Optimizing decellularization techniques to create a new nerve allograft: an in vitro study using rodent nerve segments

The Effect of Local Purified Exosome Product, Stem Cells, and Tacrolimus on Neurite Extension

PURPOSE

The combination of cellular and noncellular treatments has been postulated to improve nerve regeneration through a processed nerve allograft. This study aimed to evaluate the isolated effect of treatment with purified exosome product (PEP), mesenchymal stem cells (MSCs), and tacrolimus (FK506) alone and in combination when applied in decellularized allografts.

METHODS

A three-dimensional in vitro-compartmented cell culture system was used to evaluate the length of regenerating neurites from the neonatal dorsal root ganglion into the adjacent peripheral nerve graft. Decellularized nerve allografts were treated with undifferentiated MSCs, 5% PEP, 100 ng/mL FK506, PEP and FK506 combined, or MSCs and FK506 combined (N = 9/group) and compared with untreated nerve autografts (positive control) and nerve allografts (negative control). Neurite extension was measured to quantify nerve regeneration after 48 hours, and stem cell viability was evaluated.

RESULTS

Stem cell viability was confirmed in all MSC-treated nerve grafts. Treatments with PEP, PEP + FK506, and MSCs + FK506 combined were found to be superior to untreated allografts and not significantly different from autografts. Combined PEP and FK506 treatment resulted in the greatest neurite extension. Treatment with FK506 and MSCs was significantly superior to MSC alone. The combined treatment groups were not found to be statistically different.

CONCLUSIONS

Although all treatments improved neurite outgrowth, treatments with PEP, PEP + FK506, and MSCs + FK506 combined had superior neurite growth compared with untreated allografts and were not found to be significantly different from autografts, the current gold standard.

CLINICAL RELEVANCE

Purified exosome product, a cell-free exosome product, is a promising adjunct to enhance nerve allograft regeneration, with possible future avenues for clinical translation.

Techniques and graft materials for repairing peripheral nerve defects

Peripheral nerve defects refer to damage or destruction occurring in the peripheral nervous system, typically affecting the limbs and face. The current primary approaches to address peripheral nerve defects involve the utilization of autologous nerve transplants or the transplantation of artificial material. Nevertheless, these methods possess certain limitations, such as inadequate availability of donor nerve or unsatisfactory regenerative outcomes post-transplantation. Biomaterials have been extensively studied as an alternative approach to promote the repair of peripheral neve defects. These biomaterials include both natural and synthetic materials. Natural materials consist of collagen, chitosan, and silk, while synthetic materials consist of polyurethane, polylactic acid, and polycaprolactone. Recently, several new neural repair technologies have also been developed, such as nerve regeneration bridging technology, electrical stimulation technology, and stem cell therapy technology. Overall, biomaterials and new neural repair technologies provide new methods and opportunities for repairing peripheral nerve defects. However, these methods still require further research and development to enhance their effectiveness and feasibility.

Functional outcomes of nerve allografts augmented with mesenchymal stem cells and surgical angiogenesis in a rat sciatic nerve defect model

BACKGROUND

Motor function recovery following acellular nerve allograft (ANA) repair remains inferior to autologous nerve reconstruction. We investigated the functional recovery of ANAs after combined mesenchymal stem cell (MSC) delivery and surgical angiogenesis in a rat sciatic nerve defect model.

METHODS

In 100 Lewis rats, unilateral sciatic nerve defects were reconstructed with (I) autografts, (II) ANAs, (III) ANAs wrapped with a superficial inferior epigastric artery fascial (SIEF) flap, combined with either (IV) undifferentiated MSCs or (V) Schwann cell-like differentiated MSCs. The tibialis anterior muscle area was evaluated during the survival period using ultrasonography. Functional recovery, histomorphometry, and immunofluorescence were assessed at 12 and 16 weeks.

RESULTS

At 12 weeks, the addition of surgical angiogenesis and MSCs improved ankle contractures. The SIEF flap also significantly improved compound muscle action potential (CMAP) outcomes compared with ANAs. Autografts outperformed all groups in muscle force and weight. At 16 weeks, ankle contractures of ANAs remained inferior to autografts and SIEF, whereas the CMAP amplitude was comparable between groups. The muscle force of autografts remained superior to all other groups, and the muscle weight of ANAs remained inferior to autografts. No differences were found in histomorphometry outcomes between SIEF groups and ANAs. Vascularity, determined by CD34 staining, was significantly higher in SIEF groups compared with ANAs.

CONCLUSIONS

The combination of surgical angiogenesis and MSCs did not result in a synergistic improvement in functional outcomes. In a short nerve gap model, the adipofascial flap may provide sufficient MSCs to ANAs without additional ex vivo MSC seeding.

Enhancing Functional Recovery after Segmental Nerve Defect Using Nerve Allograft Treated with Plasma-Derived Exosome

BACKGROUND

Nerve injuries can result in detrimental functional outcomes. Currently, autologous nerve graft offers the best outcome for segmental peripheral nerve injury. Allografts are alternatives, but do not have comparable results. This study evaluated whether plasma-derived exosome can improve nerve regeneration and functional recovery when combined with decellularized nerve allografts.

METHODS

The effect of exosomes on Schwann cell proliferation and migration were evaluated. A rat model of sciatic nerve repair was used to evaluate the effect on nerve regeneration and functional recovery. A fibrin sealant was used as the scaffold for exosome. Eighty-four Lewis rats were divided into autograft, allograft, and allograft with exosome groups. Gene expression of nerve regeneration factors was analyzed on postoperative day 7. At 12 and 16 weeks, rats were subjected to maximum isometric tetanic force and compound muscle action potential. Nerve specimens were then analyzed by means of histology and immunohistochemistry.

RESULTS

Exosomes were readily taken up by Schwann cells that resulted in improved Schwann cell viability and migration. The treated allograft group had functional recovery (compound muscle action potential, isometric tetanic force) comparable to that of the autograft group. Similar results were observed in gene expression analysis of nerve regenerating factors. Histologic analysis showed no statistically significant differences between treated allograft and autograft groups in terms of axonal density, fascicular area, and myelin sheath thickness.

CONCLUSIONS

Plasma-derived exosome treatment of decellularized nerve allograft may provide comparable clinical outcomes to that of an autograft. This can be a promising strategy in the future as an alternative for segmental peripheral nerve repair.

CLINICAL RELEVANCE STATEMENT

Off-the-shelf exosomes may improve recovery in nerve allografts.

Decellularized Graft for Repairing Severe Peripheral Nerve Injuries in Sheep

BACKGROUND AND OBJECTIVES

Peripheral nerve injuries resulting in a nerve defect require surgical repair. The gold standard of autograft (AG) has several limitations, and therefore, new alternatives must be developed. The main objective of this study was to assess nerve regeneration through a long gap nerve injury (50 mm) in the peroneal nerve of sheep with a decellularized nerve allograft (DCA).

