Accuracy of reinnervation by peripheral nerve axons regenerating across a 10-mm gap within an impermeable chamber

Treatment With Nimodipine or FK506 After Facial Nerve Repair Neither Improves Accuracy of Reinnervation Nor Recovery of Mimetic Function in Rats

Purpose

Nimodipine and FK506 (Tacrolimus) are drugs that have been reported to accelerate peripheral nerve regeneration. We therefore tested these substances aiming to improve the final functional outcome of motoric reinnervation after facial nerve injury.

Methods

In 18 female rats, the transected facial nerve was repaired by an artificial nerve conduit. The rats were then treated with either placebo, nimodipine, or FK506, for 56 days. Facial motoneurons were pre-operatively double-labeled by Fluoro-Gold and again 56 days post-operation by Fast-Blue to measure the cytological accuracy of reinnervation. The whisking motion of the vibrissae was analyzed to assess the quality of functional recovery.

Results

On the non-operated side, 93–97% of those facial nerve motoneurons innervating the vibrissae were double-labeled. On the operated side, double-labeling only amounted to 38% (placebo), 40% (nimodipine), and 39% (FK506), indicating severe misdirection of reinnervation. Regardless of post-operative drug or placebo therapy, the whisking frequency reached 83–100% of the normal value (6.0 Hz), but whisking amplitude was reduced to 33–48% while whisking velocity reached 39–66% of the normal values. Compared to placebo, statistically neither nimodipine nor FK506 improved accuracy of reinnervation and function recovery.

Conclusion

Despite previous, positive data on the speed and quantity of axonal regeneration, nimodipine and FK506 do not improve the final functional outcome of motoric reinnervation in rats.

Segregation of motor and sensory axons regenerating through bicompartmental tubes by combining extracellular matrix components with neurotrophic factors

Segregation of regenerating motor and sensory axons may be a good strategy to improve selective functionality of regenerative interfaces to provide closed-loop commands. Provided that extracellular matrix components and neurotrophic factors exert guidance effects on different neuronal populations, we assessed in vivo the potential of separating sensory and motor axons regenerating in a bicompartmental Y-type tube, with each branch prefilled with an adequate combination of extracellular matrix and neurotrophic factors. The severed rat sciatic nerve was repaired using a bicompartmental tube filled with a collagen matrix enriched with fibronectin (FN) and brain-derived neurotrophic factor (BDNF) encapsulated in poly-lactic co-glycolic acid microspheres (FN + MP.BDNF) in one compartment to preferentially attract motor axons and collagen enriched with laminin (LM) and nerve growth factor (NGF) and neurotrophin-3 (NT-3) in microspheres (LM + MP.NGF/NT-3) in the other compartment for promoting sensory axons regeneration. Control animals were implanted with the same Y-tube with a collagen matrix with microspheres (MP) containing PBS (Col + MP.PBS). By using retrotracer labelling, we found that LM + MP.NGF/NT-3 did not attract higher number of regenerated sensory axons compared with controls, and no differences were observed in sensory functional recovery. However, FN + MP.BDNF guided a higher number of regenerating motor axons compared with controls, improving also motor recovery. A small proportion of sensory axons with large soma size, likely proprioceptive neurons, was also attracted to the FN + MP.BDNF compartment. These results demonstrate that muscular axonal guidance can be modulated in vivo by the addition of fibronectin and BDNF.

Role of extracellular matrix components in facial nerve regeneration: an experimental study

OBJECTIVE

The aim of this study is to evaluate the role of extracellular matrix components in nerve regeneration. Matrigel, a complex of extracellular matrix components such as laminin (the most abundant) heparan sulphate proteoglycans (HSPG), type IV collagen and fibronectin, was used.

METHODS

Forty male rabbits, which had undergone section of the right facial nerve, were later treated by reinnervation through an artificial graft of expanded polytetrafluoroethylene (ePTFE). In 20 animals the tubes of ePTFE were filled with Matrigel; in 20 control animals the tubes were filled with saline solution.

RESULTS

The Matrigel group showed a better axonal organization and a significantly higher number of regenerated axons in the early phases (at days 15 and 30 respectively) than the control group, whereas the difference of the axons number at day 60 was less significant; besides, the axon diameter and the myelin thickness were not significantly improved by Matrigel.

DISCUSSION

Our data suggest that Matrigel is an important factor in promoting and enhancing the early phases of the regeneration after nerve injuries. Tree neurite promoting agents, such as laminin, fibronectin and collagen, allow a more systematic and agonized regeneration. Extracellular matrix components may represent a direction guidance for axonal pathway.

The role of a barrier between two nerve fascicles in adjacency after transection and repair of a peripheral nerve trunk

Aberrant reinnervation of target organs caused by misdirected axonal growth at the repair site is a major reason for the poor functional outcome usually seen after peripheral nerve transection and repair. The following two studies investigate whether criss-crossing of regenerating rat sciatic nerve axons between tibial and peroneal nerve fascicles can be reduced by using a barrier at the coaption site. The left sciatic nerve was transected and repaired at mid-thigh as follows: epineural sutures (group A, A-II), fascicular repair of tibial and peroneal nerve fascicles (group B, B-II), fascicular repair of tibial and peroneal nerve fascicles separating the two fascicles with a pedicled fat flap (group C), Integra (group D) or non-vascularized autologous fascia (group C-II). In the control groups E and D-II, only the left tibial fascicle was transected and repaired. Four and 5 months postoperatively, the outcome of regeneration was evaluated by histology, by retrograde tracing, and by assessment of the muscle force of the gastrocnemius and tibial anterior muscles. The tracing experiments showed that specificity of muscle reinnervation significantly improved when a barrier was employed, which significantly or clearly improved muscle twitch tension in groups C and D. However, muscle contraction force was not better when fascia was used as barrier. The histological picture indicated that this inferior result in group C-II was due to nerve compression caused by fibrotic scar tissue at the site of the fascia graft. Results of this study show that a pedicle fat flap and Integra used as barrier significantly prevent aberrant reinnervation between two sutured nerve fascicles in adjacency resulting in improved motor recovery in rats. Non-vascularized autologous fascia however, reduces also criss-crossing of regenerating axons between the fascicles, but causes significant nerve compression.

