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 apparent at the onset of gastrulation with the formation of the primitive streak. This structure, where the first mesodermal cells arise, marks the posterior aspect of the embryo. To examine the potential role of non-mesodermal signals in specifying posterior (hematopoietic and endothelial) cell fates in the mouse embryo, we have devised a transgenic explant culture system. We show that interactions between primitive endoderm and adjacent embryonic ectoderm or nascent mesoderm are required early in gastrulation for initiation of hematopoiesis and vasculogenesis. Surprisingly, primitive endoderm signals can respecify anterior (prospective neural) ectoderm to a posterior mesodermal fate, resulting in formation of blood and activation of endothelial markers. Reprogramming of anterior ectoderm does not require cell contact 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 anterior-posterior patterning during mouse embryogenesis.