METHODS

A 5-cm long nerve gap was made in the peroneal nerve of sheep and repaired using an AG or using a DCA. Functional tests were performed once a month and electrophysiology and echography evaluations at 6.5 and 9 months postsurgery. Nerve grafts were harvested at 9 months for immunohistochemical and morphological analyses.

RESULTS

The decellularization protocol completely eliminated the cells while preserving the extracellular matrix of the nerve. No significant differences were observed in functional tests of locomotion and pain response. Reinnervation of the tibialis anterior muscles occurred in all animals, with some delay in the DCA group compared with the AG group. Histology showed a preserved fascicular structure in both AG and DCA; however, the number of axons distal to the nerve graft was higher in AG than in DCA.

CONCLUSION

The decellularized graft assayed supported effective axonal regeneration when used to repair a 5-cm long gap in the sheep. As expected, a delay in functional recovery was observed compared with the AG because of the lack of Schwann cells.

A multidisciplinary approach to the management of brachial plexus injuries: experience from the Mayo Clinic over 100 years

A multidisciplinary brachial plexus clinic has been a relatively new concept, offering different surgical speciality perspectives on the treatment of brachial plexus injuries. The resulting collaborative effort has proven to be greater than the sum of its parts. In this review, the history, philosophy of care, development/implementation and impact of a creation of a multidisciplinary brachial plexus team at the Mayo Clinic are detailed.

Optimized Decellularization Protocol for Large Peripheral Nerve Segments: Towards Personalized Nerve Bioengineering

Nerve injuries remain clinically challenging, and allografts showed great promise. Decellularized nerve allografts possess excellent biocompatibility and biological activity. However, the vast majority of decellularization protocols were established for small-size rodent nerves and are not suitable for clinical application. We aimed at developing a new method of decellularizing large-diameter nerves suitable for human transplantation. Repeated rounds of optimization to remove immunogenic material and preserve the extracellular structure were applied to the porcine sciatic nerve. Following optimization, extensive in vitro analysis of the acellular grafts via immunocytochemistry, immunohistology, proteomics and cell transplantation studies were performed. Large segments (up to 8 cm) of the porcine sciatic nerve were efficiently decellularized and histology, microscopy and proteomics analysis showed sufficient preservation of the extracellular matrix, with simultaneous consistent removal of immunogenic material such as myelin, DNA and axons, and axonal growth inhibitory molecules. Cell studies also demonstrated the suitability of these acellular grafts for 3D cell culture studies and translation to future large animal studies and clinical trials. By using non-human donors for peripheral nerve transplantation, significant drawbacks associated with the gold standard can be eliminated while simultaneously preserving the beneficial features of the extracellular matrix.

The effect of mesenchymal stem cells and surgical angiogenesis on immune response and revascularization of acellular nerve allografts in a rat sciatic defect model

BACKGROUND

Increasing evidence demonstrates an interplay between neoangiogenesis and immune cells. We investigated the immune response and revascularization of acellular nerve allografts (ANA) after combined stem cell delivery and surgical angiogenesis in a rat model.

METHODS

Unilateral sciatic nerve defects in 60 Lewis rats were repaired with (I) autografts, (II) ANAs, and (III) ANAs wrapped within a pedicled superficial inferior epigastric artery fascial flap to induce surgical angiogenesis, combined with seeding of either (IV) undifferentiated mesenchymal stem cells (uMSCs) or (V) MSCs differentiated into Schwann cell-like cells. Immune cell phenotyping was performed on days 7 and 14. The vascular volume of nerves was measured by microcomputed tomography at 12 and 16 weeks.

RESULTS

On day 7, helper T cells (CD4+) were significantly increased in groups IV and V compared to group I. Regulatory T cells (CD4+CD25+) were significantly higher in groups III-IV, and cytotoxic T cells (CD8+) were significantly reduced in groups IV and V compared to group II, respectively. Group II demonstrated the highest levels of natural killer cells (CD161+) compared to groups III-V. On day 14, group IV demonstrated the highest CD4/CD8 ratio. Vascular volume was significantly higher in groups III-V compared to group II at 12 weeks and groups IV and V compared to group II at 16 weeks. The CD4/CD8 ratio demonstrated a positive correlation to vascular volumes at 12 weeks.

CONCLUSION

Early favorable immune responses were observed in ANAs treated with surgical angiogenesis with or without stem cell delivery and demonstrated improved vascularity at longer follow-up.

Dynamic seeding versus microinjection of mesenchymal stem cells for acellular nerve allograft: an in vitro comparison

BACKGROUND

Mesenchymal stem cell (MSC)-supplemented acellular nerve allografts (ANA) are a potential strategy to improve the treatment of segmental nerve defects. Prior to clinical translation, optimal cell delivery methods must be defined. While two techniques, dynamic seeding and microinjection, have been described, the seeding efficiency, cell viability, and distribution of MSCs in ANAs are yet to be compared.

METHODS

Sciatic nerve segments of Sprague-Dawley rats were decellularized, and MSCs were harvested from the adipose tissue of Lewis rats. Cell viability was evaluated after injection of MSCs through a 27-gauge needle at different flow rates (10, 5, and 1 µL/min). MSCs were dynamically seeded or longitudinally injected into ANAs. Cell viability, seeding efficiency, and distribution were evaluated using LIVE/DEAD and MTS assays, scanning electron microscopy, and Hoechst staining.

RESULTS

No statistically significant difference in cell viability after injection at different flow rates was seen. After cell delivery, 84.1 ± 3.7% and 87.8 ± 2.8% of MSCs remained viable in the dynamic seeding and microinjection group, respectively (p = 0.41). The seeding efficiency of microinjection (100.4%±5.6) was significantly higher than dynamic seeding (48.1%±8.6) on day 1 (p = 0.001). Dynamic seeding demonstrated a significantly more uniform cell distribution over the course of the ANA compared to microinjection (p = 0.02).

CONCLUSION

MSCs remain viable after both dynamic seeding and microinjection in ANAs. Higher seeding efficiency was observed with microinjection, but dynamic seeding resulted in a more uniform distribution. In vivo studies are required to assess the effect on gene expression profiles and functional motor outcomes.

