The dose survival and regeneration characteristics of irradiated jejunal crypt stem <tex-math>$\text{cells}^{2}$</tex-math> were studied in the mouse by using a cell-cloning technique. An outstanding characteristic which was observed is that crypt cells have a large capacity for sublethal injury and its repair. It is difficult to separate the contributions of sublethal injury repair, division cycle synchrony, and cell proliferation to the increased survival measured in two-dose experiments, but we have estimated that, of an overall recovery ratio of about 70 in 24 hours, about a factor of 50 may result from repair of sublethal injury. The single-dose survival curve measured in oxygen is described by a <tex-math>$\tilde{D}_{0}$</tex-math> value of 97 rads. A second-dose survival curve measured 24 hours after an initial dose of 660 rads is roughly parallel to the single-dose curve, but at shorter fractionation intervals the slope may vary. Changes in the shoulder and slope of second-dose survival curves cause a fluctuating pattern in the recovery curve for at least the first 15 hours postirradiation. The time at which survivors of a dose of 660 rads resume cell division is not clear, but at about 2 1/2 days after a first dose, the loss of progeny through terminal differentiation probably stops, resulting in a rapid expansion of cells in proliferation. This leads to a relatively early overshoot in the size of the proliferative population, compared to its size before irradiation, followed by a return to a normal level and a resumption of steady-state kinetics within about 1 week postirradiation.

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