Osteoblastogenesis of Mesenchymal Stem Cells in 3-D Culture Enhanced by Low-Intensity Pulsed Ultrasound through Soluble Receptor Activator of Nuclear Factor Kappa B Ligand

Selective modulation of the bone remodeling regulatory system through orthodontic tooth movement-a review

Little is known about how tissues mediate the ability to selectively form or resorb bone, as required during orthodontic tooth movement (OTM), facial growth, continued tooth eruption and for healing after fractures, maxillofacial surgical repositioning or implant dentistry. OTM has the unique ability to selectively cause apposition, resorption or a combination of both at the alveolar periosteal surface and therefore, provides an optimal process to study the regulation of bone physiology at a tissue level. Our aim was to elucidate the mechanisms and signaling pathways of the bone remodeling regulatory system (BRRS) as well as to investigate its clinical applications in osteoporosis treatment, orthopedic surgery, fracture management and orthodontic treatment. OTM is restricted to a specific range in which the BRRS permits remodeling; however, surpassing this limit may lead to bone dehiscence. Low-intensity pulsed ultrasound, vibration or photobiomodulation with low-level laser therapy have the potential to modify BRRS with the aim of reducing bone dehiscence and apical root resorption or accelerating OTM. Unloading of bone and periodontal compression promotes resorption via receptor activator of nuclear factor κB-ligand, monocyte chemotactic protein-1, parathyroid hormone-related protein (PTHrP), and suppression of anti-resorptive mediators. Furthermore, proinflammatory cytokines, such as interleukin-1 (IL-1), IL-6, IL-8, tumor necrosis factor-α, and prostaglandins exert a synergistic effect on bone resorption. While proinflammatory cytokines are associated with periodontal sequelae such as bone dehiscence and gingival recessions, they are not essential for OTM. Integrins mediate mechanotransduction by converting extracellular biomechanical signals into cellular responses leading to bone apposition. Active Wnt signaling allows β-catenin to translocate into the nucleus and to stimulate bone formation, consequently converging with integrin-mediated mechanotransductive signals. During OTM, periodontal fibroblasts secrete PTHrP, which inhibits sclerostin secretion in neighboring osteocytes via the PTH/PTHrP type 1 receptor interaction. The ensuing sclerostin-depleted region may enhance stem cell differentiation into osteoblasts and subperiosteal osteoid formation. OTM-mediated BRRS modulation suggests that administering sclerostin-inhibiting antibodies in combination with PTHrP may have a synergistic bone-inductive effect. This approach holds promise for enhancing osseous wound healing, treating osteoporosis, bone grafting and addressing orthodontic treatments that are linked to periodontal complications.

Genes involved in osteogenic differentiation induced by low‑intensity pulsed ultrasound in goldfish scales

The teleost scale is a unique calcified tissue that contains osteoclasts, osteoblasts, osteocytes and the bone matrix, similar to mammalian bone. Here, the effects of low-intensity pulsed ultrasound (LIPUS) on osteoblasts and osteoclasts in goldfish scales were investigated. Scales were treated with LIPUS, which is equivalent to use under clinical conditions (30 mW/cm2 for 20 min), then cultured at 15˚C. Alkaline phosphatase activity, a marker of osteoblasts, or tartrate-resistant acid phosphatase (TRAP) activity, a marker of osteoclasts was measured. The gene expression profile was examined using RNA-sequencing. Gene network and biological function analyses were performed using the Ingenuity® Pathways Knowledge Base. A single exposure of LIPUS significantly increased ALP activity but did not affect TRAP activity. These data indicated that LIPUS induced osteoblastic activation in goldfish scales. Using RNA-sequencing, numerous genes that were significantly and differentially expressed 3, 6, and 24 h after LIPUS exposure were observed. Ingenuity® pathway analysis demonstrated that three gene networks, GN-3h, GN-6h, and GN-24h, were obtained from upregulated genes at 3, 6 and 24 h culture, respectively, and included several genes associated with osteoblast differentiation, such as protein kinase D1, prostaglandin-endoperoxide synthase 2, TNFRSF11B (tumor necrosis factor receptor superfamily, member 11b) and WNT3A (Wnt family member 3A). A significant upregulation of expression levels of these genes in scales treated with LIPUS was confirmed by reverse transcription-quantitative polymerase chain reaction. These results contribute to elucidating the molecular mechanisms of osteoblast activation induced by LIPUS.

