Exogenous matrix precursors promote functional nerve regeneration across a 15-mm gap within a silicone chamber in the rat

Strain-induced bands of Büngner formation promotes axon growth in 3D tissue-engineered constructs

Treatment of peripheral nerve lesions remains a major challenge due to poor functional recovery; hence, ongoing research efforts strive to enhance peripheral nerve repair. In this study, we aimed to establish three-dimensional tissue-engineered bands of Büngner constructs by subjecting Schwann cells (SCs) embedded in fibrin hydrogels to mechanical stimulation. We show for the first time that the application of strain induces (i) longitudinal alignment of SCs resembling bands of Büngner, and (ii) the expression of a pronounced repair SC phenotype as evidenced by upregulation of BDNF, NGF, and p75NTR. Furthermore, we show that mechanically aligned SCs provide physical guidance for migrating axons over several millimeters in vitro in a co-culture model with rat dorsal root ganglion explants. Consequently, these constructs hold great therapeutic potential for transplantation into patients and might also provide a physiologically relevant in vitro peripheral nerve model for drug screening or investigation of pathologic or regenerative processes.

In vivo near-infrared fluorescent fibrin highlights growth of nerve during regeneration across a nerve gap

SIGNIFICANCE

Exogenous extracellular matrix (ECM) proteins, such as fibrinogen and the thrombin-polymerized scaffold fibrin, are used in surgical repair of severe nerve injuries to supplement ECM produced via the injury response. Monitoring the dynamic changes of fibrin during nerve regeneration may shed light on the frequent failure of grafts in the repair of long nerve gaps.

AIM

We explored whether monitoring of fibrin dynamics can be carried out using nerve guidance conduits (NGCs) containing fibrin tagged with covalently bound fluorophores.

APPROACH

Fibrinogen was conjugated to a near-infrared (NIR) fluorescent dye. NGCs consisting of silicone tubes filled with the fluorescent fibrin were used to repair a 5-mm gap injury in rat sciatic nerve ( n = 6 ).

RESULTS

Axonal regeneration in fluorescent fibrin-filled NGCs was confirmed at 14 days after implantation. Intraoperative fluorescence imaging after implantation showed that the exogenous fibrin was embedded in the early stage regenerative tissue. The fluorescent signal temporarily highlighted a cable-like structure within the conduit and gradually degraded over two weeks.

CONCLUSIONS

This study, for the first time, visualized in vivo intraneural fibrin degradation, potentially a useful prospective indicator of regeneration success, and showed that fluorescent ECM, in this case fibrin, can facilitate imaging of regeneration in peripheral nerve conduits without significantly affecting the regeneration process.

Optimal Technique for Introducing Schwann Cells Into Peripheral Nerve Repair Sites

Peripheral nerve injury (PNI) is found in a relatively large portion of trauma patients. If the injury is severe, such as with the presence of a long segmental gap, PNI can present a challenge for treatment. The current clinical standard of nerve harvest for the repair of long segmental gap PNI can lead to many potential complications. While other methods have been utilized, recent evidence indicates the relevance of cell therapies, particularly through the use of Schwann cells, for the treatment of PNI. Schwann cells (SCs) are integral in the regeneration and restoration of function following PNI. SCs are able to dedifferentiate and proliferate, remove myelin and axonal debris, and are supportive in axonal regeneration. Our laboratory has demonstrated that SCs are effective in the treatment of severe PNI when axon guidance channels are utilized. However, in order for this treatment to be effective, optimal techniques for cellular placement must be used. Thus, here we provide relevant background information, preclinical, and clinical evidence for our method in the treatment of severe PNI through the use of SCs and axon guidance channels.

Aligned chitosan nanofiber hydrogel grafted with peptides mimicking bioactive brain-derived neurotrophic factor and vascular endothelial growth factor repair long-distance sciatic nerve defects in rats

Autologous nerve transplantation, which is the gold standard for clinical treatment of peripheral nerve injury, still has many limitations. In this study, aligned chitosan fiber hydrogel (ACG) grafted with a bioactive peptide mixture consisting of RGI (Ac-RGIDKRHWNSQGG) and KLT (Ac-KLTWQELYQLKYKGIGG), designated as ACG-RGI/KLT, was used as nerve conduit filler to repair sciatic nerve defects in rats. Methods: Chitosan nanofiber hydrogel was prepared by a combination of electrospinning and mechanical stretching methods, and was then grafted with RGI and KLT, which are peptides mimicking brain-derived neurotrophic factor (BDNF) and vascular endothelial growth factor (VEGF), respectively. The physicochemical properties of ACG-RGI/KLT were fully characterized. In vitro, the distribution, proliferation, and secretory activity of Schwann cells were analyzed. Next, the in vivo repair potential for 15-mm rat sciatic nerve defects was examined. The recovery of regenerated nerve, muscle, and motor function was evaluated by neuromuscular histology, electrophysiology, and catwalk gait analysis. Results: We first constructed directionally aligned chitosan nanofiber hydrogel grafted with RGI/KLT peptide mixture (ACG-RGI/KLT). ACG-RGI/KLT oriented the Schwann cells, and promoted the proliferation and secretion of neurotrophic factors by Schwann cells. At an early injury stage, ACG-RGI/KLT not only enhanced nerve regeneration, but also promoted vascular penetration. At 12 weeks, ACG-RGI/KLT facilitated nerve regeneration and functional recovery in rats. Conclusions: Aligned chitosan nanofiber hydrogel grafted with RGI/KLT peptide provides an effective means of repairing sciatic nerve defects and shows great potential for clinical application.

