Peripheral nerve tissue engineering: autologous Schwann cells vs. transdifferentiated mesenchymal stem cells

Recent advances in nerve tissue engineering

Nerve injury secondary to trauma, neurological disease or tumor excision presents a challenge for surgical reconstruction. Current practice for nerve repair involves autologous nerve transplantation, which is associated with significant donor-site morbidity and other complications. Previously artificial nerve conduits made from polycaprolactone, polyglycolic acid and collagen were approved by the FDA (USA) for nerve repair. More recently, there have been significant advances in nerve conduit design that better address the requirements of nerve regrowth. Innovations in materials science, nanotechnology, and biology open the way for the synthesis of new generation nerve repair conduits that address issues currently faced in nerve repair and regeneration. This review discusses recent innovations in this area, including the use of nanotechnology to improve the design of nerve conduits and to enhance nerve regeneration.

Role of mesenchymal stem cells in cell life and their signaling

Mesenchymal stem cells (MSCs) have various roles in the body and cellular environment, and the cellular phenotypes of MSCs changes in different conditions. MSCs support the maintenance of other cells, and the capacity of MSCs to differentiate into several cell types makes the cells unique and full of possibilities. The involvement of MSCs in the epithelial-mesenchymal transition is an important property of these cells. In this review, the role of MSCs in cell life, including their application in therapy, is first described, and the signaling mechanism of MSCs is investigated for a further understanding of these cells.

The role of peripheral nerve ECM components in the tissue engineering nerve construction

The extracellular matrix (ECM) is the naturally occurring substrate that provides a support structure and an attachment site for cells. It also produces a biological signal, which plays an important role in and has significant impact on cell adhesion, migration, proliferation, differentiation, and gene expression. Peripheral nerve repair is a complicated process involving Schwann cell proliferation and migration, 'bands of Büngner' formation, and newborn nerve extension. In the ECM of peripheral nerves, macromolecules are deposited among cells; these constitute the microenvironment of Schwann cell growth. Such macromolecules include collagen (I, III, IV, V), laminin, fibronectin, chondroitin sulfate proteoglycans (CSPGs), and other nerve factors. Collagen, the main component of ECM, provides structural support and guides newborn neurofilament extension. Laminin, fibronectin, CSPGs, and neurotrophic factors, are promoters or inhibitors, playing different roles in nerve repair after injury. By a chemical decellularization process, acellular nerve allografting eliminates the antigens responsible for allograft rejection and maintains most of the ECM components, which can effectively guide and enhance nerve regeneration. Thus, the composition and features of peripheral nerve ECM suggest its superiority as nerve repair material. This review focuses on the structure, function, and application in the tissue engineering nerve construction of the peripheral nerve ECM components.

In toto differentiation of human amniotic membrane towards the Schwann cell lineage

Human amniotic membrane (hAM) is a tissue containing cells with proven stem cell properties. In its decellularized form it has been successfully applied as nerve conduit biomaterial to improve peripheral nerve regeneration in injury models. We hypothesize that viable hAM without prior cell isolation can be differentiated towards the Schwann cell lineage to generate a possible alternative to commonly applied tissue engineering materials for nerve regeneration. For in vitro Schwann cell differentiation, biopsies of hAM of 8 mm diameter were incubated with a sequential order of neuronal induction and growth factors for 21 days and characterized for cellular viability and the typical glial markers glial fibrillary acidic protein (GFAP), S100β, p75 and neurotrophic tyrosine kinase receptor (NTRK) using immunohistology. The secretion of the neurotrophic factors brain-derived neurotrophic factor (BDNF) and glial cell-derived neurotrophic factor (GDNF) was quantified by ELISA. The hAM maintained high viability, especially under differentiation conditions (90.2 % ± 41.6 day 14; 80.0 % ± 44.5 day 21 compared to day 0). Both, BDNF and GDNF secretion was up-regulated upon differentiation. The fresh membrane stained positive for GFAP and p75 and NTRK, which was strongly increased after culture in differentiation conditions. Especially the epithelial layer within the membrane exhibited a change in morphology upon differentiation forming a multi-layered epithelium with intense accumulations of the marker proteins. However, S100β was expressed at equal levels and equal distribution in fresh and cultured hAM conditions. Viable hAM may be a promising alternative to present formulations used for peripheral nerve regeneration.

Tissue engineered constructs for peripheral nerve surgery

BACKGROUND

Tissue engineering has been defined as "an interdisciplinary field that applies the principles of engineering and life sciences toward the development of biological substitutes that restore, maintain, or improve tissue function or a whole organ". Traumatic peripheral nerve injury resulting in significant tissue loss at the zone of injury necessitates the need for a bridge or scaffold for regenerating axons from the proximal stump to reach the distal stump.

METHODS

A review of the literature was used to provide information on the components necessary for the development of a tissue engineered peripheral nerve substitute. Then, a comprehensive review of the literature is presented composed of the studies devoted to this goal.

RESULTS

Extensive research has been directed toward the development of a tissue engineered peripheral nerve substitute to act as a bridge for regenerating axons from the proximal nerve stump seeking the distal nerve. Ideally this nerve substitute would consist of a scaffold component that mimics the extracellular matrix of the peripheral nerve and a cellular component that serves to stimulate and support regenerating peripheral nerve axons.

