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Embryonic and fetal hemopoiesis: an overview.

Our current knowledge of embryonic and fetal hemopoiesis is critically reviewed in this article. In both murine and human systems, embryonic and fetal development is associated with multiple switching in the sites of hemopoiesis. The phenomenon is first extraembryonic, occurring in blood islands of the yolk sac. Hemopoietic stem cells (HSC) appear to derive from hemangioblasts that are of mesodermal origin. Yolk sac milieu is permissive only for erythropoiesis which proceeds synchronously and may be erythropoietin-insensitive. Yolk sac milieu is not permissive for the development of other cell lines. The final product is nucleated red cells. Yolk sac hemopoiesis is an example par excellence of primitive (as compared to definitive) form of hemopoiesis. HSC then seem to migrate via the bloodstream to the liver and spleen to seed these tissues, which then carry the burden of hemopoiesis until birth and for some time thereafter. Here also erythropoiesis predominates, but some granulopoiesis also occurs. Thus, the milieu is not totally impermissive. Hemopoiesis is in definitive form, lacking synchronicity of cell growth with the end product being anucleated cells and synthesized hemoglobin not limited to embryonic type. The site of hemopoiesis is finally transferred to the bone marrow, which is predominantly granulopoietic. Certain cellular and embryological features of these types of hemopoiesis in the context of more recent molecular understanding of stem cell homing are discussed.