties of epithelium hitherto considered the cells are, in the main, disposed upon some surface in a single layer, some, at least, of the cells usually extending from the bottom of the layer to its surface.
Stratified epithelium is distinguished from these by being of greater depth and consisting of several layers of cells. The epithelium lining the cheek or the oesophagus may be taken as a typical example of this variety.
The most deeply situated cells are small and nearly filled by the round or oval nucleus. They undergo frequent division, and as they multiply some of them are crowded toward the surface. For a time these increase in size through a growth of their cytoplasm.
But as they are pushed nearer to the surface and farther from the sources of nutrition in the vascular tissues underlying the epithelium, they become flattened and their bodies lose their cytoplasmic character, being converted into a dry, horny substance, keratin.
Upon the free surface they are reduced to thin scales, closely adhering to each other and their subjacent neighbors, but entirely devoid of both cytoplasm and nucleus.
Stratified epithelium is found upon surfaces exposed to friction, which it serves to protect against mechanical injury, and, in some cases, against desiccation. It forms the epidermis of the skin, and lines the mouth, oesophagus, rectum, and vagina. In these situations the scaly or squamous cells of the surface are constantly being removed by the attrition to which they are exposed, but are as constantly replaced by fresh cells from the deeper layers of the epithelium. Pressure and moderate friction stimulate the multiplication of the cells in the deepest layers of the tissue, so that parts, e. g. of the skin, which are especially subjected to such influences, acquire a thicker epidermis (callus).
Where the stratified epithelium consists of many layers of cells, as is the case, for instance, upon the skin, there is a provision for the nourishment of the growing cells which are somewhat removed from the vascularized subjacent tissues. The cells of the deeper layers are somewhat separated from each other, leaving a space between them through which nutrient fluids can circulate. Across this space numerous minute projections or "prickles," springing from neighboring cells, join each other, forming connecting bridges between the cells. When isolated, such cells appear covered with these small spicules (" prickle-cells "), and their presence probably increases the tenacity with which the cell-remains adhere to each other when they become hardened and toughened on the surface of the epithelial layer.
These delicate bridges connecting neighboring cells are not peculiar to stratified epithelium, though they are more conspicuous in that tissue than elsewhere. They have been observed between the cells of the columnar epithelium of the intestinal mucous membrane, and also between the cells of other elementary tissues ; e. g., smooth muscular tissue.