Application of Cyclic Strain for Accelerated Skeletal Myogenic Differentiation of Mouse Bone Marrow-Derived Mesenchymal Stromal Cells with Cell Alignment

Actuation of Soft Thermoresponsive Hydrogels Mechanically Stimulates Osteogenesis in Human Mesenchymal Stem Cells without Biochemical Factors

Mesenchymal stem cells (MSCs) have the potential to differentiate into multiple lineages and can be harvested relatively easily from adults, making them a promising cell source for regenerative therapies. While it is well-known how to consistently differentiate MSCs into adipose, chondrogenic, and osteogenic lineages by treatment with biochemical factors, the number of studies exploring how to achieve this with mechanical signals is limited. A relatively unexplored area is the effect of cyclic forces on the MSC differentiation. Recently, our group developed a thermoresponsive N-ethyl acrylamide/N-isopropylacrylamide (NIPAM/NEAM) hydrogel supplemented with gold nanorods that are able to convert near-infrared light into heat. Using light pulses allows for local hydrogel collapse and swelling with physiologically relevant force and frequency. In this study, MSCs are cultured on this hydrogel system with a patterned surface and exposed to intermittent or continuous actuation of the hydrogel for 3 days to study the effect of actuation on MSC differentiation. First, cells are harvested from the bone marrow of three donors and tested for their MSC phenotype, meeting the following criteria: the harvested cells are adherent and demonstrate a fibroblast-like bipolar morphology. They lack the expression of CD34 and CD45 but do express CD73, CD90, and CD105. Additionally, their differentiation potential into adipogenic, chondrogenic, and osteogenic lineages is validated by the addition of standardized differentiation media. Next, MSCs are exposed to intermittent or continuous actuation, which leads to a significantly enhanced cell spreading compared to nonactuated cells. Moreover, actuation results in nuclear translocation of Runt-related transcription factor 2 and the Yes-associated protein. Together, these results indicate that cyclic mechanical stimulation on a soft, ridged substrate modulates the MSC fate commitment in the direction of osteogenesis.

Covalent Immobilization of Collagen Type I to a Polydimethylsiloxane Surface for Preventing Cell Detachment by Retaining Collagen Molecules under Uniaxial Cyclic Mechanical Stretching Stress

Surface modification of polydimethylsiloxane (PDMS) with an extracellular matrix (ECM) is useful for enhancing stable cell attachment. However, few studies have investigated the correlation between the stability of deposited ECM and cell behavior on the PDMS surfaces in external stretched cell culture systems. Herein, covalent collagen type I (Col)-immobilized PDMS surfaces were fabricated using 3-aminopropyl-trimethoxysilane, glutaraldehyde, and Col molecules. The immobilized collagen molecules on the PDMS surface were more stable and uniform than the physisorbed collagen. The cells stably adhered to the Col-immobilized surface and proliferated even under uniaxial cyclic mechanical stretching stress (UnCyMSt), whereas the cells gradually detached from the Col-physisorbed PDMS surface, accompanied by a decrease in the number of deposited collagen molecules. Moreover, the immobilization of collagen molecules enhanced cell alignment under the UnCyMSt. This study reveals that cell adhesion, proliferation, and alignment under the UnCyMSt can be attributed to the retention of collagen molecules on the PDMS surface.

Optomechanically Actuated Hydrogel Platform for Cell Stimulation with Spatial and Temporal Resolution

Cells exist in the body in mechanically dynamic environments, yet the vast majority of in vitro cell culture is conducted on static materials such as plastic dishes and gels. To address this limitation, we report an approach to transition widely used hydrogels into mechanically active substrates by doping optomechanical actuator (OMA) nanoparticles within the polymer matrix. OMAs are composed of gold nanorods surrounded by a thermoresponsive polymer shell that rapidly collapses upon near-infrared (NIR) illumination. As a proof of concept, we crosslinked OMAs into laminin-gelatin hydrogels, generating up to 5 μm deformations triggered by NIR pulsing. This response was tunable by NIR intensity and OMA density within the gel and is generalizable to other hydrogel materials. Hydrogel mechanical stimulation enhanced myogenesis in C2C12 myoblasts as evidenced by ERK signaling, myocyte fusion, and sarcomeric myosin expression. We also demonstrate rescued differentiation in a chronic inflammation model as a result of mechanical stimulation. This work establishes OMA-actuated biomaterials as a powerful tool for in vitro mechanical manipulation with broad applications in the field of mechanobiology.

Insight into muscle stem cell regeneration and mechanobiology

Stem cells possess the unique ability to differentiate into specialized cell types. These specialized cell types can be used for regenerative medicine purposes such as cell therapy. Myosatellite cells, also known as skeletal muscle stem cells (MuSCs), play important roles in the growth, repair, and regeneration of skeletal muscle tissues. However, despite its therapeutic potential, the successful differentiation, proliferation, and expansion processes of MuSCs remain a significant challenge due to a variety of factors. For example, the growth and differentiation of MuSCs can be greatly influenced by actively replicating the MuSCs microenvironment (known as the niche) using mechanical forces. However, the molecular role of mechanobiology in MuSC growth, proliferation, and differentiation for regenerative medicine is still poorly understood. In this present review, we comprehensively summarize, compare, and critically analyze how different mechanical cues shape stem cell growth, proliferation, differentiation, and their potential role in disease development (Fig. 1). The insights developed from the mechanobiology of stem cells will also contribute to how these applications can be used for regenerative purposes using MuSCs.

Effect of Cyclic Uniaxial Mechanical Strain on Endothelial Progenitor Cell Differentiation

PURPOSE

Endothelial progenitor cells (EPCs) have been used as an autologous or allogeneic source in multiple tissue engineering applications. EPCs possess high proliferative and tissue regeneration potential. The effect of shear stress on EPCs has been extensively studied but the role of cyclic mechanical strain on EPCs remains to be understood. In this study, we focused on examining the role of uniaxial cyclic strain on EPCs cultured on three-dimensional (3D) anisotropic composites that mimic healthy and diseased aortic valve tissue matrix compositions.

