Long‐term haemopoietic injury in mice after repeated irradiation: Precursor‐cell cycling and its regulation

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Abstract
The cycling rate of haemopoietic stem cells (day 9 CFU‐S) and granulocyte‐macrophage colony forming cells (GM‐CFC) in mouse femora was, in response to reduced numbers, elevated at all times of sampling between 3 weeks and 10 months after 4 repeated doses of 4.5 Gy X‐rays (3 wk between doses). The level of a stimulator of CFU‐S cycling was also elevated, and this was observed in both axial and marginal regions of the marrow inside the shaft. However, the rate of production of the stimulator was low; lower than previously reported in marrow regenerating after a single dose of 4.5 Gy, indicating damage to the regulatory stromal cells. The distribution of CFU‐S across the axial and marginal zones of femoral marrow was changed from that in normal mice, where higher concentrations were found near the bone surface, to a more uniform distribution.