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Time line and main sites of hematopoiesis in the mouse conceptus. Primitive hematopoiesis begins around E7.25 within yolk sac blood islands. Definitive hematopoiesis starts around E8 in the yolk sac, E9.5 in the placenta, and E10.5 in the aorta-gonad-mesonephros. The hematopoietic stem and progenitor cells generated from the yolk sac and aorta-gonad-mesonephros are thought to migrate to the fetal liver, thymus, and spleen and finally colonize the fetal bone marrow.
