- Epithelial tissues are comprised of uninterrupted cell layers that cover most internal and external body surfaces. This means that even if the surface has been penetrated, with the help of a microscope, you can see that the penetration just barely reaches the epithelium.
- Epithelial cells are attached to one another; the attachments are referred to as cell junctions. Adhering junctions offer mechanical attachment. Tight junctions work to block diffusion and typically form a gasket or seal around the ends of cells that comprise simple epithelia. The tight junction helps to adequately separate the fluid compartments, such as the compartments between the contents of the interstitial fluid of the body and the intestines. Gap junctions are also attached and provide direct intercytoplasmic communication between the joined cells. Small molecules are able to pass through the gap junctions directly from the cytoplasm of one cell to another.
- Epithelial tissues are polarized. The epithelium has a free surface; the apical surface is exposed and the basal surface rests on the connective tissue. The base of each cell is connected to a basal membrane, and lateral surfaces are connected to neighboring epithelial cells. You will find the polarization of epithelial cell structures particularly evident in the secretion and absorption of cells.
- Epithelial cells are separated from the underlying tissues by a basement membrane. The basement membrane is a thin sheet of glycoproteins and collagen, which are all produced in part by the epithelial cells and the underlying connective tissue cells. The purpose of the basement membrane is to regulate cell behavior and reduce the spread of neoplasms.