* Skeletal Myoblast-Seeded Vascularized Tissue Scaffolds in the Treatment of a Large Volumetric Muscle Defect in the Rat Biceps Femoris Muscle

Transcriptome profiling of a synergistic volumetric muscle loss repair strategy

Volumetric muscle loss overwhelms skeletal muscle's ordinarily capable regenerative machinery, resulting in severe functional deficits that have defied clinical repair strategies. In this manuscript we pair the early in vivo functional response induced by differing volumetric muscle loss tissue engineering repair strategies that are broadly representative of those explored by the field (scaffold alone, cells alone, or scaffold + cells) to the transcriptomic response induced by each intervention. We demonstrate that an implant strategy comprising allogeneic decellularized skeletal muscle scaffolds seeded with autologous minced muscle cellular paste (scaffold + cells) mediates a pattern of increased expression for several genes known to play roles in axon guidance and peripheral neuroregeneration, as well as several other key genes related to inflammation, phagocytosis, and extracellular matrix regulation. The upregulation of several key genes in the presence of both implant components suggests a unique synergy between scaffolding and cells in the early period following intervention that is not seen when either scaffolds or cells are used in isolation; a finding that invites further exploration of the interactions that could have a positive impact on the treatment of volumetric muscle loss.

Reconstructing vascular networks promotes the repair of skeletal muscle following volumetric muscle loss by pre-vascularized tissue constructs

Current treatment for complex and large-scale volumetric muscle loss (VML) injuries remains a limited success and have substantial disadvantages, due to the irreversible loss of muscle mass, slow muscle regeneration, and rapid formation of non-functional fibrosis scars. These VML injuries are accompanied by denervation and the destruction of native vasculature which increases difficulties in the functional restoration of muscle. Here, reconstruction of the vascular network at the injury site was offered as a possible solution for improving the repair of muscle defects through the timely supply of nutrients and oxygen to surrounding cells. A hydrogel-based tissue construct containing various densities of the vascular network was successfully created in the subcutaneous space of mice by manipulating hydrogel properties, and then implanted into the VML injury site. One month after implantation, the mouse treated with the highly vascularized tissue had extensive muscle repair at the injury site and only spent a shorter time completing the inclined plane tests. These findings suggest that the reconstruction of the functional vascular network at the VML injury site accelerated muscle fiber repair through a timely supply of sufficient blood and avoided invasion by host fibroblasts.

Adipogenic Differentiation Alters Properties of Vascularized Tissue-Engineered Skeletal Muscle

Advances in the engineering of comprehensive skeletal muscle models in vitro will improve drug screening platforms and can lead to better therapeutic approaches for the treatment of skeletal muscle injuries. To this end, a vascularized tissue-engineered skeletal muscle (TE-SkM) model that includes adipocytes was developed to better emulate the intramuscular adipose tissue that is observed in skeletal muscles of patients with diseases such as diabetes. Muscle precursor cells cultured with and without microvessels derived from adipose tissue (microvascular fragments) were used to generate TE-SkM constructs, with and without a microvasculature, respectively. TE-SkM constructs were treated with adipogenic induction media to induce varying levels of adipogenesis. With a delayed addition of induction media to allow for angiogenesis, a robust microvasculature in conjunction with an increased content of adipocytes was achieved. The augmentation of vascularized TE-SkM constructs with adipocytes caused a reduction in maturation (compaction), mechanical integrity (Young's modulus), and myotube and vessel alignment. An increase in basal glucose uptake was observed in both levels of adipogenic induction, and a diminished insulin-stimulated glucose uptake was associated with the higher level of adipogenic differentiation and the greater number of adipocytes.

Microvascular fragments in microcirculation research and regenerative medicine

Adipose tissue-derived microvascular fragments (MVF) are functional vessel segments, which rapidly reassemble into new microvasculatures under experimental in vitro and in vivo conditions. Accordingly, they have been used for many years in microcirculation research to study basic mechanisms of endothelial cell function, angiogenesis and microvascular network formation in two- and three-dimensional environments. Moreover, they serve as vascularization units for musculoskeletal regeneration and implanted biomaterials as well as for the treatment of myocardial infarction and the generation of prevascularized tissue organoids. Besides, multiple factors determining the vascularization capacity of MVF have been identified, including their tissue origin and cellular composition, the conditions for their short- and long-term storage as well as their implantation site and the general health status and medication of the recipient. The next challenging step is now the successful translation of all these promising experimental findings into clinical practice. If this succeeds, a multitude of future therapeutic applications may significantly benefit from the remarkable properties of MVF.

