Physical and biological performance of a novel block copolymer nerve guide

Optimal Morphometric Characteristics of a Tubular Polymeric Scaffold to Promote Peripheral Nerve Regeneration: A Scoping Review

Cellular behavior in nerve regeneration is affected by the architecture of the polymeric nerve guide conduits (NGCs); therefore, design features of polymeric NGCs are critical for neural tissue engineering. Hence, the purpose of this scoping review is to summarize the adequate quantitative/morphometric parameters of the characteristics of NGC that provide a supportive environment for nerve regeneration, enhancing the understanding of a previous study. 394 studies were found, of which 29 studies were selected. The selected studies revealed four morphometric characteristics for promoting nerve regeneration: wall thickness, fiber size, pore size, and porosity. An NGC with a wall thickness between 250–400 μm and porosity of 60–80%, with a small pore on the inner surface and a large pore on the outer surface, significantly favored nerve regeneration; resulting in an increase in nutrient permeability, retention of neurotrophic factors, and optimal mechanical properties. On the other hand, the superiority of electrospun fibers is described; however, the size of the fiber is controversial in the literature, obtaining optimal results in the range of 300 nm to 30 µm. The incorporation of these optimal morphometric characteristics will encourage nerve regeneration and help reduce the number of experimental studies as it will provide the initial morphometric parameters for the preparation of an NGC.

Engineering Fiber-Based Nervous Tissue Constructs for Axon Regeneration

Biomaterial-based scaffolds used in nerve conduits including channels for confining regenerating axons and 3-dimensional (3D) gels as substrates for growth have made improvements in models of nerve repair. Many biomaterial strategies, however, continue to fall short of autologous nerve grafts, which remain the current gold standard in repairing severe nerve lesions (<20 mm). Intraluminal nerve conduit fibers have also shown considerable promise in directing regenerating axons in vitro and in vivo and have gained increasing interest for nerve repair. It is unknown, however, how growing axons respond to a fiber when encountered in a 3D environment. In this study, we considered a construct consisting of a compliant collagen hydrogel matrix and a fiber component to assess contact-guided axon growth. We investigated preferential axon outgrowth on synthetic and natural polymer fibers by utilizing small-diameter microfibers of poly-L-lactic acid and type I collagen representing 2 different fiber stiffnesses. We found that axons growing freely in a 3D hydrogel culture preferentially attach, turn and follow fibers with outgrowth rates and distances that far exceed outgrowth in a hydrogel alone. Wet-spun type I collagen from rat tail tendon performed the best, associated with highly aligned and accelerated outgrowth. This study also evaluated the response of dorsal root ganglion neurons from adult rats to provide data more relevant to axon regenerative potential in nerve repair. We found that ECM treatments on fibers enhanced the regeneration of adult axons indicating that both the physical and biochemical presentation of the fibers are essential for enhancing axon guidance and growth.

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.

Regenerative effects of human embryonic stem cell-derived neural crest cells for treatment of peripheral nerve injury

Surgical intervention is the current gold standard treatment following peripheral nerve injury. However, this approach has limitations, and full recovery of both motor and sensory modalities often remains incomplete. The development of artificial nerve grafts that either complement or replace current surgical procedures is therefore of paramount importance. An essential component of artificial grafts is biodegradable conduits and transplanted cells that provide trophic support during the regenerative process. Neural crest cells are promising support cell candidates because they are the parent population to many peripheral nervous system lineages. In this study, neural crest cells were differentiated from human embryonic stem cells. The differentiated cells exhibited typical stellate morphology and protein expression signatures that were comparable with native neural crest. Conditioned media harvested from the differentiated cells contained a range of biologically active trophic factors and was able to stimulate in vitro neurite outgrowth. Differentiated neural crest cells were seeded into a biodegradable nerve conduit, and their regeneration potential was assessed in a rat sciatic nerve injury model. A robust regeneration front was observed across the entire width of the conduit seeded with the differentiated neural crest cells. Moreover, the up-regulation of several regeneration-related genes was observed within the dorsal root ganglion and spinal cord segments harvested from transplanted animals. Our results demonstrate that the differentiated neural crest cells are biologically active and provide trophic support to stimulate peripheral nerve regeneration. Differentiated neural crest cells are therefore promising supporting cell candidates to aid in peripheral nerve repair.

