Biomimetic scaffolds for regeneration of volumetric muscle loss in skeletal muscle injuries

Nanoengineered myogenic scaffolds for skeletal muscle tissue engineering

Extreme loss of skeletal muscle overwhelms the natural regenerative capability of the body, results in permanent disability and substantial economic burden. Current surgical techniques result in poor healing, secondary injury to the autograft donor site, and incomplete recuperation of muscle function. Most current tissue engineering and regenerative strategies fail to create an adequate mechanical and biological environment that enables cell infiltration, proliferation, and myogenic differentiation. In this study, we present a nanoengineered skeletal muscle scaffold based on functionalized gelatin methacrylate (GelMA) hydrogel, optimized for muscle progenitors' proliferation and differentiation. The scaffold was capable of controlling the release of insulin-like growth factor 1 (IGF-1), an important myogenic growth factor, by utilizing the electrostatic interactions with LAPONITE® nanoclays (NCs). Physiologically relevant levels of IGF-1 were maintained during a controlled release over two weeks. The NC was able to retain 50% of the released IGF-1 within the hydrogel niche, significantly improving cellular proliferation and differentiation compared to control hydrogels. IGF-1 supplemented medium controls required 44% more IGF-1 than the comparable NC hydrogel composites. The nanofunctionalized scaffold is a viable option for the treatment of extreme muscle injuries and offers scalable benefits for translational interventions and the growing field of clean meat production.

Bioinspired electrospun decellularized extracellular matrix scaffolds promote muscle regeneration in a rat skeletal muscle defect model

Volumetric muscle loss is a debilitating injury that can leave patients with long-lasting or permanent structural and functional deficits. With clinical treatments failing to address these shortcomings, there is a great need for tissue-engineered therapies to promote skeletal muscle regeneration. In this study, we aim to assess the potential for electrospun decellularized skeletal muscle extracellular matrix (dECM) to promote skeletal muscle regeneration in a rat partial thickness tibialis anterior defect model. Aligned electrospun scaffolds with varying degrees of crosslinking density were implanted into the defect site and compared to an empty defect control. After 8 weeks, muscles were harvested, weighed, and cellular and morphological analyses were performed via histology and immunohistochemistry. Cell infiltration, angiogenesis, and myogenesis were observed in the defect site in both dECM groups. However, favorable mechanical properties and slower degradation kinetics resulted in greater support of tissue remodeling in the more crosslinked scaffolds and preservation of existing myofiber area in both dECM groups compared to the empty defect control. More sustained release of pro-regenerative degradation products also promoted greater myofiber formation in the defect site. This study allowed for a greater understanding of how electrospun skeletal muscle scaffolds interact with existing skeletal muscle and can inform their potential as a therapy in a wide variety of soft tissue applications.

Repairing Volumetric Muscle Loss in the Ovine Peroneus Tertius Following a 6-Month Recovery

Tissue-engineered skeletal muscle is a promising novel therapy for the treatment of volumetric muscle loss (VML). Our laboratory has developed tissue-engineered skeletal muscle units (SMUs) and engineered neural conduits (ENCs), and modularly scaled them to clinically relevant sizes for the treatment of VML in a large animal (sheep) model. In a previous study, we evaluated the effects of the SMUs and ENCs in treating a 30% VML injury in the ovine peroneus tertius muscle after a 3-month recovery period. The goal of the current study was to expand on our 3-month study and evaluate the SMUs and ENCs in restoring muscle function after a 6-month recovery period. Six months after implantation, we found that the repair groups with the SMU (VML+SMU and VML+SMU+ENC) restored muscle mass to a level that was statistically indistinguishable from the uninjured contralateral muscle. In contrast, the muscle mass in the VML-Only group was significantly less than groups repaired with an SMU. Following the 6-month recovery from VML, the maximum tetanic force was significantly lower for all VML injured groups compared to the uninjured contralateral muscle. However, we did demonstrate the ability of our ENCs to effectively regenerate nerve between the distal stump of the native nerve and the repair site in 93% of the animals.

Scaffold biomaterials and nano-based therapeutic strategies for skeletal muscle regeneration

Skeletal muscle is integral to the functioning of the human body. Several pathological conditions, such as trauma (primary lesion) or genetic diseases such as Duchenne muscular dystrophy (DMD), can affect and impair its functions or exceed its regeneration capacity. Tissue engineering (TE) based on natural, synthetic and hybrid biomaterials provides a robust platform for developing scaffolds that promote skeletal muscle regeneration, strength recovery, vascularization and innervation. Recent 3D-cell printing technology and the use of nanocarriers for the release of drugs, peptides and antisense oligonucleotides support unique therapeutic alternatives. Here, the authors present recent advances in scaffold biomaterials and nano-based therapeutic strategies for skeletal muscle regeneration and perspectives for future endeavors.

Aligned Poly(ε-caprolactone) Nanofibers Superimposed on Decellularized Human Amniotic Membrane Promoted Myogenic Differentiation of Adipose Derived Stem Cells

Objective

This study was designed to fabricate a suitable permanent scaffold for the normal aligned myotube formation and improve the process of myogenic differentiation of selected stem cells.

Materials and Methods

In this experimental study, an engineered scaffold composed of decellularized human amniotic membrane (DHAM) and electrospun fibers of poly(ε-caprolactone) (PCL) was fabricated and characterized. PCL nanofibers were superimposed on DHAM (PCL-DHAM) in two different patterns, including randomized fibers (Random) and aligned fibers (Aligned). Adipose derived stem cells (ADSCs) were isolated from adult Wistar rats and cultured on designed scaffold and induced to myotube differentiation. Using an MTT assay, the vitality of cells was determined. Then, myogenic cell differentiation was assessed using scan electron microscopy (SEM), immunofluorescence assay, and reverse transcription-polymerase chain reaction (RT-PCR).

Results

The mechanical properties of engineered PCL-DHAM composite improved significantly compared to DHAM as a control. The engineered PCL-DHAM promoted cell growth and high expression of myosin, Mhc2 and myogenin and thus enhanced the myotube formation.

Conclusion

These findings revealed that bio-composite membrane prepared from PCL nanofibers and DHAM, may represent a promising biomaterial as a desirable scaffold for applying in the bioengineered muscle repair.

Cells, scaffolds, and bioactive factors: Engineering strategies for improving regeneration following volumetric muscle loss

Severe traumatic skeletal muscle injuries, such as volumetric muscle loss (VML), result in the obliteration of large amounts of skeletal muscle and lead to permanent functional impairment. Current clinical treatments are limited in their capacity to regenerate damaged muscle and restore tissue function, promoting the need for novel muscle regeneration strategies. Advances in tissue engineering, including cell therapy, scaffold design, and bioactive factor delivery, are promising solutions for VML therapy. Herein, we review tissue engineering strategies for regeneration of skeletal muscle, development of vasculature and nerve within the damaged muscle, and achievements in immunomodulation following VML. In addition, we discuss the limitations of current state of the art technologies and perspectives of tissue-engineered bioconstructs for muscle regeneration and functional recovery following VML.

