Evaluation of peripheral nerve regeneration across an 80-mm gap using a polyglycolic acid (PGA)--collagen nerve conduit filled with laminin-soaked collagen sponge in dogs

Proliferation of endothelial cells (HUVEC) on specific-modified collagen sponges loaded with different growth factors

In general, matrices for tissue engineering must maintain structural integrity during the process of tissue formation and promote vascularization of developing tissue. Therefore, collagen sponges, manufactured by an approach that offers the potential of unidirectional pore size, were seeded with human umbilical vein endothelial cells (HUVEC) to demonstrate a positive effect on cell proliferation. In addition, vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) have been used to promote proliferation of HUVEC on optimized collagen sponges. Growth and viability of the cells were evaluated. Potential unidirectional pore structure demonstrated an improvement of both, endothelial cell growth and viability. Supplementation of growth factors showed an additional increase of endothelial cell growth on collagen sponges, which confirmed the high potential of combining this biomaterial with growth factors. The results suggest that a collagen sponge with a potential specific pore size could be a suitable scaffold for endothelial cells and might be a promising implantable biomaterial with enhanced angiogenic capabilities for future clinical applications.

3D Printed Neural Regeneration Devices

Neural regeneration devices interface with the nervous system and can provide flexibility in material choice, implantation without the need for additional surgeries, and the ability to serve as guides augmented with physical, biological (e.g., cellular), and biochemical functionalities. Given the complexity and challenges associated with neural regeneration, a 3D printing approach to the design and manufacturing of neural devices could provide next-generation opportunities for advanced neural regeneration via the production of anatomically accurate geometries, spatial distributions of cellular components, and incorporation of therapeutic biomolecules. A 3D printing-based approach offers compatibility with 3D scanning, computer modeling, choice of input material, and increasing control over hierarchical integration. Therefore, a 3D printed implantable platform could ultimately be used to prepare novel biomimetic scaffolds and model complex tissue architectures for clinical implants in order to treat neurological diseases and injuries. Further, the flexibility and specificity offered by 3D printed in vitro platforms have the potential to be a significant foundational breakthrough with broad research implications in cell signaling and drug screening for personalized healthcare. This progress report examines recent advances in 3D printing strategies for neural regeneration as well as insight into how these approaches can be improved in future studies.

The progress of biomaterials in peripheral nerve repair and regeneration

Repair and regeneration of the injured peripheral nerve (PN) is a challenging issue in clinics. Although the regeneration outcome of large PN defects is currently unsatisfactory, recently, the study of PN repair has considerably progressed. In particular, biomaterials for repairing PNs, such as nerve guidance conduits and nerve repair membranes, have been well developed. Herein, we summarize the anatomy of the PN, the pathophysiological features of the nerve injury, and the repair process post injury. Then, we highlight the progress in the development of natural and synthetic biomaterials and summarize the applications, advantages, and disadvantages of these materials. These materials can be used as nerve repair membranes and nerve conduits in the field of PN repair. Finally, we discuss the challenges encountered and development strategies for PN repair in the future.

Relevance and Recent Developments of Chitosan in Peripheral Nerve Surgery

Developments in tissue engineering yield biomaterials with different supporting strategies to promote nerve regeneration. One promising material is the naturally occurring chitin derivate chitosan. Chitosan has become increasingly important in various tissue engineering approaches for peripheral nerve reconstruction, as it has demonstrated its potential to interact with regeneration associated cells and the neural microenvironment, leading to improved axonal regeneration and less neuroma formation. Moreover, the physiological properties of its polysaccharide structure provide safe biodegradation behavior in the absence of negative side effects or toxic metabolites. Beneficial interactions with Schwann cells (SC), inducing differentiation of mesenchymal stromal cells to SC-like cells or creating supportive conditions during axonal recovery are only a small part of the effects of chitosan. As a result, an extensive body of literature addresses a variety of experimental strategies for the different types of nerve lesions. The different concepts include chitosan nanofibers, hydrogels, hollow nerve tubes, nerve conduits with an inner chitosan layer as well as hybrid architectures containing collagen or polyglycolic acid nerve conduits. Furthermore, various cell seeding concepts have been introduced in the preclinical setting. First translational concepts with hollow tubes following nerve surgery already transferred the promising experimental approach into clinical practice. However, conclusive analyses of the available data and the proposed impact on the recovery process following nerve surgery are currently lacking. This review aims to give an overview on the physiologic properties of chitosan, to evaluate its effect on peripheral nerve regeneration and discuss the future translation into clinical practice.

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.

Biomimetic neural scaffolds: a crucial step towards optimal peripheral nerve regeneration

Peripheral nerve injury is a common disease that affects more than 20 million people in the United States alone and remains a major burden to society. The current gold standard treatment for critical-sized nerve defects is autologous nerve graft transplantation; however, this method is limited in many ways and does not always lead to satisfactory outcomes. The limitations of autografts have prompted investigations into artificial neural scaffolds as replacements, and some neural scaffold devices have progressed to widespread clinical use; scaffold technology overall has yet to be shown to be consistently on a par with or superior to autografts. Recent advances in biomimetic scaffold technologies have opened up many new and exciting opportunities, and novel improvements in material, fabrication technique, scaffold architecture, and lumen surface modifications that better reflect biological anatomy and physiology have independently been shown to benefit overall nerve regeneration. Furthermore, biomimetic features of neural scaffolds have also been shown to work synergistically with other nerve regeneration therapy strategies such as growth factor supplementation, stem cell transplantation, and cell surface glycoengineering. This review summarizes the current state of neural scaffolds, highlights major advances in biomimetic technologies, and discusses future opportunities in the field of peripheral nerve regeneration.

