2558 Circulation June 17, 2014 hierarchy in hypercholesterolemic low-density lipoprotein receptor–deficient/apolipoprotein B100–only mice (LDLR−/− ApoB100/100) mice. 33 The vasa vasorum tree consists of a large main vessel from which smaller vessels branch; these in turn branch to form a plexus that occupies the space between 2 larger vessels. The vessels within the plexus collapse in response to an angiogenesis inhibitor, whereas the larger vessels of the tree remain intact. The vasa vasorum are also detectable in chow-fed mice but do not exhibit a branching pattern. 33 The cumulative data from studies using various animal models and imaging modalities clearly indicate that under disease conditions, AVV expand, frequently in a disorderly fashion. The venous vasa vasorum (VVV) drain the arterial wall into companion veins34 that are parallel to the AVV feeder or into the largest branches of the main vein, where they penetrate every 5 to 15 mm. 35 There are distinct qualitative differences between the AVV and the VVV. 36, 37 The AVV are many fewer in number, with diameters ranging from 11.6 to 36.6 μm compared with the VVV diameter range of 11.1 to 200.3 μm. Vascular corrosion casts coupled with scanning electron microscopy show that the VVV change course at acute angles and form kinks, constrictions, and outpouchings. This spatial distribution enables the VVV to withstand vessel wall distension with increased blood pressure or vessel stretching without a dramatic effect on the VVV function. 37 The autonomic nervous system is thought to control blood flow in the human saphenous vein vasa vasorum. 35, 37 Use of the saphenous vein for coronary artery bypass graft surgery is associated with spasm of vascular smooth muscle cells (SMCs) of the vein that can develop into vein-graft disease. 38, 39 The conventional surgical technique for saphenous vein harvest involves stripping the connective tissue from the vein, which injures the adventitial autonomic nerves and vasa vasorum37, 38 and may trigger venospasm. 40 Moreover, studies suggest that the VVV play a role in vein vasorelaxation; any VVV damage during saphenous vein harvesting may impair flow-induced vasodilation in the graft. 39, 41, 42