METHODS AND RESULTS

The composites were fabricated by combining centrifugal jet spun fibers with photocrosslinkable gelatin and glycosaminoglycan hydrogels. A custom-designed uniaxial cyclic stretcher was used to provide the necessary cyclic stimulation to the EPC-seeded 3D composites. The samples were cyclically strained at a rate of 1 Hz at 15% strain mimicking the physiological condition experienced by aortic valve, with static conditions serving as controls. Cell viability was high in all conditions. Immunostaining revealed reduced endothelial marker (CD31) expression with increased smooth muscle cell marker, SM22α, expression when subjected to cyclic strain. Functional analysis through Matrigel assay agreed with the immunostaining findings with reduced tubular structure formation in strained conditions compared to EPC controls. Additionally, the cells showed reduced acLDL uptake compared to controls which are in alignment with the EPCs undergoing differentiation.

CONCLUSION

Overall, we show that EPCs lose their endothelial progenitor phenotype, and have the potential to be differentiated into mesenchymal-like cells through cyclic mechanical stimulation.

Static and Dynamic Biomaterial Engineering for Cell Modulation

In the biological microenvironment, cells are surrounded by an extracellular matrix (ECM), with which they dynamically interact during various biological processes. Specifically, the physical and chemical properties of the ECM work cooperatively to influence the behavior and fate of cells directly and indirectly, which invokes various physiological responses in the body. Hence, efficient strategies to modulate cellular responses for a specific purpose have become important for various scientific fields such as biology, pharmacy, and medicine. Among many approaches, the utilization of biomaterials has been studied the most because they can be meticulously engineered to mimic cellular modulatory behavior. For such careful engineering, studies on physical modulation (e.g., ECM topography, stiffness, and wettability) and chemical manipulation (e.g., composition and soluble and surface biosignals) have been actively conducted. At present, the scope of research is being shifted from static (considering only the initial environment and the effects of each element) to biomimetic dynamic (including the concepts of time and gradient) modulation in both physical and chemical manipulations. This review provides an overall perspective on how the static and dynamic biomaterials are actively engineered to modulate targeted cellular responses while highlighting the importance and advance from static modulation to biomimetic dynamic modulation for biomedical applications.

Cell-Based Double-Screening Method to Identify a Reliable Candidate for Osteogenesis-Targeting Compounds

Small-molecule compounds strongly affecting osteogenesis can form the basis of effective therapeutic strategies in bone regenerative medicine. A cell-based high-throughput screening system might be a powerful tool for identifying osteoblast-targeting candidates; however, this approach is generally limited with using only one molecule as a cell-based sensor that does not always reflect the activation of the osteogenic phenotype. In the present study, we used the MC3T3-E1 cell line stably transfected with the green fluorescent protein (GFP) reporter gene driven by a fragment of type I collagen promoter (Col-1a1GFP-MC3T3-E1) to evaluate a double-screening system to identify osteogenic inducible compounds using a combination of a cell-based reporter assay and detection of alkaline phosphatase (ALP) activity. Col-1a1GFP-MC3T3-E1 cells were cultured in an osteogenic induction medium after library screening of 1280 pharmacologically active compounds (Lopack¹²⁸⁰). After 7 days, GFP fluorescence was measured using a microplate reader. After 14 days of osteogenic induction, the cells were stained with ALP. Library screening using the Col-1a1/GFP reporter and ALP staining assay detected three candidates with significant osteogenic induction ability. Furthermore, leflunomide, one of the three detected candidates, significantly promoted new bone formation in vivo. Therefore, this double-screening method could identify candidates for osteogenesis-targeting compounds more reliably than conventional methods.

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.

Protocol for the Use of a Novel Bioreactor System for Hydrated Mechanical Testing, Strained Sterile Culture, and Force of Contraction Measurement of Tissue Engineered Muscle Constructs

Bioreactor systems are built as controlled environments for biological processes and utilized in the field of tissue engineering to apply mechanical, spatial, and chemical cues to developing tissue grafts. Often the systems are applied to instruct differentiation and maturation of the cells grown inside. Perhaps the most obvious targets for strain and compression-based bioreactors are mechanically active tissues, as it is hypothesized that biomimetic mechanical environments instruct immature cells to form differentiated tissues. One such tissue, skeletal muscle, has been identified as a key candidate for strain application due to the close structure-function relationship of myofibers. Here we detail the multiple uses of a custom-built bioreactor system in combination with electrospun fibrin microfibers for muscle tissue engineering. Outlined below are the methods used in the system to test the mechanical properties of hydrogel-based scaffolds in an aqueous environment, including Young’s modulus and poroelasticity. Additionally, we demonstrate the application of tensile strain to sterile cell cultures grown on electrospun scaffolds and perform end-point testing of tissue contractility with the addition of an electrode.

Integrating biomaterials and food biopolymers for cultured meat production

Cultured meat has recently achieved mainstream prominence due to the emergence of societal and industrial interest. In contrast to animal-based production of traditional meat, the cultured meat approach entails laboratory cultivation of engineered muscle tissue. However, bioengineers have hitherto engineered tissues to fulfil biomedical endpoints, and have had limited experience in engineering muscle tissue for its post-mortem traits, which broadly govern consumer definitions of meat quality. Furthermore, existing tissue engineering approaches face fundamental challenges in technical feasibility and industrial scalability for cultured meat production. This review discusses how animal-based meat production variables influence meat properties at both the molecular and functional level, and whether current cultured meat approaches recapitulate these properties. In addition, this review considers how conventional meat producers employ exogenous biopolymer-based meat ingredients and processing techniques to mimic desirable meat properties in meat products. Finally, current biomaterial strategies for engineering muscle and adipose tissue are surveyed in the context of emerging constraints that pertain to cultured meat production, such as edibility, sustainability and scalability, and potential areas for integrating biomaterials and food biopolymer approaches to address these constraints are discussed. STATEMENT OF SIGNIFICANCE: Laboratory-grown or cultured meat has gained increasing interest from industry and the public, but currently faces significant impediment to market feasibility. This is due to fundamental knowledge gaps in producing realistic meat tissues via conventional tissue engineering approaches, as well as translational challenges in scaling up these approaches in an efficient, sustainable and high-volume manner. By defining the molecular basis for desirable meat quality attributes, such as taste and texture, and introducing the fundamental roles of food biopolymers in mimicking these properties in conventional meat products, this review aims to bridge the historically disparate fields of meat science and biomaterials engineering in order to inspire potentially synergistic strategies that address some of these challenges.

