Studies on the Radiosensitivity of Bone Marrow : I. The Relative Sensitivity of Erythroid and Myeloid Elements

1. Data are presented on (a) comparison of the regenerative capacity of erythroid tissue in the irradiated and nonirradiated cat after the stimulus of acute hemorrhage resulting in a 40 per cent reduction in the peripheral erythrocyte level; (b) comparison of the relative regenerative capacity of myelopoietic and erythropoietic tissue in the same animals after 200 r delivered to the whole body.

2. Recovery from standard anemia produced by hemorrhage immediately prior to irradiation injury was only slightly less rapid in the irradiated than in the control group. Estimates of functional reserve indicate that the cats exposed to 200 r were still able in the first 20 days after exposure to produce erythrocytes at nearly twice the normal production rate in the nonanemic, normal animal.

3. In sharp contrast to the findings for erythropoiesis, myelopoiesis as judged by peripheral leukocyte counts was severely depressed for about 30 days and final recovery delayed for 40 to 50 days after 200 r. In the face of the stimulus of leukopenia developing shortly after the radiation exposure, myeloid tissue appeared incapable of operating at more than a fraction of its normal productive capacity for a long period of time.

4. The data indicate that on the basis of functional impairment erythropoietic tissue is significantly less sensitive to radiation injury than is the myelopoietic tissue of the same animal under the conditions of this experiment.