The most basic characteristic of epithelial tissue is that it covers the surfaces of the body, whether external or internal. It acts as a protective covering or boundary for such surfaces including the outer layer of the skin, as well as the inner surface of "hollow" organs like the stomach, colon, and blood vessels. Beyond this, epithelial tissue is typically identified as having six characteristics. Some of the common characteristics are that it is made almost entirely of cells, contains cells joined by specialized contacts, and has distinct upper and lower surfaces. Other features are that it is not supported by blood cells, it retains the ability to regenerate, and is supported by connective tissue.