Control mechanisms of cell proliferation in intestinal epithelium

In the adult organism some organs and tissues still contain proliferating and differentiating cells, whereas other organs only consist of non-dividing specialized cells. On the basis of their proliferative activity cell populations may be classified into three categories (135, 138,208). In some cell populations, e.g. neurons, no cell divisions occur in the adult organism, and these have been referred to as "static" or "stable" cell populations. In other cell populations the number of cells keeps increasing in the adult organism at a slow rate with scattered mitoses, e.g. parenchymal cells of liver and kidney; such cell populations are called "growing" or 11 expanding" cell populations. In some organ systems cell proliferation occurs at a high rate in the adult organism, and mitotic figures are abundant. These cell populations do not increase in size, or only to a slight degree, and cell production is balanced by continuous cell loss. These cell populations are called "renewing" cell populations (141). Examples of 11 cell renewing systems" are epidermis and derivatives, gastrointestinal epithelia, testis and blood-forming tissues. The size and the functional capacity of such a cell population is dependent on the precise bqlance between cell production and cell loss, while some flexibility is needed to meet with possible perturbations of the system.