The nature of radiation damage of haemopoietic stem cells under continuous irradiation at low dose rate.

Mice irradiated continuously at a low dose rate of 70 rad/day showed not only a drop of CFU-S content in the bone marrow but also an apparent functional disturbance of haemopoietic stem cells. With increase in accumulated dose of radiation, the damage of the haemopoietic microenvironment becomes evident. From the changes caused by chronical irradiation, it is believed that haemopoietic stem cells is more sensitive than haemopoietic microenvironment, and the radiation injury by hemopoiesis involves both damage of haemopoietic stem cells and microenvironment. Furthermore, in the course of recovery of hemopoietic injury, microenvironment plays as important role. In comparison of the growth curves of CFU-S from normal and irradiated bone marrow cells in irradiated recipient spleens or cultured in diffusion chambers, it is suggested that the nature of radiation damage of haemopoietic stem cells in continuous irradiation at low dose rate is mainly attributed to the deterioration of self replicative capacity of CFU-S.