The proliferative status of microcolony-forming cells in mouse small intestine

The technique of thymidine (TdR) suicide has been used with the intestinal microcolony assay to demonstrate that in the middle of the light cycle, nearly all intestinal clonogenic cells, in the B6D2F1 mice used in these experiments, were not in S phase. Doses of tritiated thymidine [3H]TdR up to 1 mCi/mouse did not kill a significant fraction of those clonogenic cells which survived a test dose of 12 Gy gamma-rays. This finding supports some data in the literature, but conflicts with others. However, the suicide technique was found in the studies reported here to be very efficient in sterilizing clonogenic cells in the middle of the dark cycle, and also in a regenerating epithelium at day 3 after a dose of 9 Gy. This implies that the technique can discriminate well between populations of clonogenic cells which differ in their content of cells in S phase. The lack of a suicide effect in the middle of the light cycle indicates that the majority of proliferative epithelial cells are not clonogenic.