Primitive and definitive erythropoiesis in the yolk sac: a bird's eye view.

The yolk sac is the sole niche and source of cells for primitive erythropoiesis from E1 to E5 of chicken development. It is also the main niche and source of cells for early definitive erythropoiesis from E5 to E12. A transition occurs during late embryonic development, after which the bone marrow becomes the major niche and intraembryonically-derived cells the major source. How the yolk sac is involved in these three phases of erythropoiesis is discussed in this review. Prior to the establishment of circulation at E2, specification of primitive erythrocytes is discussed in relation to that of two other cell types formed in the extraembryonic mesoderm, namely the smooth muscle and endothelial cells. Concepts of blood island, hemangioblast and hemogenic endothelium are also discussed. It is concluded that the chick embryo remains a powerful model for studying developmental hematopoiesis and erythropoiesis.