The production of blood cells is based on multipotent, self-renewing stem cells designated as hematopoietic stem cells (HSCs). The first HSC appear autonomously and in small numbers at the level of the embryonic aorta. They are generated from specialized endothelial cells named hemogenic. HSCs then amplifie in the placenta and fetal liver before reaching in the bone marrow where they will remain lifelong. During these steps, they are in close contact with different cellular microenvironments that play a decisive role in their function.