Regeneration of skeletal muscle using in situ tissue engineering on an acellular collagen sponge scaffold in a rabbit model

Tissue Engineered 3D Constructs for Volumetric Muscle Loss

Severe injuries to skeletal muscles, including cases of volumetric muscle loss (VML), are linked to substantial tissue damage, resulting in functional impairment and lasting disability. While skeletal muscle can regenerate following minor damage, extensive tissue loss in VML disrupts the natural regenerative capacity of the affected muscle tissue. Existing clinical approaches for VML, such as soft-tissue reconstruction and advanced bracing methods, need to be revised to restore tissue function and are associated with limitations in tissue availability and donor-site complications. Advancements in tissue engineering (TE), particularly in scaffold design and the delivery of cells and growth factors, show promising potential for regenerating damaged skeletal muscle tissue and restoring function. This article provides a brief overview of the pathophysiology of VML and critiques the shortcomings of current treatments. The subsequent section focuses on the criteria for designing TE scaffolds, offering insights into various natural and synthetic biomaterials and cell types for effectively regenerating skeletal muscle. We also review multiple TE strategies involving both acellular and cellular scaffolds to encourage the development and maturation of muscle tissue and facilitate integration, vascularization, and innervation. Finally, the article explores technical challenges hindering successful translation into clinical applications.

Regenerative therapy in geriatric patients with low back pain

Low back pain (LBP) is a prevalent and debilitating condition, particularly among older adults, with degenerative spinal disease being a major contributor. Regenerative therapy, which aims to repair and regenerate damaged spinal structures, has shown promise in providing long-term pain relief and functional improvement. This review focuses on the application and efficacy of regenerative therapies such as mesenchymal stem cells, platelet-rich plasma, and atelocollagen in older patients with LBP. Despite the potential benefits, there is a notable scarcity of studies specifically targeting the older population, and those available often have small sample sizes and limited age-related analyses. Our findings underscore the need for more comprehensive and well-designed clinical trials to evaluate the effectiveness of these therapies in older patients. Future research should prioritize larger age-specific studies to establish regenerative therapy as a viable and effective treatment option for LBP in the aging population.

Effects of Paraspinal Intramuscular Injection of Atelocollagen in Patients with Chronic Low Back Pain: A Retrospective Observational Study

Background/Objectives: Atelocollagen is used for soft tissue repair and reconstruction by replacing defective or damaged muscles, membranes, ligaments, and tendons. This study aimed to evaluate the clinical efficacy and safety of additional paraspinal intramuscular injection of atelocollagen on lumbar epidural steroid injection for reducing pain and improving functional capacity of patients with chronic low back pain (CLBP). Methods: We retrospectively enrolled 608 consecutive patients with CLBP who received lumbar epidural steroid injection with or without additional paraspinal intramuscular injection of atelocollagen. The Numerical Rating Scale and the Oswestry Disability Index were used to assess pain and functional capacity, respectively, before the procedure, and three months after the injection. Also, we analyzed the relationship between the additional paraspinal intramuscular injection of atelocollagen and the success rate. Results: Both Numerical Rating Scale and the Oswestry Disability Index scores were significantly reduced in both groups at three months after injection. However, there was a significant difference between the two groups. Furthermore, the success rate was significantly higher in the additional paraspinal intramuscular injection of atelocollagen group. Conclusions: This study's results showed that additional paraspinal intramuscular injection of atelocollagen on lumbar epidural steroid injection reduced pain and improved functional capacity for patients with CLBP. Therefore, the paraspinal intramuscular injection of atelocollagen may be a promising option for the treatment of patients with CLBP.

Aligned Collagen Sponges with Tunable Pore Size for Skeletal Muscle Tissue Regeneration

Volumetric muscle loss (VML) is a traumatic injury where at least 20% of the mass of a skeletal muscle has been destroyed and functionality is lost. The standard treatment for VML, autologous tissue transfer, is limited as approximately 1 in 10 grafts fail because of necrosis or infection. Tissue engineering strategies seek to develop scaffolds that can regenerate injured muscles and restore functionality. Many of these scaffolds, however, are limited in their ability to restore muscle functionality because of an inability to promote the alignment of regenerating myofibers. For aligned myofibers to form on a scaffold, myoblasts infiltrate the scaffold and receive topographical cues to direct targeted myofiber growth. We seek to determine the optimal pore size for myoblast infiltration and differentiation. We developed a method of tuning the pore size within collagen scaffolds while inducing longitudinal alignment of these pores. Significantly different pore sizes were generated by adjusting the freezing rate of the scaffolds. Scaffolds frozen at -20 °C contained the largest pores. These scaffolds promoted the greatest level of cell infiltration and orientation in the direction of pore alignment. Further research will be conducted to induce higher levels of myofiber formation, to ultimately create an off-the-shelf treatment for VML injuries.

Porous biomaterial scaffolds for skeletal muscle tissue engineering

Volumetric muscle loss is a traumatic injury which overwhelms the innate repair mechanisms of skeletal muscle and results in significant loss of muscle functionality. Tissue engineering seeks to regenerate these injuries through implantation of biomaterial scaffolds to encourage endogenous tissue formation and to restore mechanical function. Many types of scaffolds are currently being researched for this purpose. Scaffolds are typically made from either natural, synthetic, or conductive polymers, or any combination therein. A major criterion for the use of scaffolds for skeletal muscle is their porosity, which is essential for myoblast infiltration and myofiber ingrowth. In this review, we summarize the various methods of fabricating porous biomaterial scaffolds for skeletal muscle regeneration, as well as the various types of materials used to make these scaffolds. We provide guidelines for the fabrication of scaffolds based on functional requirements of skeletal muscle tissue, and discuss the general state of the field for skeletal muscle tissue engineering.

Preclinical research studies for treating severe muscular injuries: focus on tissue-engineered strategies

Severe skeletal muscle injuries are a lifelong trauma with limited medical solutions. Significant progress has been made in developing in vitro surrogates for treating such trauma. However, more attention is needed when translating these approaches to the clinic. In this review, we survey the potential of tissue-engineered surrogates in promoting muscle healing, by critically analyzing data from recent preclinical models. The therapeutic advantages provided by a combination of different biomaterials, cell types, and biochemical mediators are discussed. Current therapies on muscle healing are also summarized, emphasizing their main advantages and drawbacks. We also discuss previous and ongoing clinical trials as well as highlighting future directions for the field.

