The erythroid and stem cell (colony-forming unit) responses were observed in the mouse after transfusion-induced polycythemia. Although there was a 40% decrease in erythroid differentiation 24 hours after transfusion, an increase in splenic CFU was not evident until 48 hours. This reached a level of approximately 200% by the seventh day, a time when maximum suppression of erythropoiesis was observed. With the administration of erythropoietin, a marked increase in splenic erythropoiesis was noted within 24 hours but the number of splenic CFU was not significantly changed from control values. Thereafter splenic CFU increased, following the erythroid response by approximately 24 hours. Bone marrow erythropoiesis gradually returned to the normal levels; however, quantitative estimates indicated at 15-20% decrease in marrow CFU. These responses to erythropoietin are similar to what has been observed in nonpolycythemic animals and suggest stem cell migration from the marrow to the spleen. The delays noted in stem cell accumulation after transfusion or treatment with erythropoietin are in keeping with the presence of an erythroid-committed stem cell compartment which serves as a buffer between the pluripotential compartment and the identifiable erythroid series.

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