Quantitative Studies on the Radiation Pathology of the Bone Marrow of Small Laboratory Animals: II. Marrow Response to Whole-Body X-Irradiation in Rodents of Different LD 50

In a series of experiments carried out during 1959- 1963, the acute bone marrow response to x rays (LD₅₀ dose) was investigated by direct counting techniques in strains of mice of different radiosensitivity (BALB/c, A/Crgl/2, C57B, BCF₁) and in the Chinese hamster (Cricetulus griseus), which has the highest LD/sub 50/ so far known in mammals. In all experiments, the animals were given a wholebody dose of 250-kvcp x rays (HVT = 1.5 mm Cu; absorbed dose rate about 45 rads/min; absorbed dose in soft tissue about 250 rads). The relatively uniform pattern of change in the nucleated cell count is consistent with the assumption that similar physiological mechanisms were at work in each animal but that genetic factors introduced quantitative differences in response. The principal differences were in the control counts, the percentage loss in control counts by four days after irradiation, and the percentage recovery by 18 days after irradiation. The Chinese hamster had the lowest control nucleated cell count and a recovery rate of this count that was no better than that of the mice. When the undifferentiated stem cell count was examined, however, it appeared that the hamster's recovery was remarkable; a marked hyperplasia of these cells was observed on days 4, 6, and 8 after exposure. Some recovery was observed 12 hours after exposure. It is suggested that this peculiar response of the stem cells of the hamster marrow is related to the high radioresistance of the species. (auth)