Experimental medullary aplasia

Young adult mice given 6 μCi per gram of body weight of ⁸⁹Sr, a bone-seeking beta-emitting isotope, develop bone marrow aplasia. Compensatory extramedullary hematopoiesis is observed in the spleen, and the hematocrit remains essentially unchanged. Following splenectomy, even as late as 37 days after administration of radiostrontium, most of the animals die of pancytopenia. ⁵⁹Fe incorporation by newly formed red cells remains unchanged following ⁸⁹Sr administration. ⁵⁹Fe incorporation following splenectomy is 42 per cent in control animals and 58 per cent in sham-operated control animals. However, after splenectomy it drops to 0.6 per cent in mice previously given 6 μCi per gram of ⁸⁹Sr. Therefore, red cells produced in animals after radiostrontium administration are essentially of splenic origin, while normal animals produce only a small fraction of their erythrocytes in the spleen. Peripheral counts of mice given radiostrontium reveal a persistent lymphopenia, neutropenia, and slight reticulocytosis while thrombocyte values are normal. The radiation dose received by the spleen, as determined by LiF dosimeters, is cumulatively 40.2 rads 1 day, 54.2 rads 2 days, and 75.0 rads 8 days postinjection of 6 μCi ⁸⁹Sr per gram of body weight. The marked leukopenia does not appear to be dose-related over the 2 to 6 μCi per gram range. These observations suggest intracorporeal irradiation of a radiosensitive fraction of the blood lymphocytes as they circulate through the marrow may be a factor in the production of leukopenia.