Epithelial tissues cover all body surfaces, line most internal surfaces of passageways or tubes, and are the major tissues of glands. Because epithelium covers organs, forms the inner lining of body cavities, and lines hollow organs, it always has a free surface-one that is exposed to the outside or to an open space internally. The underside of this tissue is always anchored to connective tissue by a thin, noncellular layer called the basement membrane.
As a rule, epithelial tissues lack blood vessels; however, substances diffuse from underlying capillaries in connective tissues to nourish epithelial cells. Connective tissue is usually well supplied with blood vessels.