Continuous gamma and neutron irradiation at low doses can increase the number of stromal progenitor cell (CFU-F) in mouse bone marrow

Experimental groups of male and female F1 (CBA × C57Bl/6) mice at the age of 3-4 months were exposed for 10 days to gamma irradiation (total dose 1.5 cGy, dose rate 0.15 cGy/day) or neutron irradiation (neutrons at average energy of 4.5 MeV at a total neutron flux ranging from 105 to 106 cm−2 and neutron flux density from 1 to 30 cm−2 s−1). These radiation doses were chosen so as to correspond to those received aboard spacecraft. [Mitrikas, V.G., Tsetlin, V.V., 2000. Radiation control onboard the MIR orbital manned station during the 22th solar cycle. Kosm. Issled. 38(2), 113-118.] Gamma irradiation stimulated the proliferation of femoral CFU-F, and their number increased by a factor of 1.5-4.5. The ectopic marrow grafts from γ-irradiated donors also increased in size. However, no changes in CFU-S proliferation rate and their number were observed. Neutron irradiation at a total absorbed dose of 2 × 10−1 cGy (total neutron flux 2.8 × 107 cm−2) produced a 1.5-3-fold increase in the number of femoral CFU-F, but that of CFU-S remained unchanged. At a lower total absorbed dose 0.82 × 10−2 cGy, total neutron flux 1.3 × 106 cm−2, the number of CFU-F remained at the control level. Therefore, the effect of radiation hormesis caused by neutron irradiation was observed at doses much lower than those of gamma irradiation.