Epithelial tissue

Epithelial tissue, also called epithelium, is one of the body's primary tissues, meaning one of the basic types the body is built from, and its job is to line surfaces. Its cells line the inside of the body's cavities and cover most of its surfaces, both on the outside and on the organs.

To classify it, you look at two things. The first is how many layers of cells it has: if there is only one, it is a simple epithelium; if there are two or more, it is stratified. The second is the shape of those cells, which comes in three main types. Squamous cells are flat, like tiles; cuboidal cells are as tall as they are wide, like dice; and columnar cells are taller than they are wide.

Compare two epithelia made of squamous cells. The simple one, a single layer thick, lines the blood vessels and aids gas diffusion, meaning it lets gases pass from one side to the other. The stratified one protects, but nutrients have to cross several layers and diffuse less. A single sheet lets air through; several stacked sheets keep you warm but let less of it through.