Abstract
Effects of cell concentrations on the survival and repopulation of haemopoietic stem cells after irradiation were studied in the long-term culture of mouse bone marrow cells in vitro. No difference was observed in the survival of the stem cells among cultures in which 0-107 cells were re-inoculated on the adherent cell colonies in the culture flask. Stem cells showed a significant proliferation within 1 week and the number of the stem cells exceeded the control in 3 weeks after irradiation in the cultures with less than 106 re-inoculated cells per flask. In contrast, there was a considerable delay in the onset of stem cell proliferation after irradiation in the culture with 107 cells per flask. Based on these results, a possiblity that a stimulator of stem cell proliferation, released from irradiated stromal cells, is cancelled by an inhibitory factor produced by irradiated or unirradiated haemopoietic cells is postulated.
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