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.

Translational products of adipose tissue-derived mesenchymal stem cells: Bench to bedside applications

With developments in the field of tissue engineering and regenerative medicine, the use of biological products for the treatment of various disorders has come into the limelight among researchers and clinicians. Among all the available biological tissues, research and exploration of adipose tissue have become more robust. Adipose tissue engineering aims to develop by-products and their substitutes for their regenerative and immunomodulatory potential. The use of biodegradable scaffolds along with adipose tissue products has a major role in cellular growth, proliferation, and differentiation. Adipose tissue, apart from being the powerhouse of energy storage, also functions as the largest endocrine organ, with the release of various adipokines. The progenitor cells among the heterogeneous population in the adipose tissue are of paramount importance as they determine the capacity of regeneration of these tissues. The results of adipose-derived stem-cell assisted fat grafting to provide numerous growth factors and adipokines that improve vasculogenesis, fat graft integration, and survival within the recipient tissue and promote the regeneration of tissue are promising. Adipose tissue gives rise to various by-products upon processing. This article highlights the significance and the usage of various adipose tissue by-products, their individual characteristics, and their clinical applications.

GLUT4-overexpressing engineered muscle constructs as a therapeutic platform to normalize glycemia in diabetic mice

Skeletal muscle insulin resistance is a main defect in type 2 diabetes (T2D), which is associated with impaired function and content of glucose transporter type 4 (GLUT4). GLUT4 overexpression in skeletal muscle tissue can improve glucose homeostasis. Therefore, we created an engineered muscle construct (EMC) composed of GLUT4-overexpressing (OEG4) cells. The ability of the engineered implants to reduce fasting glucose levels was tested in diet-induced obesity mice. Decrease and stabilization of basal glucose levels were apparent up to 4 months after implantation. Analysis of the retrieved constructs showed elevated expression of myokines and proteins related to metabolic processes. In addition, we validated the efficiency of OEG4-EMCs in insulin-resistant mice. Following high glucose load administration, mice showed improved glucose tolerance. Our data indicate that OEG4-EMC implant is an efficient mode for restoring insulin sensitivity and improving glucose homeostasis in diabetic mice. Such procedure is a potential innovative modality for T2D therapy.

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.

Self-aligned myofibers in 3D bioprinted extracellular matrix-based construct accelerate skeletal muscle function restoration

To achieve rapid skeletal muscle function restoration, many attempts have been made to bioengineer functional muscle constructs by employing physical, biochemical, or biological cues. Here, we develop a self-aligned skeletal muscle construct by printing a photo-crosslinkable skeletal muscle extracellular matrix-derived bioink together with poly(vinyl alcohol) that contains human muscle progenitor cells. To induce the self-alignment of human muscle progenitor cells, in situ uniaxially aligned micro-topographical structure in the printed constructs is created by a fibrillation/leaching of poly(vinyl alcohol) after the printing process. The in vitro results demonstrate that the synergistic effect of tissue-specific biochemical signals (obtained from the skeletal muscle extracellular matrix-derived bioink) and topographical cues [obtained from the poly(vinyl alcohol) fibrillation] improves the myogenic differentiation of the printed human muscle progenitor cells with cellular alignment. Moreover, this self-aligned muscle construct shows the accelerated integration with neural networks and vascular ingrowth in vivo, resulting in rapid restoration of muscle function. We demonstrate that combined biochemical and topographic cues on the 3D bioprinted skeletal muscle constructs can effectively reconstruct the extensive muscle defect injuries.

Current Strategies for the Regeneration of Skeletal Muscle Tissue

Traumatic injuries, tumor resections, and degenerative diseases can damage skeletal muscle and lead to functional impairment and severe disability. Skeletal muscle regeneration is a complex process that depends on various cell types, signaling molecules, architectural cues, and physicochemical properties to be successful. To promote muscle repair and regeneration, various strategies for skeletal muscle tissue engineering have been developed in the last decades. However, there is still a high demand for the development of new methods and materials that promote skeletal muscle repair and functional regeneration to bring approaches closer to therapies in the clinic that structurally and functionally repair muscle. The combination of stem cells, biomaterials, and biomolecules is used to induce skeletal muscle regeneration. In this review, we provide an overview of different cell types used to treat skeletal muscle injury, highlight current strategies in biomaterial-based approaches, the importance of topography for the successful creation of functional striated muscle fibers, and discuss novel methods for muscle regeneration and challenges for their future clinical implementation.