Strategic Design and Fabrication of Nerve Guidance Conduits for Peripheral Nerve Regeneration

Nerve guidance conduits (NGCs) have been drawing considerable attention as an aid to promote regeneration of injured axons across damaged peripheral nerves. Ideally, NGCs should include physical and topographic axon guidance cues embedded as part of their composition. Over the past decades, much progress has been made in the development of NGCs that promote directional axonal regrowth so as to repair severed nerves. This paper briefly reviews the recent designs and fabrication techniques of NGCs for peripheral nerve regeneration. Studies associated with versatile design and preparation of NGCs fabricated with either conventional or rapid prototyping (RP) techniques have been examined and reviewed. The effect of topographic features of the filler material as well as porous structure of NGCs on axonal regeneration has also been examined from the previous studies. While such strategies as macroscale channels, lumen size, groove geometry, use of hydrogel/matrix, and unidirectional freeze-dried surface are seen to promote nerve regeneration, shortcomings such as axonal dispersion and wrong target reinnervation still remain unsolved. On this basis, future research directions are identified and discussed.

Scaffoldless tissue-engineered nerve conduit promotes peripheral nerve regeneration and functional recovery after tibial nerve injury in rats

Damage to peripheral nerve tissue may cause loss of function in both the nerve and the targeted muscles it innervates. This study compared the repair capability of engineered nerve conduit (ENC), engineered fibroblast conduit (EFC), and autograft in a 10-mm tibial nerve gap. ENCs were fabricated utilizing primary fibroblasts and the nerve cells of rats on embryonic day 15 (E15). EFCs were fabricated utilizing primary fibroblasts only. Following a 12-week recovery, nerve repair was assessed by measuring contractile properties in the medial gastrocnemius muscle, distal motor nerve conduction velocity in the lateral gastrocnemius, and histology of muscle and nerve. The autografts, ENCs and EFCs reestablished 96%, 87% and 84% of native distal motor nerve conduction velocity in the lateral gastrocnemius, 100%, 44% and 44% of native specific force of medical gastrocnemius, and 63%, 61% and 67% of native medial gastrocnemius mass, respectively. Histology of the repaired nerve revealed large axons in the autograft, larger but fewer axons in the ENC repair, and many smaller axons in the EFC repair. Muscle histology revealed similar muscle fiber cross-sectional areas among autograft, ENC and EFC repairs. In conclusion, both ENCs and EFCs promoted nerve regeneration in a 10-mm tibial nerve gap repair, suggesting that the E15 rat nerve cells may not be necessary for nerve regeneration, and EFC alone can suffice for peripheral nerve injury repair.

A novel bioactive nerve conduit for the repair of peripheral nerve injury

The use of a nerve conduit provides an opportunity to regulate cytokines, growth factors and neurotrophins in peripheral nerve regeneration and avoid autograft defects. We constructed a poly-D-L-lactide (PDLLA)-based nerve conduit that was modified using poly{(lactic acid)-co-[(glycolic acid)-alt-(L-lysine)]} and β-tricalcium phosphate. The effectiveness of this bioactive PDLLA-based nerve conduit was compared to that of PDLLA-only conduit in the nerve regeneration following a 10-mm sciatic nerve injury in rats. We observed the nerve morphology in the early period of regeneration, 35 days post injury, using hematoxylin-eosin and methylene blue staining. Compared with the PDLLA conduit, the nerve fibers in the PDLLA-based bioactive nerve conduit were thicker and more regular in size. Muscle fibers in the soleus muscle had greater diameters in the PDLLA bioactive group than in the PDLLA only group. The PDLLA-based bioactive nerve conduit is a promising strategy for repair after sciatic nerve injury.

Past, Present, and Future of Nerve Conduits in the Treatment of Peripheral Nerve Injury

With significant advances in the research and application of nerve conduits, they have been used to repair peripheral nerve injury for several decades. Nerve conduits range from biological tubes to synthetic tubes, and from nondegradable tubes to biodegradable tubes. Researchers have explored hollow tubes, tubes filled with scaffolds containing neurotrophic factors, and those seeded with Schwann cells or stem cells. The therapeutic effect of nerve conduits is improving with increasing choice of conduit material, new construction of conduits, and the inclusion of neurotrophic factors and support cells in the conduits. Improvements in functional outcomes are expected when these are optimized for use in clinical practice.

Functionalized α-Helical Peptide Hydrogels for Neural Tissue Engineering

Trauma to the central and peripheral nervous systems often lead to serious morbidity. Current surgical methods for repairing or replacing such damage have limitations. Tissue engineering offers a potential alternative. Here we show that functionalized α-helical-peptide hydrogels can be used to induce attachment, migration, proliferation and differentiation of murine embryonic neural stem cells (NSCs). Specifically, compared with undecorated gels, those functionalized with Arg-Gly-Asp-Ser (RGDS) peptides increase the proliferative activity of NSCs; promote their directional migration; induce differentiation, with increased expression of microtubule-associated protein-2, and a low expression of glial fibrillary acidic protein; and lead to the formation of larger neurospheres. Electrophysiological measurements from NSCs grown in RGDS-decorated gels indicate developmental progress toward mature neuron-like behavior. Our data indicate that these functional peptide hydrogels may go some way toward overcoming the limitations of current approaches to nerve-tissue repair.

Human umbilical cord mesenchymal stem cell-loaded amniotic membrane for the repair of radial nerve injury

In this study, we loaded human umbilical cord mesenchymal stem cells onto human amniotic membrane with epithelial cells to prepare nerve conduits, i.e., a relatively closed nerve regeneration chamber. After neurolysis, the injured radial nerve was enwrapped with the prepared nerve conduit, which was fixed to the epineurium by sutures, with the cell on the inner surface of the conduit. Simultaneously, a 1.0 mL aliquot of human umbilical cord mesenchymal stem cell suspension was injected into the distal and proximal ends of the injured radial nerve with 1.0 cm intervals. A total of 1.75 × 10⁷ cells were seeded on the amniotic membrane. In the control group, patients received only neurolysis. At 12 weeks after cell transplantation, more than 80% of patients exhibited obvious improvements in muscular strength, and touch and pain sensations. In contrast, these improvements were observed only in 55–65% of control patients. At 8 and 12 weeks, muscular electrophysiological function in the region dominated by the injured radial nerve was significantly better in the transplantation group than the control group. After cell transplantation, no immunological rejections were observed. These findings suggest that human umbilical cord mesenchymal stem cell-loaded amniotic membrane can be used for the repair of radial nerve injury.

