Shifts in site of hematopoiesis during mouse and human development. (A) Hematopoietic development in the mouse. Formation of mesoderm during gastrulation (around E6.5), development of yolk sac blood islands (∼ E7.5), emergence of HSCs in the aorta-gonad-mesonephros region (E10.5; other sites such as large arteries and placenta not shown), active fetal liver hematopoiesis (E14.5), and hematopoiesis in the bone marrow of the late gestation fetus (E18.5) and adult animal. Active circulation begins at approximately E9.0.⁴³  (B) Hematopoietic development in the human embryo. An embryo at yolk sac stage of hematopoiesis (day 17), at the time of the first hepatic colonization by HSCs (day 23), arterial cluster formation (day 27), second hepatic colonization (day 30), and bone marrow colonization (10.5 weeks). Active circulation begins at approximately day 21.³