Skeletal muscle tissue engineering: which cell to use?

Impact of Passaging Primary Skeletal Muscle Cell Isolates on the Engineering of Skeletal Muscle

Volumetric muscle loss (VML) is a clinical state that results in impaired skeletal muscle function. Engineered skeletal muscle can serve as a treatment for VML. Currently, large biopsies are required to achieve the cells necessary for the fabrication of engineered muscle, leading to donor-site morbidity. Amplification of cell numbers using cell passaging may increase the usefulness of a single muscle biopsy for engineering muscle tissue. In this study, we evaluated the impact of passaging cells obtained from donor muscle tissue by analyzing characteristics of in vitro cellular growth and tissue-engineered skeletal muscle unit (SMU) structure and function. Human skeletal muscle cell isolates from three separate donors (P0-Control) were compared with cells passaged once (P1), twice (P2), or three times (P3) by monitoring SMU force production and determining muscle content and structure using immunohistochemistry. Data indicated that passaging decreased the number of satellite cells and increased the population doubling time. P1 SMUs had slightly greater contractile force and P2 SMUs showed statistically significant greater force production compared with P0 SMUs with no change in SMU muscle content. In conclusion, human skeletal muscle cells can be passaged twice without negatively impacting SMU muscle content or contractile function, providing the opportunity to potentially create larger SMUs from smaller biopsies, thereby producing clinically relevant sized grafts to aid in VML repair.

Therapeutic applications of biological macromolecules and scaffolds for skeletal muscle regeneration: A review

Skeletal muscle (SM) mass and strength maintenance are important requirements for human well-being. SM regeneration to repair minor injuries depends upon the myogenic activities of muscle satellite (stem) cells. However, losses of regenerative properties following volumetric muscle loss or severe trauma or due to congenital muscular abnormalities are not self-restorable, and thus, these conditions have major healthcare implications and pose clinical challenges. In this context, tissue engineering based on different types of biomaterials and scaffolds provides an encouraging means of structural and functional SM reconstruction. In particular, biomimetic (able to transmit biological signals) and several porous scaffolds are rapidly evolving. Several biological macromolecules/biomaterials (collagen, gelatin, alginate, chitosan, and fibrin etc.) are being widely used for SM regeneration. However, available alternatives for SM regeneration must be redesigned to make them more user-friendly and economically feasible with longer shelf lives. This review aimed to explore the biological aspects of SM regeneration and the roles played by several biological macromolecules and scaffolds in SM regeneration in cases of volumetric muscle loss.

Current situation and publication trends of skeletal muscle related research: A bibliometric analysis

Skeletal muscle (SM) is a highly plastic and dynamic tissue of the body and is largely responsible for body maintenance. SM is primarily responsible for body balance, movement, postural support, thermogenesis, and blood glucose homeostasis. SM regeneration depends on the activation of muscle satellite (stem) cells (MSCs) under the regulation of several muscle regulatory factors that regulate myogenesis. Bibliometric analysis involves the quantitative and qualitative assessments of research and scientific progress that provides researchers access to recent publications, research directions, and thus generates ideas that can be implemented to guide future research. In this analysis, the Web of Science database was searched for articles using the search term "skeletal muscle AND myogenesis AND muscle satellite cell", and 1777 articles (original research/review articles) published from the year 1997 to June 2023 were retrieved. After applying several other exclusion and inclusion criteria, 129 articles were considered for analysis. Types of research, keywords, journals, authors, years, institutions, funding agencies, and average annual citations were analyzed. Muscle regeneration, satellite cell, and myogenesis were often used keywords and exhibited increasing trends in research articles over the decades. Some journals were found to strongly support research publications with high impact factors and citation scores. This study aimed to examine research ideas and growth in the skeletal muscle related field for atrophy and aging improvement.

Impact of Human Recombinant Irisin on Tissue-Engineered Skeletal Muscle Structure and Function

Tissue engineering of exogenous skeletal muscle units (SMUs) through isolation of muscle satellite cells from muscle biopsies is a potential treatment method for acute volumetric muscle loss (VML). A current issue with this treatment process is the limited capacity for muscle stem cell (satellite cell) expansion in cell culture, resulting in a decreased ability to obtain enough cells to fabricate SMUs of appropriate size and structural quality and that produce native levels of contractile force. This study determined the impact of human recombinant irisin on the growth and development of three-dimensional (3D) engineered skeletal muscle. Muscle satellite cells were cultured without irisin (control) or with 50, 100, or 250 ng/mL of irisin supplementation. Light microscopy was used to analyze myotube formation with particular focus placed on the diameter and density of the monotubes during growth of the 3D SMU. Following the formation of 3D constructs, SMUs underwent measurement of maximum tetanic force to analyze contractile function, as well as immunohistochemical staining, to characterize muscle structure. The results indicate that irisin supplementation with 250 ng/mL significantly increased the average diameter of myotubes and increased the proliferation and differentiation of myoblasts in culture but did not have a consistent significant impact on force production. In conclusion, supplementation with 250 ng/mL of human recombinant irisin promotes the proliferation and differentiation of myotubes and has the potential for impacting contractile force production in scaffold-free tissue-engineered skeletal muscle.

