[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.