The epithelium is a layer of densely packed cells that form sheets that cover the surfaces of the body that are likely to associate with external substances and objects. Found in both plants and animals, the epithelium may function differently depending upon where it is located within the organism.

The primary activities of the epithelium are secretion, absorption, and protection. In some cases, for instance, the epithelial cells may produce fat or enzymes, storing them until they are ready for use by the body, at which point they are secreted. Other cells, however, are capable of absorbing granular material and some generate special outgrowths, such as hair and fingernails. These outgrowths help protect the body, by providing insulation and guarding the fingertips respectively.

A number of different kinds of cells may be present in the epithelium. Ciliated epithelial cells are a variety that exhibit small, hairlike projections. Layers of these cells comprise ciliated epithelium tissue, which can be found in a number of locations within the body. Ciliated epithelium tissue typically lines the bronchi of the lungs, the trachea, regions of the nasal pathways, and reproductive organs of both males and females. The cilia of the tissue are provided energy by mitochondria and their movement helps transport substances, such as mucus, through these regions.