Nerve tubes for peripheral nerve repair

The concept of the nerve tube has been a major topic of research in the field of peripheral nerve regeneration for more than 25 years. The first nerve tubes are currently available for clinical use. This article gives an overview of the experimental and clinical data on nerve tubes for peripheral nerve repair and critically analyzes the data on which the step from laboratory to clinical use is based. In addition, it briefly discusses the different modifications to the common single lumen nerve tubes that may improve the results of generation.

Accuracy of motor axon regeneration across autograft, single-lumen, and multichannel poly(lactic-co-glycolic acid) nerve tubes

OBJECTIVE

The accuracy of motor axon regeneration becomes an important issue in the development of a nerve tube for motor nerve repair. Dispersion of regeneration across the nerve tube may lead to misdirection and polyinnervation. In this study, we present a series of methods to investigate the accuracy of regeneration, which we used to compare regeneration across autografts and single-lumen poly(lactic-co-glycolic acid) (PLGA) nerve tubes. We also present the concept of the multichannel nerve tube that may limit dispersion by separately guiding groups of regenerating axons.

METHODS

The simultaneous tracing of the tibial and peroneal nerves with fast blue and diamidino yellow was performed 8 weeks after the repair of a 1-cm nerve gap in the rat sciatic nerve to determine the percentage of double-projecting motoneurons. Sequential tracing of the peroneal nerve with diamidino yellow 1 week before repair and fast blue 8 weeks after repair was performed to determine the percentage of correctly directed peroneal motoneurons.

RESULTS

In the cases in which there was successful regeneration across single-lumen nerve tubes, more motoneurons had double projections to both the tibial and peroneal nerve branches after single-lumen nerve tube repair (21.4%) than after autograft repair (5.9%). After multichannel nerve tube repair, this percentage was slightly reduced (16.9%), although not significantly. The direction of regeneration was nonspecific after all types of repair.

CONCLUSION

Retrograde tracing techniques provide new insights into the process of regeneration across nerve tubes. The methods and data presented in this study can be used as a basis for the development of a nerve tube for motor nerve repair.

Misdirection of regenerating motor axons after nerve injury and repair in the rat sciatic nerve model

Misdirection of regenerating axons is one of the factors that can explain the poor results often found after nerve injury and repair. In this study, we quantified the degree of misdirection and the effect on recovery of function after different types of nerve injury and repair in the rat sciatic nerve model; crush injury, direct coaptation, and autograft repair. Sequential tracing with retrograde labeling of the peroneal nerve before and 8 weeks after nerve injury and repair was performed to quantify the accuracy of motor axon regeneration. Digital video analysis of ankle motion was used to investigate the recovery of function. In addition, serial compound action potential recordings and nerve and muscle morphometry were performed. In our study, accuracy of motor axon regeneration was found to be limited; only 71% (+/-4.9%) of the peroneal motoneurons were correctly directed 2 months after sciatic crush injury, 42% (+/-4.2%) after direct coaptation, and 25% (+/-6.6%) after autograft repair. Recovery of ankle motion was incomplete after all types of nerve injury and repair and demonstrated a disturbed balance of ankle plantar and dorsiflexion. The number of motoneurons from which axons had regenerated was not significantly different from normal. The number of myelinated axons was significantly increased distal to the site of injury. Misdirection of regenerating motor axons is a major factor in the poor recovery of nerves that innervate different muscles. The results of this study can be used as basis for developing new nerve repair techniques that may improve the accuracy of regeneration.

Effect of conduit repair on aberrant motor axon growth within the nerve graft in rats

After peripheral nerve injury, minimizing axonal misdirection has been a matter of importance to obtain good functional outcomes. In general, it becomes more challenging as the nerve defect length is longer. As previous works suggested that a conduit repair leaving a short gap could induce some target-specific reinnervation, we expected that a distally placed conduit combined with nerve graft would enhance the specificity of reinnervation, especially in dealing with a long gap. To test this, a 14-mm-long gap was created in the rat sciatic nerves and repaired with either 1) whole nerve graft (WG), 2) interfascicular nerve grafts (FG), or 3) whole nerve graft combined with distally placed silicone tube leaving a 5-mm gap (TUBG). At the end of follow up, the extent of target specific reinnervation (measurement of the compound muscle action potentials evoked by stimulation of the sciatic nerve and its tibial and common peroneal fascicles) and the accuracy of motoneuronal projection (sequential retrograde labeling of the common peroneal motor pool) were assessed. Both assessments revealed that groups FG and TUBG had a similar selectivity, which was significantly higher than in group WG. Consistent with these results, the functional recovery as assessed by walking track analysis showed no significant difference between groups FG and TUBG, whereas those were significantly superior to group WG. In contrast, histomorphometric assessment of the regenerating axons and wet muscle weight showed no significant difference among the three groups. In conclusion, conduit repair would have some effects on reducing motor axonal misdirection, and it might be more effective when used in the management of a large defect in combination with nerve graft.

