Figure 1.

Timeline of fetal hematopoiesis. Primitive hematopoietic cells appear in the yolk sac for seven weeks. Hematopoiesis is then temporarily shared between the liver, spleen, and thymus. After blood circulation starts, primitive hematopoietic cells get into the circulation and mature. At 10 weeks, hematopoietic progenitor cells gradually migrate from the aorta-mesonephros-gonad (AGM) region to colonize the liver, which becomes the major hematopoietic organ. Hematopoiesis shifts from foetal liver to bone marrow at 16 gestational weeks.