Histologic characterization of orbicularis oris muscle with a new acellular dermal matrix grafts in a rabbit model

Muscular dysplasia is the key factor in influencing surgical outcomes in patients with cleft lip/palate. In this research, we attempted to evaluate a new acellular dermal matrix (ADM) as a substitute for reconstruction of the orbicularis oris muscle with growth factors such as Insulin-Like Growth Factor I (IGF-I), vascular endothelial growth factor (VEGF) in a rabbit model. 30 male New Zealand Rabbits (2-3 m, 1700-2000 g) were divided into four groups as follows; a group in which the orbicularis oris muscle of the upper lip was implanted with ADM, a group in which the orbicularis oris muscle of the upper lip was implanted with ADM + IGF-I + VEGF, a group in which the upper lip was operated without implantation of an ADM scaffold, and a normal upper lip for comparison. Macroscopic observation, histological evaluation, and immunohistochemistry were employed to evaluate levels of the muscle regeneration, vascularization, and inflammation at 1, 2, 4, 6, and 12 weeks after the operation. All wounds healed well without infection, immune rejection and so on. Histological evaluation showed that ADM was totally degraded and replaced by connective tissue. The area in which the ADM scaffold was coated with growth factors show a significant increase in the formation of new myofibers after injury, and the vascularization improved compared to the control group and the normal group. In regard to the degrees of inflammation, there were no notable differences among the groups. In conclusion, Our study indicated that ADM grafts combined with IGF-I and VEGF have potential advantages in alleviating muscular dysplasia in cleft lip treatment.

Biomaterials for recruiting and activating endogenous stem cells in situ tissue regeneration

Over the past two decades in situ tissue engineering has emerged as a new approach where biomaterials are used to harness the body's own stem/progenitor cells to regenerate diseased or injured tissue. Immunomodulatory biomaterials are designed to promote a regenerative environment, recruit resident stem cells to diseased or injured tissue sites, and direct them towards tissue regeneration. This review explores advances gathered from in vitro and in vivo studies on in situ tissue regenerative therapies. Here we also examine the different ways this approach has been incorporated into biomaterial sciences in order to create customized biomaterial products for therapeutic applications in a broad spectrum of tissues and diseases. STATEMENT OF SIGNIFICANCE: Biomaterials can be designed to recruit stem cells and coordinate their behavior and function towards the restoration or replacement of damaged or diseased tissues in a process known as in situ tissue regeneration. Advanced biomaterial constructs with precise structure, composition, mechanical, and physical properties can be transplanted to tissue site and exploit local stem cells and their micro-environment to promote tissue regeneration. In the absence of cells, we explore the critical immunomodulatory, chemical and physical properties to consider in material design and choice. The application of biomaterials for in situ tissue regeneration has the potential to address a broad range of injuries and diseases.

Functionalized Nanocellulose Drives Neural Stem Cells toward Neuronal Differentiation

Transplantation of differentiated and fully functional neurons may be a better therapeutic option for the cure of neurodegenerative disorders and brain injuries than direct grafting of neural stem cells (NSCs) that are potentially tumorigenic. However, the differentiation of NSCs into a large population of neurons has been a challenge. Nanomaterials have been widely used as substrates to manipulate cell behavior due to their nano-size, excellent physicochemical properties, ease of synthesis, and versatility in surface functionalization. Nanomaterial-based scaffolds and synthetic polymers have been fabricated with topology resembling the micro-environment of the extracellular matrix. Nanocellulose materials are gaining attention because of their availability, biocompatibility, biodegradability and bioactivity, and affordable cost. We evaluated the role of nanocellulose with different linkage and surface features in promoting neuronal differentiation. Nanocellulose coupled with lysine molecules (CNC-Lys) provided positive charges that helped the cells to attach. Embryonic rat NSCs were differentiated on the CNC-Lys surface for up to three weeks. By the end of the three weeks of in vitro culture, 87% of the cells had attached to the CNC-Lys surface and more than half of the NSCs had differentiated into functional neurons, expressing endogenous glutamate, generating electrical activity and action potentials recorded by the multi-electrode array.

Biomimetic sponges improve muscle structure and function following volumetric muscle loss

Skeletal muscle is inept in regenerating after traumatic injuries such as volumetric muscle loss (VML) due to significant loss of various cellular and acellular components. Currently, there are no approved therapies for the treatment of muscle tissue following trauma. In this study, biomimetic sponges composed of gelatin, collagen, laminin-111, and FK-506 were used for the treatment of VML in a rodent model. We observed that biomimetic sponge treatment improved muscle structure and function while modulating inflammation and limiting the extent of fibrotic tissue deposition. Specifically, sponge treatment increased the total number of myofibers, type 2B fiber cross-sectional area, myosin: collagen ratio, myofibers with central nuclei, and peak isometric torque compared to untreated VML injured muscles. As an acellular scaffold, biomimetic sponges may provide a promising clinical therapy for VML.

Effects of radiation dose and nitrogen purge on collagen scaffold properties

Sterilization of medical devices is commonly performed using radiation methods. However, collagen materials can be damaged when using standard radiation doses (25 kGy). Small increases of radiation dose can allow for increases in the acceptable initial bioburden load of aseptically manufactured devices while maintaining required sterility assurance levels, which is often critical in early stage translational settings. In this study, we hypothesized that small increases in radiation dose from 15 to 20 kGy would result in significant changes to several key characteristics of collagen scaffolds. Scaffolds were manufactured by lyophilizing the pepsin digest of dense bovine connective tissue in cylindrical molds and were irradiated at either 0, 15, 17.5, or 20 kGy with an additional group packaged in nitrogen and irradiated at 17.5 kGy. Groups were evaluated for changes to the soluble collagen and glycosaminoglycan mass fractions, protein banding patterns in electrophoresis, a collagen fragmentation assay, and resistance to enzymatic degradation. All parameters were statistically analyzed using one-way analysis of variance with Tukey's correction for multiple comparisons. The soluble collagen mass fraction was significantly decreased in the 20 kGy group; however, there was no significant effect of radiation dose or a nitrogen-rich environment on the other measured parameters, including protein banding patterns, fragmented collagen content, and resistance to enzymatic degradation.Statement of Clinical Significance: Collagen scaffolds have proven useful in clinical applications but can be damaged by standard radiation doses. Low-dose sterilization may be a viable alternative that minimally impacts key properties of these scaffolds.

