Ontogeny of the endothelial system in the avian model.

The avian model provides an experimental approach for dissecting the origin, migrations and differentiation of cell lineages in early embryos. In this model, the endothelial network was shown to take place through two processes depending on the origin of endothelial precursors: vasculogenesis when angioblasts emerge in situ, angiogenesis when angioblasts are extrinsic. Two different mesodermal territories produce angioblasts, the somite which only gives rise to endothelial cells and the splanchnopleural mesoderm which also produces hemopoietic stem cells. Potentialities of the mesoderm are determined by a positive influence from the endoderm and a negative control from the ectoderm. The presence of circulating endothelial precursors in the embryonic blood stream is also detected.