The role of conduits in nerve repair: a review

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

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

Evaluation of Processed Nerve Allograft in Peripheral Nerve Surgery: A Systematic Review and Critical Appraisal

Peripheral nerve injuries cause substantial problems when not treated properly. A specific problem is reconstruction of nerve defects, which can be treated in different ways. This study aimed to systematically review whether processed nerve allograft (PNA) is justified in reconstruction of a nerve defect in patients after posttraumatic or iatrogenic peripheral nerve injury and to compare PNA with other established methods.

Methods

A systematic review with a focused question, PICO (patient, intervention, comparison, outcome) and constraints, was performed. A structured literature search, including several databases, was done to evaluate the existing evidence for outcomes and postoperative complications related to PNA. The certainty of evidence was classified according to Grading of Recommendations, Assessment, Development and Evaluations.

Results

No conclusions, concerning differences in outcome of nerve reconstruction using PNA compared with the use of nerve autograft or conduits, could be drawn. The level of certainty for all evaluated outcomes was very low (⊕◯◯◯). Most published studies lack a control group to patients treated with PNA; being only descriptive, making it difficult to compare PNA with established methods without substantial risk of bias. For studies including a control group, the scientific evidence was of very low certainty, due to a low number of included patients, and large, undefined loss of patients during follow-up, rendering a high risk of bias. Finally, the authors often had financial disclosures.

Conclusion

Properly conducted randomized controlled trial studies on the use of PNA in reconstruction of peripheral nerve injuries are needed to establish recommendations in clinical practice.

Conducting Composite Material Based on Chitosan and Single-Wall Carbon Nanotubes for Cellular Technologies

Biocompatible electrically conducting chitosan-based films filled with single-wall carbon nanotubes were obtained. Atomic force microscopic studies of the free surface topography revealed a change in the morphology of chitosan films filled with single-wall carbon nanotubes. Introducing 0.5 wt.% of single-wall carbon nanotubes into chitosan results in an increase in tensile strength of the films (up to ~180 MPa); the tensile strain values also rise up to ~60%. It was demonstrated that chitosan films containing 0.1–3.0 wt.% of single-wall carbon nanotubes have higher conductivity (10 S/m) than pure chitosan films (10⁻¹¹ S/m). The investigation of electrical stimulation of human dermal fibroblasts on chitosan/single-wall carbon nanotubes film scaffolds showed that the biological effect of cell electrical stimulation depends on the content of single-walled carbon nanotubes in the chitosan matrix.

Mini review: Biomaterials in repair and regeneration of nerve in a volumetric muscle loss

Volumetric muscle loss (VML) following a severe trauma or injury is beyond the intrinsic regenerative capacity of muscle tissues, and hence interventional therapy is required. Extensive muscle loss concomitant with damage to neuromuscular components overwhelms the muscles' remarkable regenerative capacity. The loss of nervous and vascular tissue leads to further damage and atrophy, so a combined treatment for neuromuscular junction (NMJ) along with the volumetric muscle regeneration is important. There have been immense advances in the field of tissue engineering for skeletal muscle tissue and peripheral nerve regeneration, but very few address the interdependence of the tissues and the need for combined therapies to repair and regenerate fully functional muscle tissue. This review addresses the problem and presents an overview of the biomaterials that have been studied for tissue engineering of neuromuscular tissues associated with skeletal muscles.

Botulinum Toxin and Neuronal Regeneration after Traumatic Injury of Central and Peripheral Nervous System

Botulinum neurotoxins (BoNTs) are toxins produced by the bacteria Clostridiumbotulinum, the causing agent for botulism, in different serotypes, seven of which (A-G) are well characterized, while others, such as H or FA, are still debated. BoNTs exert their action by blocking SNARE (soluble N-ethylmale-imide-sensitive factor-attachment protein receptors) complex formation and vesicle release from the neuronal terminal through the specific cleavage of SNARE proteins. The action of BoNTs at the neuromuscular junction has been extensively investigated and knowledge gained in this field has set the foundation for the use of these toxins in a variety of human pathologies characterized by excessive muscle contractions. In parallel, BoNTs became a cosmetic drug due to its power to ward off facial wrinkles following the activity of the mimic muscles. Successively, BoNTs became therapeutic agents that have proven to be successful in the treatment of different neurological disorders, with new indications emerging or being approved each year. In particular, BoNT/A became the treatment of excellence not only for muscle hyperactivity conditions, such as dystonia and spasticity, but also to reduce pain in a series of painful states, such as neuropathic pain, lumbar and myofascial pain, and to treat various dysfunctions of the urinary bladder. This review summarizes recent experimental findings on the potential efficacy of BoNTs in favoring nerve regeneration after traumatic injury in the peripheral nervous system, such as the injury of peripheral nerves, like sciatic nerve, and in the central nervous system, such as spinal cord injury.

