Cell-matrix interactions regulate mesenchymal stem cell response to hydrostatic pressure

Variations in fluid chemical potential induce fibroblast mechano-response in 3D hydrogels

Mechanical deformation of skin creates variations in fluid chemical potential, leading to local changes in hydrostatic and osmotic pressure, whose effects on mechanobiology remain poorly understood. To study these effects, we investigate the specific influences of hydrostatic and osmotic pressure on primary human dermal fibroblasts in three-dimensional hydrogel culture models. Cyclic hydrostatic pressure and hyperosmotic stress enhanced the percentage of cells expressing the proliferation marker Ki67 in both collagen and PEG-based hydrogels. Osmotic pressure also activated the p38 MAPK stress response pathway and increased the expression of the osmoresponsive genes PRSS35 and NFAT5. When cells were cultured in two-dimension (2D), no change in proliferation was observed with either hydrostatic or osmotic pressure. Furthermore, basal, and osmotic pressure-induced expression of osmoresponsive genes differed in 2D culture versus 3D hydrogels, highlighting the role of dimensionality in skin cell mechanotransduction and stressing the importance of 3D tissue-like models that better replicate in vivo conditions. Overall, these results indicate that fluid chemical potential changes affect dermal fibroblast mechanobiology, which has implications for skin function and for tissue regeneration strategies.

Mechanical loading and orthobiologic therapies in the treatment of post-traumatic osteoarthritis (PTOA): a comprehensive review

The importance of mechanical loading and its relationship to orthobiologic therapies in the treatment of post-traumatic osteoarthritis (PTOA) is beginning to receive attention. This review explores the current efficacy of orthobiologic interventions, notably platelet-rich plasma (PRP), bone marrow aspirate (BMA), and mesenchymal stem/stromal cells (MSCs), in combating PTOA drawing from a comprehensive review of both preclinical animal models and human clinical studies. This review suggests why mechanical joint loading, such as running, might improve outcomes in PTOA management in conjunction with orthiobiologic administration. Accumulating evidence underscores the influence of mechanical loading on chondrocyte behavior and its pivotal role in PTOA pathogenesis. Dynamic loading has been identified as a key factor for optimal articular cartilage (AC) health and function, offering the potential to slow down or even reverse PTOA progression. We hypothesize that integrating the activation of mechanotransduction pathways with orthobiologic treatment strategies may hold a key to mitigating or even preventing PTOA development. Specific loading patterns incorporating exercise and physical activity for optimal joint health remain to be defined, particularly in the clinical setting following joint trauma.

Biomaterials regulates BMSCs differentiation via mechanical microenvironment

Bone mesenchymal stem cells (BMSCs) are crucial for bone tissue regeneration, the mechanical microenvironment of hard tissues, including bone and teeth, significantly affects the osteogenic differentiation of BMSCs. Biomaterials may mimic the microenvironment of the extracellular matrix and provide mechanical signals to regulate BMSCs differentiation via inducing the secretion of various intracellular factors. Biomaterials direct the differentiation of BMSCs via mechanical signals, including tension, compression, shear, hydrostatic pressure, stiffness, elasticity, and viscoelasticity, which can be transmitted to cells through mechanical signalling pathways. Besides, biomaterials with piezoelectric effects regulate BMSCs differentiation via indirect mechanical signals, such as, electronic signals, which are transformed from mechanical stimuli by piezoelectric biomaterials. Mechanical stimulation facilitates achieving vectored stem cell fate regulation, while understanding the underlying mechanisms remains challenging. Herein, this review summarizes the intracellular factors, including translation factors, epigenetic modifications, and miRNA level, as well as the extracellular factor, including direct and indirect mechanical signals, which regulate the osteogenic differentiation of BMSCs. Besides, this review will also give a comprehensive summary about how mechanical stimuli regulate cellular behaviours, as well as how biomaterials promote the osteogenic differentiation of BMSCs via mechanical microenvironments. The cellular behaviours and activated signal pathways will give more implications for the design of biomaterials with superior properties for bone tissue engineering. Moreover, it will also provide inspiration for the construction of bone organoids which is a useful tool for mimicking in vivo bone tissue microenvironments.

