Stimulation of erythroblast formation in suspension cultures of murine marrow by a factor in normal mouse serum, and its relationship to erythropoietin.

Addition of normal mouse serum to liquid suspension cultures of murine marrow cells resulted in dose-related increases in the number of heme-synthesizing erythroid cells. Removal of late normoblasts and reticulocytes by differential immune lysis prior to culture greatly enhanced the difference in heme synthesis between cultures with or without added mouse serum, permitting detection of as little as 1 microliter/l.5 ml of medium. Cell counts on these cultures indicated that the mouse serum factor actually stimulated formation of late normoblasts through proliferation and maturation of their precursors. The effect required either the presence of Ep in the medium, or the presence in the to be cultured cell suspensions of proerythroblasts which had been induced by or had contact with Ep prior to collection of the marrow.