Epithelial tissue covers surfaces (e.g., skin), lines cavities (e.g., stomach). The individual cells are extremely cohesive. They are also firmly attached to a basement membrane that separates them from the tissue beneath. There is little ifany external matrix (intercellular substance) and no fibrous material between the cells. Because they are located at interfaces between entirely different environments, epithelial cells tend to be strikingly polarized. A basal end is firmly attached to a basement membrane that separates them from the tissue beneath and to 'the connective tissue support; an apical end faces the lumen or free surface. This polarization preserves the internal milieu of the body and prevents entropy by establishing and maintaining the necessary concentration gradients across epithelial membranes. As will be seen, epithelium may demonstrate considerable specialization related to function. In organs such as the intestines, epithelium may have surface modifications related to secretion and absorption. In other areas it may be thick and tough for protective purposes.
KeywordsSquamous Epithelium Columnar Epithelium Endocrine Gland Exocrine Gland Stratify Squamous Epithelium
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