Effect of high cyclic hydrostatic pressure on osteogenesis of mesenchymal stem cells cultured in liquefied micro-compartments

Bone resident cells are constantly subjected to a range of distinct mechanical loadings, which generates a complex microenvironment. In particular, hydrostatic pressure (HP) has a key impact on modulation of cell function and fate determination. Although HP is a constant mechanical stimulus, its role in regulating the osteogenesis process within a defined 3D microenvironment has not been comprehensively elucidated. Perceiving how environmental factors regulate the differentiation of stem cells is essential for expanding their regenerative potential. Inspired by the mechanical environment of bone, this study attempted to investigate the influence of different ranges of cyclic HP on human adipose-derived mesenchymal stem cells (MSCs) encapsulated within a compartmentalized liquefied microenvironment. Taking advantage of the liquefied environment of microcapsules, MSCs were exposed to cyclic HP of 5 or 50 MPa, 3 times/week at 37 °C. Biological tests using fluorescence staining of F-actin filaments showed a noticeable improvement in cell-cell interactions and cellular network formation of MSCs. These observations were more pronounced in osteogenic (OST) condition, as confirmed by fluorescent staining of vinculin. More interestingly, there was a significant increase in alkaline phosphatase activity of MSCs exposed to 50 MPa magnitude of HP, even in the absence of osteoinductive factors. In addition, a greater staining area of both osteopontin and hydroxyapatite was detected in the 50 MPa/OST group. These findings highlight the benefit of hydrostatic pressure to regulate osteogenesis of MSCs as well as the importance of employing simultaneous biochemical and mechanical stimulation to accelerate the osteogenic potential of MSCs for biomedical purposes.

Stimulating factors for regulation of osteogenic and chondrogenic differentiation of mesenchymal stem cells

Mesenchymal stem cells (MSCs), distributed in many tissues in the human body, are multipotent cells capable of differentiating in specific directions. It is usually considered that the differentiation process of MSCs depends on specialized external stimulating factors, including cell signaling pathways, cytokines, and other physical stimuli. Recent findings have revealed other underrated roles in the differentiation process of MSCs, such as material morphology and exosomes. Although relevant achievements have substantially advanced the applicability of MSCs, some of these regulatory mechanisms still need to be better understood. Moreover, limitations such as long-term survival in vivo hinder the clinical application of MSCs therapy. This review article summarizes current knowledge regarding the differentiation patterns of MSCs under specific stimulating factors.

An assessment of the response of human MSCs to hydrostatic pressure in environments supportive of differential chondrogenesis

Mechanical stimulation can modulate the chondrogenic differentiation of stem/progenitor cells and potentially benefit tissue engineering (TE) of functional articular cartilage (AC). Mechanical cues like hydrostatic pressure (HP) are often applied to cell-laden scaffolds, with little optimization of other key parameters (e.g. cell density, biomaterial properties) known to effect lineage commitment. In this study, we first sought to establish cell seeding densities and fibrin concentrations supportive of robust chondrogenesis of human mesenchymal stem cells (hMSCs). High cell densities (15*10⁶ cells/ml) were more supportive of sGAG deposition on a per cell basis, while collagen deposition was higher at lower seeding densities (5*10⁶ cells/ml). Employment of lower fibrin (2.5 %) concentration hydrogels supported more robust chondrogenesis of hMSCs, with higher collagen type II and lower collagen type X deposition compared to 5 % hydrogels. The application of HP to hMSCs maintained in identified chondro-inductive culture conditions had little effect on overall levels of cartilage-specific matrix production. However, if hMSCs were first temporally primed with TGF-β3 before its withdrawal, they responded to HP by increased sGAG production. The response to HP in higher cell density cultures was also associated with a metabolic shift towards glycolysis, which has been linked with a mature chondrocyte-like phenotype. These results suggest that mechanical stimulation may not be necessary to engineer functional AC grafts using hMSCs if other culture conditions have been optimised. However, such bioreactor systems can potentially be employed to better understand how engineered tissues respond to mechanical loading in vivo once removed from in vitro culture environments.

Bioprinting-associated pulsatile hydrostatic pressure elicits a mild proinflammatory response in epi- and endothelial cells

During nozzle-based bioprinting, like inkjet and microextrusion, cells are subjected to hydrostatic pressure for up to several minutes. The modality of the bioprinting-related hydrostatic pressure is either constant or pulsatile depending on the technique. We hypothesized that the difference in the modality of hydrostatic pressure affects the biological response of the processed cells differently. To test this, we used a custom-made setup to apply either controlled constant or pulsatile hydrostatic pressure on endothelial and epithelial cells. Neither bioprinting procedure visibly altered the distribution of selected cytoskeletal filaments, cell-substrate adhesions, and cell-cell contacts in either cell type. In addition, pulsatile hydrostatic pressure led to an immediate increase of intracellular ATP in both cell types. However, the bioprinting-associated hydrostatic pressure triggered a pro-inflammatory response in only the endothelial cells, with an increase of interleukin 8 (IL-8) and a decrease of thrombomodulin (THBD) transcripts. These findings demonstrate that the settings adopted during nozzle-based bioprinting cause hydrostatic pressure that can trigger a pro-inflammatory response in different barrier-forming cell types. This response is cell-type and pressure-modality dependent. The immediate interaction of the printed cells with native tissue and the immune system in vivo might potentially trigger a cascade of events. Our findings, therefore, are of major relevance in particular for novel intra-operative, multicellular bioprinting approaches.

