Late effects of fission neutron and 60Co gamma radiation on the hematopoietic stem cell compartment in the mouse

The late effects of irradiation on a continually renewing cell population were investigated using the hematopoietic stem cell compartment as a model system. The CFU-S (colony-forming unit-spleen) content is suitable as a model system to quantitate late radiation effects on pluripotent stem cells in the femur and spleen. The CFU-S content is assayed by determining the number of nodules that form in the spleen of a supralethally irradiated host transplanted with a suspension of bone marrow or spleen cells. Late effects on the CFU-S content were determined at 4 times after B6CF1 male mice received a dose of fission spectrum neutrons or of 60Co gamma radiation at 17, 40, or 57 weeks of age. The effects of radiation dose, initial cell killing, age at irradiation, and radiation quality on the CFU-S content in both the femur and spleen were determined throughout the remaining life span. Three doses of fission neutrons or of gamma radiation were given to different groups of 17 week old mice to evaluate the influence of the initial cell killing on CFU-S content later in life. A relative biological effectiveness (RBE) of approximately 3 was used in dose selection. A reduced CFU-S content was observed late in life after all doses of fission neutrons or gamma irradiation, with all ages at exposure, and at all ages at sacrifice. New steady state levels of CFU-S content were attained by the first sacrifice time at 18 weeks after irradiation. After that time, relatively few changes were observed although some CFU-S contents were found to be significantly different at later sacrifice times. Thus, there is a persistent late effect of irradiation on the hematopoietic stem cell compartment which constitutes some component of residual injury