Recovery of sensory function after the implantation of oriented-collagen tube into the resected rat sciatic nerve

Introduction

In the present study, we examined the effect of oriented collagen tube (OCT) implantation on the recovery of sensory function of the resected rat sciatic nerve.

Materials and methods

After a 10-mm long portion of the sciatic nerve of a rat was resected, an OCT was placed in the site of nerve defect. Recovery of the sensory function was evaluated using Von Frey test every 3 days after surgery. The regenerated tissue were histologically and ultrastructurally analyzed 2 and 4 weeks after the surgery.

Results

The sensory reflexes of the OCT group were restored to the level of that of the intact group after 15 days. Hematoxylin and eosin staining revealed the cross-linking between the proximal and distal stumps after 2 weeks. After 4 weeks, Luxol Fast Blue and immunohistochemical staining revealed the presence of myelin sheath from the proximal to distal region of the regenerated tissue and S100B staining confirmed the presence of Schwann cells. Interestingly, no myelin sheath was ultrastructurally observed around the regenerated axons at the central region after 2 weeks.

Conclusions

These results suggest that OCTs facilitate the recovery of sensory function. Additionally, the non-myelinated axons contributed to the recovery of the sensory function.

•

Von Frey test results in the OCT group on POD 15 were comparable at the sham group.

•

OCT group showed regeneration of unmyelinated axons in 2 weeks.

•

Myelination was observed from proximal to distal after 4 weeks OCT implantation.

•

In the OCT group, a large number of blood vessels were observed in nerve in 2 weeks.

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.

Synergistic Effects of Acetyl-l-Carnitine and Adipose-Derived Stromal Cells on Improving Regenerative Capacity of Acellular Nerve Allograft in Sciatic Nerve Defect

The combination of decellularized nerve allograft and adipose-derived stromal cells (ASCs) represents a good alternative to nerve autograft for bridging peripheral nerve defects by providing physical guidance and biologic cues. However, the regeneration outcome of acellular nerve allograft (ANA) is often inferior to autograft. Therefore, we hypothesized that acetyl-l-carnitine (ALCAR) treatment and implantation of ASC-embedded ANA would work synergistically to promote nerve regeneration. Seventy rats were randomly allocated into seven experimental groups (n = 10), including the healthy control group, sham surgery group, autograft group, ANA group, ANA + ASCs group, ANA + ALCAR group (50 mg/kg for 2 weeks), and ANA + ASCs + ALCAR (50 mg/kg for 2 weeks) group. All grafts were implanted to bridge long-gap (10-mm) sciatic nerve defects. Functional, electrophysiological, and morphologic analysis was conducted during the experimental period. We found that ALCAR potentiated the survival and retention of transplanted ASCs and upregulated the expression of neurotrophic factor mRNAs in transplanted grafts. Sixteen weeks following implantation in the rat, the ANA supplemented by ASCs was capable of supporting reinnervation across a 10-mm sciatic nerve gap, with results close to that of the autografts in terms of functional, electrophysiological, and histologic assessments. Results demonstrated that ALCAR treatment improved regenerative effects of ANA combined with ASCs on reconstruction of a 10-mm sciatic nerve defect in rat comparable to those of autograft.

Tunable Enzymatically Cross-Linked Silk Fibroin Tubular Conduits for Guided Tissue Regeneration

Hollow tubular conduits (TCs) with tunable architecture and biological properties are in great need for modulating cell functions and drug delivery in guided tissue regeneration. Here, a new methodology to produce enzymatically cross-linked silk fibroin TCs is described, which takes advantage of the tyrosine groups present in silk structure that are known to allow the formation of a covalently cross-linked hydrogel. Three different processing methods are used as a final step to modulate the properties of the silk-based TCs. This approach allows to virtually adjust any characteristic of the final TCs. The final microstructure ranges from a nonporous to a highly porous network, allowing the TCs to be selectively porous to 4 kDa molecules, but not to human skin fibroblasts. Mechanical properties are dependent both on the processing method and thickness of the TCs. Bioactivity is observed after 30 days of immersion in simulated body fluid only for the TCs submitted to a drying processing method (50 °C). The in vivo study performed in mice demonstrates the good biocompatibility of the TCs. The enzymatically cross-linked silk fibroin TCs are versatile and have adjustable characteristics that can be exploited in a variety of biomedical applications, particularly in guidance of peripheral nerve regeneration.

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

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

A novel conduit-based coaptation device for primary nerve repair

BACKGROUND

Conduit-based nerve repairs are commonly used for small nerve gaps, whereas primary repair may be performed if there is no tension on nerve endings. We hypothesize that a conduit-based nerve coaptation device will improve nerve repair outcomes by avoiding sutures at the nerve repair site and utilizing the advantages of a conduit-based repair.