The application of bone marrow mesenchymal stem cells and biomaterials in skeletal muscle regeneration

Skeletal muscle injuries have bothered doctors and caused great burdens to the public medical insurance system for a long time. Once injured, skeletal muscles usually go through the processes of inflammation, repairing and remodeling. If repairing and remodeling stages are out of balance, scars will be formed to replace injured skeletal muscles. At present, clinicians usually use conventional methods to restore the injured skeletal muscles, such as flap transplantation. However, flap transplantation sometimes needs to sacrifice healthy autologous tissues and will bring extra harm to patients. In recent years, stem cells-based tissue engineering provides us new treatment ideas for skeletal muscle injuries. Stem cells are cells with multiple differentiation potential and have ability to differentiate into adult cells under special condition. Skeletal muscle tissues also have stem cells, called satellite cells, but they are in small amount and new muscle fibers that derived from them may not be enough to replace injured fibers. Bone marrow mesenchymal stem cells (BM-MSCs) could promote musculoskeletal tissue regeneration and activate the myogenic differentiation of satellite cells. Biomaterial is another important factor to promote tissue regeneration and greatly enhance physiological activities of stem cells in vivo. The combined use of stem cells and biomaterials will gradually become a mainstream to restore injured skeletal muscles in the future. This review article mainly focuses on the review of research about the application of BM-MSCs and several major biomaterials in skeletal muscle regeneration over the past decades.

Mechanical Stimulation of Adhesion Receptors Using Light-Responsive Nanoparticle Actuators Enhances Myogenesis

The application of cyclic strain is known to enhance myoblast differentiation and muscle growth in vitro and in vivo. However, current techniques apply strain to full tissues or cell monolayers, making it difficult to evaluate whether mechanical stimulation at the subcellular or single-cell scales would drive myoblast differentiation. Here, we report the use of optomechanical actuator (OMA) particles, comprised of a ∼0.6 μm responsive hydrogel coating a gold nanorod (100 × 20 nm) core, to mechanically stimulate the integrin receptors in myoblasts. When illuminated with near-infrared (NIR) light, OMA nanoparticles rapidly collapse, exerting mechanical forces to cell receptors bound to immobilized particles. Using a pulsed illumination pattern, we applied cyclic integrin forces to C2C12 myoblasts cultured on a monolayer of OMA particles and then measured the cellular response. We found that 20 min of OMA actuation resulted in cellular elongation in the direction of the stimulus and enhancement of nuclear YAP1 accumulation, an effector of ERK phosphorylation. Cellular response was dependent on direct conjugation of RGD peptides to the OMA particles. Repeated OMA mechanical stimulation for 5 days led to enhanced myogenesis as quantified using cell alignment, fusion, and sarcomeric myosin expression in myotubes. OMA-mediated myogenesis was sensitive to the geometry of stimulation but not to MEK1/2 inhibition. Finally, we found that OMA stimulation in regions proximal to the nucleus resulted in localization of the transcription activator YAP-1 to the nucleus, further suggesting the role of YAP1 in mechanotransduction in C2C12 cells. These findings demonstrate OMAs as a novel tool for studying the role of spatially localized forces in influencing myogenesis.

Effects of local injection and intravenous injection of allogeneic bone marrow mesenchymal stem cells on the structure and function of damaged anal sphincter in rats

Anal sphincter injury leads to damage to the anal structure and functions and has been identified as a major risk factor for fecal incontinence. Bone marrow mesenchymal stem cells (BMSCs) with capacities of multidifferentiation, paracrine, and low immunogenicity have been widely used in tissue repair and regeneration. The primary objective of this research was to compare the effects of different injection therapies of BMSCs on the injured anal sphincters. Ninety-six Sprague-Dawley female rats were randomly divided into four groups (n = 24 each): intravenous injection, local injection, sham operation, and normal control. For the first three groups, 25% removal of the anal sphincter complex was performed and 0.3-ml phosphate-buffered saline (PBS) (containing 10⁷ green fluorescent protein-labeled allogeneic BMSCs) was given accordingly to the treatment group 24 h after operation for 7 consecutive days. The sham operation group was injected with 0.3-ml PBS only. All cases had undergone evaluation in the 1st, 7th, 14th, and 28th postoperative days. The rats were sacrificed on the 28th postoperative day, and the anal sphincters were dissected to be analyzed by morphological examination. At 14 days postoperatively, local injection of BMSC significantly improved the peak contraction pressure, electromyography amplitude, and frequency of the injured anal sphincter compared with tail vein, but there was no significant difference in resting pressure until 28 days after sphincterectomy. Masson staining results confirmed that the local injection group had significantly more new muscles on the wound. BMSC could remarkably improve peak contraction pressure, electromyography amplitude, and muscle fibers on the wound, and local injection is superior to intravenous injection.

