Influence of glial growth factor and Schwann cells in a bioresorbable guidance channel on peripheral nerve regeneration

Multichannel Conduits with Fascicular Complementation: Significance in Long Segmental Peripheral Nerve Injury

Despite the advances in tissue engineering approaches, reconstruction of long segmental peripheral nerve defects remains unsatisfactory. Although autologous grafts with proper fascicular complementation have shown meaningful functional recovery according to the Medical Research Council Classification (MRCC), the lack of donor nerve for such larger defect sizes (>30 mm) has been a serious clinical issue. Further clinical use of hollow nerve conduits is limited to bridging smaller segmental defects of denuded nerve ends (<30 mm). Recently, bioinspired multichannel nerve guidance conduits (NGCs) gained attention as autograft substitutes as they mimic the fascicular connective tissue microarchitecture in promoting aligned axonal outgrowth with desirable innervation for complete sensory and motor function restoration. This review outlines the hierarchical organization of nerve bundles and their significance in the sensory and motor functions of peripheral nerves. This review also emphasizes the major challenges in addressing the longer nerve defects with the role of fascicular arrangement in the multichannel nerve guidance conduits and the need for fascicular matching to accomplish complete functional restoration, especially in treating long segmental nerve defects. Further, currently available fabrication strategies in developing multichannel nerve conduits and their inconsistency in existing preclinical outcomes captured in this review would seed a new process in designing an ideal larger nerve conduit for peripheral nerve repair.

The Effect of Schwann Cells/Schwann Cell-Like Cells on Cell Therapy for Peripheral Neuropathy

Peripheral neuropathy is a common neurological issue that leads to sensory and motor disorders. Over time, the treatment for peripheral neuropathy has primarily focused on medications for specific symptoms and surgical techniques. Despite the different advantages of these treatments, functional recovery remains less than ideal. Schwann cells, as the primary glial cells in the peripheral nervous system, play crucial roles in physiological and pathological conditions by maintaining nerve structure and functions and secreting various signaling molecules and neurotrophic factors to support both axonal growth and myelination. In addition, stem cells, including mesenchymal stromal cells, skin precursor cells and neural stem cells, have the potential to differentiate into Schwann-like cells to perform similar functions as Schwann cells. Therefore, accumulating evidence indicates that Schwann cell transplantation plays a crucial role in the resolution of peripheral neuropathy. In this review, we summarize the literature regarding the use of Schwann cell/Schwann cell-like cell transplantation for different peripheral neuropathies and the potential role of promoting nerve repair and functional recovery. Finally, we discuss the limitations and challenges of Schwann cell/Schwann cell-like cell transplantation in future clinical applications. Together, these studies provide insights into the effect of Schwann cells/Schwann cell-like cells on cell therapy and uncover prospective therapeutic strategies for peripheral neuropathy.

Promotion of nerve regeneration by biodegradable nanofibrous scaffold following sciatic nerve transection in rats

Peripheral nerve injuries (PNIs) are one of the common causes of morbidity and disability worldwide. Autograft is considered the gold standard treatment for PNIs. However, due to the complications associated with autografts, other sources are considered as alternatives. Recently, electrospun nanofibrous scaffolds have received wide attention in the field of tissue engineering. Exogenous tubular constructs with uniaxially aligned topographical cues to enhance the axonal re-growth are needed to bridge large nerve gaps between proximal and distal ends. Although several studies have used PLGA/PCL, but few studies have been conducted on developing a two-layer scaffold with aligned fibers properly orientated along the axis direction of the sciatic nerve to meet the physical properties required for suturing, transplantation, and nerve regeneration. In this study, we sought to design and develop PLGA-PCL-aligned nanofibers. Following the conventional examinations, we implanted the scaffolds into 7-mm sciatic nerve gaps in a rat model of nerve injury. Our in vivo evaluations did not show any adverse effects, and after eight weeks, an acceptable improvement was noted in the electrophysiological, functional, and histological analyses. Thus, it can be concluded that nanofiber scaffolds can be used as a reliable approach for repairing PNIs. However, further research is warranted.

Machine intelligence for nerve conduit design and production

Nerve guidance conduits (NGCs) have emerged from recent advances within tissue engineering as a promising alternative to autografts for peripheral nerve repair. NGCs are tubular structures with engineered biomaterials, which guide axonal regeneration from the injured proximal nerve to the distal stump. NGC design can synergistically combine multiple properties to enhance proliferation of stem and neuronal cells, improve nerve migration, attenuate inflammation and reduce scar tissue formation. The aim of most laboratories fabricating NGCs is the development of an automated process that incorporates patient-specific features and complex tissue blueprints (e.g. neurovascular conduit) that serve as the basis for more complicated muscular and skin grafts. One of the major limitations for tissue engineering is lack of guidance for generating tissue blueprints and the absence of streamlined manufacturing processes. With the rapid expansion of machine intelligence, high dimensional image analysis, and computational scaffold design, optimized tissue templates for 3D bioprinting (3DBP) are feasible. In this review, we examine the translational challenges to peripheral nerve regeneration and where machine intelligence can innovate bottlenecks in neural tissue engineering.

