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

Recent perspectives on the synergy of mesenchymal stem cells with micro/nano strategies in peripheral nerve regeneration-a review

Despite the intrinsic repair of peripheral nerve injury (PNI), it is important to carefully monitor the process of peripheral nerve repair, as peripheral nerve regeneration is slow and incomplete in large traumatic lesions. Hence, mesenchymal stem cells (MSCs) with protective and regenerative functions are utilized in synergy with innovative micro/nano technologies to enhance the regeneration process of peripheral nerves. Nonetheless, as MSCs are assessed using standard regenerative criteria including sensory-motor indices, structural features, and morphology, it is challenging to differentiate between the protective and regenerative impacts of MSCs on neural tissue. This study aims to analyze the process of nerve regeneration, particularly the performance of MSCs with and without synergistic approaches. It also focuses on the paracrine secretions of MSCs and their conversion into neurons with functional properties that influence nerve regeneration after PNI. Furthermore, the study explores new ideas for nerve regeneration after PNI by considering the synergistic effect of MSCs and therapeutic compounds, neuronal cell derivatives, biological or polymeric conduits, organic/inorganic nanoparticles, and electrical stimulation. Finally, the study highlights the main obstacles to developing synergy in nerve regeneration after PNI and aims to open new windows based on recent advances in neural tissue regeneration.

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.

Biomedical Application of Decellularized Scaffolds

Tissue loss and end-stage organ failure are serious health problems across the world. Natural and synthetic polymer scaffold material based artificial organs play an important role in the field of tissue engineering and organ regeneration, but they are not from the body and may cause side effects such as rejection. In recent years, the biomimetic decellularized scaffold based materials have drawn great attention in the tissue engineering field for their good biocompatibility, easy modification, and excellent organism adaptability. Therefore, in this review, we comprehensively summarize the application of decellularized scaffolds in tissue engineering and biomedicine in recent years. The preparation methods, modification strategies, construction of artificial tissues, and application in biomedical applications are discussed. We hope that this review will provide a useful reference for research on decellularized scaffolds and promote their application tissue engineering.

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.

Dose-response analysis after administration of a human platelet-derived exosome product on neurite outgrowth in vitro

Modulating the nerve's local microenvironment using exosomes is proposed to enhance nerve regeneration. This study aimed to determine the optimal dose of purified exosome product (PEP) required to exert maximal neurite extension. An in vitro dorsal root ganglion (DRG) neurite outgrowth assay was used to evaluate the effect of treatment with (i) 5% PEP, (ii) 10% PEP, (iii) 15% PEP, or (iv) 20% PEP on neurite extension (N = 9/group), compared to untreated controls. After 72 h, neurite extension was measured to quantify nerve regeneration. Live cell imaging was used to visualize neurite outgrowth during incubation. Treatment with 5% PEP resulted in the longest neurite extension and was superior to the untreated DRG (p = 0.003). Treatment with 10% PEP, 15% PEP, and 20% PEP was found to be comparable to controls (p = 0.12, p = 0.06, and p = 0.41, respectively) and each other. Live cell imaging suggested that PEP migrated towards the DRG neural regeneration site, compared to the persistent homogenous distribution of PEP in culture media alone. 5% PEP was found to be the optimal concentration for nerve regeneration based on this in vitro dose-response analysis.

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.

Augmenting Peripheral Nerve Regeneration with Adipose-Derived Stem Cells

Peripheral nerve injuries (PNIs) are common and debilitating, cause significant health care costs for society, and rely predominately on autografts, which necessitate grafting a nerve section non-locally to repair the nerve injury. One possible approach to improving treatment is bolstering endogenous regenerative mechanisms or bioengineering new nervous tissue in the peripheral nervous system. In this review, we discuss critical-sized nerve gaps and nerve regeneration in rats, and summarize the roles of adipose-derived stem cells (ADSCs) in the treatment of PNIs. Several regenerative treatment modalities for PNI are described: ADSCs differentiating into Schwann cells (SCs), ADSCs secreting growth factors to promote peripheral nerve growth, ADSCs promoting myelination growth, and ADSCs treatments with scaffolds. ADSCs' roles in regenerative treatment and features are compared to mesenchymal stem cells, and the administration routes, cell dosages, and cell fates are discussed. ADSCs secrete neurotrophic factors and exosomes and can differentiate into Schwann cell-like cells (SCLCs) that share features with naturally occurring SCs, including the ability to promote nerve regeneration in the PNS. Future clinical applications are also discussed.

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.

