Stem Cell Renewal in Spleen and Bone Marrow of Mice after Repeated Total-body Irradiations

SummaryTo study the effect of repeated sublethal irradiations on the multiplication of haemopoietic stem cells, mice were irradiated one, two or three times with an x-ray dose of 440 rads. The time-interval between the first and the second irradiation was 7 days. When three irradiations were given, the time-interval between the last two exposures varied between 7 and 100 days.The growth-rate of CFU's in femoral bone-marrow after two irradiations was slower than after one irradiation; at around the 15th day the overshoot in the spleen was smaller after two irradiations than after one. These findings were even more pronounced when a third dose was given within a few weeks after the second. When the third dose was given longer than 5 weeks after the second, the CFU growth curve in bone-marrow and spleen was closer to the curve obtained after one irradiation. The significance of the findings in respect of the capacity of stem cells for self-renewal are discussed.