W. Lo E, Lim H, Dias S, Wagner KR, Mao HQ, Cahan P, Lee G, Grayson WL. Engineering Skeletal Muscle Grafts with PAX7::GFP-Sorted Human Pluripotent Stem Cell-Derived Myogenic Progenitors on Fibrin Microfiber Bundles for Tissue Regeneration

Tissue engineering strategies that combine human pluripotent stem cell-derived myogenic progenitors (hPDMs) with advanced biomaterials provide promising tools for engineering 3D skeletal muscle grafts to model tissue development in vitro and promote muscle regeneration in vivo. We recently demonstrated (i) the potential for obtaining large numbers of hPDMs using a combination of two small molecules without the overexpression of transgenes and (ii) the application of electrospun fibrin microfiber bundles for functional skeletal muscle restoration following volumetric muscle loss. In this study, we aimed to demonstrate that the biophysical cues provided by the fibrin microfiber bundles induce hPDMs to form engineered human skeletal muscle grafts containing multinucleated myotubes that express desmin and myosin heavy chains and that these grafts could promote regeneration following skeletal muscle injuries. We tested a genetic PAX7 reporter line (PAX7::GFP) to sort for more homogenous populations of hPDMs. RNA sequencing and gene set enrichment analyses confirmed that PAX7::GFP-sorted hPDMs exhibited high expression of myogenic genes. We tested engineered human skeletal muscle grafts derived from PAX7::GFP-sorted hPDMs within in vivo skeletal muscle defects by assessing myogenesis, engraftment and immunogenicity using immunohistochemical staining. The PAX7::GFP-sorted groups had moderately high vascular infiltration and more implanted cell association with embryonic myosin heavy chain (eMHC) regions, suggesting they induced pro-regenerative microenvironments. These findings demonstrated the promise for the use of PAX7::GFP-sorted hPDMs on fibrin microfiber bundles and provided some insights for improving the cell-biomaterial system to stimulate more robust in vivo skeletal muscle regeneration.

Evaluation of Biomechanical and Chemical Properties of Gamma-Irradiated Polycaprolactone Microfilaments for Musculoskeletal Tissue Engineering Applications

An appropriate and reliable sterilization technique is crucial for tissue engineering scaffolds. Skeletal muscle scaffolds are often fabricated using microfilaments of a wide variety of polymers. One method for sterilization is 25 kGy of gamma irradiation. In addition, sterilization through irradiation should administer a dose within a specific range. Radiation directly affects the chemical and mechanical properties of scaffolds. The accuracy and effects of irradiation are often not considered during sterilization procedures; however, these are important since they provide insight on whether the sterilization procedure is reliable and reproducible. This study focused on the chemical and mechanical characterization of 25 kGy gamma-irradiated scaffold. The accuracy and uncertainty of the irradiation procedure were also obtained. X-ray diffraction (XRD) and differential scanning calorimetry (DSC) analyses were performed to determine whether the crystallinity of the polymer changed after irradiation and whether gamma rays influenced its thermal properties. The tensile parameters of the microfilaments were analyzed by comparing irradiated and nonirradiated scaffolds to determine whether gamma radiation changed their elastic behavior. Dose distribution and uncertainty were recorded with several dosimeters. The results showed that the irradiation process slightly affected the mechanical parameters of the scaffold; however, it did not modify its crystallinity or thermal properties. The irradiation was uniform, since the measured uncertainty was low. The scaffold was pathogen-free after 7 days; this meant sterilization was achieved. These results indicated that gamma-sterilized scaffolds were a promising material for use as a skeletal muscle analog material for tissue-engineering applications because they can be sterilized with gamma rays without changing their chemical structure and mechanical properties. This study provided the dose distribution measurement and uncertainty calculations for the sterilization procedure.

Impact of Cell-Seeding Density and Cell Confluence on Human Tissue Engineered Skeletal Muscle

Tissue engineering methodologies have the potential to treat volumetric muscle loss (VML) via the growth of exogenous skeletal muscle grafts from small autogenous muscle biopsies. A significant obstacle preventing the widespread use of engineered skeletal muscle grafts in a clinical setting is the high number of skeletal muscle stem cells, known as satellite cells, required for fabrication of human-sized skeletal muscle tissue. Additionally, there is a lack of work adapting engineered constructs created for animal models into skeletal muscle engineered from a primary human skeletal muscle cell source. For this study, we used scaffold-free tissue-engineered skeletal muscle units (SMUs) to determine the impact of cell seeding density on the ability to fabricate functional human engineered skeletal muscle. Following established protocols, human skeletal muscle isolates were cultured into SMUs at five different cell-seeding densities: 1,000 cells/cm2, 2,500 cells/cm², 5,000 cells/cm², 10,000 cells/cm², and 25,000 cells/cm². Following previous human SMU work, SMUs prepared at a cell seeding density of 10,000 cells/cm² served as controls. Additionally, the impact of cell monolayer confluency on the outcome of human cell-sourced SMU fabrication was investigated at both the 1,000 cells/cm² and 10,000 cells/cm² seeding densities. Light microscopy was used to examine myotube formation and hypertrophy in cell monolayers. After the formation of three-dimensional constructs, SMUs underwent maximum tetanic isometric force production measurements and immunohistochemical staining to examine SMU contractile function and muscle-like structure, respectively. Results indicate that the 25,000 cells/cm² cell-seeding density was detrimental to the contractile function of human cell-sourced SMUs, which had significantly lower maximum tetanic forces compared to SMUs seeded at lower densities. Compared to control, low cell-seeding densities (1,000 cells/cm² - 5,000 cells/cm²) have no detrimental impact on SMU skeletal muscle growth, maturation, or contractility. Cell cultures seeded at 1,000 cells/cm² and allowed to proliferate to 90-100% confluency prior to treatment in muscle differentiation media (MDM) resulted in SMUs with greater contractile forces and total muscle-structure compared to cell cultures switched to MDM when underconfluent or overconfluent. In conclusion, initial cell-seeding density for SMU fabrication can be decreased to as low as 1,000 cells/cm² without negatively impacting SMU muscle-like structure and function.

Replace and repair: Biomimetic bioprinting for effective muscle engineering

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

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

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

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.

