Sensitivity of murine haemopoietic stem cell populations to X-rays and 1 MeV fission neutrons in vitro and in vivo under hypoxic conditions

Abstract

Abstract. The radiosensitivity of primitive haemopoietic stem cells that repopulate the bone marrow with precursors of granulocytes and macrophages (MRA CFU-C) , mature stem cells capable of forming spleen colonies in lethally irradiated recipients (CFU-S-7) and colony-forming units in culture (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 's were compared with previously observed 0 D's after irradiation in vivo under normal oxic conditions. 0 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 irradiation. With 300kV X-rays a lower radiosensitivity for all three cell populations 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 enhancement 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 hypoxic conditions much higher OER s were observed. An OER=1.8 was obtained for CFU-S and for MRA CFU-C and for CFU-C OER 3.0 and 2.9 were observed. These results indicate that the radioresistance of primitive haemopietic stem cells (MRA CFU-C) compared to mature stem cells (CFU-S-7) is mainly due to intrinsic factors and not to differences in localization or oxygenation between primitive and mature stem cells.