Replace and repair: Biomimetic bioprinting for effective muscle engineering

The debilitating effects of muscle damage, either through ischemic injury or volumetric muscle loss (VML), can have significant impacts on patients, and yet there are few effective treatments. This challenge arises when function is degraded due to significant amounts of skeletal muscle loss, beyond the regenerative ability of endogenous repair mechanisms. Currently available surgical interventions for VML are quite invasive and cannot typically restore function adequately. In response to this, many new bioengineering studies implicate 3D bioprinting as a viable option. Bioprinting for VML repair includes three distinct phases: printing and seeding, growth and maturation, and implantation and application. Although this 3D bioprinting technology has existed for several decades, the advent of more advanced and novel printing techniques has brought us closer to clinical applications. Recent studies have overcome previous limitations in diffusion distance with novel microchannel construct architectures and improved myotubule alignment with highly biomimetic nanostructures. These structures may also enhance angiogenic and nervous ingrowth post-implantation, though further research to improve these parameters has been limited. Inclusion of neural cells has also shown to improve myoblast maturation and development of neuromuscular junctions, bringing us one step closer to functional, implantable skeletal muscle constructs. Given the current state of skeletal muscle 3D bioprinting, the most pressing future avenues of research include furthering our understanding of the physical and biochemical mechanisms of myotube development and expanding our control over macroscopic and microscopic construct structures. Further to this, current investigation needs to be expanded from immunocompromised rodent and murine myoblast models to more clinically applicable human cell lines as we move closer to viable therapeutic implementation.

Minimally invasive co-injection of modular micro-muscular and micro-vascular tissues improves in situ skeletal muscle regeneration

Various conventional treatment strategies for volumetric muscle loss (VML) are often hampered by the extreme donor site morbidity, the limited availability of quality muscle flaps, and complicated, as well as invasive surgical procedures. The conventional biomaterial-based scaffolding systems carrying myoblasts have been extensively investigated towards improving the regeneration of the injured muscle tissues, as well as their injectable forms. However, the applicability of such designed systems has been restricted due to the lack of available vascular networks. Considering these facts, here we present the development of a unique set of two minimally invasively injectable modular microtissues, consisting of mouse myoblast (C2C12)-laden poly(lactic-co-glycolic acid) porous microspheres (PLGA PMs), or the micro-muscles, and human umbilical vein endothelial cell (HUVEC)-laden poly(ethylene glycol) hollow microrods (PEG HMs), or the microvessels. Besides systematic in vitro investigations, the myogenic performance of these modular composite microtissues, when co-injected, was explored in vivo using a mouse VML model, which confirmed improved in situ muscle regeneration and remolding. Together, we believe that the construction of these injectable modular microtissues and their combination for minimally invasive therapy provides a promising method for in situ tissue healing.

Immunomodulation and Biomaterials: Key Players to Repair Volumetric Muscle Loss

Volumetric muscle loss (VML) is defined as a condition in which a large volume of skeletal muscle is lost due to physical insult. VML often results in a heightened immune response, resulting in significant long-term functional impairment. Estimates indicate that ~250,000 fractures occur in the US alone that involve VML. Currently, there is no active treatment to fully recover or repair muscle loss in VML patients. The health economics burden due to VML is rapidly increasing around the world. Immunologists, developmental biologists, and muscle pathophysiologists are exploring both immune responses and biomaterials to meet this challenging situation. The inflammatory response in muscle injury involves a non-specific inflammatory response at the injured site that is coordination between the immune system, especially macrophages and muscle. The potential role of biomaterials in the regenerative process of skeletal muscle injury is currently an important topic. To this end, cell therapy holds great promise for the regeneration of damaged muscle following VML. However, the delivery of cells into the injured muscle site poses a major challenge as it might cause an adverse immune response or inflammation. To overcome this obstacle, in recent years various biomaterials with diverse physical and chemical nature have been developed and verified for the treatment of various muscle injuries. These biomaterials, with desired tunable physicochemical properties, can be used in combination with stem cells and growth factors to repair VML. In the current review, we focus on how various immune cells, in conjunction with biomaterials, can be used to promote muscle regeneration and, most importantly, suppress VML pathology.

Old and new biomarkers for volumetric muscle loss

Volumetric muscle loss (VML) impacts skeletal muscles and causes damage to associated tissues such as blood vessels and other structural tissues. Despite progress in the VML field, current preclinical approaches are often ineffective at restoring muscle volume. Additional research is paramount to develop strategies that improve muscle mass and function, while restoring supporting tissues. We highlight mechanisms that govern normal muscle function that are also key players for VML, including intracellular calcium signaling/homeostasis, mitochondria signaling (calcium, reactiove oxidative species (ROS)/oxidative stress), and angiogenesis. We propose an integration of these processes within the context of emerging biomaterials that provide structural support for muscle regeneration. We posit that new biomarkers (i.e. myokines and lipid signaling mediators) may serve as sentinels of early muscle injury and regeneration. We conclude that as new ideas, approaches, and models come together, new treatments will emerge to allow the full rebuilding of skeletal muscles and functional recovery of skeletal muscles after VML.

Bioprinted nanocomposite hydrogels: A proposed approach to functional restoration of skeletal muscle and vascular tissue following volumetric muscle loss

Musculoskeletal conditions are the highest contributor to global disability, accounting for 16% of all ages lived with disability. Volumetric muscle loss (VML) is classified as significant damage to skeletal muscle compartments and motor units, leading to significant tissue loss, functional deficits, and long-term disability. In this review, the current tissue engineering approaches in terms of fabrication techniques, materials, cell sources, and growth factors for enhanced angiogenesis and neuromuscular junction (NMJ) in VML repair, are discussed. Review of results recently published in the literature suggested that bioprinted nanocomposite hydrogels (NC gels) seeded with adult muscle progenitor cells that promote secretion of endogenous vascular growth factors have potential applications in promoting skeletal muscle regeneration, revascularization, and NMJ repair (Figure 1). Despite recent advancements, future research is needed on NC gels and the complex processes underlying vascular infiltration and NMJ repair in VML injuries.

Functionalizing biomaterials to promote neurovascular regeneration following skeletal muscle injury

During embryogenesis, blood vessels and nerves develop with similar branching structure in response to shared signaling pathways guiding network growth. With both systems integral to physiological homeostasis, dual targeting of blood vessels and nerves to promote neurovascular regeneration following injury is an emerging therapeutic approach in biomedical engineering. A limitation to this strategy is that the nature of cross talk between emergent vessels and nerves during regeneration in an adult is poorly understood. Following peripheral nerve transection, intraneural vascular cells infiltrate the site of injury to provide a migratory pathway for mobilized Schwann cells of regenerating axons. As Schwann cells demyelinate, they secrete vascular endothelial growth factor, which promotes angiogenesis. Recent advances point to concomitant restoration of neurovascular architecture and function through simultaneous targeting of growth factors and guidance cues shared by both systems during regeneration. In the context of traumatic injury associated with volumetric muscle loss, we consider the nature of biomaterials used to engineer three-dimensional scaffolds, functionalization of scaffolds with molecular signals that guide and promote neurovascular growth, and seeding scaffolds with progenitor cells. Physiological success is defined by each tissue component of the bioconstruct (nerve, vessel, muscle) becoming integrated with that of the host. Advances in microfabrication, cell culture techniques, and progenitor cell biology hold great promise for engineering bioconstructs able to restore organ function after volumetric muscle loss.

