1. The layer of cells forming the epidermis of the skin and the surface layer of mucous and serous membranes.

The cells rest on a basement membrane and lie in close approximation with little intercellular material between them.

2. The covering of the internal and external organs of the body and the lining of vessels, body cavities, glands, and other bodily organs.

It consists of cells that are held together by connective material and varies in the number of layers and the kinds of cells.

Epithelium is classified into types on the basis of the number of layers deep and the shape of the superficial cells.

• flat or squamous
• cube-shaped or cuboidal
• cylindrical or columnar
• goblet cells that secrete mucus

Epithelium functions as a protection, the absorption and secretion, and other specialized functions; such as, movement of substances through ducts, production of germ cells, and the reception of stimuli.

Consisting of four main specialized cell lineages: absorptive enterocytes and three secretory cell types known as enteroendocrine, Paneth, and goblet cells; plus, a rare, fifth type of intestinal cell called tuft cells that also exist.

Defined by the thick brush of long microvilli that project from their apical surface, tuft cells are seen in several epithelial tissues, yet little is known about their functions because there is a lack of tuft cell–specific markers.