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Epithelial tissue, also called epithelium, is one of the four main types of tissue found in animals, including humans. It completely covers, or lines, all external body surfaces as well as nearly all internal body surfaces. Several internal organs, such as the liver, kidneys, and lungs as well as other internal structures, like some glands, are primarily made up of epithelial tissue. Epithelial tissue provides both a protective barrier from the environment for other tissues and organs but also acts as an interface with the outside world. Epithelial cells are fairly diverse and are responsible for many functions, including protection, secretion, certain types of absorption, and for some types of sensory input.
The most familiar type of epithelial tissue is made up of a special type of epithelial cell and is called the epidermis, or the skin. The epidermis protects other body tissues from damage from a hostile or dangerous environment and provides a protective barrier against many parasites and harmful microorganisms. Epithelial skin tissue is very versatile as well. Sweat glands in this tissue help to regulate body temperature, and nerve endings in epithelial tissues are part of the sensory system. It also heals rapidly after sustaining damage, especially the epidermis.
Several characteristics of epithelial tissue distinguish it from other types of tissue. Epithelial tissue forms a contiguous layer where found, without gaps, breaks, or holes. Its cells are very close together, each one attached to the others around it with little space between them.The cells are arranged so that all the cells in a layer of epithelium are aligned in the same direction. Epithelial tissue is usually separated from other tissues by a type of membrane called a basement membrane that is formed by secretions of the epithelial cells themselves.
Internally, epithelial tissue serves some of the same functions as the epidermis or external epithelium, lining the surfaces of all body cavities and providing a protective layer to internal body surfaces. Several internal organs and glands are also composed of epithelial tissue as well, and these organs are primarily involved in the absorption of food, water, air and the filtration and secretion of wastes. A good example of internal epithelial tissue is the lungs, which allow oxygen to enter the blood stream.