Novel spiral structured nerve guidance conduits with multichannels and inner longitudinally aligned nanofibers for peripheral nerve regeneration

Nerve guidance conduits (NGCs) are artificial substitutes for autografts, which serve as the gold standard in treating peripheral nerve injury. A recurring challenge in tissue engineered NGCs is optimizing the cross-sectional surface area to achieve a balance between allowing nerve infiltration while supporting maximum axonal extension from the proximal to distal stump. In this study, we address this issue by investigating the efficacy of an NGC with a higher cross-sectional surface composed of spiral structures and multi-channels, coupled with inner longitudinally aligned nanofibers and protein on aiding nerve repair in critical sized nerve defect. The NGCs were implanted into 15-mm-long rat sciatic nerve injury gaps for 4 weeks. Nerve regeneration was assessed using an established set of assays, including the walking track analysis, electrophysiological testing, pinch reflex assessment, gastrocnemius muscle measurement, and histological assessment. The results indicated that the novel NGC design yielded promising data in encouraging nerve regeneration within a relatively short recovery time. The performance of the novel NGC for nerve regeneration was superior to that of the control nerve conduits with tubular structures. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1410-1419, 2019.

Electrospun nerve guide conduits have the potential to bridge peripheral nerve injuries in vivo

Electrospinning can be used to mimic the architecture of an acellular nerve graft, combining microfibers for guidance, and pores for cellular infiltration. We made electrospun nerve guides, from polycaprolactone (PCL) or poly-L-lactic acid (PLLA), with aligned fibers along the insides of the channels and random fibers around them. We bridged a 10 mm rat sciatic nerve defect with the guides, and, in selected groups, added a cell transplant derived from autologous stromal vascular fraction (SVF). For control, we compared to hollow silicone tubes; or autologous nerve grafts. PCL nerve guides had a high degree of autotomy (8/43 rats), a negative indicator with respect to future usefulness, while PLLA supported axonal regeneration, but did not outperform autologous nerve grafts. Transplanted cells survived in the PLLA nerve guides, but axonal regeneration was not enhanced as compared to nerve guides alone. The inflammatory response was partially enhanced by the transplanted cells in PLLA nerve grafts; Schwann cells were poorly distributed compared to nerve guide without cells. Tailor-made electrospun nerve guides support axonal regeneration in vivo, and can act as vehicles for co-transplanted cells. Our results motivate further studies exploring novel nerve guides and the effect of stromal cell-derived factors on nerve generation.

Matrices, scaffolds & carriers for cell delivery in nerve regeneration

Nerve injuries can be life-long debilitating traumas that severely impact patients' quality of life. While many acellular neural scaffolds have been developed to aid the process of nerve regeneration, complete functional recovery is still very difficult to achieve, especially for long-gap peripheral nerve injury and most cases of spinal cord injury. Cell-based therapies have shown many promising results for improving nerve regeneration. With recent advances in neural tissue engineering, the integration of biomaterial scaffolds and cell transplantation are emerging as a more promising approach to enhance nerve regeneration. This review provides an overview of important considerations for designing cell-carrier biomaterial scaffolds. It also discusses current biomaterials used for scaffolds that provide permissive and instructive microenvironments for improved cell transplantation.

Biomimetic Architectures for Peripheral Nerve Repair: A Review of Biofabrication Strategies

Biofabrication techniques have endeavored to improve the regeneration of the peripheral nervous system (PNS), but nothing has surpassed the performance of current clinical practices. However, these current approaches have intrinsic limitations that compromise patient care. The "gold standard" autograft provides the best outcomes but requires suitable donor material, while implantable hollow nerve guide conduits (NGCs) can only repair small nerve defects. This review places emphasis on approaches that create structural cues within a hollow NGC lumen in order to match or exceed the regenerative performance of the autograft. An overview of the PNS and nerve regeneration is provided. This is followed by an assessment of reported devices, divided into three major categories: isotropic hydrogel fillers, acting as unstructured interluminal support for regenerating nerves; fibrous interluminal fillers, presenting neurites with topographical guidance within the lumen; and patterned interluminal scaffolds, providing 3D support for nerve growth via structures that mimic native PNS tissue. Also presented is a critical framework to evaluate the impact of reported outcomes. While a universal and versatile nerve repair strategy remains elusive, outlined here is a roadmap of past, present, and emerging fabrication techniques to inform and motivate new developments in the field of peripheral nerve regeneration.

Transplantation of human amnion prevents recurring adhesions and ameliorates fibrosis in a rat model of sciatic nerve scarring

Peripheral nerve fibrosis and painful adhesions are common, recurring pathological sequelae following injury. In this study, vital human amnion (hAM), an increasingly interesting biomaterial for regenerative medicine, was investigated as a novel therapy. hAM was first analyzed in vitro regarding its anti-adhesive characteristics. Then, the reflected region of hAM which was identified as more suitable, was transplanted into female Sprague Dawley rats with recurring sciatic nerve scarring (n = 24) and compared with untreated controls (n = 30) at one, four and twelve weeks. Immune response and fibrosis were investigated by (immuno)histochemical analysis. Nerve structure was examined and function determined using electrophysiology and gait analysis. Here we identified strongly reduced adhesions in the hAM-treated rats, displaying a significant difference at four weeks post transplantation compared to untreated controls (p = .0052). This correlated with the in vitro cell attachment test on hAM explants, which demonstrated a distinctly limited ability of fibroblasts to adhere to amniotic epithelial cells. Upon hAM transplantation, significantly less intraneural fibrosis was identified at the later time points. Moreover, hAM-treated rats exhibited a significantly higher sciatic functional index (SFI) after four weeks compared to controls (p < .05), which indicated a potentially pro-regenerative effect of hAM. As a possible explanation, an impact of hAM on the endogenous immune response, including T cell and macrophage subsets, was indicated. We conclude that hAM is strongly effective against recurring nerve scarring and induces an anti-fibrotic and pro-regenerative effect, making it highly promising for treating adhesion-related disorders.

