THE STRUCTURE OF EPITHELIAL TISSUES:
Cells of epithelial tissues fit closely together to form membranes, or "sheets" of cells. These "sheets" always have one free surface, and they are attached to a basement membrane. The basement membrane is secreted partly by the epithelial cells and partly by connective tissues (epithelium and connective tissues lie adjacent to each other).
Epithelial tissues are avascular (lack a blood supply), so nutrients must move to these tissues' cells from adjacent connective tissues. If well nourished, epithelial cells can easily regenerate themselves. An example of this process is the skin: Superficial skin cells are dead from lack of nutrition, but the deepest skin cells are well nourished. The superficial dead cells are continually being rubbed off with everyday activity. This is usually not a problem; however, as the lost cells are replaced by underlying cells that continually divide and push upward. As these cells move upward, they are removed from their nutrient supply, die, and slough off the body just like the ones they replaced.
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