[Encyclios]

Multilayered lining epithelia

These are epithelia consisting of multiple layers of cells. The type of epithelium is defined by the shape of the cells of the most superficial layer. In fact, the shape of the cells is different in the various layers constituting the epithelium; generally the cells of the basal layer, in contact with the basal lamina, are cubic.

Multilayered pavimentous epithelium: it is composed of numerous superimposed layers of cells that are flattened towards the free edge of the tissue. It has the function of protection and barrier against external aggressions. It can be not keratinized (or not corneified), or keratinized (or corneified). In the latter case, the surface cells lose their nuclei and the cytoplasm is occupied by a large amount of keratin (scleroprotein); as a result, on the surface there are no viable cells, but horny scales.

A pluristratified keratinized pavimentous epithelium is typical of the epidermis. Non-keratinized multilayered pavimentous epithelia line the cornea, oral cavity, pharynx, and most of the esophagus. The stratum corneum protects against dehydration, when it is very thick (sole of the foot, palm of the hand) it also has a protective function against mechanical insults.

In the epidermis of a mammal can be distinguished several layers of cells. Starting from the deepest (in contact with the basal membrane) we find: the basal or germinative layer, the spinous layer, the granular layer, the stratum corneum. In the epidermis of the palm of the hand and the sole of the foot, between the stratum granulosum and the stratum corneum, there is the stratum lucidum.

Cubic or isoprismatic multilayered epithelium: is formed by two or more layers of cubic cells. The stratified cubic epithelium is, in mammals, very rare and lines only a few large excretory ducts.

Multilayered cylindrical or bathyprismatic epithelium: is formed by two or more layers of overlapping cells. In the deeper layer, they are small, polyhedral and never reach the surface of the epithelium, while the more superficial layer is formed by true cylindrical cells. In preparations, the nuclei appear to be overlapping. This epithelium has the function of protecting and covering large ducts or particularly irrigated surfaces and is very rare in mammals: we find it only in large excretory ducts, in the penile urethra, in a short stretch of the epiglottis and on the inner surface of the eyelid.