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.

Biomaterials for In Situ Tissue Regeneration: A Review

This review focuses on a somewhat unexplored strand of regenerative medicine, that is in situ tissue engineering. In this approach manufactured scaffolds are implanted in the injured region for regeneration within the patient. The scaffold is designed to attract cells to the required volume of regeneration to subsequently proliferate, differentiate, and as a consequence develop tissue within the scaffold which in time will degrade leaving just the regenerated tissue. This review highlights the wealth of information available from studies of ex-situ tissue engineering about the selection of materials for scaffolds. It is clear that there are great opportunities for the use of additive manufacturing to prepare complex personalized scaffolds and we speculate that by building on this knowledge and technology, the development of in situ tissue engineering could rapidly increase. Ex-situ tissue engineering is handicapped by the need to develop the tissue in a bioreactor where the conditions, however optimized, may not be optimum for accelerated growth and maintenance of the cell function. We identify that in both methodologies the prospect of tissue regeneration has created much promise but delivered little outside the scope of laboratory-based experiments. We propose that the design of the scaffolds and the materials selected remain at the heart of developments in this field and there is a clear need for predictive modelling which can be used in the design and optimization of materials and scaffolds.

Biomaterials for In Situ Tissue Regeneration: A Review

This review focuses on a somewhat unexplored strand of regenerative medicine, that is in situ tissue engineering. In this approach manufactured scaffolds are implanted in the injured region for regeneration within the patient. The scaffold is designed to attract cells to the required volume of regeneration to subsequently proliferate, differentiate, and as a consequence develop tissue within the scaffold which in time will degrade leaving just the regenerated tissue. This review highlights the wealth of information available from studies of ex-situ tissue engineering about the selection of materials for scaffolds. It is clear that there are great opportunities for the use of additive manufacturing to prepare complex personalized scaffolds and we speculate that by building on this knowledge and technology, the development of in situ tissue engineering could rapidly increase. Ex-situ tissue engineering is handicapped by the need to develop the tissue in a bioreactor where the conditions, however optimized, may not be optimum for accelerated growth and maintenance of the cell function. We identify that in both methodologies the prospect of tissue regeneration has created much promise but delivered little outside the scope of laboratory-based experiments. We propose that the design of the scaffolds and the materials selected remain at the heart of developments in this field and there is a clear need for predictive modelling which can be used in the design and optimization of materials and scaffolds.

In vitro evaluation of functionalized decellularized muscle scaffold for in situ skeletal muscle regeneration

Current treatment options for repairing volumetric muscle loss injury involve the use of existing host tissue like muscular flaps or grafts. However, host muscle tissue may not be available and donor site morbidity, such as functional loss and volume deficiency, is often present. In this study, we developed a biofunctionalized muscle-derived decellularized extracellular matrix scaffolding system to utilize endogenous stem/progenitor cells for in situ muscle tissue regeneration. We optimized the decellularization process to enhance cellular infiltration and fabricated an insulin-like growth factor-binding protein 3 (IGFBP-3)-conjugated scaffold for controlled delivery of IGF-I. We then tested in vitro characterization including IGF-I release kinetics and cellular infiltration. In addition, we have analyzed the bioactivities of skeletal muscle cells (C2C12) to assess the indirect effect of released IGF-1 from the scaffold. The IGFBP-3 conjugated scaffolds demonstrated showed sustained release of IGF-1 and 1% SDS decellularized scaffold with IGF-1 showed higher cellular infiltration compared to control scaffolds (no conjugation). In indirect bioactivity assay, IGF-1 conjugated scaffold showed 2.1-fold increased cell activity compared to control (fresh media). Our results indicate that IGFBP-3/IGF-I conjugated scaffold has the potential to be used for in situ muscle tissue regeneration.

3D Bioprinted Human Skeletal Muscle Constructs for Muscle Function Restoration

A bioengineered skeletal muscle tissue as an alternative for autologous tissue flaps, which mimics the structural and functional characteristics of the native tissue, is needed for reconstructive surgery. Rapid progress in the cell-based tissue engineering principle has enabled in vitro creation of cellularized muscle-like constructs; however, the current fabrication methods are still limited to build a three-dimensional (3D) muscle construct with a highly viable, organized cellular structure with the potential for a future human trial. Here, we applied 3D bioprinting strategy to fabricate an implantable, bioengineered skeletal muscle tissue composed of human primary muscle progenitor cells (hMPCs). The bioprinted skeletal muscle tissue showed a highly organized multi-layered muscle bundle made by viable, densely packed, and aligned myofiber-like structures. Our in vivo study presented that the bioprinted muscle constructs reached 82% of functional recovery in a rodent model of tibialis anterior (TA) muscle defect at 8 weeks of post-implantation. In addition, histological and immunohistological examinations indicated that the bioprinted muscle constructs were well integrated with host vascular and neural networks. We demonstrated the potential of the use of the 3D bioprinted skeletal muscle with a spatially organized structure that can reconstruct the extensive muscle defects.

Biomaterials in Tendon and Skeletal Muscle Tissue Engineering: Current Trends and Challenges

Tissue engineering is a promising approach to repair tendon and muscle when natural healing fails. Biohybrid constructs obtained after cells’ seeding and culture in dedicated scaffolds have indeed been considered as relevant tools for mimicking native tissue, leading to a better integration in vivo. They can also be employed to perform advanced in vitro studies to model the cell differentiation or regeneration processes. In this review, we report and analyze the different solutions proposed in literature, for the reconstruction of tendon, muscle, and the myotendinous junction. They classically rely on the three pillars of tissue engineering, i.e., cells, biomaterials and environment (both chemical and physical stimuli). We have chosen to present biomimetic or bioinspired strategies based on understanding of the native tissue structure/functions/properties of the tissue of interest. For each tissue, we sorted the relevant publications according to an increasing degree of complexity in the materials’ shape or manufacture. We present their biological and mechanical performances, observed in vitro and in vivo when available. Although there is no consensus for a gold standard technique to reconstruct these musculo-skeletal tissues, the reader can find different ways to progress in the field and to understand the recent history in the choice of materials, from collagen to polymer-based matrices.