Promoting endogenous repair of skeletal muscle using regenerative biomaterials

Skeletal muscles normally have a remarkable ability to repair themselves; however, large muscle injuries and several myopathies diminish this ability leading to permanent loss of function. No clinical therapy yet exists that reliably restores muscle integrity and function following severe injury. Consequently, numerous tissue engineering techniques, both acellular and with cells, are being investigated to enhance muscle regeneration. Biomaterials are an essential part of these techniques as they can present physical and biochemical signals that augment the repair process. Successful tissue engineering strategies require regenerative biomaterials that either actively promote endogenous muscle repair or create an environment supportive of regeneration. This review will discuss several acellular biomaterial strategies for skeletal muscle regeneration with a focus on those under investigation in vivo. This includes materials that release bioactive molecules, biomimetic materials and immunomodulatory materials.

Methacrylic acid-based hydrogels enhance skeletal muscle regeneration after volumetric muscle loss in mice

Volumetric muscle loss (VML) impairs the regenerative ability of skeletal muscle resulting in scar tissue formation and loss of function. Current treatments are of limited efficacy as they do not fully restore function, i.e., force generation. Regenerative biomaterials, such as those containing methacrylic-acid (MAA), are proposed as a novel approach to enhancing muscle regeneration without added cells, growth factors or drugs. Here, the regenerative effects of two hydrogels were investigated: MAA-poly(ethylene glycol) (MAA-PEG) and MAA-collagen. These hydrogels were used to treat VML injuries in murine tibialis anterior muscles. The MAA-collagen hydrogel significantly increased regenerating muscle fiber size and muscle force production. While both hydrogels increased vascularization, only the MAA-collagen hydrogel increased apparent muscle innervation. The MAA-collagen hydrogel also significantly reduced a pro-inflammatory macrophage (MHCII+CD206-) population. Furthermore, the hydrogels had distinct gene expression profiles indicating that their regenerative abilities were carrier dependent. Overall, this study suggests MAA-collagen as a cell-free and drug-free approach to enhancing skeletal muscle regeneration after traumatic injury.

Agent-based model provides insight into the mechanisms behind failed regeneration following volumetric muscle loss injury

Skeletal muscle possesses a remarkable capacity for repair and regeneration following a variety of injuries. When successful, this highly orchestrated regenerative process requires the contribution of several muscle resident cell populations including satellite stem cells (SSCs), fibroblasts, macrophages and vascular cells. However, volumetric muscle loss injuries (VML) involve simultaneous destruction of multiple tissue components (e.g., as a result of battlefield injuries or vehicular accidents) and are so extensive that they exceed the intrinsic capability for scarless wound healing and result in permanent cosmetic and functional deficits. In this scenario, the regenerative process fails and is dominated by an unproductive inflammatory response and accompanying fibrosis. The failure of current regenerative therapeutics to completely restore functional muscle tissue is not surprising considering the incomplete understanding of the cellular mechanisms that drive the regeneration response in the setting of VML injury. To begin to address this profound knowledge gap, we developed an agent-based model to predict the tissue remodeling response following surgical creation of a VML injury. Once the model was able to recapitulate key aspects of the tissue remodeling response in the absence of repair, we validated the model by simulating the tissue remodeling response to VML injury following implantation of either a decellularized extracellular matrix scaffold or a minced muscle graft. The model suggested that the SSC microenvironment and absence of pro-differentiation SSC signals were the most important aspects of failed muscle regeneration in VML injuries. The major implication of this work is that agent-based models may provide a much-needed predictive tool to optimize the design of new therapies, and thereby, accelerate the clinical translation of regenerative therapeutics for VML injuries.

In Vivo Printing of Nanoenabled Scaffolds for the Treatment of Skeletal Muscle Injuries

Extremity skeletal muscle injuries result in substantial disability. Current treatments fail to recoup muscle function, but properly designed and implemented tissue engineering and regenerative medicine techniques can overcome this challenge. In this study, a nanoengineered, growth factor-eluting bioink that utilizes Laponite nanoclay for the controlled release of vascular endothelial growth factor (VEGF) and a GelMA hydrogel for a supportive and adhesive scaffold that can be crosslinked in vivo is presented. The bioink is delivered with a partially automated handheld printer for the in vivo formation of an adhesive and 3D scaffold. The effect of the controlled delivery of VEGF alone or paired with adhesive, supportive, and fibrilar architecture has not been studied in volumetric muscle loss (VML) injuries. Upon direct in vivo printing, the constructs are adherent to skeletal muscle and sustained release of VEGF. The in vivo printing of muscle ink in a murine model of VML injury promotes functional muscle recovery, reduced fibrosis, and increased anabolic response compared to untreated mice. The in vivo construction of a therapeutic-eluting 3D scaffold paves the way for the immediate treatment of a variety of soft tissue traumas.

Bioinspired electrospun dECM scaffolds guide cell growth and control the formation of myotubes

While skeletal muscle has a high capacity for endogenous repair in acute injuries, volumetric muscle loss can leave long-lasting or permanent structural and functional deficits to the injured muscle and surrounding tissues. With clinical treatments failing to repair lost tissue, there is a great need for a tissue-engineered therapy to promote skeletal muscle regeneration. In this study, we aim to assess the potential for electrospun decellularized skeletal muscle extracellular matrix (dECM) with tunable physicochemical properties to control mouse myoblast growth and myotube formation. The material properties as well as cell behavior - growth and differentiation - were assessed in response to modulation of crosslinking and scaffold architecture. The fabrication of a bioactive dECM-based system with tunable physicochemical properties that can control myotube formation has several applications in skeletal muscle engineering and may bring the field one step closer to developing a therapy to address these unmet clinical needs.

Decellularized xenografts in regenerative medicine: From processing to clinical application

Decellularized xenografts are an inherent component of regenerative medicine. Their preserved structure, mechanical integrity and biofunctional composition have well established them in reparative medicine for a diverse range of clinical indications. Nonetheless, their performance is highly influenced by their source (ie species, age, tissue) and processing (ie decellularization, crosslinking, sterilization and preservation), which govern their final characteristics and determine their success or failure for a specific clinical target. In this review, we provide an overview of the different sources and processing methods used in decellularized xenografts fabrication and discuss their effect on the clinical performance of commercially available decellularized xenografts.

