Specificity in peripheral nerve regeneration: a discussion of the issues and the research

Cauda equina repair in the rat: part 1. Stimulus-evoked EMG for identifying spinal nerves innervating intrinsic tail muscles

Cauda equina injuries may produce severe leg and pelvic floor dysfunction, for which no effective treatments exist. We are developing a rat cauda equina injury model to allow nerve root identification and surgical repair. One possible difficulty in implementing any repair strategy after trauma in humans involves the correct identification of proximal and distal ends of nerve roots separated by the injury. Two series of studies were carried out. In Series 1, we electrically stimulated segmental contributors to the dorsal and ventral caudales nerves in order to characterize the recruitment patterns of muscles controlling rat tail movements. In Series 2, we attempted to identify individual nerve roots forming the cauda equina by both level of origin and function (i.e., dorsal or ventral), based solely upon the recruitment patterns in response to electrical stimulation. For Series 1 studies, electrical stimulation of the segmental contributors showed that all nerve roots-from the sixth lumbar to the first coccygeal-contributed to recruitment of muscles found at the base of the tail. Intrinsic tail muscles lying more distally in the tail showed a more root-specific pattern of innervation. For Series 2, the rate of successful identification of an unknown nerve root as being ventral was very high (>95%), and only somewhat lower (approximately 80%) for dorsal roots. Correctly identifying the level of origin of that root was more difficult, but for ventral roots this rate still exceeded 90%. Using the rat cauda equina model, we have shown that stimulus-evoked EMG can be used to identify ventral nerve roots innervating tail muscles with a high degree of accuracy. These findings support the feasibility of using this conceptual approach for identifying and repairing damaged human cauda equina nerve roots based on stimulus-evoked recruitment of muscles in the leg and pelvic floor.

Neuregulinas 1-alfa e 1-beta na regeneração de nervos periféricos

OBJETIVO: Avaliar o efeito das neuregulinas 1-alfa e 1-beta na regeneração de nervos ciáticos de camundongos C57BL/6J, adultos, machos, através da técnica de tubulização. MÉTODOS: Utilizaram-se 18 animais, divididos em 3 grupos, implantando-se prótese de polietileno em falhas de 4,0 mm no nervo ciático esquerdo: grupo 1 contendo apenas colágeno purificado (Vitrogen®); grupo 2, colágeno associado a neuregulina 1-alfa; grupo 3 com colágeno e neuregulina 1-beta. O grupo controle foi formado por 6 segmentos de nervos ciáticos direitos. Após 4 semanas, os animais foram sacrificados; extraiu-se segmento do ponto médio do nervo regenerado no interior das próteses, padronizaram-se cortes histológicos e confecção das lâminas para análise histomorfométrica. Confrontaram-se os resultados estatisticamente. RESULTADOS: Os animais tratados com neuregulinas tiveram maior número de axônios mielinizados, com diferença estatisticamente significante quando comparados ao grupo colágeno. Não houve diferença estatística entre os grupos de neuregulinas 1-alfa e 1-beta. CONCLUSÃO: a adição de neuregulinas proporcionou aumento significativo do número de fibras mielinizadas.

Lack of topographic specificity in nerve fiber regeneration of rat forelimb mixed nerves

Multiple nerve repair by means of a Y-shaped nerve guide represents a good model for studying the specificity of peripheral nerve fiber regeneration. Here we have used it for investigating the specificity of axonal regeneration in mixed nerves of the rat forelimb model. The left median and ulnar nerves, in adult female rats, were transected and repaired with a 14-mm Y-shaped conduit. The proximal end of the Y-shaped conduit was sutured to the proximal stump of either the median nerve or the ulnar nerve. Ten months after surgery, rats were tested for functional recovery of each median and ulnar nerve. Quantitative morphology of regenerated myelinated nerve fibers was then carried out by the two-dimensional disector technique. Results showed that partial recovery of both median and ulnar nerve motor function was regained in all experimental groups. Performance in the grasping test was significantly lower when the ulnar nerve was used as the proximal stump. Ulnar test assessment showed no significant difference between the two Y-shaped repair groups. The number of regenerated nerve fibers was significantly higher in the median nerve irrespectively of the donor nerve, maintaining the same proportion of myelinated fibers between the two nerves (about 60% median and 40% ulnar). On the other hand, nerve fiber size and myelin thickness were significantly larger in both distal nerves when the median nerve was used as the proximal donor nerve stump. G-ratio and myelin thickness/axon diameter ratio returned to normal values in all experimental groups. These results demonstrate that combined Y-shaped-tubulization repair of median and ulnar nerves permits the functional recovery of both nerves, independently from the proximal donor nerve employed, and that tissue, and not topographic, specificity guides nerve fiber regeneration in major forelimb mixed nerves of rats.