Ultrasound-generated bubbles enhance osteogenic differentiation of mesenchymal stromal cells in composite collagen hydrogels

Hydrogels can improve the delivery of mesenchymal stromal cells (MSCs) by providing crucial biophysical cues that mimic the extracellular matrix. The differentiation of MSCs is dependent on biophysical cues like stiffness and viscoelasticity, yet conventional hydrogels cannot be dynamically altered after fabrication and implantation to actively direct differentiation. We developed a composite hydrogel, consisting of type I collagen and phase-shift emulsion, where osteogenic differentiation of MSCs can be non-invasively modulated using ultrasound. When exposed to ultrasound, the emulsion within the hydrogel was non-thermally vaporized into bubbles, which locally compacted and stiffened the collagen matrix surrounding each bubble. Bubble growth and matrix compaction were correlated, with collagen regions proximal (i.e., ≤ ∼60 μm) to the bubble displaying a 2.5-fold increase in Young's modulus compared to distal regions (i.e., > ∼60 μm). The viability and proliferation of MSCs, which were encapsulated within the composite hydrogel, were not impacted by bubble formation. In vitro and in vivo studies revealed encapsulated MSCs exhibited significantly elevated levels of RUNX2 and osteocalcin, markers of osteogenic differentiation, in collagen regions proximal to the bubble compared to distal regions. Additionally, alkaline phosphatase activity and calcium deposition were enhanced adjacent to the bubble. An opposite trend was observed for CD90, a marker of MSC stemness. Following subcutaneous implantation, bubbles persisted in the hydrogels for two weeks, which led to localized collagen alignment and increases in nuclear asymmetry. These results are a significant step toward controlling the 3D differentiation of MSCs in a non-invasive and on-demand manner.

Fostering tissue engineering and regenerative medicine to treat musculoskeletal disorders in bone and muscle

The musculoskeletal system, which is vital for movement, support, and protection, can be impaired by disorders such as osteoporosis, osteoarthritis, and muscular dystrophy. This review focuses on the advances in tissue engineering and regenerative medicine, specifically aimed at alleviating these disorders. It explores the roles of cell therapy, particularly Mesenchymal Stem Cells (MSCs) and Adipose-Derived Stem Cells (ADSCs), biomaterials, and biomolecules/external stimulations in fostering bone and muscle regeneration. The current research underscores the potential of MSCs and ADSCs despite the persistent challenges of cell scarcity, inconsistent outcomes, and safety concerns. Moreover, integrating exogenous materials such as scaffolds and external stimuli like electrical stimulation and growth factors shows promise in enhancing musculoskeletal regeneration. This review emphasizes the need for comprehensive studies and adopting innovative techniques together to refine and advance these multi-therapeutic strategies, ultimately benefiting patients with musculoskeletal disorders.

Photobiomodulation and low-intensity pulsed ultrasound synergistically enhance dental mesenchymal stem cells viability, migration and differentiation: an invitro study

Novel methods and technologies that improve mesenchymal stem cells (MSCs) proliferation and differentiation properties are required to increase their clinical efficacy. Photobiomodulation (PBM) and low-intensity pulsed ultrasound (LIPUS) are two strategies that can be used to enhance the regenerative properties of dental MSCs. This study evaluated the cytocompatibility and osteo/odontogenic differentiation of dental pulp, periodontal ligament, and gingival MSCs after stimulation by either PBM or LIPUS and their combined effect. MTT assay, cell migration assay, osteo/odontogenic differentiation by AR staining and ALP activity, and expression of osteo/odontogenic markers (OPG, OC, RUNX2, DSPP, DMP1) by RT-qPCR were evaluated. Statistical analysis was performed using ANOVA, followed by Tukey's post hoc test, with a p-value of less than 0.05 considered significant. The results showed that combined stimulation by PBM and LIPUS resulted in significantly the highest viability of MSCs, the fastest migration, the most dense AR staining, the most increased ALP activity, and the most elevated levels of osteogenic and odontogenic markers. The synergetic stimulation of PBM and LIPUS can be utilized in cell-based regenerative approaches to promote the properties of dental MSCs.

