Survival of spermatogonial stem cells in the mouse after split-dose irradiation with fission neutrons of 1-MeV mean energy or 300-kV X rays.

Radioresistant, colony forming stem cells in the mouse testis move into a much more radiosensitive phase of their cell cycle shortly after irradiation. This is demonstrated in experiments in which total doses of 300 rad of neutrons and 1200 rad of x rays were split into two equal fractions. With neutron fractionation a rapid decrease of stem cell survival as a function of the interval between dose fractions was observed, and survival was reduced to less than 3 percent of the single-dose value for intervals ranging from 16 to 40 hr. At that time the radiosensitivity of the survivors of the first dose is greatly increased. The D/sub 0/ value of the neutron dose--response curve 24 hr after a first dose of 150 rad is 25 +- 1 rad, much less than the D/sub 0/ value for single exposure which is equal to 77 +- 3 rad. Between 44 and 72 hr after the first dose, increased survival levels were found, on the average amounting to 15 percent of the single dose level. The increased survival is caused by an entrance of first-dose survivors into a cell cycle phase with a D/sub 0/ value higher than 25 rad of neutrons.more » With x-ray fractionation a similar decrease of stem cell survival in the 16 to 40-hr period was observed, although the decrease is not as large as with neutron fractionation. For the 4-hr interval, however, an increase in cell survival was found, amounting to 160 percent of the single-dose value. This increase can be quantitatively explained by the repair of induced sublethal damage. Evidence was obtained that the radiosensitivity of stem cells is not influenced by nembutal anaesthesia. Furthermore, no apparent diurnal rhythm in radiosensitivity was observed. The results indicate that the irradiation triggers a shorter cell cycle for the surviving stem cells.« less