Recent progress in extrusion 3D bioprinting of hydrogel biomaterials for tissue regeneration: a comprehensive review with focus on advanced fabrication techniques

Over the last decade, 3D bioprinting has received immense attention from research communities to bridge the divergence between artificially engineered tissue constructs and native tissues.

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

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

Skeletal muscle cell transplantation: models and methods

Xenografts of skeletal muscle are used to study muscle repair and regeneration, mechanisms of muscular dystrophies, and potential cell therapies for musculoskeletal disorders. Typically, xenografting involves using an immunodeficient host that is pre-injured to create a niche for human cell engraftment. Cell type and method of delivery to muscle depend on the specific application, but can include myoblasts, satellite cells, induced pluripotent stem cells, mesangioblasts, immortalized muscle precursor cells, and other multipotent cell lines delivered locally or systemically. Some studies follow cell engraftment with interventions to enhance cell proliferation, migration, and differentiation into mature muscle fibers. Recently, several advances in xenografting human-derived muscle cells have been applied to study and treat Duchenne muscular dystrophy and Facioscapulohumeral muscular dystrophy. Here, we review the vast array of techniques available to aid researchers in designing future experiments aimed at creating robust muscle xenografts in rodent hosts.

Cell therapy to improve regeneration of skeletal muscle injuries

Diseases that jeopardize the musculoskeletal system and cause chronic impairment are prevalent throughout the Western world. In Germany alone, ~1.8 million patients suffer from these diseases annually, and medical expenses have been reported to reach 34.2bn Euros. Although musculoskeletal disorders are seldom fatal, they compromise quality of life and diminish functional capacity. For example, musculoskeletal disorders incur an annual loss of over 0.8 million workforce years to the German economy. Among these diseases, traumatic skeletal muscle injuries are especially problematic because they can occur owing to a variety of causes and are very challenging to treat. In contrast to chronic muscle diseases such as dystrophy, sarcopenia, or cachexia, traumatic muscle injuries inflict damage to localized muscle groups. Although minor muscle trauma heals without severe consequences, no reliable clinical strategy exists to prevent excessive fibrosis or fatty degeneration, both of which occur after severe traumatic injury and contribute to muscle degeneration and dysfunction. Of the many proposed strategies, cell-based approaches have shown the most promising results in numerous pre-clinical studies and have demonstrated success in the handful of clinical trials performed so far. A number of myogenic and non-myogenic cell types benefit muscle healing, either by directly participating in new tissue formation or by stimulating the endogenous processes of muscle repair. These cell types operate via distinct modes of action, and they demonstrate varying levels of feasibility for muscle regeneration depending, to an extent, on the muscle injury model used. While in some models the injury naturally resolves over time, other models have been developed to recapitulate the peculiarities of real-life injuries and therefore mimic the structural and functional impairment observed in humans. Existing limitations of cell therapy approaches include issues related to autologous harvesting, expansion and sorting protocols, optimal dosage, and viability after transplantation. Several clinical trials have been performed to treat skeletal muscle injuries using myogenic progenitor cells or multipotent stromal cells, with promising outcomes. Recent improvements in our understanding of cell behaviour and the mechanistic basis for their modes of action have led to a new paradigm in cell therapies where physical, chemical, and signalling cues presented through biomaterials can instruct cells and enhance their regenerative capacity. Altogether, these studies and experiences provide a positive outlook on future opportunities towards innovative cell-based solutions for treating traumatic muscle injuries-a so far unmet clinical need.

Biomaterial and stem cell-based strategies for skeletal muscle regeneration

Adult skeletal muscle can regenerate effectively after mild physical or chemical insult. Muscle trauma or disease can overwhelm this innate capacity for regeneration and result in heightened inflammation and fibrotic tissue deposition resulting in loss of structure and function. Recent studies have focused on biomaterial and stem cell-based therapies to promote skeletal muscle regeneration following injury and disease. Many stem cell populations besides satellite cells are implicated in muscle regeneration. These stem cells include but are not limited to mesenchymal stem cells, adipose-derived stem cells, hematopoietic stem cells, pericytes, fibroadipogenic progenitors, side population cells, and CD133⁺ stem cells. However, several challenges associated with their isolation, availability, delivery, survival, engraftment, and differentiation have been reported in recent studies. While acellular scaffolds offer a relatively safe and potentially off-the-shelf solution to cell-based therapies, they are often unable to stimulate host cell migration and activity to a level that would result in clinically meaningful regeneration of traumatized muscle. Combining stem cells and biomaterials may offer a viable therapeutic strategy that may overcome the limitations associated with these therapies when they are used in isolation. In this article, we review the stem cell populations that can stimulate muscle regeneration in vitro and in vivo. We also discuss the regenerative potential of combination therapies that utilize both stem cell and biomaterials for the treatment of skeletal muscle injury and disease. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1246-1262, 2019.

iPSCs: A powerful tool for skeletal muscle tissue engineering

Both volumetric muscle loss (VML) and muscle degenerative diseases lead to an important decrease in skeletal muscle mass, condition that nowadays lacks an optimal treatment. This issue has driven towards an increasing interest in new strategies in tissue engineering, an emerging field that can offer very promising approaches. In addition, the discovery of induced pluripotent stem cells (iPSCs) has completely revolutionized the actual view of personalized medicine, and their utilization in skeletal muscle tissue engineering could, undoubtedly, add myriad benefits. In this review, we want to provide a general vision of the basic aspects to consider when engineering skeletal muscle tissue using iPSCs. Specifically, we will focus on the three main pillars of tissue engineering: the scaffold designing, the selection of the ideal cell source and the addition of factors that can enhance the resemblance with the native tissue.