Integrating Biophysics in Toxicology

Integration of biophysical stimulation in test systems is established in diverse branches of biomedical sciences including toxicology. This is largely motivated by the need to create novel experimental setups capable of reproducing more closely in vivo physiological conditions. Indeed, we face the need to increase predictive power and experimental output, albeit reducing the use of animals in toxicity testing. In vivo, mechanical stimulation is essential for cellular homeostasis. In vitro, diverse strategies can be used to model this crucial component. The compliance of the extracellular matrix can be tuned by modifying the stiffness or through the deformation of substrates hosting the cells via static or dynamic strain. Moreover, cells can be cultivated under shear stress deriving from the movement of the extracellular fluids. In turn, introduction of physical cues in the cell culture environment modulates differentiation, functional properties, and metabolic competence, thus influencing cellular capability to cope with toxic insults. This review summarizes the state of the art of integration of biophysical stimuli in model systems for toxicity testing, discusses future challenges, and provides perspectives for the further advancement of in vitro cytotoxicity studies.

Myoblast maturity on aligned microfiber bundles at the onset of strain application impacts myogenic outcomes

Engineered skeletal muscle grafts may be employed in various applications including the treatment of volumetric muscle loss (VML) and pharmacological drug screening. To recapitulate the well-defined structure of native muscle, tensile strains have been applied to the grafts. In this study, we cultured C2C12 murine myoblasts on electrospun fibrin microfiber bundles for 7 days in custom-built bioreactor units and investigated the impact of strain regimen and delayed onset of tensile straining on myogenic outcomes. The substrate topography induced uniaxial alignment of cells in all (strained and unstrained) groups. The engineered grafts in strained groups were subjected to 10% strain amplitude for 6 h per day. We found that both static and cyclic uniaxial strains resulted in similar morphological and gene expression outcomes. However, relative to 0% strain groups, there were stark increases in myotube diameter, myosin heavy chain (MHC) coverage, and expression of key myogenic genes (Pax 7, Troponin, MHC I, MHC IIb, MHC IIx) only if strain was applied at Days 5-7 rather than Days 3-7. This finding suggests that a critical indicator of myogenic improvement under strain in our system is the phenotype of the cells at the onset of strain and suggests that this is a key parameter that should be considered in studies where myoblasts are subjected to biophysical stimulation to promote tissue formation. STATEMENT OF SIGNIFICANCE: This is the first report on the impact of the timing of the initial application of mechanical strain for improving the myogenic outcomes of 3D engineered skeletal muscle grafts. In this work, immature skeletal myoblasts were grown on topographically aligned, electrospun fibrin microfiber bundles and we applied 10% uniaxial static or cyclic strain. We concluded that the maturity of myoblasts prior to strain application, rather than strain waveform, was the primary predictor of improved myogenic outcomes, including myogenic gene expression and myotube morphology. Elucidating the optimal conditions for strain application is a vital step in recapitulating physiological myogenic properties in tissue engineered skeletal muscle constructs, with applications for treating volumetric muscle loss, disease modeling, and drug testing.

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.

Deoxynivalenol induces structural alterations in epidermoid carcinoma cells A431 and impairs the response to biomechanical stimulation

Morphology together with the capability to respond to surrounding stimuli are key elements governing the spatial interaction of living cells with the environment. In this respect, biomechanical stimulation can trigger significant physiological cascades that can potentially modulate toxicity. Deoxynivalenol (DON, vomitoxin) is one of the most prevalent mycotoxins produced by Fusarium spp. and it was used to explore the delicate interaction between biomechanical stimulation and cytotoxicity in A431 cells. In fact, in addition of being a food contaminant, DON is a relevant toxin for several organ systems. The combination between biomechanical stimulation and the mycotoxin revealed how DON can impair crucial functions affecting cellular morphology, tubulin and lysosomes at concentrations even below those known to be cytotoxic in routine toxicity studies. Sub-toxic concentrations of DON (0.1-1 μM) impaired the capability of A431 cells to respond to a biomechanical stimulation that normally sustains trophic effects in these cells. Moreover, the effects of DON (0.1-10 μM) were partially modulated by the application of uniaxial stretching (0.5 Hz, 24 h, 15% deformation). Ultimately, proteomic analysis revealed the potential of DON to alter several proteins necessary for cell adhesion and cytoskeletal modulation suggesting a molecular link between biomechanics and the cytotoxic potential of the mycotoxin.

Stem Cells in Dentistry: Types of Intra- and Extraoral Tissue-Derived Stem Cells and Clinical Applications

Stem cells are undifferentiated cells, capable of renewing themselves, with the capacity to produce different cell types to regenerate missing tissues and treat diseases. Oral facial tissues have been identified as a source and therapeutic target for stem cells with clinical interest in dentistry. This narrative review report targets on the several extraoral- and intraoral-derived stem cells that can be applied in dentistry. In addition, stem cell origins are suggested in what concerns their ability to differentiate as well as their particular distinguishing quality of convenience and immunomodulatory for regenerative dentistry. The development of bioengineered teeth to replace the patient's missing teeth was also possible because of stem cell technologies. This review will also focus our attention on the clinical application of stem cells in dentistry. In recent years, a variety of articles reported the advantages of stem cell-based procedures in regenerative treatments. The regeneration of lost oral tissue is the target of stem cell research. Owing to the fact that bone imperfections that ensue after tooth loss can result in further bone loss which limit the success of dental implants and prosthodontic therapies, the rehabilitation of alveolar ridge height is prosthodontists' principal interest. The development of bioengineered teeth to replace the patient's missing teeth was also possible because of stem cell technologies. In addition, a “dental stem cell banking” is available for regenerative treatments in the future. The main features of stem cells in the future of dentistry should be understood by clinicians.