CONCLUSIONS

The field of tissue engineering should consider its challenge to not only meet the autograft "gold standard" but also to understand what drives and inhibits nerve regeneration in order to surpass the results of an autograft.

[The role of mesenchymal stem cells in the reconstruction of nerve injuries in the hand]

At present the end results of reconstruction of peripheral nerve injuries cannot be considered unequivocally advantageous. It seems that the level of reconstructive possibilities of these injuries has already peaked. Hence, ongoing research focuses on experimental studies to further improve results. One of the methods is the joint usage of pluripotent mesenchymal stem cells with tissue inductive polypeptides (growth factors) and frame structures to enhance the attachment of these cells with the aim creating new neural tissue (tissue engineering). The conditions to create new tissue can be further improved by gene technology. Based on recent literature data, the author summarizes the basic characteristics of the method related to nerve injuries, and the possibilities and modalities of clinical applications. In conclusion, future direction is a the wider use of stem cells, however, the currently established surgical and rehabilitation methods have to be performed at high levels since stem cell research data are not established in the clinical setting yet.

Distal segment extracts of the degenerated rat sciatic nerve induce bone marrow stromal cells to express Schwann cell markers in vitro

Bone marrow stromal cells (MSCs) have the ability to support nerve regeneration when transplanted into lesion sites, but the mechanism is unclear. We hypothesized that specific factors in the lesioned microenvironment induce the differentiation of transplanted MSCs into functional Schwann-like cells. To test this hypothesis and determine the origin of such factors, we investigated the effects of different extracts from degenerated rat sciatic nerves on MSCs in vitro. After 3 days of degeneration, extracts from the distal segment (Ds) and proximal segment (Ps) of the rat sciatic nerve were used in experiments. After 1 day of treatment, the morphology of MSCs cultured with Ds extracts were spindle shaped, and the cells interconnected with each other, followed by gradual loss of typical morphology during culture. After 7 days of treatment, western blotting and RT-PCR analyses indicated that the cells cultured with Ds extracts had significantly higher expression of glial fibrillary acidic protein (GFAP), Sox10, Oct6, and early growth response 2(Egr2) than that of cells cultured with Ps extracts and the untreated cells. Our study suggests that, in the microenvironments of nerve lesions, specific factors induce MSCs to differentiate into functional Schwann-like cells, which may originate from the Ds of the degenerated nerve. These results may help to elucidate the mechanisms by which MSCs function in peripheral nerve repair.

Inducible expression of neurotrophic factors by mesenchymal progenitor cells derived from traumatically injured human muscle

Peripheral nerve damage frequently accompanies musculoskeletal trauma and repair of these nerves could be enhanced by the targeted application of neurotrophic factors (NTFs), which are typically expressed by endogenous cells that support nerve regeneration. Injured muscle tissues express NTFs to promote reinnervation as the tissue regenerates, but the source of these factors from within the muscles is not fully understood. We have previously identified a population of mesenchymal progenitor cells (MPCs) in traumatized muscle tissue with properties that support tissue regeneration, and our hypothesis was that MPCs also secrete the NTFs that are associated with muscle tissue reinnervation. We determined that MPCs express genes associated with neurogenic function and measured the protein-level expression of specific NTFs with known functions to support nerve regeneration. We also demonstrated the effectiveness of a neurotrophic induction protocol to enhance the expression of the NTFs, which suggests that the expression of these factors may be modulated by the cellular environment. Finally, neurotrophic induction affected the expression of cell surface markers and proliferation rate of the MPCs. Our findings indicate that traumatized muscle-derived MPCs may be useful as a therapeutic cell type to enhance peripheral nerve regeneration following musculoskeletal injury.

Long-term outcome of the repair of 50 mm long median nerve defects in rhesus monkeys with marrow mesenchymal stem cells-containing, chitosan-based tissue engineered nerve grafts

Despite great progress in the fields of tissue engineering and stem cell therapy, the translational and preclinical studies are required to accelerate the clinical application of tissue engineered nerve grafts, as an alternative to autologous nerve grafts, for peripheral nerve repair. Rhesus monkeys (non-human primates) are more clinically relevant and more suitable for scaling up to humans as compared to other mammalians. Based on this premise, and considering a striking similarity in the anatomy and function between human and monkey hands, here we used chitosan/PLGA-based, autologous marrow mesenchymal stem cells (MSCs)-containing tissue engineered nerve grafts (TENGs) for bridging a 50-mm long median nerve defect in rhesus monkeys. At 12 months after grafting, locomotive activity observation, electrophysiological assessments, and FG retrograde tracing tests indicated that the recovery of nerve function by TENGs was more efficient than that by chitosan/PLGA scaffolds alone; histological and morphometric analyses of regenerated nerves further confirmed that the morphological reconstruction by TENGs was close to that by autografts and superior to that by chitosan/PLGA scaffolds alone. In addition, blood test and histopathological examination demonstrated that TENGs featured by addition of autologous MSCs could be safely used in the primate body. These findings suggest the efficacy of our developed TENGs for peripheral nerve regeneration and their promising perspective for clinical applications.

