20.0.1 20.3 Epithelial tissue covers most interior and exterior surfaces of the body.

From the tough soles of your feet that allow you to walk barefoot on a rough, hot sidewalk, to the lining of your mouth that burns when you eat a hot pepper, epithelial tissue (also called epithelium) is a sheet-like tissue that covers the surfaces of your—and every other vertebrate’s—body. It is made up of a single layer or a few layers of cells, called epithelial cells, that are tightly bound together so that, in most tissues, fluids and gases must pass through the cells, rather than around or between the cells, to get into or out of the body (FIGURE 20-7). When you look at a vertebrate, most of what you see is epithelium, because skin is an epithelial tissue.

Epithelial tissue is not just found on the outer surfaces of organisms, however. It also forms glands—individual cells or collections of cells producing secretions for use elsewhere in the body—and lines the internal tubes and cavities of the body, such as the stomach and intestines, lungs, and blood vessels. In each location, epithelium always has two distinctive sides. The “outside” can be in contact with the outside of the body or with an internal cavity such as the stomach, where it plays a protective role. The “inside” (or underside) faces away from the surface and is generally secured to underlying tissues. Epithelium plays multiple roles in organisms. Three of its most important functions are protection, transport, and secretion (see Figure 20-7).

1. Protection. Acting as a barrier, epithelial cells are linked together closely by tight junctions and desmosomes (see Section 3-12), which keep fluids from leaking into or out of tissue. If the strong acids in the stomach, for example, were to leak into surrounding tissue, they would seriously damage the tissue. This is what occurs in individuals with stomach ulcers, open sores in the lining of the stomach. The tight fit between epithelial cells is also why the skin of terrestrial vertebrates is usually waterproof.

2. Transport. Epithelium forms small finger-like projections in the lining of the small intestine, where nutrients are absorbed from digested food into the bloodstream. In blood vessels, epithelium controls which molecules can enter other tissues of the body. And in the kidneys, epithelium helps to regulate which molecules from the bloodstream are eliminated in the urine.

3. Secretion. Epithelium can also form glands. Exocrine glands generally secrete products—including earwax, sweat, saliva, milk, mucus, and, in the case of some frogs, toxic poisons—into ducts that lead to the external environment. Endocrine glands, on the other hand, are ductless glands that produce hormones, which are released into the fluid surrounding the glands and, from there, usually enter the bloodstream for distribution to other parts of the body, where they act as chemical messengers (see Chapter 24).

Because epithelial cells are often in contact with a large variety of materials, they tend to experience more damage than other tissues. For this reason, they tend not to last long and are replaced frequently. Human skin cells, for example, are replaced approximately every two weeks. (Dandruff is mostly dead skin cells.) Cells lining the digestive tract last about five days, and because liver epithelium experiences slightly less wear and tear, it lasts a year or two before replacement.