INITIATION OF HEMATOPOIESIS AND VASCULOGENESIS IN MURINE YOLK-SAC EXPLANTS

The blood islands of the visceral yolk sac (VYS) are the initial sites of hematopoiesis in mammals. We have developed a yolk sac explant cul ture system to study the process of blood cell and endothelial cell de velopment from extraembryonic mesoderm cells. No benzidine-positive ce lls or beta H1-globin mRNA expression was detected at the primitive st reak or neural plate stage of development (E7.5). However, when isolat ed E7.5 dissected tissues were cultured for 36 to 72 hours in serum-fr ee medium, hundreds of hemoglobin-producing cells and embryonic globin gene expression were identified in both intact yolk sec and VYS mesod erm explants. Explanted E7.5 extraembryonic mesoderm tissues thus reca pitulate in vivo primitive erythropoiesis and do not require the prese nce of a vascular network or the VYS endoderm. Yolk sac blood islands also contain endothelial cells that arise by vasculogenesis and expres s flk-1, We detected flk-1 mRNA as early as the primitive streak stage of mouse embryogenesis. Culture of embryo proper and intact VYS expla nts, which contain both mesoderm and endoderm cells, produced capillar y networks and expressed flk-1, In contrast, vascular networks were no t seen when VYS mesoderm was cultured alone, although flk-1 expression was similar to that of intact VYS explants. The addition of vascular endothelial growth factor to VYS mesoderm explants did not induce vasc ular network formation, These results suggest that the VYS endoderm or its extracellular matrix is necessary for the coalescence of developi ng endothelial cells into capillary networks. (C) 1995 by The American Society of Hematology.