Stem Cells for Skeletal Muscle Tissue Engineering

Volumetric muscle loss (VML) is a debilitating condition wherein muscle loss overwhelms the body's normal physiological repair mechanism. VML is particularly common among military service members who have sustained war injuries. Because of the high social and medical cost associated with VML and suboptimal current surgical treatments, there is great interest in developing better VML therapies. Skeletal muscle tissue engineering (SMTE) is a promising alternative to traditional VML surgical treatments that use autogenic tissue grafts, and rather uses isolated stem cells with myogenic potential to generate de novo skeletal muscle tissues to treat VML. Satellite cells are the native precursors to skeletal muscle tissue, and are thus the most commonly studied starting source for SMTE. However, satellite cells are difficult to isolate and purify, and it is presently unknown whether they would be a practical source in clinical SMTE applications. Alternative myogenic stem cells, including adipose-derived stem cells, bone marrow-derived mesenchymal stem cells, perivascular stem cells, umbilical cord mesenchymal stem cells, induced pluripotent stem cells, and embryonic stem cells, each have myogenic potential and have been identified as possible starting sources for SMTE, although they have yet to be studied in detail for this purpose. These alternative stem cell varieties offer unique advantages and disadvantages that are worth exploring further to advance the SMTE field toward highly functional, safe, and practical VML treatments. The following review summarizes the current state of satellite cell-based SMTE, details the properties and practical advantages of alternative myogenic stem cells, and offers guidance to tissue engineers on how alternative myogenic stem cells can be incorporated into SMTE research.

A Poroelastic Model of a Fibrous-Porous Tissue Engineering Scaffold

Tissue engineering scaffolds are used in conjunction with stem cells for the treatment of various diseases. A number of factors provided by the scaffolds affect the differentiation of stem cells. Mechanical cues that are part of the natural cellular microenvironment can both accelerate the differentiation toward particular cell lineages or induce differentiation to an alternative cell fate. Among such factors, there are externally applied strains and mechanical (stiffness and relaxation time) properties of the extracellular matrix. Here, the mechanics of a fibrous-porous scaffold is studied by applying a coordinated modeling and experimental approach. A force relaxation experiment is used, and a poroelastic model associates the relaxation process with the fluid diffusion through the fibrous matrix. The model parameters, including the stiffness moduli in the directions along and across the fibers as well as fluid diffusion time, are estimated by fitting the experimental data. The time course of the applied force is then predicted for different rates of loading and scaffold porosities. The proposed approach can help in a reduction of the technological and experimental efforts to produce 3-D scaffolds for regenerative medicine as well as in a higher accuracy of the estimation of the local factors sensed by stem cells.

Biophysical Stimulation for Engineering Functional Skeletal Muscle

Tissue engineering is a promising therapeutic strategy to regenerate skeletal muscle. However, ex vivo cultivation methods typically result in a low differentiation efficiency of stem cells as well as grafts that resemble the native tissues morphologically, but lack contractile function. The application of biomimetic tensile strain provides a potent stimulus for enhancing myogenic differentiation and engineering functional skeletal muscle grafts. We reviewed integrin-dependent mechanisms that potentially link mechanotransduction pathways to the upregulation of myogenic genes. Yet, gaps in our understanding make it challenging to use these pathways to theoretically determine optimal ex vivo strain regimens. A multitude of strain protocols have been applied to in vitro cultures for the cultivation of myogenic progenitors (adipose- and bone marrow-derived stem cells and satellite cells) and transformed murine myoblasts, C2C12s. Strain regimens are characterized by orientation, amplitude, and time-dependent factors (effective frequency, duration, and the rest period between successive strain cycles). Analysis of published data has identified possible minimum/maximum values for these parameters and suggests that uniaxial strains may be more potent than biaxial strains, possibly because they more closely mimic physiologic strain profiles. The application of these biophysical stimuli for engineering 3D skeletal muscle grafts is nontrivial and typically requires custom-designed bioreactors used in combination with biomaterial scaffolds. Consideration of the physical properties of these scaffolds is critical for effective transmission of the applied strains to encapsulated cells. Taken together, these studies demonstrate that biomimetic tensile strain generally results in improved myogenic outcomes in myogenic progenitors and differentiated myoblasts. However, for 3D systems, the optimization of the strain regimen may require the entire system including cells, biomaterials, and bioreactor, to be considered in tandem.

Focal Adhesion Kinase Activation Is Necessary for Stretch-Induced Alignment and Enhanced Differentiation of Myogenic Precursor Cells

Myogenic precursors sense and dynamically respond to mechanical stimulation through complex integrin-mediated mechanotransduction, in which focal adhesion kinase (FAK) is a fundamental intracellular signaling mediator. When skeletal myoblasts are exposed to uniaxial cyclic tensile strain (UCTS), they display uniform alignment and an enhanced rate of differentiation. In this work, we explored the role of FAK activation by using C2C12 myoblasts that were grown on flexible culture plates and exposed to UCTS during the early differentiation phase. After 24 h, the cells oriented perpendicularly to the direction of strain and exhibited an enhanced differentiation profile. Next, the cells were exposed to a strain field that was either kept in the same direction or rotated 90°, in the presence or not of an FAK phosphorylation inhibitor. On reorientation of the strain field by 90°, the cells reassembled their focal adhesions and actin cytoskeleton to regain the perpendicular position with respect to the engaging stress. After blocking the FAK, however, the cells failed to respond to the reoriented strain field and their differentiation was abrogated. Interestingly, when the strain field remained in the same direction, the FAK inhibitor compromised the differentiation, even though there was no evident change in cell orientation. Our data indicate that during exposure to UCTS, the activation of FAK is necessary for the myoblasts to undergo alignment and enhanced differentiation.