Sonomechanobiology: Vibrational stimulation of cells and its therapeutic implications

All cells possess an innate ability to respond to a range of mechanical stimuli through their complex internal machinery. This comprises various mechanosensory elements that detect these mechanical cues and diverse cytoskeletal structures that transmit the force to different parts of the cell, where they are transcribed into complex transcriptomic and signaling events that determine their response and fate. In contrast to static (or steady) mechanostimuli primarily involving constant-force loading such as compression, tension, and shear (or forces applied at very low oscillatory frequencies (≤ 1 Hz) that essentially render their effects quasi-static), dynamic mechanostimuli comprising more complex vibrational forms (e.g., time-dependent, i.e., periodic, forcing) at higher frequencies are less well understood in comparison. We review the mechanotransductive processes associated with such acoustic forcing, typically at ultrasonic frequencies (> 20 kHz), and discuss the various applications that arise from the cellular responses that are generated, particularly for regenerative therapeutics, such as exosome biogenesis, stem cell differentiation, and endothelial barrier modulation. Finally, we offer perspectives on the possible existence of a universal mechanism that is common across all forms of acoustically driven mechanostimuli that underscores the central role of the cell membrane as the key effector, and calcium as the dominant second messenger, in the mechanotransduction process.

Design, Fabrication, and Characterization of a Multimodal Reconfigurable Bioreactor for Bone Tissue Engineering

In the past decades, bone tissue engineering developed and exploited many typologies of bioreactors, which, besides providing proper culture conditions, aimed at integrating those bio‐physical stimulations that cells experience in vivo, to promote osteogenic differentiation. Nevertheless, the highly challenging combination and deployment of many stimulation systems into a single bioreactor led to the generation of several unimodal bioreactors, investigating one or at mostly two of the required biophysical stimuli. These systems miss the physiological mimicry of bone cells environment, and often produced contrasting results, thus making the knowledge of bone mechanotransduction fragmented and often inconsistent. To overcome this issue, in this study we developed a perfusion and electroactive‐vibrational reconfigurable stimulation bioreactor to investigate the differentiation of SaOS‐2 bone‐derived cells, hosting a piezoelectric nanocomposite membrane as cell culture substrate. This multimodal perfusion bioreactor is designed based on a numerical (finite element) model aimed at assessing the possibility to induce membrane nano‐scaled vibrations (with ~12 nm amplitude at a frequency of 939 kHz) during perfusion (featuring 1.46 dyn cm⁻² wall shear stress), large enough for inducing a physiologically‐relevant electric output (in the order of 10 mV on average) on the membrane surface. This study explored the effects of different stimuli individually, enabling to switch on one stimulation at a time, and then to combine them to induce a faster bone matrix deposition rate. Biological results demonstrate that the multimodal configuration is the most effective in inducing SaOS‐2 cell differentiation, leading to 20‐fold higher collagen deposition compared to static cultures, and to 1.6‐ and 1.2‐fold higher deposition than the perfused‐ or vibrated‐only cultures. These promising results can provide tissue engineering scientists with a comprehensive and biomimetic stimulation platform for a better understanding of mechanotransduction phenomena beyond cells differentiation.

Ultrasound Therapy: Experiences and Perspectives for Regenerative Medicine

Ultrasound has emerged as a novel tool for clinical applications, particularly in the context of regenerative medicine. Due to its unique physico-mechanical properties, low-intensity ultrasound (LIUS) has been approved for accelerated fracture healing and for the treatment of established non-union, but its utility has extended beyond tissue engineering to other fields, including cell regeneration. Cells and tissues respond to acoustic ultrasound by switching on genetic repair circuits, triggering a cascade of molecular signals that promote cell proliferation, adhesion, migration, differentiation, and extracellular matrix production. LIUS also induces angiogenesis and tissue regeneration and has anti-inflammatory and anti-degenerative effects. Accordingly, the potential application of ultrasound for tissue repair/regeneration has been tested in several studies as a stand-alone treatment and, more recently, as an adjunct to cell-based therapies. For example, ultrasound has been proposed to improve stem cell homing to target tissues due to its ability to create a transitional and local gradient of cytokines and chemokines. In this review, we provide an overview of the many applications of ultrasound in clinical medicine, with a focus on its value as an adjunct to cell-based interventions. Finally, we discuss the various preclinical and clinical studies that have investigated the potential of ultrasound for regenerative medicine.