Microfluidic devices for disease modeling in muscle tissue

Microfluidic devices have advanced significantly in recent years and are a promising technology for the field of tissue engineering. Highly sophisticated microfabrication techniques have paved the way for the development of complex ex vivo models capable of incorporating and measuring the real-time response of multiple cell types interacting together in a single system. Muscle-on-a-chip technology has drastically improved and serves as a drug screening platform for many muscular diseases such as muscular dystrophy, tendinosis, fibromyalgia, mitochondrial myopathy, and myasthenia gravis. This review seeks to communicate the gaps in knowledge of current muscular disease models and highlight the power of microfluidic devices in enabling researchers to better understand disease pathology and provide high throughput screening of therapeutics for muscular myopathies.

The Importance of Biophysical and Biochemical Stimuli in Dynamic Skeletal Muscle Models

Classical approaches to engineer skeletal muscle tissue based on current regenerative and surgical procedures still do not meet the desired outcome for patient applications. Besides the evident need to create functional skeletal muscle tissue for the repair of volumetric muscle defects, there is also growing demand for platforms to study muscle-related diseases, such as muscular dystrophies or sarcopenia. Currently, numerous studies exist that have employed a variety of biomaterials, cell types and strategies for maturation of skeletal muscle tissue in 2D and 3D environments. However, researchers are just at the beginning of understanding the impact of different culture settings and their biochemical (growth factors and chemical changes) and biophysical cues (mechanical properties) on myogenesis. With this review we intend to emphasize the need for new in vitro skeletal muscle (disease) models to better recapitulate important structural and functional aspects of muscle development. We highlight the importance of choosing appropriate system components, e.g., cell and biomaterial type, structural and mechanical matrix properties or culture format, and how understanding their interplay will enable researchers to create optimized platforms to investigate myogenesis in healthy and diseased tissue. Thus, we aim to deliver guidelines for experimental designs to allow estimation of the potential influence of the selected skeletal muscle tissue engineering setup on the myogenic outcome prior to their implementation. Moreover, we offer a workflow to facilitate identifying and selecting different analytical tools to demonstrate the successful creation of functional skeletal muscle tissue. Ultimately, a refinement of existing strategies will lead to further progression in understanding important aspects of muscle diseases, muscle aging and muscle regeneration to improve quality of life of patients and enable the establishment of new treatment options.

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.

Label-Free, High-Throughput Purification of Satellite Cells Using Microfluidic Inertial Separation

Skeletal muscle satellite cells have tremendous therapeutic potential in cell therapy or skeletal muscle tissue engineering. Obtaining a sufficiently pure satellite cell population, however, presents a significant challenge. We hypothesized that size differences between satellite cells and fibroblasts, two primary cell types obtained from skeletal muscle dissociation, would allow for label-free, inertial separation in a microfluidic device, termed a "Labyrinth," and that these purified satellite cells could be used to engineer skeletal muscle. Throughout tissue fabrication, Labyrinth-purified cells were compared with unsorted controls to assess the efficiency of this novel sorting process and to examine potential improvements in myogenic proliferation, differentiation, and tissue function. Immediately after dissociation and Labyrinth sorting, cells were immunostained to identify myogenic cells and fibroblast progenitors. Remaining cells were cultured for 14 days to form a confluent monolayer that was induced to delaminate and was captured as a 3D skeletal muscle construct. During monolayer development, myogenic proliferation (BrdU assay on Day 4), differentiation and myotube fusion index (α-actinin on Day 11), and myotube structural development (light microscopy on Day 14) were assessed. Isometric tetanic force production was measured in 3D constructs on Day 16. Immediately following sorting, unsorted cells exhibited a myogenic purity of 39.9% ± 3.99%, and this purity was enriched approximately two-fold to 75.5% ± 1.59% by microfluidic separation. The BrdU assay on Day 4 similarly showed significantly enhanced myogenic proliferation: in unsorted controls 47.0% ± 2.77% of proliferating cells were myogenic, in comparison to 61.7% ± 2.55% following purification. Myogenic differentiation and fusion, assessed by fusion index quantification, showed improvement from 82.7% ± 3.74% in control to 92.3% ± 2.04% in the purified cell population. Myotube density in unsorted controls, 18.6 ± 3.26 myotubes/mm², was significantly enriched in the purified cell population to 33.9 ± 3.74 myotubes/mm². Constructs fabricated from Labyrinth-purified cells also produced significantly greater tetanic forces (143.6 ± 16.9 μN) than unsorted controls (70.7 ± 8.03 μN). These results demonstrate the promise of microfluidic sorting in purifying isolated satellite cells. This unique technology could assist researchers in translating the regenerative potential of satellite cells to cell therapies and engineered tissues.

Mussel Inspired Polynorepinephrine Functionalized Electrospun Polycaprolactone Microfibers for Muscle Regeneration

Electrospun scaffolds with excellent mechanical properties, high specific surface area and a commendable porous network are widely used in tissue engineering. Improving the hydrophilicity and cell adhesion of hydrophobic substrates is the key point to enhance the effectiveness of electrospun scaffolds. In this study, polycaprolactone (PCL) fibrous membranes with appropriate diameter were selected and coated by mussel-inspired poly norepinephrine (pNE). And norepinephrine is a catecholamine functioning as a hormone and neurotransmitter in the human brain. The membrane with smaller diameter fibers, a relative larger specific surface area and the suitable pNE functionalization provided more suitable microenvironment for cell adhesion and proliferation both in vitro and in vivo. The regenerated muscle layer can be integrated well with fibrous membranes and surrounding tissues at the impaired site and thus the mechanical strength reached the value of native tissue. The underlying molecular mechanism is mediated via inhibiting myostatin expression by PI3K/AKT/mTOR hypertrophy pathway. The properly functionalized fibrous membranes hold the potential for repairing muscle injuries. Our current work also provides an insight for rational design and development of better tissue engineering materials for skeletal muscle regeneration.

