The in vitro radiosensitivity of hemopoietic stem cells from control and preirradiated infant mice.

The radiosensitivity of hemopoietic stem cells isolated from infant mice (6 or 9 days of life), of infant preirradiated mice (exposed to 126 rad on day 6 and assayed at day 9 of life) and of adult C57/Bl mice was assayed on the basis of their capacity to form spleen colonies and to incorporate iododeoxyuridine after transplantation into heavily irradiated hosts. Stem cells of infant non-irradiated mice have a DO of 115 rad compared to 72 rad for adult mice whereas the DO of preirradiated infant mice has diminished to 80 rad. No significant difference was seen between spleen and bone marrow cells or between total cells and cells not sensitive to 3H-thymidine. It is postulated that this sensitization of stem cells caused by a preirradiation is responsible for the greater mortality of infant mice after fractionated exposure compared to a single one.