An overview of embryonic hematopoiesis in zebrafish. (A) From 28 to 54 hpf HSPCs are born from endothelial cells in the ventral wall of the dorsal aorta and are induced by a variety of local signals, including Cxcl12b produced by somite-derived “endotome cells” in the dorsal aorta and Tnfa produced by primitive myeloid cells. In zebrafish, HSPCs bud off into the extravascular space ventral to the dorsal aorta, divide, and enter into circulation. (B) From 48 to 96 hpf HSPCs exit circulation and lodge in the CHT, where local signals, including Kitlgb, Osm, thrombopoietin, colony stimulating factor 3a, Ccl25b, Cxcl8b, and Cxcl12a, regulate their trafficking and expansion. Vascular endothelial cells remodel to form a pocket around resident HSPCs, and somite-derived stromal cells anchor HSPCs during CHT occupancy. Primitive neutrophils regulate HSPC egress through secretion of Mmp9, which cleaves locally produced Cxcl12a.