The Length of Intestinal Villi Is Determined by the Relative Rates of Cell Loss at the Tips and Cell Replenishment at the Base
Division and replication of enterocytes occur in the crypts only. Crypt enterocytes are highly mitotic and regenerate rapidly. In fact, the intestinal crypt cells are among the most rapidly regenerating cells of the body, representing the single greatest need for protein synthesis in nongrowing animals.
As crypt cells multiply, they migrate upward onto the villi, pushing other villous cells ahead of them, so there is a continuous progression of cells migrating up the villi. As the cells migrate they mature, changing from relatively undifferentiated cells in the crypts to highly specialized absorptive cells on the villi. As the cells reach the tips of the villi, they are lost because of age and exposure to gut contents. The length of villi is determined by the rate at which cells arc lost at the tips and the rate at which they are replaced by cells from the crypts. An increase in cell loss at the villi tips, relative to crypt cell replication, results in shortening of the villi. Conversely, fast replication of crypt cells, relative to cell loss, leads to lengthening of the villi. The time taken for an enterocyte to migrate from its site of origin in the crypt to the tip of a villus varies with species and physiological state; on average, however, the turnover time of enterocytes is 4 to 7 days.The rate of cell replication in the crypts appears to be stimulated by several Gl hormones. When appetite and feed intake increase, there is an overall increase in the secretion of GI hormones. This in turn leads to a rise in crypt cell proliferation. An increase in the rate of crypt cell replication adds cells to the villus at a rate faster than they are lost at the tips, resulting in longer villi. Appetite and feed intake may increase because of conditions of greater energy need, such as lactation, exercise, and cold environmental temperatures. The greater villi length provides greater digestive and absorptive capacity to match the need created by higher feed intake. Thus the functional capacity of the intestine is adjusted to match the nutrient needs of the animal.