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.

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.

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.

Microvascular Fragments: More Than Just Natural Vascularization Units

Adipose tissue-derived microvascular fragments serve as natural vascularization units in angiogenesis research and tissue engineering due to their ability to rapidly reassemble into microvascular networks. Recent studies indicate that they exhibit additional unique properties that may be beneficial for a wide range of future biomedical applications. Their angiogenic activity can be increased during short-term cultivation as a means of adapting their vascularization capacity to patient-specific needs. Moreover, they are a source of endothelial progenitor cells, multipotent mesenchymal stromal cells, and lymphatic vessel fragments. Finally, they exert immunomodulatory effects, determining the tissue integration of implanted biomaterials. Hence, microvascular fragments represent versatile building blocks for the improvement of vascularization, organotypic tissue formation, lymphatic regeneration, and implant integration.

A Straightforward Approach to Engineer Vascularized Adipose Tissue Using Microvascular Fragments

There is a need to overcome the donor-site morbidity and loss of volume over time that accompanies the current clinical approaches to treat soft tissue defects caused by disease and trauma. The development of bioactive constructs that can regenerate adipose tissue have made great progress toward addressing the limitations of current therapies, but their lack of vascularization and ability to meet the significant dimension requirements of tissue defects limit their clinical translatability. Microvascular fragments (MVFs) can form extensive vascular networks and contain resident cells that have the ability to differentiate into adipocytes. Therefore, the objective of this study was to determine if vascularized adipose tissue could be engineered using a fibrin-based hydrogel containing MVFs as the sole source of microvessels and adipocyte-forming cells. The potential for MVFs from different fat depots (epididymal, inguinal, and subcutaneous) to form microvascular networks and generate adipocytes when exposed to growth media (GM), adipogenic differentiation media (ADM), or when treated with GM before adipogenic induction (i.e., they were allowed to presprout before adipogenic induction) was evaluated. MVFs treated with adipogenic induction media, both with and without presprouting, contained lipid droplets, had an increase in expression levels of genes associated with adipogenesis (adiponectin and fatty acid synthase [FAS]), and had an increased rate of lipolysis. MVFs allowed to presprout before ADM treatment maintained their ability to form vascular networks while maintaining an elevated lipid content, adipogenic gene expression, and lipolysis rate. Collectively, these results support the contention that MVFs can serve as the sole source of biologic material for creating a vascularized adipose tissue scaffold. Impact statement Microvascular fragments have both the ability to form extensive vascular networks and function as a source of adipocytes. These phenomena were exploited as vascularized adipose tissue was generated by first allowing for a period of angiogenesis before the adipogenic induction. This strategy has the ability to provide a means of both improving soft tissue reconstruction while also serving as a model to better understand adipose tissue expansion.

Long-Term Evaluation of Functional Outcomes Following Rat Volumetric Muscle Loss Injury and Repair

