The emergence of the chorioallantoic placenta in euthe-rian mammals is a critical developmental process that brings the maternal and fetal vascular systems into close proximity thereby providing the fetus with the means to obtain nutrients, respire, and eliminate wastes. The failure to establish and maintain vascular circulation and the lack of transition from yolk-sac-based to liver-based hematopoiesis are lethal events. The fetal blood vessels constituting the chorioallantoic placenta are derived from the allantois, a structure that originates, in part, from the extraembryonic mesoderm that also gives rise to the visceral yolk sac, the chorionic mesoderm, and the am-niotic mesoderm. As the allantois enlarges during development, it fuses with the chorion, and shortly thereafter, the allantois becomes visibly vascularized throughout. Although vasculogenesis and hematopoiesis are developmentally associated, vasculogenesis maybe independent of erythropoiesis in the allantois.
Red blood cells are formed from hematopoietic stem cells that reside in the adult bone marrow and are capable of differentiating into at least eight morphologically and functionally distinct types of mature blood cells. In the mouse, hematopoietic events begin in the yolk sac at Day 7 of pregnancy, shift to the fetal liver at midgestation, and then to the bone marrow shortly before birth. Extraembryonic blood islands develop from groups of mesodermal cell aggregates in the developing yolk sac, and the cells at the interior of these aggregates become primitive blood cells. As the yolk sac becomes vascularized, the extraembryonic circulation is directly linked to that of the embryo, and the hematopoietic cells can circulate within the vasculature between intraembryonic and extraembryonic sites.