Sites of Fetal Hematopoietic Cells Production
During embryogenesis, hematopoiesis occurs in spatially and temporally distinct sites, including the extraembryonic yolk sac, the fetal liver, the thymus, and the preterm marrow. The origin of hematopoietic cells is closely tied to gastrulation, the formation of mesoderm cells, and to the emergence of the endothelial lineage. Hematopoiesis is first established soon after implantation of the blastocyst, with the appearance of primitive erythroid cells in blood islands of the yolk sac beginning at day 18 of gestation.
The spatial and temporal association of embryonic red cells and endothelial cells in these blood islands suggests that the transient erythromyeloid potential of the yolk sac arises from hemangioblast precursors that also contain endothelial potential. This concept is supported by in vitro studies of human embryonic stem cells cultured as embryoid bodies.
It now appears likely that hematopoietic stem cells containing erythromyeloid and lymphoid potential subsequently arise from intraembryonic vasculature, particularly the aorta. These hematopoietic stem cells provide for fetal and long-term postnatal blood cell production. The ontogeny of the hematopoietic system remains a topic of active research using mammalian and several nonmammalian model systems.