FIGURE 1.

Ontogeny of the hematopoietic system in mouse and human. Embryonic hematopoiesis is established in three distinct waves. The first hematopoietic cells emerge in YS blood islands, generating primitive erythrocytes, macrophages and megakaryocytes, constituting the primitive hematopoietic wave. The second hematopoietic wave initiates in the vascular plexus of the YS, generating EMPs that produce definitive erythrocytes, megakaryocytes, macrophages, neutrophils, granulocytes and mast cells, but lacks HSC activity. EMPs are the origin of tissue-resident macrophages that can persist throughout life. HSCs emerge in the AGM region and migrate to the FL, the major embryonic hematopoietic organ. In FL, HSCs expand and differentiate into all mature blood cell lineages. Small numbers of hematopoietic progenitors also colonize the fetal spleen and are still found few weeks after birth. Migration to the BM, where HSCs reside through adulthood, occurs as a continuous process and, in humans, can take several weeks. YS, yolk sac; P-Sp para-aortic splanchnopleura; FL, fetal liver; AGM, aorta-gonads-mesonephros; BM, bone marrow.