Biomaterials for skeletal muscle tissue engineering

Although skeletal muscle can naturally regenerate in response to minor injuries, more severe damage and myopathies can cause irreversible loss of muscle mass and function. Cell therapies, while promising, have not yet demonstrated consistent benefit, likely due to poor survival of delivered cells. Biomaterials can improve muscle regeneration by presenting chemical and physical cues to muscle cells that mimic the natural cascade of regeneration. This brief review describes strategies for muscle repair utilizing biomaterials that can provide signals to either transplanted or host muscle cells. These strategies range from approaches that utilize biomaterials alone to those that combine biomaterials with exogenous growth factors, ex vivo cultured cells, and extensive culture time.

Emerging Biofabrication Strategies for Engineering Complex Tissue Constructs

The demand for organ transplantation and repair, coupled with a shortage of available donors, poses an urgent clinical need for the development of innovative treatment strategies for long-term repair and regeneration of injured or diseased tissues and organs. Bioengineering organs, by growing patient-derived cells in biomaterial scaffolds in the presence of pertinent physicochemical signals, provides a promising solution to meet this demand. However, recapitulating the structural and cytoarchitectural complexities of native tissues in vitro remains a significant challenge to be addressed. Through tremendous efforts over the past decade, several innovative biofabrication strategies have been developed to overcome these challenges. This review highlights recent work on emerging three-dimensional bioprinting and textile techniques, compares the advantages and shortcomings of these approaches, outlines the use of common biomaterials and advanced hybrid scaffolds, and describes several design considerations including the structural, physical, biological, and economical parameters that are crucial for the fabrication of functional, complex, engineered tissues. Finally, the applications of these biofabrication strategies in neural, skin, connective, and muscle tissue engineering are explored.

Stem Cell Differentiation Toward the Myogenic Lineage for Muscle Tissue Regeneration: A Focus on Muscular Dystrophy

Skeletal muscle tissue engineering is one of the important ways for regenerating functionally defective muscles. Among the myopathies, the Duchenne muscular dystrophy (DMD) is a progressive disease due to mutations of the dystrophin gene leading to progressive myofiber degeneration with severe symptoms. Although current therapies in muscular dystrophy are still very challenging, important progress has been made in materials science and in cellular technologies with the use of stem cells. It is therefore useful to review these advances and the results obtained in a clinical point of view. This article focuses on the differentiation of stem cells into myoblasts, and their application in muscular dystrophy. After an overview of the different stem cells that can be induced to differentiate into the myogenic lineage, we introduce scaffolding materials used for muscular tissue engineering. We then described some widely used methods to differentiate different types of stem cell into myoblasts. We highlight recent insights obtained in therapies for muscular dystrophy. Finally, we conclude with a discussion on stem cell technology. We discussed in parallel the benefits brought by the evolution of the materials and by the expansion of cell sources which can differentiate into myoblasts. We also discussed on future challenges for clinical applications and how to accelerate the translation from the research to the clinic in the frame of DMD.

Dental and Nondental Stem Cell Based Regeneration of the Craniofacial Region: A Tissue Based Approach

Craniofacial reconstruction may be a necessary treatment for those who have been affected by trauma, disease, or pathological developmental conditions. The use of stem cell therapy and tissue engineering shows massive potential as a future treatment modality. Currently in the literature, there is a wide variety of published experimental studies utilising the different stem cell types available and the plethora of available scaffold materials. This review investigates different stem cell sources and their unique characteristics to suggest an ideal cell source for regeneration of individual craniofacial tissues. At present, understanding and clinical applications of stem cell therapy remain in their infancy with numerous challenges to overcome. In spite of this, the field displays immense capacity and will no doubt be utilised in future clinical treatments of craniofacial regeneration.

A Solvent-Free Surface Suspension Melt Technique for Making Biodegradable PCL Membrane Scaffolds for Tissue Engineering Applications

In tissue engineering, there is limited availability of a simple, fast and solvent-free process for fabricating micro-porous thin membrane scaffolds. This paper presents the first report of a novel surface suspension melt technique to fabricate a micro-porous thin membrane scaffolds without using any organic solvent. Briefly, a layer of polycaprolactone (PCL) particles is directly spread on top of water in the form of a suspension. After that, with the use of heat, the powder layer is transformed into a melted layer, and following cooling, a thin membrane is obtained. Two different sizes of PCL powder particles (100 µm and 500 µm) are used. Results show that membranes made from 100 µm powders have lower thickness, smaller pore size, smoother surface, higher value of stiffness but lower ultimate tensile load compared to membranes made from 500 µm powder. C2C12 cell culture results indicate that the membrane supports cell growth and differentiation. Thus, this novel membrane generation method holds great promise for tissue engineering.

Endothelial Network Formation Within Human Tissue-Engineered Skeletal Muscle

The size of in vitro engineered skeletal muscle tissue is limited due to the lack of a vascular network in vitro. In this article, we report tissue-engineered skeletal muscle consisting of human aligned myofibers with interspersed endothelial networks. We extend our bioartificial muscle (BAM) model by coculturing human muscle progenitor cells with human umbilical vein endothelial cells (HUVECs) in a fibrin extracellular matrix (ECM). First, the optimal medium conditions for coculturing myoblasts with HUVECs were determined in a fusion assay. Endothelial growth medium proved to be the best compromise for the coculture, without affecting the myoblast fusion index. Second, both cell types were cocultured in a BAM maintained under tension to stimulate myofiber alignment. We then tested different total cell numbers containing 50% HUVECs and found that BAMs with a total cell number of 2 × 10(6) resulted in well-aligned and densely packed myofibers while allowing for improved interspersed endothelial network formation. Third, we compared different myoblast-HUVEC ratios. Including higher numbers of myoblasts improved endothelial network formation at lower total cell density; however, improvement of network characteristics reached a plateau when 1 × 10(6) or more myoblasts were present. Finally, addition of Matrigel to the fibrin ECM did not enhance overall myofiber and endothelial network formation. Therefore, in our BAM model, we suggest the use of a fibrin extracellular matrix containing 2 × 10(6) cells of which 50-70% are muscle cells. Optimizing these coculture conditions allows for a physiologically more relevant muscle model and paves the way toward engineering of larger in vitro muscle constructs.