Nerve regeneration in rat peripheral nerve allografts: An assessment of the role of endogenous neurotrophic factors in nerve cryopreservation and regeneration

Peripheral nerve injury is a common clinical problem that often leads to significant functional impairment or even complete paralysis. Allograft has been proposed as a potential repair strategy for peripheral nerve injuries. Furthermore, peripheral nerve cryopreservation may result in nearly unlimited supply of grafts. However, the concentration of neurotrophic factors secreted by Schwann cells (SCs) in the local microenvironment after transplantation may not be sufficient for the survival of neuronal soma and axonal regeneration. Here, we investigated the effect of endogenous neurotrophic factors (ENTFs) on nerve regeneration in rats after the allograft of a cryopreserved sciatic nerve. ENTFs were highly expressed in the sciatic nerves pretreated for 14 days. Although the number of surviving cells in the sciatic nerves and their immunogenicity were low in the 14-day group after 4 weeks of cryopreservation, they continued to express high levels of ENTFs in vitro. At one week postoperation, the 14-day Allo group showed low plasma levels of interleukin-2, interferon-gamma, and tumour necrosis factor-alpha and low cellular immune response. At 20 weeks postoperation, nerve regeneration and functional recovery in the 14-day Allo group was similar to that in the fresh isograft group but better than that in the cryopreserved fresh allograft and fresh allograft groups. Thus, ENTFs were induced in vitro after pretreatment of the sciatic nerve. Following cryopreservation, the sciatic nerves with high levels of ENTFs continued to express high levels of ENTFs in vitro. The immune response after allograft was weak, which promoted recipient nerve regeneration.

A novel decellularized nerve graft for repairing peripheral nerve long gap injury in the rat

Decellularized nerve allografts are an alternative to autograft for repairing severe nerve injuries, since they have higher availability and do not induce rejection. In this study, we have assessed the regenerative potential of a novel decellularization protocol for human and rat nerves for repairing nerve resections, compared to the gold standard autograft. A 15-mm gap in the sciatic nerve was repaired with decellularized rat allograft (DC-RA), decellularized human xenograft (DC-HX), or fresh autograft (AG). Electrophysiology tests were performed monthly to evaluate muscle reinnervation, whereas histological and immunohistochemical analyses of the grafts were evaluated at 4 months. A short-term study was also performed to compare the differences between the two decellularized grafts (DC-RA and DC-HX) in early phases of regeneration. The decellularization process eliminated cellularity while preserving the ECM and endoneurial tubules of both rat and human nerves. Higher amount of reinnervation was observed in the AG group compared to the DC-RA group, while only half of the animals of the DC-HX showed distal muscle reinnervation. The number of regenerating myelinated axons in the mid-graft was similar between AG and DC-RA and lower in DC-HX graft, but significantly lower in both DC grafts distally. At short term, fibroblasts repopulated the DC-RA graft, supporting regenerated axons, whereas an important fibrotic reaction was observed around DC-HX grafts. In conclusion, the decellularized allograft sustained regeneration through a long gap in the rat although at a slower rate compared to the ideal autograft, whereas regeneration was limited or even failed when using a decellularized xenograft.

Mesenchymal stem cells and local tacrolimus delivery synergistically enhance neurite extension

BACKGROUND

The aim of this study was to investigate the combined effect of mesenchymal stem cells (MSC) and local delivery of tacrolimus (FK506) on nerve regeneration when applied to nerve autografts and decellularized allografts.

METHODS

A three-dimensional in vitro compartmented cell culture system consisting of a neonatal dorsal root ganglion adjacent to a nerve graft was used to evaluate the regenerating neurites into the peripheral nerve scaffold. Nerve autografts and allografts were treated with (i) undifferentiated MSCs, (ii) FK506 (100 ng/mL) or (iii) both (N = 9/group). After 48 hours, neurite extension was measured to quantify nerve regeneration and stem cell viability was evaluated.

RESULTS

Stem cell viability was confirmed in all MSC-treated grafts. Neurite extension was superior in autografts treated with FK506, and MSCs and FK506 combined (p < 0.001 and p = 0.0001, respectively), and autografts treated with MSCs (p = 0.12) were comparable to untreated autografts. In allografts, FK506 treatment and combined treatment were superior to controls (p < 0.001 and p = 0.0001, respectively), and treatment with MSCs (p = 0.09) was comparable to controls. All autograft groups were superior compared to their respective allograft treatment group (p < 0.05) in neurite extension.

CONCLUSIONS

Alone, either MSC or FK506 treatment improved neurite outgrowth, and combined they further enhanced neurite extension in both autografts and allografts.

Sterilization of sealed PVDF pouches containing decellularized scaffold by electrical stimulation

A terminal sterilization process for tissue engineering products, such as allografts and biomaterials is necessary to ensure complete removal of pathogenic microorganisms such as the bacteria, fungi and viruses. However, it can be difficult to sterilize allografts and artificial tissue models packaged in wet conditions without deformation. In this study, we investigated the sterilization effects of electrical stimulation (ES) and assessed its suitability by evaluating sterility assurance levels in pouches at a constant current. Stability of polyvinylidene fluoride pouches was determined by a sterility test performed after exposure to five microorganisms (Staphylococcus aureus, Bacillus subtilis, Pseudomonas aeruginosa, Escherichia coli, and Candida albicans) for 5 days; the sterility test was also performed with decellularized human dermal tissues inoculated with the five microorganisms. Sterilization using ES inactivated microorganisms both inside and outside of sealed pouches and caused no damage to the packaged tissue. Our results support the development of a novel system that involves ES sterilization for packaging of implantable biomaterials and human derived materials. This article is protected by copyright. All rights reserved.

Functional Outcomes of Nerve Allografts Seeded with Undifferentiated and Differentiated Mesenchymal Stem Cells in a Rat Sciatic Nerve Defect Model

BACKGROUND

Mesenchymal stem cells have the potential to produce neurotrophic growth factors and establish a supportive microenvironment for neural regeneration. The purpose of this study was to determine the effect of undifferentiated and differentiated mesenchymal stem cells dynamically seeded onto decellularized nerve allografts on functional outcomes when used in peripheral nerve repair.

METHODS

In 80 Lewis rats, a 10-mm sciatic nerve defect was reconstructed with (1) autograft, (2) decellularized allograft, (3) decellularized allograft seeded with undifferentiated mesenchymal stem cells, or (4) decellularized allograft seeded with mesenchymal stem cells differentiated into Schwann cell-like cells. Nerve regeneration was evaluated over time by cross-sectional tibial muscle ultrasound measurements, and at 12 and 16 weeks by isometric tetanic force measurements, compound muscle action potentials, muscle mass, histology, and immunofluorescence analyses.

RESULTS

At 12 weeks, undifferentiated mesenchymal stem cells significantly improved isometric tetanic force measurement and compound muscle action potential outcomes compared to decellularized allograft alone, whereas differentiated mesenchymal stem cells significantly improved compound muscle action potential outcomes. The autografts outperformed both stem cell groups histologically at 12 weeks. At 16 weeks, functional outcomes normalized between groups. At both time points, the effect of undifferentiated versus differentiated mesenchymal stem cells was not significantly different.

CONCLUSIONS

Undifferentiated and differentiated mesenchymal stem cells significantly improved functional outcomes of decellularized allografts at 12 weeks and were similar to autograft results in the majority of measurements. At 16 weeks, outcomes normalized as expected. Although differences between both cell types were not statistically significant, undifferentiated mesenchymal stem cells improved functional outcomes of decellularized nerve allografts to a greater extent and had practical benefits for clinical translation by limiting preparation time and costs.