Volumetric muscle loss (VML) injuries, by definition, exceed the endogenous repair capacity of skeletal muscle resulting in permanent structural and functional deficits. VML injuries present a significant burden for both civilian and military medicine. Despite progress, there is still considerable room for therapeutic improvement. In this regard, tissue-engineered constructs show promise for VML repair, as they provide an opportunity to introduce both scaffolding and cellular components. We have pioneered the development of a tissue-engineered muscle repair (TEMR) technology created by seeding muscle progenitor cells onto a porcine-derived bladder acellular matrix followed by cyclic stretch preconditioning before implantation. Our work to date has demonstrated significant functional repair (60-90% functional recovery) in progressively larger rodent models of VML injury following TEMR implantation. Notwithstanding this success, TEMR implantation in cylindrically shaped VML injuries in the tibialis anterior (TA) muscle was associated with more variable functional outcomes than has been observed in sheet-like muscles such as the latissimus dorsi. In fact, previous observations documented a dichotomy of responses following TEMR implantation in a rodent TA VML injury model; with an ≈61% functional improvement observed in fewer than half (46%) of TEMR-implanted animals at 12 weeks postinjury. This current study builds directly from those observations as we modified the geometry of both the VML injury and the TEMR construct to determine if improved matching of the implanted TEMR construct to the surgically created VML injury resulted in increased functional recovery posttreatment. Following these modifications, we observed a comparable degree of functional improvement in a larger proportion of animals (≈67%) that was durable up to 24 weeks post-TEMR implantation. Moreover, in ≈25% of all TEMR-implanted animals, functional recovery was virtually complete (TEMR max responders), and furthermore, the functional recovery in all 67% of responding animals was accompanied by the presence of native-like muscle properties within the repaired TA muscle, including fiber cross-sectional area, fiber type, vascularization, and innervation. This study emphasizes the importance of tuning the application of tissue engineering technology platforms to the specific requirements of diverse VML injuries to improve functional outcomes. Impact Statement This report confirms and extends previous observations with our implantable tissue-engineered technology platform for repair of volumetric muscle loss (VML) injuries. Based on our prior work, we addressed factors hypothesized to be responsible for significant outcome variability following treatment of VML injuries in a rat tibialis anterior model. Through customization of the muscle repair technology to a specific VML injury, we were able to significantly increase the frequency at which functional recovery occurred, and furthermore, demonstrate durability out to 6 months. In addition, the enhanced biomimetic qualities of repaired muscle tissue were associated with the most robust functional outcomes.

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.

Myogenic differentiation of human amniotic mesenchymal cells and its tissue repair capacity on volumetric muscle loss

Stem cell-based tissue engineering therapy is the most promising method for treating volumetric muscle loss. Human amniotic mesenchymal cells possess characteristics similar to those of embryonic stem cells. In this study, we verified the stem cell characteristics of human amniotic mesenchymal cells by the flow cytometry analysis, and osteogenic and adipogenic differentiation. Through induction with the DNA demethylating agent 5-azacytidine, human amniotic mesenchymal cells can undergo myogenic differentiation and express skeletal muscle cell-specific markers such as desmin and MyoD. The Wnt/β-catenin signaling pathway also plays an important role. After 5-azacytidine-induced human amniotic mesenchymal cells were implanted into rat tibialis anterior muscle with volumetric muscle loss, we observed increased angiogenesis and improved local tissue repair. We believe that human amniotic mesenchymal cells can serve as a potential source of cells for skeletal muscle tissue engineering.

Biofabrication of thick vascularized neo-pedicle flaps for reconstructive surgery

Wound chronicity due to intrinsic and extrinsic factors perturbs adequate lesion closure and reestablishment of the protective skin barrier. Immediate and proper care of chronic wounds is necessary for a swift recovery and a reduction of patient vulnerability to infection. Advanced therapies supplemented with standard wound care procedures have been clinically implemented to restore aberrant tissue; however, these treatments are ineffective if local vasculature is too compromised to support minimally-invasive strategies. Autologous "flaps", which are tissues equipped with their own hierarchical vascular supply, can be harvested from one region of the patient and transplanted to the wound where it is reperfused upon microsurgical anastomosis to appropriate recipient vessels. Despite the success of autologous flap transfer, these procedures are extremely invasive, incur obligatory donor-site morbidity, and require sufficient donor-tissue availability, microsurgical expertise, and specialized equipment. 3D-bioprinting modalities, such as extrusion-based bioprinting, can be used to address the clinical constraints of autologous flap transfer, primarily addressing donor-site morbidity and tissue availability. This advancement in regenerative medicine allows the biofabrication of heterogeneous tissue structures with high shape fidelity and spatial resolution to generate biomimetic constructs with the anatomically-precise geometries of native tissue to ensure tissue-specific function. Yet, meaningful progress toward this clinical application has been limited by the lack of vascularization required to meet the nutrient and oxygen demands of clinically relevant tissue volumes. Thus, various criteria for the fabrication of functional tissues with hierarchical, patent vasculature must be considered when implementing 3D-bioprinting technologies for deep, chronic wounds.

