Introduction: In this study, the spatial-temporal progression of peripheral nerve regeneration, macrophage recruitment, and Schwann cell migration across a 10 mm gap bridged by a venous conduit was examined using immunoctyochemical techniques.

Methods: In male Lewis rats, a 10 mm segment of sciatic nerve was resected. The femoral vein was resected and interposed between the proximal and distal sciatic nerve stumps. Interrupted 10-0 nylon sutures were used to approximate the full thickness of the epineurium with the endothelium of the vein graft. AVNC samples were harvested every other day beginning POD 1 and ending POD 31. Silver staining was used to identify nerve axons. Macrophage cells were labeled with anti-ED1 monoclonal antibody immunoperoxidase staining. Schwann cells were identified with anti-S100 polyclonal antibody immunoperoxidase staining.

Results: In the post-vein graft period up to POD 5, the conduit distends secondary to endoneurial vessel bleeding and red blood cell clot formation. As the clot is resorbed, a pauci-cellular matrix is left behind. By POD 9, macrophage cells are observed in the lumen of the vein conduit and throughout the length of the distal nerve stump. Schwann cells proliferate at both nerve stumps and begin to migrate into the matrix. By POD 11, regenerating axons grow from the proximal nerve stump and the density of macrophage cells in the proximal stump is markedly decreased. At the distal stump, macrophage density is markedly increased and extensive Wallerian degeneration is observed. Schwann cells completely bridge the gap by 13 days. By POD 14, regenerating axons have reached the middle of the venous conduit. Macrophage cells are not observed at the proximal stump or within segments of the conduit that have filled with regenerating axons. By POD 30, axons regenerate into the remnant of the distal nerve stump. Macrophage cells are only localized to areas of the distal nerve stump still undergoing Wallerian degeneration.

Conclusions: Within 1 day, endoneurial vessel bleeding fills the venous conduit thereby preventing collapse of the vessel lumen and also gives rise to a fibrin and red blood cell clot. Macrophage cells are localized to the tips of proximal and distal nerve stumps. Schwann cells proliferate at the proximal and distal stumps and migrate into the matrix. Axonal regeneration from the proximal stump consistently lags behind Schwann cell migration into the conduit. Macrophage density at the more proximal portions of the conduit begins to decrease as Schwann cells and axons advance into the conduit.

These observations suggest that macrophage cells may induce Schwann cell activation, proliferation, and migration. In turn the activated Schwann cells may modify the matrix making it suitable for axonal regeneration or more likely they provide the necessary cellular framework for axonal growth by offering their own cell surfaces as substrata, or some combination thereof.