Deriving vascular smooth muscle cells from mesenchymal stromal cells: Evolving differentiation strategies and current understanding of their mechanisms

Vascular smooth muscle cells (VSMCs) play essential roles in regulating blood vessel form and function. Regeneration of functional vascular smooth muscle tissue to repair vascular diseases is an area of intense research in tissue engineering and regenerative medicine. For functional vascular smooth muscle tissue regeneration to become a practical therapy over the next decade, the field will need to have access to VSMC sources that are effective, robust and safe. While pluripotent stem cells hold good future promise to this end, more immediate translation is expected to come from approaches that generate functional VSMCs from adult sources of multipotent adipose-derived and bone marrow-derived mesenchymal stromal cells (ASCs and BMSCs). The research to this end is extensive and is dominated by studies relating to classical biochemical signalling molecules used to induce differentiation of ASCs and BMSCs. However, prolonged use of the biochemical induction factors is costly and can cause potential endotoxin contamination in the culture. Over recent years several non-traditional differentiation approaches have been devised to mimic defined aspects of the native micro-environment in which VSMCs reside to contribute to the differentiation of VSMC-like cells from ASCs and BMSCs. In this review, the promises and limitations of several non-traditional culture approaches (e.g., co-culture, biomechanical, and biomaterial stimuli) targeting VSMC differentiation are discussed. The extensive crosstalk between the underlying signalling cascades are delineated and put into a translational context. It is expected that this review will not only provide significant insight into VSMC differentiation strategies for vascular smooth muscle tissue engineering applications, but will also highlight the fundamental importance of engineering the cellular microenvironment on multiple scales (with consideration of different combinatorial pathways) in order to direct cell differentiation fate and obtain cells of a desired and stable phenotype. These strategies may ultimately be applied to different sources of stem cells in the future for a range of biomaterial and tissue engineering disciplines.

Codelivery of Infusion Decellularized Skeletal Muscle with Minced Muscle Autografts Improved Recovery from Volumetric Muscle Loss Injury in a Rat Model

Skeletal muscle is capable of robust self-repair following mild trauma, yet in cases of traumatic volumetric muscle loss (VML), where more than 20% of a muscle's mass is lost, this capacity is overwhelmed. Current autogenic whole muscle transfer techniques are imperfect, which has motivated the exploration of implantable scaffolding strategies. In this study, the use of an allogeneic decellularized skeletal muscle (DSM) scaffold with and without the addition of minced muscle (MM) autograft tissue was explored as a repair strategy using a lower-limb VML injury model (n = 8/sample group). We found that the repair of VML injuries using DSM + MM scaffolds significantly increased recovery of peak contractile force (81 ± 3% of normal contralateral muscle) compared to unrepaired VML controls (62 ± 4%). Similar significant improvements were measured for restoration of muscle mass (88 ± 3%) in response to DSM + MM repair compared to unrepaired VML controls (79 ± 3%). Histological findings revealed a marked decrease in collagen dense repair tissue formation both at and away from the implant site for DSM + MM repaired muscles. The addition of MM to DSM significantly increased MyoD expression, compared to isolated DSM treatment (21-fold increase) and unrepaired VML (37-fold) controls. These findings support the further exploration of both DSM and MM as promising strategies for the repair of VML injury.

Development of an infusion bioreactor for the accelerated preparation of decellularized skeletal muscle scaffolds

The implantation of decellularized tissue has shown effectiveness as a strategy for the treatment of volumetric muscle loss (VML) injuries. The preparation of decellularized tissue typically relies on the diffusion driven removal of cellular debris. For bulky tissues like muscle, the process can be lengthy, which introduces opportunities for both tissue contamination and degradation of key ECM molecules. In this study we report on the accelerated preparation of decellularized skeletal muscle (DSM) scaffolds using a infusion system and examine scaffold performance for the repair of VML injuries. The preparation of DSM scaffolds using infusion was dramatically accelerated. As the infusion rate (1% SDS) was increased from 0.1 to 1 and 10ml/hr, the time needed to remove intracellular myoglobin and actin decreased from a maximum of 140 ± 3hrs to 45 ± 3hrs and 10 ± 2hrs respectively. Although infusion appeared to remove cellular debris more aggressively, it did not significantly decrease the collagen or glycosaminoglycan composition of DSM samples when compared to un-infused controls. Infusion prepared DSM samples retained the aligned network structure and mechanical integrity of control samples. Infusion prepared DSM samples supported the attachment and in-vitro proliferation of myoblast cells and was well tolerated by the host when examined in-vivo. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:745-755, 2016.

Scaffold-Free Fabrication of Osteoinductive Cellular Constructs Using Mouse Gingiva-Derived Induced Pluripotent Stem Cells

Three-dimensional (3D) cell constructs are expected to provide osteoinductive materials to develop cell-based therapies for bone regeneration. The proliferation and spontaneous aggregation capability of induced pluripotent stem cells (iPSCs) thus prompted us to fabricate a scaffold-free iPSC construct as a transplantation vehicle. Embryoid bodies of mouse gingival fibroblast-derived iPSCs (GF-iPSCs) were seeded in a cell chamber with a round-bottom well made of a thermoresponsive hydrogel. Collected ball-like cell constructs were cultured in osteogenic induction medium for 30 days with gentle shaking, resulting in significant upregulation of osteogenic marker genes. The constructs consisted of an inner region of unstructured cell mass and an outer osseous tissue region that was surrounded by osteoblast progenitor-like cells. The outer osseous tissue was robustly calcified with elemental calcium and phosphorous as well as hydroxyapatite. Subcutaneous transplantation of the GF-iPSC constructs into immunodeficient mice contributed to extensive ectopic bone formation surrounded by teratoma tissue. These results suggest that mouse GF-iPSCs could facilitate the fabrication of osteoinductive scaffold-free 3D cell constructs, in which the calcified regions and surrounding osteoblasts may function as scaffolds and drivers of osteoinduction, respectively. With incorporation of technologies to inhibit teratoma formation, this system could provide a promising strategy for bone regenerative therapies.