Nanofibrous Nerve Conduits with Nerve Growth Factors and Bone Marrow Stromal Cells Pre-Cultured in Bioreactors for Peripheral Nerve Regeneration

Peripheral nerve injury (PNI) was the leading cause of permanent dysfunction in movement and sensation. Synthesized nerve guide conduits (NGCs) with Schwann Cells (SCs) can help peripheral nerve regeneration. However, poor accessibility of SCs and lack of full coverage of seeded cells on NGCs can lead to failure of nerve regeneration across long gaps and full functional recovery. To overcome these limitations, bone marrow stromal cells (BMSCs) and a novel culture method were proposed in the current study. BMSCs were harvested and seeded on a never growth factor (NGF)-loaded PCL nanofibrous NGCs and cultured with a rotary cell culture system (RCCS) before implantation. The NGCs were tested in vitro with PC-12 cells to validate the bioactivity of released NGF and to access its ability to promote neurite extension. Also, the NGCs were tested in vivo with rat sciatic nerve model to exam its potential in bridging the long gap (15 mm segmental defect). The efficacy of the NGCs was investigated based on the results of the functional test, electrophysiology test, muscle atrophy, and histological analysis. The results of in vitro PC-12 cell study confirmed the bioactivity of released NGF and showed a significant increase in the neurite extension with the help of PEG-diamine and BSA. These results showed that the novel loading method could preserve the bioactivity of growth factors and achieve a sustained release in vitro. Besides, the results of the in vivo study exhibited a significant increase with the combination of all additives. These results showed that with the help of NGF and RCCS, the NGCs with the seeded BMSCs could enhance peripheral nerve regeneration across long nerve injury gaps.

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.

Therapeutic strategies for peripheral nerve injury: decellularized nerve conduits and Schwann cell transplantation

In recent years, the use of Schwann cell transplantation to repair peripheral nerve injury has attracted much attention. Animal-based studies show that the transplantation of Schwann cells in combination with nerve scaffolds promotes the repair of injured peripheral nerves. Autologous Schwann cell transplantation in humans has been reported recently. This article reviews current methods for removing the extracellular matrix and analyzes its composition and function. The development and secretory products of Schwann cells are also reviewed. The methods for the repair of peripheral nerve injuries that use myelin and Schwann cell transplantation are assessed. This survey of the literature data shows that using a decellularized nerve conduit combined with Schwann cells represents an effective strategy for the treatment of peripheral nerve injury. This analysis provides a comprehensive basis on which to make clinical decisions for the repair of peripheral nerve injury.

Synthetic bioresorbable poly-α-hydroxyesters as peripheral nerve guidance conduits; a review of material properties, design strategies and their efficacy to date

Implantable tubular devices known as nerve guidance conduits (NGCs) have drawn considerable interest as an alternative to autografting in the repair of peripheral nerve injuries.

Designs of Biomaterials and Microenvironments for Neuroengineering

Recent clinical research on neuroengineering is primarily focused on biocompatible materials, which can be used to provide electroactive and topological cues, regulate the microenvironment, and perform other functions. Novel biomaterials for neuroengineering have been received much attention in the field of research, including graphene, photonic crystals, and organ-on-a-chip. Graphene, which has the advantage of high mechanical strength and chemical stability with the unique electrochemical performance for electrical signal detection and transmission, has significant potential as a conductive scaffolding in the field of medicine. Photonic crystal materials, known as a novel concept in nerve substrates, have provided a new avenue for neuroengineering research because of their unique ordered structure and spectral attributes. The "organ-on-a-chip" systems have shown significant prospects for the developments of the solutions to nerve regeneration by mimicking the microenvironment of nerve tissue. This paper presents a review of current progress in the designs of biomaterials and microenvironments and provides case studies in developing nerve system stents upon these biomaterials. In addition, we compose a conductive patterned compounded biomaterial, which could mimic neuronal microenvironment for neuroengineering by concentrating the advantage of such biomaterials.

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.

Laminin-chitosan-PLGA conduit co-transplanted with Schwann and neural stem cells to repair the injured recurrent laryngeal nerve

The aim of the present study was to assess the possibility and efficacy of utilizing a laminin-chitosan-poly (lactic-co-glycolic acid), otherwise known as laminin-chitosan-PLGA, nerve conduit with the co-transplantation of Schwann and neural stem cells to repair peripheral nerve defects. Previous in vitro experiments have demonstrated that the three-dimensional structure of the built in fiber filament electrospinning of laminin-chitosan-PLGA nerve conduit is beneficial to the migration and regeneration of nerve cells, and has notable mechanical strength and plasticity. It is able to provide support in the neural tissue regeneration process, and has the ability to degrade itself once peripheral nerves complete their regeneration, providing more advantages than other biological and synthetic materials. In the present study, 132 female Sprague Dawley rats were used to establish an animal model of laryngeal nerve injury, and the rats were randomly divided into six groups for experimentation. The nerve conduit was prepared and co-cultured with Schwann and neural stem cells, and micro-surgical techniques were used to repair the 5-mm-long recurrent laryngeal nerve injuries. Functional and histological assessments were performed at 8 and 12 weeks post-surgery, respectively. The results revealed that the laminin-chitosan-PLGA nerve conduit combined with Schwann and neural stem cells was able to promote nerve regeneration (P<0.05), and its effect was superior to those of the autograft (P<0.05). The results of the present study suggest that this is the ideal method for repairing peripheral nerve defects, and cells in the graft may promote nerve regeneration.