Trimethylene carbonate-caprolactone conduit with poly-p-dioxanone microfilaments to promote regeneration after spinal cord injury

Spinal cord injury (SCI) is often associated with scarring and cavity formation and therefore bridging strategies are essential to provide a physical substrate for axonal regeneration. In this study we investigated the effects of a biodegradable conduit made from trimethylene carbonate and ε-caprolactone (TC) containing poly-p-dioxanone microfilaments (PDO) with longitudinal grooves on regeneration after SCI in adult rats. In vitro studies demonstrated that different cell types including astrocytes, meningeal fibroblasts, Schwann cells and adult sensory dorsal root ganglia neurons can grow on the TC and PDO material. For in vivo experiments, the TC/PDO conduit was implanted into a small 2-3 mm long cavity in the C3-C4 cervical segments immediately after injury (acute SCI) or at 2-5 months after initial surgery (chronic SCI). At 8 weeks after implantation into acute SCI, numerous 5HT-positive descending raphaespinal axons and sensory CGRP-positive axons regenerated across the conduit and were often associated with PDO microfilaments and migrated host cells. Implantation into chronically injured SCI induced regeneration mainly of the sensory CGRP-positive axons. Although the conduit had no effect on the density of OX42-positive microglial cells when compared with SCI control, the activity of GFAP-positive astrocytes was reduced. The results suggest that a TC/PDO conduit can support axonal regeneration after acute and chronic SCI even without addition of exogenous glial or stem cells.

STATEMENT OF SIGNIFICANCE

Biosynthetic conduits can support regeneration after spinal cord injury but often require addition of cell therapy and neurotrophic factors. This study demonstrates that biodegradable conduits made from trimethylene carbonate and ε-caprolactone with poly-p-dioxanone microfilaments alone can promote migration of different host cells and stimulate axonal regeneration after implantation into acute and chronic spinal cord injury. These results can be used to develop biosynthetic conduits for future clinical applications.

Transplantation of Schwann Cells Inside PVDF-TrFE Conduits to Bridge Transected Rat Spinal Cord Stumps to Promote Axon Regeneration Across the Gap

Among various models for spinal cord injury in rats, the contusion model is the most often used because it is the most common type of human spinal cord injury. The complete transection model, although not as clinically relevant as the contusion model, is the most rigorous method to evaluate axon regeneration. In the contusion model, it is difficult to distinguish regenerated from sprouted or spared axons due to the presence of remaining tissue post injury. In the complete transection model, a bridging method is necessary to fill the gap and create continuity from the rostral to the caudal stumps in order to evaluate the effectiveness of the treatments. A reliable bridging surgery is essential to test outcome measures by reducing the variability due to the surgical method. The protocols described here are used to prepare Schwann cells (SCs) and conduits prior to transplantation, complete transection of the spinal cord at thoracic level 8 (T8), insert the conduit, and transplant SCs into the conduit. This approach also uses in situ gelling of an injectable basement membrane matrix with SC transplantation that allows improved axon growth across the rostral and caudal interfaces with the host tissue.

Providing the right cues in nerve guidance conduits: Biofunctionalization versus fiber profile to facilitate oriented neuronal outgrowth

Following peripheral nerve injury, rapid and spatially oriented axonal outgrowth from the proximal nerve stump is required for successful tissue regeneration. Regenerative strategies such as introducing fiber bundles into the nerve guidance conduits improve the directional growth of neurons and Schwann cells. Recently, it has been proposed that fiber profiling increases cell alignment and could accelerate neuronal growth. Here, we evaluate the impact of fiber profiling on the extent of neurite outgrowth in vitro as compared to non-profiled round fibers. We developed novel profiled trilobal poly(lactic acid) (PLA) fibers and systematically tested their potency to support nerve regeneration in vitro. The profiled fibers did not improve neurite outgrowth as compared to the round fibers. Instead, we show that growing neurites are merely guided by the type and quantity of proteins adsorbed on the polymer surface. Together this data has significant implications for in vivo experiments focusing on directional regrowth of severed axons across lesion sites during peripheral nerve regeneration.