Recent progress in stem cell differentiation directed by material and mechanical cues

Stem cells play essential roles in tissue regeneration in vivo via specific lineage differentiation induced by environmental factors. In the past, biochemical signals were the focus of induced stem cell differentiation. As reported by Engler et al (2006 Cell 126 677-89), biophysical signal mediated stem cell differentiation could also serve as an important inducer. With the advancement of material science, it becomes a possible strategy to generate active biophysical signals for directing stem cell fate through specially designed material microstructures. In the past five years, significant progress has been made in this field, and these designed biophysical signals include material elasticity/rigidity, micropatterned structure, extracellular matrix (ECM) coated materials, material transmitted extracellular mechanical force etc. A large number of investigations involved material directed differentiation of mesenchymal stem cells, neural stem/progenitor cells, adipose derived stem cells, hematopoietic stem/progenitor cells, embryonic stem cells and other cells. Hydrogel based materials were commonly used to create varied mechanical properties via modifying the ratio of different components, crosslinking levels, matrix concentration and conjugation with other components. Among them, polyacrylamide (PAM) and polydimethylsiloxane (PDMS) hydrogels remained the major types of material. Specially designed micropatterning was not only able to create a unique topographical surface to control cell shape, alignment, cell-cell and cell-matrix contact for basic stem cell biology study, but also could be integrated with 3D bioprinting to generate micropattered 3D structure and thus to induce stem cell based tissue regeneration. ECM coating on a specific topographical structure was capable of inducing even more specific and potent stem cell differentiation along with soluble factors and mechanical force. The article overviews the progress of the past five years in this particular field.

Controlled Osteogenic Differentiation of Mouse Mesenchymal Stem Cells by Tetracycline-Controlled Transcriptional Activation of Amelogenin

Regenerative dental therapies for bone tissues rely on efficient targeting of endogenous and transplanted mesenchymal stem cells (MSCs) to guide bone formation. Amelogenin is the primary component of Emdogain, which is used to regenerate periodontal defects; however, the mechanisms underlying the therapeutic effects on alveolar bone remain unclear. The tetracycline (Tet)-dependent transcriptional regulatory system is a good candidate to investigate distinct roles of genes of interest during stem cell differentiation. Here, we investigated amelogenin-dependent regulation of osteogenesis in MSCs by establishing a Tet-controlled transcriptional activation system. Clonal mouse bone marrow-derived MSCs were lentivirally transduced with the Tet repressor (TetR) expression vector followed by drug selection to obtain MSCs constitutively expressing TetR (MSCs-TetR). Expression vectors that contained the Tet operator and amelogenin-coding (Amelx) cDNA fragments were constructed using the Gateway system and lentivirally introduced into MSCs-TetR to generate a Tet regulation system in MSCs (MSCs-TetR/Amelx). MSCs-TetR/Amelx significantly overexpressed the Amelx gene and protein in the presence of the tetracycline derivative doxycycline. Concomitant expression of osterix, bone sialoprotein (BSP), osteopontin, and osteocalcin was modulated by addition or removal of doxycycline under osteogenic guidance. During osteogenic induction, MSCs-TetR/Amelx treated with doxycycline showed significantly increased gene expression of osterix, type I collagen, BSP, and osteocalcin in addition to increased alkaline phosphatase activity and mineralized nodule formation. Enhanced extracellular matrix calcification was observed when forced Amelx expression commenced at the early stage but not at the intermediate or late stages of osteogenesis. These results suggest that a Tet-controlled Amelx gene regulation system for mouse MSCs was successfully established, in which transcriptional activation of Amelx was associated with enhanced osteogenic differentiation, especially in the early stage of biomineralization.

Spontaneous and specific myogenic differentiation of human mesenchymal stem cells on polyethylene glycol-linked multi-walled carbon nanotube films for skeletal muscle engineering

This study explored the influence of polyethylene glycol-linked multi-walled carbon nanotube (PEG-CNT) films on skeletal myogenic differentiation of human mesenchymal stem cells (hMSCs). PEG-CNT films were prepared with nanoscale surface roughness, orderly arrangement of PEG-CNTs, high hydrophilicity and high mechanical strength. Notably, PEG-CNT films alone could direct the skeletal myogenic differentiation of hMSCs in the absence of myogenic induction factors. The quantitative real-time polymerase chain reaction (RT-PCR) showed that the non-induced hMSCs plated on the PEG-CNT films, compared to the negative control, presented significant up-regulation of general myogenic markers including early commitment markers of myoblast differentiation protein-1 (MyoD) and desmin, as well as a late phase marker of myosin heavy chain-2 (MHC). Corresponding protein analysis by immunoblot assays corroborated these results. Skeletal muscle-specific markers, fast skeletal troponin-C (TnC) and ryanodine receptor-1 (Ryr) were also significantly increased in the non-induced hMSCs on PEG-CNT films by RT-PCR. For these cells, the commitment to specific skeletal myoblasts was further proved by the absence of enhanced adipogenic, chondrogenic and osteogenic markers. This study elucidated that PEG-CNT films supported a dedicated differentiation of hMSCs into a skeletal myogenic lineage and can work as a promising material towards skeletal muscle injury repair.

Three dimensional multi-cellular muscle-like tissue engineering in perfusion-based bioreactors

Conventional tissue engineering strategies often rely on the use of a single progenitor cell source to engineer in vitro biological models; however, multi-cellular environments can better resemble the complexity of native tissues. Previous described co-culture models used skeletal myoblasts, as parenchymal cell source, and mesenchymal or endothelial cells, as stromal component. Here, we propose instead the use of adipose tissue-derived stromal vascular fraction cells, which include both mesenchymal and endothelial cells, to better resemble the native stroma. Percentage of serum supplementation is one of the crucial parameters to steer skeletal myoblasts toward either proliferation (20%) or differentiation (5%) in two-dimensional culture conditions. On the contrary, three-dimensional (3D) skeletal myoblast culture often simply adopts the serum content used in monolayer, without taking into account the new cell environment. When considering 3D cultures of mm-thick engineered tissues, homogeneous and sufficient oxygen supply is paramount to avoid formation of necrotic cores. Perfusion-based bioreactor culture can significantly improve the oxygen access to the cells, enhancing the viability and the contractility of the engineered tissues. In this study, we first investigated the influence of different serum supplementations on the skeletal myoblast ability to proliferate and differentiate during 3D perfusion-based culture. We tested percentages of serum promoting monolayer skeletal myoblast-proliferation (20%) and differentiation (5%) and suitable for stromal cell culture (10%) with a view to identify the most suitable condition for the subsequent co-culture. The 10% serum medium composition resulted in the highest number of mature myotubes and construct functionality. Co-culture with stromal vascular fraction cells at 10% serum also supported the skeletal myoblast differentiation and maturation, hence providing a functional engineered 3D muscle model that resembles the native multi-cellular environment.