A rat model for long-gap peripheral nerve reconstruction

BACKGROUND

The rat model has had limited utility for the study of long nerve gaps because of the small size of the animal. The authors sought to develop a simple, effective rat model for reconstruction of long nerve gap defects.

METHODS

Fifteen rats had a sciatic nerve transection followed by reconstruction. Positive control rats received a 1-cm isograft. Negative control rats received a 3.5-cm hollow silicone conduit, and experimental rats received a 4-cm isograft; these were implanted in a looped configuration to accommodate the long length. Nerves were harvested at 6 weeks (1-cm grafts) and 12 weeks (3.5-cm conduits and 4-cm grafts) for histologic and histomorphometric evaluation.

RESULTS

The 1-cm and 4-cm isograft groups showed robust regeneration in the distal nerve segment. The 3.5-cm hollow conduits showed absolutely no initiation of nerve regeneration. Histomorphometric values were as expected for the specified gap length.

CONCLUSIONS

This study describes a simple and effective long nerve gap rat model for experiments on nerve grafts and nerve conduits. The long nerve graft model can be useful for studying techniques such as processed nerve grafts, which are currently a topic of frequent investigation. The 3.5-cm hollow conduit "no-regrowth" long-gap model is ideal for investigating conduit-based tissue-engineering solutions for long-gap nerve repair. The authors' approach overcomes the size limitation of the small animal while exploiting the features that make the rat the model of choice for preliminary nerve studies.

Evaluation of electronspun silk fibroin-based transplants used for facial nerve repair

OBJECTIVE

To study whether regenerated electrospun silk fibroin (SF) nanofibers as nerve conduits could improve nerve regeneration microenvironment and induce the facial nerve regeneration of Sprague-Dawley rats.

DESIGN

Electrospun SF nanofibers were prepared to bridge a 5-mm facial nerve defect in Sprague-Dawley rats. Three months after implantation, a comprehensive morphologic and functional evaluation was performed by electrophysiology, histology, Fluorogold retrograde tracing, and transmission electron micrograph.

RESULTS

The SF nanofiber tube exhibited good biocompatibility in vivo, and no distinct regional inflammation response and scar formation was observed. After 3 months of operation, the morphologic and functional investigation has shown a positive evaluation on the nerve repair outcome elicited by SF nanofiber graft and autograft.

CONCLUSION

Electrospun SF grafts could promote nerve regeneration after facial nerve injury and become a potential possibility of newly developed nerve grafts as an alternative of autografts to peripheral nerve regeneration.

Design of super-elastic biodegradable scaffolds with longitudinally oriented microchannels and optimization of the channel size for Schwann cell migration

We newly designed super-elastic biodegradable scaffolds with longitudinally oriented microchannels for repair and regeneration of peripheral nerve defects. Four-armed poly(ε-caprolactone-co-D,L-lactide)s (P(CL-co-DLLA)s) were synthesized by ring-opening copolymerization of CL and DLLA from terminal hydroxyl groups of pentaerythritol, and acryloyl chloride was then reacted with the ends of the chains. The end-functionalized P(CL-co-DLLA) was crosslinked in a cylindrical mold in the presence of longitudinally oriented silica fibers as the templates, which were later dissolved by hydrofluoric acid. The elastic moduli of the crosslinked P(CL-co-DLLA)s were controlled between 10^-1 and 10² MPa at 37 °C, depending on the composition. The scaffolds could be elongated to 700% of their original size without fracture or damage ('super-elasticity'). Scanning electron microscopy images revealed that well-defined and highly aligned multiple channels consistent with the mold design were produced in the scaffolds. Owing to their elastic nature, the microchannels in the scaffolds did not collapse when they were bent to 90°. To evaluate the effect of the channel diameter on Schwann cell migration, microchannels were also fabricated in transparent poly(dimethylsiloxane), allowing observation of cell migration. The migration speed increased with channel size, but the Young's modulus of the scaffold decreased as the channel diameter increased. These findings may serve as the basis for designing tissue-engineering scaffolds for nerve regeneration and investigating the effects of the geometrical and dimensional properties on axonal outgrowth.

Neurite extension of primary neurons on electrospun piezoelectric scaffolds

Neural tissue engineering may be a promising option for neural repair treatment, for which a well-designed scaffold is essential. Smart materials that can stimulate neurite extension and outgrowth have been investigated as potential scaffolding materials. A piezoelectric polymer polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE) was used to fabricate electrospun aligned and random scaffolds having nano- or micron-sized fiber dimensions. The advantage of using a piezoelectric polymer is its intrinsic electrical properties. The piezoelectric characteristics of PVDF-TrFE scaffolds were shown to be enhanced by annealing. Dorsal root ganglion (DRG) neurons attached to all fibrous scaffolds. Neurites extended radially on random scaffolds, whereas aligned scaffolds directed neurite outgrowth for all fiber dimensions. Neurite extension was greatest on aligned, annealed PVDF-TrFE having micron-sized fiber dimensions in comparison with annealed and as-spun random PVDF-TrFE scaffolds. DRG on micron-sized aligned, as-spun and annealed PVDF-TrFE also had the lowest aspect ratio amongst all scaffolds, including non-piezoelectric PVDF and collagen-coated substrates. Findings from this study demonstrate the potential use of a piezoelectric fibrous scaffold for neural repair applications.

