Mitotic activity in vitro of erythroblasts previously exposed to erythropoietin.

erythropoietin, seems at present well established, and investigative work in progress in several laboratories is aimed at specific problems such as the chemical nature of erythropoietin, its site of production, metabolism and mechanism of action. Relatively little experimental work has been done on the mechanism of action of erythropoietin. Theoretically, erythropoietin can increase erythropoiesis in one of three ways: it can induce more stem cells to differentiate into erythroblasts, increase the mitotic activity of differentiated erythroblasts, or do both. Evidence obtained recently with autoradiographic technics has been interpreted as favoring an increased flow of stem cells into the erythroblast compartment under erythropoietin stimulation, whereas the proliferation and maturation of erythroblasts are not affected.1’2 A different conclusion, implicating marrow erythroid cells as the target organ of erythropoietin, has been reached by one of us,3 who observed an increased mitotic activity of erythroblasts incubated in the presence of erythropoietin in marrow cultures. In the present report, further experimental work in support of a direct effect of erythropoietin on the mitotic activity of erythroblasts will be presented and discussed in relation to findings by other workers and to current theories on the mechanism of action of erythropoietin.