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.

Estimations of topographically correct regeneration to nerve branches and skin after peripheral nerve injury and repair

Peripheral nerve injury is typically associated with long-term disturbances in sensory localization, despite nerve repair and regeneration. Here, we investigate the extent of correct reinnervation by back-labeling neuronal soma with fluorescent tracers applied in the target area before and after sciatic nerve injury and repair in the rat. The subpopulations of sensory or motor neurons that had regenerated their axons to either the tibial branch or the skin of the third hindlimb digit were calculated from the number of cell bodies labeled by the first and/or second tracer. Compared to the normal control side, 81% of the sensory and 66% of the motor tibial nerve cells regenerated their axons back to this nerve, while 22% of the afferent cells from the third digit reinnervated this digit. Corresponding percentages based on quantification of the surviving population on the experimental side showed 91%, 87%, and 56%, respectively. The results show that nerve injury followed by nerve repair by epineurial suture results in a high but variable amount of topographically correct regeneration, and that proportionally more neurons regenerate into the correct proximal nerve branch than into the correct innervation territory in the skin.

A sequential double labeling technique for studying changes in motoneuronal projections to muscle following nerve injury and reinnervation

The purpose of this study was to develop an anatomical technique that could directly demonstrate the motoneuron projections to the muscle both before injury and again following reinnervation. Investigation focused on the identification of a long-term retrograde fluorescent tracer that would label original motoneurons and persist long enough for reinnervating motoneurons to become labeled by a second fluorescent tracer. True Blue (TB) was evaluated as a potential long-term tracer, Fluoro-ruby (FR) and Fluoro-emerald (FE) were tested as potential short-term tracers in 45 adult Sprague-Dawley rats. In the initial phase of the study, TB was injected into the tibialis anterior (TA) muscle in 16 rats and sacrificed 1 week to 6 months later, to study its persistence. During the second stage, a short-term tracer was injected into the TA muscles bilaterally in 15 rats with survival time ranging from 4 to 28 days. Sequential double labeling was subsequently performed using the combination of TB and FR in 14 rats. The number and brightness of TB cells did not change over 6 months time, a period sufficient for complete reinnervation. FR and FE showed maximum labeling of motoneurons at 1 week after tracer application. In the double labeling study, we could easily distinguish double-labeled cells from those labeled only by TB or FR. These results suggest that sequential double labeling of TB and FR is a valuable method for long-term muscle reinnervation studies.

Correlation between target reinnervation and distribution of motor axons in the injured rat sciatic nerve

Peripheral nerve injuries are rarely followed by complete return of function. Deficits are particularly important for motor function, resulting in paralysis and muscle atrophy. In different groups, the sciatic nerve was either crushed or transected and repaired by direct suture or by tube repair using silicone or collagen tubes. After 60 days, nerve regeneration was assessed by electrophysiological and functional tests, nerve morphology and immunohistochemistry against choline acetyltransferase (ChAT) for labeling motor axons. Suture and tube repair resulted in similar levels of muscle reinnervation, but significantly lower than after nerve crush. Recovery of walking track pattern was poor in all groups after nerve section. The numbers of regenerated myelinated fibers and of ChAT+ fibers were similar to control values after nerve crush, but increased after section and repair. The normal fascicular architecture and grouping of ChAT+ fibers were maintained after nerve crush, but lost after section and repair, where motor fibers were scattered within small regenerated fascicles throughout the nerve. The loss of fascicular organization was related to the deficient recovery of locomotor function. Thus, labeling of motor axons by ChAT immunohistochemistry provides useful information for the study of the degree and specificity of nerve regeneration.

Morphological and functional evaluation of leg-muscle reinnervation after coupler coaptation of the divided rat sciatic nerve

Mechanical couplers are successfully used for microvascular venous anastomoses. The advantages include a simple and fast technique and a high patency rate. Couplers offer a secluded coaptation site, and might also be of use in peripheral nerve repair. The present study was designed to investigate coupler coaptation of the rat sciatic nerve, evaluating the number and locations of motor and sensory neurons projecting to the selected muscles as well as stimulation-induced muscle contraction force. Adult rats underwent either suture or coupler repair after left sciatic nerve transection. In all rats, the experimental side was compared to the healthy right side. Evaluation after 20 weeks included retrograde labeling of motoneurons and dorsal root ganglion neurons projecting to the tibial anterior muscle and to the tibial posterior muscle, histology, muscle contraction force (tibial anterior muscle and gastrocnemius muscle), and a pinch reflex test. The results show that the suture and the coupler groups did not differ significantly regarding the examined parameters, except for discrete signs of nerve compression at the coaptation site after coupler repair due to fibrous tissue ingrowth. However, this did not impair axonal regeneration. Importantly, axonal outgrowth from the repair site to the surrounding tissue was not observed after coupler coaptation, but it was observed after suture repair. These results suggest that couplers may be of value for repair of nerves in adjacency to avoid axonal crisscrossing between nerves during regeneration.