Extracellular matrix scaffolds for treatment of large volume muscle injuries: A review

OBJECTIVE

Large muscular or musculotendinous defects present a dilemma because of the inadequacies of current treatment strategies. Extracellular matrices (ECM) are potential clinically applicable regenerative biomaterials. This review summarizes information from the preclinical literature evaluating the use of ECM for muscle regeneration in animal models of volumetric muscle loss (VML).

STUDY DESIGN

Literature review.

SAMPLE POPULATION

Animal models of VML in which surgical repair was performed with an ECM product, with or without added cell populations.

METHODS

PubMed, Google Scholar, CAB abstracts, and Scopus were searched for preclinical studies using ECM in animal models of VML. The search terms "extracellular matrix," "VML," "muscle regeneration," "cell seeded," and "scaffold" identified 40 articles that met inclusion criteria of an animal model of VML in which surgical repair was performed with an ECM product, with or without added cell populations. Key skeletal muscle repair mechanisms and experimental findings on scaffold type, VML location, and experimental animal species were summarized.

CONCLUSIONS

Satellite cells and basal lamina are key endogenous contributors to skeletal muscle regeneration. ECM as a dynamic tissue component may provide structural integrity, signaling molecules, and a 3-dimensional topography conducive to muscle regeneration. Preclinical models of muscle repair most commonly used mice and rats (88%). Most experimental lesions were created in abdominal wall (33%), anterior tibialis (33%), latissimus dorsi (10%), or quadriceps (10%) muscles. Matrices varied markedly in source and preparation. Experimental outcomes of ECM and cell-seeded ECM implantation for muscle regeneration in VML were highly variable and dependent on matrix tissue source, preparation method, and anatomic site of injury. Scar tissue formation likely contributes to load transfer. Nonappendicular lesions had better regenerative results compared with appendicular VML.

CLINICAL SIGNIFICANCE

The preponderance of current evidence supports the use of ECM for muscle defect repair only in specific instances, such as nonappendicular and/or partial-thickness defects. Consequently, clinical use of ECM in veterinary patients requires careful consideration of the specific ECM product, lesion size and location, and loading circumstances.

Preferred M2 Polarization by ASC-Based Hydrogel Accelerated Angiogenesis and Myogenesis in Volumetric Muscle Loss Rats

Volumetric muscle loss (VML) injury resulted from massive muscle defects and diseases for which there are still no effective therapeutic treatments. This study aimed to investigate the effects of rat adipose-derived mesenchymal stem cells (rASCs) and rASCs-conditioned medium- (CM-) based type I collagen hydrogel on macrophage (MP) transition, myogenesis, and vascularization in the rat VML model. Laser Doppler results demonstrated much higher blood flow in the rASC- and CM-based hydrogel groups. qRT-PCR, hematoxylin and eosin, immunofluorescence, and Sirius Red staining manifested that both rASCs and CM-based hydrogel implantation accelerated muscle repair with upregulated angiogenesis and myogenesis, attenuated inflammation while facilitating M2 transition, and decreased the collagen deposition compared with the hydrogel group. In vitro experiments indicated that factors secreted from polarized M2 MPs could accelerate the migration and tube formation capacities of HUVECs. These results suggested that rASCs exerted immunomodulatory effects on MPs which further enhanced the proangiogenic potential on ECs to promote myogenesis and angiogenesis during muscle repair. These fundamental results support further clinical applications of ASCs for muscle loss injury.

Achieving Acetylcholine Receptor Clustering in Tissue-Engineered Skeletal Muscle Constructs In vitro through a Materials-Directed Agrin Delivery Approach

Volumetric muscle loss (VML) can result from trauma, infection, congenital anomalies, or surgery, and produce permanent functional and cosmetic deficits. There are no effective treatment options for VML injuries, and recent advances toward development of muscle constructs lack the ability to achieve innervation necessary for long-term function. We sought to develop a proof-of-concept biomaterial construct that could achieve acetylcholine receptor (AChR) clustering on muscle-derived cells (MDCs) in vitro. The approach consisted of the presentation of neural (Z+) agrin from the surface of microspheres embedded with a fibrin hydrogel to muscle cells (C2C12 cell line or primary rat MDCs). AChR clustering was spatially restricted to areas of cell (C2C12)-microsphere contact when the microspheres were delivered in suspension or when they were incorporated into a thin (2D) fibrin hydrogel. AChR clusters were observed from 16 to 72 h after treatment when Z+ agrin was adsorbed to the microspheres, and for greater than 120 h when agrin was covalently coupled to the microspheres. Little to no AChR clustering was observed when agrin-coated microspheres were delivered from specially designed 3D fibrin constructs. However, cyclic stretch in combination with agrin-presenting microspheres led to dramatic enhancement of AChR clustering in cells cultured on these 3D fibrin constructs, suggesting a synergistic effect between mechanical strain and agrin stimulation of AChR clustering in vitro. These studies highlight a strategy for maintaining a physiological phenotype characterized by motor endplates of muscle cells used in tissue engineering strategies for muscle regeneration. As such, these observations may provide an important first step toward improving function of tissue-engineered constructs for treatment of VML injuries.

The Potential of Combination Therapeutics for More Complete Repair of Volumetric Muscle Loss Injuries: The Role of Exogenous Growth Factors and/or Progenitor Cells in Implantable Skeletal Muscle Tissue Engineering Technologies

Despite the robust regenerative capacity of skeletal muscle, there are a variety of congenital and acquired conditions in which the volume of skeletal muscle loss results in major permanent functional and cosmetic deficits. These latter injuries are referred to as volumetric muscle loss (VML) injuries or VML-like conditions, and they are characterized by the simultaneous absence of multiple tissue components (i.e., nerves, vessels, muscles, satellite cells, and matrix). There are currently no effective treatment options. Regenerative medicine/tissue engineering technologies hold great potential for repair of these otherwise irrecoverable VML injuries. In this regard, three-dimensional scaffolds have been used to deliver sustained amounts of growth factors into a variety of injury models, to modulate host cell recruitment and extracellular matrix remodeling. However, this is a nascent field of research, and more complete functional improvements require more precise control of the spatiotemporal distribution of critical growth factors over a physiologically relevant range. This is especially true for VML injuries where incorporation of a cellular component into the scaffolds might provide not only a source of new tissue formation but also additional signals for host cell migration, recruitment, and survival. To this end, we review the major features of muscle repair and regeneration for largely recoverable injuries, and then discuss recent cell- and/or growth factor-based approaches to repair the more profound and irreversible VML and VML-like injuries. The underlying supposition is that more rationale incorporation of exogenous growth factors and/or cellular components will be required to optimize the regenerative capacity of implantable therapeutics for VML repair.

