Outgrowth, long-term self-renewal, and terminal maturation of human erythroid progenitors derived from umbilical cord blood in serum-free medium can be modulated by steroid hormones. Homogeneous erythroid cultures, as characterized by flow cytometry and dependence on a specific mixture of physiologic proliferation factors, were obtained within 8 days from a starting population of mature and immature mononuclear cells. Due to previous results in mouse and chicken erythroblasts, the proliferation-promoting effect of glucocorticoids was not unexpected. Surprisingly, however, androgen had a positive effect on the sustained expansion of human female but not male erythroid progenitors. Under optimal conditions, sustained proliferation of erythroid progenitors resulted in a more than 10 9 -fold expansion within 60 days. Terminal erythroid maturation was significantly improved by adding human serum and thyroid hormone ( IntroductionHematopoietic development within the erythroid lineage requires a delicate balance between the opposing effects of proliferationpromoting factors maintaining the renewal capacity of immature erythroid progenitors and differentiation-inducing factors required for successful terminal maturation of erythroid progenitors into red blood cells. 1 In adult humans, erythroid differentiation produces about 2 ϫ 10 11 (roughly 20 g) red cells per day, representing the most vigorous proliferation process in the entire body. Because low numbers of erythrocytes are immediately life-threatening, erythropoiesis needs regulatory mechanisms temporarily altering the balance toward enhanced proliferation to cope with stress situations such as low oxygen levels at elevated altitudes (hypoxia) or massive blood loss.In vivo, both renewal and maturation of human erythroid progenitors proceed in parallel in the bone marrow. It has thus been difficult to assess the contribution of particular signaling pathways and their deregulation during aberrant depletion of these progenitors, or their death on differentiation in anemia, versus unscheduled progenitor proliferation (as occurring in leukemogenesis). Most information was obtained from established cell lines and, more recently, primary animal cell models of chicken and mouse in defined media. [1][2][3] The latter approach even allowed analysis of cells from genetically modified mice. [4][5][6] For sustained proliferation, both avian and murine primary erythroblasts require cooperation between erythropoietin (Epo), stem cell factor (SCF), and glucocorticoids (dexamethasone [Dex]). Involvement of Epo and SCF was expected from their contribution to proliferation and survival of erythroid cells, 7-9 whereas the stress hormone Dex massively prolonged progenitor proliferation (from 10 to 30 days in chickens, 7 to 20 days in mice), at the same time reducing their rate of spontaneous differentiation. Our idea that this might point to specific roles for glucocorticoids in stress erythropoiesis could be verified in vivo using mutant mice carrying a dimerizationdefective glucoco...

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