The acute and long-term effects of total-body X irradiation (TBI) on early erythroid progenitors, burst-forming units (BFU-E), in the bone marrow of beagles were studied for midline tissue doses of 1.6 and 2.4 Gy. After both radiation doses the initial reduction in the concentration of BFU-E was greater than that found for the granulocyte-macrophage progenitor cells (GM-CFC) and thus was in general agreement with the higher in vitro radiosensitivity of BFU-E compared to GM-CFC. After TBI with 1.6 Gy the GM-CFC and BFU-E returned to their normal levels within 2-4 weeks without showing long-term radiation effects. In contrast, after TBI with 2.4 Gy the concentrations of GM-CFC and BFU-E remained below the pretreatment levels up to 1 year after exposure. For a given midline tissue dose, the extent of the long-term effect of radiation on the BFU-E in a certain bone marrow site seems to be dependent on the local radiation dose in the respective bone marrow space. The minor radiation effects observed in the erythrocyte concentration in the peripheral blood, the hematocrit, and the hemoglobin concentration point to the enormous compensatory capacity of the more mature erythropoietic transit population to increase the proliferative capacity upon demand.

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