Angiogenesis precedes myogenesis during regeneration following biopsy injury of skeletal muscle.
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BACKGROUND: Acute injury to skeletal muscle damages myofibers and fragment capillaries, impairing contractile function and local perfusion. Myofibers and microvessels regenerate from satellite cells and from surviving microvessel fragments, respectively, to restore intact muscle. Established models of injury have used myotoxins and physical trauma to demonstrate the concurrence of myogenesis and angiogenesis during regeneration. In these models, efferocytosis removes cellular debris while basal laminae persist to provide guidance during myofiber and microvessel regeneration. It is unknown whether the spatiotemporal coupling between myofiber and microvascular regeneration persists when muscle tissue is completely removed and local guidance cues are lost. METHODS: To test whether complete removal of skeletal muscle tissue affects the spatiotemporal relationship between myogenesis and angiogenesis during regeneration, subthreshold volumetric muscle loss was created with a biopsy punch (diameter, 2mm) through the center of the gluteus maximus (GM) in adult mice. Regeneration into the void was evaluated through 21days post-injury (dpi). Microvascular perfusion was evaluated in vivo by injecting fluorescent dextran into the circulation during intravital imaging. Confocal imaging and histological analyses of whole-mount GM preparations and tissue cross-sections assessed the growth of microvessels and myofibers into the wound. RESULTS: A provisional matrix filled with PDGFR+ and CD45+ cells spanned the wound within 1 dpi. Regenerating microvessels advanced from the edges of the wound into the matrix by 7 dpi. Nascent microvascular networks formed by 10 dpi with blood-perfused networks spanning the wound by 14 dpi. In striking contrast, the wound remained devoid of myofibers at 7 and 10 dpi. Myogenesis into the wound was apparent by 14 dpi and traversed the wound by 21 dpi. Regenerated myofibers and microvessels were disorganized compared to the uninjured muscle. CONCLUSIONS: Following punch biopsy of adult skeletal muscle, regenerating microvessels span the wound and become perfused with blood prior to myofiber regeneration. The loss of residual guidance cues with complete tissue removal disrupts the spatiotemporal correspondence between microvascular and myofiber regeneration. We conclude that angiogenesis precedes myogenesis during regeneration following subthreshold volumetric muscle loss.
