Epithelial Tissue and Epithelium
Epithelial tissue is the tissue that covers surface areas of the body. There are three different types of epithelial tissue. Taken together they are called the epithelium. In any given region of the body, the epithelial tissue is arranged in certain ways, with varying layers of cells. The aggregate is collectively referred to as the epithelium.
The three types of epithelial tissue are: covering and lining tissue, glandular tissue, and sensory tissue. The covering and lining epithelia are associated with, as in these three examples, the skin, the lining of the Gastrointestinal tract, and the lining of the respiratory tract. Their location determines the function of these cells. Again using the above examples, the skin epithelia is a protective layer, the epithelial tissue of the Gastrointestinal tract is concerned with the absorption of nutrients, and the epithelial tissue of the lung alveoli functions in the diffusion of gases across the lung-blood barrier.
Epithilial cells have several characteristics. They are contiguous; that is, the cells are attached to one another. The cells rest on a material called the basement membrane, which consists of a basal lamina produced by the epithelial cells and a fiber-like reticular layer. All epithelial cells have a so-called apical side, which is oriented toward the outside world (be that the lung or the intestinal tract or the true outside world, in the case of the skin) and a so-called basal side, which is oriented towards the basement membrane. As well, all epithelial cells lack blood vessels. Finally, epithelial cells are composed largely of a material called keratin.
The basement membrane aids epithelial cells in orienting themselves in relation to other tissues. After an injury to the cells, such as an abrasion, the basement membrane acts as a scaffolding upon which new cells can attach themselves during healing.
Epithilium acts as an interface between the outside world and the rest of the body. For this function, the epithelium needs to be a permeability barrier, capable of letting certain molecules through while excluding other molecules from entry. This function is accomplished by the presence of proteins at the meeting point between adjacent epithelial cells. The proteins span the cell, from its basal surface to its apical surface. These regions are called junctional complexes. Several types of junctional complexes exist. Anchoring junctions facilitate the tight association of adjacent epithelial cells, allowing cells such as skin cells to resist stretching forces that might otherwise pull the cells apart. Tight junctions plug up the space between adjacent cells, so those molecules that are absorbed across a layer of epithelial cells must pass through the cells. This provides a mechanism whereby epithelial tissue can control absorption. Also, gap junctions do allow the passage of molecules, but can do so in a regulated way. This allows cells, such as muscle cells and nervous tissue, to communicate and coordinate their activity.
Epithilial cells are classified according to several criteria: cell shape (squamous, cuboidal, columnar), the number of layers of cells in the epithelium (simple, stratified, pseudostratified), and the function of the cells (microvilli, cilia). Their varied shape, arrangement and function makes possible a number of critical activities, including absorption of molecules (in a non-specific or specific manner), secretion of substances such as tears and hormones from glands, supporting and maintaining form within the body, as in cartilage, bones, tendons and ligaments, and in the functioning of sensory cells.
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