Surgical Angiogenesis of Decellularized Nerve Allografts Improves Early Functional Recovery in a Rat Sciatic Nerve Defect Model

BACKGROUND

Surgical angiogenesis applied to nerve grafts has been suggested to enhance nerve regeneration after nerve injury. The authors hypothesized that surgical angiogenesis to decellularized nerve allografts would improve functional recovery in a rat sciatic nerve defect model.

METHODS

Sixty Lewis rats were divided in three groups of 20 animals each. Unilateral sciatic nerve defects were repaired with (1) autografts, (2) decellularized allografts, and (3) decellularized allografts wrapped with a superficial inferior epigastric artery fascial flap to add surgical angiogenesis. Twelve and 16 weeks after surgery, nerve regeneration was assessed using functional, electrophysiologic, histologic, and immunofluorescence analyses. Ultrasonography was used during the survival period to noninvasively evaluate muscle atrophy and reinnervation by measuring cross-sectional muscle area.

RESULTS

Surgical angiogenesis of allografts demonstrated significantly improved isometric tetanic force recovery at 12 weeks, compared to allograft alone, which normalized between groups at 16 weeks. Cross-sectional muscle areas showed no differences between groups. Electrophysiology showed superiority of autografts at both time points. No differences were found in histologic analysis, besides a significantly inferior N ratio in allografts at 12 weeks. Immunofluorescent expression of CD34, indicating vascularity, was significantly enhanced in the superficial inferior epigastric artery fascial group compared to allografts at 12 weeks, with highest expression at 16 weeks compared to all groups.

CONCLUSION

Surgical angiogenesis with an adipofascial flap to the nerve allograft increases vascularity in the nerve graft, with subsequent improvement of early muscle force recovery, comparable to autografts.

Recellularization of Native Tissue Derived Acellular Scaffolds with Mesenchymal Stem Cells

The functionalization of decellularized scaffolds is still challenging because of the recellularization-related limitations, including the finding of the most optimal kind of cell(s) and the best way to control their distribution within the scaffolds to generate native mimicking tissues. That is why researchers have been encouraged to study stem cells, in particular, mesenchymal stem cells (MSCs), as alternative cells to repopulate and functionalize the scaffolds properly. MSCs could be obtained from various sources and have therapeutic effects on a wide range of inflammatory/degenerative diseases. Therefore, in this mini-review, we will discuss the benefits using of MSCs for recellularization, the factors affecting their efficiency, and the drawbacks that may need to be overcome to generate bioengineered transplantable organs.

Decellularized scaffold and its elicited immune response towards the host: the underlying mechanism and means of immunomodulatory modification

The immune response of the host towards a decellularized scaffold is complex. Not only can a number of immune cells influence this process, but also the characteristics, preparation and modification of the decellularized scaffold can significantly impact this reaction. Such factors can, together or alone, trigger immune cells to polarize towards either a pro-healing or pro-inflammatory direction. In this article, we have comprehensively reviewed factors which may influence the immune response of the host towards a decellularized scaffold, including the source of the biomaterial, biophysical properties or modifications of the scaffolds with bioactive peptides, drugs and cytokines. Furthermore, the underlying mechanism has also been recapitulated.

Evaluation of the Use of Nerve Allograft Preserved in Glycerol

Background:

We aimed to evaluate the use of nerve allograft preserved in glycerol. We compared the efficiency of glycerol-preserved allografts with autogenous nerve grafting, cryopreserved grafts, and detergent-processed grafts in the axonal regeneration. Secondarily, we evaluated the effectiveness of each preservation method in maintaining the extracellular matrix free of cellular components.

Methods:

This was a prospective experimental, longitudinal, unblinded, nonrandomized, controlled animal model study. Three different allograft preservation techniques for the repair of sciatic nerve injuries were compared, including cold preservation, glycerol preservation, and detergent preservation. Functional assessment was performed, and histomorphometric analyses were further performed, which enabled the allograft structure evaluation and an estimation of the nerve regeneration efficacy based on the myelinated axons count and on their diameters.

Results:

After the 14^th week, all groups were already balanced and similar (P = 0.265): all groups present near-zero SFIs, thus confirming their efficiency in promoting nerve regeneration. In the histomorphometric evaluations, all groups were equivalent, presenting a similar efficiency in nerve regeneration (P = 0.716 and P = 0.577, respectively). Similarly, histomorphometric evaluations showed a reduction in the number of axons and in their diameters, but none of them effectively eliminated all cellular debris. Comparing the groups with each other, the groups preserved in glycerol and detergent solution were similar, both presenting better results than the cooled group.

Conclusion:

By evaluating the presence of cell debris after the treatment using glycerol, it was found to be similar to the treatment using detergent and significantly better than the cold-preservation treatment.

Sterilization and disinfection methods for decellularized matrix materials: Review, consideration and proposal

Sterilization is the process of killing all microorganisms, while disinfection is the process of killing or removing all kinds of pathogenic microorganisms except bacterial spores. Biomaterials involved in cell experiments, animal experiments, and clinical applications need to be in the aseptic state, but their physical and chemical properties as well as biological activities can be affected by sterilization or disinfection. Decellularized matrix (dECM) is the low immunogenicity material obtained by removing cells from tissues, which retains many inherent components in tissues such as proteins and proteoglycans. But there are few studies concerning the effects of sterilization or disinfection on dECM, and the systematic introduction of sterilization or disinfection for dECM is even less. Therefore, this review systematically introduces and analyzes the mechanism, advantages, disadvantages, and applications of various sterilization and disinfection methods, discusses the factors influencing the selection of sterilization and disinfection methods, summarizes the sterilization and disinfection methods for various common dECM, and finally proposes a graphical route for selecting an appropriate sterilization or disinfection method for dECM and a technical route for validating the selected method, so as to provide the reference and basis for choosing more appropriate sterilization or disinfection methods of various dECM.

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Asepsis is the prerequisite for the experiment and application of biomaterials.

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Sterilization or disinfection affects physic-chemical properties of biomaterials.

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Mechanism, advantages and disadvantages of sterilization or disinfection methods.

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Factors influencing the selection of sterilization or disinfection methods.

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Selection of sterilization or disinfection methods for decellularized matrix.

Microcomputed analysis of nerve angioarchitecture after combined stem cell delivery and surgical angiogenesis to nerve allograft

INTRODUCTION

A detailed three-dimensional (3D) evaluation of microvasculature is evolving to be a powerful tool, providing mechanistic understanding of angiomodulating strategies. The aim of this study was to evaluate the microvascular architecture of nerve allografts after combined stem cell delivery and surgical angiogenesis in a rat sciatic nerve defect model.