Stem cell-based therapy for spinal cord injury

Stem cells (SCs) represent a new therapeutic approach for spinal cord injury (SCI) by enabling improved sensory and motor functions in animal models. The main goal of SC-based therapy for SCI is the replacement of neurons and glial cells that undergo cell death soon after injury. Stem cells are able to promote remyelination via oligodendroglia cell replacement to produce trophic factors enhancing neurite outgrowth, axonal elongation, and fiber density and to activate resident or transplanted progenitor cells across the lesion cavity. While several SC transplantation strategies have shown promising yet partial efficacy, mechanistic proof is generally lacking and is arguably the largest impediment toward faster progress and clinical application. The main challenge ahead is to spur on cooperation between clinicians, researchers, and patients in order to define and optimize the mechanisms of SC function and to establish the ideal source/s of SCs that produce efficient and also safe therapeutic approaches.

Functional regeneration of a severed peripheral nerve with a 7-mm gap in rats through the use of an implantable electrical stimulator and a conduit electrode with collagen coating

OBJECTIVE

This paper examined the efficacy of an implantable electrical stimulator in rats for the functional regeneration of peripheral nerves.

MATERIALS AND METHODS

The implantable electrical stimulator was fabricated on a polyimide-based conduit with an integrated electrode, a stimulation chip, and a battery; 3 mg/mL of collagen gel was coated onto the conduit surface and electrical stimulation (20 µ A, 100 µ s, and 100 Hz biphasic current) was continuously applied between the nerve stumps for four weeks. The stimulator was tested on a severed sciatic nerve with a 7-mm gap in rats. The effects of both the electrical stimulation and the collagen application were examined.

RESULTS

Functionality was evaluated through walk track assessments and by recording the action potential of the regenerated nerve. Immunohistochemical staining of the regenerated nerve was done using peripheral myelin protein 22.

CONCLUSION

The results suggest that the functional recovery of a severed peripheral nerve by the proposed implantable electrical stimulator was achieved through electrical current stimulation along the use of a collagen coating on the conduit surface.

Mesenchymal progenitor cells derived from traumatized muscle enhance neurite growth

The success of peripheral nerve regeneration is governed by the rate and quality of axon bridging and myelination that occurs across the damaged region. Neurite growth and the migration of Schwann cells is regulated by neurotrophic factors produced as the nerve regenerates, and these processes can be enhanced by mesenchymal stem cells (MSCs), which also produce neurotrophic factors and other factors that improve functional tissue regeneration. Our laboratory has recently identified a population of mesenchymal progenitor cells (MPCs) that can be harvested from traumatized muscle tissue debrided and collected during orthopaedic reconstructive surgery. The objective of this study was to determine whether the traumatized muscle-derived MPCs exhibit neurotrophic function equivalent to that of bone marrow-derived MSCs. Similar gene- and protein-level expression of specific neurotrophic factors was observed for both cell types, and we localized neurogenic intracellular cell markers (brain-derived neurotrophic factor and nestin) to a subpopulation of both MPCs and MSCs. Furthermore, we demonstrated that the MPC-secreted factors were sufficient to enhance in vitro axon growth and cell migration in a chick embryonic dorsal root ganglia (DRG) model. Finally, DRGs in co-culture with the MPCs appeared to increase their neurotrophic function via soluble factor communication. Our findings suggest that the neurotrophic function of traumatized muscle-derived MPCs is substantially equivalent to that of the well-characterized population of bone marrow-derived MPCs, and suggest that the MPCs may be further developed as a cellular therapy to promote peripheral nerve regeneration.

Differentiated adipose-derived stem cells promote myelination and enhance functional recovery in a rat model of chronic denervation

Transplantation of autologous Schwann cells (SCs) is a promising approach for treating various peripheral nerve disorders, including chronic denervation. However, given their drawbacks, such as invasive biopsy and lengthy culture in vitro, alternative cell sources would be needed. Adipose-derived stem cells (ASCs) are a candidate, and in this study rat ASCs transdifferentiated into a SC phenotype (dASC) cocultured with dorsal root ganglion neurons were shown to associate with neurites and to express myelin basic protein (MBP)-positive myelin protein. Furthermore, dASCs transplanted into a chronically denervated rat common peroneal nerve survived for at least for 10 weeks, maintaining their differentiated state. Immunohistochemical analysis revealed that transplanted dASCs associated with regenerating axons, forming MBP-/protein zero-positive myelin sheaths. The cell survival and myelin expression assessed by double labelling with S100 and glial fibrillary acidic protein were similar between the dASC- and SC-transplanted nerves. Importantly, transplantation of dASCs resulted in dramatically improved motor functional recovery and nerve regeneration, with a level comparable to, or even superior to, transplantation of SCs. In conclusion, dASCs are capable of myelinating axons in vivo and enhancing functional outcome after chronic denervation.