Bioactive Organic Rosette Nanotubes Support Sensory Neurite Outgrowth

Regardless of the intervention for peripheral nerve repair, slow rates of axonal regeneration often result in poor clinical outcomes. Thus, using new materials such as biologically inspired, biocompatible, organic rosette nanotubes (RNTs) could provide a tailorable scaffold to modulate neurite extension and attachment for improved nerve repair. RNTs are obtained through the spontaneous self-assembly of a synthetic DNA base analogue featuring the hydrogen bond triads of both guanine and cytosine, the G∧C base. Here, we investigated the potential of RNTs functionalized with lysine and Arg-Gly-Asp-Ser-Lys (RGDSK) peptide to support neural growth. We hypothesized that (a) due to their dimensions, the RNTs would support neuron attachment, and (b) their conjugation to the integrin-binding peptide RGDSK would further enhance neurite outgrowth compared to unfunctionalized RNT. Neurite extension was examined on a variety of RNT structures, including RNT with a lysine side chain (K1), a mixture of the K1 and a free RGDS peptide, RNT alone, an RGDSK-functionalized RNT, in addition to poly-d-lysine and laminin controls. Both whole dorsal root ganglion (DRG) and single dissociated DRG neurons were seeded onto RNT-coated substrates containing various ratios of peptides. Analysis of neuron morphometrics showed that RNT blends support DRG neuron attachment and neurite extension, with RGDS presentation increasing neurite outgrowth from whole DRG by up to 47% over a 7-day period compared to K1 alone (p < 0.013). In addition, while RNTs increased the sprouting of primary neurites extending from dissociated DRG neurons, the total neurite outgrowth per neuron remained the same. These results show that functionalized biomimetic RNTs provide a support for neurite growth and extension and have the ability to modulate neuronal morphology. These results also pave the way for the design of injectable RNT-based nanomaterials that support guided neural regeneration following traumatic injury.

Effects of nerve cells and adhesion molecules on nerve conduit for peripheral nerve regeneration

Background

For peripheral nerve regeneration, recent attentions have been paid to the nerve conduits made by tissue-engineering technique. Three major elements of tissue-engineering are cells, molecules, and scaffolds.

Methods

In this study, the attachments of nerve cells, including Schwann cells, on the nerve conduit and the effects of both growth factor and adhesion molecule on these attachments were investigated.

Results

The attachment of rapidly-proliferating cells, C6 cells and HS683 cells, on nerve conduit was better than that of slowly-proliferating cells, PC12 cells and Schwann cells, however, the treatment of nerve growth factor improved the attachment of slowly-proliferating cells. In addition, the attachment of Schwann cells on nerve conduit coated with fibronectin was as good as that of Schwann cells treated with glial cell line-derived neurotrophic factor (GDNF).

Conclusions

Growth factor changes nerve cell morphology and affects cell cycle time. And nerve growth factor or fibronectin treatment is indispensable for Schwann cell to be used for implantation in artificial nerve conduits.

First human experience with autologous Schwann cells to supplement sciatic nerve repair: report of 2 cases with long-term follow-up

OBJECTIVE Long-segment injuries to large peripheral nerves present a challenge to surgeons because insufficient donor tissue limits repair. Multiple supplemental approaches have been investigated, including the use of Schwann cells (SCs). The authors present the first 2 cases using autologous SCs to supplement a peripheral nerve graft repair in humans with long-term follow-up data. METHODS Two patients were enrolled in an FDA-approved trial to assess the safety of using expanded populations of autologous SCs to supplement the repair of long-segment injuries to the sciatic nerve. The mechanism of injury included a boat propeller and a gunshot wound. The SCs were obtained from both the sural nerve and damaged sciatic nerve stump. The SCs were expanded and purified in culture by using heregulin β1 and forskolin. Repair was performed with sural nerve grafts, SCs in suspension, and a Duragen graft to house the construct. Follow-up was 36 and 12 months for the patients in Cases 1 and 2, respectively. RESULTS The patient in Case 1 had a boat propeller injury with complete transection of both sciatic divisions at midthigh. The graft length was approximately 7.5 cm. In the postoperative period the patient regained motor function (Medical Research Council [MRC] Grade 5/5) in the tibial distribution, with partial function in peroneal distribution (MRC Grade 2/5 on dorsiflexion). Partial return of sensory function was also achieved, and neuropathic pain was completely resolved. The patient in Case 2 sustained a gunshot wound to the leg, with partial disruption of the tibial division of the sciatic nerve at the midthigh. The graft length was 5 cm. Postoperatively the patient regained complete motor function of the tibial nerve, with partial return of sensation. Long-term follow-up with both MRI and ultrasound demonstrated nerve graft continuity and the absence of tumor formation at the repair site. CONCLUSIONS Presented here are the first 2 cases in which autologous SCs were used to supplement human peripheral nerve repair in long-segment injury. Both patients had significant improvement in both motor and sensory function with correlative imaging. This study demonstrates preliminary safety and efficacy of SC transplantation for peripheral nerve repair.

Proteomic Profiling of Neuroblastoma Cells Adhesion on Hyaluronic Acid-Based Surface for Neural Tissue Engineering

The microenvironment of neuron cells plays a crucial role in regulating neural development and regeneration. Hyaluronic acid (HA) biomaterial has been applied in a wide range of medical and biological fields and plays important roles in neural regeneration. PC12 cells have been reported to be capable of endogenous NGF synthesis and secretion. The purpose of this research was to assess the effect of HA biomaterial combining with PC12 cells conditioned media (PC12 CM) in neural regeneration. Using SH-SY5Y cells as an experimental model, we found that supporting with PC12 CM enhanced HA function in SH-SY5Y cell proliferation and adhesion. Through RP-nano-UPLC-ESI-MS/MS analyses, we identified increased expression of HSP60 and RanBP2 in SH-SY5Y cells grown on HA-modified surface with cotreatment of PC12 CM. Moreover, we also identified factors that were secreted from PC12 cells and may promote SH-SY5Y cell proliferation and adhesion. Here, we proposed a biomaterial surface enriched with neurotrophic factors for nerve regeneration application.