Peripheral Nerve Regeneration Using Different Germ Layer-Derived Adult Stem Cells in the Past Decade

Peripheral nerve injuries (PNIs) are some of the most common types of traumatic lesions affecting the nervous system. Although the peripheral nervous system has a higher regenerative ability than the central nervous system, delayed treatment is associated with disturbances in both distal sensory and functional abilities. Over the past decades, adult stem cell-based therapies for peripheral nerve injuries have drawn attention from researchers. This is because various stem cells can promote regeneration after peripheral nerve injuries by differentiating into neural-line cells, secreting various neurotrophic factors, and regulating the activity of in situ Schwann cells (SCs). This article reviewed research from the past 10 years on the role of stem cells in the repair of PNIs. We concluded that adult stem cell-based therapies promote the regeneration of PNI in various ways.

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.

Combined local delivery of tacrolimus and stem cells in hydrogel for enhancing peripheral nerve regeneration

The application of scaffold-based stem cell transplantation to enhance peripheral nerve regeneration has great potential. Recently, the neuroregenerative potential of tacrolimus (a U.S. Food and Drug Administration-approved immunosuppressant) has been explored. In this study, a fibrin gel-based drug delivery system for sustained and localized tacrolimus release was combined with rat adipose-derived mesenchymal stem cells (MSC) to investigate cell viability in vitro. Tacrolimus was encapsulated in poly(lactic-co-glycolic) acid (PLGA) microspheres and suspended in fibrin hydrogel, using concentrations of 0.01 and 100 ng/ml. Drug release over time was measured. MSCs were cultured in drug-released media collected at various days to mimic systemic exposure. MSCs were combined with (i) hydrogel only, (ii) empty PLGA microspheres in the hydrogel, (iii) 0.01, and (iv) 100 ng/ml of tacrolimus PLGA microspheres in the hydrogel. Stem cell presence and viability were evaluated. A sustained release of 100 ng/ml tacrolimus microspheres was observed for up to 35 days. Stem cell presence was confirmed and cell viability was observed up to 7 days, with no significant differences between groups. This study suggests that combined delivery of 100 ng/ml tacrolimus and MSCs in fibrin hydrogel does not result in cytotoxic effects and could be used to enhance peripheral nerve regeneration.

Gene expression profiles of human adipose-derived mesenchymal stem cells dynamically seeded on clinically available processed nerve allografts and collagen nerve guides

It was hypothesized that mesenchymal stem cells (MSCs) could provide necessary trophic factors when seeded onto the surfaces of commonly used nerve graft substitutes. We aimed to determine the gene expression of MSCs when influenced by Avance® Nerve Grafts or NeuraGen® Nerve Guides. Human adipose-derived MSCs were cultured and dynamically seeded onto 30 Avance® Nerve Grafts and 30 NeuraGen® Nerve Guides for 12 hours. At six time points after seeding, quantitative polymerase chain reaction analyses were performed for five samples per group. Neurotrophic [nerve growth factor (NGF), glial cell line-derived neurotrophic factor (GDNF), pleiotrophin (PTN), growth associated protein 43 (GAP43) and brain-derived neurotrophic factor (BDNF)], myelination [peripheral myelin protein 22 (PMP22) and myelin protein zero (MPZ)], angiogenic [platelet endothelial cell adhesion molecule 1 (PECAM1/CD31) and vascular endothelial cell growth factor alpha (VEGFA)], extracellular matrix (ECM) [collagen type alpha I (COL1A1), collagen type alpha III (COL3A1), Fibulin 1 (FBLN1) and laminin subunit beta 2 (LAMB2)] and cell surface marker cluster of differentiation 96 (CD96) gene expression was quantified. Unseeded Avance® Nerve Grafts and NeuraGen® Nerve Guides were used to evaluate the baseline gene expression, and unseeded MSCs provided the baseline gene expression of MSCs. The interaction of MSCs with the Avance® Nerve Grafts led to a short-term upregulation of neurotrophic (NGF, GDNF and BDNF), myelination (PMP22 and MPZ) and angiogenic genes (CD31 and VEGFA) and a long-term upregulation of BDNF, VEGFA and COL1A1. The interaction between MSCs and the NeuraGen® Nerve Guide led to short term upregulation of neurotrophic (NGF, GDNF and BDNF) myelination (PMP22 and MPZ), angiogenic (CD31 and VEGFA), ECM (COL1A1) and cell surface (CD96) genes and long-term upregulation of neurotrophic (GDNF and BDNF), angiogenic (CD31 and VEGFA), ECM genes (COL1A1, COL3A1, and FBLN1) and cell surface (CD96) genes. Analysis demonstrated MSCs seeded onto NeuraGen® Nerve Guides expressed significantly higher levels of neurotrophic (PTN), angiogenic (VEGFA) and ECM (COL3A1, FBLN1) genes in the long term period compared to MSCs seeded onto Avance® Nerve Grafts. Overall, the interaction between human MSCs and both nerve graft substitutes resulted in a significant upregulation of the expression of numerous genes important for nerve regeneration over time. The in vitro interaction of MSCs with the NeuraGen® Nerve Guide was more pronounced, particularly in the long term period (> 14 days after seeding). These results suggest that MSC-seeding has potential to be applied in a clinical setting, which needs to be confirmed in future in vitro and in vivo research.