Nerve regeneration with aid of nanotechnology and cellular engineering

Repairing nerve defects with large gaps remains one of the most operative challenges for surgeons. Incomplete recovery from peripheral nerve injuries can produce a diversity of negative outcomes, including numbness, impairment of sensory or motor function, possibility of developing chronic pain, and devastating permanent disability. In the last few years, numerous microsurgical techniques, such as coaptation, nerve autograft, and different biological or polymeric nerve conduits, have been developed to reconstruct a long segment of damaged peripheral nerve. A few of these techniques are promising and have become popular among surgeons. Advancements in the field of tissue engineering have led to development of synthetic nerve conduits as an alternative for the nerve autograft technique, which is the current practice to bridge nerve defects with gaps larger than 30 mm. However, to date, despite significant progress in this field, no material has been found to be an ideal alternative to the nerve autograft. This article briefly reviews major up-to-date published studies using different materials as an alternative to the nerve autograft to bridge peripheral nerve gaps in an attempt to assess their ability to support and enhance nerve regeneration and their prospective drawbacks, and also highlights the promising hope for nerve regeneration with the next generation of nerve conduits, which has been significantly enhanced with the tissue engineering approach, especially with the aid of nanotechnology in development of the three-dimensional scaffold. The goal is to determine potential alternatives for nerve regeneration and repair that are simply and directly applicable in clinical conditions.

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

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

Peripheral nerve reconstruction with collagen tubes filled with denatured autologous muscle tissue in the rat model

Conventional nerve conduits lack cellular and extracellular guidance structures critical for bridging larger defects. In this study, an exogenous matrix for axonal regeneration was provided by pretreated muscle tissue. In 24 rats, 14-mm sciatic nerve segments were resected and surgically reconstructed using one of the following methods: autograft (AG); bovine type I collagen conduit; (MDM) collagen tube filled with modified denatured autologous muscle tissue. For 8 weeks, functional regeneration was evaluated by footprint and video gait analysis. Evaluation was complemented by electrophysiology, as well as qualitative and quantitative structural assessment of nerves and target muscles. Group AG was superior both structurally and functionally, showing higher axon counts, a more normal gait pattern, and less severe muscle atrophy. Fiber quality (fiber size and myelin thickness) was highest in group MDM, possibly related to the myelin-producing effect of muscular laminin. However, axon count was lowest in this group, and ultrastructural analysis of the denatured muscle tissue showed areas of incomplete denaturation that had acted as a mechanical barrier for regenerating axons. In light of these results, the often advocated use of muscular exogenous matrix for peripheral nerve reconstruction is reviewed in the literature, and its clinical application is critically discussed. In conclusion, combined muscle tubes may have a positive influence on nerve fiber maturation. However, muscle pretreatment is not without risks, and denaturation processes need to be further refined.

Combined use of decellularized allogeneic artery conduits with autologous transdifferentiated adipose-derived stem cells for facial nerve regeneration in rats

Natural biological conduits containing seed cells have been widely used as an alternative strategy for nerve gap reconstruction to replace traditional nerve autograft techniques. The purpose of this study was to investigate the effects of a decellularized allogeneic artery conduit containing autologous transdifferentiated adipose-derived stem cells (dADSCs) on an 8-mm facial nerve branch lesion in a rat model. After 8 weeks, functional evaluation of vibrissae movements and electrophysiological assessment, retrograde labeling of facial motoneurons and morphological analysis of regenerated nerves were performed to assess nerve regeneration. The transected nerves reconstructed with dADSC-seeded artery conduits achieved satisfying regenerative outcomes associated with morphological and functional improvements which approached those achieved with Schwann cell (SC)-seeded artery conduits, and superior to those achieved with artery conduits alone or ADSC-seeded artery conduits, but inferior to those achieved with nerve autografts. Besides, numerous transplanted PKH26-labeled dADSCs maintained their acquired SC-phenotype and myelin sheath-forming capacity inside decellularized artery conduits and were involved in the process of axonal regeneration and remyelination. Collectively, our combined use of decellularized allogeneic artery conduits with autologous dADSCs certainly showed beneficial effects on nerve regeneration and functional restoration, and thus represents an alternative approach for the reconstruction of peripheral facial nerve defects.

Repair of facial nerve defects with decellularized artery allografts containing autologous adipose-derived stem cells in a rat model

The purpose of this study was to investigate the effects of a decellularized artery allograft containing autologous adipose-derived stem cells (ADSCs) on an 8-mm facial nerve branch lesion in a rat model. At 8 weeks postoperatively, functional evaluation of unilateral vibrissae movements, morphological analysis of regenerated nerve segments and retrograde labeling of facial motoneurons were all analyzed. Better regenerative outcomes associated with functional improvement, great axonal growth, and improved target reinnervation were achieved in the artery-ADSCs group (2), whereas the cut nerves sutured with artery conduits alone (group 1) achieved inferior restoration. Furthermore, transected nerves repaired with nerve autografts (group 3) resulted in significant recovery of whisking, maturation of myelinated fibers and increased number of labeled facial neurons, and the latter two parameters were significantly different from those of group 2. Collectively, though our combined use of a decellularized artery allograft with autologous ADSCs achieved regenerative outcomes inferior to a nerve autograft, it certainly showed a beneficial effect on promoting nerve regeneration and thus represents an alternative approach for the reconstruction of peripheral facial nerve defects.

