REGENERATION THROUGH NERVE ALLOGRAFTS IN THE CYNOMOLGUS MONKEY (MACACA-FASCICULARIS)

With the use of ulnar nerves of cynomolgus monkeys, the present study examined whether basal laminae of Schwann cells can serve as conduits for regenerating axons in nerve allografts from non-human primates. A segment of ulnar nerve was transected distal to the elbow joint one week before grafting. In Group A, a distal segment of the transected nerve was transplanted, after freezing and thawing, into the ulnar nerve of another monkey, at a level that corresponded to that from which the graft was taken. In Group B (the control group), the segment of nerve was grafted in the same manner but without cryotreatment. Two weeks, five weeks, eight weeks, and five months after grafting, the graft and the host nerve were examined with light and electron microscopy. Within two weeks after grafting in Group A, after degradation of the cellular components of the Schwann cells, the basal laminae of the Schwann cells were intact in the form of tubes. Within five weeks, many regenerating axons grew out into these basal lamina tubes in the three-centimeter-long grafts and extended into the host nerve. As seen at the wrist (seven centimeters from the distal suture) five months after grafting, the axons exhibited fully mature myelination both in the graft and in the host nerve. In contrast, in Group B, in which the Schwann cells had not been disrupted by cryotreatment, cellular components and connective-tissue matrices, including basal laminae, had been degraded and had been replaced by invading cells, which filled the endoneurial spaces of the graft. Five months after grafting, axonal growth had been arrested in the graft one centimeter distal to the proximal suture. The beneficial effect in Group A appears to have been the result of the retention and preservation of intact basal laminae of Schwann cells after rapid removal of killed Schwann cells and myelin debris. Killing of Schwann cells by freezing before grafting may abolish the immune response to the Schwann cells in allografts and lead to fragmentation and disruption of myelin, which facilitates the rapid removal of myelin by macrophages. CLINICAL RELEVANCE: At present, defects in injured nerves are treated with autogenous nerve grafts. However, the autogenous graft is not always large enough to fill large gaps in injured nerves. In addition, excision of the donor nerve results in sensory and sometimes motor deficits. For these reasons, the possibility of the use of allografts is being actively investigated. The results of the present study indicate that when the Schwann cells are killed by freezing before the use of nerve allografts, the basal laminae of the Schwann cells may be effective conduits for regenerating axons.