Effects of motor and sensory nerve transplants on amount and specificity of sciatic nerve regeneration

Nerve regeneration after complete transection does not allow for adequate functional recovery mainly because of lack of selectivity of target reinnervation. We assessed if transplanting a nerve segment from either motor or sensory origin may improve specifically the accuracy of sensory and motor reinnervation. For this purpose, the rat sciatic nerve was transected and repaired with a silicone guide containing a predegenerated segment of ventral root (VR) or dorsal root (DR), compared to a silicone guide filled with saline. Nerve regeneration and reinnervation was assessed during 3 months by electrophysiologic and functional tests, and by nerve morphology and immunohistochemistry against choline acetyltransferase (ChAT) for labeling motor axons. Functional tests showed that reinnervation was successful in all the rats. However, the two groups with a root allotransplant reached higher degrees of reinnervation in comparison with the control group. Group VR showed the highest reinnervation of muscle targets, whereas Group DR had higher levels of sensory reinnervation than VR and saline groups. The total number of regenerated myelinated fibers was similar in the three groups, but the number of ChAT+ fibers was slightly lower in the VR group in comparison with DR and saline groups. These results indicate that a predegenerated root nerve allotransplant enhances axonal regeneration, leading to faster and higher levels of functional recovery. Although there is not clear preferential reinnervation, regeneration of motor axons is promoted at early times by a motor graft, whereas reinnervation of sensory pathways is increased by a sensory graft.

The course of aberrant reinnervation following nerve repair with fresh or denatured muscle autografts

Denatured muscle grafts obtained by freeze thawing have been proposed to replace losses in the peripheral nerves. In the present report, we compare the performance of such grafts with fresh grafts in the rat median nerve. A long-term effect of muscle interposition on reinnervation was studied by behavioral assessment, muscle ATPase histochemistry, and retrograde labeling of motoneurons. There was no difference in grasping strength recovery between fresh and denatured 10-mm-long muscle grafts. Recovery was delayed and incomplete. Twelve months after surgery, only 50% of the normal grasping strength was attained. Grasping recovery was not observed in the 20-mm-long graft groups. Pathway reinnervation was non-specific with a huge amount of motor fiber misdirection. A decrease in the number of misdirected motor fibers occurred with time and activity recovery. Muscle reinnervation was not specific with disturbance of the mosaic pattern and type-grouping formation. Preference of type I axons for reinnervating deeper zones was observed. Type I aberrant reinnervation was demonstrated in the muscle periphery. The mosaic distribution of type I and II muscle fibers was not stable, and readjustments were observed with time, correlating with grasping improvement. During grasping strength recovery, there was a decrease in the number of type I fibers peripherally located and an increase of those deeply disposed. A time- and activity-related recovery was associated with readjustment in the pathways and muscle fiber rearrangement. We suggest that muscle activity generates specificity.

Differential growth of axons from sensory and motor neurons through a regenerative electrode: A stereological, retrograde tracer, and functional study in the rat

Polyimide regenerative electrodes (RE) constitute a promising neural interface to selectively stimulate regenerating fibers in injured nerves. The characteristics of the regeneration through an implanted RE, however, are only beginning to be established. It was recently shown that the number of myelinated fibers distal to the implant reached control values 7 months postimplant; however, the functional recovery remained substantially below normal [J Biomed Mater Res 60 (2002) 517]. In this study we sought to determine the magnitude, and possible selectivity, of axonal regeneration through the RE by counting sensory and motor neurons that were retrogradely labeled from double tracer deposits in the sciatic nerve. Adult rats had their right sciatic nerves transected, and the stumps were placed in silicone tubes; some simply were filled with saline (Tube group), and others held a RE in its center (RE group). Simultaneously, the proximal stump was exposed to Diamidino Yellow. Two months later the nerves were bilaterally excised distal to the implant, and exposed to Fast Blue. Electrophysiological recordings, and skin nociceptive responses confirmed previous findings of partial functional recovery. In controls, an average of 20,000 and 3080 neurons were labeled in L4-L5 dorsal root ganglia (with minor contributions from L3 and/or L6), and in the ventral horn of the lumbar spinal cord, respectively. In the regenerating side, 35% of the DRG neurons were double-labeled, without differences between groups. In contrast, only 7.5% of motoneurons were double-labeled in the RE group, vs. 21% in the Tube group. Moreover, smaller ganglion cells regenerated better than large neurons by a significant 13.8%. These results indicate that the RE is not an obstacle for the re-growth of sensory fibers, but partially hinders fiber regeneration from motoneurons. They also suggest that fine fibers may be at an advantage over large ones to regenerate through the RE.