The Use of Autologous Schwann Cells to Supplement Sciatic Nerve Repair With a Large Gap: First in Human Experience

Insufficient donor nerve graft material in peripheral nerve surgery remains an obstacle for successful long-distance regeneration. Schwann cells (SCs) can be isolated from adult mammalian peripheral nerve biopsies and can be grown in culture and retain their capacity to enhance peripheral nerve regeneration within tubular repair strategies in multiple animal models. Human Schwann cells (hSCs) can be isolated, expanded in number, and retain their ability to promote regeneration and myelinate axons, but have never been tested in a clinical case of peripheral nerve injury. A sural nerve biopsy and peripheral nerve tissue from the traumatized sciatic nerve stumps was obtained after Food and Drug Administration (FDA) and Institutional Review Board (IRB) approval as well as patient consent. The SCs were isolated after enzymatic digestion of the nerve and expanded with the use of heregulin β1 (0.1 µg/ml) and forskolin (15 mM). After two passages the Schwann cell isolates were combined with sural nerve grafts to repair a large sciatic nerve defect (7.5 cm) after a traumatic nerve injury. The sural nerve and the traumatized sciatic nerve ends both served as an excellent source of purified (90% and 97%, respectively) hSCs. Using ultrasound and magnetic resonance imaging (MRI) we were able to determine continuity of the nerve graft repair and the absence of tumor formation. The patient had evidence of proximal sensory recovery and definitive motor recovery distal to the repair in the distribution of the tibial and common peroneal nerve. The patient did experience an improvement in her pain scores over time. The goals of this approach were to determine the safety and clinical feasibility of implementing a new cellular repair strategy. In summary, this approach represents a novel strategy in the treatment of peripheral nerve injury and represents the first reported use of autologous cultured SCs after human peripheral nerve injury.

Ribosomal trafficking is reduced in Schwann cells following induction of myelination

Local synthesis of proteins within the Schwann cell periphery is extremely important for efficient process extension and myelination, when cells undergo dramatic changes in polarity and geometry. Still, it is unclear how ribosomal distributions are developed and maintained within Schwann cell projections to sustain local translation. In this multi-disciplinary study, we expressed a plasmid encoding a fluorescently labeled ribosomal subunit (L4-GFP) in cultured primary rat Schwann cells. This enabled the generation of high-resolution, quantitative data on ribosomal distributions and trafficking dynamics within Schwann cells during early stages of myelination, induced by ascorbic acid treatment. Ribosomes were distributed throughout Schwann cell projections, with ~2-3 bright clusters along each projection. Clusters emerged within 1 day of culture and were maintained throughout early stages of myelination. Three days after induction of myelination, net ribosomal movement remained anterograde (directed away from the Schwann cell body), but ribosomal velocity decreased to about half the levels of the untreated group. Statistical and modeling analysis provided additional insight into key factors underlying ribosomal trafficking. Multiple regression analysis indicated that net transport at early time points was dependent on anterograde velocity, but shifted to dependence on anterograde duration at later time points. A simple, data-driven rate kinetics model suggested that the observed decrease in net ribosomal movement was primarily dictated by an increased conversion of anterograde particles to stationary particles, rather than changes in other directional parameters. These results reveal the strength of a combined experimental and theoretical approach in examining protein localization and transport, and provide evidence of an early establishment of ribosomal populations within Schwann cell projections with a reduction in trafficking following initiation of myelination.

Chitosan conduits combined with nerve growth factor microspheres repair facial nerve defects

Microspheres containing nerve growth factor for sustained release were prepared by a compound method, and implanted into chitosan conduits to repair 10-mm defects on the right buccal branches of the facial nerve in rabbits. In addition, chitosan conduits combined with nerve growth factor or normal saline, as well as autologous nerve, were used as controls. At 90 days post-surgery, the muscular atrophy on the right upper lip was more evident in the nerve growth factor and normal sa-line groups than in the nerve growth factor-microspheres and autologous nerve groups. physiological analysis revealed that the nerve conduction velocity and amplitude were significantly higher in the nerve growth factor-microspheres and autologous nerve groups than in the nerve growth factor and normal saline groups. Moreover, histological observation illustrated that the di-ameter, number, alignment and myelin sheath thickness of myelinated nerves derived from rabbits were higher in the nerve growth factor-microspheres and autologous nerve groups than in the nerve growth factor and normal saline groups. These findings indicate that chitosan nerve conduits bined with microspheres for sustained release of nerve growth factor can significantly improve facial nerve defect repair in rabbits.

Carriers in cell-based therapies for neurological disorders

There is a pressing need for long-term neuroprotective and neuroregenerative therapies to promote full function recovery of injuries in the human nervous system resulting from trauma, stroke or degenerative diseases. Although cell-based therapies are promising in supporting repair and regeneration, direct introduction to the injury site is plagued by problems such as low transplanted cell survival rate, limited graft integration, immunorejection, and tumor formation. Neural tissue engineering offers an integrative and multifaceted approach to tackle these complex neurological disorders. Synergistic therapeutic effects can be obtained from combining customized biomaterial scaffolds with cell-based therapies. Current scaffold-facilitated cell transplantation strategies aim to achieve structural and functional rescue via offering a three-dimensional permissive and instructive environment for sustainable neuroactive factor production for prolonged periods and/or cell replacement at the target site. In this review, we intend to highlight important considerations in biomaterial selection and to review major biodegradable or non-biodegradable scaffolds used for cell transplantation to the central and peripheral nervous system in preclinical and clinical trials. Expanded knowledge in biomaterial properties and their prolonged interaction with transplanted and host cells have greatly expanded the possibilities for designing suitable carrier systems and the potential of cell therapies in the nervous system.

Olfactory ensheathing cells seeded muscle-stuffed vein as nerve conduit for peripheral nerve repair: a nerve conduction study

We evaluated bridging of 15 mm nerve gap in rat sciatic nerve injury model with muscle-stuffed vein seeded with olfactory ensheathing cells as a substitute for nerve autograft. Neurophysiological recovery, as assessed by electrophysiological analysis was faster in the constructed biological nerve conduit compared to that of autograft.

