Early vascularization as a key feature of muscle-in-vein grafts for peripheral nerve repair

Peripheral nerve injuries significantly impair quality of life due to limited regenerative capacity, which is affected by factors such as neuroma formation, injury severity, scarring, and comorbidities. The Muscle-in-Vein (MIV) repair technique, consisting of a vein filled with skeletal muscle fibers, has emerged as a promising alternative to nerve autografts. This approach supports regeneration by providing growth factors, guiding axonal growth, enhancing Schwann cell migration, and limiting scar and neuroma formation. However, its clinical use is currently restricted mainly to short gaps in sensory digital nerves, and the biological mechanisms underlying its effectiveness remain incompletely understood. In this study, we investigated the role of muscle fibers in the early phases of nerve regeneration, with a particular focus on vascularization. An 8 mm gap in rat median nerves was repaired using the MIV technique and analyzed at 3, 7, 14, and 21 days post-injury. Immunofluorescence analysis demonstrated complete macrophage infiltration and well-organized vascularization throughout the entire graft as early as 7 days post-injury. Consistently, RNA sequencing at early time points revealed significant enrichment of pathways associated with vascular development and identified key angiogenesis-related genes. Notably, our findings indicate partial anastomosis between vessels originating from the nerve stumps and those within the muscle component of the graft. These results suggest that the success of muscle-in-vein nerve repair strategy may be due to an early vascularization process mediated by the synergistic contribution of both muscle and vein.