The outer surface of the body and the inner body cavities of the animals are covered by this tissue.
Epithelial tissue performs various functions in the body, such as protection of the body (skin), absorption of useful substances (intestinal epithelium) and perception of sensations (skin), depending on the organ in which they are located.
The epithelial tissues or epithelia have cells perfectly juxtaposed, joined by a small amount of cementitious material, with very little intercellular space. The epithelia are not vascularized and do not bleed when injured. The cells are nourished by diffusion from the capillaries in another tissue, the connective tissue, adjacent to the epithelium attached to it. The arrangement of epithelial cells can be compared to that of tightly fitted tiles or bricks.
Epithelia can be classified by cell number:
- When epithelia are formed by a single layer of cells, they are called uniestratified or simple epitheliums (from Latin uni one and stratum, layer).
- Already the epithelia formed by more than one layer of cells are called stratified.
- There are also epithelia that, although formed by a single cell layer, have cells of different heights, which gives the impression of being stratified. Therefore, they are often called pseudo-stratified.
Regarding the shape of cells, epithelia can be classified into:
- Pavement, when cells are flattened as tiles;
- Cubicwhen the cells are cube shaped, or
- Prismaticwhen the cells are elongated, columnar.
In the epithelium that lines the bladder, the shape of the cells is originally cubic, but they become flattened when stretched by dilation of the organ. Therefore, this type of epithelium is called, by some authors, transitional epithelium.
Epithelial tissues, also called epithelia, are classified into two main types: lining epithelia and glandular epithelia.