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

Skeletal muscle injuries typically result from traumatic incidents such as combat injuries where soft-tissue extremity injuries are present in one of four cases. Further, about 4.5 million reconstructive surgical procedures are performed annually as a result of car accidents, cancer ablation, or cosmetic procedures. These combat- and trauma-induced skeletal muscle injuries are characterized by volumetric muscle loss (VML), which significantly reduces the functionality of the injured muscle. While skeletal muscle has an innate repair mechanism, it is unable to compensate for VML injuries because large amounts of tissue including connective tissue and basement membrane are removed or destroyed. This results in a significant need to develop off-the-shelf biomimetic scaffolds to direct skeletal muscle regeneration. Here, the structure and organization of native skeletal muscle tissue is described in order to reveal clear design parameters that are necessary for scaffolds to mimic in order to successfully regenerate muscular tissue. We review the literature with respect to the materials and methodologies used to develop scaffolds for skeletal muscle tissue regeneration as well as the limitations of these materials. We further discuss the variety of cell sources and different injury models to provide some context for the multiple approaches used to evaluate these scaffold materials. Recent findings are highlighted to address the state of the field and directions are outlined for future strategies, both in scaffold design and in the use of different injury models to evaluate these materials, for regenerating functional skeletal muscle.

STATEMENT OF SIGNIFICANCE

Volumetric muscle loss (VML) injuries result from traumatic incidents such as those presented from combat missions, where soft-tissue extremity injuries are represented in one of four cases. These injuries remove or destroy large amounts of skeletal muscle including the basement membrane and connective tissue, removing the structural, mechanical, and biochemical cues that usually direct its repair. This results in a significant need to develop off-the-shelf biomimetic scaffolds to direct skeletal muscle regeneration. In this review, we examine current strategies for the development of scaffold materials designed for skeletal muscle regeneration, highlighting advances and limitations associated with these methodologies. Finally, we identify future approaches to enhance skeletal muscle regeneration.

Rapid release of growth factors regenerates force output in volumetric muscle loss injuries

A significant challenge in the design and development of biomaterial scaffolds is to incorporate mechanical and biochemical cues to direct organized tissue growth. In this study, we investigated the effect of hepatocyte growth factor (HGF) loaded, crosslinked fibrin (EDCn-HGF) microthread scaffolds on skeletal muscle regeneration in a mouse model of volumetric muscle loss (VML). The rapid, sustained release of HGF significantly enhanced the force production of muscle tissue 60 days after injury, recovering more than 200% of the force output relative to measurements recorded immediately after injury. HGF delivery increased the number of differentiating myoblasts 14 days after injury, and supported an enhanced angiogenic response. The architectural morphology of microthread scaffolds supported the ingrowth of nascent myofibers into the wound site, in contrast to fibrin gel implants which did not support functional regeneration. Together, these data suggest that EDCn-HGF microthreads recapitulate several of the regenerative cues lost in VML injuries, promote remodeling of functional muscle tissue, and enhance the functional regeneration of skeletal muscle. Further, by strategically incorporating specific biochemical factors and precisely tuning the structural and mechanical properties of fibrin microthreads, we have developed a powerful platform technology that may enhance regeneration in other axially aligned tissues.

Repair of articular osteochondral defects of the knee joint using a composite lamellar scaffold

Objectives

The major problem with repair of an articular cartilage injury is the extensive difference in the structure and function of regenerated, compared with normal cartilage. Our work investigates the feasibility of repairing articular osteochondral defects in the canine knee joint using a composite lamellar scaffold of nano-ß-tricalcium phosphate (ß-TCP)/collagen (col) I and II with bone marrow stromal stem cells (BMSCs) and assesses its biological compatibility.

Methods

The bone–cartilage scaffold was prepared as a laminated composite, using hydroxyapatite nanoparticles (nano-HAP)/collagen I/copolymer of polylactic acid–hydroxyacetic acid as the bony scaffold, and sodium hyaluronate/poly(lactic-co-glycolic acid) as the cartilaginous scaffold. Ten-to 12-month-old hybrid canines were randomly divided into an experimental group and a control group. BMSCs were obtained from the iliac crest of each animal, and only those of the third generation were used in experiments. An articular osteochondral defect was created in the right knee of dogs in both groups. Those in the experimental group were treated by implanting the composites consisting of the lamellar scaffold of ß-TCP/col I/col II/BMSCs. Those in the control group were left untreated.

Results

After 12 weeks of implantation, defects in the experimental group were filled with white semi-translucent tissue, protruding slightly over the peripheral cartilage surface. After 24 weeks, the defect space in the experimental group was filled with new cartilage tissues, finely integrated into surrounding normal cartilage. The lamellar scaffold of ß-TCP/col I/col II was gradually degraded and absorbed, while new cartilage tissue formed. In the control group, the defects were not repaired.

Conclusion

This method can be used as a suitable scaffold material for the tissue-engineered repair of articular cartilage defects.

Cite this article: Bone Joint Res 2015;4:56–64

Biomaterial-based delivery for skeletal muscle repair

Skeletal muscle possesses a remarkable capacity for regeneration in response to minor damage, but severe injury resulting in a volumetric muscle loss can lead to extensive and irreversible fibrosis, scarring, and loss of muscle function. In early clinical trials, the intramuscular injection of cultured myoblasts was proven to be a safe but ineffective cell therapy, likely due to rapid death, poor migration, and immune rejection of the injected cells. In recent years, appropriate therapeutic cell types and culturing techniques have improved progenitor cell engraftment upon transplantation. Importantly, the identification of several key biophysical and biochemical cues that synergistically regulate satellite cell fate has paved the way for the development of cell-instructive biomaterials that serve as delivery vehicles for cells to promote in vivo regeneration. Material carriers designed to spatially and temporally mimic the satellite cell niche may be of particular importance for the complete regeneration of severely damaged skeletal muscle.

