SURVIVAL OF SPERMATOGONIAL STEM-CELLS IN THE MOUSE AFTER SPLIT-DOSE IRRADIATION WITH FISSION NEUTRONS OF 1-MEV MEAN ENERGY - EFFECT OF THE FRACTIONATION INTERVAL

Spermatogonial stem cells, which survive a neutron dose of 150 rad, all belong to a radioresistant population, characterized in the present study by a D0 value for neutrons of 96 ± 5 rad. Shortly after a dose of 150-rad neutrons the surviving stem cells greatly increased in radiosensitivity; at 24 hr after this dose they were characterized by a D0 of 25 ± 1 rad. At 8 days after 150 rad the D0 value was higher: 46 ± 4 rad and it increased further to 54 ± 3 rad at 15 days. The increase in D0 ceased between the 15th and 26th day after irradiation; at the latter time a D0 of 49 ± 2 rad was found. These data show that during the first 26 days after a dose of 150-rad neutrons the radiosensitivity of the surviving stem cell population first increases and then slowly decreases to stay at a relatively high level of radiosensitivity from 15 days on without signs of a return to the preirradiation value. The stem cell population, although expanding after the first dose, did not return to its normal size within the period investigated.