Myogenic Differentiation of Stem Cells for Skeletal Muscle Regeneration

Stem cells have become a hot research topic in the field of regenerative medicine due to their self-renewal and differentiation capabilities. Skeletal muscle tissue is one of the most important tissues in the human body, and it is difficult to recover when severely damaged. However, conventional treatment methods can cause great pain to patients. Stem cell-based tissue engineering can repair skeletal muscle to the greatest extent with little damage. Therefore, the application of stem cells to skeletal muscle regeneration is very promising. In this review, we discuss scaffolds and stem cells for skeletal muscle regeneration and put forward our ideas for future development.

Pelvic floor muscle function recovery using biofabricated tissue constructs with neuromuscular junctions

Damages in pelvic floor muscles often cause dysfunction of the entire pelvic urogenital system, which is clinically challenging. A bioengineered skeletal muscle construct that mimics structural and functional characteristics of native skeletal muscle could provide a therapeutic option to restore normal muscle function. However, most of the current bioengineered muscle constructs are unable to provide timely innervation necessary for successful grafting and functional recovery. We previously have demonstrated that post-synaptic acetylcholine receptors (AChR) clusters can be pre-formed on cultured skeletal muscle myofibers with agrin treatment and suggested that implantation of AChR clusters containing myofibers could accelerate innervation and recovery of muscle function. In this study, we develop a 3-dimensional (3D) bioprinted human skeletal muscle construct, consisting of multi-layers bundles with aligned and AChR clusters pre-formed human myofibers, and investigate the effect of pre-formed AChR clusters in bioprinted skeletal muscle constructs and innervation efficiency in vivo. Agrin treatment successfully pre-formed functional AChR clusters on the bioprinted muscle constructs in vitro that increased neuromuscular junction (NMJ) formation in vivo in a transposed nerve implantation model in rats. In a rat model of pelvic floor muscle injury, implantation of skeletal muscle constructs containing the pre-formed AChR clusters resulted in functional muscle reconstruction with accelerated construct innervation. This approach may provide a therapeutic solution to the many challenges associated with pelvic floor reconstruction resulting from the lack of suitable bioengineered tissue for efficient innervation and muscle function restoration.

Muscle functional recovery is driven by extracellular vesicles combined with muscle extracellular matrix in a volumetric muscle loss murine model

Biological scaffolds derived from decellularized tissues are being investigated as a promising approach to repair volumetric muscle losses (VML). Indeed, extracellular matrix (ECM) from decellularized tissues is highly biocompatible and mimics the original tissue. However, the development of fibrosis and the muscle stiffness still represents a major problem. Intercellular signals mediating tissue repair are conveyed via extracellular vesicles (EVs), biologically active nanoparticles secreted by the cells. This work aimed at using muscle ECM and human EVs derived from Wharton Jelly mesenchymal stromal cells (MSC EVs) to boost tissue regeneration in a VML murine model. Mice transplanted with muscle ECM and treated with PBS or MSC EVs were analyzed after 7 and 30 days. Flow cytometry, tissue analysis, qRT-PCR and physiology test were performed. We demonstrated that angiogenesis and myogenesis were enhanced while fibrosis was reduced after EV treatment. Moreover, the inflammation was directed toward tissue repair. M2-like, pro-regenerative macrophages were significantly increased in the MSC EVs treated group compared to control. Strikingly, the histological improvements were associated with enhanced functional recovery. These results suggest that human MSC EVs can be a naturally-derived boost able to ameliorate the efficacy of tissue-specific ECM in muscle regeneration up to the restored tissue function.

Nanomaterial Additives for Fabrication of Stimuli-Responsive Skeletal Muscle Tissue Engineering Constructs

Volumetric muscle loss necessitates novel tissue engineering strategies for skeletal muscle repair, which have traditionally involved cells and extracellular matrix-mimicking scaffolds and have thus far been unable to successfully restore physiologically relevant function. However, the incorporation of various nanomaterial additives with unique physicochemical properties into scaffolds has recently been explored as a means of fabricating constructs that are responsive to electrical, magnetic, and photothermal stimulation. Herein, several classes of nanomaterials that are used to mediate external stimulation to tissue engineered skeletal muscle are reviewed and the impact of these stimuli-responsive biomaterials on cell growth and differentiation and in vivo muscle repair is discussed. The degradation kinetics and biocompatibilities of these nanomaterial additives are also briefly examined and their potential for incorporation into clinically translatable skeletal muscle tissue engineering strategies is considered. Overall, these nanomaterial additives have proven efficacious and incorporation in tissue engineering scaffolds has resulted in enhanced functional skeletal muscle regeneration.

3D Printing Decellularized Extracellular Matrix to Design Biomimetic Scaffolds for Skeletal Muscle Tissue Engineering

Functional engineered muscles are still a critical clinical issue to be addressed, although different strategies have been considered so far for the treatment of severe muscular injuries. Indeed, the regenerative capacity of skeletal muscle (SM) results inadequate for large-scale defects, and currently, SM reconstruction remains a complex and unsolved task. For this aim, tissue engineered muscles should provide a proper biomimetic extracellular matrix (ECM) alternative, characterized by an aligned/microtopographical structure and a myogenic microenvironment, in order to promote muscle regeneration. As a consequence, both materials and fabrication techniques play a key role to plan an effective therapeutic approach. Tissue-specific decellularized ECM (dECM) seems to be one of the most promising material to support muscle regeneration and repair. 3D printing technologies, on the other side, enable the fabrication of scaffolds with a fine and detailed microarchitecture and patient-specific implants with high structural complexity. To identify innovative biomimetic solutions to develop engineered muscular constructs for the treatment of SM loss, the more recent (last 5 years) reports focused on SM dECM-based scaffolds and 3D printing technologies for SM regeneration are herein reviewed. Possible design inputs for 3D printed SM dECM-based scaffolds for muscular regeneration are also suggested.

Holistic Approach of Swiss Fetal Progenitor Cell Banking: Optimizing Safe and Sustainable Substrates for Regenerative Medicine and Biotechnology

Safety, quality, and regulatory-driven iterative optimization of therapeutic cell source selection has constituted the core developmental bedrock for primary fetal progenitor cell (FPC) therapy in Switzerland throughout three decades. Customized Fetal Transplantation Programs were pragmatically devised as straightforward workflows for tissue procurement, traceability maximization, safety, consistency, and robustness of cultured progeny cellular materials. Whole-cell bioprocessing standardization has provided plethoric insights into the adequate conjugation of modern biotechnological advances with current restraining legislative, ethical, and regulatory frameworks. Pioneer translational advances in cutaneous and musculoskeletal regenerative medicine continuously demonstrate the therapeutic potential of FPCs. Extensive technical and clinical hindsight was gathered by managing pediatric burns and geriatric ulcers in Switzerland. Concomitant industrial transposition of dermal FPC banking, following good manufacturing practices, demonstrated the extensive potential of their therapeutic value. Furthermore, in extenso, exponential revalorization of Swiss FPC technology may be achieved via the renewal of integrative model frameworks. Consideration of both longitudinal and transversal aspects of simultaneous fetal tissue differential processing allows for a better understanding of the quasi-infinite expansion potential within multi-tiered primary FPC banking. Multiple fetal tissues (e.g., skin, cartilage, tendon, muscle, bone, lung) may be simultaneously harvested and processed for adherent cell cultures, establishing a unique model for sustainable therapeutic cellular material supply chains. Here, we integrated fundamental, preclinical, clinical, and industrial developments embodying the scientific advances supported by Swiss FPC banking and we focused on advances made to date for FPCs that may be derived from a single organ donation. A renewed model of single organ donation bioprocessing is proposed, achieving sustained standards and potential production of billions of affordable and efficient therapeutic doses. Thereby, the aim is to validate the core therapeutic value proposition, to increase awareness and use of standardized protocols for translational regenerative medicine, potentially impacting millions of patients suffering from cutaneous and musculoskeletal diseases. Alternative applications of FPC banking include biopharmaceutical therapeutic product manufacturing, thereby indirectly and synergistically enhancing the power of modern therapeutic armamentariums. It is hypothesized that a single qualifying fetal organ donation is sufficient to sustain decades of scientific, medical, and industrial developments, as technological optimization and standardization enable high efficiency.

