Hematopoietic induction and respecification of A-P identity by visceral endoderm signaling in the mouse embryo

The anteroposterior axis of the developing embryo becomes morphologically a pparent at the onset of gastrulation with the formation of the primitive st reak. This structure, where the first mesodermal cells arise, marks the pos terior aspect of the embryo. To examine the potential role of non-mesoderma l signals in specifying posterior (hematopoietic and endothelial) cell fate s in the mouse embryo, we have devised a transgenic explant culture system, We show that interactions between primitive endoderm and adjacent embryoni c ectoderm or nascent mesoderm are required early in gastrulation for initi ation of hematopoiesis and vasculogenesis, Surprisingly, primitive endoderm signals can respecify anterior (prospective neural) ectoderm to a posterio r mesodermal fate, resulting in formation of blood and activation of endoth elial markers, Reprogramming of anterior ectoderm does not require cell con tact and is effected by stage-dependent, short-range, diffusible signal(s), Therefore, primitive endoderm signaling is a critical early determinant of hematopoietic and vascular development and plays a decisive role in anteri or-posterior patterning during mouse embryogenesis.