STATEMENT OF SIGNIFICANCE

Abnormal fibrotic bonding of tissues, frequently involving peripheral nerves, affects millions of people worldwide. These so-called adhesions usually cause severe pain and drastically reduce quality of life. To date, no adequate treatment exists and none is routinely used in the clinical practice. In this study, vital human amnion, the innermost of the fetal membranes, was transplanted in a rat model of peripheral nerve scarring and recurring adhesions as novel therapeutic approach. Amniotic cells have already demonstrated to feature stem-cell like properties and produce pro-regenerative factors, which makes the amnion an increasingly promising biomaterial for regenerative medicine. We identified that its transplantation was very effective against peripheral nerve scarring and distinctly reduced recurring adhesions. Moreover, we identified a pro-regenerative effect. This study showed that the amnion is a highly promising novel therapeutic approach for adhesion-related disorders.

Decellularized sciatic nerve matrix as a biodegradable conduit for peripheral nerve regeneration

The use of autologous nerve grafts remains the gold standard for treating nerve defects, but current nerve repair techniques are limited by donor tissue availability and morbidity associated with tissue loss. Recently, the use of conduits in nerve injury repair, made possible by tissue engineering, has shown therapeutic potential. We manufactured a biodegradable, collagen-based nerve conduit containing decellularized sciatic nerve matrix and compared this with a silicone conduit for peripheral nerve regeneration using a rat model. The collagen-based conduit contains nerve growth factor, brain-derived neurotrophic factor, and laminin, as demonstrated by enzyme-linked immunosorbent assay. Scanning electron microscopy images showed that the collagen-based conduit had an outer wall to prevent scar tissue infiltration and a porous inner structure to allow axonal growth. Rats that were implanted with the collagen-based conduit to bridge a sciatic nerve defect experienced significantly improved motor and sensory nerve functions and greatly enhanced nerve regeneration compared with rats in the sham control group and the silicone conduit group. Our results suggest that the biodegradable collagen-based nerve conduit is more effective for peripheral nerve regeneration than the silicone conduit.

Prompt peripheral nerve regeneration induced by a hierarchically aligned fibrin nanofiber hydrogel

Fibrin plays a crucial role in peripheral nerve regeneration, which could occur spontaneously in the format of longitudinally oriented fibrin cables during the initial stage of nerve regeneration. This fibrin cable can direct migration and proliferation of Schwann cells and axonal regrowth, which is very important to nerve regeneration. In the present study, we prepared a three-dimensional hierarchically aligned fibrin nanofiber hydrogel (AFG) through electrospinning and molecular self-assembly to resemble the architecture and biological function of the native fibrin cable. The AFG displayed a hierarchically aligned topography as well as low elasticity (∼1.5kPa) that were similar to nerve extracellular matrix (ECM) and the native fibrin cable. Rapid, directional cell adhesion and migration of Schwann cells (SCs) and dorsal root ganglions were observed in vitro. The AFG was then used as a potential intraluminal substrate in a bioengineered chitosan tube to bridge a 10-mm-long sciatic nerve gap in rats. We found that the AFG served as a beneficial microenvironment to support SCs cable formation and axonal regrowth within 2weeks. Further histological and morphological analyses as well as electrophysiological and functional examinations were performed after AFG implantation for up to 12weeks. The results from morphological analysis and electrophysiological examination indicated that regenerative outcomes achieved by our developed graft were close to those by an autologous nerve graft, but superior to those by hollow chitosan tubes (hCST) and random fibrin nanofiber hydrogel (RFG). Our results demonstrate that the AFG creates an instructive microenvironment by mimicking the native fibrin cable as well as the oriented and soft features of nerve ECM to accelerate axonal regrowth, thus showing great promising potential for applications in neural regeneration.

STATEMENT OF SIGNIFICANCE

In peripheral nervous system defect repair, a wide variety of strategies have been proposed for preparing functionalized nerve guidance conduits (NGC) with more complex configurations to obtain optimal repair effects. Longitudinally oriented fibrin cables were reported to form spontaneously during the initial stages of peripheral nerve regeneration in an empty NGC, which can direct the migration and proliferation of Schwann cells and promote axonal regrowth. Therefore, based on the biomimetic idea, we prepared a three-dimensional hierarchically aligned fibrin nanofiber hydrogel (AFG) through electrospinning and molecular self-assembly, resembling the architecture and biological function of the native fibrin cable and serving as an intraluminal filling to accelerate axon regeneration. We found that the AFG was a beneficial microenvironment to support SCs cable formation and accelerate axonal regrowth with improved motor functional recovery.

The effects of venous ensheathment on facial nerve repair in the rat

OBJECTIVE

To investigate the protective effect of autologous venous ensheathment on sutured rat facial nerve and to test whether the ensheathment could improve the functional recovery of repaired nerve and accuracy of axonal growth.

STUDY DESIGN

In vivo study.

METHODS

Forty-six rats were examined, with six rats serving as normal controls and 40 receiving facial nerve transection and suture repair (SR) or transection and suture repair with an additional venous ensheathment (VE). The rats were then subjected to functional testing, histological assessment of nerve specimens, or retrograde tracing, respectively.