MATERIALS AND METHODS

In 25 Lewis rats, sciatic nerve gaps were repaired with (i) autografts, (ii) allografts, (iii) allografts wrapped in a pedicled superficial inferior epigastric artery fascia (SIEF) flap to provide surgical angiogenesis, combined with (iv) undifferentiated mesenchymal stem cells (MSC) and (v) MSCs differentiated into Schwann cell-like cells. At two weeks, vascular volume was measured using microcomputed tomography, and percentage and volume of vessels at different diameters were evaluated and compared with controls.

RESULTS

The vascular volume was significantly greatest in allografts treated with undifferentiated MSCs and surgical angiogenesis combined as compared to all experimental groups (P<0.01 as compared to autografts, P<0.0001 to allografts, and P<0.05 to SIEF and SIEF combined with differentiated MSCs, respectively). Volume and diameters of vessel segments in nerve allografts were enhanced by surgical angiogenesis. These distributions were further improved when surgical angiogenesis was combined with stem cells, with greatest increase found when combined with undifferentiated MSCs.

CONCLUSIONS

The interaction between vascularity and stem cells remains complex, however, this study provides some insight into its synergistic mechanisms. The combination of surgical angiogenesis with undifferentiated MSCs specifically, results in the greatest increase in revascularization, size of vessels, and stimulation of vessels to reach the middle longitudinal third of the nerve allograft.

Functional Outcome after Reconstruction of a Long Nerve Gap in Rabbits Using Optimized Decellularized Nerve Allografts

BACKGROUND

Processed nerve allografts are a promising alternative to nerve autografts, providing an unlimited, readily available supply and avoiding donor-site morbidity and the need for immunosuppression. Currently, clinically available nerve allografts do not provide satisfactory results for motor reconstruction. This study evaluated motor recovery after reconstruction of a long nerve gap using a processed nerve allograft and the influence of storage techniques.

METHODS

Nerve allografts were decellularized using elastase and detergents and stored at either 4° or -80°C. In 36 New Zealand White rabbits, a 3-cm peroneal nerve gap was repaired with either an autograft (group 1, control) or a cold-stored (group 2) or frozen-stored (group 3) processed nerve allograft. Nerve recovery was evaluated using longitudinal ultrasound measurements, electrophysiology (compound muscle action potentials), isometric tetanic force, wet muscle weight, and histomorphometry after 24 weeks.

RESULTS

Longitudinal ultrasound measurements showed that the cold-stored allograft provided earlier regeneration than the frozen-stored allograft. Furthermore, ultrasound showed significantly inferior recovery in group 3 than in both other groups (p < 0.05). Muscle weight and isometric tetanic force showed similar outcomes in the autograft and cold-stored allograft groups [p = 0.096 (muscle weight) and p = 0.286 (isometric tetanic force)], and confirmed the inferiority of the frozen-stored allograft to the autograft [p < 0.01 (muscle weight) and p = 0.02 (isometric tetanic force)].

CONCLUSIONS

Frozen storage of the nerve allograft significantly impairs functional recovery and should be avoided. The cold-stored optimized nerve allograft yields functional recovery similar to the gold standard autograft in the reconstruction of a 3-cm motor nerve defect. Future studies should focus on further improvement of the nerve allograft.

Description and validation of a simple histological nerve tissue scoring system for nerve allografts

BACKGROUND

In nerve allograft development, currently used subjective histological scoring systems to evaluate nerve structure in experimental studies are not uniform and have not been validated. The aim of this study was to describe and validate a simple, fast and inexpensive method to compare structural properties of nerve allografts on a histological level.

MATERIALS AND METHODS

A total of 113 histological sections of rat (sciatic nerves) and human peripheral nerve segments (thoracodorsal and long thoracic nerve for motor and sural for sensory nerve) treated with various decellularization protocols were analyzed. Slides were stained with Hematoxylin & Eosin (H&E), Toluidine Blue and Laminin. A subjective scoring system using a 5-point scale was used to score each nerve section by four independent researchers. For validation of this score both inter- and intra-rater reliability were calculated using the Intraclass Correlation Coefficient (ICC).

RESULTS

Overall, an excellent correlation between the different observers was found for the Toluidine Blue (ICC = 0.919, 95% CI: 0.891-0.941), H&E (ICC = 0.9, 95% CI: 0.865-0.927) and Laminin staining (ICC = 0.897, 95% CI: 0.860-0.926). Intra-rater reliability was good for all three staining techniques (ICC >0.8).

CONCLUSION

This study demonstrates the validity of this simple scoring system to evaluate nerve structure after different processing protocols. All three evaluated staining techniques can reliably be used for both rat and human nerve tissue. We believe this method is suitable to compare different processing techniques to create processed nerve allografts, especially when the average scores of multiple raters are used.

Adipose derived mesenchymal stem cells seeded onto a decellularized nerve allograft enhances angiogenesis in a rat sciatic nerve defect model

PURPOSE

Adipose derived mesenchymal stem cells (MSCs) are hypothesized to supplement tissues with growth factors essential for regeneration and neovascularization. The purpose of this study was to determine the effect of MSCs with respect to neoangiogenesis when seeded onto a decellularized nerve allograft in a rat sciatic nerve defect model.

METHODS

Allograft nerves were harvested from Sprague-Dawley rats and decellularized. MSCs were obtained from Lewis rats. 10 mm sciatic nerve defects in Lewis rats were reconstructed with reversed autograft nerves, decellularized allografts, decellularized allografts seeded with undifferentiated MSC or decellularized allografts seeded with differentiated MSCs. At 16 weeks, the vascular surface area and volume were evaluated.

RESULTS

The vascular surface area in normal nerves (34.9 ± 5.7%), autografts (29.5 ± 8.7%), allografts seeded with differentiated (38.9 ± 7.0%) and undifferentiated MSCs (29.2 ± 3.4%) did not significantly differ from each other. Unseeded allografts (21.2 ± 6.2%) had a significantly lower vascular surface area percentage than normal nonoperated nerves (13.7%, p = .001) and allografts seeded with differentiated MSCs (17.8%, p = .001). Although the vascular surface area was significantly correlated to the vascular volume (r = .416; p = .008), no significant differences were found between groups concerning vascular volumes. The vascularization pattern in allografts seeded with MSCs consisted of an extensive nonaligned network of microvessels with a centripetal pattern, while the vessels in autografts and normal nerves were more longitudinally aligned with longitudinal inosculation patterns.

CONCLUSIONS

Neoangiogenesis of decellularized allograft nerves was enhanced by stem cell seeding, in particular by differentiated MSCs. The pattern of vascularization was different between decellularized allograft nerves seeded with MSCs compared to autograft nerves.

