Control mechanisms of crypt cell production in the small intestinal epithelium.

The small intestinal epithelium constitutes the cell renewing system with the largest turnover activity in the body. It is influenced by numerous factors, and has a great potential for adaptation. The control mechanisms responsible for the balance between cell production and cell loss in this system are still largely unknown. In this paper we present the hypothesis for two control mechanisms of crypt cell production. The first mechanism, the feedback control mechanism of crypt cell production by the functional villus cell population, regulates the relative size of the proliferative cell compartment in the crypt (i.e. the growth fraction), and it is a strictly local control mechanism. The second mechanism, the adaptive control mechanism of crypt cell reduction, regulates the total size of the crypt without changing the relative size of the proliferative cell compartment in the crypt. This control mechanism may act in a local as well as in a systemic way. Some of the problems encountered in studies on the control of crypt cell production in the small intestinal epithelium, are briefly discussed.