Are Schwann cells essential for axonal regeneration into muscle autografts?

When axons regenerate through frozen-thawed (FT) muscle grafts, they are accompanied by co-migrating Schwann cells derived from the nerve stumps. Although acellular, FT muscle grafts contain an internal scaffold of basal laminae rich in components capable of supporting neurite outgrowth in vitro such as laminin and fibronectin: it is not known whether Schwann cells are essential for axonal regrowth within these grafts. In this paper we test the hypothesis that sarcolemmal basal laminae will support axonal regeneration in the absence of Schwann cells. Two groups of 12 adult Wistar rats were used. All rats received a 0.5 cm FT muscle graft, and 12 rats also received a subperineurial injection of the anti-mitotic agent mitomycin C (400 micrograms/ml in physiological saline) prior to grafting. Previous studies have shown that this dose effectively depresses cell proliferation within the endoneurium for 3-4 weeks [17, 18, 28]. Rats were killed (n = 3) 1, 2, 3 or 4 weeks later. The spatio-temporal sequence of axonal regeneration into the grafts was assessed histologically, by immunofluorescence using antibodies against GAP-43; S-100; RT97; laminin and macrophages (ED1), and by transmission electron microscopy. Outgrowth of almost all axons from the mitomycin C-treated proximal stumps was delayed for up to 3 weeks, after which time vigorous regeneration occurred into the persisting tubes of sarcolemmal basal lamina. All axons regenerating within the grafts (irrespective of mitomycin C-treatment) were accompanied by co-migrating Schwann cells. The results suggest that Schwann cells play an important role in axonal regeneration across FT muscle autografts and that sarcolemmal basal laminae alone are insufficient to support axonal regeneration.