Repair of rat sciatic nerve gap by a silk fibroin-based scaffold added with bone marrow mesenchymal stem cells

Tissue-engineered nerve grafts (TENGs), typically consisting of a neural scaffold included with support cells and/or growth factors, represent a promising alternative to autologous nerve grafts for surgical repair of large peripheral nerve gaps. Here, we developed a new design of TENGs by introducing bone marrow mesenchymal stem cells (MSCs) of rats, as support cells, into a silk fibroin (SF)-based scaffold, which was composed of an SF nerve guidance conduit and oriented SF filaments as the conduit lumen filler. The biomaterial SF had been tested to possess good biocompatibility and noncytoxicity with MSCs before the TENG was implanted to bridge a 10-mm-long gap in rat sciatic nerve. Functional and histological assessments showed that at 12 weeks after nerve grafting, TENGs yielded an improved outcome of nerve regeneration and functional recovery, which was better than that achieved by SF scaffolds and close to that by autologous nerve grafts. During 1-4 weeks after nerve grafting, MSCs contained in the TENG significantly accelerated axonal growth, displaying a positive reaction to S-100 (a Schwann cell marker). During 1-3 weeks after nerve grafting, MSCs contained in the TENG led to gene expression upregulation of S100 and several growth factors (brain-derived neurotrophic factor, ciliary neurotrophic factor, and basic fibroblast growth factor). These results suggest that the cell behaviors and neurotrophic functions of MSCs might be responsible for their promoting effects on peripheral nerve regeneration.

Regeneration and functional recovery of intrapelvic nerves removed during extensive surgery by a new artificial nerve conduit: a breakthrough to radical operation for locally advanced and recurrent rectal cancers

PURPOSE

In the current strategy against locally advanced and recurrent rectal cancers possibly involving intrapelvic nerves, there has been a serious dilemma between extensive surgery and limited surgery. The former can attain high tumor curability by sacrificing the nerve functions while the latter prioritizes the patient quality of life by preserving the nerve functions but with a compromised curability. Here we present a new surgical strategy for locally advanced and recurrent rectal cancers, which realize both high tumor curability and good quality of life.

METHODS

A new artificial nerve conduit (polyglycolic acid collagen tube) developed by in site tissue engineering technology was applied to recovery the disturbed functions after removing the nerves from 11 patients undergoing extensive surgery for intrapelvic advanced or recurrent colorectal cancers. The reconstructed nerves included eight autonomic nerves which are essential for the genitourinary function and three somatic nerves which control the sensation and mobility of the legs.

RESULTS

Out of ten cases followed up more than 2 years and evaluated fully, eight including two report cases showed a functional recovery of the disturbed autonomic and somatic nerves clinically. The nerve function started to recover from 3 to 6 months after the operation and continued to improve with times. No specific complications associated with the nerve repair have been noted.

CONCLUSIONS

The new strategy utilizing the nerve conduit can be a breakthrough in radical operations for locally advanced and recurrent rectal cancers to resolve the problems between tumor curability and the patient quality of life.

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.

Use of tissue-engineered nerve grafts consisting of a chitosan/poly(lactic-co-glycolic acid)-based scaffold included with bone marrow mesenchymal cells for bridging 50-mm dog sciatic nerve gaps

Bone marrow mesenchymal cells (MSCs) have attracted increasing research interest due to their possible use as support cells for nerve tissue-engineering approaches. We developed a novel design of tissue-engineered nerve grafts consisting of a chitosan/poly(lactic-co-glycolic acid) (PLGA)-based neural scaffold included with autologous MSCs. The graft was used as an alternative to nerve autografts for bridging 50-mm-long gaps in dog sciatic nerve, and the repair outcome at 6 months after nerve grafting was evaluated by a combination of electrophysiological assessment, FluoroGold retrograde tracing, and histological investigation to regenerated nerve tissue and reinnervated target muscle. The experimental results indicated that introduction of autologous MSCs to the chitosan/PLGA-based neural scaffold promoted sciatic nerve regeneration and functional recovery, demonstrating significant efficacy that was, to a certain degree, close to that by nerve autografting, a gold standard for treating large peripheral nerve gaps, and better than that by grafting with the chitosan/PLGA-based scaffold alone.

Controlling dispersion of axonal regeneration using a multichannel collagen nerve conduit

Single channel conduits are used clinically in nerve repair as an alternative to the autologous nerve graft. Axons regenerating across single channel tubes, however, may disperse resulting in inappropriate target reinnervation. This dispersion may be limited by multichannel nerve conduits as they resemble the structure of nerve multiple basal lamina tubes. In this study, we investigated the influence of channel number on the axonal regeneration using a series of 1-, 2-, 4-, and 7-channel collagen conduits and commercial (NeuraGen) single channel conduits. Nerve conduits were implanted in rats with a 1 cm gap of sciatic nerve. After four months, quantitative results of regeneration were evaluated with nerve morphometry and the accuracy of regeneration was assessed using retrograde tracing: two tracers being applied simultaneously to tibial and peroneal nerves to determine the percentage of motor neurons with double projections. Recovery of function was investigated with compound muscle action potential recordings and ankle motion analysis. We showed that the fabricated 1-channel and 4-channel conduits are superior to other types of conduits in axonal regeneration. Simultaneous tracing showed a significantly lower percentage of motor neurons with double projections after 2- and 4-channel compared with 1-channel conduit repair. This study shows the potential influence of multichannel guidance on limiting dispersion without decreasing quantitative results of regeneration.