MECHANOBIOLOGICAL APPROACHES FOR STIMULATING CHONDROGENESIS OF STEM CELLS

Chondrogenesis is the process of differentiation of stem cells into mature chondrocytes. Such a process consists of chemical, functional, and structural changes which are initiated and mediated by the host environment of the cells. To date, the mechanobiology of chondrogenesis has not been fully elucidated. Hence, experimental activity is focused on recreating specific environmental conditions for stimulating chondrogenesis, and to look for a mechanistic interpretation of the mechanobiological response of cells in the cartilaginous tissues. There are a large number of studies on the topic that vary considerably in their experimental protocols used for providing environmental cues to cells for differentiation, making generalizable conclusions difficult to ascertain. The main objective of this contribution is to review the mechanobiological stimulation of stem cell chondrogenesis and methodological approaches utilized to date to promote chondrogenesis of stem cells in-vitro. In-vivo models will also be explored, but this area is currently limited. An overview of the experimental approaches used by different research groups may help the development of unified testing methods that could be used to overcome existing knowledge gaps, leading to an accelerated translation of experimental findings to clinical practice.

From Vial to Vein: Crucial Gaps in Mesenchymal Stromal Cell Clinical Trial Reporting

Retrospective analysis of clinical trial outcomes is a vital exercise to facilitate efficient translation of cellular therapies. These analyses are particularly important for mesenchymal stem/stromal cell (MSC) products. The exquisite responsiveness of MSCs, which makes them attractive candidates for immunotherapies, is a double-edged sword; MSC clinical trials result in inconsistent outcomes that may correlate with underlying patient biology or procedural differences at trial sites. Here we review 45 North American MSC clinical trial results published between 2015 and 2021 to assess whether these reports provide sufficient information for retrospective analysis. Trial reports routinely specify the MSC tissue source, autologous or allogeneic origin and administration route. However, most methodological aspects related to cell preparation and handling immediately prior to administration are under-reported. Clinical trial reports inconsistently provide information about cryopreservation media composition, delivery vehicle, post-thaw time and storage until administration, duration of infusion, and pre-administration viability or potency assessments. In addition, there appears to be significant variability in how cell products are formulated, handled or assessed between trials. The apparent gaps in reporting, combined with high process variability, are not sufficient for retrospective analyses that could potentially identify optimal cell preparation and handling protocols that correlate with successful intra- and inter-trial outcomes. The substantial preclinical data demonstrating that cell handling affects MSC potency highlights the need for more comprehensive clinical trial reporting of MSC conditions from expansion through delivery to support development of globally standardized protocols to efficiently advance MSCs as commercial products.

“Musical dish” efficiently induces osteogenic differentiation of mesenchymal stem cells through music derived micro‐stretch with variable frequency

Nonuniform microstretching (NUMS) naturally occurs in real bone tissues in vivo, but its profound effects have not been identified yet. In order to explore the biological effects of NUMS and static stretch (uniform stretch [US]) on cells, a new “musical dish” device was developed. Musical signal was used to provide NUMS to cells. More stress fibers, arranging along the long axis of cells, were formed throughout the cells under NUMS, compared with US and untreated control group, although cell morphology did not show any alteration. Whole transcriptome sequencing revealed enhanced osteogenic differentiation of cells after NUMS treatment. Cells in the NUMS group showed a higher expression of bone‐related genes, while genes related to stemness and other lineages were down‐regulated. Our results give insights into the biological effects of NUMS and US on stem cell osteogenic differentiation, suggesting beneficial effects of micromechanical stimulus for osteogenesis. The newly developed device provides a basis for the development of NUMS derived rehabilitation technology to promote bone healing.

Mechanosignalling in cartilage: an emerging target for the treatment of osteoarthritis

Mechanical stimuli have fundamental roles in articular cartilage during health and disease. Chondrocytes respond to the physical properties of the cartilage extracellular matrix (ECM) and the mechanical forces exerted on them during joint loading. In osteoarthritis (OA), catabolic processes degrade the functional ECM and the composition and viscoelastic properties of the ECM produced by chondrocytes are altered. The abnormal loading environment created by these alterations propagates cell dysfunction and inflammation. Chondrocytes sense their physical environment via an array of mechanosensitive receptors and channels that activate a complex network of downstream signalling pathways to regulate several cell processes central to OA pathology. Advances in understanding the complex roles of specific mechanosignalling mechanisms in healthy and OA cartilage have highlighted molecular processes that can be therapeutically targeted to interrupt pathological feedback loops. The potential for combining these mechanosignalling targets with the rapidly expanding field of smart mechanoresponsive biomaterials and delivery systems is an emerging paradigm in OA treatment. The continued advances in this field have the potential to enable restoration of healthy mechanical microenvironments and signalling through the development of precision therapeutics, mechanoregulated biomaterials and drug systems in the near future.

