Radiation Sensitivity and Proliferative Recovery of Hemopoietic Stem Cells in Weanling as Compared to Adult Mice

Abstract

Effects of radiation on survival and proliferation of stem cells were compared in weanling and adult mice, and the observed differences related to the differences in granulocyte count and 30 -day LD50. Colony-forming units (CFU) of both spleen and femur were more radiosensitive in weanlings. In nonirradiated weanlings the number of CFU in the spleen was greater than in adults, so that after irradiation the numbers were approximately the same in both. Endogenous spleen colony counts were also the same in the 2 age groups given the same radiation dose, although mortality was higher in the weanlings. In the femur there were fewer CFU initially as well as a lower percentage survival, which resulted in the weanlings having less than half as many CFU as adults after irradiation. Granulocytic areas were rare in the recovering spleen but numerous in the femur. Thus, the greater damage to bone marrow, rather than to spleen, was probably responsible for the lower postirradiation granulocyte counts in weanlings. There was no difference in the pattern of proliferative recovery of surviving CFU of weanlings and adults. The principal difference in damage affecting survival was therefore in the death of marrow stem cells. Extrapolating from the dose range studied, This would account for approximately half of the difference in 30-day LD50.