Information-Driven Design as a Potential Approach for 3D Printing of Skeletal Muscle Biomimetic Scaffolds

Severe muscle injuries are a real clinical issue that still needs to be successfully addressed. Tissue engineering can represent a potential approach for this aim, but effective healing solutions have not been developed yet. In this regard, novel experimental protocols tailored to a biomimetic approach can thus be defined by properly systematizing the findings acquired so far in the biomaterials and scaffold manufacturing fields. In order to plan a more comprehensive strategy, the extracellular matrix (ECM), with its properties stimulating neomyogenesis and vascularization, should be considered as a valuable biomaterial to be used to fabricate the tissue-specific three-dimensional structure of interest. The skeletal muscle decellularized ECM can be processed and printed, e.g., by means of stereolithography, to prepare bioactive and biomimetic 3D scaffolds, including both biochemical and topographical features specifically oriented to skeletal muscle regenerative applications. This paper aims to focus on the skeletal muscle tissue engineering sector, suggesting a possible approach to develop instructive scaffolds for a guided healing process.

Biological Basis of Treatments of Acute Muscle Injuries: A Short Review

Muscle strains occur frequently in recreational and professional sports. This article considers various treatment options in a biological context and reviews evidence of their efficacy. Treatments reviewed include the PRICE principle (P: rotection, R: est, I: ce, C: ompression, E: levation), early mobilization, physical therapy, hematoma aspiration, platelet-rich plasma injections, use of nonsteroidal anti-inflammatory drugs, corticosteroids, and local anesthetics, cellular therapies, and surgery.

Pre-Clinical Cell Therapeutic Approaches for Repair of Volumetric Muscle Loss

Extensive damage to skeletal muscle tissue due to volumetric muscle loss (VML) is beyond the inherent regenerative capacity of the body, and results in permanent functional debilitation. Current clinical treatments fail to fully restore native muscle function. Recently, cell-based therapies have emerged as a promising approach to promote skeletal muscle regeneration following injury and/or disease. Stem cell populations, such as muscle stem cells, mesenchymal stem cells and induced pluripotent stem cells (iPSCs), have shown a promising capacity for muscle differentiation. Support cells, such as endothelial cells, nerve cells or immune cells, play a pivotal role in providing paracrine signaling cues for myogenesis, along with modulating the processes of inflammation, angiogenesis and innervation. The efficacy of cell therapies relies on the provision of instructive microenvironmental cues and appropriate intercellular interactions. This review describes the recent developments of cell-based therapies for the treatment of VML, with a focus on preclinical testing and future trends in the field.

Recent advances in tissue engineering scaffolds based on polyurethane and modified polyurethane

Organ repair, regeneration, and transplantation are constantly in demand due to various acute, chronic, congenital, and infectious diseases. Apart from traditional remedies, tissue engineering (TE) is among the most effective methods for the repair of damaged tissues via merging the cells, growth factors, and scaffolds. With regards to TE scaffold fabrication technology, polyurethane (PU), a high-performance medical grade synthetic polymer and bioactive material has gained significant attention. PU possesses exclusive biocompatibility, biodegradability, and modifiable chemical, mechanical and thermal properties, owing to its unique structure-properties relationship. During the past few decades, PU TE scaffold bioactive properties have been incorporated or enhanced with biodegradable, electroactive, surface-functionalised, ayurvedic products, ceramics, glass, growth factors, metals, and natural polymers, resulting in the formation of modified polyurethanes (MPUs). This review focuses on the recent advances of PU/MPU scaffolds, especially on the biomedical applications in soft and hard tissue engineering and regenerative medicine. The scientific issues with regards to the PU/MPU scaffolds, such as biodegradation, electroactivity, surface functionalisation, and incorporation of active moieties are also highlighted along with some suggestions for future work.

Skeletal Muscle Tissue Engineering: Biomaterials-Based Strategies for the Treatment of Volumetric Muscle Loss

Millions of Americans suffer from skeletal muscle injuries annually that can result in volumetric muscle loss (VML), where extensive musculoskeletal damage and tissue loss result in permanent functional deficits. In the case of small-scale injury skeletal muscle is capable of endogenous regeneration through activation of resident satellite cells (SCs). However, this is greatly reduced in VML injuries, which remove native biophysical and biochemical signaling cues and hinder the damaged tissue's ability to direct regeneration. The current clinical treatment for VML is autologous tissue transfer, but graft failure and scar tissue formation leave patients with limited functional recovery. Tissue engineering of instructive biomaterial scaffolds offers a promising approach for treating VML injuries. Herein, we review the strategic engineering of biophysical and biochemical cues in current scaffold designs that aid in restoring function to these preclinical VML injuries. We also discuss the successes and limitations of the three main biomaterial-based strategies to treat VML injuries: acellular scaffolds, cell-delivery scaffolds, and in vitro tissue engineered constructs. Finally, we examine several innovative approaches to enhancing the design of the next generation of engineered scaffolds to improve the functional regeneration of skeletal muscle following VML injuries.

3D cellular alignment and biomimetic mechanical stimulation enhance human adipose-derived stem cell myogenesis

Determination of a stem cell source with sufficient myogenic differentiation capacity that can be easily obtained in large quantities is of great importance in skeletal muscle regeneration therapies. Adipose-derived stem cells (ASCs) are readily available, can be isolated from fat tissue with high yield and possess myogenic differentiation capacity. Consequently, ASCs have high applicability in muscle regenerative therapies. However, a key challenge is their low differentiation efficiency. In this study, we have explored the potential of mimicking the natural microenvironment of the skeletal muscle tissue to enhance ASC myogenesis by inducing 3D cellular alignment and using dynamic biomimetic culture. ASCs were entrapped and 3D aligned in parallel within fibrin-based microfibers and subjected to uniaxial cyclic stretch. 3D cell alignment was shown to be necessary for achieving and maintaining the stiffness of the construct mimicking the natural tissue (12 ± 1 kPa), where acellular aligned fibers and cell-laden random fibers had stiffness values of 4 ± 1 and 5 ± 2 kPa, respectively, at the end of 21 d. The synergistic effect of 3D cell alignment and biomimetic dynamic culture was evaluated on cell proliferation, viability and the expression of muscle-specific markers (immunofluorescent staining for MyoD1, myogenin, desmin and myosin heavy chain). It was shown that the myogenic markers were only expressed on the aligned-dynamic culture samples on day 21 of dynamic culture. These results demonstrate that 3D skeletal muscle grafts can be developed using ASCs by mimicking the structural and physiological muscle microenvironment.

