Comparison of radiation damage and recovery of hematopoietic system in mice of different ages

Objective To observe and compare the effects of radiation on the damage and recovery of hematopoietic system in mice of different ages (juvenile, adult, and aged mice). Methods Sixty male C57BL/6 mice of various ages were randomly divided into control, 2 Gy irradiation, and 4 Gy irradiation groups. The control group received sham irradiation, and the other two were administered with 137Cs γ total body irradiation. On the 3rd, 7th, 14th, and 28th days after irradiation, peripheral blood was collected, white blood cells, red blood cells and platelets were counted, unilateral femur bone marrow nucleated cell counts were determined, and percentages of hematopoietic stem cell (HSC), hematopoietic progenitor cell (HPC) and colony forming unit-granulocyte macrophage were calculated. One way ANOVA was used for comparison among groups, while t test was used for comparison between two groups. Results White blood cell count, unilateral femur bone marrow nucleated cell count, hematopoietic stem cell and progenitor cell count of irradiated mice were reduced to the lowest values on the 3rd day. Compared with the control group, the white blood cell count of juvenile mice decreased to 34.8% (2 Gy irradiation) (t=6.129, P<0.05) and 19.6% (4 Gy irradiation) (t=7.084, P<0.05), while adult mice decreased to 43.8% (2 Gy irradiation) (t=3.043, P<0.05) and 20.0% (4 Gy irradiation) (t=7.084, P<0.05), aged mice decreased to 19.0% (2 Gy irradiation) (t=22.080, P<0.05) and 8.4% (4 Gy irradiation) (t=24.590, P<0.05). The decrease in 4 Gy group was more serious than that in 2 Gy group in a dose-dependent manner. At 28th day after irradiation, unilateral femur bone marrow nucleated cell count in irradiated juvenile mice did not return to normal, the HSC percentage in the juvenile mice of 4 Gy group was still lower than 50%, and the HPC percentage of irradiated juvenile mice recovered to approximately 50% of that of the control group. At 28th day after irradiation, the percentage of colony forming unit-granulocyte macrophage returned to normal for the irradiated adult mice, however, the irradiated juvenile mice (2 Gy irradiation: t=2.067, P<0.05; 4 Gy irradiation: t=3.358, P<0.05) and aged mice (2 Gy irradiation: t=2.586, P<0.05; 4 Gy irradiation: t=4.772, P<0.05) were significantly lower than those of adult mice, which did not return to normal level. Conclusions After irradiation, majority of the hematopoietic system of the irradiated group showed acute, dose-dependent inhibition on the 3rd day. After the recovery period, most of the indicators returned to normal values on the 28th day. The results suggest that the hematopoietic stem cell functions in juvenile mice are more sensitive to radiation injury and poorly recover after radiation compared with those in adult mice. In addition, the recovery for bone marrow cell proliferation was decreased in aged mice. Key words: Mice; Radiation injuries; Hematopoietic system; Repair; Age difference