Enhanced femoral nerve regeneration after tubulization with a tyrosine-derived polycarbonate terpolymer: effects of protein adsorption and independence of conduit porosity

Following complete nerve transection, entubulation of the nerve stumps helps guide axons to reconnect distally. In this study, a biodegradable and noncytotoxic tyrosine-derived polycarbonate terpolymer composed of 89.5 mol% desaminotyrosyl tyrosine ethyl ester (DTE), 10 mol% desaminotyrosyl tyrosine (DT), and 0.5 mol% poly(ethylene glycol) (PEG, molecular weight [Mw]=1 kDa) [designated as E10-0.5(1K)] was used to fabricate conduits for peripheral nerve regeneration. These conduits were evaluated against commercially available nonporous polyethylene (PE) tubes. The two materials are characterized in vitro for differences in surface properties, and the conduits are then evaluated in vivo in a critical-sized nerve defect in the mouse femoral nerve model. Conduits were fabricated from E10-0.5(1K) in both porous [P-E10-0.5(1K)] and nonporous [NP-E10-0.5(1K)] configurations. The results illustrate that adsorption of laminin, fibronectin, and collagen type I was enhanced on E10-0.5(1K) compared to PE. In addition, in vivo the E10-0.5(1K) conduits improved functional recovery over PE conduits, producing regenerated nerves with a fivefold increase in the number of axons, and an eightfold increase in the percentage of myelinated axons. These increases were observed for both P-E10-0.5(1K) and NP-E10-0.5(1K) after 15 weeks. When conduits were removed at 7 or 14 days following implantation, an increase in Schwann cell proteins and fibrin matrix formation was observed in E10-0.5(1K) conduits over PE conduits. These results indicate that E10-0.5(1K) is a pro-regenerative material for peripheral nerves and that the porosity of P-E10-0.5(1K) conduits was inconsequential in this model of nerve injury.

Fabrication and characterization of biomimetic multichanneled crosslinked-urethane-doped polyester tissue engineered nerve guides

Biomimetic scaffolds that replicate the native architecture and mechanical properties of target tissues have been recently shown to be a very promising strategy to guide cellular growth and facilitate tissue regeneration. In this study, porous, soft, and elastic crosslinked urethane-doped polyester (CUPE) tissue engineered nerve guides were fabricated with multiple longitudinally oriented channels and an external non-porous sheath to mimic the native endoneurial microtubular and epineurium structure, respectively. The fabrication technique described herein is highly adaptable and allows for fine control over the resulting nerve guide architecture in terms of channel number, channel diameter, porosity, and mechanical properties. Biomimetic multichanneled CUPE guides were fabricated with various channel numbers and displayed an ultimate peak stress of 1.38 ± 0.22 MPa with a corresponding elongation at break of 122.76 ± 42.17%, which were comparable to that of native nerve tissue. The CUPE nerve guides were also evaluated in vivo for the repair of a 1 cm rat sciatic nerve defect. Although histological evaluations revealed collapse of the inner structure from CUPE TENGs, the CUPE nerve guides displayed fiber populations and densities comparable with nerve autograft controls after 8 weeks of implantation. These studies are the first report of a CUPE-based biomimetic multichanneled nerve guide and warrant future studies towards optimization of the channel geometry for use in neural tissue engineering.

Sciatic nerve repair with tissue engineered nerve: Olfactory ensheathing cells seeded poly(lactic-co-glygolic acid) conduit in an animal model

BACKGROUND AND AIM

Synthetic nerve conduits have been sought for repair of nerve defects as the autologous nerve grafts causes donor site morbidity and possess other drawbacks. Many strategies have been investigated to improve nerve regeneration through synthetic nerve guided conduits. Olfactory ensheathing cells (OECs) that share both Schwann cell and astrocytic characteristics have been shown to promote axonal regeneration after transplantation. The present study was driven by the hypothesis that tissue-engineered poly(lactic-co-glycolic acid) (PLGA) seeded with OECs would improve peripheral nerve regeneration in a long sciatic nerve defect.

MATERIALS AND METHODS

Sciatic nerve gap of 15 mm was created in six adult female Sprague-Dawley rats and implanted with PLGA seeded with OECs. The nerve regeneration was assessed electrophysiologically at 2, 4 and 6 weeks following implantation. Histopathological examination, scanning electron microscopic (SEM) examination and immunohistochemical analysis were performed at the end of the study.

RESULTS

Nerve conduction studies revealed a significant improvement of nerve conduction velocities whereby the mean nerve conduction velocity increases from 4.2 ΁ 0.4 m/s at week 2 to 27.3 ΁ 5.7 m/s at week 6 post-implantation (P < 0.0001). Histological analysis revealed presence of spindle-shaped cells. Immunohistochemical analysis further demonstrated the expression of S100 protein in both cell nucleus and the cytoplasm in these cells, hence confirming their Schwann-cell-like property. Under SEM, these cells were found to be actively secreting extracellular matrix.

CONCLUSION

Tissue-engineered PLGA conduit seeded with OECs provided a permissive environment to facilitate nerve regeneration in a small animal model.

Comparison of Schwann cells and olfactory ensheathing cells for peripheral nerve gap bridging

INTRODUCTION

Previously, we introduced the biogenic conduit (BC) as a novel autologous nerve conduit for bridging peripheral nerve defects and tested its regenerative capacity in a short- and long-term setting. The aim of the present study was to clarify whether intraluminal application of regeneration-promoting glial cells, including Schwann cells (SC) and olfactory ensheathing cells (OEC), displayed differential effects after sciatic nerve gap bridging.

