Abstract: Abstract: Objective To observe the difference of radiosensitivity between hematopoietic stem and progenitor cells toward difference dose of 137Csγ--ray irradiation. Methods Seventy-two C57BL/6 mice were randomly divided into control group and irradiated groups (n=18 for each). Mice of control group received sham irradiation, and the rest accepted 2、4 and 6Gy total body irradiation respectively, dose rate 1.01Gy/min. After irradiation 14 day、35day and 56day, the peripheral bloods were collected by balls enucleation. The number of bone marrow mononuclear cells、hematopoietic stem and progenitor cells were counted. Results Peripheral blood of irradiated mice had significant changes in the number of white blood cells and platelets in a dose-response relationship; at 14d post-radiation compared with the control group, the number of bone marrow mononuclear cells (BMMNCs) was decreased 21.9%、39.9% and 54.4%（t=4.311、6.401、8.007，P<0.05）；the number of BMMNCs and hematopoietic progenitor cells (HPCs) were significantly lower at 35d and 56d after 2Gy、4Gy and 6Gy irradiation（t=4.185、3.596，P<0.05）； the number of bone marrow hematopoietic stem cells in the irradiated group were lower than that in the control group at 14d、35d and 56d （t=9.706、3.427~7.465，P<0.05）. Conclusions 137Cs γ-ray irradiation on mouse hematopoietic system has some damage, the radiosensitivity of HPC is markedly higher than that of the HSC, where the difference ; the damage caused by radiation to hematopoietic stem cells is persistent.

Key words: γ-ray, peripheral blood, hematopoietic stem cell, hematopoietic progenitor cell, 137Cs