Extracts obtained from predegenerated nerves improve functional recovery after sciatic nerve transection

Gap injuries of peripheral nerves, resulting from trauma or neurosurgical procedures, presage badly, for the presence of the distal stump of the nerve seems to be indispensable for regeneration. The standard grafting method requires a lesion of a healthy nerve, and therefore various substitutional materials are under consideration. The aim of the present work was to examine the recovery of rat sciatic nerves after supplying 10-mm-long gaps with an autologous connective-tissue chambers filled with fibrin only or fibrin and various neuroactive substances (brain-derived neurotrophic factor (BDNF), extracts from predegenerated or non-predegenerated nerves). The nerves were allowed to regenerate for 16 weeks. Recovery was measured functionally using the sciatic functional index, and by comparing the weight ratios of calf muscles. The histologic features of regeneration were assessed by counting the number of acetylcholinesterase-positive nerve fibers present inside implanted chambers. We found that chambers filled with fibrin and predegenerated peripheral nerve extracts or BDNF supported functional nerve regeneration much more strongly than chambers filled with fibrin only or fibrin and non-predegenerated peripheral nerve extracts. We conclude that autologous connective-tissue chambers filled with fibrin and predegenerated peripheral nerve extracts or BDNF seem to be a promising tool in peripheral nerve gap injury treatment, with likely clinical implications.

Peripheral and Spinal Motor Reorganization after Nerve Injury and Repair

Functional recovery after peripheral nerve injury depends on the amount as well as on the accuracy of reinnervation by regenerative axons. In this study, the rat sciatic nerve was subjected to crush injury or complete transection repaired by either (1) straight nerve suture, (2) crossed nerve suture of tibial and peroneal fascicles, or (3) silicone tubulization leaving a gap of 4 mm. The compound muscle action potentials (CMAP) of gastrocnemius, tibialis anterior and plantar muscles were recorded 90 days post operation to assess functional reinnervation and Fast Blue, Fluoro Gold and DiI were applied to the nerve branches projecting into these muscles to quantify morphological reinnervation. The CMAP amplitude achieved in gastrocnemius, tibialis anterior and plantar muscles was higher after nerve crush (86%, 82%, 65% of control) than after any surgical nerve repair (straight suture: 49%, 53%, 32%; crossed suture: 56%, 50%, 31%; silicone tube: 42%, 44%, 25%). The total number of labeled motoneurons, however, did not significantly differ between groups (control: 1238 +/- 82, crush: 1048 +/- 49, straight suture: 1175 +/- 106, crossed suture: 1085 +/- 84, silicone tube: 1250 +/- 182). The volume occupied by labeled motoneurons within the spinal cord was larger after surgical nerve repair than in crush or normal control animals, and fewer neurons showed abnormal multiple projections after crush (2.5%) or straight suture (2.2%) than following crossed suture (5%) or silicone tube (6%). In conclusion, nerve repair with a silicone tube leaving a short gap does not increase accuracy of reinnervation.

Recurrent laryngeal nerve regeneration by tissue engineering

The recurrent laryngeal nerve (RLN) does not regenerate well after it has been cut, and no current surgical methods achieve functional regeneration. Here, we evaluate the functional regeneration of the RLN after reconstruction using a biodegradable nerve conduit or an autologous nerve graft. The nerve conduit was made of a polyglycolic acid (PGA) tube coated with collagen. A 10-mm gap in the resected nerve was bridged by a PGA tube in 6 adult beagle dogs (group 1) and by an autologous nerve graft in 3 dogs (group 2). Fiberscopic observation revealed functional regeneration of the RLN in 4 of the 6 dogs in group 1. No regeneration of the RLN was observed in any dog in group 2. We also tested for axonal transport, and measured the compound muscle action potential. The RLN can be functionally regenerated with a PGA tube, which may act as a scaffold for the growth of regenerating axons.

Functional impact of axonal misdirection after peripheral nerve injuries followed by graft or tube repair

Accuracy of reinnervation is one of the main factors conditioning functional recovery after brain, spinal, or peripheral axonal damage. Using the peripheral nerve as an experimental model, we studied the amount of inaccurate muscle reinnervation and its consequences on walking. Adult rats were submitted to an 8-mm resection of the sciatic nerve repaired by autograft (AG, n = 9), silicone (SIL, n = 13) or poly-L-lactate-epsilon-caprolactone (PLC, n = 11) single guides, and fascicular tubulization of peroneal and tibial branches with a dual silicone tube (FSIL, n = 9). At the end of follow-up, the sciatic nerve and its tibial and peroneal fascicles were dissected and stimulated by means of a suction electrode. In control rats, gastrocnemius and plantar muscles are fully innervated by the tibial fascicle and the tibialis anterior muscle by the peroneal nerve. None of the groups had noticeable recovery of locomotion assessed by the walking track index (SFI around -70 in all groups). After resection, all animals of groups AG, SIL, and PLC showed aberrant muscle reinnervation by axons from a non-corresponding fascicle, whereas in group FSIL only one of six regenerated animals showed misdirected activity. The proportion of inaccurate muscle activation was similar in group AG (47% for gastrocnemius, 54% for tibialis anterior, and 44% for plantar muscles) and in group SIL (42%, 42%, and 42%), and reduced in group PLC (26%, 38%, and 27%). In conclusion, fascicular silicone tubulization allowed the highest degree of accuracy but the lowest recovery, whereas resorbable PLC guides provided for the best balance between amount and accuracy of reinnervation after nerve resection.