In situ regeneration of skeletal muscle tissue through host cell recruitment

Standard reconstructive procedures for restoring normal function after skeletal muscle defects involve the use of existing host tissues such as muscular flaps. In many instances, this approach is not feasible and delays the rehabilitation process and restoration of tissue function. Currently, cell-based tissue engineering strategies have been used for reconstruction; however, donor tissue biopsy and ex vivo cell manipulation are required prior to implantation. The present study aimed to overcome these limitations by demonstrating mobilization of muscle cells into a target-specific site for in situ muscle regeneration. First, we investigated whether host muscle cells could be mobilized into an implanted scaffold. Poly(l-lactic acid) (PLLA) scaffolds were implanted in the tibialis anterior (TA) muscle of rats, and the retrieved scaffolds were characterized by examining host cell infiltration in the scaffolds. The host cell infiltrates, including Pax7+ cells, gradually increased with time. Second, we demonstrated that host muscle cells could be enriched by a myogenic factor released from the scaffolds. Gelatin-based scaffolds containing a myogenic factor were implanted in the TA muscle of rats, and the Pax7+ cell infiltration and newly formed muscle fibers were examined. By the second week after implantation, the Pax7+ cell infiltrates and muscle formation were significantly accelerated within the scaffolds containing insulin-like growth factor 1 (IGF-1). Our data suggest an ability of host stem cells to be recruited into the scaffolds with the capability of differentiating to muscle cells. In addition, the myogenic factor effectively promoted host cell recruitment, which resulted in accelerating muscle regeneration in situ.

In vivo skeletal muscle biocompatibility of composite, coaxial electrospun, and microfibrous scaffolds

One weakness with currently researched skeletal muscle tissue replacement is the lack of contraction and relaxation during the regenerative process. A biocompatible scaffold that can act similar to the muscle would be a pivotal innovation. Coaxial electrospun scaffolds, capable of movement with electrical stimulation, were created using poly(ɛ-caprolactone) (PCL), multiwalled carbon nanotubes (MWCNT), and a (83/17 or 40/60) poly(acrylic acid)/poly(vinyl alcohol) (PAA/PVA) hydrogel. The two scaffolds were implanted into Sprague-Dawley rat vastus lateralis muscle and compared with a phosphate-buffered saline injection sham surgery and an unoperated control. No complications or adverse effects were observed. Rats were sacrificed on days 7, 14, 21, and 28 postimplantation and biocompatibility assessed using enzymatic activity, fibrosis formation, inflammation, scaffold cellular infiltration, and neovascularization. Serum creatine kinase and lactate dehydrogenase levels were significantly higher in scaffold-implanted rats compared with the control on day 7, but returned to baseline by day 14. Day 7 scaffolds showed significant inflammation and fibrosis that decreased over time. Fibroblasts infiltrated the scaffolds early, but decreased with time, while myogenic cell numbers increased. Neovascularization of both scaffolds occurred as early as day 7. We conclude that the PCL-MWCNT-PAA/PVA scaffolds are biocompatible and suitable for muscle regeneration as myogenic cell growth was supported.

In situ tissue regeneration through host stem cell recruitment

The field of tissue engineering has made steady progress in translating various tissue applications. Although the classical tissue engineering strategy, which involves the use of culture-expanded cells and scaffolds to produce a tissue construct for implantation, has been validated, this approach involves extensive cell expansion steps, requiring a lot of time and laborious effort before implantation. To bypass this ex vivo process, a new approach has been introduced. In situ tissue regeneration utilizes the body's own regenerating capacity by mobilizing host endogenous stem cells or tissue-specific progenitor cells to the site of injury. This approach relies on development of a target-specific biomaterial scaffolding system that can effectively control the host microenvironment and mobilize host stem/progenitor cells to target tissues. An appropriate microenvironment provided by implanted scaffolds would facilitate recruitment of host cells that can be guided to regenerating structural and functional tissues.

Muscle-derived decellularised extracellular matrix improves functional recovery in a rat latissimus dorsi muscle defect model

PURPOSE

Craniofacial maxillary injuries represent nearly 30% of all battlefield wounds, often involving volumetric muscle loss (VML). The physical loss of muscle results in functional deficits and cosmetic disfigurement. Although surgical solutions are limited, advances in biomaterials offer great promise for the restoration of form and function following VML. The primary purpose of this study was to determine whether muscle function could be restored in a novel VML rat model using muscle-derived extracellular matrix (M-ECM).

METHODS

Ten percent of the mass of the latissimus dorsi (LD) was excised. Three groups were examined: 1) no repair of defect (DEF), 2) repair with M-ECM and 3) sham (all procedures except muscle excision). Four and 8 weeks post-surgery, the isometric contractile properties of the LD were assessed in situ and selected histological properties were evaluated.

RESULTS

The defect resulted in an initial reduction in peak isometric force (Po) of 30%. At 8 weeks the difference between DEF and sham was 20.5%. At the same time, M-ECM was only 8.4% below sham. Although the histological analysis revealed a narrow, but well-formed band of muscle running along the middle of the M-ECM, it was judged to be too small to account for the observed improvement in muscle force.

CONCLUSIONS

Repair of VML with M-ECM can dramatically improve muscle function independent of muscle regeneration by providing a physical bridge that accommodates force transmission across the injury site. This method of repair may provide an easily translatable surgical method for selected forms of VML.

A standardized rat model of volumetric muscle loss injury for the development of tissue engineering therapies

Soft tissue injuries involving volumetric muscle loss (VML) are defined as the traumatic or surgical loss of skeletal muscle with resultant functional impairment and represent a challenging clinical problem for both military and civilian medicine. In response, a variety of tissue engineering and regenerative medicine treatments are under preclinical development. A wide variety of animal models are being used, all with critical limitations. The objective of this study was to develop a model of VML that was reproducible and technically uncomplicated to provide a standardized platform for the development of tissue engineering and regenerative medicine solutions to VML repair. A rat model of VML involving excision of ∼20% of the muscle's mass from the superficial portion of the middle third of the tibialis anterior (TA) muscle was developed and was functionally characterized. The contralateral TA muscle served as the uninjured control. Additionally, uninjured age-matched control rats were also tested to determine the effect of VML on the contralateral limb. TA muscles were assessed at 2 and 4 months postinjury. VML muscles weighed 22.7% and 19.5% less than contralateral muscles at 2 and 4 months postinjury, respectively. These differences were accompanied by a reduction in peak isometric tetanic force (Po) of 28.4% and 32.5% at 2 and 4 months. Importantly, Po corrected for differences in body weight and muscle wet weights were similar between contralateral and age-matched control muscles, indicating that VML did not have a significant impact on the contralateral limb. Lastly, repair of the injury with a biological scaffold resulted in rapid vascularization and integration with the wound. The technical simplicity, reliability, and clinical relevance of the VML model developed in this study make it ideal as a standard model for the development of tissue engineering solutions for VML.

