Fig. 6. Developmental history of the aorta in relationship with endothelial remodeling and smooth muscle formation. (A) Before fusion of the aortae, aortic ECs (red) derive from the splanchnopleura. The dorsolateral quadrant of the somite displays a population of ECs (yellow). (B) Immediately before fusion, the initial roof of splanchnopleural origin (red) has been replaced by ECs from the somite (yellow). (C) After fusion, the roof is of somite origin, the sides and floor remain of splanchnopleural origin. (D) During early hematopoiesis, the floor begins to loose endothelial markers and acquires of hematopoietic traits (Jaffredo et al., 1998). (E) At the hematopoietic clusters stage, clusters are budding into the lumen, whereas some HCs ingress into the mesentery. At the same time, somitic ECs replace the initial floor, whereas numerous somitic ECs are found in the floor either underneath the clusters. At this time, cells of somite origin reach the aorta (gray). When these cells reach the aorta, they begin to express SMA (green). (F) Completion of hematopoiesis. The aortic floor has disappeared and is replaced by somitic ECs. Aortic ECs are entirely of somite origin. Somite cells in abluminal position now express SMA (green).