All epithelial tissue rests on a basement membrane. The basement membrane acts as a substrate on which epithelium can attach to as well as grow and regenerate after injuries. Epithelial tissues have a nerve supply, but they do not have a blood supply. Therefore, epithelia must be nourished by substances diffusing from vascular structures in the underlying tissue.
The primary functions of epithelial tissues are:
- To protect the tissues that lie beneath it from injury such as desiccation, invasion by pathogens, and physical trauma
- The regulation and exchange of molecules between the underlying tissues another compartment
- The secretion of hormones into the blood vascular system, and/or the secretion of sweat, mucus, enzymes, and other products that are delivered by ducts
Epithelium lines both the outside (skin) and the inside cavities and lumina of bodies. For instance, the uppermost layer of skin is composed of dead stratified squamous, keratinized epithelial cells while the outer lining of many organs is comprised of mesothelium.
The type of epithelium that is present in an organ system depends on the function of the system. Surfaces that require a water resistant surface and protection from abrasion often are composed of keratinized stratified squamous epithelium (skin) or nonkeratinized stratified squamous epithelium (oral cavity, esophagus,rectum). Other surfaces that separate body cavities within the body are lined by simple squamous, columnar, or pseudostratified epithelial cells. Some types of epithelium can form solid masses of tissue or the majority of an organ. Hepatocytes comprise a majority of the hepatic parenchyma and are considered epithelial cells. Endothelium (the inner cell lining of vascular structures) is a specialized form of epithelium. Another type of specialized epithelium, mesothelium, forms the outside surface layer of the pericardium, pleura, and peritoneum. Some epithelial cells are organized into secretory structures at different locations in an animal’s body known as glands.
All epithelial cells types share the following characteristics:
- Close apposition of cells.
- Free surface of epithelial cells is adjacent to the space.
- Basal surface is adjacent to connective tissue.
- Sheets of epithelial cells may be modified into tubes forming glands.
- Absence of blood vessels within epithelial layer