The origin of the endothelium in the developing marginal vein of the chick wing-bud.

The origin of the endothelium of the developing limb vasculature has received little attention. From early accounts of the pattern of vessel development in the limb it has been assumed that the endothelia are provided by sprouting and growth of the earliest formed vessels such as the aortae, large veins (the 'genetic' vessels). An alternative origin of the endothelium is the transformation in situ of mesenchymal cells of the limb bud. The origin of the vascular endothelium in the developing marginal vein was investigated by the creation of an enlarged avascular zone of mesenchyme at the distal tip of early chick and quail wing-buds. This was achieved by grafting, heterospecifically, avascular tips of correspondingly aged wing-buds. Using the nucleolar marker of the quail cells, the newly formed vessels within the grafted tissue were shown to originate from the host wing-bud vasculature. Thus, it was concluded that the endothelium in developing vessels of the limb bud are derived from the preexisting vessels rather than by the recruitment of undifferentiated limb mesenchyme cells.