On the use of fast blue, fluoro-gold and diamidino yellow for retrograde tracing after peripheral nerve injury: uptake, fading, dye interactions, and toxicity

The usefulness of three retrograde fluorescent dyes for tracing injured peripheral axons was investigated. The rat sciatic was transected bilaterally and the proximal end briefly exposed to either Fast Blue (FB), Fluoro-Gold (FG) or to Diamidino Yellow (DY) on the right side, and to saline on the left side, respectively. The nerves were then resutured and allowed to regenerate. Electrophysiological tests 3 months later showed similar latencies and amplitudes of evoked muscle and nerve action potentials between tracer groups. The nerves were then cut distal to the original injury and exposed to a second (different) dye. Five days later, retrogradely labelled neurones were counted in the dorsal root ganglia (DRGs) and spinal cord ventral horn. The number of neurones labelled by the first tracer was similar for all three dyes in the DRG and ventral horn except for FG, which labelled fewer motoneurones. When used as second tracer, DY labelled fewer neurones than FG and FB in some experimental situations. The total number of neurones labelled by the first and/or second tracer was reduced by about 30% compared with controls. The contributions of cell death as well as different optional tracer combinations for studies of nerve regeneration are discussed.

Superior muscle reinnervation after autologous nerve graft or poly-L-lactide-epsilon-caprolactone (PLC) tube implantation in comparison to silicone tube repair

Recovery after peripheral nerve injury depends not only on the amount of reinnervation, but also on its accuracy. The rat sciatic nerve was subjected to an 8 mm long gap lesion repaired either by autograft (AG, n = 6) or tubulization with impermeable silicone tube (SIL, n = 6) or permeable tube of poly-L-lactide-epsilon-caprolactone (PLC, n = 8). Recordings of the compound muscle action potential (CMAP) from gastrocnemius (mGC), tibialis anterior (mTA) and plantar (mPL) muscles were performed 90 days after injury to assess the amount of muscle reinnervation. The CMAP amplitude achieved in mGC, mTA and mPL was similar in after nerve autograft (39%, 42%, 22% of control values) and PLC tube implantation (37%, 36%, 24%) but lower with SIL tube (29%, 30%, 14%). The nerve fascicles projecting into each of these muscles were then transected and retrograde tracers (Fluoro Gold, Fast Blue, DiI) were applied to quantify the percentage of motoneurons with single or multiple branches to different targets. The total number of labeled motoneurons for the three muscles did not differ in autografted rats (1186 +/- 56; mean +/- SEM) with respect to controls (1238 +/- 82), but was reduced with PLC tube (802 +/- 101) and SIL tube (935 +/- 213). The percentage of neurons with multiple projections was lower after autograft and PLC tube (6%) than with SIL tube (10%). Considering the higher CMAP amplitude and lower number of neurons with multiple projections, PLC nerve conduits seem superior to SIL tubes and a suitable alternative to autografts for the repair of long gaps.

Interposition of a pedicle fat flap significantly improves specificity of reinnervation and motor recovery after repair of transected nerves in adjacency in rats

Despite highest standards in nerve repair, functional recovery following nerve transection still remains unsatisfactory. Nonspecific reinnervation of target organs caused by misdirected axonal growth at the repair site is regarded as one reason for a poor functional outcome. This study was conducted to establish a method for preventing aberrant reinnervation between transected and repaired nerves in adjacency. Rat sciatic nerve was transected and repaired as follows: epineural sutures of the sciatic nerve (group A, n = 6), fascicular repair of tibial and peroneal nerves respectively (group B, n = 8), and, as in group B, separating both nerves using a pedicle fat flap as barrier (group C, n = 8). As control only, the tibial nerve was transected and repaired (group D, n = 5). Muscle contraction force of the gastrocnemius muscle was significantly higher in group C as compared with groups A and B after 4 months. Muscle weight showed significantly lower values in group A as compared with groups B, C, and D. Histologic examination in group C revealed little growth of axons from the tibial to the peroneal nerve and vice versa. This axon crossing was observed only when gaps between the fat cells were available. These findings were confirmed by a significantly lower rate of misdirected axonal growth as compared with groups A and B using sequential retrograde double labeling technique of the soleus motoneuron pool. We conclude that a pedicle fat flap significantly prevents aberrant reinnervation between repaired adjacent nerves resulting in significantly improved motor recovery in rats. Clinically, this is of importance for brachial plexus, sciatic nerve, and facial nerve repair.

An anterograde degeneration study of the distribution of regenerating rat myelinated fibers in the silicone chamber model

Specificity of reinnervation after a peripheral nerve lesion has given rise to considerable controversy. As a contribution to solving this issue we have evaluated the specificity of reinnervation of the peroneal nerve after a complete transection of the sciatic nerve repaired with an 8 mm silicone tube, leaving a 4 mm gap between the nerve stumps. Our findings reveal unspecificity of reinnervation of the distal peroneal branch. This lack of specificity is shown by a random distribution of fibers originating from both proximal branches at the level of the tube and at distal peroneal and tibial branches, argue against specificity of regeneration in this model.

Differential effects of NT-3 on reinnervation of the fast extensor digitorum longus (EDL) and the slow soleus muscle of rat

Previous studies of gastrocnemius muscle reinnervation showed specific normalization of the proportion and diameter of fast type 2b muscle fibres following NT-3 delivery to the proximal stump of the cut sciatic nerve. Here, we investigate if normalization was related to greater improvement of muscle reinnervation of fast (extensor digitorum longus; EDL) than slow (soleus) motor units. NT-3-impregnated (NT-3 group) or plain fibronectin (FN group) mats were inserted into a sciatic nerve gap. Neuromuscular junctions (NMJs) labelled with TRITC-alpha-bungarotoxin were colabelled with calcitonin gene-related peptide (CGRP) or 4E2 antisera and imaged using confocal microscopy. CGRP and 4E2 were used as markers for newly reinnervated and structurally mature NMJs, respectively. At 40 days postsurgery, denervated NMJs in EDL and soleus muscles of both groups presented a 50% decrease of surface area due to decreased width. At day 80 in EDL, more NMJs were reinnervated by CGRP-immunoreactive terminals in the NT-3 (7.1%) than in the FN group (4.2%); there was no difference between groups for soleus. At 120 days, 4E2-immunoreactive NMJs were more numerous in EDL of the NT-3 (40.0%) than in the FN group (7.3%), unlike in soleus (NT-3, 1. 6%; FN, 1.8%), and presented a partial size recovery. These results indicate that NT-3 preferentially improves reinnervation of fast muscles over slow muscle, although the mechanism of this improvement is still unclear.

