In vitro hematopoietic and endothelial potential of flk-1^-/- embryonic stem cells and embryos

Mice deficient in the Flk-1 receptor tyrosine kinase are known to die in utero because of defective vascular and hematopoietic development. Here, we show that flk-1^-/- embryonic stem cells are nevertheless able to differentiate into hematopoietic and endothelial cells in vitro, although they give rise to a greatly reduced number of blast colonies, a measure of hemangioblast potential. Furthermore, normal numbers of hematopoietic progenitors are found in 7.5-day postcoitum flk-1^-/- embryos, even though 8.5-day postcoitum flk-1^-/- embryos are known to be deficient in such cells. Our results suggest that hematopoietic endothelial progenitors arise independently of Flk-1, but that their subsequent migration and expansion require a Flk-1-mediated signal.