Decorin-supplemented collagen hydrogels for the co-delivery of bone morphogenetic protein-2 and microvascular fragments to a composite bone-muscle injury model with impaired vascularization

Traumatic musculoskeletal injuries that result in bone defects or fractures often affect both bone and the surrounding soft tissue. Clinically, these types of multi-tissue injuries have increased rates of complications and long-term disability. Vascular integrity is a key clinical indicator of injury severity, and revascularization of the injury site is a critical early step of the bone healing process. Our lab has previously established a pre-clinical model of composite bone-muscle injury that exhibits impaired bone healing; however, the vascularization response in this model had not yet been investigated. Here, the early revascularization of a bone defect following composite injury is shown to be impaired, and subsequently the therapeutic potential of combined vascularization and osteoinduction was investigated to overcome the impaired regeneration in composite injuries. A decorin (DCN)-supplemented collagen hydrogel was developed as a biomaterial delivery vehicle for the co-delivery microvascular fragments (MVF), which are multicellular segments of mature vasculature, and bone morphogenetic protein-2 (BMP-2), a potent osteoinductive growth factor. We hypothesized that collagen + DCN would increase BMP-2 retention over collagen alone due to DCN's ability to sequester TGF-ß growth factors. We further hypothesized that MVF would increase both early vascularization and subsequent BMP-2-mediated bone regeneration. Contrary to our hypothesis, BMP + MVF decreased the number of blood vessels relative to BMP alone and had no effect on bone healing. However, collagen + DCN was demonstrated to be a BMP-2 delivery vehicle capable of achieving bridging in the challenging composite defect model that is comparable to that achieved with a well-established alginate-based delivery system. STATEMENT OF SIGNIFICANCE: We have previously established a model of musculoskeletal trauma that exhibits impaired bone healing. For the first time, this work shows that the early revascularization response is also significantly, albeit modestly, impaired. A decorin-supplemented collagen hydrogel was used for the first time in vivo as a delivery vehicle for both a cell-based vascular therapeutic, MVF, and an osteoinductive growth factor, BMP-2. While MVF did not improve vascular volume or bone healing, collagen + DCN is a BMP-2 delivery vehicle capable of achieving bridging in the challenging composite defect model. Based on its support of robust angiogenesis in vitro, collagen + DCN may be extended for future use with other vascular therapeutics such as pre-formed vascular networks.

Tissue Engineering of the Microvasculature

The ability to generate new microvessels in desired numbers and at desired locations has been a long-sought goal in vascular medicine, engineering, and biology. Historically, the need to revascularize ischemic tissues nonsurgically (so-called therapeutic vascularization) served as the main driving force for the development of new methods of vascular growth. More recently, vascularization of engineered tissues and the generation of vascularized microphysiological systems have provided additional targets for these methods, and have required adaptation of therapeutic vascularization to biomaterial scaffolds and to microscale devices. Three complementary strategies have been investigated to engineer microvasculature: angiogenesis (the sprouting of existing vessels), vasculogenesis (the coalescence of adult or progenitor cells into vessels), and microfluidics (the vascularization of scaffolds that possess the open geometry of microvascular networks). Over the past several decades, vascularization techniques have grown tremendously in sophistication, from the crude implantation of arteries into myocardial tunnels by Vineberg in the 1940s, to the current use of micropatterning techniques to control the exact shape and placement of vessels within a scaffold. This review provides a broad historical view of methods to engineer the microvasculature, and offers a common framework for organizing and analyzing the numerous studies in this area of tissue engineering and regenerative medicine. © 2019 American Physiological Society. Compr Physiol 9:1155-1212, 2019.

Osteopontin-derived synthetic peptide SVVYGLR has potent utility in the functional regeneration of oral and maxillofacial skeletal muscles

