SENSITIVITY OF MURINE HEMATOPOIETIC STEM-CELL POPULATIONS TO X-RAYS AND 1 MEV FISSION NEUTRONS IN-VITRO AND IN-VIVO UNDER HYPOXIC CONDITIONS

The radiosensitivity of primitive haemopoietic stem cells that repopul ate the bone marrow with precursors of granulocytes and macrophages (M RA[CFU-C]), mature stem cells capable of forming spleen colonies in le thally irradiated recipients (CFU-S-7) and colony-forming units in cul ture (CFU-C) were determined in vitro and under hypoxic conditions in vivo for 1 MeV fission neutrons and 300 kV X-rays. The obtained D-0's were compared with previously observed D-0's after irradiation in vivo under normal oxic conditions. With 1 MeV fission neutron irradiation no significant difference in radiosensitivity of the cell populations was observed between normal in vivo irradiation and in vitro irradiati on. With 300 kV X-rays a lower radiosensitivity for all three cell pop ulations was observed after in vitro compared to in vivo irradiation. In vivo irradiation with fission neutrons under hypoxic conditions led to a small decrease in radiosensitivity. The obtained oxygen enhancem ent ratio (OER) for fission neutrons varied from 1.2 for MRA[CFU-C] to 1.5 for CFU-C. After in vivo irradiation with 300 kV X-rays under hyp oxic conditions much higher OERs were observed. An OER = 1.8 was obtai ned for CFUS and for MRA[CFU-C] and for CFU-C OER 3.0 and 2.9 were obs erved. These results indicate that the radioresistance of primitive ha emopietic stem cells (MRA[CFU-C]) compared to mature stem cells (CFU-S -7) is mainly due to intrinsic factors and not to differences in local ization or oxygenation between primitive and mature stem cells.