METHODS

The left sciatic nerves of female Sprague-Dawley rats were transected and repaired using a novel conduit-based device. The conduit-based device group was compared to a control group of rats that underwent a standard end-to-end microsurgical repair of the sciatic nerve. Animals underwent behavioral assessments at weekly intervals post-operatively using the sciatic functional index (SFI) test. Animals were sacrificed at four weeks to obtain motor axon counts from immunohistochemistry. A sub-group of animals were sacrificed immediately post repair to obtain MRI images.

RESULTS

SFI scores were superior in rats which received conduit-based repairs compared to the control group. Motor axon counts distal to the injury in the device group at four weeks were statistically superior to the control group. MRI tractography was used to demonstrate repair of two nerves using the novel conduit device.

CONCLUSIONS

A conduit-based nerve coaptation device avoids sutures at the nerve repair site and leads to improved outcomes in a rat model. Conduit-based nerve repair devices have the potential to standardize nerve repairs while improving outcomes.

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

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

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

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

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

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

Preparation and biocompatibility of electrospinning PDLLA/β-TCP/collagen for peripheral nerve regeneration

A unique nerve conduit composed of poly(d,l-lactic acid) (PDLLA), β-tricalcium phosphate (β-TCP) and collagen was prepared by electrospinning for the first time.

Experimental immunological demyelination enhances regeneration in autograft-repaired long peripheral nerve gaps

Peripheral nerve long gap defects are a clinical challenge in the regeneration field. Despite the wide variety of surgical techniques and therapies, autografting is the "gold standard" for peripheral nerve gap reconstruction. The pathological process of Wallerian degeneration from the time of acute injury to efficient regeneration requires several weeks. Regeneration time is critical for nerve reconstruction. Immunological demyelination induced by anti-galactocerebroside antibodies plus guinea pig complement was used to shorten the treatment time. Based on an antigen-antibody complex reaction, the demyelinating agent induced an acute and severe demyelination, leading to the pathological process of Wallerian degeneration during the demyelinating period. This method was used to treat a 12 mm-long sciatic nerve defect in rats. The control groups were injected with one of the demyelinating agent components. The results indicated that anti-galactocerebroside antibodies plus guinea pig complement can significantly shorten treatment time and promote nerve regeneration and functional recovery. In addition, the demyelinating agent can increase the mRNA levels of nerve growth factors and can regulate inflammation. In conclusion, treatment with anti-galactocerebroside antibodies plus guinea pig complement can promote axonal regeneration. This therapy provides a novel method to improve functional recovery in the treatment of long nerve defects.

Polylactic acid (PLA) controlled delivery carriers for biomedical applications

Polylactic acid (PLA) and its copolymers have a long history of safety in humans and an extensive range of applications. PLA is biocompatible, biodegradable by hydrolysis and enzymatic activity, has a large range of mechanical and physical properties that can be engineered appropriately to suit multiple applications, and has low immunogenicity. Formulations containing PLA have also been Food and Drug Administration (FDA)-approved for multiple applications making PLA suitable for expedited clinical translatability. These biomaterials can be fashioned into sutures, scaffolds, cell carriers, drug delivery systems, and a myriad of fabrications. PLA has been the focus of a multitude of preclinical and clinical testing. Three-dimensional printing has expanded the possibilities of biomedical engineering and has enabled the fabrication of a myriad of platforms for an extensive variety of applications. PLA has been widely used as temporary extracellular matrices in tissue engineering. At the other end of the spectrum, PLA's application as drug-loaded nanoparticle drug carriers, such as liposomes, polymeric nanoparticles, dendrimers, and micelles, can encapsulate otherwise toxic hydrophobic anti-tumor drugs and evade systemic toxicities. The clinical translation of these technologies from preclinical experimental settings is an ever-evolving field with incremental advancements. In this review, some of the biomedical applications of PLA and its copolymers are highlighted and briefly summarized.

c-Jun gene-modified Schwann cells: upregulating multiple neurotrophic factors and promoting neurite outgrowth