Microchip with Single-Cell Impedance Measurements for Monitoring Osteogenic Differentiation of Mesenchymal Stem Cells under Electrical Stimulation

Effective induction methods and in situ monitoring are essential for studying the mechanism of biological responses in stem cell differentiation. This article proposes an induction method incorporating electrical stimulation under an inhomogeneous field with single-cell impedance monitoring for studying osteogenic differentiation of mesenchymal stem cells (MSCs) using a microchip. The microchip contains an array of sextupole-electrode units for implementing a combination of controllable electrical stimulation and single-cell impedance measurements. MSCs are inducted to osteogenic differentiation under electrical stimulation using quadrupole electrodes and single-cell impedances are monitored in situ using a pair of microelectrodes at each unit center. The proposed microchip adopts an array design to monitor a number of MSCs in parallel, which improves measurement throughput and facilitates to carry out statistic tests. We perform osteogenic differentiation of MSCs on the microchip with and without electrical stimulation meanwhile monitoring single-cell impedance in real time for 21 days. The recorded impedance results show the detailed characteristic change of MSCs at the single-cell level during osteogenic differentiation, which demonstrates a significant difference between the conditions with and without electrical stimulation. The cell morphology and various staining analyses are also used to validate osteogenesis and correlate with the impedance expression. Correlation analysis of the impedance measurement, cell morphology, and various staining assays proves the great acceleration effect of the proposed electrical stimulation on osteogenic differentiation of MSCs. The proposed impedance method can monitor the dynamic process of cell development and study heterogeneity of stem cell differentiation at the single-cell level.

The role of ultrasound in enhancing mesenchymal stromal cell-based therapies

Mesenchymal stromal cells (MSCs) have been a popular platform for cell-based therapy in regenerative medicine due to their propensity to home to damaged tissue and act as a repository of regenerative molecules that can promote tissue repair and exert immunomodulatory effects. Accordingly, a great deal of research has gone into optimizing MSC homing and increasing their secretion of therapeutic molecules. A variety of methods have been used to these ends, but one emerging technique gaining significant interest is the use of ultrasound. Sound waves exert mechanical pressure on cells, activating mechano-transduction pathways and altering gene expression. Ultrasound has been applied both to cultured MSCs to modulate self-renewal and differentiation, and to tissues-of-interest to make them a more attractive target for MSC homing. Here, we review the various applications of ultrasound to MSC-based therapies, including low-intensity pulsed ultrasound, pulsed focused ultrasound, and extracorporeal shockwave therapy, as well as the use of adjunctive therapies such as microbubbles. At a molecular level, it seems that ultrasound transiently generates a local gradient of cytokines, growth factors, and adhesion molecules that facilitate MSC homing. However, the molecular mechanisms underlying these methods are far from fully elucidated and may differ depending on the ultrasound parameters. We thus put forth minimal criteria for ultrasound parameter reporting, in order to ensure reproducibility of studies in the field. A deeper understanding of these mechanisms will enhance our ability to optimize this promising therapy to assist MSC-based approaches in regenerative medicine.

Effect of Low Intensity Pulsed Ultrasound (LIPUS) on Tooth Movement and Root Resorption: A Prospective Multi-Center Randomized Controlled Trial

The aim of this study was to evaluate the possible effect of low intensity pulsed ultrasound (LIPUS) on tooth movement and root resorption in orthodontic patients. Twenty-one patients were included in a split-mouth study design (group 1). Ten additional patients were included with no LIPUS device being used and this group was used as the negative control group (group 2). Group 1 patients were given LIPUS devices that were randomly assigned to right or left side on upper or lower arches. LIPUS was applied to the assigned side that was obtained by randomization, using transducers that produce ultrasound with a pulse frequency of 1.5 MHz, a pulse repetition rate of 1 kHz, and average output intensity of 30 mW/cm². Cone-beam computed tomography (CBCT) images were taken before and after treatment. The extraction space dimensions were measured every four weeks and root lengths of canines were measured before and after treatment. The data were analyzed using paired t-test. The study outcome showed that the mean rate of tooth movement in LIPUS side was 0.266 ± 0.092 mm/week and on the control side was 0.232 ± 0.085 mm/week and the difference was statistically significant. LIPUS increased the rate of tooth movement by an average of 29%. For orthodontic root resorption, the LIPUS side (0.0092 ± 0.022 mm/week) showed a statistically significant decrease as compared to control side (0.0223 ± 0.022 mm/week). The LIPUS application accelerated tooth movement and minimized orthodontically induced tooth root resorption at the same time.