Engineered in vitro disease models

The ultimate goal of most biomedical research is to gain greater insight into mechanisms of human disease or to develop new and improved therapies or diagnostics. Although great advances have been made in terms of developing disease models in animals, such as transgenic mice, many of these models fail to faithfully recapitulate the human condition. In addition, it is difficult to identify critical cellular and molecular contributors to disease or to vary them independently in whole-animal models. This challenge has attracted the interest of engineers, who have begun to collaborate with biologists to leverage recent advances in tissue engineering and microfabrication to develop novel in vitro models of disease. As these models are synthetic systems, specific molecular factors and individual cell types, including parenchymal cells, vascular cells, and immune cells, can be varied independently while simultaneously measuring system-level responses in real time. In this article, we provide some examples of these efforts, including engineered models of diseases of the heart, lung, intestine, liver, kidney, cartilage, skin and vascular, endocrine, musculoskeletal, and nervous systems, as well as models of infectious diseases and cancer. We also describe how engineered in vitro models can be combined with human inducible pluripotent stem cells to enable new insights into a broad variety of disease mechanisms, as well as provide a test bed for screening new therapies.

Gene and cell therapy for muscle regeneration

Skeletal muscle injury and healing are multifactorial processes, involving three steps of healing: (1) degeneration and inflammation, (2) regeneration, and (3) fibrosis. Fibrous tissue hinders the muscle's complete recovery and current therapies fail in achieving total muscle recovery. Gene and cell therapy (or both) are potential future treatments for severe muscular injuries. Stem cells' properties associated with growth factors or/and cytokines can improve muscle healing and permit long-term recovery.

Cell Therapy for Stress Urinary Incontinence

Urinary incontinence (UI) is the involuntary loss of urine and is a common condition in middle-aged and elderly women and men. Stress urinary incontinence (SUI) is caused by leakage of urine when coughing, sneezing, laughing, lifting, and exercise, even standing leads to increased intra-abdominal pressure. Other types of UI also exist such as urge incontinence (also called overactive bladder), which is a strong and unexpected sudden urge to urinate, mixed forms of UI that result in symptoms of both urge and stress incontinence, and functional incontinence caused by reduced mobility, cognitive impairment, or neuromuscular limitations that impair mobility or dexterity. However, for many SUI patients, there is significant loss of urethral sphincter muscle due to degeneration of tissue, the strain and trauma of pregnancy and childbirth, or injury acquired during surgery. Hence, for individuals with SUI, a cell-based therapeutic approach to regenerate the sphincter muscle offers the advantage of treating the cause rather than the symptoms. We discuss current clinically relevant cell therapy approaches for regeneration of the external urethral sphincter (striated muscle), internal urethral sphincter (smooth muscle), the neuromuscular synapse, and blood supply. The use of mesenchymal stromal/stem cells is a major step in the right direction, but they may not be enough for regeneration of all components of the urethral sphincter. Inclusion of other cell types or biomaterials may also be necessary to enhance integration and survival of the transplanted cells.

Biomaterials based strategies for skeletal muscle tissue engineering: existing technologies and future trends

Skeletal muscles have a robust capacity to regenerate, but under compromised conditions, such as severe trauma, the loss of muscle functionality is inevitable. Research carried out in the field of skeletal muscle tissue engineering has elucidated multiple intrinsic mechanisms of skeletal muscle repair, and has thus sought to identify various types of cells and bioactive factors which play an important role during regeneration. In order to maximize the potential therapeutic effects of cells and growth factors, several biomaterial based strategies have been developed and successfully implemented in animal muscle injury models. A suitable biomaterial can be utilized as a template to guide tissue reorganization, as a matrix that provides optimum micro-environmental conditions to cells, as a delivery vehicle to carry bioactive factors which can be released in a controlled manner, and as local niches to orchestrate in situ tissue regeneration. A myriad of biomaterials, varying in geometrical structure, physical form, chemical properties, and biofunctionality have been investigated for skeletal muscle tissue engineering applications. In the current review, we present a detailed summary of studies where the use of biomaterials favorably influenced muscle repair. Biomaterials in the form of porous three-dimensional scaffolds, hydrogels, fibrous meshes, and patterned substrates with defined topographies, have each displayed unique benefits, and are discussed herein. Additionally, several biomaterial based approaches aimed specifically at stimulating vascularization, innervation, and inducing contractility in regenerating muscle tissues are also discussed. Finally, we outline promising future trends in the field of muscle regeneration involving a deeper understanding of the endogenous healing cascades and utilization of this knowledge for the development of multifunctional, hybrid, biomaterials which support and enable muscle regeneration under compromised conditions.

Mesenchymal stromal cells for sphincter regeneration

Stress urinary incontinence (SUI), defined as the involuntary loss of considerable amounts of urine during increased abdominal pressure (exertion, effort, sneezing, coughing, etc.), is a severe problem to the individuals affected and a significant medical, social and economic challenge. SUI is associated with pelvic floor debility, absence of detrusor contraction, or a loss of control over the sphincter muscle apparatus. The pathology includes an increasing loss of muscle cells, replacement of muscular tissue with fibrous tissue, and general aging associated processes of the sphincter complex. When current therapies fail to cure or improve SUI, application of regeneration-competent cells may be an alternative therapeutic option. Here we discuss different aspects of the biology of mesenchymal stromal cells, which are relevant to their clinical applications and for regenerating the sphincter complex. However, there are reports in favor of and against cell-based therapies. We therefore summarize the potential and the risks of cell-based therapies for the treatment of SUI.