ADDITION OF CONDUCTIVE ELEMENTS TO POLYMERIC SCAFFOLDS FOR MUSCLE TISSUE ENGINEERING

Cardiac and skeletal muscles are two tissues that would benefit from an electrically conductive scaffold to regenerate lost or lower functioning areas. By augmenting polymeric scaffolds with conductive elements, the contractile process for both muscles could increase. In this review, the components reviewed include polyaniline (PANi), gold (Au) nanoparticles, and carbon nanotubes (CNT). PANi has been combined with several polymers and increased the conductivity of the scaffolds. It is biocompatible, but increases mechanical properties and decreases scaffold elongation. Tissue engineering using nanoparticles is an emerging area and considerable research focuses on determining possible toxicity due to nanoparticle concentration. Contradicting data exists for both Au nanoparticles and CNT. Smaller Au nanoparticles damage cardiac tissue in vivo while larger ones do not. By comparison, in vitro data shows no harmful results for skeletal muscle cells. Data for CNT is just as diverse as the amount, orientation and further purification or functionalization could all play a role in determining biocompatibility. Future research should focus on establishing the conductivity level needed for each muscle tissue to ascertain the amount of conductive element needed so the most suitable one can be utilized.

Matrigel, but not collagen I, maintains the differentiation capacity of muscle derived cells in vitro

Satellite cells are key cells for post-natal muscle growth and regeneration and they play a central role in the search for therapies to treat muscle injuries. In this study the proliferation and differentiation capacity of muscle progenitor cells was studied in 2D and 3D cultures with collagen type I and Matrigel, which contain the niche factors laminin and collagen type IV. Muscle progenitor cells were cultured to induce proliferation and differentiation in collagen- or Matrigel-coated surfaces (2D) or in gels (3D). In the 2D cultures, muscle progenitor cells proliferated faster in Matrigel than in collagen. The numbers of Pax7(+) and MyoD(+) cells were also significantly higher in Matrigel than in collagen. During differentiation, muscle progenitor cells formed more and larger MyoD(+) and myogenin(+) myotubes in Matrigel. In the 3D cultures, muscle progenitor cells in Matrigel expressed higher mRNA levels of MyoD and myogenin, and formed elongated myotubes expressing myogenin and myosin. In collagen gels, the myotubes were short and rounded. In conclusion, muscle progenitor cells, both in 2D and 3D, lose their differentiation capacity in collagen but not in Matrigel. Although Matrigel contains growth factors, our results indicate that the kind of biomaterial steers the maintenance of the myogenic potential and their proper differentiation to achieve optimal skeletal muscle restoration.

Further development of a tissue engineered muscle repair construct in vitro for enhanced functional recovery following implantation in vivo in a murine model of volumetric muscle loss injury

Volumetric muscle loss (VML) can result from trauma and surgery in civilian and military populations, resulting in irrecoverable functional and cosmetic deficits that cannot be effectively treated with current therapies. Previous work evaluated a bioreactor-based tissue engineering approach in which muscle derived cells (MDCs) were seeded onto bladder acellular matrices (BAM) and mechanically preconditioned. This first generation tissue engineered muscle repair (TEMR) construct exhibited a largely differentiated cellular morphology consisting primarily of myotubes, and moreover, significantly improved functional recovery within 2 months of implantation in a murine latissimus dorsi (LD) muscle with a surgically created VML injury. The present report extends these initial observations to further document the importance of the cellular phenotype and composition of the TEMR construct in vitro to the functional recovery observed following implantation in vivo. To this end, three distinct TEMR constructs were created by seeding MDCs onto BAM as follows: (1) a short-term cellular proliferation of MDCs to generate primarily myoblasts without bioreactor preconditioning (TEMR-1SP), (2) a prolonged cellular differentiation and maturation period that included bioreactor preconditioning (TEMR-1SPD; identical to the first generation TEMR construct), and (3) similar treatment as TEMR-1SPD but with a second application of MDCs during bioreactor preconditioning (TEMR-2SPD); simulating aspects of "exercise" in vitro. Assessment of maximal tetanic force generation on retrieved LD muscles in vitro revealed that TEMR-1SP and TEMR-1SPD constructs promoted either an accelerated (i.e., 1 month) or a prolonged (i.e., 2 month postinjury) functional recovery, respectively, of similar magnitude. Meanwhile, TEMR-2SPD constructs promoted both an accelerated and prolonged functional recovery, resulting in twice the magnitude of functional recovery of either TEMR-1SP or TEMR-1SPD constructs. Histological and molecular analyses indicated that TEMR constructs mediated functional recovery via regeneration of functional muscle fibers either at the interface of the construct and the native tissue or within the BAM scaffolding independent of the native tissue. Taken together these findings are encouraging for the further development and clinical application of TEMR constructs as a VML injury treatment.

A tissue-engineered muscle repair construct for functional restoration of an irrecoverable muscle injury in a murine model

There are no effective clinical treatments for volumetric muscle loss (VML) resulting from traumatic injury, tumor excision, or other degenerative diseases of skeletal muscle. The goal of this study was to develop and characterize a more clinically relevant tissue-engineered muscle repair (TE-MR) construct for functional restoration of a VML injury in the mouse lattissimus dorsi (LD) muscle. To this end, TE-MR constructs developed by seeding rat myoblasts on porcine bladder acellular matrix were preconditioned in a bioreactor for 1 week and implanted in nude mice at the site of a VML injury created by excising 50% of the native LD. Two months postinjury and implantation of TE-MR, maximal tetanic force was ∼72% of that observed in native LD muscle. In contrast, injured LD muscles that were not repaired, or were repaired with scaffold alone, produced only ∼50% of native LD muscle force after 2 months. Histological analyses of LD tissue retrieved 2 months after implantation demonstrated remodeling of the TE-MR construct as well as the presence of desmin-positive myofibers, blood vessels, and neurovascular bundles within the TE-MR construct. Overall, these encouraging initial observations document significant functional recovery within 2 months of implantation of TE-MR constructs and provide clear proof of concept for the applicability of this technology in a murine VML injury model.

