Epithelium is a medical concept that is usually learned by medical students in their second year of medical study as part of a course in medical histology. Histology refers to a medical science which specializes in the research and investigation of the various body tissues. The general definition of epithelium is that it is a collection of cells that are arranged in a sheetlike form and are connected with each other by special form of connection that differs according to the tissue that is involved.
These types of cellular connections can permit or prevent the passage of cellular content from one cell to the other. This will depend on the type of connection between the cells. For example, tight junction is a type of connection between cells that does not permit any passage of molecules between the cells and the external environment. An example of such a connection occurs in the blood-testes barrier. In this case the environment in the testicular tissue is separated from the contents of the blood by tight junctions so that there is a constant environment inside the testes.
Other type of connections between the cells can be more flexible as far as molecules passage through them. The simplest type of epithelium that can be seen in the body is a monolayered epithelium. This type of epithelium is composed of a sheet of cells that are only one layer in thickness. Epithelium like this specializes in absorption of nutrients such as occurs in the epithelium of the intestine. This type of epithelium can be cuboidal or squamous or columnar.
Cuboidal epithelium refers to a type of epithelium in which its cellular length is equivalent to its width. In squamous epithelium the width of the cell is larger than its length. An example of a squamous epithelium occurs in the epidermis of the skin. Columnar epithelium refers to an epithelium in which the length of the cell is bigger than its width. Columnar epithelium is seen, for example, in the epithelium that lines the inner surface of the lungs.
The type of epithelium in which there is only one cellar layer is called simple epithelium. The type of layer in this epithelium dictates its name. For example, simple columnar epithelium is spread in the lungs and means that columnar cells are the type of cells in this epithelium. The epithelium in the lungs has a special function to secrete mucus in addition to performing ciliary motion in order to expel the contents in the bronchi outside the lungs which are subsequently coughed or swallowed in to the stomach.
Some sort of epithelia such as those occurring in the small intestine have special structures which are called microvilli. These are protrusions in the free end of the cell which has the function of increasing the absorptive surface of these cells in order to absorb maximum nutrients in the small intestine.
Other types of epithelium include the multilayered epithelium or the stratified epithelium. This type of epithelium is characterized by a several layers epithelium which has mostly a protective function in the body against trauma and against pathogens. The epidermis of the skin is a typical example of such a type of epithelium. The epidermis of the skin is a stratified squamous epithelium which is also keratinized and has a protective function as a formidable barrier against pathogens and harmful chemicals.
The epithelium is connected at the base of the cells with basement membrane. Epithelial tissues are usually avascular, namely they do not have blood supply but they may have nerve supply such as occurs in the epidermis of the skin. Simple epithelia are usually found in places where there is exchange of gases such as in the lungs alveoli. It can also occur in organs where filtration of blood occurs such as the blood vessels and the kidney cells. Stratified epithelium occurs for example in sweat glands and the urethra.