Origins and assembly of avian embryonic blood vessels.

Two processes by which embryonic blood vessels develop are well-known: angiogenesis (growth by budding and branching of existing vessels) and local formation of endothelial vesicles that coalesce with elongating vessels. The former process appears to be more prevalent, with the latter restricted to vessels that form near the endoderm-mesoderm interface. The contributions of endothelial cells formed by each of these processes to specific blood vessels has not been defined, however, nor have the origins of precursors (angioblasts) of intraembryonic endothelial populations been established. To identify the origins of endothelial cells, precursor populations from quail embryos were transplanted into chick embryos. Antibodies that recognize quail endothelial cells were applied to sections from chimeric embryos fixed 2-5 days after surgery. These experiments reveal that all intraembryonic mesodermal tissues, except the notochord and prechordal plate, contain angiogenic precursors. Many angioblasts emigrate from the grafted tissue, invading surrounding mesenchyme and contributing to the formation of arteries, veins, and capillaries in a wide area. The invasive behavior of these angioblasts is unlike that of any other embryonic mesenchymal cell type and represents a third process operating during embryonic blood vessel formation. Transplanted angioblasts, even those excised from quail trunk regions, form normal craniofacial vascular channels, including the cardiac outflow tract. These results demonstrate that the control over blood vessel assembly resides within the connective tissue-forming mesenchyme of the embryo, not within endothelial precursors.