Signaling cue presentation and cell delivery to promote nerve regeneration

Limitations in current nerve regeneration techniques have stimulated the development of various approaches to mimic the extrinsic cues available in the natural nerve regeneration environment. Biomaterials approaches modulate the microenvironment of a regenerating nerve through tailored presentation of signaling molecules, creating physical and biochemical guidance cues to direct axonal regrowth across nerve lesion sites. Cell-based approaches center on increasing the neurotrophic support, adhesion guidance and myelination capacity of Schwann cells and other alternative cell types to enhance nerve regrowth and functional recovery. Recent advances in presenting directional guidance cues in nerve guidance conduits and improving the regenerative outcomes of cell delivery provide inspirations to engineering the next generation of nerve repair solutions.

Intrinsic and therapeutic factors determining the recovery of motor function after peripheral nerve transection

Insufficient recovery after peripheral nerve injury has been attributed to (i) poor pathfinding of regrowing axons, (ii) excessive collateral axonal branching at the lesion site and (iii) polyneuronal innervation of the neuromuscular junctions (NMJ). The facial nerve transection model has been used initially to measure restoration of function after varying therapies and to examine the mechanisms underlying their effects. Since it is very difficult to control the navigation of several thousand axons, efforts concentrated on collateral branching and NMJ-polyinnervation. Treatment with antibodies against trophic factors to combat branching improved the precision of reinnervation, but had no positive effects on functional recovery. This suggested that polyneuronal reinnervation--rather than collateral branching--may be the critical limiting factor. The former could be reduced by pharmacological agents known to perturb microtubule assembly and was followed by recovery of function. Because muscle polyinnervation is activity-dependent and can be manipulated, attempts to design a clinically feasible therapy were performed by electrical stimulation or by soft tissue massage. Electrical stimulation applied to the transected facial nerve or to paralysed facial muscles did not improve vibrissal motor performance and failed to diminish polyinnervation. In contrast, gentle stroking of the paralysed muscles (vibrissal, orbicularis oculi, tongue musculature) resulted in full recovery of function. This manual stimulation was also effective after hypoglossal-facial nerve suture and after interpositional nerve grafting, but not after surgical reconstruction of the median nerve. All these findings raise hopes that clinically feasible and effective therapies could be soon designed and tested.

In vitro evaluation of polyester-based scaffolds seeded with adipose derived stem cells for peripheral nerve regeneration

To overcome the disadvantages of autografts for peripheral nerve repair, different methods such as artificial nerve conduits have been investigated for an alternative approach. This study demonstrated that solvent casting is a simple but efficient method to create thin polyester-based scaffolds for stem cell delivery. Using poly (ε-caprolactone) and poly (D,L-lactic acid), we produced scaffold films containing heterogenous depressions (pits) on the air surface with a size ranging from 0.5 to 30 μm(2). These scaffolds were moderately hydrophobic; however, they supported the differentiation of adipose derived stem cells (ADSC) into a Schwann cell-like phenotype. The differentiated ADSC (dADSC) expressed S100 protein and glial fibrillary acidic protein and readily adhered to the films and proliferated at a similar rate to those cultured on tissue culture polystyrene. Cells were also positive for proliferating cell nuclear antigen. Furthermore, dADSC retained functional activity and significantly enhanced neurite outgrowth from dorsal root ganglia neurons. This study suggests polymer scaffolds combined with dADSCs could be a promising therapy for peripheral nerve injuries.

Effect of mesenchymal stem cells on skin graft to flap prefabrication: an experimental study

Angiogenetic potential has been reported for bone marrow-derived stem cells (BSCs) and adipose-derived stem cells (ASCs). The superficial femoral artery, vein, and fascia were used as a vascular crane for prefabrication model of skin graft to flap. BSCs or ASCs were injected before the adaptation of the graft to the vascular crane depending on the group. The prefabricated grafts were then transferred to inguinal region in every 7 days to observe the viability. In experiment part I (n = 18), the critical time for the prefabrication was found to be 1 week. In experiment part II (n = 12), the control and experiment assays were performed on the same animal to support the data of the experiment part I. The viability of flaps was evaluated. The vascular density was higher in BSC, and ASC groups. The Vascular Endothelial Growth Factor immunohistochemical staining was quantified. Furthermore, mesenchymal stem cells could be helpful in any prefabrication procedure in which neovascularization is indispensable.