Fast blue and diamidino yellow as retrograde tracers in peripheral nerves: efficacy of combined nerve injection and capsule application to transected nerves in the adult rat

Capsule application of Diamidino Yellow (DY) to the cut end of the sciatic nerve immediately followed by capsule application of Fast Blue (FB) resulted in approximately 95% double-labelled dorsal root ganglion neurones (DRGn) and motoneurones (Mn). Nerve injection of DY followed either immediately or 2 months later by capsule application of FB resulted in approximately 90% double-labelled DRGn and Mn, indicating that DY and FB label similar populations of DRGn and Mn, and that insignificant DY fading occurred during this period. Inversing the order of application, however, i.e. nerve injection of FB followed immediately by capsule application of DY, resulted in double labelling in only approximately 10% of the DRGn and Mn. These percentages increased to 70% of the DRGn and 60% of the Mn when the FB injection was followed 1 or 2 months after by the DY application, indicating that DY uptake is blocked by recent administration of FB. The results indicate that DY and FB might be useful for sequential labelling before and after nerve injury as a tool to investigate the accuracy of sensory and motor regeneration.

Sciatic nerve transection in the adult rat: abnormal EMG patterns during locomotion by aberrant innervation of hindleg muscles

The effects of lesions in the sciatic nerve were studied in adult rats. In the left hindleg, a segment 12 mm long was resected from the proximal part of the nerve, before the bifurcation into the peroneal and tibial nerves. This segment in a reversed orientation was used as a nerve graft. EMG patterns in the tibialis anterior and the gastrocnemius muscles at both sides were recorded during locomotion in six rats after recovery periods varying from 15 to 21 weeks. The specificity of axonal outgrowth was studied in nine rats by retrogradely labeling the motoneurons with unconjugated Cholera Toxin subunit B (CTB) after injections into the gastrocnemius, the soleus, and the tibialis anterior muscles at both sides. EMG patterns at the operated side were irregular and we often observed coactivation of the gastrocnemius and tibialis anterior muscle. Moreover, burst activity was badly adjusted to the phases of the stepcycle. Retrogradely labeling indicated that the pools of motoneurons innervating the respective muscles at the left side had increased in volume. Neuronal diameters were slightly decreased but a considerable decrease was observed in dendritic branching and dendrite bundles in the pools of the SOL and in the GC were absent. No consistent trends in neuronal numbers at the affected side in comparison to the right side were detected. We conclude that axons, sprouting from the proximal stump of the sciatic nerve, innervate the muscles aselectively and that the motoneurons of origin maintain their original activation pattern.

Effects of Systemically Applied IGF-1 on Motor Nerve Recovery After Peripheral Nerve Transection and Repair in the Rat - A Functional Study

The trophic effects of systemically applied Rh insulin-like growth factor-1 (rhIGF-1) on peripheral motor nerve regeneration following transection and epineural repair in rats median nerve have been examined. RhIGF-1 (0.5 mg/kg/rat) was administered subcutaneously to the neck region of the repaired side for 14 days post-operation. Motor recovery was tested with the grasping test that is an objective quantitative behavioural assessment of regeneration of the rats median nerve. Muscle twitch tension and muscle weight were measured in the flexor digitorum sublimus muscle. No significant differences between experimental and control animals regarding onset of muscle function, recovery of muscle power, and muscle weight were found. These results demonstrate that subcutaneously applied rhIGF-1 cannot improve functional motor recovery after nerve transection and repair in the rat as has been demonstrated after nerve crushing injury. This is regarded as a consequence of specificity failure during reinnervation, which occurs after nerve transection and repair, whereas after crushing injury specific reinnervation is a common feature.

Sciatic nerve regeneration through venous or nervous grafts in the rat

This study analyses the interest of isologous venous grafts filled with saline or with Schwann cells versus nerve grafts as guides for regeneration of the sciatic nerve in 35 Wistar rats. Electrophysiological parameters (conduction velocities and distal latencies of motor responses) and the functional index of De Medinacelli were measured several times from 1 month to 1 year after surgery. An histological analysis was performed on 2 control rats and on 3 rats killed 6 or 12 months after surgery: the total number of fibers was counted on a montage photoprint of the whole nerve, and the diameters of axons and the thickness of the myelin sheath were measured on digitized images. With a portion of nerve as guide, the regeneration is faster than with a vein. However, regeneration after 6 months is at least as good with a venous graft filled with Schwann cells, as assessed by electrophysiological, functional, and histological analysis. The addition of Schwann cells in grafted veins allows the nerve to regenerate through longer gaps than previously described (25 vs 15 mm). In order to assess the quality of nerve regeneration, functional, electrophysiological, and histological analysis are complementary.

