It has previously been established that the intestinal crypt epithelium has a remarkable capability to react to injury by increasing the production of new cells. Accordingly, the seriously injured nonviable and/or dying cells are replaced and the critical steady state necessary for the survival of the animal is reestablished. The number of cells in mitosis (MF/crypt) and in DNA synthesis (LN/crypt) in the mouse duodenal epithelium have been studied at closely spaced time intervals following a series of single-dose, whole-body,60 Co gamma irradiation. These studies show that the pattern of change in the numbers of cells in mitosis (M) and synthesis (S) are qualitatively similar at all dose levels. The degree of change is directly related to dose, increasing as dose increases. The results show that (1) cells are immediately blocked in G1 and G2, (2) following an immediate drop in numbers of cells in S and M the number of cells in these two phases rise rapidly and overshoot control values, (3) during the recovery period, the number of S and M cells shows a series of peaks and troughs which are similar to the diurnal fluctuations found in nonirradiated controls, and (4) cell production returns to normal control levels.

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