Erythropoietin (Epo) and EpoR expression and 2 waves of erythropoiesis.

Erythropoiesis occurs in 2 distinct waves during embryogenesis: the primitive wave in the extra-embryonic yolk sac (YS) followed by the definitive wave in the fetal liver and spleen. Even though progenitors for both cell types are present in the YS blood islands, only primitive cells are formed in the YS during early embryogenesis. In this study, it is proposed that erythropoietin (Epo) expression and the resultant EpoR activation regulate the timing of the definitive wave. First, it was demonstrated that Epo and EpoR gene expressions are temporally and spatially segregated: though EpoR is expressed early (embryonic days 8.0-9.5) in the yolk sac blood islands, no Epo expression can be detected in this extra-embryonic tissue. Only at a later stage can Epo expression be detected intra-embryonically, and the onset of Epo expression correlates with the initiation of definitive erythropoiesis. It was further demonstrated that the activation of the EpoR signaling pathway by knocking-in a constitutively active form of EpoR (R129C EpoR) can lead to earlier onset of definitive erythropoiesis in the YS. Thus, these results provide the first in vivo mechanism as to how 2 erythroid progenitor populations can coexist concurrently in the YS yet always differentiate successively during embryogenesis.