RESULTS

At the postoperative day (POD) 60, the venous ensheathment showed no adhesion at the surrounding tissues. No significant difference in neuroma formation was found between the two surgical manipulations (SR and VE groups) (P < 0.05). Retrogradely labeled motoneurons in facial nuclei were extremely disorganized after the facial nerve undertook surgical manipulation. In all manipulated groups, double retrogradely labeled neurons, indicative of aberrant axonal branching during regeneration, could be observed after peripheral manipulation across all time points. With the two facial surgical manipulations, the average count of double-labeled neurons at POD 60 was significantly less than at POD 21 (P < 0.05).

CONCLUSION

Autologous venous ensheathment could not help with the functional recovery of facial nerve or improve the accuracy of axonal regeneration. Further studies are warranted to elucidate the effects of venous ensheathment in other motor and sensory nerve models.

LEVEL OF EVIDENCE

NA. Laryngoscope, 127:1558-1564, 2017.

Platelet-Rich Plasma Promotes Axon Regeneration, Wound Healing, and Pain Reduction: Fact or Fiction

Platelet-rich plasma (PRP) has been tested in vitro, in animal models, and clinically for its efficacy in enhancing the rate of wound healing, reducing pain associated with injuries, and promoting axon regeneration. Although extensive data indicate that PRP-released factors induce these effects, the claims are often weakened because many studies were not rigorous or controlled, the data were limited, and other studies yielded contrary results. Critical to assessing whether PRP is effective are the large number of variables in these studies, including the method of PRP preparation, which influences the composition of PRP; type of application; type of wounds; target tissues; and diverse animal models and clinical studies. All these variables raise the question of whether one can anticipate consistent influences and raise the possibility that most of the results are correct under the circumstances where PRP was tested. This review examines evidence on the potential influences of PRP and whether PRP-released factors could induce the reported influences and concludes that the preponderance of evidence suggests that PRP has the capacity to induce all the claimed influences, although this position cannot be definitively argued. Well-defined and rigorously controlled studies of the potential influences of PRP are required in which PRP is isolated and applied using consistent techniques, protocols, and models. Finally, it is concluded that, because of the purported benefits of PRP administration and the lack of adverse events, further animal and clinical studies should be performed to explore the potential influences of PRP.

C3 toxin and poly-DL-lactide-ε-caprolactone conduits in the critically damaged peripheral nervous system: a combined therapeutic approach

INTRODUCTION

Peripheral nerve regeneration over longer distances through conduits is limited. In the presented study, critical size nerve gap bridging with a poly-DL-lactide-ε-caprolactone (PLC) conduit was combined with application of C3 toxin to facilitate axonal sprouting.

MATERIALS AND METHODS

The PLC filled with fibrin (n = 10) and fibrin gel loaded with 1-μg C3-C2I and 2-μg C2II (n = 10) were compared to autologous nerve grafts (n = 10) in a 15-mm sciatic nerve gap lesion model of the rat. Functional and electrophysiological analyses were performed before histological evaluation.

RESULTS

Evaluation of motor function and nerve conduction velocity at 16 weeks revealed no differences between the groups. All histological parameters and muscle weight were significantly elevated in nerve graft group. No differences were observed in both PLC groups.

CONCLUSIONS

The PLCs are permissive for nerve regeneration over a 15-mm defect in rats. Intraluminal application of C3 toxin did not lead to significant enhancement of nerve sprouting.

Peripheral nerve injury: principles for repair and regeneration

Peripheral Nerve Injuries are one of the most common causes of hand dysfunction caused by upper limb trauma but still current management has remained suboptimal. This review aims to explain the traditional view of pathophysiology of nerve repair and also describe why surgical management is still inadequate in using the new biological research that has documented the changes that occur after the nerve injury, which, could cause suboptimal clinical outcomes. Subsequently presentation and diagnosis will be described for peripheral nerve injuries. When traditional surgical repair using end-to-end anastomosis is not adequate nerve conduits are required with the gold standard being the autologous nerve. Due to associated donor site morbidity and poor functional outcome documented with autologous nerve repair several new advancements for alternatives to bridge the gap are being investigated. We will summarise the new and future advancements of non-biological and biological replacements as well as gene therapy, which are being considered as the alternatives for peripheral nerve repair.

Development and characterization of novel agar and gelatin injectable hydrogel as filler for peripheral nerve guidance channels

Injectable hydrogels are becoming of increasing interest in the field of tissue engineering thanks to their versatile properties and to the possibility of being injected into tissues or devices during surgery. In peripheral nerve tissue engineering, injectable hydrogels having shear-thinning properties are advantageous as filler of nerve guidance channels (NGCs) to improve the regeneration process. In the present work, gelatin-based hydrogels were developed and specifically designed for the insertion into the lumen of hollow NGCs through a syringe during surgery. Injectable hydrogels were obtained using an agar-gelatin 20:80 weight ratio, (wt/wt) blend crosslinked by the addition of genipin (A/GL_GP). The physicochemical properties of the A/GL_GP hydrogels were analysed, including their injectability, rheological, swelling and dissolution behaviour, and their mechanical properties under compression. The hydrogel developed showed shear-thinning properties and was applied as filler of NGCs. The A/GL_GP hydrogel was tested in vitro using different cell lines, among them Schwann cells which have been used because they have an important role in peripheral nerve regeneration. Viability assays demonstrated the lack of cytotoxicity. In vitro experiments showed that the hydrogel is able to promote cell adhesion and proliferation. Two- and three-dimensional migration assays confirmed the capability of the cells to migrate both on the surface and within the internal framework of the hydrogel. These data show that A/GL_GP hydrogel has characteristics that make it a promising scaffold material for tissue engineering and nerve regeneration. Copyright © 2014 John Wiley & Sons, Ltd.

