Abstract
The survival rate of mice after acute high dose irradiation is markedly improved by low dose whole-body preirradiation. The number of hematopoietic stem cells (HSCs) is related to the kinetics of the improvement in survival rate. One of the possible mechanisms underlying the adaptive response of mice is the acquisition of radioresistance by HSCs. Conventionally, preirradiation is carried out at a high dose rate (HDR). Therefore, the adaptive response induced by low dose-rate chronic irradiation is virtually unknown. We have been investigating the induction of such adaptive response in vivo in terms of the radioresistance of HSCs under very low dose-rate (around 1 mGy/hr) irradiation for different periods. As results, the survival rate of HSCs after acute challenge irradiation was enhanced by the low dose-rate irradiation but the acquisition showed specific intervals. We are interested in how HSCs acquire radioresistance with low dose-rate chronic irradiation to establish methods of evaluating adaptive response without performing challenge irradiation. Thus far, we have observed colony forming units in spleen (CFU-S), which was originally established for HSCs. Alternatively, we have evaluated HSC populations that indicate a more primitive stage than CFU-S populations without performing challenge irradiation. Further analysis of the effect of low dose-rate irradiation on such primitive stem cells will provide new directions of future research on the adaptive response of mice.
