Epithelium

The epithelium is a tissue composed of layers of epithelial cells.

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.

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 called stratified. Some epithelial cells have many protrusions called cilia that act to increase the surface area.

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.
Pseudostratified - Although through a microscope these cells can appear to be more than one layer, there is in fact just one layer of cells.
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.)

Glands are composed of epithelial cells.