Adipose Stem Cell-Derived Extracellular Vesicles Induce Proliferation of Schwann Cells via Internalization

Recent studies showed a beneficial effect of adipose stem cell-derived extracellular vesicles (ADSC-EVs) on sciatic nerve repair, presumably through Schwann cell (SC) modulation. However, it has not yet been elucidated whether ADSC-EVs exert this supportive effect on SCs by extracellular receptor binding, fusion to the SC membrane, or endocytosis mediated internalization. ADSCs, ADSC-EVs, and SCs were isolated from rats and characterized according to associated marker expression and properties. The proliferation rate of SCs in response to ADSC-EVs was determined using a multicolor immunofluorescence staining panel followed by automated image analysis. SCs treated with ADSC-EVs and silica beads were further investigated by 3-D high resolution confocal microscopy and live cell imaging. Our findings demonstrated that ADSC-EVs significantly enhanced the proliferation of SCs in a time- and dose-dependent manner. 3-D image analysis revealed a perinuclear location of ADSC-EVs and their accumulation in vesicular-like structures within the SC cytoplasm. Upon comparing intracellular localization patterns of silica beads and ADSC-EVs in SCs, we found striking resemblance in size and distribution. Live cell imaging visualized that the uptake of ADSC-EVs preferentially took place at the SC processes from which the EVs were transported towards the nucleus. This study provided first evidence for an endocytosis mediated internalization of ADSC-EVs by SCs and underlines the therapeutic potential of ADSC-EVs in future approaches for nerve regeneration.

Gene expression and growth factor analysis in early nerve regeneration following segmental nerve defect reconstruction with a mesenchymal stromal cell-enhanced decellularized nerve allograft

The purpose of this study was to evaluate the molecular mechanisms underlying nerve repair by a decellularized nerve allograft seeded with adipose-derived mesenchymal stromal cells (MSCs) and compare it to the unseeded allograft and autograft nerve.

METHODS

Undifferentiated MSCs were seeded onto decellularized nerve allografts and used to reconstruct a 10 mm gap in a rat sciatic nerve model. Gene expression profiles of genes essential for nerve regeneration and immunohistochemical staining (IHC) for PGP9.5, NGF, RECA-1, and S100 were obtained 2 weeks postoperatively.

RESULTS

Semi-quantitative RT-PCR analysis showed that the angiogenic molecule VEGFA was significantly increased in seeded allografts, and transcription factor SOX2 was downregulated in seeded allografts. Seeded grafts showed a significant increase in immunohistochemical markers NGF and RECA-1, when compared with unseeded allografts.

CONCLUSIONS

MSCs contributed to the secretion of trophic factors. A beneficial effect of the MSCs on angiogenesis was found when compared with the unseeded nerve allograft, but implanted MSCs did not show evidence of differentiation into Schwann cell-like cells.

Revascularization patterns of nerve allografts in a rat sciatic nerve defect model

INTRODUCTION

The specific patterns of revascularization of allograft nerves after the addition of vascularization remain unknown. The aim of this study was to determine the revascularization patterns of optimized processed allografts (OPA) after surgically induced angiogenesis to the wound bed in a rat sciatic nerve model.

MATERIALS AND METHODS

In 51 Lewis rats, sciatic nerve gaps were repaired with (i) autografts, (ii) OPA and (iii) OPA wrapped in a pedicled superficial inferior epigastric artery fascia flap (SIEF) to provide vascularization to the wound bed. At 2, 12, and 16 weeks, the vascular volume and vascular surface area in nerve samples were measured using micro CT and photography. Cross-sectional images were obtained and the number of vessels was quantified in the proximal, mid, and distal sections of the nerve samples.

RESULTS

At 2 weeks, the vascular volume of SIEF nerves was comparable to control (P = 0.1). The vascular surface area in SIEF nerves was superior to other groups (P<0.05). At 12 weeks, vascularity in SIEF nerves was significantly higher than allografts (P<0.05) and superior compared to all other groups (P<0.0001) at 16 weeks. SIEF nerves had a significantly increased number of vessels compared to allografts alone in the proximal (P<0.05) and mid-section of the graft (P<0.05).

CONCLUSIONS

Addition of surgical angiogenesis to the wound bed greatly improves revascularization. It was demonstrated that revascularization occurs primarily from proximal to distal (proximal inosculation) and not from both ends as previously believed and confirms the theory of centripetal revascularization.

Gene expression profiles of differentiated and undifferentiated adipose derived mesenchymal stem cells dynamically seeded onto a processed nerve allograft

BACKGROUND

Differentiation of mesenchymal stem cells (MSCs) into Schwann-like cells onto processed nerve allografts may support peripheral nerve repair. The purpose of this study was to understand the biological characteristics of undifferentiated and differentiated MSCs before and after seeding onto a processed nerve allograft by comparing gene expression profiles.

METHODS

MSCs from Lewis rats were cultured in maintenance media or differentiated into Schwann-like cells. Both treatment groups were dynamically seeded onto decellularized nerve allografts derived from Sprague-Dawley rats. Gene expression was quantified by quantitative polymerase chain reaction (qPCR) analysis of representative biomarkers, including neurotrophic (GDNF, PTN, GAP43, PMP22), angiogenic (CD31, VEGF1), extracellular matrix (ECM) (COL1A1, COL3A1, FBLN1, LAMB2) or cell cycle (CAPS3, CCBN2) genes. Gene expression values were statistically evaluated using a 2-factor ANOVA with repeated measures.

RESULTS

Baseline gene expression of undifferentiated and differentiated MSCs was significantly altered upon interaction with processed nerve allografts. Interaction between processed allografts and undifferentiated MSCs enhanced expression of neurotrophic (NGF, GDNF, PMP22), ECM (FBLN1, LAMB2) and regulatory cell cycle genes (CCNB2) during a 7-day time course. Interactions of differentiated MSCs with nerve allografts enhanced expression of neurotrophic (NGF, GDNF, GAP43), angiogenic (VEGF1), ECM (FBLN1) and regulatory cell cycle genes (CASP3, CCNB2) within one week.

CONCLUSIONS

Dynamic seeding onto processed nerve allografts modulates temporal gene expression profiles of differentiated and undifferentiated MSCs. These changes in gene expressions may support the reparative functions of MSCs in supporting nerve regeneration in different stages of axonal growth.

Decellularization techniques and their applications for the repair and regeneration of the nervous system

A variety of surgical and non-surgical approaches have been used to address the impacts of nervous system injuries, which can lead to either impairment or a complete loss of function for affected patients. The inherent ability of nervous tissues to repair and/or regenerate is dampened due to irreversible changes that occur within the tissue remodeling microenvironment following injury. Specifically, dysregulation of the extracellular matrix (i.e., scarring) has been suggested as one of the major factors that can directly impair normal cell function and could significantly alter the regenerative potential of these tissues. A number of tissue engineering and regenerative medicine-based approaches have been suggested to intervene in the process of remodeling which occurs following injury. Decellularization has become an increasingly popular technique used to obtain acellular scaffolds, and their derivatives (hydrogels, etc.), which retain tissue-specific components, including critical structural and functional proteins. These advantageous characteristics make this approach an intriguing option for creating materials capable of stimulating the sensitive repair mechanisms associated with nervous system injuries. Over the past decade, several diverse decellularization methods have been implemented specifically for nervous system applications in an attempt to carefully remove cellular content while preserving tissue morphology and composition. Each application-based decellularized ECM product requires carefully designed treatments that preserve the unique biochemical signatures associated within each tissue type to stimulate the repair of brain, spinal cord, and peripheral nerve tissues. Herein, we review the decellularization techniques that have been applied to create biomaterials with the potential to promote the repair and regeneration of tissues within the central and peripheral nervous system.

