Recovery of number and radiosensitivity of myeloid stem cells (CFU-C) in the sublethally X-irradiated mice.

After whole-body irradiation of mice with 300R X-rays, frequency of myeloid stem cells (CFU-C) in the femoral marrow decreased markedly within 24 hrs and returned to a nearly normal level in 3 or 4 days, while it decreased again thereafter. Bone marrow cells isolated from these mice were plated in semi-solid agar, and exposed against X-rays for the second time. After the cells were cultured for 1 week for colony formation, no significant difference was observed in the survival ratio between the CFU-C of the control and the CFU-C of the animals irradiated 24 hrs before. The result supports the assumption that recovery of the cells occurred quickly during the fractionated irradiation. In contrast, a slightly increased radiosensitivity of CFU-C was observed 3 or 4 days after the initial irradiation, while it was no longer observed 7 days after irradiation. It seems that latent injury was expressed through amplification during repeated cell divisions, and that a mechanism operated rather slowly for the recovery of this kind of injury.