Influence of physical parameters of nerve chambers on peripheral nerve regeneration and reinnervation

Beyond the limiting gap length: peripheral nerve regeneration through implantable nerve guidance conduits

Peripheral nerve damage results in the loss of sensorimotor and autonomic functions, which is a significant burden to patients. Furthermore, nerve injuries greater than the limiting gap length require surgical repair. Although autografts are the preferred clinical choice, their usage is impeded by their limited availability, dimensional mismatch, and the sacrifice of another functional donor nerve. Accordingly, nerve guidance conduits, which are tubular scaffolds engineered to provide a biomimetic environment for nerve regeneration, have emerged as alternatives to autografts. Consequently, a few nerve guidance conduits have received clinical approval for the repair of short-mid nerve gaps but failed to regenerate limiting gap damage, which represents the bottleneck of this technology. Thus, it is still necessary to optimize the morphology and constituent materials of conduits. This review summarizes the recent advances in nerve conduit technology. Several manufacturing techniques and conduit designs are discussed, with emphasis on the structural improvement of simple hollow tubes, additive manufacturing techniques, and decellularized grafts. The main objective of this review is to provide a critical overview of nerve guidance conduit technology to support regeneration in long nerve defects, promote future developments, and speed up its clinical translation as a reliable alternative to autografts.

Repair of Long Peripheral Nerve Defects in Sheep: A Translational Model for Nerve Regeneration

Despite advances in microsurgery, full functional recovery of severe peripheral nerve injuries is not commonly attained. The sheep appears as a good preclinical model since it presents nerves with similar characteristics to humans. In this study, we induced 5 or 7 cm resection in the peroneal nerve and repaired with an autograft. Functional evaluation was performed monthly. Electromyographic and ultrasound tests were performed at 6.5 and 9 months postoperation (mpo). No significant differences were found between groups with respect to functional tests, although slow improvements were seen from 5 mpo. Electrophysiological tests showed compound muscle action potentials (CMAP) of small amplitude at 6.5 mpo that increased at 9 mpo, although they were significantly lower than the contralateral side. Ultrasound tests showed significantly reduced size of tibialis anterior (TA) muscle at 6.5 mpo and partially recovered size at 9 mpo. Histological evaluation of the grafts showed good axonal regeneration in all except one sheep from autograft 7 cm (AG7) group, while distal to the graft there was a higher number of axons than in control nerves. The results indicate that sheep nerve repair is a useful model for investigating long-gap peripheral nerve injuries.

Repair of Long Nerve Defects with a New Decellularized Nerve Graft in Rats and in Sheep

Decellularized nerve allografts (DC) are an alternative to autografts (AG) for repairing severe peripheral nerve injuries. We have assessed a new DC provided by VERIGRAFT. The decellularization procedure completely removed cellularity while preserving the extracellular matrix. We first assessed the DC in a 15 mm gap in the sciatic nerve of rats, showing slightly delayed but effective regeneration. Then, we assayed the DC in a 70 mm gap in the peroneal nerve of sheep compared with AG. Evaluation of nerve regeneration and functional recovery was performed by clinical, electrophysiology and ultrasound tests. No significant differences were found in functional recovery between groups of sheep. Histology showed a preserved fascicular structure in the AG while in the DC grafts regenerated axons were grouped in small units. In conclusion, the DC was permissive for axonal regeneration and allowed to repair a 70 mm long gap in the sheep nerve.

Neural tissue engineering: From bioactive scaffolds and in situ monitoring to regeneration

Peripheral nerve injury is a large-scale problem that annually affects more than several millions of people all over the world. It remains a great challenge to effectively repair nerve defects. Tissue engineered nerve guidance conduits (NGCs) provide a promising platform for peripheral nerve repair through the integration of bioactive scaffolds, biological effectors, and cellular components. Herein, we firstly describe the pathogenesis of peripheral nerve injuries at different orders of severity to clarify their microenvironments and discuss the clinical treatment methods and challenges. Then, we discuss the recent progress on the design and construction of NGCs in combination with biological effectors and cellular components for nerve repair. Afterward, we give perspectives on imaging the nerve and/or the conduit to allow for the in situ monitoring of the nerve regeneration process. We also cover the applications of different postoperative intervention treatments, such as electric field, magnetic field, light, and ultrasound, to the well-designed conduit and/or the nerve for improving the repair efficacy. Finally, we explore the prospects of multifunctional platforms to promote the repair of peripheral nerve injury.

Stimulating effect of thyroid hormones in peripheral nerve regeneration: research history and future direction toward clinical therapy

Injury to peripheral nerves is often observed in the clinic and severe injuries may cause loss of motor and sensory functions. Despite extensive investigation, testing various surgical repair techniques and neurotrophic molecules, at present, a satisfactory method to ensuring successful recovery does not exist. For successful molecular therapy in nerve regeneration, it is essential to improve the intrinsic ability of neurons to survive and to increase the speed of axonal outgrowth. Also to induce Schwann cell phenotypical changes to prepare the local environment favorable for axonal regeneration and myelination. Therefore, any molecule that regulates gene expression of both neurons and Schwann cells could play a crucial role in peripheral nerve regeneration. Clinical and experimental studies have reported that thyroid hormones are essential for the normal development and function of the nervous system, so they could be candidates for nervous system regeneration. This review provides an overview of studies devoted to testing the effect of thyroid hormones on peripheral nerve regeneration. Also it emphasizes the importance of combining biodegradable tubes with local administration of triiodothyronine for future clinical therapy of human severe injured nerves. We highlight that the local and single administration of triiodothyronine within biodegradable nerve guide improves significantly the regeneration of severed peripheral nerves, and accelerates functional recovering. This technique provides a serious step towards future clinical application of triiodothyronine in human severe injured nerves. The possible regulatory mechanism by which triiodothyronine stimulates peripheral nerve regeneration is a rapid action on both axotomized neurons and Schwann cells.

Stabilization, Rolling, and Addition of Other Extracellular Matrix Proteins to Collagen Hydrogels Improve Regeneration in Chitosan Guides for Long Peripheral Nerve Gaps in Rats

BACKGROUND: Autograft is still the gold standard technique for the repair of long peripheral nerve injuries. The addition of biologically active scaffolds into the lumen of conduits to mimic the endoneurium of peripheral nerves may increase the final outcome of artificial nerve devices. Furthermore, the control of the orientation of the collagen fibers may provide some longitudinal guidance architecture providing a higher level of mesoscale tissue structure.

OBJECTIVE: To evaluate the regenerative capabilities of chitosan conduits enriched with extracellular matrix-based scaffolds to bridge a critical gap of 15 mm in the rat sciatic nerve.

METHODS: The right sciatic nerve of female Wistar Hannover rats was repaired with chitosan tubes functionalized with extracellular matrix-based scaffolds fully hydrated or stabilized and rolled to bridge a 15 mm nerve gap. Recovery was evaluated by means of electrophysiology and algesimetry tests and histological analysis 4 months after injury.

RESULTS: Stabilized constructs enhanced the success of regeneration compared with fully hydrated scaffolds. Moreover, fibronectin-enriched scaffolds increased muscle reinnervation and number of myelinated fibers compared with laminin-enriched constructs.

