Embryonic stem cells release potentially novel hematopoietic factors

We have observed that a severe decrease in the number of erythroid progenitor cells follows the long-term culture (LTC) of human primitive stem cells on an established mouse fibroblast feeder layer. This suggests that one, or several, factors necessary to support erythroid differentiation might be missing in these culture conditions. To improve the erythroid differentiation and stem cell output of LTCs, we have examined the hypothesis that the factors that regulate pluripotent stem cell proliferation and erythroid commitment can be found in media conditioned by embryonic stem (ES) cells. We have found that media conditioned by undifferentiated and differentiating ES cells can affect differentiation patterns in both short-term culture and LTC assays. Medium conditioned by undifferentiated ES cells increases the numbers of estimated LTC-initiating cells (est.LTC-IC) and potentiates granulocytic differentiation. In contrast, medium conditioned by ES cells undergoing differentiation increases the number of est.LTC-IC and is a powerful promoter of erythroid differentiation. In the presence of this medium, the number of erythroid progenitors generated after 5 weeks of LTC was increased up to 100-fold as compared with controls, and the number of est.LTC-IC was amplified up to 140-fold. These results offer a new approach for the identification of factors implicated in stem cell proliferation, self-renewal and commitment. In addition, the improved LTC conditions for the expansion of stem cells without reduction of their in vitro differentiation potential should be useful for a wide range of applications.