Peripheral nerve regeneration: experimental strategies and future perspectives

Peripheral nerve injuries represent a substantial clinical problem with insufficient or unsatisfactory treatment options. This review summarises all the events occurring after nerve damage at the level of the cell body, the site of injury and the target organ. Various experimental strategies to improve neuronal survival, axonal regeneration and target reinnervation are described including pharmacological approaches and cell-based therapies. Given the complexity of nerve regeneration, further studies are needed to address the biology of nerve injury, to improve the interaction with implantable scaffolds, and to implement cell-based therapies in nerve tissue engineering.

Clinical applications of naturally derived biopolymer-based scaffolds for regenerative medicine

Naturally derived polymeric biomaterials, such as collagens, silks, elastins, alginates, and fibrins are utilized in tissue engineering due to their biocompatibility, bioactivity, and tunable mechanical and degradation kinetics. The use of these natural biopolymers in biomedical applications is advantageous because they do not release cytotoxic degradation products, are often processed using environmentally-friendly aqueous-based methods, and their degradation rates within biological systems can be manipulated by modifying the starting formulation or processing conditions. For these reasons, many recent in vivo investigations and FDA-approval of new biomaterials for clinical use have utilized natural biopolymers as matrices for cell delivery and as scaffolds for cell-free support of native tissues. This review highlights biopolymer-based scaffolds used in clinical applications for the regeneration and repair of native tissues, with a focus on bone, skeletal muscle, peripheral nerve, cardiac muscle, and cornea substitutes.

Isolation and Purification of Satellite Cells for Skeletal Muscle Tissue Engineering

Engineered skeletal muscle holds promise as a source of graft tissue for the repair of traumatic injuries such as volumetric muscle loss. The resident skeletal muscle stem cell, the satellite cell, has been identified as an ideal progenitor for tissue engineering due to its role as an essential player in the potent skeletal muscle regeneration mechanism. A significant challenge facing tissue engineers, however, is the isolation of sufficiently large satellite cell populations with high purity. The two common isolation techniques, single fiber explant culture and enzymatic dissociation, can yield either a highly pure satellite cell population or a suitably large number or cells but fail to do both simultaneously. As a result, it is often necessary to use a purification technique such as pre-plating or cell sorting to enrich the satellite cell population post-isolation. Furthermore, the absence of complex chemical and biophysical cues influencing the in vivo satellite cell "niche" complicates the culture of isolated satellite cells. Techniques under investigation to maximize myogenic proliferation and differentiation in vitro are described in this article, along with current methods for isolating and purifying satellite cells.

Operating RegenMed: development of better in-theater strategies for handling tissue-engineered organs and tissues

Tissue engineering ex vivo and direct cellular application with bioscaffolds in vivo has allowed surgeons to restore and establish function throughout the human body. The evidence for regenerative surgery is growing, and consequently there is a need for the development of more advanced regenerative surgery facilities. Regenerative medicine in the surgical field is changing rapidly and this must be reflected in the design of any future operating suite. The theater environment needs to be highly adaptable to account for future significant advances within the field. Development of purpose built, combined operating suites and tissue-engineering laboratories will provide the facility for modern surgeons to treat patients with organ deficits, using bespoke, regenerated constructs without the need for immunosuppression.

Skeletal muscle tissue engineering: methods to form skeletal myotubes and their applications

Skeletal muscle tissue engineering (SMTE) aims to repair or regenerate defective skeletal muscle tissue lost by traumatic injury, tumor ablation, or muscular disease. However, two decades after the introduction of SMTE, the engineering of functional skeletal muscle in the laboratory still remains a great challenge, and numerous techniques for growing functional muscle tissues are constantly being developed. This article reviews the recent findings regarding the methodology and various technical aspects of SMTE, including cell alignment and differentiation. We describe the structure and organization of muscle and discuss the methods for myoblast alignment cultured in vitro. To better understand muscle formation and to enhance the engineering of skeletal muscle, we also address the molecular basics of myogenesis and discuss different methods to induce myoblast differentiation into myotubes. We then provide an overview of different coculture systems involving skeletal muscle cells, and highlight major applications of engineered skeletal muscle tissues. Finally, potential challenges and future research directions for SMTE are outlined.

Bioengineering Strategies for Polymeric Scaffold for Tissue Engineering an Aortic Heart Valve: An Update

The occurrence of dysfunctional aortic valves is increasing every year, and current replacement heart valves, although having been shown to be clinically successful, are only short-term solutions and suffer from many agonizing long-term drawbacks. The tissue engineering of heart valves is recognized as one of the most promising answers for aortic valve disease therapy, but overcoming current shortcomings will require multidisciplinary efforts. The use of a polymeric scaffold to guide the growth of the tissue is the most common approach to generate a new tissue for an aortic heart valve. However, optimizing the design of the scaffold, in terms of biocompatibility, surface morphology for cell attachments and the correct rate of degradation is critical in creating a viable tissue-engineered aortic heart valve. This paper highlights the bioengineering strategies that need to be followed to construct a polymeric scaffold of sufficient mechanical integrity, with superior surface morphologies, that is capable of mimicking the valve dynamics in vivo. The current challenges and future directions of research for creating tissue-engineered aortic heart valves are also discussed.