CONCLUSION: A mixed combination of collagen and fibronectin may be a promising internal filler for neural conduits for the repair of peripheral nerve injuries, and their stabilization may increase the quality of regeneration over long gaps.

PEOT/PBT Guides Enhance Nerve Regeneration in Long Gap Defects

Development of new nerve guides is required for replacing autologous nerve grafts for the repair of long gap defects after nerve injury. A nerve guide comprised only of electrospun fibers able to bridge a critical (15 mm) nerve gap in a rat animal model is reported for the first time. The nerve conduits are made of poly(ethylene oxide terephthalate) and poly(butylene terephthalate) (PEOT/PBT), a biocompatible copolymer composed of alternating amorphous, hydrophilic poly(ethylene oxide terephthalate), and crystalline, hydrophobic poly(butylene terephthalate) segments. These guides show suitable mechanical properties, high porosity, and fibers aligned in the longitudinal axis of the guide. In vitro studies show that both neurites and Schwann cells exhibit growth alignment with PA fibers. In vivo studies reveal that, after rat sciatic nerve transection and repair with PEOT/PBT guides, axons grow occupying a larger area compared to silicone tubes. Moreover, after repair of limiting (10 mm) and critical (15 mm) nerve gaps, PEOT/PBT guides significantly increase the percentage of regenerated nerves, the number of regenerated myelinated axons, and improve motor, sensory, and autonomic reinnervation in both gaps. This nerve conduit design combines the properties of PEOT/PBT with electrospun structure, demonstrating that nerve regeneration through long gaps can be achieved through the design of instructive biomaterial constructs.

Controlled Release of Nerve Growth Factor and Basic Fibroblast Growth Factor Combined with Small-Gap Anastomosis Enhances Sciatic Nerve Regeneration

Objectives

Nerve regeneration after peripheral nerve injury is a slow process with a limited degree of functional recovery, resulting in a high disability rate. Thus, accelerating the rate of nerve regeneration and improving the degree of nerve repair is a clinical challenge. This study aimed to investigate the role of growth factor gel combined with small-gap nerve anastomosis in the regeneration of sciatic nerve injury in rats. This was achieved by injecting nerve growth factor (NGF) and basic fibroblast growth factor (bFGF) gel into a silicon chamber that bridged the transection of the nerve.

Methods

In 27 randomly chosen Sprague Dawley rats, a sharp blade was used to transect the right hind leg sciatic nerve. The rats were divided into 3 groups: in groups A and B, silicon tubes containing NGF and bFGF gel or saline, respectively, were used to bridge the nerve proximal and distal ends (3-mm gap), and in group C, the nerve proximal and distal ends were directly sutured. Eight weeks after surgery, the sciatic nerve function index, neural electrophysiology, and muscle wet weight as well as histological, ultrastructural, and immunohistochemical parameters were evaluated.

Results

The sciatic nerve function index, nerve conduction velocity, muscle wet weight, density of regenerated nerve fibers, and myelination in group A were better than those in group B or C, but the sciatic nerve function index, muscle wet weight, and thickness of myelination in the 3 groups were not significantly different (P > 0.05). There were no significant differences innerve conduction velocity between groups A and B (P > 0.05), but it was higher in both groups than that of group C (P < 0.05). The regenerated nerve fiber density in the 3 groups showed significant differences (P < 0.05).

Conclusions

Small-gap nerve anastomosis can provide a good regenerative microenvironment for rat sciatic nerve regeneration, and the combined strategy of growth factor gel with small-gap nerve anastomosis appears to have a superior effect on nerve repair.

Inducible depletion of adult skeletal muscle stem cells impairs the regeneration of neuromuscular junctions

Skeletal muscle maintenance depends on motor innervation at neuromuscular junctions (NMJs). Multiple mechanisms contribute to NMJ repair and maintenance; however muscle stem cells (satellite cells, SCs), are deemed to have little impact on these processes. Therefore, the applicability of SC studies to attenuate muscle loss due to NMJ deterioration as observed in neuromuscular diseases and aging is ambiguous. We employed mice with an inducible Cre, and conditionally expressed DTA to deplete or GFP to track SCs. We found SC depletion exacerbated muscle atrophy and type transitions connected to neuromuscular disruption. Also, elevated fibrosis and further declines in force generation were specific to SC depletion and neuromuscular disruption. Fate analysis revealed SC activity near regenerating NMJs. Moreover, SC depletion aggravated deficits in reinnervation and post-synaptic morphology at regenerating NMJs. Therefore, our results propose a mechanism whereby further NMJ and skeletal muscle decline ensues upon SC depletion and neuromuscular disruption.

DOI:http://dx.doi.org/10.7554/eLife.09221.001

New muscle fibers are made throughout our lives to replace those that have been damaged by normal wear and tear, and to meet new physical demands. These new muscle fibers develop from a pool of muscle stem cells. To create and maintain fully working muscles, nerve cells called motor neurons must also properly attach to the muscle fibers. These nerve cells transmit messages from the brain that tell the muscles what to do. If the muscle-nerve connections do not form correctly, or are severed, muscles can waste away. This may occur as part of a neuromuscular disease, and also happens to some extent as a normal part of aging.

It was thought that muscle stem cells do not affect how the muscle-nerve connections form. By studying genetically engineered mice, Liu et al. now show that this is not the case. These mice had modifications to their muscle stem cells that allowed the number of these cells to be artificially reduced, and some cells also produced a fluorescent protein that allowed them to be tracked.

Surgically severing some of the muscle-nerve connections in the mice triggered the rebuilding of the connections, but also weakened the muscles and caused some disease-related changes in the muscle tissue. During the healing process, the muscle stem cells are active near the regenerating connections. Reducing the number of muscle stem cells in the mice while these broken connections were healing further weakened the muscles. Closer inspection of the muscle-nerve connections also revealed poorer quality connections were formed in the stem-cell deficient mice.

Further study of how stem cells help to form strong nerve-muscle connections may allow scientists to develop new treatments for age- or disease-related muscle loss.

DOI:http://dx.doi.org/10.7554/eLife.09221.002

Neuroregenerative effects of olfactory ensheathing cells transplanted in a multi-layered conductive nanofibrous conduit in peripheral nerve repair in rats

Background

The purpose of this study was to evaluate the efficacy of a multi-layered conductive nanofibrous hollow conduit in combination with olfactory ensheathing cells (OEC) to promote peripheral nerve regeneration. We aimed to harness both the topographical and electrical cues of the aligned conductive nanofibrous single-walled carbon nanotube/ poly (_L-lactic acid) (SWCNT/PLLA) scaffolds along with the neurotrophic features of OEC in a nerve tissue engineered approach.

Results

We demonstrated that SWCNT/PLLA composite scaffolds support the adhesion, growth, survival and proliferation of OEC. Using microsurgical techniques, the tissue engineered nerve conduits were interposed into an 8 mm gap in sciatic nerve defects in rats. Functional recovery was evaluated using sciatic functional index (SFI) fortnightly after the surgery. Histological analyses including immunohistochemistry for S100 and NF markers along with toluidine blue staining (nerve thickness) and TEM imaging (myelin sheath thickness) of the sections from middle and distal parts of nerve grafts showed an increased regeneration in cell/scaffold group compared with cell-free scaffold and silicone groups. Neural regeneration in cell/scaffold group was very closely similar to autograft group, as deduced from SFI scores and histological assessments.

