Epithelium is characterized by close cell apposition and presence at a free surface
Epithelial cells, whether arranged in a single layer or in multiple layers, are always contiguous with one another. In addition, they are usually joined by specialized cell-to-cell junctions, which create a barrier between the free surface and the adjacent connective tissue. The intercellular space between epithelial cells is minimal and is devoid of any structure except where junctional attachments are present.
Free surfaces are characteristic of the exterior of the body, the outer surface of many internal organs, and the lining of the body cavities, tubes, and ducts, both those that ultimately communicate with the exterior of the body and those that are enclosed. The enclosed body cavities and tubes include the pleural, pericardial, and peritoneal cavities as well as the cardiovascular system. All of these are lined by epithelium.
Subclassifications of epithelium are usually based on the shape of the cells and the number of cell layers rather than on function. Cell shapes include squamous (flattened), cuboidal, and columnar. Layers are described as simple (single layer) or stratified (multiple layers). Figure 3.1
that the cells are much taller than the lining cells of the pancreatic duct. The free surface of the epithelial cells is exposed to the lumen of the gallbladder, and the basal surface is in apposition to the adjacent connective tissue. x540.
shows epithelia from two sites. Both are simple epithelia, i.e., one cell layer thick. The major distinction between the two examples is the shape of the cells, cuboidal versus columnar. In both epithelia, however, the cells occupy a surface position.
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