A microelectrode array chip for osteogenic differentiation of mesenchymal stem cells under electrical stimulation

Electrical stimulation (ES) as an easy and effective inducing method has been widely used in induction differentiation of stem cells, e.g. osteogenic differentiation of mesenchymal stem cells (MSCs) for bone healing and bone tissue therapies. However, the micro-effect of an inhomogeneous electric field has rarely been investigated for ES in induction differentiation, and conventionally used ex situ assays may preclude accurate assessment due to variation from cell inoculation and treatments. Here, a novel electrical stimulation method with a microelectrode array chip is proposed for osteogenic differentiation of MSCs. The electric field applied onto the MSCs by the microelectrode array is designed similarly with a natural aggregation distribution of differentiated MSCs. The proposed ES method accelerates osteoblast proliferation and differentiation in the electrode array region and generates a larger amount of mineralized deposits, which are assayed via in situ alizarin red staining and morphology observation as well as immunocytochemistry. In addition, this method allows a direct in situ assessment to compare the osteogenic differentiation of MSCs with and without ES on a single chip to avoid culture environment difference. The method provides a fundamental platform for investigating induced differentiation of stem cells and allows integration with multifunctional cell assays to achieve in situ tracking for the differentiation process of stem cells.

Impact of Ultrasound Therapy on Stem Cell Differentiation - A Systematic Review

OBJECTIVE

This is a systematic review of the effects of low-intensity pulsed ultrasound (LIPUS) on stem cell differentiation.

BACKGROUND DATA

Recent studies have investigated several types of stem cells from different sources in the body. These stem cells should strictly be certified and promoted for cell therapies before being used in medical applications. LIPUS has been used extensively in treatment centers and in research to promote stem cell differentiation, function, and proliferation.

MATERIALS AND METHODS

The databases of PubMed, Google Scholar, and Scopus were searched for abstracts and full-text scientific papers published from 1989-2019 that reported the application of LIPUS on stem cell differentiation. Related English language articles were found using the following defined keywords: low-intensity pulsed ultrasound, stem cell, differentiation. Criteria for inclusion in the review were: LIPUS with frequencies of 1-3 MHz and pulsed ultrasound intensity of <500 mW/cm2. Duration, exposure time, and cell sources were taken into consideration.

RESULTS

Fifty-two articles were selected based on the inclusion criteria. Most articles demonstrated that the application of LIPUS had positive effects on stem cell differentiation. However, some authors recommended that LIPUS combined with other physical therapy aides was more effective in stem cell differentiation.

CONCLUSION

LIPUS significantly increases the level of stem cell differentiation in cells derived mainly from bone marrow mesenchymal stem cells. There is a need for further studies to analyze the effect of LIPUS on cells derived from other sources, particularly adipose tissue-derived mesenchymal stem cells, for treating hard diseases, such as osteoporosis and diabetic foot ulcer. Due to a lack of reporting on standard LIPUS parameters in the field, more experiments comparing the protocols for standardization of LIPUS parameters are needed to establish the best protocol, which would allow for the best results.

Dual-Frequency Ultrasound Induces Neural Stem/Progenitor Cell Differentiation and Growth Factor Utilization by Enhancing Stable Cavitation

Neural stem/progenitor cells (NSPCs) have the potential to serve as the basic materials for treating severe neural diseases and injuries. Ultrasound exposure is an effective therapy for nonunion fractures and healing fresh wounds through an easy and noninvasive application. According to the results of our preliminary study, low-intensity ultrasound (LIUS) promotes the attachment and differentiation of NSPCs. However, the parameters of and mechanisms by which LIUS induces NSPC differentiation remain unclear. To the best of our knowledge, no published studies have reported and compared the biological effects of dual-frequency and single-frequency LIUS on NSPCs. The purpose of this study is to systematically compare several LIUS parameters, including single-frequency, single-transducer dual-frequency ultrasound, burst, and continuous cycling stimulation at several intensities. Furthermore, synergistic effects of single-/dual-frequency LIUS combined with neural growth factor addition on NSPCs were also evaluated. Based on the results of the cytotoxicity assay, low-intensity (40 kPa) ultrasound does not damage NSPCs compared with that observed in the control group. The morphology and immunostaining results show that all experimental groups exposed to ultrasound exhibit neurite outgrowth and NSPC differentiation. In particular, dual-frequency ultrasound promotes NSPCs differentiation to a greater extent than single-frequency ultrasound. In addition, more complicated and denser neural networks are observed in the dual-frequency group. Neural growth factor addition increased the percentage of neurons formed, particularly in the groups stimulated with ultrasound. Among these groups, the dual-frequency group exhibited significant differences in the percentage of differentiated neurons compared with the single-frequency group. This study may the first to prove that dual-frequency LIUS exposure further enhances NSPC differentiation and the utilization of growth factors than single-frequency LIUS. Moreover, the result also revealed that dual-frequency ultrasound generated higher calcium ion influx and extended the channel opening time. A potential explanation is that dual-frequency ultrasound generates more stable cavitation than single-frequency LIUS, which may stimulate cell membrane mechanochannels and enhance calcium ion influx but does not damage them. This in vitro study may serve as a useful alternative for ultrasound therapy.