Comparison of systematically combined detergent and nuclease-based decellularization methods for acellular nerve graft: An ex vivo characterization and in vivo evaluation

Little consensus exists regarding which decellularization technique best removes the cellular components while maintaining structural integrity. We aimed to identify the most efficient and safest decellularization method by combining previously established chemical (detergent based) and biological (nuclease based) methods in a systematic manner. Sixty sciatic nerves were harvested from Sprague-Dawley rats and prepared in 120 nerve fragments with 1-cm length. Nerve fragments were randomly divided into six groups and decellularized with six different methods: A, nonionic detergent + amphoteric detergent; B, nonionic detergent + anionic detergent; C, anionic detergent + amphoteric detergent; D, nonionic detergent + nuclease; E, amphoteric detergent + nuclease; and F, anionic detergent + nuclease. The remaining cellular components were evaluated with H&E, DAPI, and S-100 immunohistochemical staining, and DNA content was measured in each sample. The remaining extracellular matrix (ECM) integrity was evaluated with H&E, Masson's trichrome, periodic acid-Schiff, Luxol fast blue, and laminin immunohistochemical staining, and collagen content was measured in each sample. The amphoteric detergent + nuclease method was the best protocol for both cell removal and ECM preservation. In the in vivo study, the nerve allograft that was decellularized with amphoteric detergent + nuclease showed an inferior recovery rate based on the tibialis anterior muscle weight to autograft, but considerable recovery was observed. In conclusion, among the possible systematic combinations of detergent- and nuclease-based methods, the combination of amphoteric detergent and nuclease is currently the most suitable for nerve decellularization in terms of adequate cell removal and sufficient preservation of the ECM.

In Vivo Survival of Mesenchymal Stromal Cell-Enhanced Decellularized Nerve Grafts for Segmental Peripheral Nerve Reconstruction

PURPOSE

Adipose-derived mesenchymal stromal cells (MSCs) have emerged as promising tools for peripheral nerve reconstruction. There is a paucity of information regarding the ultimate survivorship of implanted MSCs or whether these cells remain where they are placed. The aim of the present study was to track the in vivo distribution and survival of MSCs seeded on a decellularized nerve allograft reconstruction of a peripheral nerve defect using luciferase-based bioluminescence imaging (BLI).

METHODS

To determine the in vivo survivability of MSCs, autologous Lewis rat MSCs were stably labeled with luciferase by lentiviral particles. Labeled cells were dynamically seeded onto a Sprague Dawley decellularized rat nerve allograft and used to bridge a 10-mm sciatic nerve defect. The MSC survival was determined by performing in vivo BLI to detect living cells. Twelve animals were examined at 24 hours after implantation, 3, 7, 9, 11, and 14 days, and at daily intervals thereafter if signals were still present.

RESULTS

Labeled MSCs could be detected for up to 29 days. Gradually diminishing BLI signals were observed within the first week following implantation. Implanted MSCs were not detected anywhere other than the site of surgery.

CONCLUSIONS

The MSCs seeded on decellularized nerve allografts can survive in vivo but have finite survival after implantation. There was no evidence of migration of MSCs to surrounding tissues.

CLINICAL RELEVANCE

The findings support a therapeutic approach that combines MSCs with a biological scaffold for peripheral nerve surgery. It provides understanding of the viability and distribution of implanted MSCs, which is a prerequisite before clinical translation can be considered.

Adhesion, distribution, and migration of differentiated and undifferentiated mesenchymal stem cells (MSCs) seeded on nerve allografts

BACKGROUND

Although undifferentiated MSCs and MSCs differentiated into Schwann-like cells have been extensively compared in vitro and in vivo, studies on the ability and efficiency of differentiated MSCs for delivery into nerve allografts are lacking. As this is essential for their clinical potential, the purpose of this study was to determine the ability of MSCs differentiated into Schwann-like cells to be dynamically seeded on decellularized nerve allografts and to compare their seeding potential to that of undifferentiated MSCs.

METHODS

Fifty-six sciatic nerve segments from Sprague Dawley rats were decellularized, and MSCs were harvested from Lewis rat adipose tissue. Control and differentiated MSCs were dynamically seeded on the surface of decellularized allografts. Cell viability, seeding efficiencies, cell adhesion, distribution, and migration were evaluated.

RESULTS

The viability of both cell types was not influenced by the processed nerve allograft. Both cell types achieved maximal seeding efficiency after 12 h of dynamic seeding, albeit that differentiated MSCs had a significantly higher mean seeding efficiency than control MSCs. Dynamic seeding resulted in a uniform distribution of cells among the surface of the nerve allograft. No cells were located inside the nerve allograft after seeding.

CONCLUSION

Differentiated MSCs can be dynamically seeded on the surface of a processed nerve allograft, in a similar fashion as undifferentiated MSCs. Schwann-like differentiated MSCs have a significantly higher seeding efficiency after 12 h of dynamic seeding. We conclude that differentiation of MSCs into Schwann-like cells may improve the seeding strategy and the ability of nerve allografts to support axon regeneration.

Seeding decellularized nerve allografts with adipose-derived mesenchymal stromal cells: An in vitro analysis of the gene expression and growth factors produced

Mesenchymal stromal cells (MSCs) secrete many soluble growth factors and have previously been shown to stimulate nerve regeneration. MSC-seeded processed nerve allografts could potentially be a promising method for large segmental motor nerve injuries. Further progress in our understanding of how the functions of MSCs can be leveraged for peripheral nerve repair is required before making clinical translation. The present study, therefore, investigated whether interactions of adipose-derived MSCs with decellularized nerve allografts can improve gene and protein expression of growth factors that may support nerve regeneration. Human nerve allografts (n = 30) were decellularized and seeded with undifferentiated human adipose-derived MSCs. Subsequently, the MSCs and MSC-seeded grafts were isolated on days 3, 7, 14, and 21 in culture for RNA expression analysis by qRT-PCR. Evaluated genes included NGF, BDNF, PTN, GAP43, MBP, PMP22, VEGF, and CD31. Growth factor production was evaluated and quantified using enzyme-linked immunosorbent assay (ELISA). On day 21, semi-quantitative RT-PCR analysis showed that adherence of MSCs to nerve allografts significantly enhances mRNA expression of neurotrophic, angiogenic, endothelial, and myelination markers (e.g., BDNF, VEGF, CD31, and MBP). ELISA results revealed an upregulation of BDNF and reduction of both VEGF and NGF protein levels. This study demonstrates that seeding of undifferentiated adipose-derived MSCs onto processed nerve allografts permits the secretion of neurotrophic and angiogenic factors that can stimulate nerve regeneration. These favorable molecular changes suggest that MSC supplementation of nerve allografts may have potential in improving nerve regeneration.

