Anatomical sites of primitive and definitive hematopoiesis. The anterior and posterior intermediate cell masses (ICMs) are sites of primitive hematopoeisis, whereas the aorta-gonad-mesonephros (AGM)-like region, caudal hematopoietic tissue (CHT), thymus, and pronephros are sites of definitive hematopoiesis. Blood cell types that arose from each hematopoietic site are indicated. The AGM-like region is located between the dorsal aorta and axial vein. Hematopoietic stem cells (HSCs) and hematopoietic progenitor cells (HPCs) emerge from the AGM-like region and enter the blood circulation via the axial vein. HSCs and HPCs expand in the CHT and then migrate to the thymus and pronephros. Anatomical sites of erythropoiesis are shown in the red-framed boxes. YS, yolk sac.