Oral and maxillofacial skeletal muscles are critical for oral motor functions, and severe damage to these muscles by trauma or surgery may lead to persistent functional impairment. This study investigated the effects of SVVYGLR (SV) peptide, a thrombin-cleaved osteopontin-derived motif, on histopathological wound healing and functional repair after severe injury of skeletal muscles. A rat model of volumetric muscle loss bilateral masseter muscle was developed. A single dose of SV-peptide or phosphate-buffered saline (PBS) was separately injected into the injured muscle belly. Histopathological and functional analyses were performed 1-8 weeks after the treatment. Behavioral analysis during free-feeding revealed that the feeding rate markedly increased in the SV-peptide group, in contrast, the PBS group showed fewer changes after the injury. Electromyogram recordings from injured muscles demonstrated amplification of rectified burst activity over time accompanied by increased maximal amplitude and duration in the SV-peptide group, in contrast, the PBS group showed moderate changes. A lissajous figure for bilateral masseter muscle activities also revealed superior functional recovery by the SV-peptide treatment. The SV-peptide also facilitated regeneration of muscles composed of matured myofibers with a greater diameter compared to the PBS group. In addition, granulation in the earlier period and fibrosis in the later period of wound healing were significantly inhibited by the SV-peptide treatment but not by the PBS treatment. Therefore, local application of the SV-peptide could help facilitate regeneration of muscles, inhibition of fibrosis, and improvement of functional impairment of oral and maxillofacial skeletal muscles damaged by severe trauma or surgery.

Effects of BMP-2 dose and delivery of microvascular fragments on healing of bone defects with concomitant volumetric muscle loss

Traumatic composite bone-muscle injuries, such as open fractures, often require multiple surgical interventions and still typically result in long-term disability. Clinically, a critical indicator of composite injury severity is vascular integrity; vascular damage alone is sufficient to assign an open fracture to the most severe category. Challenging bone injuries are often treated with bone morphogenetic protein 2 (BMP-2), an osteoinductive growth factor, delivered on collagen sponge. Previous studies in a composite defect model found that a minimally bridging dose in the segmental defect model was unable to overcome concomitant muscle damage, but the effect of BMP dose on composite injuries has not yet been studied. Here, we test the hypotheses that BMP-2-mediated functional regeneration of composite extremity injuries is dose dependent and can be further enhanced via co-delivery of adipose-derived microvascular fragments (MVF), which have been previously shown to increase tissue vascular volume. Although MVF did not improve healing outcomes, we observed a significant BMP-2 dose-dependent increase in regenerated bone volume and biomechanical properties. This is the first known report of an increased BMP-2 dose improving bone healing with concomitant muscle damage. While high dose BMP-2 delivery can induce heterotopic ossification (HO) and increased inflammation, the maximum 10 μg dose used in this study did not result in HO and was associated with a lower circulating inflammatory cytokine profile than the low dose (2.5 μg) group. These data support the potential benefits of an increased, though still moderate, BMP-2 dose for treatment of bone defects with concomitant muscle damage. Published 2019. This article is a U.S. Government work and is in the public domain in the USA. J Orthop Res.

Determination of a Critical Size Threshold for Volumetric Muscle Loss in the Mouse Quadriceps

IMPACT STATEMENT

The goal of this study was to determine the threshold for a critically sized, nonhealing muscle defect by characterizing key components in the balance between fibrosis and regeneration as a function of injury size in the mouse quadriceps. There is currently limited understanding of what leads to a critically sized muscle defect and which muscle regenerative components are functionally impaired. With the substantial increase in preclinical VML models as testbeds for tissue engineering therapeutics, defining the critical threshold for VML injuries will be instrumental in characterizing therapeutic efficacy and potential for subsequent translation.

Engineering functional and histological regeneration of vascularized skeletal muscle

Tissue engineering strategies to treat patients with volumetric muscle loss (VML) aim to recover the structure and contractile function of lost muscle tissue. Here, we assessed the capacity of novel electrospun fibrin hydrogel scaffolds seeded with murine myoblasts to regenerate the structure and function of damaged muscle within VML defects to the mouse tibialis anterior muscle. The electrospun fibrin scaffolds provide pro-myogenic alignment and stiffness cues, myomimetic hierarchical structure, suturability, and scale-up capabilities. Myoblast-seeded scaffolds enabled remarkable muscle regeneration with high myofiber and vascular densities after 2 and 4 weeks, mimicking that of native skeletal muscle, while acellular scaffolds lacked muscle regeneration. Both myoblast-seeded and acellular scaffolds fully recovered muscle contractile function to uninjured values after 2 and 4 weeks. Electrospun scaffolds pre-vascularized with co-cultured human endothelial cells and human adipose-derived stem cells implanted into VML defects for 2 weeks anastomosed with host vasculature and were perfused with host red blood cells. These data demonstrate the significant potential of electrospun fibrin scaffolds seeded with myoblasts to fully regenerate the structure and function of volumetric muscle defects and these scaffolds offer a promising treatment option for patients with VML.