Limitations of nerve repair of segmental defects using acellular conduits

The authors present the case of a 20-year-old man who, 3 months after his initial injury, underwent repair of a 1.7-cm defect of the ulnar nerve at the wrist; repair was performed with an acellular nerve allograft. Given the absence of clinical or electrophysiological recovery at 8 months postrepair, the patient underwent reexploration, excision of the “regenerated cable,” and rerepair of the ulnar nerve with sural nerve autografts. Histology of the cable demonstrated minimal axonal regeneration at the midpoint of the repair. At the 6- and 12-month follow-ups of the sural nerve graft repair, clinical and electrophysiological evidence of both sensory and motor reinnervation of the ulnar nerve and associated hand muscles was demonstrated. In this report, the authors describe a single case of failed acellular nerve allograft and correlate the results with basic science and human studies reporting length and diameter limitations in human nerve repair utilizing grafts or conduits devoid of viable Schwann cells.

Transplantation of Schwann cells in a collagen tube for the repair of large, segmental peripheral nerve defects in rats

Object

Segmental nerve defects pose a daunting clinical challenge, as peripheral nerve injury studies have established that there is a critical nerve gap length for which the distance cannot be successfully bridged with current techniques. Construction of a neural prosthesis filled with Schwann cells (SCs) could provide an alternative treatment to successfully repair these long segmental gaps in the peripheral nervous system. The object of this study was to evaluate the ability of autologous SCs to increase the length at which segmental nerve defects can be bridged using a collagen tube.

Methods

The authors studied the use of absorbable collagen conduits in combination with autologous SCs (200,000 cells/μl) to promote axonal growth across a critical size defect (13 mm) in the sciatic nerve of male Fischer rats. Control groups were treated with serum only–filled conduits of reversed sciatic nerve autografts. Animals were assessed for survival of the transplanted SCs as well as the quantity of myelinated axons in the proximal, middle, and distal portions of the channel.

Results

Schwann cell survival was confirmed at 4 and 16 weeks postsurgery by the presence of prelabeled green fluorescent protein–positive SCs within the regenerated cable. The addition of SCs to the nerve guide significantly enhanced the regeneration of myelinated axons from the nerve stump into the proximal (p < 0.001) and middle points (p < 0.01) of the tube at 4 weeks. The regeneration of myelinated axons at 16 weeks was significantly enhanced throughout the entire length of the nerve guide (p < 0.001) as compared with their number in a serum–only filled tube and was similar in number compared with the reversed autograft. Autotomy scores were significantly lower in the animals whose sciatic nerve was repaired with a collagen conduit either without (p < 0.01) or with SCs (p < 0.001) when compared with a reversed autograft.

Conclusions

The technique of adding SCs to a guidance channel significantly enhanced the gap distance that can be repaired after peripheral nerve injury with long segmental defects and holds promise in humans. Most importantly, this study represents some of the first essential steps in bringing autologous SC-based therapies to the domain of peripheral nerve injuries with long segmental defects.

Extracellular matrix components in peripheral nerve regeneration

Injured axons of the peripheral nerve are able to regenerate and, eventually, reinnervate target organs. However, functional recovery is usually poor after severe nerve injuries. The switch of Schwann cells to a proliferative state, secretion of trophic factors, and the presence of extracellular matrix (ECM) molecules (such as collagen, laminin, or fibronectin) in the distal stump are key elements to create a permissive environment for axons to grow. In this review, we focus attention on the ECM components and their tropic role in axonal regeneration. These components can also be used as molecular cues to guide the axons through artificial nerve guides in attempts to better mimic the natural environment found in a degenerating nerve. Most used scaffolds tested are based on natural molecules that form the ECM, but use of synthetic polymers and functionalization of hydrogels are bringing new options. Progress in tissue engineering will eventually lead to the design of composite artificial nerve grafts that may replace the use of autologous nerve grafts to sustain regeneration over long gaps.

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.

Recent advances and developments in neural repair and regeneration for hand surgery

End-to-end suture of nerves and autologous nerve grafts are the 'gold standard' for repair and reconstruction of peripheral nerves. However, techniques such as sutureless nerve repair with tissue glues, end-to-side nerve repair and allografts exist as alternatives. Biological and synthetic nerve conduits have had some success in early clinical studies on reconstruction of nerve defects in the hand. The effectiveness of nerve regeneration could potentially be increased by using these nerve conduits as scaffolds for delivery of Schwann cells, stem cells, neurotrophic and neurotropic factors or extracellular matrix proteins. There has been extensive in vitro and in vivo research conducted on these techniques. The clinical applicability and efficacy of these techniques needs to be investigated fully.

Poly(amidoamine) Hydrogels as Scaffolds for Cell Culturing and Conduits for Peripheral Nerve Regeneration

Biodegradable and biocompatible poly(amidoamine)-(PAA-) based hydrogels have been considered for different tissue engineering applications. First-generation AGMA1 hydrogels, amphoteric but prevailing cationic hydrogels containing carboxylic and guanidine groups as side substituents, show satisfactory results in terms of adhesion and proliferation properties towards different cell lines. Unfortunately, these hydrogels are very swellable materials, breakable on handling, and have been found inadequate for other applications. To overcome this problem, second-generation AGMA1 hydrogels have been prepared adopting a new synthetic method. These new hydrogels exhibit good biological propertiesin vitrowith satisfactory mechanical characteristics. They are obtained in different forms and shapes and successfully testedin vivofor the regeneration of peripheral nerves. This paper reports on our recent efforts in the use of first-and second-generation PAA hydrogels as substrates for cell culturing and tubular scaffold for peripheral nerve regeneration.

