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Neuromuscular Reinnervation Efficacy After Nerve Repair Or Graft In Yfp Mice
Alex C. Woollard, BM BSc MRCS1, Jeff W. Lichtman, PhD2, Kerstin Rolfe, PhD1, Adriaan O. Grobbelaar, MBChB, FCS (SA), MMed, FRCS(Plast)1.
1The Royal Free Hospital, London, United Kingdom, 2Harvard University, Cambridge, MA, USA.
Our objective was to find a convenient animal model to assess to the reanimation of the paralyzed human face. Reanimation is achieved through the functional muscle transfer of a skeletal muscle into the paralyzed mid-face, re-innervated by the opposite facial nerve through a nerve graft. Results however are variable: 80% are good or excellent but 20% poor. A poor result is where there is little or no excursion of the transferred muscle, or where an initially good result develops tightness within cthe muscle over the course of a few years. The underlying cause of these poor results is unknown.
Transgenic mice which express yellow fluorescent protein in all peripheral axons (YFP-16) were used to study the re-innervation patterns within the interscutularis ear muscle at 3 times after repair or graft. Using confocal microscopy, re-innervated neuromuscular junctions (i.e., expressing both YFP and fluorescently tagged bungarotoxin) were assayed. Extra-muscularly the number of axonal processes projecting to the regenerating nerve was assessed using 2-photon microscopy.
In most cases nerve repair or graft provided good regrowth of axons and re-innervation of most neuromuscular junctions. Axons initially grow wildly at the site of repair until finding distal nerve stumps where they re-achieve fasciculation. Distal to the repair site regenerating axons show increased branching and reduced overall caliber. The number of axons present at the point where the nerve enters the muscle is an overestimate of the actual number of axons that regenerate. We estimate that approximately half the original number of axons re-innervate the muscle. This number is the same for both nerve repair and nerve graft, however remodeling to a stable number occurs more slowly in the graft group. The number of re-innervated neuromuscular junctions was not significantly different from control animals and far greater than following nerve cut without repair. There were however significant deficits in the completeness of the reinnervation of the neuromuscular junctions themselves.
In conclusion approximately half he original number of axons return to muscle following nerve repair or graft. Axons exhibit considerable branching to expand their sphere of influence resulting in a doubling of the mean motor unit size. In addition the reduction in the caliber of regenerating axons may represent additional inadequacies of re-innervation due to the wrong axons finding their way to the muscle, or axons that are overextended due to excessive branching. This is presently being studied. There is no change in the number of neuromuscular junctions after re-innervation however the quality of the junction reoccupation is not optimal. The reduced force and excursion generated by functional muscle transfers in facial reanimation could be a result of alterations in the motor unit size and changes seen at the neuromuscular junction.
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