In zootomy, epithelium is a tissue composed of epithelial cellss. Such tissue tends to surround parts of the body, somewhat like a cell membrane. It is also used to form glands.
The epithelium is usually the layer of cells 'closer' to the outside world. The outermost layer of our skin is composed of dead squamous epithelial cells, as are the mucous membranes lining the inside of mouths and body cavities. Other epithelial cells line the insides of the lungs, the gastrointestinal tract and the reproductive and urinary tracts.
Functions of epithelial cells include secretion, absorption and protection. Some epithelial cells have cilia, which act to increase the surface area.
Types of epithelium
Epithelial cells are named according to their shape. If there is more than one layer of cells (such as skin), it is stratified; otherwise, it is simple. There are also pseudostratified cells, which "appear" to consist of more than one layer.
- Squamous - These are flat cells. Such as the cells of skin. These tend to slough off easily, providing resistance to abrasion.
- Cuboidal - These cells are cuboidal in shape. Epithelial cells in glands such as the pancreas and the salivary glands tend to be cuboidal cells.
- Columnar - The epithelial cells of the intestines are columnar, with numerous cilia. They are designed to absorb nutrients from the intestinal contents.
- Transitional - These are present in the urinary bladder and ureters. (Transitional zone also refers to the epithelium of part of the cervix prone to metaplasia.)