Genetically modified Schwann cells (SCs) that overexpress neurotrophic factors (NFs), especially those that overexpress multiple NFs, hold great potential for promoting nerve regeneration. Currently, only one NF can be upregulated in most genetically modified SCs, and simultaneously upregulating multiple NFs in SCs remains challenging. In this study, we found that the overexpression of c-Jun, a component of the AP-1 transcription factor, effectively upregulated the expression and secretion of multiple NFs, including glial cell line-derived neurotrophic factor, brain-derived neurotrophic factor, artemin, leukemia inhibitory factor, and nerve growth factor. The c-Jun gene-modified SCs showed a normal morphology in scanning electron microscopy and fluorescent staining analysis. In addition, the c-Jun-modified SCs showed enhanced proliferation and migration abilities compared with vector control cells. We used transwell chambers to establish coculture systems imitating the in vivo conditions in which transplanted SCs might influence native SCs and neurons. We found that the c-Jun-modified SCs enhanced native SC migration and promoted the proliferation of native SCs in the presence of axons. Further analysis revealed that in the c-Jun group, the average length and the total area of neurites divided by the total area of the explant body were μm 1180±25 and 6.4±0.4, respectively, which were significantly greater compared with the other groups. These findings raise the possibility of constructing an optimal therapeutic alternative for nerve repair using c-Jun-modified SCs, which have the potential to promote axonal regeneration and functional recovery by upregulating multiple NFs. In addition, these cells exhibit enhanced migration and proliferation abilities, enhance the biological functions of native SCs, and promote neurite outgrowth.

Peripheral nerve repair: a hot spot analysis on treatment methods from 2010 to 2014

Therapeutic strategies for neurological deficits and for promoting nerve regeneration after peripheral nerve injuries have received much focus in clinical research. Advances in basic research in recent years have increased our understanding of the anatomy of peripheral nerves and the importance of the microenvironment. Various new intervention methods have been developed, but with varying effectiveness. In the present study, we selected 911 papers on different repair methods for peripheral nerve injury from the Web of Science and indexed in the Science Citation Index from 2010 to 2014. We quantitatively examine new repair methods and strategies using bibliometrics, and we discuss the present state of knowledge and the problems and prospects of various repair methods, including nerve transfer, neural transplantation, tissue engineering and genetic engineering. Our findings should help in the study and development of repair methods for peripheral nerve injury.

In vivo evaluation of nerve guidance channels of PTMC/PLLA porous biomaterial

INTRODUCTION

Peripheral nerve disruptions, frequently occurring during limb injuries, give rise to serious complications of patients recovery resulting from limitations in neural tissue regeneration capabilities. To overcome this problem bridging techniques utilizing guidance channels gain their importance. Biodegradable polymeric tubes seem to be more prospective then non-degradable materials - no necessity of implant removal and possibilities of release of incorporated drugs or biologically active agents that may support nerve regeneration process are the main advantages.

MATERIAL AND METHODS

Polymer blend of commercial poly(L-lactic acid) (PLLA) and in-house synthesized poly(trimethylene carbonate) (PTMC) were processed in an organic solvent - phase inversion process on a supporting rod - to form a guidance porous tube of 1.1 mm inner diameter. In vivo experiments on rat's cut femoral nerve by using either the tubes or end-to-end suturing (control group) involved 22 and 19 rats, respectively. Motor recovery of operated limbs, neuroma occurrence and histopathology of explanted nerves were evaluated after 30, 60 and 90 days of implantation.

RESULTS

Motor recovery of the limbs was of similar rate for the two animal groups. The neuroma formation was evident in over 90% control specimens, while for the bridging group it was less than 40% of all evaluable samples (p = 0.0022). Biocompatibility of applied materials was affirmed by moderate tissue response.

CONCLUSIONS

Application of the biodegradable PLLA/PTMC polymeric tubes effectively supports regeneration of discontinued nerves. The applied material prevents neuroma formation, by reducing the scar tissue formation time and, thus, accelerating the process of neural tissue restoration.

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

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

Sciatic nerve repair using adhesive bonding and a modified conduit

When repairing nerves with adhesives, most researchers place glue directly on the nerve stumps, but this method does not fix the nerve ends well and allows glue to easily invade the nerve ends. In this study, we established a rat model of completely transected sciatic nerve injury and repaired it using a modified 1 cm-length conduit with inner diameter of 1.5 mm. Each end of the cylindrical conduit contains a short linear channel, while the enclosed central tube protects the nerve ends well. Nerves were repaired with 2-octyl-cyanoacrylate and suture, which complement the function of the modified conduit. The results demonstrated that for the same conduit, the average operation time using the adhesive method was much shorter than with the suture method. No significant differences were found between the two groups in sciatic function index, motor evoked potential latency, motor evoked potential amplitude, muscular recovery rate, number of medullated nerve fibers, axon diameter, or medullary sheath thickness. Thus, the adhesive method for repairing nerves using a modified conduit is feasible and effective, and reduces the operation time while providing an equivalent repair effect.

Improvement of peripheral nerve defects using a silicone conduit filled with hepatocyte growth factor

OBJECTIVE

To assess local effects of hepatocyte growth factor (HGF) on peripheral nerve repair in a rat sciatic nerve transection model.