Conclusions

Our results indicated that the tissue engineered construct made of rolled sheet of SWCNT/PLLA nanofibrous scaffolds and OEC could promote axonal outgrowth and peripheral nerve regeneration suggesting them as a promising alternative in nerve tissue engineering.

Effect of transgenic human insulin-like growth factor-1 on spinal motor neurons following peripheral nerve injury

The aim of the present study was to observe the protective effect of exogenous human insulin-like growth factor-1 (hIGF-1) on spinal motor neurons, following its local transfection into an area of peripheral nerve injury. A total of 90 male Wistar rats that had been established as sciatic nerve crush injury models were randomly divided into three groups: hIGF-1 treatment, sham-transfected control and blank control groups. The different phases of hIGF-1 expression were observed in the spinal cord via postoperative immunostaining and the apoptosis of motor neurons was observed using the terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling method. Pathological changes of the motor neurons and Nissl bodies within cell bodies were observed via Marsland and Luxol fast blue double staining, while changes in the neuropil of the spinal cord anterior horn were investigated via ultrastructural observation. It was found that hIGF-1, locally transfected into an area of peripheral nerve injury, was expressed in the spinal anterior horn following axoplasmic transport; the peak hIGF-1 expression occurred approximately a week following transfection. The number of apoptotic spinal cord motor neurons observed in the hIGF-1 treatment group was fewer than that in the sham-transfected and blank control groups at days 7, 14 and 21 following transfection (P<0.01). Furthermore, the quantity of motor neuron cells in the anterior horn of the spinal cord in the hIGF-1 treatment group was higher compared with those in the sham-transfected and blank control groups at days 2, 7, 14 and 28 following transfection (P<0.01). The degenerative changes of Nissl bodies within the cytoplasm of the hIGF-1 treatment group were less severe compared with those of the sham-transfected and blank control groups. At day 56 following transfection, the spinal anterior horn neuropil ultrastructure in the hIGF-1 treatment group was generally normal, while the sham-transfected and blank control groups exhibited an increased number of protruding gaps and local cavities. These results indicate that the application of exogenous hIGF-1 is capable of protecting spinal cord motor neurons following peripheral nerve injury.

Comparison of results between chitosan hollow tube and autologous nerve graft in reconstruction of peripheral nerve defect: An experimental study

OBJECT

This study evaluated a chitosan tube for regeneration of the injured peripheral nerve in a rodent transected sciatic nerve model in comparison to autologous nerve graft repair.

METHODS

Chitosan hollow tube was used to bridge a 10-mm gap between the proximal and distal ends in 11 rats. In the control group, an end-to-end coaptation of 10-mm long autologous nerve graft was performed in 10 rats for nerve reconstruction.

RESULTS

SFI showed an insignificant advantage to the autologous group both at 30 days (P = 0.177) and at 90 days post procedure (P = 0.486). Somato-sensory evoked potentials (SSEP) and compound muscle action potentials (CMAP) tests showed similar results between chitosan tube (group 1) and autologous (group 2) groups with no statistically significant differences. Both groups presented the same pattern of recovery with 45% in group 1 and 44% in group 2 (P = 0.96) showing SSEP activity at 30 days. At 90 days most rats showed SSEP activity (91% vs.80% respectively, P = 0.46). The CMAP also demonstrated no statistically significant differences in latency (1.39 ms in group 1 vs. 1.63 ms in group 2; P = 0.48) and amplitude (6.28 mv vs. 6.43 mv respectively; P = 0.8). Ultrasonography demonstrated tissue growth inside the chitosan tube. Gastrocnemius muscle weight showed no statistically significant difference. Histomorphometry of the distal sciatic nerve, 90 days post reconstructive procedure, showed similar number of myelinated fibers and size parameters in both groups (P ≥ 0.05).

CONCLUSIONS

Chitosan hollow tube used for peripheral nerve reconstruction of rat sciatic nerve showed similar results in comparison to autologous nerve grafting. © 2015 Wiley Periodicals, Inc. Microsurgery 36:664-671, 2016.

Tubulization with chitosan guides for the repair of long gap peripheral nerve injury in the rat

Biosynthetic guides can be an alternative to nerve grafts for reconstructing severely injured peripheral nerves. The aim of this study was to evaluate the regenerative capability of chitosan tubes to bridge critical nerve gaps (15 mm long) in the rat sciatic nerve compared with silicone (SIL) tubes and nerve autografts (AGs). A total of 28 Wistar Hannover rats were randomly distributed into four groups (n = 7 each), in which the nerve was repaired by SIL tube, chitosan guides of low (∼2%, DAI) and medium (∼5%, DAII) degree of acetylation, and AG. Electrophysiological and algesimetry tests were performed serially along 4 months follow-up, and histomorphometric analysis was performed at the end of the study. Both groups with chitosan tubes showed similar degree of functional recovery, and similar number of myelinated nerve fibers at mid tube after 4 months of implantation. The results with chitosan tubes were significantly better compared to SIL tubes (P < 0.01), but lower than with AG (P < 0.01). In contrast to AG, in which all the rats had effective regeneration and target reinnervation, chitosan tubes from DAI and DAII achieved 43 and 57% success, respectively, whereas regeneration failed in all the animals repaired with SIL tubes. This study suggests that chitosan guides are promising conduits to construct artificial nerve grafts.

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.

Standardized criterion to analyze and directly compare various materials and models for peripheral nerve regeneration

Progress in understanding conditions for optimal peripheral nerve regeneration has been stunted due to lack of standardization of experimental conditions and assays. In this paper we review the large database that has been generated using the Lundborg nerve chamber model and compare various theories for their ability to explain the experimental data. Data were normalized based on systematic use of the critical axon elongation, the gap length at which the probability of axon reconnection between the stumps is just 50%. Use of this criterion has led to a rank-ordering of devices or treatments and has led, in turn, to conclusions about the conditions that facilitate regeneration. Experimental configurations that have maximized facilitation of peripheral nerve regeneration are those in which the tube wall comprised degradable polymers, including collagen and certain synthetic biodegradable polymers, and was cell-permeable rather than protein-permeable. Tube fillings that showed very high regenerative activity were suspensions of Schwann cells, a solution either of acidic or basic fibroblast growth factor, insoluble ECM substrates rather than solutions or gels, polyamide filaments oriented along the tube axis and highly porous, insoluble analogs of the ECM with specific structure and controlled degradation rate. It is suggested that the data are best explained by postulating that the quality of regeneration depends on two critical processes. The first is compression of stumps and regenerating nerve by a thick myofibroblast layer that surrounds these tissues and blocks synthesis of a nerve of large diameter (pressure cuff theory). The second is synthesis of linear columns of Schwann cells that serve as tracks for axon elongation (basement membrane microtube theory). It is concluded that experimental configurations that show high regenerative activity suppress the first process while facilitating the second.