Integrins in the Regulation of Mesenchymal Stem Cell Differentiation by Mechanical Signals

Mesenchymal stem cells (MSCs) can sense and convert mechanical stimuli signals into a chemical response. Integrins are involved in the mechanotransduction from inside to outside and from outside to inside, and ultimately affect the fate of MSCs responding to different mechanical signals. Different integrins participate in different signaling pathways to regulate MSCs multi-differentiation. In this review, we summarize the latest advances in the effects of mechanical signals on the differentiation of MSCs, the importance of integrins in mechanotransduction, the relationship between integrin heterodimers and different mechanical signals, and the interaction among mechanical signals. We put forward our views on the prospect and challenges of developing mechanical biology in tissue engineering and regenerative medicine.

Remote-Controlled 3D Porous Magnetic Interface toward High-Throughput Dynamic 3D Cell Culture

Mechanical stimuli have been shown to play a large role in cellular behavior, including cellular growth, differentiation, morphology, homeostasis, and disease. Therefore, developing bioreactor systems that can create complex mechanical environments for both tissue engineering and disease modeling drug screening is appealing. However, many of existing systems are restricted because of their bulky size with external force generators, destructive microenvironment control, and low throughput. These shortcomings have preceded to the utilization of magnetic stimuli responsive materials, given their untethered, fast, and tunable actuation potential at both the microscale and macroscale level, for seamless integration into cell culture wells and microfluidic systems. Nevertheless, magnetic soft materials for cell culture have been limited due to the inability to develop well-defined 3D structures for more complex and physiological relevant mechanical actuation. Herein, we introduce a facile fabrication process to develop magnetic-PDMS (polydimethylsiloxane) porous composite designs with both well-defined and controllable microlevel and macrolevel features to dynamically manipulate 3D cell-laden gel at the scale. The intrinsic stiffness of the magnetic-PDMS porous composites is also modulated to control the deformation potential to mimic physiological relevant strain levels, with 2.89-11% observed in magnetic actuation studies. High cell viability was achieved with the culturing of both human adipose stem cells (hADMSCs) and human umbilical cord mesenchymal stem cells (hUCMSCs) in 3D cell-laden gel interfaced with the magnetic-PDMS porous composite. Also, the highly interconnected porous network of the magnetic-PDMS composites facilitated free diffusion throughout the porous structure showcasing the potential of a multisurface contact 3D porous magnetic structure in both reservoir and 96-well plate insert designs for more complex dynamic mechanical actuation. In conclusion, these studies provide a means for establishing a biocompatible, tunable magnetic-PDMS porous composite with fast and programmable dynamic strain potential making it a suitable platform for high-throughput, dynamic 3D cell culture.

Bioprintable Lung Extracellular Matrix Hydrogel Scaffolds for 3D Culture of Mesenchymal Stromal Cells

Mesenchymal stromal cell (MSC)-based cell therapy in acute respiratory diseases is based on MSC secretion of paracrine factors. Several strategies have proposed to improve this are being explored including pre-conditioning the MSCs prior to administration. We here propose a strategy for improving the therapeutic efficacy of MSCs based on cell preconditioning by growing them in native extracellular matrix (ECM) derived from the lung. To this end, a bioink with tunable stiffness based on decellularized porcine lung ECM hydrogels was developed and characterized. The bioink was suitable for 3D culturing of lung-resident MSCs without the need for additional chemical or physical crosslinking. MSCs showed good viability, and contraction assays showed the existence of cell–matrix interactions in the bioprinted scaffolds. Adhesion capacity and length of the focal adhesions formed were increased for the cells cultured within the lung hydrogel scaffolds. Also, there was more than a 20-fold increase of the expression of the CXCR4 receptor in the 3D-cultured cells compared to the cells cultured in plastic. Secretion of cytokines when cultured in an in vitro model of lung injury showed a decreased secretion of pro-inflammatory mediators for the cells cultured in the 3D scaffolds. Moreover, the morphology of the harvested cells was markedly different with respect to conventionally (2D) cultured MSCs. In conclusion, the developed bioink can be used to bioprint structures aimed to improve preconditioning MSCs for therapeutic purposes.