Carriers in cell-based therapies for neurological disorders

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

Controlled formation of cross-linked collagen fibers for neural tissue engineering applications

Fibrous scaffolds engineered to direct the growth of tissues can be important in forming architecturally functional tissue such as aligning regenerating nerves with their target. Collagen is a commonly used substrate used for neuronal growth applications in the form of surface coatings and hydrogels. The wet spinning technique can create collagen fibers without the use of organic solvents and is typically accomplished by extruding a collagen dispersion into a coagulation bath. To create well-controlled and uniform collagen fibers, we developed an automatic wet spinning device with precise control over the spinning and fiber collection parameters. A fiber collection belt allowed the continuous formation of very soft and delicate fibers up to half a meter in length. Wet-spun collagen fibers were characterized by tensile and thermal behavior, diameter uniformity, the swelling response in phosphate buffered saline and their biocompatibility with dorsal root ganglion (DRG) neurons and Schwann cells. Fibers formed from 0.75% weight by volume (w/v) collagen dispersions formed the best fibers in terms of tensile behavior and fiber uniformity. Fibers post-treated with the cross-linkers glutaraldehyde and genipin exhibited increased mechanical stability and reduced swelling. Importantly, genipin-treated fibers were conducive to DRG neurons and Schwann cell survival and growth, which validated the use of this cross-linker for neural tissue engineering applications.

A prospective randomized study comparing woven polyglycolic acid and autogenous vein conduits for reconstruction of digital nerve gaps

PURPOSE

The optimal management of a nerve gap within the fingers remains an unanswered question in hand surgery. The purpose of this study was to compare the sensory recovery, cost, and complication profile of digital nerve repair using autogenous vein and polyglycolic acid conduits.

METHODS

We enrolled patients undergoing repair of digital nerve injuries with gaps precluding primary repair. The minimum gap that was found to preclude primary repair was 4 mm. Each nerve repair was randomized to the type of nerve repair with either a woven polyglycolic acid conduit or autogenous vein. Time required for repair was recorded. We performed sensory testing, consisting of static and moving 2-point discrimination, at 6 and 12 months after repair. We compared patient factors between the 2 groups using chi-square and Student's t-test. We compared sensory recovery between the 2 groups at each time point using Student's t-test and compared time and cost of repair.

RESULTS

We enrolled 42 patients with 76 nerve repairs. Of these, 37 patients (representing 68 repairs) underwent sensory evaluation at the 6-month time point. The median age in this group was 35 years. We repaired 36 nerves with synthetic conduit and 32 with vein. Nerve gaps ranged from 4 to 25 mm (mean, 10 mm). Study groups were not significantly different regarding age, time to repair, gap length, medical history, smoking history, or worker's compensation status. Time to harvest the vein was longer but the average cost of materials and surgery in the vein group was $1,220, compared with $1,269 for synthetic conduit repairs. These differences were not statistically significant. Mean static and moving 2-point discrimination at 6 months for the synthetic conduit group were 8.3 ± 2.0 and 6.6 ± 2.3, respectively, compared with 8.5 ± 1.8 and 7.1 ± 2.2 for the vein group. Values at 12 months for the synthetic conduit group were 7.5 ± 1.9 and 5.6 ± 2.2, compared with 7.6 ± 2.6 and 6.6 ± 2.9 for the vein group. These differences were not statistically significant. Smokers and worker's compensation patients had a worse sensory recovery at 12 months postrepair. There were 2 extrusions in the synthetic conduit group requiring reoperation; however, the difference in extrusion rate was not found to be statistically significant.

CONCLUSIONS

Sensory recovery after digital nerve reconstruction with autogenous vein conduit was equivalent to that using polyglycolic acid conduit, with a similar cost profile and fewer postoperative complications.

