Hematopoietic cells in cultures of the murine embryonic aorta-gonad-mesonephros region are induced by c-Myb

Definitive hematopoiesis begins in the para-aortic, splanchnopleural (P-Sp) and aorta-gonad-mesonephros (AGM) regions of mouse embryos and then switches to the fetal liver [1-3]. Gene-targeted mice lacking the c-Myb transcription factor have severe hematopoietic defects in the fetal liver [4], The role of c-Myb, if any, in P-Sp/AGM hematopoiesis has not been examined, however, Recently, we reported that oncostatin M can effectively expand both hematopoietic and endothelial-like cells from in vitro cultures of the AGM region [5]. Using this cell culture system, we examined the involvement of c-Myb in definitive hematopoiesis in the P-Sp and AGM regions. When primary cultures from the P-Sp or AGM regions of wildtype mouse embryos were probed with an anti-c-Myb antibody, hematopoietic cells but not endothelial like cells showed positive staining. In contrast, in the P-Sp/AGM culture from c-myb(-/-) embryos, no hematopoietic cells were generated and endothelial like cells predominated, indicating that the impairment of hematopoiesis in the liver of c-myb(-/-) embryos is actually preceded by a defect in P-Sp/AGM hematopoiesis. Hematogenic precursor cells were, however, still present in an inert but competent form among the endothelial-like, adherent cell population of c-myb(-/-) P-Sp/AGM cultures. When infected with a retrovirus carrying c-myb cDNA, these cultures gave rise to a significant number of hematopoietic cells. The rescued cells, unlike wild-type hematopoietic cells, were negative for c-Kit (a marker of hematopoietic progenitors), but did express other hematopoietic cell surface markers such as Mac-1, Gr-1 (myeloid markers), CD19, B220, Thy-1.2 (lymphoid markers), and Ter119 (an erythroid marker). Thus, c-Myb plays a role in the generation of hematopoietic cells in the embryonic P-Sp and AGM regions.