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Blood flow through new microvessels: factors that affect regrowth of vasa vasorum.

A segment of abdominal aorta in dogs was transplanted to its original position (high PO2) or to the femoral vein (low PO2), and a segment of thoracic aorta was transplanted to its original position. Two days after grafting to each location, there was no blood flow (measured with microspheres) through vasa vasorum. After 2 and 4 wk, blood flow through vasa was restored to normal, or above normal, levels in all grafts. Vasa in grafts did not have smooth muscle, and they were not responsive to adenosine. Medial necrosis occurred in grafts of thoracic aorta but not in grafts of abdominal aorta. There was cellular degeneration and medial atrophy in grafts of abdominal aorta to the femoral vein. We conclude that 1) revascularization of the aorta by vasa vasorum is rapid; 2) new vasa do not have smooth muscle, and they are not responsive to vasoactive stimuli; and 3) medial necrosis occurs in grafts in which delivery of oxygen to media is compromised, either by low luminal PO2 or by interruption of medial vasa (in thoracic aorta), but not when the abdominal aorta is excised and reanastomosed. Absence of medial necrosis in grafts of abdominal aorta indicates that diffusion from the lumen of the vessel is adequate to nourish the vessel. This finding provides evidence that adventitial vasa, in contrast to medial vasa, are not essential for nourishment of the vessel wall.