2 DISTINCT ENDOTHELIAL LINEAGES IN ONTOGENY, ONE OF THEM RELATED TO HEMATOPOIESIS

We have shown previously by means of quail/chick transplantations that external and visceral organs, i,e,, somatopleural and splanchnopleura l derivatives, acquire their endothelial network through different mec hanisms, namely immigration (termed angiogenesis) versus in situ emerg ence of precursors (or vasculogenesis), We have traced the distributio n of QH1-positive cells in chick hosts after replacement of the last s omites by quail somites (orthotopic grafts) or lateral plate mesoderm (heterotopic grafts), The results lead to the conclusion that the embr yo becomes vascularized by endothelial precursors from two distinct re gions, splanchnopleural mesoderm and paraxial mesoderm, The territorie s respectively vascularized are complementary, precursors from the par axial mesoderm occupy the body wall and kidney, i,e,, they settle alon g with the other paraxial mesoderm derivatives and colonize the somato pleure, The precursors from the two origins have distinct recognition and potentialities properties: endothelial precursors of paraxial orig in are barred from vascularizing visceral organs and from integrating into the door of the aorta, and are never associated with hemopoiesis; splanchnopleural mesoderm grafted in the place of somites, gives off endothelial cells to body wall and kidney but also visceral organs, It gives rise to hemopoietic precursors in addition to endothelial cells .