Kinetics and possible regulation of crypt cell populations under normal and stress conditions.

The proliferative organisation of the crypts of the small intestine is considered with special reference to the existence, location and numbers of stem cells. It is concluded that the crypt contains a minority population of cells at its base that are the true stem cells. These cells provide an input of cells for the larger proliferative compartment higher up the crypt. The presumptive stem cells may be pluripotent and produce Paneth, goblet and columnar cells. They are probably also the cells which are capable of regenerating the crypt after X-ray depopulation. Radiobiological experiments indicate that the number of cryptogenic cells is less than 80, while the results of several experiments on the kinetics of the cell populations indicate that the number of stem cells is about 20. The stem cells are located in the Paneth cell zone of the crypt, and are apparently passing through the cell cycle at about half the speed of the proliferative cells. It is these vital stem cells that will determine the response of the mucosa to therapeutic agents, probably play a role in carcinogenesis and play a dominant role in mechanisms controlling cell proliferation.