Construction of tissue engineered nerve grafts and their application in peripheral nerve regeneration

Surgical repair of severe peripheral nerve injuries represents not only a pressing medical need, but also a great clinical challenge. Autologous nerve grafting remains a golden standard for bridging an extended gap in transected nerves. The formidable limitations related to this approach, however, have evoked the development of tissue engineered nerve grafts as a promising alternative to autologous nerve grafts. A tissue engineered nerve graft is typically constructed through a combination of a neural scaffold and a variety of cellular and molecular components. The initial and basic structure of the neural scaffold that serves to provide mechanical guidance and optimal environment for nerve regeneration was a single hollow nerve guidance conduit. Later there have been several improvements to the basic structure, especially introduction of physical fillers into the lumen of a hollow nerve guidance conduit. Up to now, a diverse array of biomaterials, either of natural or of synthetic origin, together with well-defined fabrication techniques, has been employed to prepare neural scaffolds with different structures and properties. Meanwhile different types of support cells and/or growth factors have been incorporated into the neural scaffold, producing unique biochemical effects on nerve regeneration and function restoration. This review attempts to summarize different nerve grafts used for peripheral nerve repair, to highlight various basic components of tissue engineered nerve grafts in terms of their structures, features, and nerve regeneration-promoting actions, and finally to discuss current clinical applications and future perspectives of tissue engineered nerve grafts.

Microscopic evidences that bone marrow mononuclear cell treatment improves sciatic nerve regeneration after neurorrhaphy

Cell therapy constitutes a possibility for improving nerve regeneration, increasing the success of nerve repair. We evaluate the use of mononuclear cells in the regeneration of the sciatic nerve after axotomy followed by end-to-end neurorrhaphy. Forty adult male Wistar rats (250-300 g) were divided into four groups: (1) sham, (2) neurorrhaphy: the sciatic nerve was sectioned and repaired using epineural sutures, (3) culture medium: after the suture, received an injection of 10 μL of culture medium into the nerve, and (4) mononuclear cell: after the suture, a concentration of 3 × 10(6) of mononuclear cell was injected in epineurium region. Mononuclear cells were obtained from the bone marrow aspirates and separated by Ficoll-Hypaque method. The histological analyses were performed at the 4th postoperative day. The sciatic functional index, histological, and morphometric analyzes were used to evaluate nerve regeneration at the 6th postoperative week. Six rats were used for immunohistochemical analysis on the 4th postoperative day. In the group 4, on the fourth day, the histological analysis demonstrated a more accelerated degenerative process and an increase of the neurotrophic factors was observed. In the 6th week, all the morphometric results of the group 4 were statistically better compared with groups 2 and 3. There was a statistically significant improvement in the sciatic functional index for group 4 compared with groups 2 and 3. Mononuclear cells stimulated nerve regeneration, most probably by speeding up the Wallerian degeneration process as well as stimulating the synthesis of neurotrophic factors.

In vivo study of novel nanofibrous intra-luminal guidance channels to promote nerve regeneration

A novel nanofibrous construct for promoting peripheral nerve repair was fabricated and tested in a rat sciatic nerve defect model. The conduit is made out of bilayered nanofibrous membranes with the nanofibers longitudinally aligned in the lumen and randomly oriented on the outer surface. The intra-luminal guidance channel is made out of aligned nanofibrous yarns. In addition, biomolecules such as laminin and nerve growth factor were incorporated in the nanofibrous nerve construct to determine their efficacy in in vivo nerve regeneration. Muscle reinnervation, withdrawal reflex latency, histological, axon density and electrophysiology tests were carried out to compare the efficacy of nanofibrous constructs with an autograft. Our study showed mixed results when comparing the artificial constructs with an autograft. In some cases, the nanofibrous conduit with aligned nanofibrous yarn as an intra-luminal guidance channel performs better than the autograft in muscle reinnervation and withdrawal reflex latency tests. However, the axon density count is highest in the autograft at mid-graft. Functional recovery was improved with the use of the nerve construct which suggested that this nerve implant has the potential for clinical usage in reconstructing peripheral nerve defects.

Axon growth-promoting properties of human bone marrow mesenchymal stromal cells

Mesenchymal stromal cells are promising candidate donor cells for promoting functional tissue repair following traumatic spinal cord injury (SCI), however, the mechanism(s) of action remain poorly defined. Here, we describe an in vitro study of the axon growth-promoting properties of highly enriched populations of adult human mesenchymal stromal cells (hMSC). A random, non-oriented pattern of neuritic outgrowth was observed from dissociated adult rat DRG neurons seeded onto confluent A431 cells and PLL/laminin positive control substrata. Confluent hMSC formed arrays of similarly orientated cell bodies and processes which supported the regeneration of significantly more primary neurites but a slightly lower overall neuritic length than was observed over the PLL/laminin control substrate. The hMSC exerted a strong influence on the direction of neuritic outgrowth, with many regenerating processes following the orientation of underlying hMSC. The production of extracellular matrix appeared to be responsible for neuritic directionality, but the release of growth factors was a significant promoter for DRG neuritic outgrowth. This suggests that further investigations into the properties of hMSC may be of particular interest in the development of transplant-mediated strategies intending to promote functional axonal regeneration after SCI.

Biocompatibility of Different Nerve Tubes

Bridging nerve gaps with suitable grafts is a major clinical problem. The autologous nerve graft is considered to be the gold standard, providing the best functional results; however, donor site morbidity is still a major disadvantage. Various attempts have been made to overcome the problems of autologous nerve grafts with artificial nerve tubes, which are “ready-to-use” in almost every situation. A wide range of materials have been used in animal models but only few have been applied to date clinically, where biocompatibility is an inevitable prerequisite. This review gives an idea about artificial nerve tubes with special focus on their biocompatibility in animals and humans.