Hydrogels for Large-Scale Expansion of Stem Cells

Stem cells demonstrate considerable promise for various preclinical and clinical applications, including drug screening, disease treatments, and regenerative medicine. Producing high-quality and large amounts of stem cells is in demand for these applications. Despite challenges, as hydrogel-based cell culture technology has developed, tremendous progress has been made in stem cell expansion and directed differentiation. Hydrogels are soft materials with abundant water. Many hydrogel properties, including biodegradability, mechanical strength, and porosity, have been shown to play essential roles in regulating stem cell proliferation and differentiation. The biochemical and physical properties of hydrogels can be specifically tailored to mimic the native microenvironment that various stem cells reside in vivo. A few hydrogel-based systems have been developed for successful stem cell cultures and expansion in vitro. In this review, we summarize various types of hydrogels that have been designed to effectively enhance the proliferation of hematopoietic stem cells (HSCs), mesenchymal stem/stromal cells (MSCs), and pluripotent stem cells (PSCs), respectively. According to each stem cell type's preference, we also discuss strategies for fabricating hydrogels with biochemical and mechanical cues and other characteristics representing microenvironments of stem cells in vivo. STATEMENT OF SIGNIFICANCE: In this review article we summarize current progress on the construction of hydrogel systems for the culture and expansion of various stem cells, including hematopoietic stem cells (HSCs), mesenchymal stem/stromal cells (MSCs), and pluripotent stem cells (PSCs). The Significance includes: (1) Provide detailed discussion on the stem cell niches that should be considered for stem cell in vitro expansion. (2) Summarize various strategies to construct hydrogels that can largely recapture the microenvironment of native stem cells. (3) Suggest a few future directions that can be implemented to improve current in vitro stem cell expansion systems.

Hydrostatic Pressure Regulates the Volume, Aggregation and Chondrogenic Differentiation of Bone Marrow Derived Stromal Cells

The limited ability of articular cartilage to self-repair has motivated the development of tissue engineering strategies that aim to harness the regenerative potential of mesenchymal stem/marrow stromal cells (MSCs). Understanding how environmental factors regulate the phenotype of MSCs will be central to unlocking their regenerative potential. The biophysical environment is known to regulate the phenotype of stem cells, with factors such as substrate stiffness and externally applied mechanical loads known to regulate chondrogenesis of MSCs. In particular, hydrostatic pressure (HP) has been shown to play a key role in the development and maintenance of articular cartilage. Using a collagen-alginate interpenetrating network (IPN) hydrogel as a model system to tune matrix stiffness, this study sought to investigate how HP and substrate stiffness interact to regulate chondrogenesis of MSCs. If applied during early chondrogenesis in soft IPN hydrogels, HP was found to downregulate the expression of ACAN, COL2, CDH2 and COLX, but to increase the expression of the osteogenic factors RUNX2 and COL1. This correlated with a reduction in SMAD 2/3, HDAC4 nuclear localization and the expression of NCAD. It was also associated with a reduction in cell volume, an increase in the average distance between MSCs in the hydrogels and a decrease in their tendency to form aggregates. In contrast, the delayed application of HP to MSCs grown in soft hydrogels was associated with increased cellular volume and aggregation and the maintenance of a chondrogenic phenotype. Together these findings demonstrate how tailoring the stiffness and the timing of HP exposure can be leveraged to regulate chondrogenesis of MSCs and opens alternative avenues for developmentally inspired strategies for cartilage tissue regeneration.

Predicting Bone Formation in Mesenchymal Stromal Cell-Seeded Hydrogels Using Experiment-Based Mathematical Modeling

In vitro bone formation by mesenchymal stromal cells encapsulated in type-1 collagen hydrogels is demonstrated after a 28-day in vitro culture period. Analysis of the hydrogels is carried out by X-ray microcomputed tomography, histology, and immunohistochemistry, which collectively demonstrates that bone formation in the hydrogels was quantifiably proportional to the initial collagen concentration, and subsequently the population density of seeded cells. This was established by varying the initial collagen concentration at a constant cell seeding density (3 × 10⁵ cells/0.3 mL hydrogel), and separately varying cell seeding density at a constant collagen concentration (1 mg/mL). Using these data, a mathematical model is presented for the total hydrogel volume and mineralization volume based on the observed linear contraction dynamics of cell-seeded collagen gels. The model parameters are fitted by comparing the predictions of the mathematical model for the hydrogel and mineralized volumes on day 28 with the experimental data. The model is then used to predict the hydrogel and mineralization volumes for a range of hydrogel collagen concentrations and cell seeding densities, providing comprehensive input/output descriptors for generating mineralized hydrogels for bone tissue engineering. It is proposed that this quantitative approach will be a useful tool for generating in vitro manufactured bone tissue, defining input parameters that yield predictable output measures of tissue maturation. Impact statement This article describes a simple yet powerful quantitative description of in vitro tissue-engineered bone by combining experimental data with mathematical modeling. The overall aim of the article is to examine what is currently known about cell-mediated collagen contraction, and demonstrate that this phenomenon can be exploited to tailor bone formation by choosing a specific set of input parameters in the form of cell seeding density and collagen hydrogel concentration. Our study utilizes a clinically relevant cell source (human mesenchymal stem cells) with a biomaterial that has received regulatory approval for use in humans (collagen type 1), and hence could be useful for clinical applications, as well as furthering our understanding of cell/extracellular matrix interactions in determining in vitro bone tissue formation.