Thyroid hormone enhances transected axonal regeneration and muscle reinnervation following rat sciatic nerve injury

Improvement of nerve regeneration and functional recovery following nerve injury is a challenging problem in clinical research. We have already shown that following rat sciatic nerve transection, the local administration of triiodothyronine (T3) significantly increased the number and the myelination of regenerated axons. Functional recovery is a sum of the number of regenerated axons and reinnervation of denervated peripheral targets. In the present study, we investigated whether the increased number of regenerated axons by T3-treatment is linked to improved reinnervation of hind limb muscles. After transection of rat sciatic nerves, silicone or biodegradable nerve guides were implanted and filled with either T3 or phosphate buffer solution (PBS). Neuromuscular junctions (NMJs) were analyzed on gastrocnemius and plantar muscle sections stained with rhodamine alpha-bungarotoxin and neurofilament antibody. Four weeks after surgery, most end-plates (EPs) of operated limbs were still denervated and no effect of T3 on muscle reinnervation was detected at this stage of nerve repair. In contrast, after 14 weeks of nerve regeneration, T3 clearly enhanced the reinnervation of gastrocnemius and plantar EPs, demonstrated by significantly higher recovery of size and shape complexity of reinnervated EPs and also by increased acetylcholine receptor (AChRs) density on post synaptic membranes compared to PBS-treated EPs. The stimulating effect of T3 on EP reinnervation is confirmed by a higher index of compound muscle action potentials recorded in gastrocnemius muscles. In conclusion, our results provide for the first time strong evidence that T3 enhances the restoration of NMJ structure and improves synaptic transmission.

Local delivery of glial cell line-derived neurotrophic factor improves facial nerve regeneration after late repair

OBJECTIVES/HYPOTHESIS

Facial nerve regeneration is limited in some clinical situations: in long grafts, by aged patients, and when the delay between nerve lesion and repair is prolonged. This deficient regeneration is due to the limited number of regenerating nerve fibers, their immaturity and the unresponsiveness of Schwann cells after a long period of denervation. This study proposes to apply glial cell line-derived neurotrophic factor (GDNF) on facial nerve grafts via nerve guidance channels to improve the regeneration.

METHODS

Two situations were evaluated: immediate and delayed grafts (repair 7 months after the lesion). Each group contained three subgroups: a) graft without channel, b) graft with a channel without neurotrophic factor; and c) graft with a GDNF-releasing channel. A functional analysis was performed with clinical observation of facial nerve function, and nerve conduction study at 6 weeks. Histological analysis was performed with the count of number of myelinated fibers within the graft, and distally to the graft. Central evaluation was assessed with Fluoro-Ruby retrograde labeling and Nissl staining.

RESULTS

This study showed that GDNF allowed an increase in the number and the maturation of nerve fibers, as well as the number of retrogradely labeled neurons in delayed anastomoses. On the contrary, after immediate repair, the regenerated nerves in the presence of GDNF showed inferior results compared to the other groups.

CONCLUSIONS

GDNF is a potent neurotrophic factor to improve facial nerve regeneration in grafts performed several months after the nerve lesion. However, GDNF should not be used for immediate repair, as it possibly inhibits the nerve regeneration.

Collagen biomaterial doped with colominic acid for cell culture applications with regard to peripheral nerve repair

Colominic acid (CA) is a homopolymer of sialic acid residues and is solely composed of polymerised units of alpha-2,8-linked N-acetylneuraminic acid. CA is a specific derivative of polysialic acid (PSA), produced as the capsular polysaccharide of Escherichia coli K1 derived molecule of PSA. PSA in vivo plays a significant role in synaptic plasticity and neural development. The use of collagen materials doped with defined CA is presented for the cultivation of various cell lines relevant for possible applications in Tissue Engineering. First, the release behaviour under culture conditions of the collagen-based (C-CA) materials was investigated by thiobarbituric acid assay. Additionally, the established cell lines, PC-12 and immortalised Schwann cells (ISC), used for neurobiological and neurochemical studies and the model liver cell line Hep-G2 as indicator for biocompatibility testing, were cultured on the C-CA matrix. Cell proliferation (MTT-test) and cell adhesion (DAPI-staining) of the cell lines on the matrices were observed. Likewise, gene expression of the marker genes thyrosine hydroxylase for the PC-12 cells, and albumin, transferrin and CYP3A4 for the Hep-G2 cells was evaluated via RT-PCR. The results indicate that CA integration in established biomaterial constructs enhances cell proliferation and offers promising features as conduits additive in regarding peripheral nerve regeneration.

Fast and efficient screening system for new biomaterials in tissue engineering: a model for peripheral nerve regeneration

The use of three-dimensional biodegradable matrices is one major issue in tissue engineering. Numerous materials, fabrication techniques, and modifications have been used and tested in different areas of tissue engineering recently. But nevertheless, technology is far from being optimized and optimal constructs with bioidentical and mechanical properties have not been described in the literature so far. Hence, there is great demand of new suitable biomaterials for tissue engineering applications. In this study, a fast and efficient screening system for initial testing of biomaterials for cell culture application was developed. The set up for the screening system and the decision criteria applied for the determination of suitability of new materials are presented. Hep-G2 and PC-12 cells were seeded onto different matrices and cultured over a period of 2 weeks. The viability of the cells was monitored via the MTT assay. Cell spreading was investigated by DAPI-staining of cell nuclei. Furthermore, the adhesion of the cells on the different matrices was examined by counting the number of attached cells. With these general assays a classification of materials is possible with regard to their suitability. Optimal cell models must be chosen for the defined applications and at least two cell lines are necessary for a differentiating interpretation.

Neural plasticity after peripheral nerve injury and regeneration

Injuries to the peripheral nerves result in partial or total loss of motor, sensory and autonomic functions conveyed by the lesioned nerves to the denervated segments of the body, due to the interruption of axons continuity, degeneration of nerve fibers distal to the lesion and eventual death of axotomized neurons. Injuries to the peripheral nervous system may thus result in considerable disability. After axotomy, neuronal phenotype switches from a transmitter to a regenerative state, inducing the down- and up-regulation of numerous cellular components as well as the synthesis de novo of some molecules normally not expressed in adult neurons. These changes in gene expression activate and regulate the pathways responsible for neuronal survival and axonal regeneration. Functional deficits caused by nerve injuries can be compensated by three neural mechanisms: the reinnervation of denervated targets by regeneration of injured axons, the reinnervation by collateral branching of undamaged axons, and the remodeling of nervous system circuitry related to the lost functions. Plasticity of central connections may compensate functionally for the lack of specificity in target reinnervation; plasticity in human has, however, limited effects on disturbed sensory localization or fine motor control after injuries, and may even result in maladaptive changes, such as neuropathic pain, hyperreflexia and dystonia. Recent research has uncovered that peripheral nerve injuries induce a concurrent cascade of events, at the systemic, cellular and molecular levels, initiated by the nerve injury and progressing throughout plastic changes at the spinal cord, brainstem relay nuclei, thalamus and brain cortex. Mechanisms for these changes are ubiquitous in central substrates and include neurochemical changes, functional alterations of excitatory and inhibitory connections, atrophy and degeneration of normal substrates, sprouting of new connections, and reorganization of somatosensory and motor maps. An important direction for ongoing research is the development of therapeutic strategies that enhance axonal regeneration, promote selective target reinnervation, but are also able to modulate central nervous system reorganization, amplifying those positive adaptive changes that help to improve functional recovery but also diminishing undesirable consequences.