MATERIAL AND METHODS

BCs were generated as previously described. The conduits filled with fibrin/SC (n = 8) and fibrin/OEC (n = 8) were compared to autologous nerve transplants (NT; n = 8) in the 15-mm sciatic nerve gap lesion model of the rat. The sciatic functional index was evaluated every 4 weeks. After 16 weeks, histological evaluation followed regarding nerve area, axon number, myelination index and N ratio.

RESULTS

Common to all groups was a continual improvement in motor function during the observation period. Recovery was significantly better after SC transplantation compared to OEC (p < 0.01). Both cell transplantation groups showed significantly worse function than the NT group (p < 0.01). Whereas nerve area and axon number were correlated to function, being significantly lowest in the OEC group (p < 0.001), both cell groups showed lowered myelination (p < 0.001) and lower N ratio compared to the NT group.

DISCUSSION

SC-filled BCs led to improved regeneration compared to OEC-filled BCs in a 15-mm-long nerve gap model of the rat.

Spatiotemporal expression profiling of proteins in rat sciatic nerve regeneration using reverse phase protein arrays

Background

Protein expression profiles throughout 28 days of peripheral nerve regeneration were characterized using an established rat sciatic nerve transection injury model. Reverse phase protein microarrays were used to identify the spatial and temporal expression profile of multiple proteins implicated in peripheral nerve regeneration including growth factors, extracellular matrix proteins, and proteins involved in adhesion and migration. This high-throughput approach enabled the simultaneous analysis of 3,360 samples on a nitrocellulose-coated slide.

Results

The extracellular matrix proteins collagen I and III, laminin gamma-1, fibronectin, nidogen and versican displayed an early increase in protein levels in the guide and proximal sections of the regenerating nerve with levels at or above the baseline expression of intact nerve by the end of the 28 day experimental course. The 28 day protein levels were also at or above baseline in the distal segment however an early increase was only noted for laminin, nidogen, and fibronectin. While the level of epidermal growth factor, ciliary neurotrophic factor and fibroblast growth factor-1 and -2 increased throughout the experimental course in the proximal and distal segments, nerve growth factor only increased in the distal segment and fibroblast growth factor-1 and -2 and nerve growth factor were the only proteins in that group to show an early increase in the guide contents. As expected, several proteins involved in cell adhesion and motility; namely focal adhesion kinase, N-cadherin and β-catenin increased earlier in the proximal and distal segments than in the guide contents reflecting the relatively acellular matrix of the early regenerate.

Conclusions

In this study we identified changes in expression of multiple proteins over time linked to regeneration of the rat sciatic nerve both demonstrating the utility of reverse phase protein arrays in nerve regeneration research and revealing a detailed, composite spatiotemporal expression profile of peripheral nerve regeneration.

Allotransplanted neurons used to repair peripheral nerve injury do not elicit overt immunogenicity

A major problem hindering the development of autograft alternatives for repairing peripheral nerve injuries is immunogenicity. We have previously shown successful regeneration in transected rat sciatic nerves using conduits filled with allogeneic dorsal root ganglion (DRG) cells without any immunosuppression. In this study, we re-examined the immunogenicity of our DRG neuron implanted conduits as a potential strategy to overcome transplant rejection. A biodegradable NeuraGen® tube was infused with pure DRG neurons or Schwann cells cultured from a rat strain differing from the host rats and used to repair 8 mm gaps in the sciatic nerve. We observed enhanced regeneration with allogeneic cells compared to empty conduits 16 weeks post-surgery, but morphological analyses suggest recovery comparable to the healthy nerves was not achieved. The degree of regeneration was indistinguishable between DRG and Schwann cell allografts although immunogenicity assessments revealed substantially increased presence of Interferon gamma (IFN-γ) in Schwann cell allografts compared to the DRG allografts by two weeks post-surgery. Macrophage infiltration of the regenerated nerve graft in the DRG group 16 weeks post-surgery was below the level of the empty conduit (0.56 fold change from NG; p<0.05) while the Schwann cell group revealed significantly higher counts (1.29 fold change from NG; p<0.001). Major histocompatibility complex I (MHC I) molecules were present in significantly increased levels in the DRG and Schwann cell allograft groups compared to the hollow NG conduit and the Sham healthy nerve. Our results confirmed previous studies that have reported Schwann cells as being immunogenic, likely due to MHC I expression. Nerve gap injuries are difficult to repair; our data suggest that DRG neurons are superior medium to implant inside conduit tubes due to reduced immunogenicity and represent a potential treatment strategy that could be preferable to the current gold standard of autologous nerve transplant.

Next generation nerve guides: materials, fabrication, growth factors, and cell delivery

Nerve guides are increasingly being used surgically to repair acute peripheral nerve injuries. This is not only due to an increase in the number of commercially available devices, but also clinical acceptance. However, regeneration distance is typically limited to 20-25 mm, in part due to the basic tubular design. A number of experimental studies have shown improvements in nerve regeneration distance when conduits incorporate coatings, internal scaffolds, topographical cues, or the delivery of support cells. Current studies on designing nerve guides for maximizing nerve regeneration focus both on cell-containing and cell-free devices, the latter being clinically attractive as "off the shelf" products. Arguably better results are obtained when conduits are used in conjunction with support cells (e.g., Schwann cells or stem cells) that can improve regeneration distance and speed of repair, and provide informative experimental data on how Schwann and neuronal cells respond in regenerating injured nerves. In this review we discuss the range of current nerve guides commercially available and appraise experimental studies in the context of the future design of nerve guides for clinical use.