In vivo study of ethyl-2-cyanoacrylate applied in direct contact with nerves regenerating in a novel nerve-guide

Stitch suture is still the most recommended method to hold a nerve-guide in place but stitch suture is a well known cause of local inflammatory response. Glues of several kinds have been proposed as an alternative but they are not easy to apply in a real surgical setting. In 2006 authors developed a new concept of nerve-guide termed "NeuroBox" which is double-halved, not-degradable and rigid, and allows the use of cyanoacrylic glues. In this study, Authors analyzed histologically the nerve-glue interface. Wistar rats were used as animal model. In group 1, animals were implanted a NeuroBox to promote the regeneration of an experimentally produced 4 mm gap in the sciatic nerve. In group 2, the gap was left without repair ("sham-operated" group). Group 3 was assembled by harvesting 10 contralateral intact nerves to document the normal anatomy. Semi-thin sections for visible light microscopy and ultra-thin sections for Transmission Electron Microscopy were analyzed. Results showed that application of ethyl-2-cyanoacrylate directly to the epineurium produced no significative insult to the underlining nerve fibers nor impaired nerve regeneration. No regeneration occurred in the "sham-operated" group.

Polysialic acid immobilized on silanized glass surfaces: a test case for its use as a biomaterial for nerve regeneration

The immobilization of polysialic acid (polySia) on glass substrates has been investigated with regard to the applicability of this polysaccharide as a novel, biocompatible and bioresorbable material for tissue engineering, especially with regard to its use in nerve regeneration. PolySia, a homopolymer of alpha-2,8-linked sialic acid, is involved in post-translational modification of the neural cell adhesion molecule (NCAM). The degradation of polySia can be controlled which makes it an interesting material for coating and for scaffold construction in tissue engineering. Here, we describe the immobilization of polySia on glass surfaces via an epoxysilane linker. Whereas glass surfaces will not actually be used in nerve regeneration scaffolds, they provide a simple and efficient means for testing various methods for the investigation of immobilized polySia. The modified surfaces were investigated with contact angle measurements and the quantity of immobilized polySia was examined by the thiobarbituric acid assay and a specific polySia-ELISA. The interactions between the polySia-modified surface and immortalized Schwann cells were evaluated via cell adhesion and cell viability assays. The results show that polySia can be immobilized on glass surfaces via the epoxysilane linker and that surface-bound polySia has no toxic effects on Schwann cells. Therefore, as a key substance in the development of vertebrates and as a favourable substrate for the cultivation of Schwann cells, it offers interesting features for the use in nerve guidance tubes for treatment of peripheral nerve injuries.

Polymers for Fabricating Nerve Conduits

Peripheral nerve regeneration is a complicated and long-term medical challenge that requires suitable guides for bridging nerve injury gaps and restoring nerve functions. Many natural and synthetic polymers have been used to fabricate nerve conduits as well as luminal fillers for achieving desired nerve regenerative functions. It is important to understand the intrinsic properties of these polymers and techniques that have been used for fabricating nerve conduits. Previously extensive reviews have been focused on the biological functions and in vivo performance of polymeric nerve conduits. In this paper, we emphasize on the structures, thermal and mechanical properties of these naturally derived synthetic polymers, and their fabrication methods. These aspects are critical for the performance of fabricated nerve conduits. By learning from the existing candidates, we can advance the strategies for designing novel polymeric systems with better properties for nerve regeneration.

Strategies for inducing the formation of bands of Büngner in peripheral nerve regeneration

Peripheral human nerves fail to regenerate across longer tube implants (>2 cm), most likely because implants lack the microarchitecture of native nerves, including bands of Büngner. Bands of Büngner comprise longitudinally aligned Schwann cell strands that guide selectively regrowing axons. We aim to optimize tubular implants by integrating artificial bands of Büngner. Three principle strategies for inducing the formation of bands of Büngner were investigated: (a) an aligned extracellular matrix, (b) polarizing differentiation factors, and (c) microstructured biomaterial filaments. In vitro oriented collagen and a combination of differentiation factors (NGF, neuregulin-1, TGF-beta) induced Schwann cell alignment to some extent. The most pronounced Schwann cell alignment was evident on ultrathin, endless poly-epsilon-caprolactone (PCL) filaments with longitudinal microgrooves. Precoated PCL filaments proved to be non-cytotoxic, displayed good cell attachment, and supported Schwann cell proliferation as well as guided axonal outgrowth. In vitro on PCL filaments Schwann cells displayed a polarized expression of the cell adhesion molecule L1 similar to that seen in vivo in bands of Büngner after sciatic nerve crush in adult rats. In summary, the integration of bioengineered bands of Büngner based on microstructured polymer filaments in nerve conduits promises to be the most valuable approach to initiating a more efficient regeneration across longer nerve lesions.