Recent Trends in Injury Models to Study Skeletal Muscle Regeneration and Repair

Skeletal muscle injuries that occur from traumatic incidents, such as those caused by car accidents or surgical resections, or from injuries sustained on the battlefield, result in the loss of functionality of the injured muscle. To understand skeletal muscle regeneration and to better treat these large scale injuries, termed volumetric muscle loss (VML), in vivo injury models exploring the innate mechanisms of muscle injury and repair are essential for the creation of clinically applicable treatments. While the end result of a muscle injury is often the destruction of muscle tissue, the manner in which these injuries are induced as well as the response from the innate repair mechanisms found in muscle in each animal models can vary. This targeted review describes injury models that assess both skeletal muscle regeneration (i.e., the response of muscle to myotoxin or ischemic injury) and skeletal muscle repair (i.e., VML injury). We aimed to summarize the injury models used in the field of skeletal muscle tissue engineering, paying particular attention to strategies to induce muscle damage and how to standardize injury conditions for future experiments.

The effect of BMP-mimetic peptide tethering bioinks on the differentiation of dental pulp stem cells (DPSCs) in 3D bioprinted dental constructs

The goal of this study was to use 3D bioprinting technology to create a bioengineered dental construct containing human dental pulp stem cells (hDPSCs). To accomplish this, we first developed a novel bone morphogenetic protein (BMP) peptide-tethering bioink formulation and examined its rheological properties, its printability, and the structural stability of the bioprinted construct. Second, we evaluated the survival and differentiation of hDPSCs in the bioprinted dental construct by measuring cell viability, proliferation, and gene expression, as well as histological and immunofluorescent analyses. Our results showed that the peptide conjugation into the gelatin methacrylate-based bioink formulation was successfully performed. We determined that greater than 50% of the peptides remained in the bioprinted construct after three weeks in vitro cell culture. Human DPSC viability was >90% in the bioprinted constructs immediately after the printing process. Alizarin Red staining showed that the BMP peptide construct group exhibited the highest calcification as compared to the growth medium, osteogenic medium, and non-BMP peptide construct groups. In addition, immunofluorescent and quantitative reverse transcription-polymerase chain reaction analyses showed robust expression of dentin sialophosphoprotein and osteocalcin in the BMP peptide dental constructs. Together, these results strongly suggested that BMP peptide-tethering bioink could accelerate the differentiation of hDPSCs in 3D bioprinted dental constructs.

Pre-innervated tissue-engineered muscle promotes a pro-regenerative microenvironment following volumetric muscle loss

Volumetric muscle loss (VML) is the traumatic or surgical loss of skeletal muscle beyond the inherent regenerative capacity of the body, generally leading to severe functional deficit. Formation of appropriate somato-motor innervations remains one of the biggest challenges for both autologous grafts as well as tissue-engineered muscle constructs. We aim to address this challenge by developing pre-innervated tissue-engineered muscle comprised of long aligned networks of spinal motor neurons and skeletal myocytes on aligned nanofibrous scaffolds. Motor neurons led to enhanced differentiation and maturation of skeletal myocytes in vitro. These pre-innervated tissue-engineered muscle constructs when implanted in a rat VML model significantly increased satellite cell density, neuromuscular junction maintenance, graft revascularization, and muscle volume over three weeks as compared to myocyte-only constructs and nanofiber scaffolds alone. These pro-regenerative effects may enhance functional neuromuscular regeneration following VML, thereby improving the levels of functional recovery following these devastating injuries.

Advances in biomaterials for skeletal muscle engineering and obstacles still to overcome

Repair of injured skeletal muscle is a sophisticated process that uses immune, muscle, perivascular, and neural cells. In acute injury, the robust endogenous repair process can facilitate complete regeneration with little to no functional deficit. However, in severe injury, the damage is beyond the capacity for self-repair, often resulting in structural and functional deficits. Aside from the insufficiencies in muscle function, the aesthetic deficits can impact quality of life. Current clinical treatments are significantly limited in their capacity to structurally and functionally repair the damaged skeletal muscle. Therefore, alternative approaches are needed. Biomaterial therapies for skeletal muscle engineering have leveraged natural materials with sophisticated scaffold fabrication techniques to guide cell infiltration, alignment, and differentiation. Advances in biomaterials paired with a standardized and rigorous assessment of resulting tissue formation have greatly advanced the field of skeletal muscle engineering in the last several years. Herein, we discuss the current trends in biomaterials-based therapies for skeletal muscle regeneration and present the obstacles still to be overcome before clinical translation is possible. With millions of people affected by muscle trauma each year, the development of a therapy that can repair the structural and functional deficits after severe muscle injury is pivotal.

Elastin-Like Recombinamer Hydrogels for Improved Skeletal Muscle Healing Through Modulation of Macrophage Polarization

Large skeletal muscle injuries, such as a volumetric muscle loss (VML), often result in an incomplete regeneration due to the formation of a non-contractile fibrotic scar tissue. This is, in part, due to the outbreak of an inflammatory response, which is not resolved over time, meaning that type-1 macrophages (M1, pro-inflammatory) involved in the initial stages of the process are not replaced by pro-regenerative type-2 macrophages (M2). Therefore, biomaterials that promote the shift from M1 to M2 are needed to achieve optimal regeneration in VML injuries. In this work, we used elastin-like recombinamers (ELRs) as biomaterials for the formation of non- (physical) and covalently (chemical) crosslinked bioactive and biodegradable hydrogels to fill the VML created in the tibialis anterior (TA) muscles of rats. These hydrogels promoted a higher infiltration of M2 within the site of injury in comparison to the non-treated control after 2 weeks (p<0.0001), indicating that the inflammatory response resolves faster in the presence of both types of ELR-based hydrogels. Moreover, there were not significant differences in the amount of collagen deposition between the samples treated with the chemical ELR hydrogel at 2 and 5 weeks, and this same result was found upon comparison of these samples with healthy tissue after 5 weeks, which implies that this treatment prevents fibrosis. The macrophage modulation also translated into the formation of myofibers that were morphologically more similar to those present in healthy muscle. Altogether, these results highlight that ELR hydrogels provide a friendly niche for infiltrating cells that biodegrades over time, leaving space to new muscle tissue. In addition, they orchestrate the shift of macrophage population toward M2, which resulted in the prevention of fibrosis in the case of the chemical hydrogel treatment and in a more healthy-like myofiber phenotype for both types of hydrogels. Further studies should focus in the assessment of the regeneration of skeletal muscle in larger animal models, where a more critical defect can be created and additional methods can be used to evaluate the functional recovery of skeletal muscle.

