Epithelial tissues are a well-defined group. They are the tissues that cover and line surfaces. Most glands are also epithelial tissues because they developed from multiple foldings of surface tissues during development. Examples of epithelia would be the skin, the lining of the gastrointestinal tract and lining of airways to the lungs and blood vessels. Also, sweat glands, thyroid gland and the liver, which originates as a very large gland.
Epithelial tissues are often on vulnerable surfaces where they will receive damage and wear and tear, like the stomach and the skin. For this reason they need to be able to regenerate well. They generally do. Everyone is familiar with the healing process after sunburn, or a slight abrasion or thin scratch to the skin. If the damage is not too extensive, the regenerations appears to be perfect. Coupled with this ability to grow and recover from damage, epithelial tissues are also the most vulnerable to cancer. This is because cancer is basically a disease of uncontrolled growth and is uncommon in tissues which have limited capacity for growth.
Epithelial tissues have a straightforward classification system. There are just two factors:
- The number of layers
- The shape of the cells.
The classification by layers is the simplest possible: One layer, or more than one . One layer is called simple, and more than one layer is called stratified. The classification of shapes is also simple, there are only three shapes: Short, medium and tall. The short cells are flat and are called squamous. The medium cells are as tall as they are broad, often appear like squares under the microscope, and are termed cuboidal. The tall cells are taller than they are broad and are termed columnar.
So three shapes can be combined with two terms for number of layers to give six possible epithelia: Simple squamous (single layer of flat cells), simple cuboidal (single layer of medium cells), and simple columnar (single layer of tall cells), stratified squamous (more than one layer of flat cells), stratified cuboidal and stratified columnar. This simple mathematical regularity is ruined by the biology and there are actually two more important epithelia: Transitional and pseudostratified columnar. These tissues include the skin and the inner surfaces of the body, such as those of the lungs, stomach, intestines, and blood vessels. Because its primary function is to protect the body from injury and infection, epithelium is made up of tightly packed cells with little intercellular substance between them. About 12 kinds of epithelial tissue occur. One kind is stratified squamous tissue found in the skin and the linings of the esophagus and vagina. It is made up of thin layers of flat, scalelike cells that form rapidly above the blood capillaries and are pushed toward the tissue surface, where they die and are shed. Another is simple columnar epithelium, which lines the digestive system from the stomach to the anus; these cells stand upright and not only control the absorption of nutrients but also secrete mucus through individual goblet cells. Glands are formed by the inward growth of epithelium-for example, the sweat glands of the skin and the gastric glands of the stomach. Outward growth results in hair, nails, and other structures. See Epithelium
"Tissue," Microsoft® Encarta® Encyclopedia 2000. © 1993-1999 Microsoft Corporation. All rights reserved.