Everything that enters or leaves the human body must pass through an epithelial layer. Embryonically, the epithelia derive from all three embryonic tissues depending on location. The linings of the mouth, nose, anus and skin are of ectodermal origin. The respiratory and digestive system are of endodermal origin. Other epithelial linings, such as those lining blood vessels, derive from embryonic mesoderm.
Epithelial cells range in shape from columnar to cuboidal to flattened squamous cells.
The cell nuclei vary from sperical to elongated or elliptical and often conform to the overall cell shape. Single layers are found where a barrier is needed that minimally impedes diffusion such as in the lining of the alveoli of the lungs. Stratified epithelia are seen when protection is more important such as in the skin or lining of the mouth. Epithlial tissue is often associated with a basement membrane made of thin fibers and ground substance. Basal lamina have several functions such as structural support, filtering, cell polarity, cell migration, cell growth and differentiation and have an influence on cellular metabolism.