Schematic of primitive and definitive hematopoiesis during zebrafish embryogenesis. Cells functioning in both primitive and definitive hematopoiesis are derived from mesoderm. After gastrulation, hemangioblasts in the intermediate cell mass (ICM) differentiate into either primitive hematopoietic stem cells (HSCs) and hematopoietic progenitor cells (HPCs) or vascular endothelial cells. Primitive HSCs and HPCs give rise to erythrocytes, granulocytes, and macrophages. For definitive hematopoiesis, HSCs and HPCs emerge from hemogenic endothelial cells lining the ventral wall of dorsal aorta in the aorta-gonad-mesonephros (AGM)-like region. Definitive HSCs and HPCs enter the circulation and migrate to the caudal hematopoietic tissue, thymus, and pronephros, where they differentiate into erythroid/myeloid progenitors or lymphoid progenitors. Those progenitors further differentiate into the indicated cell types. Primitive erythropoiesis and definitive erythropoiesis are highlighted in blue boxes. Transcription factors critical for HPCs and erythroid/myeloid progenitor cells are shown in blue in italics. cmyb, transcription factor cmyb; csf1ra, colony-stimulating factor 1 receptor a; gata1a, GATA-binding protein 1a; gata2a, GATA-binding protein 2a; runx1, runt-related transcription factor 1; spi1, spleen focus-forming virus proviral integration oncogene spi1; tal1, T-cell acute lymphocytic leukemia.