Refinement of Symptoms of Neuropathic Pain Measurements After Various Transections of the Nerve Endings of the Sciatic and Femoral Nerve in Rats: An Exploratory Behavioral Analysis

BACKGROUND

Many animal models can be used to study the underlying pathophysiological mechanisms of neuropathic pain. Most of these models rely on a partial denervation of the limb of the animal by ligating a selected nerve. In this study, we performed nerve lesions on three peripheral nerves supplying the plantar side of the rat hindpaw by differentially transecting the saphenous, the tibial, and the sural nerves alone or in paired combinations.

METHODS

The development of neuropathic pain symptoms at three different anatomical areas (medial, central, and lateral) of the glabrous skin of the hindpaw was evaluated by sensory testing over a 12-wk period. Mechanical hyperalgesia (pinprick), cold allodynia (acetone), and abnormalities of hindpaw posture were continuously present in animals with tibial and tibial and saphenous nerve transection.

RESULTS

Transection of the tibial and sural nerves induced cold allodynia and moderate mechanical hyperalgesia. Transection of the sural, the saphenous, or both nerves simultaneously induced no signs of specific neuropathic pain behavior and no abnormalities in posture of the affected hindpaw were noted after adequate stimulation.

CONCLUSIONS

The overlapping innervation of nerve distribution can complicate the interpretation of nerve ligation studies of peripheral neuropathies.

Thyroid Hormone in Biodegradable Nerve Guides Stimulates Sciatic Nerve Regeneration: A Potential Therapeutic Approach for Human Peripheral Nerve Injuries

It has been already demonstrated that thyroid hormone (T3) is one of the most important stimulating factors in peripheral nerve regeneration. We have recently shown that local administration of T3 in silicon tubes at the level of the transected rat sciatic nerve enhanced axonal regeneration and improved functional recovery. Silicon, however, cannot be used in humans because it causes a chronic inflammatory reaction. Therefore, in order to provide future clinical applications of thyroid hormone in human peripheral nerve lesions, we carried out comparative studies on the regeneration of transected rat sciatic nerve bridged either by biodegradable P(DLLA-(-CL) or by silicon nerve guides, both guides filled with either T3 or phosphate buffer. Our macroscopic observation revealed that 85% of the biodegradable guides allowed the expected regeneration of the transected sciatic nerve. The morphological, morphometric and electrophysiological analysis showed that T3 in biodegradable guides induces a significant increase in the number of myelinated regenerated axons (6862 +/- 1831 in control vs. 11799 +/- 1163 in T3-treated). Also, T3 skewed the diameter of myelinated axons toward larger values than in controls. Moreover, T3 increases the compound muscle action potential amplitude of the flexor and extensor muscles of the treated rats. This T3 stimulation in biodegradable guides was equally well to that obtained by using silicone guides. In conclusion, the administration of T3 in biodegradable guides significantly improves sciatic nerve regeneration, confirming the feasibility of our technique to provide a serious step towards future clinical application of T3 in human peripheral nerve injuries.

Cross-chest median nerve transfer: A new model for the evaluation of nerve regeneration across a 40mm gap in the rat

A new animal model for the study of nerve regeneration in rats across a 40 mm gap between both median nerves is described. For autologous grafting, the ulnar nerves were dissected and sutured together. From the left median nerve, they were transplanted across the chest to the right median nerve. Animals having undergone this operation were observed for 12 months and periodically assessed using the grasping test and measurements of body-weight. For histological analysis rats were sacrificed after this period and axon counts were determined at the suture points of operated animals and in the median nerve of non-operated animals. Functional recovery could be seen, although partially, beginning as early as the fifth postoperative month, as demonstrated by the grasping test. Quantification of the number of axons demonstrated axonal regeneration across all three coaptation points. This model provides a new approach for analysis of long distance peripheral nerve regeneration without impairment of behaviour.

Comparação entre duas técnicas de neurorrafia do digital palmar em eqüinos

Foram comparadas duas técnicas de neurorrafia em seis eqüinos, divididos em três grupos (G), conforme o tempo para a biópsia. Os animais foram submetidos a neurectomia dos nervos digitais palmares (NDP) e aplicaram-se duas suturas epineurais (SE) e suturas com tubos de silicone (STS) em cada animal. Avaliaram-se os animais mensalmente pelo teste de sensibilidade e exame do aparelho locomotor até a realização das biópsias dos NDP. Aos 30 dias pós-cirurgia foi realizada biópsia no GI, aos 60 dias no GII e aos 180 dias no GIII. Macroscopicamente, o NDP encontrou-se envolvido por tecido conjuntivo fibroso. Microscopicamente, constataram-se proliferação axonal em uma amostra do GI e neuromas nas amostras dos GI, GII e GIII. Houve proliferação de tecido conjuntivo em todos os grupos no local de reparação para SE e adentrando no interior do tubo na STS. Visibilizaram-se infiltrado de células inflamatórias, alterações no coto proximal e degeneração no coto distal na SE e na STS. As técnicas não apresentaram resultados satisfatórios quanto ao grau de regeneração do coto proximal até o coto distal.

Permeable guidance channels containing microfilament scaffolds enhance axon growth and maturation

Successful peripheral nerve regeneration is still limited in artificial conduits, especially for long lesion gaps. In this study, porous poly(L-lactide-co-DL-lactide, 75:25) (PLA) conduits were manufactured with 16 poly(L-lactide) (PLLA) microfilaments aligned inside the lumen. Fourteen and 18 mm lesion gaps were created in a rat sciatic nerve lesion model. To evaluate the combined effect of permeable PLA conduits and microfilament bundles on axon growth, four types of implants were tested for each lesion gap: PLA conduits with 16 filaments; PLA conduits without filaments; silicone conduits with 16 filaments; and silicone conduits without filaments. Ten weeks following implantation, regeneration within the distal nerve was compared between corresponding groups. Antibodies against the markers S100, calcitonin gene related peptide (CGRP), RMDO95, and P0 were used to identify Schwann cells, unmyelinated axons, myelinated axons, and myelin, respectively. Results demonstrated that the filament scaffold enhanced tissue cable formation and Schwann cell migration in all groups. The filament scaffold enhanced axonal regeneration toward the distal stump, especially across long lesion gaps, but significance was only achieved with PLA conduits. When compared to corresponding silicone conduits, permeable PLA conduits enhanced myelinated axon regeneration across both lesion gaps and achieved significance only in combination with filament scaffolds. Myelin staining indicated PLA conduits supported axon myelination with better myelin quantity and quality when compared to silicone conduits.