STUDY DESIGN

Sixty male, healthy, white Wistar rats were randomized into 4 experimental groups: In the sham-operated group, sciatic nerve was exposed and manipulated. In the transected control group, the left sciatic nerve was transected. In the silicone graft group (SIL), a 10-mm defect was made and bridged using a silicone tube. The graft was filled with phosphate-buffered saline in the SIL group and with HGF in the SIL/HGF group.

RESULTS

Behavioral testing, sciatic nerve functional study, gastrocnemius muscle mass measurement, and morphometric indices found earlier regeneration of axons in the SIL/HGF than in the SIL group (P < .05). Immunohistochemical study clearly found more positive location of reactions to S-100 in the SIL/HGF group than in the SIL group.

CONCLUSIONS

HGF may have clinical implications for the surgical management of patients after facial nerve transection.

Promoting nerve regeneration in a neurotmesis rat model using poly(DL-lactide-ε-caprolactone) membranes and mesenchymal stem cells from the Wharton's jelly: in vitro and in vivo analysis

In peripheral nerves MSCs can modulate Wallerian degeneration and the overall regenerative response by acting through paracrine mechanisms directly on regenerating axons or upon the nerve-supporting Schwann cells. In the present study, the effect of human MSCs from Wharton's jelly (HMSCs), differentiated into neuroglial-like cells associated to poly (DL-lactide-ε-caprolactone) membrane, on nerve regeneration, was evaluated in the neurotmesis injury rat sciatic nerve model. Results in vitro showed successful differentiation of HMSCs into neuroglial-like cells, characterized by expression of specific neuroglial markers confirmed by immunocytochemistry and by RT-PCR and qPCR targeting specific genes expressed. In vivo testing evaluated during the healing period of 20 weeks, showed no evident positive effect of HMSCs or neuroglial-like cell enrichment at the sciatic nerve repair site on most of the functional and nerve morphometric predictors of nerve regeneration although the nociception function was almost normal. EPT on the other hand, recovered significantly better after HMSCs enriched membrane employment, to values of residual functional impairment compared to other treated groups. When the neurotmesis injury can be surgically reconstructed with an end-to-end suture or by grafting, the addition of a PLC membrane associated with HMSCs seems to bring significant advantage, especially concerning the motor function recovery.

The proximal medial sural nerve biopsy model: a standardised and reproducible baseline clinical model for the translational evaluation of bioengineered nerve guides

Autologous nerve transplantation (ANT) is the clinical gold standard for the reconstruction of peripheral nerve defects. A large number of bioengineered nerve guides have been tested under laboratory conditions as an alternative to the ANT. The step from experimental studies to the implementation of the device in the clinical setting is often substantial and the outcome is unpredictable. This is mainly linked to the heterogeneity of clinical peripheral nerve injuries, which is very different from standardized animal studies. In search of a reproducible human model for the implantation of bioengineered nerve guides, we propose the reconstruction of sural nerve defects after routine nerve biopsy as a first or baseline study. Our concept uses the medial sural nerve of patients undergoing diagnostic nerve biopsy (≥ 2 cm). The biopsy-induced nerve gap was immediately reconstructed by implantation of the novel microstructured nerve guide, Neuromaix, as part of an ongoing first-in-human study. Here we present (i) a detailed list of inclusion and exclusion criteria, (ii) a detailed description of the surgical procedure, and (iii) a follow-up concept with multimodal sensory evaluation techniques. The proximal medial sural nerve biopsy model can serve as a preliminary nature of the injuries or baseline nerve lesion model. In a subsequent step, newly developed nerve guides could be tested in more unpredictable and challenging clinical peripheral nerve lesions (e.g., following trauma) which have reduced comparability due to the different nature of the injuries (e.g., site of injury and length of nerve gap).

Peripheral nerve regeneration following transection injury to rat sciatic nerve by local application of adrenocorticotropic hormone

The objective of this study was to assess local effect of adrenocorticotropic hormone (ACTH) on the functional recovery of the sciatic nerve in a transection model. Sixty male healthy white Wistar rats were randomized into four experimental groups of 15 animals each: In the sham-operated group (SHAM), the sciatic nerve was exposed and manipulated. In the transected group (TC), the left sciatic nerve was transected and the cut nerve ends were fixed in the adjacent muscle. In the silicone graft group (SIL) a 10-mm defect was made and bridged using a silicone tube. The graft was filled with phosphated-buffer saline alone. In the treatment group a silicone tube (SIL/ACTH) was filled with 10 μL ACTH (0.1 mg/mL). Each group was subdivided into three subgroups of five animals each and regenerated nerve fibres were studied at 4, 8 and 12 weeks post operation. Behavioral testing, functional, gastrocnemius muscle mass and morphometric indices showed earlier regeneration of axons in SIL/ACTH than in SIL group (p < 0.05). Immunohistochemistry clearly showed more positive location of reactions to S-100 in SIL/ACTH than in SIL group. ACTH improved functional recovery and morphometric indices of sciatic nerve. This finding supports role of ACTH after peripheral nerve repair and may have clinical implications for the surgical management of patients after nerve transection.

