[The aortic endothelium in the embryo: genesis and role in hematopoiesis].

Intra-aortic haematopoiesis is a transient phenomenon, characterised by the emergence of Hematopoietic Stem Cells (HSC) from the ventral aortic endothelium through an endothelial cell (EC) to HSC lineage switch. HSC differentiation is followed by the colonization of definitive haematopoietic organs. Since intra-aortic haematopoiesis is born from EC of the aortic floor, we wondered how vascular integrity was maintained during hematopoietic production. We have used interspecific quail to chick grafts to study the aortic morphogenesis during hematopoiesis. We have demonstrated that: 1) before haematopoiesis, the aortic endothelium, originally entirely from splanchnic origin, was colonized by somitic EC, creating a new roof and sides derived from the somite, whereas the floor was contributed by splanchnopleural-derived EC. 2) As haematopoiesis proceeded, somite-derived EC colonized the aortic floor, where they settled underneath the HSC clusters. 3) After haematopoiesis, splanchnopleural ECs have disappeared from the aortic floor and have been replaced by somite-derived EC. At this stage, the whole aortic endothelium originated from somitic cells. 4) We have identified that the somite contributed to the vascular smooth muscle cells (VSMC). 5) Using grafts of either single quail dermomyotome or sclerotome in the chick, we showed that EC originated from the dermomyotome whereas the vascular smooth muscle cells originated from the sclerotome. Taken together, our results bring about new insights on aorta morphogenesis and the time-restricted production of HSCs.