On the in vivo action of erythropoietin: a quantitative analysis.

The composite response of the erythron to exogenous erythropoietin has been studied in normal, splenectomized, and polycythemic mice. After stimulation the normal animal doubled its marrow nucleated red cells by the 3rd day with little further change by the 5th. Nucleated red cells within the spleen began to increase sharply on the 2nd day and, by the 5th, exceeded those in the marrow. The total nucleated erythroid response represented a fourfold increase. Reticulocytes lagged behind the expansion of the nucleated red cell mass, but by the 5th day the original ratio was re-established. Hemoglobin synthesis was increased, but the ratio of hemoglobin synthesized in nucleated red cells and reticulocytes was basically unchanged. Early displacement of marrow reticulocytes into circulation and the production of a larger red cell also occurred. No evidence of a change in the number of erythroid mitoses was found; only a slight decrease in the average cell cycle time was demonstrated. Thus, whereas erythropoietin stimulation induced several changes in erythropoiesis, the increased number of cells entering into the maturing pool appeared to be of greatest quantitative significance.Splenectomy reduced the proliferative response of the erythron over 5 days stimulation to three-fourths that found in the normal animal. This difference, also reflected in a proportionately lower reticulocyte response and increment in circulating red cell mass, suggests that erythropoiesis within the mouse marrow is spatially or otherwise restricted and that the spleen provided a supplemental area of erythroid expansion.