3D Graphene Scaffolds for Skeletal Muscle Regeneration: Future Perspectives

Although skeletal muscle can regenerate after injury, in chronic damages or in traumatic injuries its endogenous self-regeneration is impaired. Consequently, tissue engineering approaches are promising tools for improving skeletal muscle cells proliferation and engraftment. In the last decade, graphene and its derivates are being explored as novel biomaterials for scaffolds production for skeletal muscle repair. This review describes 3D graphene-based materials that are currently used to generate complex structures able not only to guide cell alignment and fusion but also to stimulate muscle contraction thanks to their electrical conductivity. Graphene is an allotrope of carbon that has indeed unique mechanical, electrical and surface properties and has been functionalized to interact with a wide range of synthetic and natural polymers resembling native musculoskeletal tissue. More importantly, graphene can stimulate stem cell differentiation and has been studied for cardiac, neuronal, bone, skin, adipose, and cartilage tissue regeneration. Here we recapitulate recent findings on 3D scaffolds for skeletal muscle repairing and give some hints for future research in multifunctional graphene implants.

Nanohybrid biodegradable scaffolds for TGF-β3 release for the chondrogenic differentiation of human mesenchymal stem cells

An ideal scaffold for bone tissue engineering should have chondroinductive, biodegradable, and biocompatible properties, as well as the ability to absorb and slowly release the biological molecules. In order to develop such a system to support bone tissue regeneration, in the present study, we developed a three-dimensional poly(L-lactic-co-glycolic acid) (PLGA)/Polycaprolactone (PCL) nanohybrid scaffold embedded with PLGA macroparticles (MPs) conjugated with TGF-β3 for the growth and chondrogenic differentiation of human mesenchymal stem cells (hMSCs). First, a microfluidic device was used to fabricate porous PLGA MPs with the sizes ranging from 10 to 50 µm. Next, the PLGA MPs were loaded with TGF-β3, mixed with PCL solution, and then electrospun to obtain PLGA-TGF-β3 MPs/PCL nanohybrid scaffold. Our results demonstrated that PLGA MPs fabricated using a microfluidic-based approach exhibited enhanced conjugation of TGF-β3 with over 80% loading efficiency and sustained release of TGF-β3. Furthermore, the results of glycosaminoglycan (GAG) content measurement and Safranin O staining revealed that the PLGA-TGF-β3 MPs and PLGA-TGF-β3 MPs/PCL nanohybrid scaffold can promote the proliferation and chondrogenic differentiation of hMSCs in vitro. Therefore, the PLGA-TGF-β3 MPs/PCL nanohybrid scaffold could pave the way for cartilage regeneration and have wide applications in regenerative medicine.

Analysis of sensitivity and specificity: precise recognition of neutrophils during regeneration of contused skeletal muscle in rats

In this report, we applied the TissueFAXS 200 digital pathological analysis system to rapidly and accurately identify neutrophils during regeneration of contused skeletal muscle, and to provide information for follow-up studies on neutrophils to estimate wound age. Rat injury model was established, and skeletal muscle samples were obtained from the control group and contusion groups at 1, 1.5, 2, 3, 4, and 6 h, as well as at 1, 3, 5, and 15 d post-injury (n = 5 per group). The expression of nuclei and neutrophils was detected by hematoxylin and eosin (HE) staining and immunohistochemical (IHC) staining. A total of 20 injury site areas of 0.25 mm² (0.5 mm × 0.5 mm) were then randomly selected at all time points. A TissueFAXS 200 digital pathological analysis system was used to identify the positive and negative numbers. Knowledge of five professional medical workers were considered the gold standard to measure the false positive rate (FPR), false negative rate (FNR), sensitivity, specificity, and area under the curve (AUC) of receiver operating characteristic (ROC) curves. As a result, with a staining area of neutrophils from 8 µm² to 15 µm², the FPR was 4.28%–12.14%, the FNR was 12.42%–64.08%, the sensitivity was 35.92%–87.58%, the specificity was 87.86%–95.72%, the Youden index was 0.316–0.754, the accuracy was 82.80%–88.30%, and the AUC was 0.771–0.826. The AUC was largest when the cut-off value of the staining area was 12 µm². Our results show that this software-based method is more accurate than the human eye in evaluating neutrophil infiltration. Based on the sensitivity and specificity, neutrophils can be accurately identified during regeneration of contused skeletal muscle. The TissueFAXS 200 digital pathological analysis system can also be used to optimize conditions for different cell types under various injury conditions to determine the optimal cut-off value of the staining area and provide optimal conditions for further study. Furthermore, it will provide evidence for forensic pathology cases.

Paradoxical Changes Underscore Epigenetic Reprogramming During Adult Zebrafish Extraocular Muscle Regeneration

Purpose

Genomic reprogramming and cellular dedifferentiation are critical to the success of de novo tissue regeneration in lower vertebrates such as zebrafish and axolotl. In tissue regeneration following injury or disease, differentiated cells must retain lineage while assuming a progenitor-like identity in order to repopulate the damaged tissue. Understanding the epigenetic regulation of programmed cellular dedifferentiation provides unique insights into the biology of stem cells and cancer and may lead to novel approaches for treating human degenerative conditions.

Methods

Using a zebrafish in vivo model of adult muscle regeneration, we utilized chromatin immunoprecipitation followed by massively parallel DNA sequencing (ChIP-seq) to characterize early changes in epigenetic signals, focusing on three well-studied histone modifications-histone H3 trimethylated at lysine 4 (H3K4me3), and histone H3 trimethylated or acetylated at lysine 27 (H3K27me3 and H3K27Ac, respectively).

Results

We discovered that zebrafish myocytes undergo a global, rapid, and transient program to drive genomic remodeling. The timing of these epigenetic changes suggests that genomic reprogramming itself represents a distinct sequence of events, with predetermined checkpoints, to generate cells capable of de novo regeneration. Importantly, we uncovered subsets of genes that maintain epigenetic marks paradoxical to changes in expression, underscoring the complexity of epigenetic reprogramming.

Conclusions

Within our model, histone modifications previously associated with gene expression act for the most part as expected, with exceptions suggesting that zebrafish chromatin maintains an easily editable state with a number of genes paradoxically marked for transcriptional activity despite downregulation.

Neural cell integration into 3D bioprinted skeletal muscle constructs accelerates restoration of muscle function

A bioengineered skeletal muscle construct that mimics structural and functional characteristics of native skeletal muscle is a promising therapeutic option to treat extensive muscle defect injuries. We previously showed that bioprinted human skeletal muscle constructs were able to form multi-layered bundles with aligned myofibers. In this study, we investigate the effects of neural cell integration into the bioprinted skeletal muscle construct to accelerate functional muscle regeneration in vivo. Neural input into this bioprinted skeletal muscle construct shows the improvement of myofiber formation, long-term survival, and neuromuscular junction formation in vitro. More importantly, the bioprinted constructs with neural cell integration facilitate rapid innervation and mature into organized muscle tissue that restores normal muscle weight and function in a rodent model of muscle defect injury. These results suggest that the 3D bioprinted human neural-skeletal muscle constructs can be rapidly integrated with the host neural network, resulting in accelerated muscle function restoration.

