[Encyclios]

Simple lining epithelia

These are epithelia consisting of a single layer of cells that are wider than tall, with a strongly flattened nucleus and arranged to form a thin pavement. Examples of simple pavement epithelia are: the endothelium, epithelium that forms the tonaca intima of vessels; the mesothelium that forms the serosae (pericardium, pleura, and peritoneum); Bowman’s capsule of the renal corpuscle; and the epithelium that forms the wall of the pulmonary alveoli. Cells may have different types of free surface specializations, such as cilia, stereocilia, and microvilli.

The shape of the cells defines the type of epithelium:

• if the cells are flattened it is called simple pavimentous epithelium if the cells have a square profile it is called simple cubic epithelium;
• if the cells have a rectangular profile, i.e. the prevailing size is the height, we speak of simple cylindrical epithelium; nuclei are elongated and located in basal, central or (rarely) apical position;
• in the human organism we find simple pavimentous epithelia forming the wall of the membranous labyrinth and the external surface of the tympanic membrane, the lining of the pulmonary alveoli, the “rete testis” and some parts of the nephrons; the mesothelium and the endothelium, both of mesodermal origin, are simple pavimentous epithelia.

Simple cubic or isoprismatic epithelium: it is formed by a layer of cells generally as high as wide, with a nucleus always round and placed centrally to the cell. Under conditions of activation the cells of this epithelium can take a more elongated shape but are distinguishable from the simple cylindrical epithelium because they always have a round nucleus and arranged centrally.

This epithelium has a containment function: it usually lines the excretory ducts of exocrine glands, some tracts of renal tubules, thyroid follicles (normofunctioning), in the lens and in the retina (pigmented epithelium).

Simple cylindrical or bathyprismatic epithelium: it is formed by cells that are much taller than wide arranged to form a palisade and that have an oval nucleus located in the first third of the cell, in the part closest to the basal lamina. This type of epithelium is widespread, has multiple functions and therefore can have different specializations on its surface.

In the intestinal canal, for example, the simple cylindrical epithelium covers the luminal surface of the mucous membrane and the cells that compose it have numerous microvilli on the apical face, that is, digitiform expansions of the cytoplasmic membrane, designed to increase the surface of absorption and facilitate the exchange of substances.

In the salpinx, the simple cylindrical epithelium is ciliated because its function is no longer that of exchange, but that of creating amorphous currents that favor the descent of the oocyte towards the uterus and, at the same time, of hindering the ascent of the spermatozoa along the oviduct. The simple cylindrical epithelium is also found, without any specialization, in excretory ducts, bronchial tracts, renal tubules, in the small intestine (intestinal villi), mucosa of the oviduct, some areas of the uterus.