Repairing injured peripheral nerves: Bridging the gap

Peripheral nerve injuries that induce gaps larger than 1-2 cm require bridging strategies for repair. Autologous nerve grafts are still the gold standard for such interventions, although alternative treatments, as well as treatments to improve the therapeutic efficacy of autologous nerve grafting are generating increasing interest. Investigations are still mostly experimental, although some clinical studies have been undertaken. In this review, we aim to describe the developments in bridging technology which aim to replace the autograft. A multi-disciplinary approach is of utmost importance to develop and optimise treatments of the most challenging peripheral nerve injuries.

Effects of nerve growth factor from genipin-crosslinked gelatin in polycaprolactone conduit on peripheral nerve regeneration--in vitro and in vivo

The gelatin solution crosslinked by genipin (0, 0.1, 0.5, 1.0, and 1.5% w/w) was studied as a nerve growth factor (NGF) carrier (GGp0, GGp0.1, GGp0.5, GGp1.0, and GGp1.5) in a polycaprolactone conduit in large-gap nerve regeneration. The GGp0 and GGp0.1 displayed the highest activity of PC12 cells and inhibited the reduction of 3-(4,5-dimethylthiazol-2-yl)2,5-diphenyl-tetrazolium bromide (MTT). No cytotoxicity was found in all groups by lactate dehydrogenase (LDH) release. The NGF-releasing characters were obtained by ELISA tests. A relatively fast release rate appeared during the first 10 days and then a subsequent slower release profile followed. NGF was higher in GGp0.1 than in GGp0 and GGp0.1 after 10 days. The bioactivity of the released NGF remains the same when measuring the neurite outgrowth of PC 12 cells. Finally, the controlled-release conduits were implanted into 12-mm long sciatic nerve gaps of rats. In addition, the best site of NGF carrier was determined either by filling carrier into the conduit lumen or by sucking carrier to the conduit wall. Four and 8 weeks after implantation, morphological analysis revealed that GGp0.1 conduits had markedly larger and more number of myelin axons in the midconduit and distal nerve. Further, sucking the carrier into the conduit wall was an efficient and convenient way to prevent the regeneration of axons and vessels from being impaired by the lumen's carrier. The genipin-crosslinked gelatin is a promising carrier in producing a high release concentration and a long release period of NGF to promote the regeneration over a large-gap nerve injury.

Functional recovery after implantation of artificial nerve grafts in the rat- a systematic review

Purpose

The aim of this study was to compare functional data of different nerve-gap bridging materials evaluated in rat experiments by means of a systematic review.

Materials and methods

A systematic review was conducted, searching MEDLINE, HTS and CENTRAL to identify all trials evaluating functional recovery of artificial nerve conduits in the rat model.

Results

There was a trend towards a favourable outcome of conduits coated with Schwann-cells compared to the plain synthetics. Histomorphometry, electrophysiology and muscle-weight correlated poorly with functional outcome.

Conclusion

Schwann-cell coated conduits showed promising results concerning functional recovery. Further standardization in outcome reporting is encouraged.

Current applications and future perspectives of artificial nerve conduits

Artificial nerve guide conduits have the advantage over autografts in terms of their availability and ease of fabrication. However, clinical outcomes associated with the use of artificial nerve conduits are often inferior to that of autografts, particularly over long lesion gaps. There have been significant advances in the designs of artificial nerve conduits over the years. In terms of materials selection and design, a wide variety of new synthetic polymers and biopolymers have been evaluated. The inclusion of nerve conduit lumen fillers has also been demonstrated as essential to enable nerve regeneration across large defect gaps. These lumen filler designs have involved the integration of physical cues for contact guidance and biochemical signals to control cellular function and differentiation. Novel conduit architectural designs using porous and fibrous substrates have also been developed. This review highlights the recent advances in synthetic nerve guide designs for peripheral nerve regeneration, and the in vivo applicability and future prospects of these nerve guide conduits.

Composite PHB-GGF conduit for long nerve gap repair: A long-term evaluation

Two to four cm nerve gaps in the rabbit common peroneal nerve were bridged with poly-3-hydroxybutyrate (PHB) conduits containing either glial growth factor (GGF) (PHB-GGF) or alginate matrix (PHB-ALG), and with empty PHB conduit (E-PHB). PHB-GGF significantly increased nerve regeneration up to 63 days following repair of long nerve gaps and the regeneration was sustained long term leading to motor organ reinnervation. At 120 days postoperatively, GGF addition significantly increased the quantity of Schwann cell and axonal regeneration compared to those in control conduits. In PHB-GGF conduits there were more minifascicles of myelinated fibres compared to the controls. The distal nerve of PHB-GGF and E-PHB conduits showed greater regeneration than that of PHB-ALG grafts, although all distal nerves contained fewer myelinated fibres than grafted conduits. Consistently, PHB-GGF conduits significantly reduced the muscle mass percentage loss compared to controls. In conclusion, GGF-containing conduits promoted sustained axonal regeneration and improved target muscle reinnervation.

Nerve repair: experimental and clinical evaluation of biodegradable artificial nerve guides

Several methods have been used for bridging nerve gaps. Much of the focus in nerve repair of peripheral nerves has focussed on creating either natural or synthetic tubular nerve guidance channels, as an alternative to nerve autografts. These conduits act to guide axons sprouting from the regenerating nerve end, provide a conduit for diffusion of neurotrophic and neurotropic factors secreted by the injured nerve stump, as well as help protect against infiltration of fibrous tissue. Among the conduits that have been studied are autogenous veins, arteries, mesothelial chambers, synthetic tubes, collagen tubes, amnion tubes, cardiac and skeletal muscle, and silicon tubes. This paper briefly reviews major studies in which bioabsorbable nerve guides were used for peripheral nerve repair, with a particular emphasis on polymeric guidance channels, in an effort to evaluate their use, their ability to support or enhance nerve regeneration and any potential problems.