A novel basic fibroblast growth factor delivery system fabricated with heparin-incorporated fibrin-fibronectin matrices for repairing rat sciatic nerve disruptions

Autologous nerve grafts are widely used in bridging critical gaps of peripheral nerves, but they remain associated with high morbidity of the donor site and lack of full recovery. As an alternative, we have focused on chitosan nerve conduits filled with a heparin-incorporated fibrin-fibronectin matrix serving as delivery systems for basic fibroblast growth factor (bFGF). The artificial nerve conduits were used for repairing sciatic nerve defects of 10 mm in adult rats. Three months post-operation, the conduction velocity recovery index (CVRI) and the muscle restoration rate (MRR) in animals of the experimental group were 32 +/- 4.1 and 77.4 +/- 7.9%, respectively, which were significantly higher than those of the PBS control group (17.8 +/- 1.9 and 66.7 +/- 6.5%), and similar to those of the autograft group (38.4 +/- 3.9 and 81.3 +/- 7.8%). These results were also consistent with the densities of regenerated axons in the three groups, which were demonstrated by histomorphological analysis.

Transplantation of autologous Schwann cells for the repair of segmental peripheral nerve defects

Peripheral nerve injuries are a source of chronic disability. Incomplete recovery from such injuries results in motor and sensory dysfunction and the potential for the development of chronic pain. The repair of human peripheral nerve injuries with traditional surgical techniques has limited success, particularly when a damaged nerve segment needs to be replaced. An injury to a long segment of peripheral nerve is often repaired using autologous grafting of "noncritical" sensory nerve. Although extensive axonal regeneration can be observed extending into these grafts, recovery of function may be absent or incomplete if the axons fail to reach their intended target. The goal of this review was to summarize the progress that has occurred in developing an artificial neural prosthesis consisting of autologous Schwann cells (SCs), and to detail future directions required in translating this promising therapy to the clinic. In the authors' laboratory, methods are being explored to combine autologous SCs isolated using cell culture techniques with axon guidance channel (AGC) technology to develop the potential to repair critical gap length lesions within the peripheral nervous system. To test the clinical efficacy of such constructs, it is critically important to characterize the fate of the transplanted SCs with regard to cell survival, migration, differentiation, and myelin production. The authors sought to determine whether the use of SC-filled channels is superior or equivalent to strategies that are currently used clinically (for example, autologous nerve grafts). Finally, although many nerve repair paradigms demonstrate evidence of regeneration within the AGC, the authors further sought to determine if the regeneration observed was physiologically relevant by including electrophysiological, behavioral, and pain assessments. If successful, the development of this reparative approach will bring together techniques that are readily available for clinical use and should rapidly accelerate the process of bringing an effective nerve repair strategy to patients with peripheral nerve injury prior to the development of pain and chronic disability.

Thin-film enhanced nerve guidance channels for peripheral nerve repair

It has been demonstrated that nerve guidance channels containing stacked thin-films of aligned poly(acrylonitrile-co-methylacrylate) fibers support peripheral nerve regeneration across critical sized nerve gaps, without the aid of exogenous cells or proteins. Here, we explore the ability of tubular channels minimally supplemented with aligned nanofiber-based thin-films to promote endogenous nerve repair. We describe a technique for fabricating guidance channels in which individual thin-films are fixed into place within the lumen of a polysulfone tube. Because each thin-film is <10 microm thick, this technique allows fine control over the positioning of aligned scaffolding substrate. We evaluated nerve regeneration through a 1-film guidance channel--containing a single continuous thin-film of aligned fibers--in comparison to a 3-film channel that provided two additional thin-film tracks. Thirty rats were implanted with one of the two channel types, and regeneration across a 14 mm tibial nerve gap was evaluated after 6 weeks and 13 weeks, using a range of morphological and functional measures. Both the 1-film and the 3-film channels supported regeneration across the nerve gap resulting in functional muscular reinnervation. Each channel type characteristically influenced the morphology of the regeneration cable. Interestingly, the 1-film channels supported enhanced regeneration compared to the 3-film channels in terms of regenerated axon profile counts and measures of nerve conduction velocity. These results suggest that minimal levels of appropriately positioned topographical cues significantly enhance guidance channel function by modulating endogenous repair mechanisms, resulting in effective bridging of critically sized peripheral nerve gaps.

Approaches to neural tissue engineering using scaffolds for drug delivery

This review seeks to give an overview of the current approaches to drug delivery from scaffolds for neural tissue engineering applications. The challenges presented by attempting to replicate the three types of nervous tissue (brain, spinal cord, and peripheral nerve) are summarized. Potential scaffold materials (both synthetic and natural) and target drugs are discussed with the benefits and drawbacks given. Finally, common methods of drug delivery, including degradable/diffusion-based delivery systems, affinity-based delivery systems, immobilized drug delivery systems, and electrically controlled drug delivery systems, are examined and critiqued. Based on the current body of work, suggestions for future directions of research in the field of neural tissue engineering are presented.

Luminal fillers in nerve conduits for peripheral nerve repair

The use of nerve conduits as an alternative for nerve grafting has a long experimental and clinical history. Luminal fillers, factors introduced into these nerve conduits, were later developed to enhance the nerve regeneration through conduits. Though many luminal fillers have been reported to improve nerve regeneration, their use has not been subjected to systematic review. This review categorizes the types of fillers used, the conduits associated with fillers, and the reported performance of luminal fillers in conduits to present a preference list for the most effective fillers to use over specific distances of nerve defect.

