Embryonic origins and assembly of blood vessels.

Embryonic blood vessels develop in two ways: angiogenesis, which is growth by budding, branching, and elongation of existing vessels, and in situ formation of endothelial vesicles that coalesce with elongating vessels. It is assumed that the former is more prevalent, with the latter restricted to vessels that form near the endoderm:mesoderm interface. Neither the relative contributions of each of these processes in the formation of specific blood vessels nor the origins of precursors (angioblasts) of these intraembryonic endothelial populations are known. Antibodies that recognize quail endothelial cells can be used to follow the movements and differentiation of endothelial cell precursors after the transplantation of putative precursor populations from quail into chick embryos. Using this method, it has been shown that all intraembryonic mesodermal tissues, except the prechordal plate, contain angiogenic precursors. After transplantation some angioblasts move in all directions away from the site of implantation, invading surrounding mesenchyme and contributing to the formation of arteries, veins, and capillaries in a wide area. Although it is clear that these invasive angioblasts, which behave unlike any other embryonic mesenchymal cell type, are found throughout the embryo, it is not known whether they represent a unique endothelial cell type in mature blood vessels. Irrespective of their original location in the donor embryo, transplanted angioblasts will form vascular channels that are appropriate for the tissues surrounding their site of implantation. These results indicate that the control over vascular assembly resides within the connective-tissue-forming mesenchyme of the embryo.