Definitive erythropoiesis in chicken yolk sac

The first wave of erythropoiesis in amniotic animals generates all primitive erythrocytes and takes place exclusively in yolk sac mesoderm. It is less clear, however, to what extent and for how long the yolk sac contributes to the second wave of erythropoiesis which gives rise to definitive erythrocytes for later embryonic and adult use. Here, we examine the initiation, duration, and site of definitive erythrocyte formation in chicken yolk sac. We show that the earliest definitive erythrocytes are generated in yolk sac venous vessels surrounding major arteries at embryonic day (E) 4-4.5, and that mature definitive erythrocytes enter circulating at E4.5-E5. This takes place at a time when yolk sac vasculature remodels extensively to generate paired arterial/venous vessels. The yolk sac remains the predominant site for definitive erythropoiesis from E5 to E10, and continues to generate definitive erythrocytes at least until E15. Similar to primitive erythropoiesis, definitive erythropoiesis in the yolk sac is accompanied by the expression of transcriptional regulators gata1, scl, and lmo2. Furthermore, our data suggest that one main source of definitive erythropoietic cells is the pre-existing vascular endothelial cells. It remains unclear whether yolk sac derived hematopoietic progenitors that do not undergo erythropoiesis in the yolk sac may take up intraembryonic niches and contribute to erythropoietic stem cell population after hatching.