FIGURE 19.

Migration of hematopoietic precursors during development. A: hematopoiesis begins with the development of blood islands surrounded by endothelial cells in the yolk sac at embryonic day 8.5 (E8.5). Around E10.5, the site of hematopoiesis has migrated to the aortogonadal ridge/mesonephros region (AGM), and by E11.5 it has moved to the fetal liver. B: the concept of multisite hematopoietic development is also outlined to indicate multiple potential sources in early development (the yolk sac, placenta, umbilical cord, and AGM), and eventual migration via the fetal liver to the spleen, thymus, and bone marrow. [A and B are from Medvinsky et al. (162), with permission from Development.]