Cell turnover in epithelial populations.

Abstract

In its simplest terms, cell turnover in a tissue is the process whereby cell formation is balanced by cell loss. Close examination of the turnover systems in epithelia, such as the epidermis and intestinal epithelium, reveals that the mechanism of the process is, in fact, very complex and may differ widely between epithelia of different morphology and function. The existence of cell turnover in an epithe1ium may be shown morphologically. Cell formation is indicated by the presence of mitotic activity. This may be restricted to one region of the epithelium, as in the deeper layers of the epidermis and the crypts of the intestinal epithelium. Regions of cell loss are indicated in the epidermis by cornification and death of the surface cells, and in the intestinal epithelium, by the presence of extrusion zones at the tips of the villi. Cell turnover in an epithelium is probably an inherent property rather than a repair process induced by trauma. However, the rate of cell turnover may be altered by a number of internal influences (e.g., hormones) as well as by factors in the external environment (e.g., temperature, feeding). Alteration of the turnover rate may be shown by changes in mitotic activity, or in size of the cell population of the epithelium. Thus, in contrast with non-renewing tissues, whose architecture is always constant, the histologic picture of an epithelium undergoing renewal may vary with the interplay of factors controlling its cell turnover.