La réparation nerveuse périphérique : 30 siècles de recherche

INTRODUCTION

Nerve injury compromises sensory and motor functions. Techniques of peripheral nerve repair are based on our knowledge regarding regeneration. Microsurgical techniques introduced in the late 1950s and widely developed for the past 20 years have improved repairs. However, functional recovery following a peripheral mixed nerve injury is still incomplete.

STATE OF ART

Good motor and sensory function after nerve injury depends on the reinnervation of the motor end plates and sensory receptors. Nerve regeneration does not begin if the cell body has not survived the initial injury or if it is unable to initiate regeneration. The regenerated axons must reach and reinnervate the appropriate target end-organs in a timely fashion. Recovery of motor function requires a critical number of motor axons reinnervating the muscle fibers. Sensory recovery is possible if the delay in reinnervation is short. Many additional factors influence the success of nerve repair or reconstruction. The timing of the repair, the level of injury, the extent of the zone of injury, the technical skill of the surgeon, and the method of repair and reconstruction contribute to the functional outcome after nerve injury.

CONCLUSION

This review presents the recent advances in understanding of neural regeneration and their application to the management of primary repairs and nerve gaps.

Selection of biomaterials for peripheral nerve regeneration using data from the nerve chamber model

Peripheral nerve regeneration has been studied in a variety of animal models. Of these, the nerve chamber model has clearly dominated. It has been used to generate a large base of data that, however, cannot be analyzed usefully due to lack of standardization of experimental conditions and assays. Lack of standardization of critical experimental parameters of the model has, however, greatly limited the opportunity to compare directly data from independent investigators; as a result, progress in understanding conditions for optimal nerve regeneration has been stunted. In this article, we provide an overview of the major experimental parameters that must be controlled in order to generate data from independent investigators that can be compared directly (normalized data). Such parameters include the gap length, animal species, and the identity of assays used to evaluate the product of the regenerative process. Use of the recently introduced concept of critical axon elongation, the gap length at which the probability of axonal outgrowth (reinnervation) across the gap is 50%, leads to generation of a normalized database that includes data from several independent investigators. Conclusions are drawn about the relative efficacy of the various biomaterials and devices employed. Nerve chamber configurations that had the highest regenerative activity were those in which the tube wall comprised collagen and certain synthetic biodegradable polymers rather than silicone, and was cell-permeable rather than protein-permeable. In addition, the following tube fillings showed very high regenerative activity: suspensions of Schwann cells; a solution either of acidic or basic fibroblast growth factor; insoluble ECM substrates rather than solutions or gels; polyamide filaments oriented along the tube axis; highly porous, insoluble analogs of the ECM with specific structure and controlled degradation rate.

Neuroregenerative and neuroprotective actions of neuroimmunophilin compounds in traumatic and inflammatory neuropathies

FK506 (tacrolimus, Prograf is an immunosuppressant drug that also has profound neuroregenerative and neuroprotective actions independent of its immunosuppressant activity. The separation of these properties has led to the development of non-immunosuppressant derivatives that retain the neurotrophic activity. This review focuses on the peripheral nerve actions of these compounds following mechanical injury (nerve crush or transection with graft repair) and in models of inflammatory neuropathies. Whereas FK506 may be indicative for the treatment of inflammatory neuropathies where its immunosuppressive action would be advantageous, non-immunosuppressant derivatives represent a new class of potential therapeutic agents for the treatment of human neurological conditions in general. Moreover, these studies have led to the discovery of a novel mechanism whereby these compounds activate intrinsic neuroregenerative and neuroprotective pathways in the neuron.

Single-cell laser-capture microdissection and RNA amplification

Generating gene-expression profiles from laser-captured cells requires the successful combination of laser-capture microdissection, RNA extraction, RNA amplification, and microarray analysis. To permit single-cell gene-expression profiling, the RNA amplification method has to be sufficiently powerful to bridge the gap between the amount of RNA available from a single cell to what is required by the microarray, a gap that spans 5 to 6 orders of magnitude. This chapter focuses on the amplification of RNA using a two-round T7 RNA amplification method. The protocols described are adapted for laser-captured material and have been used to generate gene expression profiles from single laser-captured cells.

FK506 enhances regeneration of axons across long peripheral nerve gaps repaired with collagen guides seeded with allogeneic Schwann cells

We assessed the effects of FK506 administration on regeneration after a 6-mm gap repair with a collagen guide seeded with allogeneic Schwann cells (SCs) in the mouse sciatic nerve. SCs were isolated from predegenerated adult sciatic nerves and expanded in culture using a defined medium, before being seeded in the collagen guide embedded in Matrigel. Functional reinnervation was evaluated by noninvasive methods to determine recovery of motor, sensory, and autonomic functions in the hindpaw over 4 months postoperation. Histological analysis of the regenerated nerves was performed at the end of the study. Using simple collagen guides for tubulization repair, treatment with an immunosuppressant dose of FK506 (5 mg/kg/day) resulted in significant improvement of the onset and the degree of reinnervation. While the introduction of allogeneic SCs did not improve regeneration versus a collagen guide filled only with Matrigel, treatment with FK506 allowed for successful regeneration in all the mice and for significant improvement in the levels of functional recovery. Compared with the untreated group, there was greater survival of transplanted pre-labeled SCs in the FK506-treated animals. Morphologically, the best nerve regeneration (in terms of nerve caliber and numbers of myelinated axons) was obtained with SC-seeded guides from FK506-treated animals. Thus, FK506 should be considered as adjunct therapy for various types of tubulization repair.

Axonal regeneration across long gaps in silicone chambers filled with Schwann cells overexpressing high molecular weight FGF-2

Basic fibroblast growth factor (FGF-2) has been shown to enhance the survival and neurite extension of various types of neurons including spinal ganglion neurons. In addition, endogenous FGF-2 and FGF receptors are upregulated following peripheral nerve lesion in ganglia and at the lesion site. FGF-2 protein is expressed in different isoforms (18 kDa, 21 kDa, 23 kDa) and differentially regulated after nerve injury. In the rat we analyzed the regenerative capacity of the high molecular weight (HMW) FGF-2 isoforms (21/23 kDa) to support the regeneration of the axotomized adult sciatic nerve across long gaps. The nerve stumps were inserted into the opposite ends of a silicone chamber resulting in an interstump gap of 15 mm. Silicone tubes were filled with Matrigel or a mixture of Schwann cells (SC) and Matrigel. SC were prepared from newborn rats and transfected to overexpress HMW FGF-2. Four weeks after the operation procedure, channels were analyzed with regard to tissue cables bridging both nerve stumps and myelinated axons distal to the original proximal nerve stump. Peripheral nerves interposed with HMW Schwann cells displayed significantly enhanced nerve regeneration, with the greatest number of tissue cables containing myelinated axons and the highest number of myelinated axons. These results suggest that a cellular substrate together with a source of a trophic factor could be a promising tool to promote nerve regeneration and, therefore, become useful also for a clinical approach to repair long gaps.

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.