Chapter 8: Current techniques and concepts in peripheral nerve repair

Despite the progress in understanding the pathophysiology of peripheral nervous system injury and regeneration, as well as advancements in microsurgical techniques, peripheral nerve injuries are still a major challenge for reconstructive surgeons. Thorough knowledge of anatomy, pathophysiology, and surgical reconstruction is a prerequisite of proper peripheral nerve injury management. This chapter reviews the currently available surgical treatment options for different types of nerve injuries in clinical conditions. In overview of direct nerve repair, various end-to-end coaptation techniques and the role of end-to-side repair for proximal nerve injuries is described. When primary repair cannot be performed without undue tension, nerve grafting or tubulization techniques are required. Current gold standard for bridging nerve gaps is nerve autografting. However, disadvantages of this approach, such as donor site morbidity and limited length of available graft material encouraged the search for alternative means of nerve gap reconstruction. Nerve allografting was introduced for repair of extensive nerve injuries. Tubulization techniques with natural or artificial conduits are applicable as an alternative for bridging short nerve defects without the morbidities associated with harvesting of autologous nerve grafts. Achieving better outcomes depends both on the advancements in microsurgical techniques and introduction of molecular biology discoveries into clinical practice. The field of peripheral nerve research is dynamically developing and concentrates on more sophisticated approaches tested at the basic science level. Future directions in peripheral nerve reconstruction including, tolerance induction and minimal immunosuppression for nerve allografting, cell based supportive therapies and bioengineering of nerve conduits are also reviewed in this chapter.

Artificial nerve tubes and their application for repair of peripheral nerve injury: an update of current concepts

Over the last 20 years, an increasing number of research articles have reported on the use of artificial nerve tubes to repair nerve defects. The development of an artificial nerve tube as an alternative to autogenous nerve grafting is currently a focus of interest for peripheral nerve repair. The clinical employment of tubes as an alternative to autogenous nerve grafts is mainly justified by the limited availability of donor tissue for nerve autografts and the related morbidity. Numerous studies indicate that short-distance defects in humans can be successfully treated by implantation of artificial nerve guides. This review provides a brief overview of various preclinical and clinical trials conducted to evaluate the utility of artificial nerve tubes for the regeneration of peripheral nerves. This review is also intended to help update hand surgeons on the rapid advances in tubulization techniques, and to provide them with indications of the various directions toward which future research can proceed. Future studies need to provide us with as much comparative information as possible on the effectiveness of different tubulization techniques, in order to guide the surgeon in choosing the best indications for their optimal clinical employment. Future progress in implant development can be expected from interdisciplinary approaches involving both materials and life sciences, leading to advances in neuro-tissue engineering that will be needed to effectively treat larger nerve defects.

Permeability testing of biomaterial membranes

The permeability characteristics of biomaterials are critical parameters for a variety of implants. To analyse the permeability of membranes made from crosslinked ultrathin gelatin membranes and the transmigration of cells across the membranes, we combined three technical approaches: (1) a two-chamber-based permeability assay, (2) cell culturing with cytochemical analysis and (3) biochemical enzyme electrophoresis (zymography). Based on the diffusion of a coloured marker molecule in conjunction with photometric quantification, permeability data for a gelatin membrane were determined in the presence or absence of gelatin degrading fibroblasts. Cytochemical evaluation after cryosectioning of the membranes was used to ascertain whether fibroblasts had infiltrated the membrane inside. Zymography was used to investigate the potential release of proteases from fibroblasts, which are known to degrade collagen derivatives such as gelatin. Our data show that the diffusion equilibrium of a low molecular weight dye across the selected gelatin membrane is approached after about 6-8 h. Fibroblasts increase the permeability due to cavity formation in the membrane inside without penetrating the membrane for an extended time period (>21 days in vitro). Zymography indicates that cavity formation is most likely due to the secretion of matrix metalloproteinases. In summary, the combination of the depicted methods promises to facilitate a more rational development of biomaterials, because it provides a rapid means of determining permeability characteristics and bridges the gap between descriptive methodology and the mechanistic understanding of permeability alterations due to biological degradation.