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.

Skeletal muscle tissue engineering: strategies for volumetric constructs

Skeletal muscle tissue is characterized by high metabolic requirements, defined structure and high regenerative potential. As such, it constitutes an appealing platform for tissue engineering to address volumetric defects, as proven by recent works in this field. Several issues common to all engineered constructs constrain the variety of tissues that can be realized in vitro, principal among them the lack of a vascular system and the absence of reliable cell sources; as it is, the only successful tissue engineering constructs are not characterized by active function, present limited cellular survival at implantation and possess low metabolic requirements. Recently, functionally competent constructs have been engineered, with vascular structures supporting their metabolic requirements. In addition to the use of biochemical cues, physical means, mechanical stimulation and the application of electric tension have proven effective in stimulating the differentiation of cells and the maturation of the constructs; while the use of co-cultures provided fine control of cellular developments through paracrine activity. This review will provide a brief analysis of some of the most promising improvements in the field, with particular attention to the techniques that could prove easily transferable to other branches of tissue engineering.

Role of biomechanical forces in stem cell vascular lineage differentiation

Mechanical forces have long been known to play a role in the maintenance of vascular homeostasis in the mature animal and in developmental regulation in the fetus. More recently, it has been shown that stem cells play a role in vascular repair and remodeling in response to biomechanical stress. Laminar shear stress can directly activate growth factor receptors on stem/progenitor cells, initiating signaling pathways leading toward endothelial cell differentiation. Cyclic strain can stimulate stem cell differentiation toward smooth muscle lineages through different mechanisms. In vivo, blood flow in the coronary artery is significantly altered after stenting, leading to changes in biomechanical forces on the vessel wall. This disruption may activate stem cell differentiation into a variety of cells and cause delayed re-endothelialization. Based on progress in the research field, the present review aims to explore the role of mechanical forces in stem cell differentiation both in vivo and in vitro and to examine what this means for the application of stem cells in the clinic, in tissue engineering, and for the management of aberrant stem cell contribution to disease.

Comparative analysis of mouse-induced pluripotent stem cells and mesenchymal stem cells during osteogenic differentiation in vitro

Induced pluripotent stem cells (iPSCs) can differentiate into mineralizing cells and are, therefore, expected to be useful for bone regenerative medicine; however, the characteristics of iPSC-derived osteogenic cells remain unclear. Here, we provide a direct in vitro comparison of the osteogenic differentiation process in mesenchymal stem cells (MSCs) and iPSCs from adult C57BL/6J mice. After 30 days of culture in osteogenic medium, both MSCs and iPSCs produced robustly mineralized bone nodules that contained abundant calcium phosphate with hydroxyapatite crystal formation. Mineral deposition was significantly higher in iPSC cultures than in MSC cultures. Scanning electron microscopy revealed budding matrix vesicles in early osteogenic iPSCs; subsequently, the vesicles propagated to exhibit robust mineralization without rich fibrous structures. Early osteogenic MSCs showed deposition of many matrix vesicles in abundant collagen fibrils that became solid mineralized structures. Both cell types demonstrated increased expression of osteogenic marker genes, such as runx2, osterix, dlx5, bone sialoprotein (BSP), and osteocalcin, during osteogenesis; however, real-time reverse transcription-polymerase chain reaction array analysis revealed that osteogenesis-related genes encoding mineralization-associated molecules, bone morphogenetic proteins, and extracellular matrix collagens were differentially expressed between iPSCs and MSCs. These data suggest that iPSCs are capable of differentiation into mature osteoblasts whose associated hydroxyapatite has a crystal structure similar to that of MSC-associated hydroxyapatite; however, the transcriptional differences between iPSCs and MSCs could result in differences in the mineral and matrix environments of the bone nodules. Determining the biological mechanisms underlying cell-specific differences in mineralization during in vitro iPSC osteogenesis may facilitate the development of clinically effective engineered bone.

Stem cells in dentistry--Part II: Clinical applications

New technologies that facilitate solid alveolar ridge augmentation are receiving considerable attention in the field of prosthodontics because of the growing requirement for esthetic and functional reconstruction by dental implant treatments. Recently, several studies have demonstrated potential advantages for stem-cell-based therapies in regenerative treatments. Mesenchymal stem/stromal cells (MSCs) are now an excellent candidate for tissue replacement therapies, and tissue engineering approaches and chair-side cellular grafting approaches using autologous MSCs represent the clinical state of the art for stem-cell-based alveolar bone regeneration. Basic studies have revealed that crosstalk between implanted donor cells and recipient immune cells plays a key role in determining clinical success that may involve the recently observed immunomodulatory properties of MSCs. Part II of this review first overviews progress in regenerative dentistry to consider the implications of the stem cell technology in dentistry and then highlights cutting-edge stem-cell-based alveolar bone regenerative therapies. Factors that affect stem-cell-based bone regeneration as related to the local immune response are then discussed. Additionally, pre-clinical stem cell studies for the regeneration of teeth and other oral organs as well as possible applications of MSC-based immunotherapy in dentistry are outlined. Finally, the marketing of stem cell technology in dental stem cell banks with a view toward future regenerative therapies is introduced.