The interaction of stem cells and vascularity in peripheral nerve regeneration

The degree of nerve regeneration after peripheral nerve injury can be altered by the microenvironment at the site of injury. Stem cells and vascularity are postulated to be a part of a complex pathway that enhances peripheral nerve regeneration; however, their interaction remains unexplored. This review aims to summarize current knowledge on this interaction, including various mechanisms through which trophic factors are promoted by stem cells and angiogenesis. Angiogenesis after nerve injury is stimulated by hypoxia, mediated by vascular endothelial growth factor, resulting in the growth of pre-existing vessels into new areas. Modulation of distinct signaling pathways in stem cells can promote angiogenesis by the secretion of various angiogenic factors. Simultaneously, the importance of stem cells in peripheral nerve regeneration relies on their ability to promote myelin formation and their capacity to be influenced by the microenvironment to differentiate into Schwann-like cells. Stem cells can be acquired through various sources that correlate to their differentiation potential, including embryonic stem cells, neural stem cells, and mesenchymal stem cells. Each source of stem cells serves its particular differentiation potential and properties associated with the promotion of revascularization and nerve regeneration. Exosomes are a subtype of extracellular vesicles released from cell types and play an important role in cell-to-cell communication. Exosomes hold promise for future transplantation applications, as these vesicles contain fewer membrane-bound proteins, resulting in lower immunogenicity. This review presents pre-clinical and clinical studies that focus on selecting the ideal type of stem cell and optimizing stem cell delivery methods for potential translation to clinical practice. Future studies integrating stem cell-based therapies with the promotion of angiogenesis may elucidate the synergistic pathways and ultimately enhance nerve regeneration.

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.

Progress of Mesenchymal Stem Cell-Derived Exosomes in Tissue Repair

Mesenchymal stem cells (MSCs) are a kind of adult stem cells with self-replication and multidirectional differentiation, which can differentiate into tissue-specific cells under physiological conditions, maintaining tissue self-renewal and physiological functions. They play a role in the pathological condition by lateral differentiation into tissue-specific cells, replacing damaged tissue cells by playing the role of a regenerative medicine , or repairing damaged tissues through angiogenesis, thereby, regulating immune responses, inflammatory responses, and inhibiting apoptosis. It has become an important seed cell for tissue repair and organ reconstruction, and cell therapy based on MSCs has been widely used clinically. The study found that the probability of stem cells migrating to the damaged area after transplantation or differentiating into damaged cells is very low, so the researchers believe the leading role of stem cell transplantation for tissue repair is paracrine secretion, secreting growth factors, cytokines or other components. Exosomes are biologically active small vesicles secreted by MSCs. Recent studies have shown that they can transfer functional proteins, RNA, microRNAs, and lncRNAs between cells, and greatly reduce the immune response. Under the premise of promoting proliferation and inhibition of apoptosis, they play a repair role in tissue damage, which is caused by a variety of diseases. In this paper, the biological characteristics of exosomes (MSCs-exosomes) derived from mesenchymal stem cells, intercellular transport mechanisms, and their research progress in the field of stem cell therapy are reviewed.

Adipose tissue stem cells in peripheral nerve regeneration-In vitro and in vivo

After peripheral nerve injury, Schwann cells (SCs) are crucially involved in several steps of the subsequent regenerative processes, such as the Wallerian degeneration. They promote lysis and phagocytosis of myelin, secrete numbers of neurotrophic factors and cytokines, and recruit macrophages for a biological debridement. However, nerve injuries with a defect size of >1 cm do not show proper tissue regeneration and require a surgical nerve gap reconstruction. To find a sufficient alternative to the current gold standard-the autologous nerve transplant-several cell-based therapies have been developed and were experimentally investigated. One approach aims on the use of adipose tissue stem cells (ASCs). These are multipotent mesenchymal stromal cells that can differentiate into multiple phenotypes along the mesodermal lineage, such as osteoblasts, chondrocytes, and myocytes. Furthermore, ASCs also possess neurotrophic features, that is, they secrete neurotrophic factors like the nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3, ciliary neurotrophic factor, glial cell-derived neurotrophic factor, and artemin. They can also differentiate into the so-called Schwann cell-like cells (SCLCs). These cells share features with naturally occurring SCs, as they also promote nerve regeneration in the periphery. This review gives a comprehensive overview of the use of ASCs in peripheral nerve regeneration and peripheral nerve tissue engineering both in vitro and in vivo. While the sustainability of differentiation of ASCs to SCLCs in vivo is still questionable, ASCs used with different nerve conduits, such as hydrogels or silk fibers, have been shown to promote nerve regeneration.

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.