Discussion

Abstract

In Tupaia, development of the arteries of the pelvic extremity starts in the 17-day embryo. At this stage, the pelvic limb bud is penetrated by a capillary plexus which receives blood from small branches of the a. umbilicalis. The venous return is effected by the marginal vein. Contradictory hypotheses have been published on the formation and on the fate of this capillary plexus which precedes the development of single vessels. Arey (1963) considered the “selection” of vessels from a capillary plexus to be a fundamental mechanism of vessel formation. Woollard and Harpman (1937) stated that arteries developed by the enlargement and the fusion of single capillaries, and described intermediate stages in which the fusion of the capillaries was not yet completed. Additionally, the anlagen of vessels have been reported to sprout directly from a preexisting capillary plexus and to remain independent vessels (Starck 1975). Seifert et al. (1992) distinguished between two separate mechanisms of vessel formation: vasculogenesis and angiogenesis. “Vasculogenesis” is the in situ differentiation of endothelial cells from mesenchymal precursors, whereas “angiogenesis” is characterized by the sprouting of capillaries from preexisting vessels. From our investigations, it is impossible to decide reliably whether in Tupaia vasculogenesis or angiogenesis might predominate during the formation of the arteries. The a. ischiadica, which is first recognizable in the 18-day embryo, transforms into a large artery which is joined by the a. femoralis in the 22-day embryo (Fig. 6).