Electrospun micro- and nanofiber tubes for functional nervous regeneration in sciatic nerve transections

Background

Although many nerve prostheses have been proposed in recent years, in the case of consistent loss of nervous tissue peripheral nerve injury is still a traumatic pathology that may impair patient's movements by interrupting his motor-sensory pathways. In the last few decades tissue engineering has opened the door to new approaches;: however most of them make use of rigid channel guides that may cause cell loss due to the lack of physiological local stresses exerted over the nervous tissue during patient's movement. Electrospinning technique makes it possible to spin microfiber and nanofiber flexible tubular scaffolds composed of a number of natural and synthetic components, showing high porosity and remarkable surface/volume ratio.

Results

In this study we used electrospun tubes made of biodegradable polymers (a blend of PLGA/PCL) to regenerate a 10-mm nerve gap in a rat sciatic nerve in vivo. Experimental groups comprise lesioned animals (control group) and lesioned animals subjected to guide conduits implantated at the severed nerve stumps, where the tubular scaffolds are filled with saline solution. Four months after surgery, sciatic nerves failed to reconnect the two stumps of transected nerves in the control animal group. In most of the treated animals the electrospun tubes induced nervous regeneration and functional reconnection of the two severed sciatic nerve tracts. Myelination and collagen IV deposition have been detected in concurrence with regenerated fibers. No significant inflammatory response has been found. Neural tracers revealed the re-establishment of functional neuronal connections and evoked potential results showed the reinnervation of the target muscles in the majority of the treated animals.

Conclusion

Corroborating previous works, this study indicates that electrospun tubes, with no additional biological coating or drug loading treatment, are promising scaffolds for functional nervous regeneration. They can be knitted in meshes and various frames depending on the cytoarchitecture of the tissue to be regenerated. The versatility of this technique gives room for further scaffold improvements, like tuning the mechanical properties of the tubular structure or providing biomimetic functionalization. Moreover, these guidance conduits can be loaded with various fillers like collagen, fibrin, or self-assembling peptide gels or loaded with neurotrophic factors and seeded with cells. Electrospun scaffolds can also be synthesized in different micro-architectures to regenerate lesions in other tissues like skin and bone.

Obstetrical brachial plexus palsy

Obstetrical brachial plexus palsy, one of the most complex peripheral nerve injuries, presents as an injury during the neonatal period. The majority of the children recover with either no deficit or a minor functional deficit, but it is almost certain that some will not regain adequate limb function. These few cases must be managed in an optimal way. Considerable medical and legal debate has surrounded the etiologic factors of this traumatic lesion, and obstetricians are often considered responsible for the injury. According to recent studies, spontaneous endogenous forces may contribute substantially to this type of neonatal trauma. All obstetric circumstances that predispose to brachial plexus damage and that could be anticipated should be assessed. Correct diagnosis is necessary for the accurate estimation of prognosis and treatment. The most important aspect of therapy is timely recognition and referral, to prevent the various possible sequelae affecting the shoulder, elbow, or forearm. Since the early 1990s, research has increased the understanding of obstetrical brachial plexus palsy. Further research is needed, focused on developing strategies to predict brachial injury. This review focuses on emerging data relating to obstetrical brachial plexus palsy and discusses the present controversies regarding natural history, prognosis, and treatment in infants with brachial plexus birth palsies.

A comparison of polyglycolic acid versus type 1 collagen bioabsorbable nerve conduits in a rat model: an alternative to autografting

PURPOSE

Severe nerve injury with segmental loss requires nerve graft or conduit repair. We compared 2 synthetic, bioabsorbable nerve conduits with the gold standard of autogenous nerve grafting using histopathologic and neurophysiologic analyses.

METHODS

A 10-mm segment of the sciatic nerve of 45 Sprague-Dawley rats was resected, leaving a gap defect. Three experimental groups were used: 15 coaptations using type I collagen nerve conduits, 15 coaptations using polyglycolic acid (PGA) nerve conduits, and 15 coaptations using the excised segments as autogenous nerve grafts. The contralateral legs were used as unoperated controls. After 15 weeks, nerve regeneration was evaluated by measuring isometric muscle contraction force, axonal counting, wet muscle weights, and histology.

RESULTS

Statistically significant differences in the isometric muscle contraction force, axonal counts, and wet muscle weights were found between type I collagen conduit and nerve graft compared to the PGA conduit. Axonal sprouting was less organized and less dense with the PGA conduits when compared to nerve reconstruction with the type I collagen conduits and nerve grafts.

CONCLUSIONS

Type I collagen conduits and autografts produced comparable results, which were significantly better than PGA conduits. The use of type I collagen conduit is a reliable alternative to nerve grafting for gaps up to 10 mm in length.

Experience with nerve allograft transplantation

Nerve allograft transplantation should be used for the repair of devastating peripheral nerve injuries that cannot be reconstructed through traditional means such as autologous nerve grafting or nerve transfer procedures. The risks of required systemic immunosuppression, although only temporary for nerve allograft recipients, preclude widespread use of this treatment modality. Translational research has led to several advancements in this field including the use of preoperative allograft cold preservation in University of Wisconsin organ preservation solution and inclusion of tacrolimus as part of the immunosuppressive regimen. Investigation of how to further diminish nerve allograft immunogenicity, speed neuroregeneration by use of agents such as tacrolimus, and promote preferential motor regeneration will further advance this field with the goal of restoring optimal function while minimizing patient morbidity.