Effects of collagen membranes enriched with in vitro-differentiated N1E-115 cells on rat sciatic nerve regeneration after end-to-end repair

Peripheral nerves possess the capacity of self-regeneration after traumatic injury but the extent of regeneration is often poor and may benefit from exogenous factors that enhance growth. The use of cellular systems is a rational approach for delivering neurotrophic factors at the nerve lesion site, and in the present study we investigated the effects of enwrapping the site of end-to-end rat sciatic nerve repair with an equine type III collagen membrane enriched or not with N1E-115 pre-differentiated neural cells. After neurotmesis, the sciatic nerve was repaired by end-to-end suture (End-to-End group), end-to-end suture enwrapped with an equine collagen type III membrane (End-to-EndMemb group); and end-to-end suture enwrapped with an equine collagen type III membrane previously covered with neural cells pre-differentiated in vitro from N1E-115 cells (End-to-EndMembCell group). Along the postoperative, motor and sensory functional recovery was evaluated using extensor postural thrust (EPT), withdrawal reflex latency (WRL) and ankle kinematics. After 20 weeks animals were sacrificed and the repaired sciatic nerves were processed for histological and stereological analysis. Results showed that enwrapment of the rapair site with a collagen membrane, with or without neural cell enrichment, did not lead to any significant improvement in most of functional and stereological predictors of nerve regeneration that we have assessed, with the exception of EPT which recovered significantly better after neural cell enriched membrane employment. It can thus be concluded that this particular type of nerve tissue engineering approach has very limited effects on nerve regeneration after sciatic end-to-end nerve reconstruction in the rat.

Histologic and electrophysiological study of nerve regeneration using a polyglycolic acid-collagen nerve conduit filled with collagen sponge in canine model

OBJECTIVES

To determine the rate of achieving electrophysiologically proved functional recovery by autonomic nerve regeneration, with the aid of an artificial nerve conduit.

METHODS

A polyglycolic acid (PGA) collagen nerve conduit filled with collagen sponge was interposed in a 10-mm-long gap of the right hypogastric nerve (HGN) in 16 dogs. Histologic evaluation of nerve regeneration and electrophysiological analysis at 2 weeks and 2, 3, 4, 5, 6, 7, and 8 months (n = 2, each) after surgery was performed, measuring the responses for the spermatic ducts (SD), bladder neck (BN), and prostate contraction, by stimulating the right lumbar splanchnic nerves (LSNs) from L2 to L4, after transection of the left HGN to eliminate substitutive pathways.

RESULTS

Two months after implantation, the regenerated neurofilaments were successfully extended through the graft from the proximal-to-distal direction. In 2 control dogs, electrostimulation of the right LSNs induced elevation of the intraluminal pressure of the SD, elevation of the BN pressure, and prostate contraction. No responses were observed in all dogs up to 6 months of follow-up after implantation. In 1 dog with a 7-month follow-up, electrostimulation elicited elevation of BN pressure alone. In both dogs with an 8-month follow-up, electrostimulation induced similar responses to control in all SD, BN, and prostate; however, after excision of the area of the interposed right HGN, no response was observed.

CONCLUSIONS

These results proved that regeneration of a 10-mm gap of the HGN, using a novel PGA-collagen nerve conduit could be achieved within 8 months.

Nerve fibroblast impact on Schwann cell behavior

In order to reveal non-neuronal cell interactions after peripheral nerve lesions, we began to analyze the impact of sciatic nerve fibroblasts on Schwann cells in vitro. Both cell types are considered to have opposite effects on axonal regeneration. Few data are available on how repulsive nerve fibroblasts affect neuritotrophic Schwann cells and thus might indirectly influence axonal regrowth. Using different culture systems in conjunction with time-lapse video recording, metabolic labeling, pharmacological intervention, RNAi knockdown, Western blotting and RT-PCR analysis, we found that nerve fibroblasts differentially modify the various responses of Schwann cells. In the presence of collagen type IV and heparan sulfate proteoglycan but not of laminin, diffusible fibroblast factors slow down Schwann cell proliferation. In contrast, fibroblast factors increase the migratory activity of Schwann cells without being chemoattractive. One pro-migratory fibroblast factor turned out to be neuregulin. The pro-migratory activity of nerve fibroblasts and of recombinant neuregulin-1beta1 can be counteracted by neuregulin-specific pharmacological intervention and by neuregulin RNA interference. We show for the first time that nerve fibroblasts play antagonistic and agonistic roles for Schwann cells in a context-dependent manner. The data shed light on cellular mechanisms and have implications for some neuro-tissue engineering strategies.

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.

The biodegradability of poly(Pro-Hyp-Gly) synthetic polypeptide and the promotion of a dermal wound epithelialization using a poly(Pro-Hyp-Gly) sponge

Collagens are widely used in medical applications, but animal-derived collagens have several drawbacks, such as low thermal stability, nonspecific cell attachment, and susceptibility to contamination by infectious pathogens, such as prions, which may transfect humans. We have previously reported the chemical synthesis of polypeptides consisting of a Pro-Hyp-Gly sequence and the high thermostability of their triple-helical structure. To clarify the biomaterial characteristics of the poly(Pro-Hyp-Gly) polypeptide, we assessed its biodegradability and its capability for skin regeneration. Eight weeks after implantation, a poly(Pro-Hyp-Gly) freeze-dried sponge embedded subcutaneously into a rat dorsal area degraded at the same rate as Terudermis, which is made from bovine type I atelocollagen and is used as an artificial dermis. Surprisingly, compared with Terudermis, the poly(Pro-Hyp-Gly) sponge significantly promoted epithelialization of a full-thickness wound on a rabbit's ear pad. This chemically synthesized polypeptide may be useful as a scaffold for tissue engineering and tissue regeneration.

Development and evaluation of silk fibroin-based nerve grafts used for peripheral nerve regeneration

Silk fibroin (SF), derived from natural silk long used as a textile material, has recently become an important biomaterial for tissue engineering applications. We have previously reported on good in vitro biocompatibility of SF fibers with peripheral nerve tissues and cells. In the present study, we developed a novel biomimetic design of the SF-based nerve graft (SF graft) which was composed of a SF-nerve guidance conduit (NGC) inserted with oriented SF filaments. The SF-NGC prepared via well-established procedures exhibits an eggshell-like microstructure that is responsible for its superior mechanical and permeable properties beneficial to nerve regeneration. The SF graft was used for bridge implantation across a 10-mm long sciatic nerve defect in rats, and the outcome of peripheral nerve repair at 6 months post-implantation was evaluated by a combination of electrophysiological assessment, FluoroGold retrograde tracing and histological investigation. The examined functional and morphological parameters show that SF grafts could promote peripheral nerve regeneration with effects approaching those elicited by nerve autografts which are generally considered as the gold standard for treating large peripheral nerve defects, thus raising a potential possibility of using these newly developed nerve grafts as a promising alternative to nerve autografts.