Improvement of functional recovery of transected peripheral nerve by means of chitosan grafts filled with vitamin E, pyrroloquinoline quinone and their combination

Effects of vitamin E and pyrroloquinoline quinone on peripheral nerve regeneration were studied using a rat sciatic nerve transection model. Ninety male healthy White Wistar rats were divided into three experimental groups (n = 15), randomly: Sham-operation (SHAM), transected control (TC), chitosan conduit (Chit) and three treatment groups (Vit E, PQQ and PQQ + Vit E). In SHAM group after anesthesia, left sciatic nerve was exposed through a gluteal muscle incision and after homeostasis muscle was sutured. In Chit group left sciatic nerve was exposed the same way and transected proximal to tibio-peroneal bifurcation leaving a 10-mm gap. Proximal and distal stumps were each inserted into a chitosan tube. In treatment groups the tube was implanted the same way and filled with Vit E, PQQ and PQQ + Vit E. Each group was subdivided into three subgroups of six animals each and were studied 4, 8, 12 weeks after surgery. Functional and electrophysiological studies, and gastrocnemius muscle mass measurement confirmed faster and better recovery of regenerated axons in Vit E + PQQ combination compared to Vit E or PQQ solely (P < 0.05). Morphometric indices of regenerated fibers showed number and diameter of the myelinated fibers in PQQ + Vit E was significantly higher than in other treatment groups. In immunohistochemistry, location of reactions to S-100 in PQQ + Vit E was clearly more positive than in other treatment groups. Response to PQQ + Vit E treatment demonstrates that it influences and improves functional recovery of peripheral nerve regeneration.

Neuregulin 1 role in Schwann cell regulation and potential applications to promote peripheral nerve regeneration

Neuregulin 1 (NRG1) is a multifunctional and versatile protein: its numerous isoforms can signal in a paracrine, autocrine, or juxtacrine manner, playing a fundamental role during the development of the peripheral nervous system and during the process of nerve repair, suggesting that the treatment with NRG1 could improve functional outcome following injury. Accordingly, the use of NRG1 in vivo has already yielded encouraging results. The aim of this review is to focus on the role played by the different NRG1 isoforms during peripheral nerve regeneration and remyelination and to identify good candidates to be used for the development of tissue engineered medical devices delivering NRG1, with the objective of promoting better nerve repair.

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.

Hybrid conducting polymer-hydrogel conduits for axonal growth and neural tissue engineering

Successfully and efficiently bridging peripheral nerve gaps without the use of autografts is a substantial clinical advance for peripheral nerve reconstructions. Novel templating methods for the fabrication of conductive hydrogel guidance channels for axonal regeneration are designed and developed. PEDOT is electrodeposited inside the lumen to create fully coated-PEDOT agarose conduits and partially coated-PEDOT agarose conduits.

A systematic review of animal models used to study nerve regeneration in tissue-engineered scaffolds

Research on biomaterial nerve scaffolds has been carried out for 50 years. Only three materials (collagen, polycaprolactone and polyglycollic acid) have progressed to clinical use. Pre-clinical animal models are critical for testing nerve scaffolds prior to implementation in clinical practice. We have conducted a systematic review of 416 reports in which animal models were used for evaluation of nerve regeneration into synthetic conduits. A valid animal model of nerve regeneration requires it to reproduce the specific processes that take place in regeneration after human peripheral nerve injury. No distinct animal species meets all the requirements for an ideal animal model. Certain models are well suited for understanding regenerative neurobiology while others are better for pre-clinical evaluation of efficacy. The review identified that more than 70 synthetic materials were tested in eight species using 17 different nerves. Nerve gaps ranged from 1 to 90 mm. More than 20 types of assessment methodology were used with no standardization of methods between any of the publications. The review emphasizes the urgent need for standardization or rationalization of animal models and evaluation methods for studying nerve repair.