Effects of neuroactive steroids on myelin of peripheral nervous system

Peripheral nervous system (PNS) possess both classical (e.g. progesterone receptor, PR, androgen receptor, AR) and non-classical (e.g. GABA(A) receptor) steroid receptors and consequently may represent a target for the action of neuroactive steroids. Our data have indicated that neuroactive steroids, like for instance, progesterone, dihydroprogesterone, tetrahydroprogesterone, dihydrotestosterone and 3alpha-diol, stimulate both in vivo and in vitro (Schwann cell cultures), the expression of two important proteins of the myelin of peripheral nerves, the glycoprotein Po (Po) and the peripheral myelin protein 22 (PMP22). It is important to highlight that the mechanisms by which neuroactive steroids exert their effects on the expression of Po and PMP22 involve different kind of receptors depending on the steroid and on the myelin protein considered. In particular, at least in culture of Schwann cells, the expression of Po seems to be under the control of PR, while that of PMP22 needs the GABA(A) receptor. Because Po and PMP22 play an important physiological role for the maintenance of the multilamellar structure of the myelin of the PNS, the present observations might suggest the utilization of neuroactive steroids as new therapeutically approaches for the rebuilding of the peripheral myelin.

Effect of unilateral partial facial paralysis on periosteal growth at the muscle-bone interface of facial muscles and facial bones

In a previous study, the influence of the midfacial musculature upon growth and development of the maxilla and mandible was established macroscopically. Dry skull measurements revealed a reduced premaxillary, maxillary, mandibular, and anterior corpus length with a simultaneous increase in mandibular ramal height on the paralyzed side. It was demonstrated that these reduced premaxillary and maxillary lengths were among others the result of reduced nasofrontal growth, whereas the increased ramal height was accompanied by condylar growth alterations. This study investigated whether the growth alterations at the mandibular corpus region could be explained by altered periosteal growth at the muscle-bone interface of the zygomatico-auricular muscle and the mandibular corpus, caused by altered muscle activity acting upon the periosteal sleeve. Fifty-six 12-day-old New Zealand White rabbits were randomly assigned to either a control or an experimental group. In the experimental group, left-sided partial facial paralysis was induced surgically when the animals were 12 days old. To study the muscle-bone interface, seven follow-up time intervals were defined between 3.5 and 60 days following the surgery. At these time intervals, four randomly selected control animals and four randomly selected experimental animals were killed. The anterior mandibular corpus region with the muscle-bone interface of the left control hemimandible and the left and right experimental hemimandibles was processed for undecalcified tissue preparation. Quantitative analysis of the total bone area at the muscle-bone interface revealed no significant differences between the left control hemimandible and the left and right experimental hemimandibles. Also, qualitative study of the histologic sections showed no major changes in the appearance or development of the trabecular pattern between the groups. However, slight differences in the distribution pattern of osteoblasts and osteoclasts along the bony surface were found between the left control hemimandible and the left and right experimental hemimandibles, which seemed to explain the alterations in mandibular corpus shape between these groups. It was suggested that these changes in the distribution pattern of osteoblasts and osteoclasts were the result of changes in the loading distribution pattern acting upon the mandible, caused by an altered neuromuscular recruitment pattern of the remaining functionally intact, mandibularly attached muscles. The latter was probably the result of adaptive mandibular positioning in response to an altered occlusal relationship, which was induced by the abnormal maxillary growth as a result of the unilateral partial facial paralysis.

Engineering an artificial nerve graft for the repair of severe nerve injuries

Nerve repair with tubes has a limit to regeneration depending upon the length of the gap. The characteristics of the guide, in terms of permeability, durability and adhesiveness, also influence regeneration. Considering the importance of the cellular component in regeneration, the development of artificial grafts, composed of a biocompatible nerve guide filled with a neurotropic matrix and seeded with Schwann cells (SCs), is an interesting option to enhance nerve regeneration and provide an alternative to the classical autologous nerve graft. We evaluated the ability of SCs transplanted into a nerve guide to improve regeneration after sciatic nerve resection, leaving a 6-mm gap, in the mouse. Syngeneic, isogeneic and autologous SCs were suspended in Matrigel and seeded in resorbable guides, and compared to acellular guides and to nerve autografts. The immunogenicity of the transplanted SCs clearly influenced the outcome. Transplants of autologous SCs resulted in only slightly lower levels of reinnervation than autografts, but higher recovery and number of regenerated axons than transplants of isologous and syngeneic SCs, and than acellular guides. Thus, by combined developments on nerve guides, extracellular matrix components and cell transplantation, an artificial graft has been designed that allows axonal regeneration across long gaps to levels comparable with an autograft.

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.

Changes in crossed spinal reflexes after peripheral nerve injury and repair

We investigated the changes induced in crossed extensor reflex responses after peripheral nerve injury and repair in the rat. Adults rats were submitted to non repaired sciatic nerve crush (CRH, n = 9), section repaired by either aligned epineurial suture (CS, n = 11) or silicone tube (SIL4, n = 13), and 8 mm resection repaired by tubulization (SIL8, n = 12). To assess reinnervation, the sciatic nerve was stimulated proximal to the injury site, and the evoked compound muscle action potential (M and H waves) from tibialis anterior and plantar muscles and nerve action potential (CNAP) from the tibial nerve and the 4th digital nerve were recorded at monthly intervals for 3 mo postoperation. Nociceptive reinnervation to the hindpaw was also assessed by plantar algesimetry. Crossed extensor reflexes were evoked by stimulation of the tibial nerve at the ankle and recorded from the contralateral tibialis anterior muscle. Reinnervation of the hindpaw increased progressively with time during the 3 mo after lesion. The degree of muscle and sensory target reinnervation was dependent on the severity of the injury and the nerve gap created. The crossed extensor reflex consisted of three bursts of activity (C1, C2, and C3) of gradually longer latency, lower amplitude, and higher threshold in control rats. During follow-up after sciatic nerve injury, all animals in the operated groups showed recovery of components C1 and C2 and of the reflex H wave, whereas component C3 was detected in a significantly lower proportion of animals in groups with tube repair. The maximal amplitude of components C1 and C2 recovered to values higher than preoperative values, reaching final levels between 150 and 245% at the end of the follow-up in groups CRH, CS, and SIL4. When reflex amplitude was normalized by the CNAP amplitude of the regenerated tibial nerve, components C1 (300-400%) and C2 (150-350%) showed highly increased responses, while C3 was similar to baseline levels. In conclusion, reflexes mediated by myelinated sensory afferents showed, after nerve injuries, a higher degree of facilitation than those mediated by unmyelinated fibers. These changes tended to decline toward baseline values with progressive reinnervation but still remained significant 3 mo after injury.

Formation and effects of neuroactive steroids in the central and peripheral nervous system

This chapter summarizes several observations that emphasize the importance of neuroactive steroids in the physiology of the central and peripheral nervous systems. A new, and probably important, concept is emerging: Neuroactive steroids not only modify neuronal physiology but also intervene in the control of glial cell functions. The data presented here underscore that (1) the mechanism of action of the various steroidal molecules may involve both classical (progesterone and androgens) and nonclassical steroid receptors [gamma-aminobutyric acid type A (GABAA) receptor], (2) in many instances, the actions of hormonal steroids are not due to their native molecular forms but to their 5 alpha- and 3 alpha,5 alpha-reduced metabolites, (3) several neuroactive steroids exert dramatic actions on the proteins proper of the peripheral myelin (e.g., glycoprotein Po and peripheral myelin protein 22), and (4) the effects of steroids and of their metabolites might have clinical significance in cases in which the rebuilding of the peripheral myelin is needed (e.g., aging, peripheral injury).