[Bridging peripheral nerve defects by means of nerve conduits]

BACKGROUND

The nerve autograft is the gold standard for the reconstruction of peripheral nerve defects. In short gaps, nerve repair by means of tubulization has become an alternative. This technique is discussed based on the current literature and our own experience.

PATIENTS AND METHODS

Nerve reconstruction by means of tubulization was performed in 11 patients. Nerve gaps in the hand of up to 18 mm were reconstructed. Sensibility was assessed using static and dynamic two point discrimination (s-, d2PD) and monofilament testing 3, 6 and 12 months postoperatively.

RESULTS

Three out of 11 patients complained of a temporary foreign body sensation in the area of the implant; this was persistent in one case. Four out of six patients showed excellent results with s2PD of <or=7 mm (Highet S4) assessed 12 months postoperatively. One patient regained no (s2PD>15 mm, S0), another poor sensibility (s2PD 15 mm, S3). D2PD and monofilament testing confirmed these results.

CONCLUSIONS

Nerve reconstruction by means of tubulization seems to be a suitable method for certain indications in the reconstruction of short defects of digital and palmar nerves. Donor site morbidity can be avoided. Similarly to nerve transplantation, the operation requires microsurgical skills.

Nerve conduits and growth factor delivery in peripheral nerve repair

Peripheral nerves possess the capacity of self-regeneration after traumatic injury. Transected peripheral nerves can be bridged by direct surgical coaptation of the two nerve stumps or by interposing autografts or biological (veins) or synthetic nerve conduits (NC). NC are tubular structures that guide the regenerating axons to the distal nerve stump. Early synthetic NC have primarily been made of silicone because of the relative flexibility and biocompatibility of this material and because medical-grade silicone tubes were readily available in various dimensions. Nowadays, NC are preferably made of biodegradable materials such as collagen, aliphatic polyesters, or polyurethanes. Although NC assist in guiding regenerating nerves, satisfactory functional restoration of severed nerves may further require exogenous growth factors. Therefore, authors have proposed NC with integrated delivery systems for growth factors or growth factor-producing cells. This article reviews the most important designs of NC with integrated delivery systems for localized release of growth factors. The various systems discussed comprise NC with growth factors being released from various types of matrices, from transplanted cells (Schwann cells or mesenchymal stem cells), or through genetic modification of cells naturally present at the site of injured tissue. Acellular delivery systems for growth factors include the NC wall itself, biodegradable microspheres seeded onto the internal surface of the NC wall, or matrices that are filled into the lumen of the NC and immobilize the growth factors through physical-chemical interactions or specific ligand-receptor interactions. A very promising and elegant system appears to be longitudinally aligned fibers inserted in the lumen of a NC that deliver the growth factors and provide additional guidance for Schwann cells and axons. This review also attempts to appreciate the most promising approaches and emphasize the importance of growth factor delivery kinetics.

Experimental repair of phrenic nerve using a polyglycolic acid and collagen tube

OBJECTIVE

The feasibility of a nerve guide tube for regeneration of the phrenic nerve with the aim of restoring diaphragmatic function was evaluated in a canine model.

METHODS

The nerve tube, made of woven polyglycolic acid mesh, had a diameter of 3 mm and was filled with collagen sponge. This polyglycolic acid-collagen tube was implanted into a 10-mm gap created by transection of the right phrenic nerve in 9 beagle dogs. The tubes were implanted without a tissue covering in 5 of the 9 dogs (group I), and the tubes were covered with a pedicled pericardial fat pad in 4 dogs (group II). Chest x-ray films, muscle action potentials, and histologic samples were examined 4 to 12 months after implantation.

RESULTS

All of the dogs survived without any complications. x-ray film examination showed that the right diaphragm was paralyzed and elevated in all dogs until 3 months after implantation. At 4 months, movement of the diaphragm in the implanted side was observed during spontaneous breathing in 1 dog of group I and in 3 dogs of group II. In the dogs showing diaphragm movement, muscle action potentials were evoked in the diaphragm muscle, indicating restoration of nerve function. Regeneration of the phrenic nerve structure was also examined on the reconstructed site using electron microscopy.

CONCLUSION

The polyglycolic acid-collagen tube induced functional recovery of the injured phrenic nerve and was aided by coverage with a pedicled pericardial fat pad.

Luminal fillers in nerve conduits for peripheral nerve repair

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

Combination of engineered neural cell adhesion molecules and GDF-5 for improved neurite extension in nerve guide concepts

Current therapeutical approaches for the treatment of severe lesions in the peripheral nervous system rely on the use of autologous tissue or the body's own Schwann cells. However, these approaches are limited and alternative strategies for peripheral nerve regeneration are required. Here we evaluate combinations of a variety of neuronal regeneration factors including engineered cell adhesion molecules and growth factors in embryonic model neurons to test the possible improvement of artificial nerve guides by cooperative mechanisms. Cell adhesion molecules L1 and neurofascin synergistically promote neurite elongation. The outgrowth promoting properties of both proteins can be combined and further increased within one chimeric protein. Addition of growth and differentiation factor 5 (GDF-5) further enhances neurite outgrowth in a substrate-independent manner. This effect is not due to a protective mode of action of GDF-5 against pro-apoptotic stimuli. Consequently, the study supports the idea that different modes of action of pro-regenerative factors may contribute synergistically to neurite outgrowth and emphasizes the applicability of combinations of proteins specifically involved in development of the nervous system for therapeutical approaches.

