Spontaneous Regeneration after Hypoglossal Nerve Injury with Long Nerve Resection in Rats

Spontaneous neural regeneration may occur in peripheral nerve injury after nerve resection with a relatively short gap, whereas regeneration does not occur after nerve resection with a relatively long gap. We have examined spontaneous neural regeneration after severe nerve injury modeled by the resection of a relatively long nerve segment in adult rats. After hypoglossal (XII) nerve resection at lengths of 0, 3, 6, 9, or 12 mm, motoneurons in the XII nucleus were labeled with 1,1’dioctadecyl-3,3,3’,3’-tetramethylindocarbocyanine perchlolate (DiI); the latter was multiply injected into the tongue 12 weeks after the nerve resection. In the 0 mm length nerve-resected rats that were transected, the numbers of DiI-labeled XII neurons were as high as that in the control. In the 3 mm nerve-resected rats, the number of DiI-labeled neurons was slightly less than that in the control, and the numbers were clearly smaller than that in the control and in the 6 and 9 mm nerve-resected rats. Moreover, in the 12 mm nerve-resected rats, the number of DiI-labeled neurons was minimal, or only single such cells were observed. We conclude that spontaneous neural regeneration of the XII motoneurons might occur after moderately severe XII nerve injuries with a long nerve gap.