Effects of FK506 on nerve regeneration and reinnervation after graft or tube repair of long nerve gaps

We compared the effects of FK506 administration on regeneration and reinnervation after sciatic nerve resection and repair with an autologous graft or with a silicone tube leaving a 6-mm gap in the mouse. Functional reinnervation was assessed by noninvasive methods to determine recovery of motor, sensory, and sweating functions in the hindpaw over 4 months after operation. Morphometric analysis of the regenerated nerves was performed at the end of follow-up. The nerve graft allowed for faster and higher levels of reinnervation in the four functions tested than silicone tube repair. Treatment with FK506 (for the first 9 weeks only) resulted in a slight, although not significant, improvement of the onset of reinnervation and of the maximal degree of recovery achieved after autografting. The recovery of pain sensibility and of the compound nerve action potentials in the digital nerves, which directly depend on axonal regeneration, showed better progression with FK506 than reinnervation of muscles and sweat glands, which require reestablishment of synaptic contacts with target cells. The myelinated fibers in the regenerated nerve showed a more mature appearance in the FK506-treated rats. However, FK506 showed a marginal effect in situations in which regeneration was limited, as in a silicone tube bridging a 6-mm gap in the mouse sciatic nerve. In conclusion, treatment with FK506 improved the rate of functional recovery after nerve resection and autograft repair.

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.

Peripheral nerve regeneration using bioresorbable macroporous polylactide scaffolds

The ability of DRG-derived neurons to survive and attach onto macroporous polylactide (PLA) foams was assessed in vitro. The foams were fabricated using a thermally induced polymer-solvent phase separation. Two types of pore structures, namely oriented or interconnected pores, can be produced, depending on the mechanism of phase separation, which in turn can be predicted by the thermodynamics of the polymer-solvent pair. Coating of the porous foams with polyvinylalcohol (PVA) considerably improved the wettability of the foams and allowed for cell culture. The in vitro biocompatibility of the PVA-coated supports was demonstrated by measuring cell viability and neuritogenesis. Microscopic observations of the cells seeded onto the polymer foams showed that the interconnected pore networks were more favorable to cell attachment than the anisotropic ones. The capacity of highly oriented foams to support in vivo peripheral nerve regeneration was studied in rats. A sciatic nerve gap of 5-mm length was bridged with a polymer implant showing macrotubes of 100 microm diameter. At 4 weeks postoperatively, the polymer implant was still present. It was well integrated and had restored an anatomic continuity. An abundant cell migration was observed at the outer surface of the polymer implant, but not within the macrotubes. This dense cellular microenvironment was found to be favorable for axogenesis.

Partial sciatic nerve transection as a model of neuropathic pain: a qualitative and quantitative neuropathological study

One of the most commonly used experimental animal models for neuropathic pain is the chronic constriction injury (CCI) where four loose ligatures are tied around the sciatic nerve. One disadvantage of this model is the introduction of foreign material into the wound, which causes a local inflammatory reaction. Thus the distinction between the neuropathic and the inflammatory component of pain is difficult in this model. In order to produce a pure nerve lesion, we performed a partial sciatic nerve transection (PST; a modification of the Seltzer model) in female Sprague-Dawley rats and compared behavior and nerve pathology. These rats developed thermal hyperalgesia and mechanical allodynia comparable to the CCI model. Recovery of these symptoms was found between days 40 and 60 after the nerve lesion. Some animals still showed symptoms on day 101, which was associated with a neuroma formation. The main pathological findings in the endoneurium in nerve segments distal to the lesion were edema, loss of myelinated fibers and increase in endoneurial cells, especially macrophages. In the epineurium the number of macrophages was strikingly increased after CCI compared with PST, indicating that the response of the immune system is different in a structural lesion with and without foreign material. In conclusion, PST is a pure nerve injury model without an epineurial inflammatory component due to foreign material and is therefore well suited for studying the role of local endoneurial processes in the development and maintenance of neuropathic pain. Also, the importance of regeneration in the termination of hyperalgesia can convincingly be shown in this model.

Post reinnervation maturation of myelinated nerve fibers in the cat tibial nerve: chronic electrophysiological and morphometric studies

The extent to which the long-term recovery of nerve fibers differs according to the cause of Wallerian degeneration is not clear, although outgrowth of axons is better after lesions with continuity of basal lamina of the Schwann cell tubes (nerve crush) compared with lesions with interruption of basal lamina (nerve section). Post-reinnervation maturation of myelinated nerve fibers of the cat tibial nerve was followed in chronic electrophysiologic studies after crushing, sectioning, and section+freeze lesions, and compared with morphometric analysis of the same nerves. The amplitudes of the compound nerve action potentials (CNAPs) recovered to a much lesser extent after sectioning than after crushing the nerve. This difference could be related to a smaller number of large fibers, a greater degree of sprouting after sectioning than after crushing, or less synchronization of conduction in regenerated fibers. In comparison, the compound muscle action potentials (CMAPs) recovered to a greater extent than the CNAP after sectioning and section+freeze, though not to the same degree or as fast as after crushing. The difference between the recovery of the CNAP and the CMAP could be due to better regeneration of motor fibers, to differences in the size of motor units or to a better summation of motor unit action potentials. The maximal conduction velocities (CV) in mixed nerve and in motor fibers increased faster after crushing than after sectioning and section+freeze to 60%-70% of control values. The diameters of the largest myelinated fibers increased in all lesions to about 80% of controls. The relation between fiber diameter and CV was influenced by remodeling of myelin during maturation. Hence, long-term functional recovery is influenced by the nature of the nerve lesion, and a smaller proportion of fibers recovered functionally after nerve section than after crush.

Polyimide cuff electrodes for peripheral nerve stimulation

This paper describes a new tripolar spiral cuff electrode, composed of a thin (10 microm) and flexible polyimide insulating carrier and three circumneural platinum electrodes, suitable for stimulation of peripheral nerves. The cuffs were implanted around the sciatic nerve of two groups of ten rats each, one in which the polyimide ribbon was attached to a plastic connector to characterize the in vivo stimulating properties of the electrode, and one without a connector for testing possible mechanical nerve damage by means of functional and histological methods. The polyimide cuff electrodes induced only a very mild foreign body reaction and did not change the nerve shape over a 2-6 month implantation period. There were no changes in the motor and sensory nerve conduction tests, nociceptive responses and walking track pattern over follow-up, and no morphological evidence of axonal loss or demyelination, except in one case with partial demyelination of some large fibers after 6 months. By delivering single electrical pulses through the cuff electrodes graded recruitment curves of alpha-motor nerve fibers were obtained. Recruitment of all motor units was achieved with a mean charge density lower than 4 microC/cm(2) for a pulse width of 50 micros at the time of implantation as well as 45 days thereafter. These data indicate that the polyimide cuff electrode is a stable stimulating device, with physical properties and dimensions that avoid nerve compression or activity-induced axonal damage.