Persisting radiation effect on murine 7-day and 12-day CFU-S.

If radiation-induced reduction in the proliferative ability of bone marrow cells is due to a change in the age structure of stem cells, it might be measurable by an increase in the ratio of 7- to 12-day spleen colonies. This increase has not been detected at either three weeks or one year after 5 Gy gamma-irradiation. It was found, however, that diameters of colonies and weights of hemopoietic tissue per colony were reduced after both periods of recovery. Thus, slow colony growth may have masked a change in the stem cell age structure. It is concluded that injury persists in a proportion of stem cells that continues the production of progeny and may contribute to late effects. Persisting cellular injury, in modified form, may affect all mitotically active tissues.