Matrix degradation regulates osteoblast protrusion dynamics and individual migration

Protrusions are one of the structures that cells use to sense their surrounding environment in a probing and exploratory manner as well as to communicate with other cells. In particular, osteoblasts embedded within a 3D matrix tend to originate a large number of protrusions compared to other type of cells. In this work, we study the role that mechanochemical properties of the extracellular matrix (ECM) play on the dynamics of these protrusions, namely, the regulation of the size and number of emanating structures. In addition, we also determine how the dynamics of the protrusions may lead the 3D movement of the osteoblasts. Significant differences were found in protrusion size and cell velocity, when degradation activity due to metalloproteases was blocked by means of an artificial broad-spectrum matrix metalloproteinase inhibitor, whereas stiffening of the matrix by introducing transglutaminase crosslinking, only induced slight changes in both protrusion size and cell velocity, suggesting that the ability of cells to create a path through the matrix is more critical than the matrix mechanical properties themselves. To confirm this, we developed a cell migration computational model in 3D including both the mechanical and chemical properties of the ECM as well as the protrusion mechanics, obtaining good agreement with experimental results.

Evaluation of alginate modification effect on cell-matrix interaction, mechanotransduction and chondrogenesis of encapsulated MSCs

Mesenchymal stem cells (MSCs) are promising cell candidates for cartilage regeneration. Furthermore, it is important to control the cell-matrix interactions that have a direct influence on cell functions. Providing an appropriate microenvironment for cell differentiation in response to exogenous stimuli is a critical step towards the clinical utilization of MSCs. In this study, hydrogels consisted of different proportions of alginates that were modified using gelatin, collagen type I and arginine-glycine-aspartic acid (RGD) and were evaluated regarding their effects on mesenchymal stem cells. The effect of applying hydrostatic pressure on MSCs encapsulated in collagen-modified alginate with and without chondrogenic medium was evaluated 7, 14 and 21 days after culture, which is a comprehensive evaluation of chondrogenesis in 3D hydrogels with mechanical and chemical stimulants. Alcian blue, safranin O and dimethyl methylene blue (DMMB) staining showed the chondrogenic phenotype of cells seeded in the collagen- and RGD-modified alginate hydrogels with the highest intensity after 21 days of culture. The results of real-time PCR for cartilage-specific extracellular matrix genes indicated the chondrogenic differentiation of MSCs in all hydrogels. Also, the synergic effects of chemical and mechanical stimuli are indicated. The highest expression levels of the studied genes were observed in the cells embedded in collagen-modified alginate by loading after 14 days of exposure to the chondrogenic medium. The effect of using IHP on encapsulated MSCs in modified alginate with collagen type I is equal or even higher than using TGF-beta on encapsulated cells. The results of immunohistochemical assessments also confirmed the real-time PCR data.

A novel construct with biomechanical flexibility for articular cartilage regeneration

BACKGROUND

Although tissue-engineered cartilage has been broadly studied, complete integration of regenerated cartilage with residual cartilage is still difficult for the inferior mechanical and biochemical feature of neocartilage. Chondrogenesis of mesenchymal stem cells can be induced by biophysical and biochemical factors.

METHODS

In this study, autologous platelet-rich fibrin (PRF) membrane was used as a growth factor-rich scaffold that may facilitate differentiation of the transplanted bone marrow mesenchymal stem cells (BMSCs). At the same time, hydrostatic pressure was adopted for pre-adjustment of the seed cells before transplantation that may promote the mechanical flexibility of neocartilage.

RESULTS

An in vitro study showed that the feasible hydrostatic pressure stimulation substantially promoted the chondrogenic potential of in vitro-cultured BMSC/PRF construct. In vivo results revealed that at every time point, the newborn tissues were the most favorable in the pressure-pretreated BMSC/PRF transplant group. Besides, the transplantation of feasible hydrostatic pressure-pretreated construct by BMSC sheet fragments and PRF granules could obviously improve the integration between the regenerated cartilage and host cartilage milieu, and thereby achieve boundaryless repair between the neocartilage and residual host cartilage tissue in rabbit temporomandibular joints. It could be concluded that feasible hydrostatic pressure may effectively promote the proliferation and chondrogenic differentiation of BMSCs in a BMSC/PRF construct.

CONCLUSION

This newly formed construct with biomechanical flexibility showed a superior capacity for cartilage regeneration by promoting the mechanical properties and integration of neocartilage.