Influence of the substrate's hydrophilicity on thein vitro Schwann cells viability

A series of polymeric biomaterials including poly (methyl acrylate) (PMA), chitosan (CHT), poly(ethyl acrylate) (PEA), poly(hydroxyethyl acrylate) (PHEA), and a series of random copolymers containing ethyl acrylate and hydroxyethyl acrylate monomeric units were tested in vitro as culture substrates and compared for their impact on the proliferation and expansion of Schwann cells (SCs). Immunocytochemical staining assay and scanning electron microscopy techniques were applied to perform a quantitative analysis to determine the correct maintenance of the cultured glial cells on the different biomaterials. The results strongly suggest that cell attachment and proliferation is influenced by the substrate's surface chemistry, and that hydrophobic biomaterials based on PMA, PEA, and the copolymers PEA and PHEA in a narrow composition window are suitable substrates to promote cell attachment and proliferation of SCs in vitro.

Metabosensitive Afferent Fiber Responses after Peripheral Nerve Injury and Transplantation of an Acellular Muscle Graft in Association with Schwann Cells

Studies dedicated to the repair of peripheral nerve focused almost exclusively on motor or mechanosensitive fiber regeneration. Poor attention has been paid to the metabosensitive fibers from group III and IV (also called ergoreceptor). Previously, we demonstrated that the metabosensitive response from the tibialis anterior muscle was partially restored when the transected nerve was immediately sutured. In the present study, we assessed motor and metabosensitive responses of the regenerated axons in a rat model in which 1 cm segment of the peroneal nerve was removed and immediately replaced by an autologous nerve graft or an acellular muscle graft. Four groups of animals were included: control animals (C, no graft), transected animals grafted with either an autologous nerve graft (Gold Standard-GS) or an acellular muscle filled with Schwann Cells (MSC) or Culture Medium (MCM). We observed that (1) the tibialis anterior muscle was atrophied in GS, M(SC) and M(CM) groups, with no significant difference between grafted groups; (2) the contractile properties of the reinnervated muscles after nerve stimulation were similar in all groups; (3) the metabosensitive afferent responses to electrically induced fatigue was smaller in M(SC) and MCM groups; and (4) the metabosensitive afferent responses to two chemical agents (KCl and lactic acid) was decreased in GS, M(SC) and M(CM) groups. Altogether, these data indicate a motor axonal regeneration and an immature metabosensitive afferent fiber regrowth through acellular muscle grafts. Similarities between the two groups grafted with acellular muscles suggest that, in our conditions, implanted Schwann cells do not improve nerve regeneration. Future studies could include engineered conduits that mimic as closely as possible the internal organization of uninjured nerve.

Basic fibroblast growth factor promotes nerve regeneration in a C- -ion-implanted silicon chamber

We reported previously that a silicone tube whose inner surface has been implanted with negatively charged carbon ions (C-) enables a nerve to regenerate across a 15-mm inter-stump gap. In this study, we investigated whether a C- -ion-implanted tube pretreated with basic fibroblast growth factor promotes peripheral nerve regeneration. The C- -ion-implanted tube significantly accelerated nerve regeneration, and this effect was enhanced by basic fibroblast growth factor.

Nerve regeneration with the use of a poly(l-lactide-co-glycolic acid)-coated collagen tube filled with collagen gel

AIM

The aim of this study was to develop a novel artificial nerve conduit and to evaluate its efficiency based on the promotion of peripheral nerve regeneration in rabbits.

MATERIAL AND METHODS

The nerve conduit was made of a poly (l-lactide-co-glycolic acid)-coated collagen tube filled with collagen gel. The conduits were implanted into a 15 mm gap in the peroneal nerves of five rabbits. On the contralateral side, the defects were bridged with collagen-filled vein grafts.

RESULTS

Twelve weeks postoperatively nerve regeneration was superior to the vein graft in the PLGA-coated collagen tube, both morphologically and electrophysiologically.

CONCLUSION

The results indicate the superiority of the PLGA-coated collagen tube over vein grafts. Furthermore, they show that entubulation repair with this type of tube can support nerve regeneration over a nerve gap distance of at least 15 mm.

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.

Autologous fibrin glue in peripheral nerve regeneration in vivo

The activity of several growth factors on peripheral nerve regeneration is reported. Autologous fibrin glue contains a large number of platelets, which release significant quantities of growth factors. In order to understand the role of autologous fibrin glue in peripheral nerve regeneration, a 15-mm rabbit peroneal nerve defect was repaired using a vein graft filled with autologous fibrin glue. Axonal regeneration was examined using histological and electrophysiological methods. The extent of axonal regeneration was superior when treated with autologous fibrin glue. Our data suggest that fibrin nets formed by fibrinogen, in combination with growth factors present in autologous fibrin glue, might effectively promote peripheral nerve regeneration in nerve defects.

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.

