Enhancement of the growth of human early erythroid progenitors by bone marrow conditioned media.

The inclusion of condition media (CM) obtained from cultures of human bone marrow cells increased the number of erythroid bursts observed after a 14-day culture period. In contrast, CM had no effect on the number of erythroid colonies observed at five days. The effect of CM on burst growth was more pronounced at limiting growth conditions--low serum or low erythropoietin (EPO) concentrations. Burst-forming units (BFUe) survived in culture and were able to respond to CM by increased colony growth and hemoglobin synthesis as late as eight days following the initiation of cultures. CM prevented the decline in the number of burst observed in the absence of EPO during the first four days of culture; however, when EPO was added at later times, a decrease in burst formation was seen. Increased sensitivity of burst-forming cells to low concentrations of EPO and a reduced serum requirement were observed in cultures containing CM. The incorporation of 59Fe into heme was also increased by CM, giving a more sensitive test of CM activity than burst number. CM obtained from bone marrow cultures, therefore, probably contains burst-promoting activity (BPA), one of the factors required for the proliferation of early erythroid progenitors.