The role of anthrax toxin protein receptor 1 as a new mechanosensor molecule and its mechanotransduction in BMSCs under hydrostatic pressure

Anthrax toxin protein receptor (ANTXR) 1 has many similarities to integrin and is regarded in some respects as a single-stranded integrin protein. However, it is not clear whether ANTXR1 responds to mechanical signals secondary to the activation of integrins or whether it is a completely new, independent and previously undiscovered mechanosensor that responds to an undefined subset of mechanical signaling molecules. Our study demonstrates that ANTXR1 is a novel mechanosensor on the cell membrane, acting independently from the classical mechanoreceptor molecule integrinβ1. We show that bone marrow stromal cells (BMSCs) respond to the hydrostatic pressure towards chondrogenic differentiation partly through the glycogen synthase kinase (GSK) 3β/β-Catenin signaling pathway, which can be partly regulated by ANTXR1 and might be related to the direct binding between ANTXR1 and low-density lipoprotein receptor-related protein (LRP) 5/6. In addition, ANTXR1 specifically activates Smad2 and upregulates Smad4 expression to facilitate the transport of activated Smad2 to the nucleus to regulate chondrogenesis, which might be related to the direct binding between ANTXR1 and Actin/Fascin1. We also demonstrate that ANTXR1 binds to some extent with integrinβ1, but this interaction does not affect the expression and function of either protein under pressure. Thus, we conclude that ANTXR1 plays a crucial role in BMSC mechanotransduction and controls specific signaling pathways that are distinct from those of integrin to influence the chondrogenic responses of BMSCs under hydrostatic pressure.

Regulation of Cell Behavior by Hydrostatic Pressure

Hydrostatic pressure (HP) regulates diverse cell behaviors including differentiation, migration, apoptosis, and proliferation. Abnormal HP is associated with pathologies including glaucoma and hypertensive fibrotic remodeling. In this review, recent advances in quantifying and predicting how cells respond to HP across several tissue systems are presented, including tissues of the brain, eye, vasculature and bladder, as well as articular cartilage. Finally, some promising directions on the study of cell behaviors regulated by HP are proposed.

Differential adhesion and fibrinolytic activity of mesenchymal stem cells from human bone marrow, placenta, and Wharton's jelly cultured in a fibrin hydrogel

Mesenchymal stem cells isolated from different tissues should share associated markers and the capability to differentiate to mesodermal lineages. However, their behavior varies in specific microenvironments. Herein, adhesion and fibrinolytic activity of mesenchymal stem cells from placenta, bone marrow, and Wharton’s jelly were evaluated in fibrin hydrogels prepared with nonpurified blood plasma and compared with two-dimensional cultures. Despite the source, mesenchymal stem cells adhered through focal adhesions positive for vinculin and integrin αV in two dimensions, while focal adhesions could not be detected in fibrin hydrogels. Moreover, some cells could not spread and stay rounded. The proportions of elongated and round phenotypes varied, with placenta mesenchymal stem cells having the lowest percentage of elongated cells (~10%). Mesenchymal stem cells degraded fibrin at distinct rates, and placenta mesenchymal stem cells had the strongest fibrinolytic activity, which was achieved principally through the plasminogen–plasmin axis. These findings might have clinical implications in tissue engineering and wound healing therapy.

The Importance of Physioxia in Mesenchymal Stem Cell Chondrogenesis and the Mechanisms Controlling Its Response

Articular cartilage covers the surface of synovial joints and enables joint movement. However, it is susceptible to progressive degeneration with age that can be accelerated by either previous joint injury or meniscectomy. This degenerative disease is known as osteoarthritis (OA) and it greatly affects the adult population. Cell-based tissue engineering provides a possible solution for treating OA at its earliest stages, particularly focal cartilage lesions. A candidate cell type for treating these focal defects are Mesenchymal Stem Cells (MSCs). However, present methods for differentiating these cells towards the chondrogenic lineage lead to hypertrophic chondrocytes and bone formation in vivo. Environmental stimuli that can stabilise the articular chondrocyte phenotype without compromising tissue formation have been extensively investigated. One factor that has generated intensive investigation in MSC chondrogenesis is low oxygen tension or physioxia (2⁻5% oxygen). In vivo articular cartilage resides at oxygen tensions between 1⁻4%, and in vitro results suggest that these conditions are beneficial for MSC expansion and chondrogenesis, particularly in suppressing the cartilage hypertrophy. This review will summarise the current literature regarding the effects of physioxia on MSC chondrogenesis with an emphasis on the pathways that control tissue formation and cartilage hypertrophy.

Regeneration of Osteochondral Defects Using Developmentally Inspired Cartilaginous Templates

IMPACT STATEMENT

Successfully treating osteochondral defects involves regenerating both the damaged articular cartilage and the underlying subchondral bone, in addition to the complex interface that separates these tissues. In this study, we demonstrate that a cartilage template, engineered using bone marrow-derived mesenchymal stem cells, can enhance the regeneration of such defects and promote the development of a more mechanically functional repair tissue. We also use a computational mechanobiological model to understand how joint-specific environmental factors, specifically oxygen levels and tissue strains, regulate the conversion of the engineered template into cartilage and bone in vivo.