3D bioprinting of skeletal muscle using primary human muscle progenitor cells results in correct muscle architecture, but functional restoration in rodent models is limited. Here the authors include human neural stem cells into bioprinted skeletal muscle and observe improved architecture and function in vivo.

Recent advances in the application of mesoporous silica-based nanomaterials for bone tissue engineering

Silica nanomaterials (SNMs) and their composites have recently been investigated as scaffolds for bone tissue engineering. SNM scaffolds possess the ability to encourage bone cell growth and also allow the simultaneous delivery of biologically active biomolecules that are encapsulated in the mesopores. Their high mechanical strength, low cytotoxicity, ability to stimulate both the proliferation and osteogenic differentiation of progenitor cells make the SNMs appropriate scaffolds. Their physiochemical properties facilitate the cell spreading process, allow easy access to nutrients and help the cell-cell communication process during bone tissue engineering. The ability to deliver small biomolecules, such as dexamethasone, different growth factors, vitamins and mineral ions depends on the morphology, porosity, and crystallinity of SNMs and their composites with other polymeric materials. In this review, the abilities of SNMs to perform as suitable scaffolds for bone tissue engineering are comprehensively discussed.

Glass-activated regeneration of volumetric muscle loss

Volumetric muscle loss (VML) resulting from injuries to skeletal muscles has profound consequences in healthcare. Current VML treatment based on the use of soft materials including biopolymers and decellularized extracellular matrix (dECM) is challenging due to their incapability of stimulating the formation of satellite cells (SCs), muscle stem cells, which are required for muscle regeneration. Additional stem cells and/or growth factors have to be incorporated in these constructs for improved efficacy. Here we report an approach by using bioactive glasses capable of regenerating VML without growth factors or stem cells. One silicate and two borate compositions with different degradation rates (2.4% for silicate 45S5; 5.3% and 30.4% for borate 8A3B and 13-93B3, respectively, in simulated body fluid (SBF) at 37 °C for 30 days) were used for this study. Our in vitro models demonstrate the ability of ions released from bioactive glasses in promoting angiogenesis and stimulating cells to secrete critical muscle-related growth factors. We further show the activation of SCs and the regeneration of skeletal muscles in a rat VML model. Considering these promising results, this work reveals a potentially simple and safe approach to regenerating skeletal muscle defects. STATEMENT OF SIGNIFICANCE: (1) This is the first report on an inorganic material used in skeletal muscle regeneration through in vitro and in vivo models. (2) Bioactive glass is found to activate the production of satellite cells (SCs), muscle stem cells, without the incorporation of extra stem cells or growth factors. (3) The work represents a simple, safe, low-cost yet efficient means for healing muscle defects.

Large-Volume Vascularized Muscle Grafts Engineered From Groin Adipose Tissue in Perfusion Bioreactor Culture

BACKGROUND

Muscle tissue engineering still remains a major challenge. An axial vascular pedicle and a perfusion bioreactor are necessary for the development and maintenance of a large-volume engineered muscle tissue to provide circulation within the construct. This study aimed to determine whether large-volume vascularized muscle-like constructs could be made from rat groin adipose tissue in a perfusion bioreactor.

METHODS

Epigastric adipofascial flaps based on the inferior superficial epigastric vessels were elevated bilaterally in male Lewis rats and connected to the bioreactor. The system was run using a cable pump and filled with myogenic differentiation medium in the perfusion bioreactor for 1, 3, 5, or 7 weeks. The resulting tissue constructs were characterized with respect to the morphology and muscle-related expression of genes and proteins.

RESULTS

The histological examination demonstrated intact muscle-like tissue fibers; myogenesis was verified by the expression of myosin, MADS box transcription enhancer factor 2 D, desmin-a disintegrin and metalloproteinase domain (ADAM) 12-and M-cadherin using reverse transcription-polymerase chain reaction. Western blot analysis for desmin, MyoD1, N-cadherin, and ADAM12 was performed to verify the myogenic phenotype of the extracted differentiated tissue and prove the formation of muscle-like constructs.

CONCLUSIONS

A large-volume vascularized muscle tissue could be engineered in a perfusion bioreactor. The resulting tissue had muscle-like histological features and expressed muscle-related genes and proteins, indicating that the trans-differentiation of adipose tissue into muscle tissue occurred.

Vascularized and Innervated Skeletal Muscle Tissue Engineering

Volumetric muscle loss (VML) is a devastating loss of muscle tissue that overwhelms the native regenerative properties of skeletal muscle and results in lifelong functional deficits. There are currently no treatments for VML that fully recover the lost muscle tissue and function. Tissue engineering presents a promising solution for VML treatment and significant research has been performed using tissue engineered muscle constructs in preclinical models of VML with a broad range of defect locations and sizes, tissue engineered construct characteristics, and outcome measures. Due to the complex vascular and neural anatomy within skeletal muscle, regeneration of functional vasculature and nerves is vital for muscle recovery following VML injuries. This review aims to summarize the current state of the field of skeletal muscle tissue engineering using 3D constructs for VML treatment with a focus on studies that have promoted vascular and neural regeneration within the muscle tissue post-VML.

Redox Control of IL-6-Mediated Dental Pulp Stem-Cell Differentiation on Alginate/Hydroxyapatite Biocomposites for Bone Ingrowth

Composites and porous scaffolds produced with biodegradable natural polymers are very promising constructs which show high biocompatibility and suitable mechanical properties, with the possibility to be functionalized with growth factors involved in bone formation. For this purpose, alginate/hydroxyapatite (Alg/HAp) composite scaffolds using a novel production design were successfully developed and tested for their biocompatibility and osteoconductive properties in vitro. Redox homeostasis is crucial for dental pulp stem cell (DPSC) differentiation and mineralized matrix deposition, and interleukin-6 (IL-6) was found to be involved not only in immunomodulation but also in cell proliferation and differentiation. In the present study, we evaluated molecular pathways underlying the intracellular balance between redox homeostasis and extracellular matrix mineralization of DPSCs in the presence of composite scaffolds made of alginate and nano-hydroxyapatite (Alg/HAp). Prostaglandin-2 (PGE2) and IL-6 secretion was monitored by ELISA assays, and protein expression levels were quantified by Western blotting. This work aims to demonstrate a relationship between DPSC capacity to secrete a mineralized matrix in the presence of Alg/HAp scaffolds and their immunomodulatory properties. The variation of the molecular axis Nrf2 (nuclear factor erythroid 2-related factor 2)/PGE2/IL-6 suggests a tight intracellular balance between oxidative stress responses and DPSC differentiation in the presence of Alg/HAp scaffolds.