Synthesis of poly(Pro-Hyp-Gly)(n) by direct poly-condensation of (Pro-Hyp-Gly)(n), where n=1, 5, and 10, and stability of the triple-helical structure

Pro-Hyp-Gly is a characteristic amino acid sequence found in fibrous collagens, and (Pro-Hyp-Gly)(10), which has been widely used as a collagen-model peptide, forms a stable triple-helical structure. Here, we synthesized polypeptides consisting of the Pro-Hyp-Gly sequence by direct poly-condensation of (Pro-Hyp-Gly)(n), where n=1, 5, and 10, using 1-hydroxybenzotriazole and 1-ethyl-3-(3-dimethyl-aminopropyl)-carbodiimide hydrochloride in both phosphate buffer (pH=7.4) and dimethylsulfoxide (DMSO) solutions for 48 h at 20 degrees C. The reaction of (Pro-Hyp-Gly)(5) and (Pro-Hyp-Gly)(10) in DMSO successfully gave polypeptides with molecular weights over 10,000, whereas low molecular weight products were obtained by reaction in phosphate buffer (pH=7.4). In contrast, Pro-Hyp-Gly at a concentration of 50 mg/mL in phosphate buffer (pH=7.4) gave polypeptides with molecular weights over 10,000. The Fourier transform infrared (FTIR) and (1)H nuclear magnetic resonance (NMR) spectra of poly(Pro-Hyp-Gly)(10) revealed that the polymerization of (Pro-Hyp-Gly)(10) described in this report had no side reactions. Each polypeptide obtained shows a collagen-like triple-helical structure, and the triple-helical structures of poly(Pro-Hyp-Gly) and poly(Pro-Hyp-Gly)(10) were stable up to T=80 degrees C, which suggests that the high molecular weight promotes stability of the triple-helical structure, in addition to the high Hyp content. Furthermore, transmission electron microscopy (TEM) observations show that poly(Pro-Hyp-Gly)(10) aggregates to form nanofiber-like structures about 10 nm in width, which suggests that a Pro-Hyp-Gly repeating sequence contains enough information for triple-helix formation, and for subsequent nanofiber-like structure formation.

Surgical relief of causalgia with an artificial nerve guide tube: Successful surgical treatment of causalgia (Complex Regional Pain Syndrome Type II) by in situ tissue engineering with a polyglycolic acid-collagen tube

Two patients with causalgia associated with allodynia and finger contracture were treated surgically with a bioresorbable nerve guide tube made from polygycolic acid and collagen: the injured segment of the digital nerve was resected and the resulting gap (25 and 36mm) was bridged with the tube. In both cases, a neuroma was found on the injured nerve and many sprouting branches were. After reconstruction, the causalgia and allodynia disappeared and movement of the fingers recovered during the following 6 months. Functional recovery was objectively identified for 1 year and 9 months. Both patients regained full use of their finger and were free of discomfort for up to 24 and 18 months, respectively. Since the first description of causalgia in 1864, there has been no definitive treatment for this intractable burning pain. Our experience shows that at least some types of causalgia can be resolved successfully by surgery.

Protection against photic damage in retinitis pigmentosa

Experiments on photic damage to the retinas of rats with hereditary retinal dystrophy and some tentative clinical evidence on human patients suggest that, in human retinitis pigmentosa, one could try to protect the retina and especially the rods from bright light in an attempt to delay the retinal degeneration and to prolong the period of useful vision. Several theoretical criteria have been proposed for protection of RP patients from possible photic retinal damage. Observing these criteria, Adrian developed a brownish ophthalmic filter which absorbs the short wavelengths preferentially, thus protecting the rods primarily. Whether or not use of these filters will be efficacious has yet to be determined and will require careful experimentation and the accumulation of clinical experience. Several brown ophthalmic filters also have been tested against the criteria for a protective device. The NoIR Amber 7% plastic glasses satisfy these criteria quite well and thus can be considered as a substitute for the Adrian lens. The characteristics of the two types of filters are compared. Experience with different methods of protection may show whether it is better to attempt to delay degeneration of both retinas simultaneously by decreasing their illuminations with filters or to exclude light completely from one eye in an attempt to preserve it while the other eye degenerates in the usual course of the disease. In any event, given the present state of knowledge, it seems to be appropriate, especially in the early stages of the disease, to suggest that RP patients protect their retinas from excessive light.

Antigenicity and immunogenicity of collagen

Pertinent issues of collagen antigenicity and immunogenicity are concisely reviewed as they relate to the design and application of biomedical devices. A brief discussion of the fundamental concepts of collagen immunochemistry is presented, with a subsequent review of documented clinical responses to devices containing reconstituted soluble or solubilized collagen. The significance of atelocollagen, concerns regarding collagen-induced autoimmunity, and other relevant topics are also addressed in the context of current understanding of the human immune response to collagen.

Physical and biological performance of a novel block copolymer nerve guide

Although the ability to regenerate is evident in the nervous system, lesioned neurites are unable to cross gaps in neuronal pathways. In order to bridge gaps, guiding cues are essential to direct neurite regrowth. To overcome many of the shortcomings of polymer-based nerve guides, we developed a bioresorbable nerve guide composed of a novel trimethylene carbonate-caprolacton block copolymer (TMC-CL). Pore formation was controlled by using special solvent/precipitation media compositions in combination with the pore forming agent poly ethylene glycol (PEG). NMR spectroscopy, shear force-, compression-, and permeation assays were used for conduit characterization. The polymer conduit has a semipermeable wall with submicron pores to allow free metabolite/drug exchange. In order to investigate the principle of temporally controlled expression of therapeutic proteins in nerve guides, Neuro-2a cells were genetically engineered to express the reporter gene product green fluorescent protein (GFP) under the control of the Tet-On system. When these transduced cells were encapsulated in nerve guides, GFP expression could be induced for days by adding the antibiotic tetracycline derivative doxycycline to the nerve guide environment. Furthermore, encapsulated dorsal root ganglia (DRG) produced long neurites in vitro. In subsequent in vivo experiments, nerve guides filled with Schwann cells (SC) were implanted into lesioned spinal cords of adult rats. Regeneration of spinal cord axons into nerve guides was promoted by co-implanted Schwann cells. The data suggest that the novel TMC-CL nerve guides provide a promising tool for neuroregeneration.

