Primitive erythropoiesis and megakaryopoiesis in the yolk sac are independent of c-myb

Hematopoiesis initiates within the yolk sac of mammalian embryos in overlapping primitive and definitive waves, each containing erythroid and megakaryocyte progenitors. c-myb–null mouse fetuses lack definitive erythrocytes but contain primitive erythroblasts and hepatic megakaryocytes. However, it is unclear if c-myb–null embryos harbor definitive erythroid or any megakaryocyte progenitors. We determined that c-myb was not expressed in primitive erythroid precursors and that c-myb–null embryos had normal primitive erythroid and megakaryocyte progenitor numbers and kinetics between embryonic day (E) 7.0 and E9.0. While primitive hematopoiesis is c-myb–independent, no definitive erythroid potential was detected in c-myb–null embryos, confirming that definitive erythropoiesis, beginning at E8.25 in the yolk sac, is completely c-myb–dependent. In contrast, reduced numbers of megakaryocyte progenitors with restricted proliferative capacity persist in E10.5 yolk sac and E11.5 liver. Despite this impaired megakaryocyte potential, c-myb–null fetuses had normal platelet numbers at E12.5 but became thrombocytopenic by E15.5, suggesting that c-myb is required for sustained thrombopoiesis.