Model for muscle regeneration around fibrotic lesions in recurrent strain injuries

PURPOSE

The purpose of this study was to establish an in vivo model for muscle regeneration after strain injury in the presence of a fibrotic discontinuity.

METHODS

The musculus soleus of 5-wk-old male rats was exposed, completely lacerated, and sutured together with or without a collagen scaffold in between the muscle ends. The scaffold represents a fibrotic discontinuity in the muscle. Muscle healing was evaluated after 14 d by general histology and staining for myofibroblasts, satellite cells (activated), and inflammatory cells.

RESULTS

Around all wounds, satellite cells were activated. Inside the collagen scaffolds, satellite cells were absent, indicating that muscle regeneration was impaired. In the wounds without a collagen scaffold, the lacerated and the sutured myofibers contacted and had already started to regenerate, whereas this did not occur with an implanted scaffold.

CONCLUSIONS

A fibrotic discontinuity, such as an implanted collagen scaffold, delays muscle regeneration in skeletal muscle. This model is suitable to study skeletal muscle regeneration in the presence of a fibrotic lesion and to evaluate new treatment modalities for muscle strain injuries.

Repair of traumatic skeletal muscle injury with bone-marrow-derived mesenchymal stem cells seeded on extracellular matrix

Skeletal muscle injury resulting in tissue loss poses unique challenges for surgical repair. Despite the regenerative potential of skeletal muscle, if a significant amount of tissue is lost, skeletal myofibers will not grow to fill the injured area completely. Prior work in our lab has shown the potential to fill the void with an extracellular matrix (ECM) scaffold, resulting in restoration of morphology, but not functional recovery. To improve the functional outcome of the injured muscle, a muscle-derived ECM was implanted into a 1 x 1 cm(2), full-thickness defect in the lateral gastrocnemius (LGAS) of Lewis rats. Seven days later, bone-marrow-derived mesenchymal stem cells (MSCs) were injected directly into the implanted ECM. Partial functional recovery occurred over the course of 42 days when the LGAS was repaired with an MSC-seeded ECM producing 85.4 +/- 3.6% of the contralateral LGAS. This was significantly higher than earlier recovery time points (p < 0.05). The specific tension returned to 94 +/- 9% of the contralateral limb. The implanted MSC-seeded ECM had more blood vessels and regenerating skeletal myofibers than the ECM without cells (p < 0.05). The data suggest that the repair of a skeletal muscle defect injury by the implantation of a muscle-derived ECM seeded with MSCs can improve functional recovery after 42 days.

Clinical application of an acellular biologic scaffold for surgical repair of a large, traumatic quadriceps femoris muscle defect

Many battlefield injuries involve penetrating soft tissue trauma often accompanied by skeletal muscle defects, known as volumetric muscle loss. This article presents the first known case of a surgical technique involving an innovative tissue engineering approach for the repair of a large volumetric muscle loss. A 19-year-old Marine presented with large volumentric muscle loss of the right thigh as a result of an explosion. The patient reported muscle weakness with right knee extension, secondary to volumentric muscle loss, primarily involving the vastus medialis muscle. This persisted 3 years postinjury, despite extensive physical therapy. With all existing management options exhausted, restoration of a portion of the lost vastus medialis muscle was attempted by surgical implantation of a multi-layered scaffold composed of extracellular matrix derived from porcine intestinal submucossa. The patient had no complications, was discharged home on postoperative day 5, and resumed physical therapy after 4 weeks. Four months postoperatively, the patient demonstrated marked gains in isokinetic performance. Computer tomography indicated new tissue at the implant site. This approach offers a treatment option to a heretofore untreatable injury and will allow us to improve future surgical treatments for volumetric muscle loss.

Functional assessment of skeletal muscle regeneration utilizing homologous extracellular matrix as scaffolding

The loss of a portion of skeletal muscle poses a unique challenge for the normal regeneration of muscle tissue. A transection injury with tissue loss will not heal due to the gap between muscle segments. A damage model was developed by removing a portion of the lateral gastrocnemius (GAS) of Sprague-Dawley rats. Maximal isometric, tetanic tension (P(o)) was measured after the removal of either a small defect (0.5 x 1.0 cm) or a large defect (1.0 x 1.0 cm) piece of the GAS. In situ P(o) immediately after creation of the defect was 88.3 +/- 2.0% of the nonoperated contralateral GAS force for small defect and 76.9 +/- 3.2% of control for large defect. No functional recovery occurred in either group over the course of 28 days. To enhance recovery, a homologous, decellularized, muscle extracellular matrix (ECM) was implanted into the 1 x 1 cm defect of the lateral GAS of Lewis rats. After 42 days, growth of blood vessels and myofibers into the ECM was apparent, but no restoration of P(o) occurred. These data demonstrate the ability of the ECM to support muscle and blood vessel regeneration, but full recovery of function does not occur after 42 days.

A transitional extracellular matrix instructs cell behavior during muscle regeneration

Urodele amphibians regenerate appendages through the recruitment of progenitor cells into a blastema that rebuilds the lost tissue. Blastemal formation is accompanied by extensive remodeling of the extracellular matrix. Although this remodeling process is important for appendage regeneration, it is not known whether the remodeled matrix directly influences the generation and behavior of blastemal progenitor cells. By integrating in vivo 3-dimensional spatiotemporal matrix maps with in vitro functional time-lapse imaging, we show that key components of this dynamic matrix, hyaluronic acid, tenascin-C and fibronectin, differentially direct cellular behaviors including DNA synthesis, migration, myotube fragmentation and myoblast fusion. These data indicate that both satellite cells and fragmenting myofibers contribute to the regeneration blastema and that the local extracellular environment provides instructive cues for the regenerative process. The fact that amphibian and mammalian myoblasts exhibit similar responses to various matrices suggests that the ability to sense and respond to regenerative signals is evolutionarily conserved.