Vascular endothelial growth factor-loaded poly(lactic-co-glycolic acid) microspheres-induced lateral axonal sprouting into the vein graft bridging two healthy nerves: nerve graft prefabrication using controlled release system

The most commonly used surgical technique for repairing segmental nerve defects is autogenous nerve grafting; however, this method causes donor site morbidity. In this study, we sought to produce prefabricated nerve grafts that can serve as a conduit instead of autologous nerve using a controlled release system created with vascular endothelial growth factor (VEGF)-loaded poly(lactic-co-glycolic acid) (PLGA) microspheres. The study was performed in vitro and in vivo. For the in vitro studies, VEGF-loaded PLGA microspheres were prepared. Thirty rats were used for the in vivo studies. Vein grafts were sutured between the tibial and peroneal nerves in all animals. Three groups were created, and an epineural window, partial incision, and microsphere application were performed, respectively. Walking track analysis, morphologic, and electron microscopic assessment were performed at the end of the eight weeks. Microspheres were produced in spherical shapes as required. Controlled release of VEGF was achieved during a 30-days period. Although signs of nerve injury occurred initially in the partial incision groups according to the indexes of peroneal and tibial function, it improved gradually. The index values were not affected in the other groups. There were many myelinated fibers with large diameters in the partial incision and controlled release groups, while a few myelinated fibers that passed through vein graft in the epineural window group. Thereby, prefabrication was carried out for the second and third groups. It was demonstrated that nerve graft can be prefabricated by the controlled delivery of VEGF.

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.

The long-term functional recovery of repair of sciatic nerve transection with biogenic conduits

INTRODUCTION

The aim of this study was to evaluate long-term regenerative capacity over a 15-mm nerve gap of an autologous nerve conduit, the biogenic conduit (BC), 16 weeks after sciatic nerve transection in the rat.

METHODS

A 19-mm long polyvinyl chloride (PVC) tube was implanted parallely to the sciatic nerve. After implantation, a connective tissue cover developed around the PVC-tube, the so-called BC. After removal of the PVC-tube the BCs filled with fibrin (n = 8) were compared to autologous nerve grafts (n = 8). Sciatic functional index (SFI) was evaluated every 4 weeks, histological evaluation was performed at 16 weeks postimplantation. Regenerating axons were visualized by retrograde labelling.

RESULTS

SFI revealed no significant differences. Nerve area and axon number in the BC group were significantly lower than in the autologous nerve group (P < 0.05; P < 0.01). Analysis of myelin formation showed no significant difference in both groups. Analysis of N-ratio revealed lower values in the BC group (P < 0.001).

CONCLUSION

This study reveals the suitability of BC for nerve gap bridging over a period of 16 weeks with functional recovery to comparable extent as the autologous nerve graft despite impaired histomorphometric parameters.

C3 peptide promotes axonal regeneration and functional motor recovery after peripheral nerve injury

Peripheral nerve injuries are frequently seen in trauma patients and due to delayed nerve repair, lifelong disabilities often follow this type of injury. Innovative therapies are needed to facilitate and expedite peripheral nerve regeneration. The purpose of this study was to determine the effects of a 1-time topical application of a 26-amino-acid fragment (C3(156-181)), derived from the Clostridium botulinum C3-exoenzyme, on peripheral nerve regeneration in 2 models of nerve injury and repair in adult rats. After sciatic nerve crush, different dosages of C3(156-181) dissolved in buffer or reference solutions (nerve growth factor or C3(bot)-wild-type protein) or vehicle-only were injected through an epineurial opening into the lesion sites. After 10-mm nerve autotransplantation, either 8.0 nmol/kg C3(156-181) or vehicle were injected into the proximal and distal suture sites. For a period of 3 to 10 postoperative weeks, C3(156-181)-treated animals showed a faster motor recovery than control animals. After crush injury, axonal outgrowth and elongation were activated and consequently resulted in faster motor recovery. The nerve autotransplantation model further elucidated that C3(156-181) treatment accounts for better axonal elongation into motor targets and reduced axonal sprouting, which are followed by enhanced axonal maturation and better axonal functionality. The effects of C3(156-181) are likely caused by a nonenzymatic down-regulation of active RhoA. Our results indicate the potential of C3(156-181) as a therapeutic agent for the topical treatment of peripheral nerve repair sites.

EXPERIMENTAL NERVE GRAFTING — TOWARDS FUTURE SOLUTIONS OF A CLINICAL PROBLEM

Restoration of function following complete nerve injuries with subsequent nerve repair is still not satisfactory and in many cases poor, especially when a gap has to be bridged by a graft. In such situations, there may be insufficient access to autologous graft material. Alternatives have to be developed and a close collaboration between basic scientists and clinicians is required. In the present article, current studies on experimental nerve grafts are discussed and some new alternatives to autologous nerve grafts are reviewed.

Regeneration and repair of peripheral nerves with different biomaterials: review

Peripheral nerve injury may cause gaps between the nerve stumps. Axonal proliferation in nerve conduits is limited to 10-15 mm. Most of the supportive research has been done on rat or mouse models which are different from humans. Herein we review autografts and biomaterials which are commonly used for nerve gap repair and their respective outcomes. Nerve autografting has been the first choice for repairing peripheral nerve gaps. However, it has been demonstrated experimentally that tissue engineered tubes can also permit lead to effective nerve repair over gaps longer than 4 cm repair that was previously thought to be restorable by means of nerve graft only. All of the discoveries in the nerve armamentarium are making their way into the clinic, where they are, showing great potential for improving both the extent and rate of functional recovery compared with alternative nerve guides.

