during development

Notch1 is known to play a critical role in regulating fates in numerous cell types, including those of the hematopoietic lin-eage. Multiple defects exhibited by Notch1-deficient embryos confound the determination of Notch1 function in early hematopoietic development in vivo. To overcome this limitation, we examined the developmental potential of Notch1/ embryonic stem (ES) cells by in vitro differentiation and by in vivo chimera analysis. Notch1 was found to affect primi-tive erythropoiesis differentially during ES cell differentiation and in vivo, and this result reflected an important differ-ence in the regulation of Notch1 expres-sion during ES cell differentiation relative to the developing mouse embryo. Notch1 was dispensable for the onset of defini-tive hematopoiesis both in vitro and in vivo in that Notch1/ definitive progeni-tors could be detected in differentiating ES cells as well as in the yolk sac and early fetal liver of chimeric mice. Despite the fact that Notch1/ cells can give rise to multiple types of definitive progenitors in early development, Notch1/ cells failed to contribute to long-term definitive hematopoiesis past the early fetal liver stage in the context of a wild-type environ-ment in chimeric mice. Thus, Notch1 is required, in a cell-autonomous manner, for the establishment of long-term, defini-tive hematopoietic stem cells (HSCs)