Inaccurate projection of rat soleus motoneurons: a comparison of nerve repair techniques

The objectives of this study were 1) to determine the degree to which soleus motoneurons find their appropriate target following crush and transection injuries to the sciatic nerve, and 2) to determine whether repair of a transected nerve with a silicone tube leads to greater specificity of reinnervation and recovery of muscle function than the standard epineurial suture repair method. Sixty adult female Sprague-Dawley rats were randomly assigned to one of three sciatic nerve injury groups: crush injury, transection with epineurial suture repair, or transection with a silicone tube repair. The degree to which soleus motoneurons were able to find their appropriate target following a sciatic nerve injury was examined using a double labeling dye technique in which the original soleus motor pool was labeled with fast blue and reinnervating motoneurons were labeled with Dil. Soleus motoneurons were able to find their appropriate target following a crush injury. The accuracy of reinnervation following a transection injury and repair, however, was relatively poor. Only 14% of the original soleus motoneurons found the correct target following a transection injury. Repair of a lesioned nerve with a silicone tube and a 5-mm gap as opposed to epineurial sutures did not increase the specificity of reinnervation or the degree of muscle recovery.

Nerve regeneration across a 25-mm gap bridged by a polyglycolic acid-collagen tube: a histological and electrophysiological evaluation of regenerated nerves

In the study reported here we have examined the nerve regeneration that occurs over a 25-mm gap using a novel biodegradable nerve guide tube. The tube was a composite of polyglycolic acid (PGA) mesh coated with collagen which was filled with neurotrophic factors. The left sciatic nerve of ten adult cats was dissected. The stumps were connected by the tube, and fixed gap. Histological examinations carried out 4-16 months after implantation of the tube revealed regeneration of well vascularized nerve tissue. Regeneration of both myelinated, unmyelinated axons and Schwann cells was confirmed by electron microscopy 5 months after surgery. Following injection of horseradish peroxidase (HRP) into a site peripheral to the regenerated segment of the sciatic nerves, motoneurons in the ventral horn of the spinal cord, afferent terminals in the medial portion of the dorsal column of the medulla oblongata, and sensory afferent nerve terminals in the dorsal horn of the spinal cord were labelled. Electrophysiological examinations revealed restoration of evoked electromyograms and sensory evoked potentials (SEPs) recorded from the cerebral cortex as well as the spinal cord. We also found that some of the regenerated motor axons exhibited branching in the regenerated segments. In two cases, a single motoneuronal axon from the regenerated side projected to both flexors and extensors, simultaneously. Our results indicate that the PGA-collagen composite tube is a promising tool for use as a nerve guide tube in peripheral nerve regeneration.

The role of conduits in nerve repair: a review

The restoration of effective and meaningful axonal function following peripheral nerve injury continues to be a considerable clinical challenge. The use of conduits to bridge the gap between severed ends is a contemporary experimental maneuver that isolates the microenvironment of regenerating axons. Entubulation has allowed analysis and manipulation of putative influences upon nerve regeneration. A review is provided of the research efforts that have explored the neurobiological and mechanical factors that guide nerve regeneration within conduits. Levels of specificity, from tissue specific growth to end-organ specific growth, are outlined within the framework of the theories of Neurotropism, Contact Guidance and Neurotrophism. Included are investigations utilizing different conduit materials and the few clinical applications of these conduits. A number of chamber manipulations, extra-cellular matrix substrates and growth factors and their molecular receptors have been implicated in enhanced regeneration specificity. This information has been extended to the conduit model. The interposition of healthy nerve segments into conduits is proposed as a means of extending the length of successful nerve regeneration.

Joseph H. Boyes Award. Dispersion of regenerating axons across enclosed neural gaps

Tubular prostheses support peripheral axon regeneration across gaps of up to 3 cm in the primate. However, the precision with which axons cross a gap and reinnervate the periphery remains controversial. These experiments use continuous tracing of regenerated rat sciatic nerve axons with HRP-WGA to examine the dispersion of axons as they cross a gap, and the effects on this dispersion of gap distance and fascicular orientation. Proximal and distal tibial and peroneal fascicles were precisely oriented about the longitudinal midplane of a silicon tube, with correct or reversed fascicular alignment and gaps of 2 mm and 5 mm. After 6 weeks of regeneration, HRP-WGA was applied to the distal peroneal fascicle to continuously label its reinnervating axons. These axons tended to grow straight across the tube, with dispersion increasing as a factor of distance when correct fascicular alignment was maintained. However, when fascicular alignment was reversed, axonal dispersion was determined by fascicular size rather than fascicular identity. These experiments provide no evidence for neurotropic interactions promoting "correct" fascicular reinnervation. Progressive axonal dispersion and the absence of factors to promote fascicular specificity should result in an increase of random reinnervation and functional disruption with larger gaps. An enclosed gap is not an acceptable substitute for nerve graft when reconstructing a nerve that serves multiple functions.

Effects of predegeneration on nerve regeneration through silicone Y-chambers

The effects of nerve predegeneration on the preferential growth of regenerating axons were studied using a silicone Y-chamber model. This system provided a choice for axons to grow towards two distal nerve options, either a 7-day predegenerated nerve segment (PNS) or a fresh nerve segment (FNS). The rat peroneal or tibial nerve was inserted into the proximal intlet and the PNS and FNS of the corresponding nerve were inserted into the distal outlets. At 28 days postoperative, the size of the distal regenerate was significantly greater (26%) towards the PNS for the tibial nerve group. The density and number of regenerated myelinated axons in the distal nerve segment was greater on the PNS for both the tibial (97 and 88%, respectively) and peroneal (221 and 221%, respectively) nerve groups. In contrast, the elevated density and number of nonvascular nuclei was relatively constant for both PNS and FNS. Immunocytochemical and ultrastructural evidence support the hypothesis that the early activation of Schwann cells is primarily responsible for the enhanced regeneration and maturation observed in PNS. It is suggested that PNS might improve the outcome after clinical repair of injured peripheral nerves.