Schwann-like mesenchymal stem cells within vein graft facilitate facial nerve regeneration and remyelination

To compare the ability of bone marrow mesenchymal stem cells (MSCs) and transdifferentiated Schwann-like MSCs (tMSCs) in promoting transected facial nerve branches repair in a rabbit model of injury, rabbit tMSCs were induced from bone marrow MSCs, and Schwann cells markers were assessed by Western blot analysis. The left facial nerve buccal branch was transected to form a 1-cm gap in 54 rabbits, and the gaps were immediately bridged using autologous vein grafts. Animals were then randomly assigned to three groups: vein graft (VG); VG+MSCs, and VG+tMSCs (n=18/group). Saline, autologous MSCs, and Schwann-like tMSCs were injected into vein conduits. Rabbits were sacrificed at week 4, 8, and 16 post-surgery. Facial nerves regeneration and myelination were analyzed by functional, immunohistochemical, and morphological tests. In addition, myelin protein genes expression, including peripheral myelin protein 22 (PMP22), myelin protein zero (P0), and myelin basic protein (MBP), in transplanted cells in vivo were assayed using real time quantitative-reverse transcription-polymerase chain reaction (RT-PCR). Rabbit tMSCs expressed Schwann cells markers, and results demonstrated better facial nerve functional recovery in the VG+tMSCs group, with earlier horseradish peroxidase (HRP) positive neurons appearance and a greater number of MBP positive myelinated axons since 4weeks after transplantation. Moreover, RT-PCR analysis showed transplanted tMSCs in vivo expressed higher myelin proteins at mRNA level than those of MSCs during the first 8weeks of neural regeneration. This study suggests that rabbit transdifferentiated Schwann-like MSCs within autogenous vein graft accelerate transected axons regeneration and achieve better remyelinization.

Inducible nerve growth factor delivery for peripheral nerve regeneration in vivo

BACKGROUND

HEK-293 cells can be genetically modified to release and regulate nerve growth factor (NGF) in vitro. The aim of this study was to evaluate the impact of this NGF delivery system on peripheral nerve regeneration in vivo.

METHODS

HEK-293 cells were transfected with an ecdysone receptor, NGF cDNA, and herpes simplex virus-thymidine kinase suicide vector. NGF production is induced by ponasterone A and stopped by ganciclovir. A 13-mm sciatic nerve gap was bridged with Silastic conduits in 120 nude rats, and transfected HEK-293 cells were added, induced, and boostered to secrete bioactive NGF.

RESULTS

The induction of the cell line and additional booster with ponasterone A demonstrated significantly higher levels of bioactive NGF, enhanced macroscopic nerve growth, improved functional recovery, and histologic regeneration when compared with control groups after 7, 14, and 21 days, and 2 and 4 months. The treatment with ganciclovir resulted in suppression of the NGF production and decreased functional and histologic outcomes.

CONCLUSIONS

Transfected HEK-293 cells can be regulated to inducibly produce bioactive NGF in vivo over prolonged periods. This tissue-engineered nerve construct including the NGF delivery system is able to improve peripheral nerve regeneration and functional recovery and appears to be superior to nerve isografts.

In vitro and in vivo testing of novel ultrathin PCL and PCL/PLA blend films as peripheral nerve conduit

In an attempt to obviate the drawbacks of nerve autograft, ultrathin microporous biodegradable PCL and PCL/PLA films were tested for their compatibility with motor neuron-like NG108-15 cells and primary Schwann cells. Data obtained from MTS colorimetric and DNA fluorimetric assays showed that both cell lines readily attached and proliferated on these materials. Images taken using scanning electron microscope and fluorescence microscope confirmed these observations. Enhanced cell-surface interaction was achieved by pretreating the films in NaOH solution. Importantly, NG108-15 cells could be induced into differentiated phenotype with long, un-branched neurites growing across the surface of the materials. The bipolar spindle-shaped phenotype of Schwann cells was also retained on these scaffolds. Positive immunochemical staining using antibodies against neurofilament for NG108-15 cells and S100 for Schwann cells indicated the expression of these marker proteins. In a small-scaled pilot testing, the performance of PCL conduits in bridging up a 10 mm gap in rat sciatic nerve model was assessed. Immunohistochemical staining showed that regenerated nerve tissue and penetrated Schwann cells have the potential to span the whole length of the conduit in 2 weeks.