Effect of continuous irradiation upon bone marrow haemopoietic stem cells in mice

The effect upon the stem cells in mouse bone marrow of continuous irradiation at different daily exposure rates ranging from 1R--1,000R was investigated by means of the exogenous spleen colony assay. At exposure rates between 1 and 50R/day, a stabilized state in the CFU value becomes established, its level decreasing with the increase in the daily exposure. Exposure rates between 80 and 1,000R/day lead to an initial steep, but later on flatter, decrease in CFU number until death of the animals. Mass deaths occur in mice when the CFU number falls below 5%. The results further delimit the range of exposure doses for which a dependence was found of the CFU number upon the total exposure dose of continuous radiation.