Primitive macrophages control HSPC mobilization and definitive haematopoiesis

In vertebrates, haematopoietic stem/progenitor cells (HSPCs) first emerge in the aorta–gonad–mesonephros (AGM) before colonizing transitory and subsequently definitive haematopoietic organs allowing haematopoiesis throughout adult life. Here we identify an unexpected primitive macrophage population accumulated in the dorsal mesenteric mesoderm surrounding the dorsal aorta of the human embryo and study its function in the transparent zebrafish embryo. Our study reveals dynamic interactions occurring between the HSPCs and primitive macrophages in the AGM. Specific chemical and inducible genetic depletion of macrophages or inhibition of matrix metalloproteinases (Mmps) leads to an accumulation of HSPCs in the AGM and a decrease in the colonization of haematopoietic organs. Finally, in vivo zymography demonstrates the function of primitive macrophages in extracellular matrix degradation, which allows HSPC migration through the AGM stroma, their intravasation, leading to the colonization of haematopoietic organs and the establishment of definitive haematopoiesis. Haematopoietic stem/progenitor cells (HSPCs) transform from aortic endothelium into migratory cells that move through stroma and enter circulation to colonize haematopoietic tissues. Here the authors show that HSPCs' passage is facilitated by primitive macrophages that secrete extracellular matrix-degrading enzymes.