The word epithelial is of Greek origin (epi = upon; thele = nipple).The epithelium covers or lines surfaces: the outer skin (epidermis), mucous membranes, gastrointestinal tract, urinary tracts, and inner linings of the blood vessels.
Epithelial tissue takes on various functions:
- General protection (eg, in the outer skin and mucous membranes)
- Respiration (eg, in the ciliated epithelium of the airways)
- Absorption (ie, absorbing substances in the epithelium of the intestinal villi)
- Secretion (ie, releasing substances from the glands; special groups of epithelial cells are designed to secrete fluids; Fig 2-5)
- Receiving stimuli (ie, sensory function, eg, in the retina of the eye)
Depending on the partial functions undertaken, epithelial tissue can be classified as surface or tegumentary epithelium, glandular epithelium, or sensory or neuroepithelium (Fig 2-6).
Distinguishing features of surface epithelium are that it is divided from other tissues by a basement membrane, it contains no blood vessels, and it is found as a flat collection of cells on the inner and outer surfaces of the body. The cells of the surface epithelium can be classified according to their form as follows (Fig 2-7):
- Squamous (flat): in the lungs, the blood and lymphatic vessels, and the inner surfaces of the joint capsules
- Cuboidal: such as the epithelium of the eye lens and the retinal pigment epithelium
- Columnar: such as the mucous membrane lining the digestive tract; often also as ciliated epithelium
The epithelial cells can be found in various arrangements:
- Single-layer (simple) epithelium: in the digestive tract
- Multilayered (stratified) epithelium: for example, on the outer skin as horny or keratinized squamous epithelium or on the mucous membrane as nonkeratinized squamous epithelium
- Transitional epithelium: for organ coverings where there are great fluctuations in volume (eg, urinary bladder)
The horny layers of keratinized squamous epi-thelia are made up of dead, hardened epithelial cells.