Topology of Schwann cells and sympathetic innervation along preglomerular vessels: a confocal microscopic study in protein S100B/EGFP transgenic mice

Schwann cells (Sc), associated axons, and nearby vascular endothelium constitute a functional trilogy of major importance during the development and regrowth of peripheral vascular nerves. The goal of the present study is to provide a technique of triple fluorescence confocal imaging of these cell types along renal preglomerular vessels. We took advantage of a protein S100B/EGFP transgenic mouse to visualize Sc. The endothelium was labeled with an intravenous injection of fluorescently tagged lectin, and after tissue processing, adrenergic nerves were revealed with an antibody against the marker protein synaptophysin. As a validation step, we found that EGFP-positive perivascular cells with prominent cell bodies and extensive, multidirectional cell processes were protein S100B positive. They were identified as Sc and indirectly assumed to be unmyelinated Sc. By contrast, we found strong EGFP expression in proximal epithelial cells and in the epithelium lining thin limbs of Henle. This epithelial fluorescence was not associated with immunoreactive protein S100B and thus corresponded to ectopic EGFP expressions in this mouse strain. Sc were organized in bundles or as a meshwork surrounding the preglomerular vasculature from arcuate arteries to afferent arterioles. No Sc were detected in the medulla. Although most Sc were closely apposed to adrenergic varicosities, many varicosities were not associated with detectable Sc processes. The present technique, and the capacity of confocal microscopy to yield three-dimensional imaging, allow the study of the microtopology of Sc and related sympathetic axons in the renal perivascular interstitium.

Approaches to neural tissue engineering using scaffolds for drug delivery

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

Aging Schwann cells in vitro

Schwann cells (SCs) can support the regeneration of lesioned fiber tracts of the peripheral and central nervous system and have been transplanted alone or in combination with synthetic nerve guides. For neuronal tissue engineering purposes, the cells must be isolated from small biopsies and expanded in vitro. In this study we analyze the impact of cell expansion on 9 different cell parameters, comparing short- and long-term cultured rat SCs, which we refer to as 'young' and 'old' or 'aged' cells, respectively. In comparison to young SCs, old SCs doubled the axonal outgrowth from dorsal root ganglion explants and displayed only one-third as much adhesion to the gray and white matter of spinal cord cryosections. In a 3-dimensional extracellular matrix the two cell populations showed very different cellular responses with regard to cell morphology and cell-cell adhesion. Cell proliferation of old SCs was independent of serum components and was not hampered by contact inhibition. In addition, population doubling times were reduced by a factor of almost three compared to those of young SCs. Despite considerable karyotype changes, with an average of 68.7 chromosomes versus 42 in native rat cells, old SCs did not show any increase in telomerase activity and loss of anchorage dependence--characteristics that are typical of tumor cells. The data also provide biological insights into which cell characteristics (proliferation and adhesion, for example) are functionally clustered and either change or remain constant with aging in vitro. Though the data indicate a lack of tumorigenic transformation coupled with increased neurite outgrowth-promoting activity after extensive SC expansion in vitro, thus suggesting better regeneration qualities, we strongly recommend that in vitro aged rat SCs (>11 passages) should not be employed for tissue engineering.

Synthetic nerve guide implants in humans: a comprehensive survey

OBJECTIVE

Lesions of the peripheral nervous system result in the loss of sensory and motor function and may in addition be accompanied by severe neuropathic syndromes originating from aberrant axonal regrowth. The transplantation of autologous nerve grafts represents the current "gold standard" during reconstructive surgery, despite obvious side effects. Depending on the demands of the lesion site, various donor nerves may be used for grafting (e.g., the sural, saphenous), sacrificing native functions in their target areas. Recently, several synthetic nerve guide implants have been introduced and approved for clinical use to replace autologous transplants. This alternative therapy is based on pioneering studies with experimental nerve guides.

METHODS

We present a comprehensive review of all published human studies involving synthetic nerve guides.

RESULTS

Data from some 300 patients suggest that for short nerve defects of a few centimeters, resorbable implants provide promising results, whereas a number of late compression syndromes have been documented for nonresorbable implants.

CONCLUSIONS

To treat longer defects, further implant development is needed, a goal that could be achieved, for example, by more closely imitating the intact nerve architecture and regulatory cell-cell interactions.