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, the tissue is typically identified as having six characteristics: it is made almost entirely of cells, contains cells joined by specialized contacts, has distinct upper and lower surfaces, is not supported by blood cells, retains the ability to regenerate, and is supported by connective tissue.
All epithelial tissues share a feature that is sometimes referred to as cellularity. Cellularity simply means that the tissue is made almost completely of cells, with very little space in between them. In other tissues, particularly connective tissues, an area of nonliving matter called the extracelluar matrix exists between cells. Epithelial cells all share special points of contact as well. These contacts are made by possible by special proteins called integral membrane proteins.
Another shared characteristic of this tissue is that its upper cells are different from its lower cells. This feature is known as polarity, and the upper and lower surfaces are called apical and basal, respectively. The apical cells are "free" in the sense that they are not attached to other cells, except for neighboring epithelial cells. The basal cells are attached to what is called a basement membrane, which is necessary to attach the tissue to nearby connective tissue. All of these tissues are supported by connective tissue, another common feature among them.
Epithelial tissue contains no blood vessels and is therefore said to be avascular. It is instead supported by capillaries in connective tissues, which supply it with nutrients through a process called diffusion. Although the tissue contains no blood vessels, it does contain nerve endings.
The cells are capable of rapid division, which is the process that creates new cells. Many epithelial cells in different parts of the body are lost due to friction or exposure to harmful substances. The skin, for instance, constantly produces new cells to replace the dead cells closest to the outer surface. The cells of tissues lining the digestive tract, including those of the esophagus, stomach, and intestines, also undergo continuous division.