Blood vessel formation in the avian limb bud involves angioblastic and angiotrophic growth

The vasculature of the avian limb bud takes its origin from the intersomitic vessels as can be shown by ink perfusion of the embryo. While the primitive vessels form a central network in the early limb bud, an area of about 100 microns in width from the ectoderm inward remains free from lumenized vessels. However, this subectodermal avascular zone contains isolated angioblasts, which can be demonstrated by confocal laser scanning microscopy in connection with QH-1-staining. QH-1-positive cells from the avascular zone are capable of giving rise to endothelial cells when grafted ectopically into a "permissive" environment such as the dorso-lateral paraxial mesoderm. Several grafting sites are compared regarding their permissiveness for capillary formation. In order to investigate the origin of the QH-1-positive angioblasts we carried out injections of DiI-Ac-LDL, which is specifically taken up by endothelial cells and macrophages, and found the lumenized vessels and a few isolated cells in the peripheral limb mesoderm stained. In double-labelling studies combining DiI-Ac-LDL and QH-1, it can be shown that there exists a pool of isolated angioblasts that are only QH-1-positive, but have not incorporated DiI-Ac-LDL. In contrast to the lumenized vessels in the core of the limb bud, we found that angioblasts in the avascular zone do not proliferate, as shown by proliferation studies applying the BrdU-method to semithin sections in connection with QH-1-labelled parallel sections. We conclude that the vascularization of the avian limb bud is achieved by a combination of angiotrophic growth (sprouting of vessels) and angioblastic growth (recruitment of angioblasts from the limb mesoderm.