Figure 2.

Ontogeny of erythroid lineage cells in the circulation. Embryonic erythrocytes (primitive red blood cells [RBCs]) are made by the yolk sac at E7.5 and are found in the circulation until ∼E11/12. At E9, the yolk sac and placenta generate definitive progenitors that migrate to the fetal liver, where they differentiate to definitive RBCs (expressing fetal/adult globin) and enter the circulation. At E10.5, the AGM generates the first HSCs that migrate to the fetal liver and differentiate to the erythroid lineage (among other lineages), and these definitive RBCs enter the circulation. Fetal liver HSCs migrate and colonize the bone marrow at birth, where they provide lifelong production of definitive RBCs for the circulation. The spleen also is a site of differentiation for erythroid cells (not shown).