Remodeling of the vascular bed and progressive loss of capillaries in denervated skeletal muscle

Very little is known regarding structural and functional responses of the v ascular bed of skeletal muscle to denervation and about the role of microci rculatory changes in the pathogenesis of post-denervation muscle atrophy. T he purpose of the present study was to investigate the changes of the anato mical pattern of vascularization of the extensor digitorum longus muscle in WI/HicksCar rats 1, 2, 4, 7, 12, and 18 months following denervation of th e limb. We found that the number of capillaries related to the number of mu scle fibers, i.e. the capillary-to-fiber ratio (CFR), decreased by 88%, fro m 1.55 +/- 0.35 to 0.19 +/- 0.04, during the first 7 months after denervati on and then slightly declined at a much lower rate during the next II month s of observation to 10% of the CFR in normal muscle. Between months 2 and 4 after denervation, the CRF decreased by 2.4 times, from 58% to 24% of the control value. The loss of capillaries during the first 4 months following nerve transection was nearly linear and progressed with an average decremen t of 4.16% per week. Electron microscopy demonstrated progressive degenerat ion of capillaries following nerve transection. In muscle cells close to de generating capillaries, the loss of subsarcolemmal and intermyofibrillar mi tochondria, local disassembly of myofibrils and other manifestations of pro gressive atrophy were frequently observed. The levels of devascularization and the degree of degenerative changes varied greatly within different topo graphical areas, resulting in significant heterogeneity of intercapillary d istances and local capillary densities within each sample of denervated mus cle. Perivascular and interstitial fibrosis that rapidly developed after de nervation resulted in the spatial separation of blood vessels from muscle c ells and their embedment in a dense lattice of collagen. As a result of thi s process, diffusion distances between capillaries and the surfaces of musc le fibers increased 10-400 times. Eighteen months after denervation most of the capillaries mere heavily cushioned with collagen, and on the average 4 0% of the muscle cells were completely avascular. Devascularization of the tissue was accompanied by degeneration and death of muscle cells that had b ecome embedded in a dense lattice of collagen. Immunofluorescent staining f or the vascular isoform of alpha-actin revealed preservation of major blood vessels and a greater variability in thickness of their medial layer. Hype rplastic growth of the medial layer in some blood vessels resulted in narro wing of their lumens. By the end of month 7 after denervation, large deposi ts of collagen around arterioles often exceeded their diameters. Identifica tion of oxidative muscle fibers after immunostaining for slow-twitch myosin , as well as using ultrastructural criteria, has shown that after 2 months of denervation oxidative muscle fibers were less susceptible to atrophy tha n glycolytic fibers. The lower rate of atrophy of type I muscle fibers at e arly stages of denervation may be explained by their initially better vascu larization in normal muscle and their higher capacity to retain capillaries shortly after denervation. Thus, degeneration and loss of capillaries afte r denervation occurs more rapidly than the loss of muscle fibers, which res ults in progressive decrease of the CFR in denervated muscle. The change of capillary number in denervated muscle is biphasic: the phase of a rapid de crease of the CFR during the first 7 months after nerve transection is foll owed by the phase of stabilization. The presence of areas completely devoid of capillaries in denervated muscle and the virtual absence of such areas in normal muscle indicate the develo pment of foci of regional hypoxia during long-term denervation. The anatomi cal pattern of muscle microvascularization changes dramatically after nerve transection. Each muscle fiber in normal muscle directly contacts on avera ge 3-5 capillaries. In contrast, in denervated muscle, groups of muscle fib ers are served by single capillaries spatially separated from them by dense collagen insulation. Taken together, these results suggest that the remode ling of the vascular bed in the direction of a less oxygen-dependent metabo lism and impairment of microcirculation are integral components in the path ogenesis of post-denervation muscle atrophy. Insufficient vascularization a nd a vast accumulation of collagen in denervated muscle appear to be among the factors that block regeneration of long-term denervated muscle after ex perimental or clinical reinnervation. Anat Rec 258: 292-304, 2000. (C) 2000 Wiley-Liss, Inc.