Comparative timeline of embryonic zebrafish and mouse hematopoiesis. From 12 to 24 hours post fertilization (hpf), primitive hematopoietic cells (proerythroblasts, erythroblasts, and macrophage precursors) appear in the intermediate cell mass, which is equivalent to the mouse yolk sac, from embryonic day (E) 7.5 to E10.0. After blood circulation starts at 24 hpf, primitive hematopoietic cells enter the circulation and mature. From 26 to 48 hpf, definitive hematopoietic precursors emerge from endothelial cells lining the ventral wall of dorsal aorta. The space between the dorsal aorta and axial vein is equivalent to the mouse aorta-gonad-mesonephros (AGM) region (E10.5 to E12.5). At 2 to 6 days post fertilization (dpf), definitive hematopoietic precursors enter the circulation, colonize, and expand in the caudal hematopoietic tissue, which is equivalent to mouse fetal liver (E11.5 to E18.5). In fish, hematopoiesis then shifts to the kidney and thymus. In the mouse, hematopoiesis shifts from fetal liver to bone marrow at E18.0. At 3 dpf, lymphoblasts appear in the thymus and differentiate into T lymphocytes. In fish, all hematopoietic lineages appear in the kidney marrow at 4 dpf, whereas B lymphocytes appear at 19 dpf. Thus, the kidney marrow and thymus are major hematopoietic organs throughout the lifespan of adult zebrafish. Sites where erythropoiesis occurs are shown by red boxes.