The interaction of Schwann cells with chitosan membranes and fibers in vitro

The bridging of nerve gaps is still one of the major problems in peripheral nerve regeneration. A promising alternative for the repair of peripheral nerve injuries is the bioartificial nerve graft, comprised of a biomaterial pre-seeded with Schwann cells (SCs), which is an effective substrate for enhancing nerve regeneration. Interaction between cultured SCs and biomaterials is of importance. For the purposes of this study, culture systems of normal SCs were used. The biocompatibility of chitosan, including chitosan membranes and chitosan fibers, was evaluated in vitro. The growth of SCs was observed by light and scanning electron microscopy at regular intervals. SCs were identified by immunocytochemical staining and the viability of SCs was measured by MTT assay. The experimental results indicated that SCs could grow onto chitosan materials with two different shapes: spherical and long olivary. They contacted with the extensions. The long olivary cells inclined to encircle chitosan fibers up. It was also found that the cells on the chitosan fibers migrated faster than those on the chitosan membranes. There was a good biological compatibility between chitosan and SCs. Compared with the chitosan membranes, SCs migrated more easily onto the stereoframe of chitosan fibers. These studies contribute information necessary to enhancing our understanding of biocompatibility of chitosan.

Use of Skeletal Muscle Tissue in Peripheral Nerve Repair: Review of the Literature

The management of peripheral nerve injury continues to be a major clinical challenge. The most widely used technique for bridging defects in peripheral nerves is the use of autologous nerve grafts. This technique, however, necessitates a donor nerve and corresponding deficit. Many alternative techniques have thus been developed. The use of skeletal muscle tissue as graft material for nerve repair is one example. The rationale regarding the use of the skeletal muscle tissue technique is the availability of a longitudinally oriented basal lamina and extracellular matrix components that direct and enhance regenerating nerve fibers. These factors provide superiority over other bridging methods as vein grafts or (non)degradable nerve conduits. The main disadvantages of this technique are the risk that nerve fibers can grow out of the muscle tissue during nerve regeneration, and that a donor site is necessary to harvest the muscle tissue. Despite publications on nerve conduits as an alternative for peripheral nerve repair, autologous nerve grafting is still the standard care for treatment of a nerve gap in the clinical situation; however, the use of the skeletal muscle tissue technique can be added to the surgeon's arsenal of peripheral nerve repair tools, especially for bridging short nerve defects or when traditional nerve autografts cannot be employed. This technique has been investigated both experimentally and clinically and, in this article, an overview of the literature on skeletal muscle grafts for bridging peripheral nerve defects is presented.

Peripheral nerve regeneration through guidance tubes

Biological nerve grafts have been extensively utilized in the past to repair peripheral nerve injuries. More recently, the use of synthetic guidance tubes in repairing these injuries has gained in popularity. This review focuses on artificial conduits, nerve regeneration through them, and an account of various synthetic materials that comprise these tubes in experimental animal and clinical trials. It also lists and describes several biomaterial considerations one should regard when designing, developing, and manufacturing potential guidance channel candidates. In the future, it it likely that the most successful synthetic nerve conduit will be one that has been fabricated with some of these strategies in mind.

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.

The Role of Topically Administered FK506 (Tacrolimus) at the Time of Facial Nerve Repair Using Entubulation Neurorrhaphy in a Rabbit Model

Peripheral facial nerve palsy is a common sequela of traumatic craniofacial injury, often resulting in dramatic and sometimes permanent functional deficits. Exogenous agents and methods of repair that accelerate axonal regeneration would be of great benefit to the multitude of patients with facial nerve injuries. The objective of this study was to evaluate the effect of FK506 at the time of facial nerve repair using entubulation neurorrhaphy, and to compare entubulation neurorrhaphy versus interposition autograft in critical facial nerve gap defects. The study design was a prospective, randomized, blinded animal study with a control group. Twenty-five New Zealand White rabbits were assigned to 4 experimental groups and a control group. The buccal branch of the facial nerve was used in all procedures. Group 1 was the control group. Rabbits in group 2 underwent sham surgery. Group 3 was an interposition autograft group in which a 6-mm segment of nerve was transacted, flipped, and followed by epineural repair. Groups 4 and 5 underwent transection followed by entubulation neurorrhaphy with topical administration of either a carrier molecule (group 4) or an FK506 carrier molecule (group 5). Outcome measures included daily subjective assessment of upper lip movement; electromyographic studies at weeks 3, 5, and 8 postoperatively; and blinded quantitative histomorphometric evaluation after 8 weeks. All rabbits in all groups were noted to have spontaneous movement after 8 weeks, with 1 rabbit in group 5 obtaining the highest functional score among all study groups. Electrophysiologic studies showed polyphasic potentials, indicating reinnervation in 1 rabbit in group 5. Histomorphometric examination of group 5 rabbits revealed a similar cross-sectional area distal to transection and remyelination. Other groups showed decreased cross-sectional area and/or incomplete remyelination distal to the transection. FK506 applied topically at the time of facial nerve repair using entubulation neurorrhaphy demonstrated superior results in nerve regeneration versus entubulation neurorrhaphy carrier protein alone, and interposition autograft.

Functional assessment of sciatic nerve recovery: biodegradable poly (DLLA-epsilon-CL) nerve guide filled with fresh skeletal muscle

The aim of this study was to compare functional peripheral nerve recovery in the rat sciatic nerve model after reconstruction of a 10-mm gap with a biodegradable poly (DLLA-epsilon-CL) nerve guide, as filled with either fresh skeletal muscle or phosphate-buffered saline (PBS). During 24 weeks of recovery, motor and sensory functional evaluation was tested by extensor postural thrust (EPT) and withdrawal reflex latency (WRL), respectively. At the end of the experiment, anesthetized animals were prepared for motor nerve conduction velocity (MNCV) studies, followed by gastrocnemius and soleus muscle weight measurement. Motor functional recovery was greater in the muscle-grafted group, and reached a significant difference from weeks 8-12 (P < 0.05). The results of this investigation suggest that filling a nerve guide with fresh skeletal muscle induces faster maturation of regenerated nerve fibers in comparison with traditional tubular repair.