Current applications and future perspectives of artificial nerve conduits

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

Long-term survival and integration of transplanted engineered nervous tissue constructs promotes peripheral nerve regeneration

Although peripheral nerve injury is a common consequence of trauma or surgery, there are insufficient means for repair. In particular, there is a critical need for improved methods to facilitate regeneration of axons across major nerve lesions. Here, we engineered transplantable living nervous tissue constructs to provide a labeled pathway to guide host axonal regeneration. These constructs consisted of stretch-grown, longitudinally aligned living axonal tracts inserted into poly(glycolic acid) tubes. The constructs (allogenic) were transplanted to bridge an excised segment of sciatic nerve in the rat, and histological analyses were performed at 6 and 16 weeks posttransplantation to determine graft survival, integration, and host regeneration. At both time points, the transplanted constructs were found to have maintained their pretransplant geometry, with surviving clusters of graft neuronal somata at the extremities of the constructs spanned by tracts of axons. Throughout the transplanted region, there was an intertwining plexus of host and graft axons, suggesting that the transplanted axons mediated host axonal regeneration across the lesion. By 16 weeks posttransplant, extensive myelination of axons was observed throughout the transplant region. Further, graft neurons had extended axons beyond the margins of the transplanted region, penetrating into the host nerve. Notably, this survival and integration of the allogenic constructs occurred in the absence of immunosuppression therapy. These findings demonstrate the promise of living tissue-engineered axonal constructs to bridge major nerve lesions and promote host regeneration, potentially by providing axon-mediated axonal outgrowth and guidance.

Posttraumatic survival of sensory neurons during allotransplantation of rat embryonic tissues into the nerve

Allotransplantation of the liver, hindlimb, and bone marrow tissues from 14-day-old embryos into the sciatic nerve had a modulatory effect on survival of various populations of axotomized neurons in L5 spinal ganglion in an adult rat.

Peripheral glial cell differentiation from neurospheres derived from adipose mesenchymal stem cells

Mesenchymal stem cells derived from bone marrow and adipose tissue are being considered for use in neural repair because they can differentiate after appropriate induction in culture into neurons and glia. The question we asked was if neurospheres could be harvested from adipose-derived stem cells and if they then could differentiate in culture to peripheral glial-like cells. Here, we demonstrate that adipose-derived mesenchymal stem cells can form nestin-positive non-adherent neurosphere cellular aggregates when cultured with basic fibroblast growth factor and epidermal growth factor. Dissociation of these neurospheres and removal of mitogens results in expression of the characteristic Schwann cell markers S100 and p75 nerve growth factor receptor and GFAP. The simultaneous expression of these glia markers are characteristic features of Schwann cells and olfactory ensheathing cells which have unique properties regarding remyelination and enhancement of axonal regeneration. When co-cultured with dorsal root ganglion neurons, the peripheral glial-like cells derived from adipose mesenchymal stem cells aligned with neuritis and stimulated neuritic outgrowth. These results indicate that neurospheres can be generated from adipose-derived mesenchymal stem cells, and upon mitogen withdrawal can differentiate into peripheral glial cells with neurotrophic effects.

Delivery of adipose-derived precursor cells for peripheral nerve repair

To test the hypothesis that the transplantation of adipose precursor cells (APCs) improves nerve regeneration and functional recovery, human APCs were transplanted into the lumen of a nerve guide in a 6-mm unilateral sciatic nerve defect in athymic rats. The three control groups for the study were biodegradable, polycaprolactone-based nerve conduit without APCs, autograft, and empty defect. Behavioral tests were performed every 3 weeks, and the sciatic functional index (SFI) was calculated based on measurements from the hindlimb prints. After 12 weeks, the nerve as well as right and left gastrocnemius muscles were removed and preserved for histological evaluation. Full regeneration of the sciatic nerve occurred on the rats that received the autograft, the guide, and the guide with APCs; no regeneration was observed on any of the rats in which the defect was left untreated (empty defect). APCs survived transplantation for up to 12 weeks in the injured peripheral nerve. No significant colocalization was observed between the immunostaining for glial fibrillary protein and anti-human lamin A/C, implying that the APCs did not differentiate into Schwann cells at the site of injury. In comparison with the rats with untreated defects, a decrease in muscle atrophy was observed on those rats that received the autograft and the guide with cells as indicated by the gastrocnemius muscle weight ratio and the muscle fiber ratio. Significant differences in SFI were observed 3 weeks postinjury between the rats in which the guide was left empty and those that received the guide with APCs; however, these differences were not observed at 12 weeks. The transplantation of APCs promoted the formation of a more robust nerve as evidenced by the results from the cross-sectional area of regenerated nerve, and the transplantation of APCs produced a decrease in muscle atrophy.