Microfabrication and microfluidics for muscle tissue models

The relatively recent development of microfluidic systems with wide-ranging capabilities for generating realistic 2D or 3D systems with single or multiple cell types has given rise to an extensive collection of platform technologies useful in muscle tissue engineering. These new systems are aimed at (i) gaining fundamental understanding of muscle function, (ii) creating functional muscle constructs in vitro, and (iii) utilizing these constructs a variety of applications. Use of microfluidics to control the various stimuli that promote differentiation of multipotent cells into cardiac or skeletal muscle is first discussed. Next, systems that incorporate muscle cells to produce either 2D sheets or 3D tissues of contractile muscle are described with an emphasis on the more recent 3D platforms. These systems are useful for fundamental studies of muscle biology and can also be incorporated into drug screening assays. Applications are discussed for muscle actuators in the context of microrobotics and in miniaturized biological pumps. Finally, an important area of recent study involves coculture with cell types that either activate muscle or facilitate its function. Limitations of current designs and the potential for improving functionality for a wider range of applications is also discussed, with a look toward using current understanding and capabilities to design systems of greater realism, complexity and functionality.

Airway tissue engineering: an update

INTRODUCTION

Prosthetic materials, autologous tissues, cryopreserved homografts and allogeneic tissues have thus far proven unsuccessful in providing long-term functional solutions to extensive upper airway disease and damage. Research is therefore focusing on the rapidly expanding fields of regenerative medicine and tissue engineering in order to provide stem cell-based constructs for airway reconstruction, substitution and/or regeneration.

AREAS COVERED

Advances in stem cell technology, biomaterials and growth factor interactions have been instrumental in guiding optimization of tissue-engineered airways, leading to several first-in-man studies investigating stem cell-based tissue-engineered tracheal transplants in patients. Here, we summarize current progress, outstanding research questions, as well as future directions within the field.

EXPERT OPINION

The complex immune interaction between the transplant and host in vivo is only beginning to be untangled. Recent progress in our understanding of stem cell biology, decellularization techniques, biomaterials and transplantation immunobiology offers the prospect of transplanting airways without the need for lifelong immunosuppression. In addition, progress in airway revascularization, reinnervation and ever-increasingly sophisticated bioreactor design is opening up new avenues for the construction of a tissue-engineered larynx. Finally, 3D printing is a novel technique with the potential to render microscopic control over how cells are incorporated and grown onto the tissue-engineered airway.

Engineering muscle constructs for the creation of functional engineered musculoskeletal tissue

Volumetric muscle loss (VML) is a disabling condition in which current clinical procedures are suboptimal. The field of tissue engineering has many promising strategies for the creation of functional skeletal muscle in vitro. However, there are still two key limitations that prevent it from becoming a solution for treating VML. First, engineered muscle tissue must be biocompatible to facilitate muscle tissue regrowth without generating an immune response. Second, engineered muscle constructs must be scaled up to facilitate replacement of clinically relevant volumes of tissue (centimeters in diameter). There are currently no tissue engineering strategies to produce tissue constructs that are both biocompatible and large enough to facilitate clinical repair. However, recent advances in tissue engineering using synthetic scaffolds, native scaffolds, or scaffold-free approaches may lead to a solution for repair of VML injuries.

Photocrosslinked ultrathin anionic polysaccharide supports for accelerated growth of human mesenchymal stem cells

OBJECTIVES

Properties of cell culture supports obtained from ultrathin multilayer films containing anionic natural polysaccharides (PSacs) and a synthetic polycation were studied.

MATERIALS AND METHODS

Supports were prepared via a layer-by-layer (LbL) self-assembly deposition method. Polymers used were: heparin (Hep), chondroitin sulphate (CS), hyaluronic acid (HA), and ι-carrageenan (Car) as polyanions, and diazoresin (DR) as a polycation. PSac layers were crosslinked with DR layers by irradiation with UV light absorbed by DR resin.

RESULTS

DR/PSac films are very efficient cell culture growth supports as found from experiments with human mesenchymal stem cells (hMSCs). Irradiation of the films resulted in changing zeta potential of outermost layers of both DR and PSac to more negative values, and in increased film hydrophobicity, as found from the contact angle measurements. Photocrosslinking of the supports led to their increased stability.

CONCLUSIONS

The supports allow for obtaining intact cell monolayers faster than when typical polystyrene tissue culture plates are used. Moreover, these monolayers spontaneously detach permitting formation of new cell layers on these surfaces relatively early during culture, compared to cells cultured on commonly used tissue culture plastic.

Physiology and metabolism of tissue-engineered skeletal muscle

Skeletal muscle is a major target for tissue engineering, given its relative size in the body, fraction of cardiac output that passes through muscle beds, as well as its key role in energy metabolism and diabetes, and the need for therapies for muscle diseases such as muscular dystrophy and sarcopenia. To date, most studies with tissue-engineered skeletal muscle have utilized murine and rat cell sources. On the other hand, successful engineering of functional human muscle would enable different applications including improved methods for preclinical testing of drugs and therapies. Some of the requirements for engineering functional skeletal muscle include expression of adult forms of muscle proteins, comparable contractile forces to those produced by native muscle, and physiological force-length and force-frequency relations. This review discusses the various strategies and challenges associated with these requirements, specific applications with cultured human myoblasts, and future directions.

Stem cell-based organ replacements-airway and lung tissue engineering

Tissue engineering requires the use of cells seeded onto scaffolds, often in conjunction with bioactive molecules, to regenerate or replace tissues. Significant advances have been made in recent years within the fields of stem cell biology and biomaterials, leading to some exciting developments in airway tissue engineering, including the first use of stem cell-based tissue-engineered tracheal replacements in humans. In addition, recent advances within the fields of scaffold biology and decellularization offer the potential to transplant patients without the use of immunosuppression.