[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.

[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.

[Encyclios]

Pseudostratified lining epithelia

In this type of epithelia the nuclei appear at different heights, but in reality there is a single cell layer, because all cells are in contact with the basal lamina but only some reach the free surface. The shape of the cells is varied, predominantly elongated.

Pseudostratified epithelium: it is an epithelium in which all cells are in contact with the basal lamina, but only some reach the surface. In the preparations, the nuclei are on different but staggered planes, never overlapping. The cells constituting this epithelium have a decidedly variable shape. They present a cytoplasmic swelling in which the nucleus is housed: in some cells this swelling is located towards the base in others towards the apex. This organization gives the impression, under the light microscope, of a multilayered epithelium, without being so (hence the name of pseudostratified or pluriseriated epithelium). There are two types of pseudostratified epithelium: ciliated pseudostratified epithelium and pseudostratified epithelium with stereocilia.

Pseudostratified ciliated epithelium: the function of this epithelium is to purify the inhaled air of dust and atmospheric pathogens. The calyciform mucipar cells secrete mucus that has the function of trapping impurities, the mucus is then spread evenly and moved by the high hair cells, as happens for example in the trachea where the mucus is moved by the hair cells from the bottom to the top (muco-ciliary elevator). This epithelium is found in most of the airways, the Eustachian tube, part of the tympanic cavity, and the lacrimal sac.

Transitional (or polymorphic) epithelium: represents a particular example of pseudostratified epithelium, has a very variable morphology because it lines organs, such as the urinary bladder or ureter, subject to strong volume variation during their function. This epithelium is formed by three types of cells: in the deepest layer, in direct contact with the basal lamina, we find cells of cubic or cylindrical shape (basal or germinative layer), immediately above there are some layers of elongated cells (called clavate or piriform), finally the most superficial layer consists of cells, sometimes binucleate, orthogonal with respect to clavate and with the upper surface convex called cupoliform or umbrella.

The possibility of the clavate cells to deform stretching on a horizontal plane, while remaining embedded in the umbrella-shaped, allows this epithelium to increase its surface in accordance with the dilation of the organ it covers. For example, in the released state (empty bladder) the transitional epithelium appears as consisting of 5-6 layers of cells, the most basal ones cubic, the superficial ones globular and sometimes binucleate, often with prominent nucleoli (cupoliform cells). In the distended state (full bladder) the transitional epithelium appears as consisting of only 2-3 layers of cells; the superficial cells are very flattened. Such variations in the number of layers are only apparent, there being always a single layer of cells.