The epithelium protects your body’s outer skin and internal pathways. Its functions include protection and absorption of nutrients. It also secretes fluids in our stomach, intestines, trachea, and other body pathways.

This tissue covers body surfaces, lines cavities, and produces glands. Specialized epithelia act as receptors for specific senses. Their quantity and closeness allow them to segregate connective tissues from free surfaces.

Some of these free surfaces include the outer surface of internal organs. Cavity linings, tubes, and ducts are also free surfaces. The epithelial extracellular matrix is minimal and lacks organization.

Epithelia form sheets. On each membrane, there is one detached surface or edge. This apical surface faces the outside of the body or an internal organ cavity.

The epithelium’s anchoring, or basal surface, rests atop a structureless material. The cells deep in the epithelium secrete this material called the basement membrane.

They are avascular, meaning epithelial tissues have no blood supply of their own. Instead, they rely on capillary diffusion for their blood supply. As a result, epithelial cells regenerate with ease if they are well-nourished.

You can classify epithelia according to (1) the shape of the cells and (2) the presence of layers.

According to shape:
• Squamous: These appear as flattened, keratinized, or nonkeratinized. You can find them in capillary walls and the skin.
• Cuboidal: These cube-shaped cells are present in the kidney’s nephrons. You can observe their involvement in secretion and absorption.
• Columnar: Rectangular cells with cilia make up the inner lining of the gut tube.

According to the number of layers:
• Simple: One cell layer.
• Stratified: Two or more layers.
• Pseudostratified: It looks stratified in cross-section despite a single layer.