Practical considerations concerning the use of stem cells for peripheral nerve repair

In this review the authors intend to demonstrate the need for supplementing conventional repair of the injured nerve with alternative therapies, namely transplantation of stem or progenitor cells. Although peripheral nerves do exhibit the potential to regenerate axons and reinnervate the end organ, outcome following severe nerve injury, even after repair, remains relatively poor. This is likely because of the extensive injury zone that prevents axon outgrowth. Even if outgrowth does occur, a relatively slow growth rate of regeneration results in prolonged denervation of the distal nerve. Whereas denervated Schwann cells (SCs) are key players in the early regenerative success of peripheral nerves, protracted loss of axonal contact renders Schwann cells unreceptive for axonal regeneration. Given that denervated Schwann cells appear to become effete, one logical approach is to support the distal denervated nerve environment by replacing host cells with those derived exogenously. A number of different sources of stem/precursor cells are being explored for their potential application in the scenario of peripheral nerve injury. The most promising candidate, transplant cells are derived from easily accessible sources such as the skin, bone marrow, or adipose tissue, all of which have demonstrated the capacity to differentiate into Schwann cell-like cells. Although recent studies have shown that stem cells can act as promising and beneficial adjuncts to nerve repair, considerable optimization of these therapies will be required for their potential to be realized in a clinical setting. The authors investigate the relevance of the delivery method (both the number and differentiation state of cells) on experimental outcomes, and seek to clarify whether stem cells must survive and differentiate in the injured nerve to convey a therapeutic effect. As our laboratory uses skin-derived precursor cells (SKPCs) in various nerve injury paradigms, we relate our findings on cell fate to other published studies to demonstrate the need to quantify stem cell survival and differentiation for future studies.

Growth factor and stem cell enhanced conduits in peripheral nerve regeneration and repair

OBJECTIVE

Despite the capacity for spontaneous axonal regeneration, recovery after severe peripheral nerve injury remains variable and often very poor. In addition, autologous nerve grafts, considered to be the 'gold standard' in nerve repair technique, are plagued by restricted donor tissue availability and donor site morbidity. Our primary objective is to highlight new and emerging methods of nerve repair, which have the potential to significantly improve both the functional and behavioral outcome after clinical nerve injury.

METHODS

A critical analysis of nerve injury and regeneration literature concentrating on outcome measures from both immediate and chronically denervated experimental works was conducted.

RESULTS

Results of numerous works employing both growth factor and stem cell enhanced nerve guidance conduits have shown encouraging results. However, further research is needed to optimize guidance conduit dynamics, bioavailability and delivery of both growth factors and stem cells to enhance peripheral nerve regeneration and functional recovery.

DISCUSSION

This review discusses current animal and clinical growth factor and stem cell studies, specifically focusing on future bio-engineering approaches in developing a nerve guidance conduit in the future.

Characterisation of human mesenchymal stem cells following differentiation into Schwann cell-like cells

Cell-based therapies provide a clinically applicable and available alternative to nerve autografts. Our previous studies have characterised rat-derived mesenchymal stem cells (MSC) and here we have investigated the phenotypic, molecular and functional characteristics of human-derived MSC (hMSC) differentiated along a Schwann cell lineage. The hMSC were isolated from healthy human donors and the identity of the undifferentiated hMSC was confirmed by the detection of MSC specific cells surface markers. The hMSC were differentiated along a glial cell lineage using an established cocktail of growth factors including glial growth factor-2. Following differentiation, the hMSC expressed the key Schwann cell (SC) markers at both the transcriptional and translational level. More importantly, we show the functional effect of hMSC on neurite outgrowth using an in vitro co-culture model system with rat-derived primary sensory neurons. The number of DRG sprouting neurites was significantly enhanced in the presence of differentiated hMSC; neurite length and density (branching) were also increased. These results provide evidence that hMSC can undergo molecular, morphological and functional changes to adopt a SC-like behaviour and, therefore, could be suitable as SC substitutes for nerve repair in clinical applications.

Mesenchymal stem cells: immunobiology and role in immunomodulation and tissue regeneration

Mesenchymal stem cells (MSC) are multipotent cells that differentiate into osteoblasts, myocytes, chondrocytes and adipocytes as well as insulin-producing cells. The mechanism underlying their in vivo differentiation is not clear and is thought to be caused by spontaneous cell fusion or factors present in the microenvironment. However, their ease of isolation, high 'ex-vivo' expansion potential and ability to differentiate into multiple lineages make them attractive tools for potential use in cell therapy. MSC have been isolated from several tissues, including bone/bone marrow, fat, Wharton's jelly, umbilical cord blood, placenta and pancreas. The 'immunosuppressive' property of human MSC makes them an important candidate for cellular therapy in allogeneic settings. Use of allogeneic MSC for repair of large defects may be an alternative to autologous and allogeneic tissue-grafting procedures. An allogeneic approach would enable MSC to be isolated from any donor, expanded and cryopreserved, providing a readily available source of progenitors for cell replacement therapy. Their immunomodulatory properties have raised the possibility of establishing allogeneic MSC banks for tissue regeneration. These facts are strongly reflected in the current exponential growth in stem cell research in the pharmaceutical and biotechnology communities. Current knowledge regarding the immunobiology and clinical application of MSC needs to be strengthened further to establish MSC as a safe and effective therapeutic tool in regenerative medicine. This paper discusses human MSC with particular reference to the expression of their surface markers, their role as immunomodulators and their multilineage differentiation potential and possible use in tissue regeneration and repair.