Nerve regeneration: might this be the only solution for functional problems of the urinary tract?

PURPOSE OF REVIEW

To assess the potential role of nerve regeneration in restoring urinary tract function, the rapidly developing and exciting area of central and peripheral nerve repair and regeneration is reviewed, with particular reference to papers in which animal models of nerve damage resulting in urogenital dysfunction have been used. The difficulties and potential of these techniques for therapeutic application to human subjects with functional problems of the urinary tract are discussed.

RECENT FINDINGS

Methods for encouraging regeneration of cut axons and directed growth in the inhibitory environment of the central nervous system are being extensively explored. The recent discovery of the potential of olfactory ensheathing cells has proved a significant advance. Olfactory ensheathing cells are a type of glial cell which can be harvested from the olfactory mucosa. Transplantation of these cells, in conjunction with a biodegradable synthetic nerve guide or conduit, has been shown to restore urinary tract function after spinal cord injury. Artificial, biodegradable conduits have also restored bladder and spermatic duct function after sympathetic nerve damage. Other adjuvants facilitating the process of axonal recovery include the use of neurotrophins to accelerate and guide the formation of new nerve-fibre growth.

SUMMARY

These revolutionary technologies may, in the future, provide a means of treating urinary tract dysfunction with some types of aetiology, including acute spinal cord injury, and injury to nerves following pelvic surgery. It is, however, less likely that these treatments will be used successfully in the near future in patients in which the neural damage is long term, or associated with death of post-ganglionic neurons.

Neural tissue engineering: strategies for repair and regeneration

Nerve regeneration is a complex biological phenomenon. In the peripheral nervous system, nerves can regenerate on their own if injuries are small. Larger injuries must be surgically treated, typically with nerve grafts harvested from elsewhere in the body. Spinal cord injury is more complicated, as there are factors in the body that inhibit repair. Unfortunately, a solution to completely repair spinal cord injury has not been found. Thus, bioengineering strategies for the peripheral nervous system are focused on alternatives to the nerve graft, whereas efforts for spinal cord injury are focused on creating a permissive environment for regeneration. Fortunately, recent advances in neuroscience, cell culture, genetic techniques, and biomaterials provide optimism for new treatments for nerve injuries. This article reviews the nervous system physiology, the factors that are critical for nerve repair, and the current approaches that are being explored to aid peripheral nerve regeneration and spinal cord repair.

Mechanical properties of acellular peripheral nerve

BACKGROUND

Acellular nerve has been used in experimental models as a peripheral nerve substitute. Our objective was to determine the difference in tensile strength between fresh and chemically treated acellularized peripheral nerve.

MATERIALS AND METHODS

F344 rat sciatic nerves were either fresh or acellularized and tested either whole (Part A) or transected and repaired (Part B). For all constructs, the mean ultimate stress, mean ultimate strain, Young's modulus, and total mechanical work to fracture were calculated. The average ultimate strains for Groups A-1 and A-2 were 0.480 +/- 0.117 and 0.810 +/- 0.114, respectively. The Young's moduli in Groups A-1 and A-2 were 576 +/- 160 and 580 +/- 150 kPa, respectively. In Groups A-1 and A-2, the normalized work to failure was 0.35 +/- 0.14 and 1.11 +/- 0.38 N. The specimens in Group B-1 withstood an average ultimate stress of 780 +/- 280 kPa. The specimens in Group B-2 withstood an average ultimate stress of 405 +/- 20 kPa.

RESULTS

The average ultimate strains for Groups B-1 and B-2 were 0.319 +/- 0.087 and 0.266 +/- 0.019, respectively. The Young's moduli in Groups B-1 and B-2 were 4,030 +/- 1360 and 2,290 +/- 280 kPa, respectively. The normalized work to failure in Groups B-1 and B-2 was calculated as 0.22 +/- 0.04 and 0.11 +/- 0.02 N.

CONCLUSIONS

Although adequately robust for reconstructive procedures, the acellular peripheral nerve had decreased tensile strength compared with fresh nerve either when tested whole or when transected and repaired.

Effect of basic fibroblast growth factor on vascularization in esophagus tissue engineering

We carried out an experimental study to evaluate the effect of basic fibroblast growth factor (bFGF)-containing collagen gel on vascularization in esophageal tissue engineering. We compared an acellular collagen sponge scaffold and an acellular collagen gel scaffold in combination with bFGF using a canine model. The construct was implanted in the cervical esophagus and the regenerated tissue was evaluated one month after surgery. Histological analysis confirmed a significantly large amount of blood vessels in the bFGF-containing collagen gel group as compared to the collagen gel group without bFGF (bFGF (-)). However, in the collagen sponge groups, no difference was observed between the bFGF (+) group and the bFGF (-) group. These results showed that bFGF-containing collagen gel is suitable not only for an acellular scaffold for tissue engineering but also for an effective tropic factor vehicle in vivo.

Regeneration of hypogastric nerve using a polyglycolic acid (PGA)-collagen nerve conduit filled with collagen sponge proved electrophysiologically in a canine model

The hypogastric nerve (HGN) is a sympathetic nerve in the peritoneal cavity and controls urinary and seminal functions. In this study, the regeneration of HGN was determined by using a new type of an artificial nerve conduit, polyglycolic acid (PGA)-collagen nerve conduit filled with collagen sponge in two dogs. A PGA-collagen nerve conduit (diameter=2 mm) was interposed in a 10 mm gap of the right HGN. The regeneration of the HGN was evaluated electrophysiologically 8 months after the operation. The intraluminal pressure of spermatic duct and the bladder neck were elevated 80 mmHg and 25 mmHg respectively by the stimulation across the regenerated HGN. The prostate contraction was also elicited. The responses diminished after the excision of the regenerated portion of HGN. These results proved the regeneration of HGN and this nerve conduit will be great help for patients who suffer from urinary and seminal disturbances.