Timeline of hematopoietic events.
The scheme represents major events in embryonic hematopoiesis. The indicated time (in days) represents the onset of hematopoiesis at each indicated site/niche, and the arrows represent the direction taken by each hematopoietic progenitor, or stem cell, toward its colonization site. First, the liver bud (fetal liver) is populated by hematopoietic cells from the yolk sac and the aorta-gonad-mesonephros (AGM). After their expansion in the fetal liver, hematopoietic stem cells migrate and colonize the bone marrow, spleen and thymus. The yolk sac as a source of definitive hematopoiesis is still controversial.
Abbreviations: AGM, Aorta-Gonad-Mesonephros.