Effect of Low-Intensity Pulsed Ultrasound after Mesenchymal Stromal Cell Injection to Treat Osteochondral Defects: An In Vivo Study

We investigated the effect of low-intensity pulsed ultrasound (LIPUS) treatment combined with mesenchymal stromal cell (MSC) injection for cartilage repair and subchondral bone reconstitution for treatment of osteochondral defects. An osteochondral defect was created on both femur grooves of Wistar rats. Four weeks later, bone marrow MSCs were injected into the right knee joint. The rats were divided into two intervention groups: without or with LIPUS irradiation. Cartilage repair was evaluated histologically based on the Wakitani cartilage repair score. Subchondral bone reconstitution was evaluated as bone volume (BV)/tissue volume (TV) by micro-computed tomography analysis. MSC injection improved the cartilage repair score, and LIPUS irradiation improved BV/TV. Combination treatment promoted both cartilage repair and BV/TV improvement. Thus, MSC injection combined with LIPUS irradiation is more effective than either treatment alone in promoting concurrent cartilage repair and subchondral reconstitution.

A newly identified mechanism involved in regulation of human mesenchymal stem cells by fibrous substrate stiffness

Stiffness of biomaterial substrates plays a critical role in regulation of cell behavior. Although the effect of substrate stiffness on cell behavior has been extensively studied, molecular mechanisms of regulation rather than those involving cytoskeletal activities still remain elusive. In this study, we fabricated aligned ultrafine fibers and treated the fiber with different annealing temperatures to produce fibrous substrates with different stiffness. Human mesenchymal stem cells (hMSCs) were then cultured on these fibrous substrates. Our results showed that annealing treatment did not change the diameter of electrospun fibers but increased their polymer crystallinity and mechanical properties. The mRNA expression of RUNX2 was upregulated while the mRNA expression of scleraxis was downregulated in response to an increase in substrate stiffness, suggesting that increased stiffness favorably drives hMSCs into the osteogenic lineage. With subsequent induction of osteogenic differentiation, osteogenesis of hMSCs on stiffer substrates was increased compared to that of the cells on control substrates. Cells on stiffer substrates increasingly activated AKT and YAP and upregulated transcript expression of YAP target genes compared to those on control substrates, and inhibition of AKT led to decreased expression of YAP and RUNX2. Furthermore, macrophage migration inhibitory factor (MIF) was increasingly produced by the cell on stiffer substrates, and knocking down MIF by siRNA resulted in decreased AKT phosphorylation. Taken together, we hereby demonstrate that simply using the annealing approach can manipulate stiffness of an aligned fibrous substrate without altering the material chemistry, and substrate stiffness dictates hMSC differentiation through the MIF-mediated AKT/YAP/RUNX2 pathway.

STATEMENT OF SIGNIFICANCE

Stiffness of biomaterial substrates plays a critical role in regulation of cell behavior. Although the effect of substrate stiffness on cell behavior has been extensively studied, molecular mechanisms of regulation rather than those involving cytoskeletal activities still remain elusive. In this manuscript, we report our new findings that simply using the annealing approach can manipulate stiffness of an aligned fibrous substrate without altering the material chemistry, and substrate stiffness dictates human mesenchymal stem cell (hMSC) differentiation through the macrophage migration inhibitory factor-mediated AKT/YAP/RUNX2 pathway. The findings are novel and interesting because we have identified a new mechanism rather than those involving cytoskeleton activity, by which substrate stiffness regulates hMSC behavior.