Modulation of low-affinity nerve growth factor receptor in injured adult rat spinal cord motoneurons

Spinal and brainstem motoneurons of the adult rat reexpress low-affinity nerve growth factor receptor (LNGFR) and its mRNA after axotomy. We have previously reported the time courses of this reexpression after cut (no regeneration) or crush (followed by regeneration) of the sciatic nerve. We have shown that the length of the different phases of this reexpression (appearance, maintenance and disappearance) can vary according to the type of axotomy. With the present study we expand our previous data and describe and analyze the modulation the LNGFR expression in adult spinal cord motoneurons following different lesion paradigms. In one approach we have imposed three traumatic injuries that still allow regeneration of the sciatic nerve but with a different time course with respect to the crush injury (application of a silicone regeneration chamber, multiple crushes and delayed repair of ligated nerves). In a second approach, we have determined the capability of three toxic or metabolic injuries to induce LNGFR expression without any direct trauma of the nerve (experimental diabetogenesis, botulinum and alpha-bungarotoxin intoxication and 2,5-hexanedione intoxication). In a third approach, we have investigated the effect of the block of the axoplasmic transport on the LNGFR expression following different topical applications of vincristine combined with a nerve crush. The results we present are consistent with the idea that: (1) LNGFR immunoreactivity in adult motoneurons is expressed by motoneurons that are attending to an axonal outgrowth and not a generic signal of cellular damage or impairment of the motor function; (2) LNGFR expression in these motoneurons is related to and parallels the outgrowth process time frame, and (3) the signal/s that trigger and sustain this reexpression may be retrogradely transported from the periphery.

Nerve repair using freezing and fibrin glue: immediate histologic improvement of axonal coaptation

Despite the fine microsurgical techniques available, injuries to peripheral nerves are still a surgical problem. Sutures placed in the epineurium or perineurium cause compression, brushing, and misdirection of endoneural tissue. A technique of nerve repair using freezing to trim the nerve and fibrin glue to coat it before thawing is described. The entire surgical repair procedure is carried out with the nerve stumps frozen. The observed axonal alignment with this technique was much better than that obtained by microsuture alone.

Experimental repair of the oculomotor nerve: the anatomical paradigms of functional regeneration

In adult guinea pigs, the oculomotor nerve was sectioned proximally (at the tentorial edge) or more distally (at the orbital fissure) and immediately repaired by reapproximation. During a 24-week postoperative period, extrinsic eye motility was assessed by analyzing the vestibulo-ocular reflexes. The regenerated oculomotor nerve was studied morphometrically on semi-thin histological sections at 16 and 24 weeks postinjury. The selectivity of muscle reinnervation was investigated by injection of both single (horseradish peroxidase) and double (fluorescent dyes) retrograde axonal tracers into the eye muscles. Following proximal repair of the oculomotor nerve, the degree of recovery of extraocular motility varied among different animals and remained stable over long-term observations. In animals with poor recovery, aberrant eye movements were always found, and the somatotopic map of the reinnervated eye muscles was greatly altered. Distortions of the central representation were also seen in those animals in which a good level of functional recovery was seen. However, in animals with good recovery, a topographic bias was re-established by about 65% of the original neuronal population, as opposed to 26% in the animals with poor recovery. Neurons located contralateral to the axotomized nucleus sprouted intra-axially and projected their axons to denervated eye muscles. The number and diameter of the regenerated axons, the number and soma diameter of the axotomized neurons, and the ratio of distal axonal branches to proximal supporting neurons were all related to the degree of functional recovery. Following repair of the oculomotor nerve at the orbital fissure, extraocular motility had recovered in all of the animals at 16 weeks without aberrant phenomena. Functional regeneration of the distally transected oculomotor nerve is thought to be the result of selective muscle reinnervation.

Reinnervation of muscles in rats after repair of transsected sciatic nerves with Y-shaped and X-shaped silicone tubes. Muscle reinnervation after nerve repair

Reinnervation of the gastrocnemius and anterior tibial muscles was assessed by measurements of tetanic force after repair of sciatic nerves with Y-shaped or X-shaped silicone tubes in rats. The transsected proximal stump of either the tibial or the peroneal fascicle was introduced into the opening of a Y-shaped silicone tube, or both fascicles were introduced into an X-shaped tube. The distal tibial and peroneal fascicles were inserted into the distal outlets of the tubes leaving a gap of 4 mm between proximal and distal stumps. In the X-shaped tubes the proximal inserts were placed opposite or adjacent to their respective distal parts. Sixteen weeks later reinnervation was evaluated by measurements of tetanic force of the gastrocnemius and anterior tibial muscles after electrical stimulation of the fascicles. There was preferential reinnervation in both types of tubes. In Y-shaped tubes about 90% of the tetanic force could be recorded from both muscles after stimulation of the peroneal and tibial fascicles, respectively. Recovery was lower in the X-shaped tubes, amounting to about 75%. Contractions evoked by misrouted fibres were similar (roughly 40%) in both models. We conclude that motor axons preferentially, but not exclusively, selected a path to reinnervate their original target muscle.