Epithelial tissues are classified according to the shape of the cells and the number of cell layers formed. Cell shapes can be squamous (flattened and thin), cuboidal (square-like, as wide as it is tall), or columnar (rectangular, taller than it is wide). Additionally, the nucleus shape helps identify the type of epithelial cells. Squamous cells have flattened disc-shaped nuclei, cuboidal cells have spherical nuclei, and columnar cells have elongated nuclei.

Based on the number of cell layers, epithelial tissues can be further classified as simple or stratified. Simple epithelium consists of a single layer of cells resting on a basement membrane. If more than one layer is present, it is called stratified epithelium and only the bottom layer of cells rests on the basal membrane.

Simple epithelium contains cells of the same shape in a single layer. This thin and fragile epithelium lines the internal compartments, body cavities, and blood vessels. The thinness of simple epithelia facilitates rapid absorption and secretion of substances - such as nutrient absorption in the intestines or the exchange of gases in alveoli.

In contrast, the stratified epithelium can have different cell shapes in each layer. Hence, it is named after the shape of the cells in the top layer. It covers exposed body areas such as the skin surface or mouth lining and protects against mechanical abrasions, chemical attacks, and invading pathogens.

This text is partially adapted from Openstax, Anatomy and Physiology 2e, Section 4.2: Epithelial Tissues.