Epithelia are tissues that serve as protective layers and/or secretory components of body organs and systems. In the special senses, some epithelia act as receptors in a sensory capacity. Most epithelia possess common characteristics that contribute to their functional capabilities. These common attributes are:
1. Limited intercellular space-specialized intercellular junctions hold the membranes of epithelial cells tightly together. This results in restricted intercellular space between cells. The junctions can enable epithelial cells to form effective barriers even though some are but a single layer in thickness. For example, fusion of the membranes effectively blocks infiltration of fluids through the spaces between cells(i.e. diffusion and osmosis). This enables epithelial cells to control substances that pass from one side of the cell to the other. In this way epithelia serve in absorptive, secretory, and excretory functions.
2. Single or multiple layers of cells-most epithelia form linings or coverings. These can consist of a single cohesive layer or multiple layers of cells cemented together. The location and functional needs of a site dictate what type of epithelium is present. For example, where diffusion or filtration processes are important, epithelial coverings are only a single layer thick, consisting of flattened cells. Where protection from abrasion, friction, or environmental elements is necessary, epithelia can be many layers in thickness.
3. Free apical surfaces-all epithelia possess a free apical surface at some time(during development or at maturity). The apical surface is where cells of an epithelium face internal lumens(i.e. body cavities or vessels), or lumens continuous with external environments(i.e. a lumen of a sweat gland tubule opening onto the body surface). The apical surface is typically where secretion, absorption, and excretion functions occur. Opposite to the apical surface is the basal surface that attaches to underlying tissues via a specialized basement membrane resting on connective tissues.
4. Avascular-epithelia lack blood vessels and therefore are classified as avascular tissues. Nutrient delivery to, and waste elimination from epithelia are always functions of blood vessels found in nearby connective tissues.
5. Mitotic capability-epithelia serving as covering or lining tissues are constantly subjected to wear and tear. Mitotic capability(the ability to divide and form new cells) enables epithelial cells to engage in repair and regenerative activities.
Epithelia as lining or covering layers.
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