A Simple Dynamic Strategy to Deliver Stem Cells to Decellularized Nerve Allografts

BACKGROUND

The addition of adipose-derived mesenchymal stromal cells to decellularized nerve allografts may improve outcomes of nerve reconstruction. Prior techniques used for cell seeding are traumatic to both the mesenchymal stromal cells and nerve graft. An adequate, reliable, and validated cell seeding technique is an essential step for evaluating the translational utility of mesenchymal stromal cell-enhanced decellularized nerve grafts. The purpose of this study was to develop a simple seeding strategy with an optimal seeding duration.

METHODS

A dynamic bioreactor was used to seed rat and human mesenchymal stromal cells separately onto rat and human decellularized nerve allografts. Cell viability was evaluated by MTS assays and cellular topology after seeding was determined by scanning electron microscopy. Cell density and distribution were determined by Live/Dead assays and Hoechst staining at four different time points (6, 12, 24, and 72 hours). The validity and reliability of the seeding method were calculated.

RESULTS

Cells remained viable at all time points, and mesenchymal stromal cells exhibited exponential growth in the first 12 hours of seeding. Seeding efficiency increased significantly from 79.5 percent at 6 hours to 89.2 percent after 12 hours of seeding (p = 0.004). Both intrarater reliability (r = 0.97) and interrater reliability (r = 0.92) of the technique were high.

CONCLUSIONS

This study describes and validates a new method of effectively seeding decellularized nerve allografts with mesenchymal stromal cells. This method is reproducible, distributes cells homogenously over the graft, and does not traumatize the intraneural architecture of the allograft. Use of this validated seeding technique will permit critical comparison of graft outcomes.

Emerging Technologies in Upper Extremity Surgery: Polyvinyl Alcohol Hydrogel Implant for Thumb Carpometacarpal Arthroplasty and Processed Nerve Allograft and Nerve Conduit for Digital Nerve Repairs

In the field of upper extremity surgery there are myriad new and developing technologies. The purpose of this article is to highlight a few of the most compelling new technologies and review their background, indications for use, and most recently reported outcomes in clinical practice.

Nerve Repair Using Decellularized Nerve Grafts in Rat Models. A Review of the Literature

Peripheral nerve regeneration after severe traumatic nerve injury is a relevant clinical problem. Several different strategies have been investigated to solve the problem of bridging the nerve gap. Among these, the use of decellularized nerve grafts has been proposed as an alternative to auto/isografts, which represent the current gold standard in the treatment of severe nerve injury. This study reports the results of a systematic review of the literature published between January 2007 and October 2017. The aim was to quantitatively analyze the effectiveness of decellularized nerve grafts in rat experimental models. The review included 33 studies in which eight different decellularization protocols were described. The decellularized nerve grafts were reported to be immunologically safe and able to support both functional and morphological regeneration after nerve injury. Chemical protocols were found to be superior to physical protocols. However, further research is needed to optimize preparation protocols, including recellularization, improve their effectiveness, and substitute the current gold standard, especially in the repair of long nerve defects.

Comparable functional motor outcomes after repair of peripheral nerve injury with an elastase-processed allograft in a rat sciatic nerve model

BACKGROUND

A bridging nerve autograft is the gold standard for the repair of segmental nerve injury that cannot be repaired directly. However, limited availability and donor site morbidity remain major disadvantages of autografts. Here, a nerve allograft decellularized with elastase was compared with an autograft regarding functional motor outcome in a rat sciatic segmental nerve defect model. Furthermore, the effect of storage on this allograft was studied.

METHODS

Sixty-six Lewis rats (250-300 g) underwent a 10-mm sciatic nerve reconstruction using either a cold- (n = 22) or frozen-stored (n = 22) decellularized nerve allograft or an autograft (n = 22). Sprague-Dawley rats (300-350 g) served as full major histocompatibility complex-mismatched donors. Functional motor outcome was evaluated after 12 and 16 weeks. Ankle angle, compound muscle action potential (CMAP), isometric tetanic force, wet muscle weight, and histomorphometry were tested bilaterally.

RESULTS

For CMAP and isometric tetanic force, no significant differences were observed between groups. In contrast, for ankle angle, histomorphometry and muscle weight, the cold-stored allograft performed comparable to the autograft, while the frozen-stored allograft performed significantly inferior to the autograft. At week 16, ankle angle was 88.0 ± 3.1% in the cold-stored group, 77.4 ± 3.6% in the frozen-stored group, and 74.1 ± 3.1% in the autograft group (P < .001); At week 16, the muscle weight showed a recovery up to 71.1 ± 4.8% in the autograft group, 67.0 ± 6.6% in the cold-stored group, and 64.7 ± 3.7% in the frozen-stored group (P < .05).

CONCLUSIONS

A nerve allograft decellularized with elastase, if stored under the right conditions, results in comparable functional motor outcomes as the gold standard, the autograft.

Various changes in cryopreserved acellular nerve allografts at -80°C

The experimental design evaluated histological, mechanical, and biological properties of allogeneic decellularized nerves after cryopreservation in a multi-angle, multi-directional manner to provide evidence for long-term preservation. Acellular nerve allografts from human and rats were cryopreserved in a cryoprotectant (10% fetal bovine serum, 10% dimethyl sulfoxide, and 5% sucrose in RPMI1640 medium) at −80°C for 1 year, followed by thawing at 40°C or 37°C for 8 minutes. The breaking force of acellular nerve allografts was measured using a tensile test. Cell survival was determined using L-929 cell suspensions. Acellular nerve allografts were transplanted into a rat model with loss of a 15-mm segment of the left sciatic nerve. Immunohistochemistry staining was used to measure neurofilament 200 expression. Hematoxylin-eosin staining was utilized to detect relative muscle area in gastrocnemius muscle. Electron microscopy was applied to observe changes in allograft ultrastructure. There was no obvious change in morphological appearance or ultrastructure, breaking force, or cytotoxicity of human acellular nerve allografts after cryopreservation at −80°C. Moreover, there was no remarkable change in neurofilament 200 expression, myelin sheath thickness, or muscle atrophy when fresh or cryopreserved rat acellular nerve allografts were applied to repair nerve injury in rats. These results suggest that cryopreservation can greatly extend the storage duration of acellular nerve tissue allografts without concomitant alteration of the physiochemical and biological properties of the engineered tissue to be used for transplantation.