Response of Stem Cells in the Mouse Testis to Fission Neutrons of 1 MeV Mean Energy and 300 kV X Rays. Methodology, Dose-Response Studies, Relative Biological Effectiveness

Abstract

The response of spermatogonial stem cells in the CBA mouse to 1 MeV fission neutrons was investigated. Spermatogenetic clones arising from surviving stem cells were counted indirectly by determining the fraction of repopulated tubules in testicular sections at four time intervals (3, 5, 8, and 11 weeks) after irradiation with graded doses (50 to 410 rad) of fast neutrons. This fraction, the repopulation index (RI), is proved to be a linear parameter of the number of surviving stem cells when determined between 5 and 11 weeks after irradiation. The mean D/sub 0/ value of the stem cell survival curves in this period amounts to 81 +- 2 rad, pointing to the existence in the mouse testis of a highly radioresistant population of stem cells. Evidence was obtained that age of the animal does not influence stem cell sensitivity once the animal is adult, though clones grow at a slower rate in older animals. Our findings permit an estimate of the number of stem cells present in the normal unirradiated testis. This number amounts to about 1800. The existence of a population of more radiosensitive stem cells cannot be excluded. Stem cell survival after irradiation with 300 kV x rays wasmore » also investigated. The x-ray curve is exponential from 400 rad onward; the value of D/sub 0/ is 242 +- 7 rad, and the quasi-threshold dose D/sub q/ amounts to 165 rad. By comparison with the neutron curve for the same time interval the RBE of 1 MeV fission neutrons was calculated. The RBE decreases from 5.5, at a surviving fraction of stem cells of 0.30, to 4.1 at a fraction of 0.01.« less