Epithelial Tissues and Cells
Epithelial tissue covers all of the surfaces of the body that come into contact with the environment. This includes the outer surface of the skin, as well as the linings of the respiratory, digestive, and urogenital tracts. Some epithelial cells are specialized to secrete mucus (which acts as a lubricant and protects inner surfaces from infection), while others often cluster together to form glands (structures that synthesize and secrete specific substances, often enzymes or hormones). Since epithelial tissue exists wherever the body comes into contact with the environment, it serves a protective role and also regulates the absorption of materials (as in the digestive and respiratory systems) and their excretion (as in the respiratory system and sweat glands). Any particular epithelial tissue is classified according to the shape and arrangement of its cells:
- Squamous epithelium consists of cells that are thin and flattened; typically, substances diffuse through these cells rather easily.
- Cuboidal epithelium is made up of cube-shaped cells.
- Columnar epithelium is composed of elongated, rectangular cells.
- Simple epithelium contains cells arranged in a single layer.
- Stratified epithelium contains cells arranged in sheets several layers thick.
The epithelial tissue that makes up the outer layer of the skin is stratified squamous epithelium, while the nutrient-absorbing tissue of the small intestine is simple columnar epithelium.
Epithelial tissues of all types are generally attached to underlying connective tissue by a basement membrane, a non-living extracellular conglomeration of proteins and carbohydrates that is secreted by the epithelial cells themselves. Apart from the basement membrane, epithelial tissues generally contain little non-cellular material and are usually continuously turned over. An example of this is the epithelium of the skin, whose cells are constantly shed and replaced by new ones.