A stretchable conductive Polypyrrole Polydimethylsiloxane device fabricated by simple soft lithography and oxygen plasma treatment

This paper reports a simple method used to fabricate a stretchable conductive polypyrrole (PPy) rough pore-shape polydimethylsiloxane (p-PDMS) device. An abrasive paper is first used to imprint rough micro-structures on the SU-8 micromold. The p-PDMS microchannel is then fabricated using a standard soft-lithography process. An oxygen plasma treatment is then applied to form an irreversible sealing between the microchannel and a blank cover PDMS. The conductive layer is formed by injecting the PPy mixture into the microchannel which polymerizes in the rough pore-shape micro-structures; The PPy/p-PDMS hybrid device shows good electrical property and stretchability. The electrical properties of different geometrical designs of the PPy/p-PDMS microchannel under stretching were investigated, including straight, curved, and serpentine. Mouse embryonic fibroblasts (NIH/3 T3) were also cultured inside the PPy/p-PDMS device to demonstrate good biocompatibility and feasibility using the conductive and stretchable microchannel in cell culture microfluidics applications. Finally, cyclic stretching and bending tests were performed to evaluate the reliability of PPy/p-PDMS microchannel.

3D scaffolds for brain tissue regeneration: architectural challenges

Critical analysis of experimental studies on 3D scaffolds for brain tissue engineering.

Mechanical Stimulation of Adipose-Derived Stem Cells for Functional Tissue Engineering of the Musculoskeletal System via Cyclic Hydrostatic Pressure, Simulated Microgravity, and Cyclic Tensile Strain

It is critical that human adipose stem cell (hASC) tissue-engineering therapies possess appropriate mechanical properties in order to restore function of the load bearing tissues of the musculoskeletal system. In an effort to elucidate the hASC response to mechanical stimulation and develop mechanically robust tissue engineered constructs, recent research has utilized a variety of mechanical loading paradigms including cyclic tensile strain, cyclic hydrostatic pressure, and mechanical unloading in simulated microgravity. This chapter describes methods for applying these mechanical stimuli to hASC to direct differentiation for functional tissue engineering of the musculoskeletal system.

Mechanical stimulation of mesenchymal stem cells: Implications for cartilage tissue engineering

Articular cartilage is a load-bearing tissue playing a crucial mechanical role in diarthrodial joints, facilitating joint articulation, and minimizing wear. The significance of biomechanical stimuli in the development of cartilage and maintenance of chondrocyte phenotype in adult tissues has been well documented. Furthermore, dysregulated loading is associated with cartilage pathology highlighting the importance of mechanical cues in cartilage homeostasis. The repair of damaged articular cartilage resulting from trauma or degenerative joint disease poses a major challenge due to a low intrinsic capacity of cartilage for self-renewal, attributable to its avascular nature. Bone marrow-derived mesenchymal stem cells (MSCs) are considered a promising cell type for cartilage replacement strategies due to their chondrogenic differentiation potential. Chondrogenesis of MSCs is influenced not only by biological factors but also by the environment itself, and various efforts to date have focused on harnessing biomechanics to enhance chondrogenic differentiation of MSCs. Furthermore, recapitulating mechanical cues associated with cartilage development and homeostasis in vivo, may facilitate the development of a cellular phenotype resembling native articular cartilage. The goal of this review is to summarize current literature examining the effect of mechanical cues on cartilage homeostasis, disease, and MSC chondrogenesis. The role of biological factors produced by MSCs in response to mechanical loading will also be examined. An in-depth understanding of the impact of mechanical stimulation on the chondrogenic differentiation of MSCs in terms of endogenous bioactive factor production and signaling pathways involved, may identify therapeutic targets and facilitate the development of more robust strategies for cartilage replacement using MSCs. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:52-63, 2018.

Purinergic Signaling Regulates the Transforming Growth Factor-β3-Induced Chondrogenic Response of Mesenchymal Stem Cells to Hydrostatic Pressure

Although hydrostatic pressure (HP) is known to regulate chondrogenic differentiation of mesenchymal stromal/stem cells (MSCs), improved insight into the mechanotransduction of HP may form the basis for novel tissue engineering strategies. Previously, we demonstrated that matrix stiffness and calcium ion (Ca(++)) mobility regulate the mechanotransduction of HP; however, the mechanisms, by which these Ca(++) signaling pathways are initiated, are currently unknown. The purinergic pathway, in which adenosine triphosphate (ATP) is released and activates P-receptors to initiate Ca(++) signaling, plays a key role in the mechanotransduction of compression, but has yet to be investigated with regard to HP. Therefore, the objective of this study was to investigate the interplay between purinergic signaling, matrix stiffness, and the chondrogenic response of MSCs to HP. Porcine bone marrow-derived MSCs were seeded into soft or stiff agarose hydrogels and subjected to HP (10 MPa at 1 Hz for 4 h/d for 21 days) or kept in free swelling conditions. Stiff constructs were incubated with pharmacological inhibitors of extracellular ATP, P2 receptors, or hemichannels, or without any inhibitors as a control. As with other loading modalities, HP significantly increased ATP release in the control group; however, inhibition of hemichannels completely abrogated this response. The increase in sulfated glycosaminoglycan (sGAG) synthesis and vimentin reorganization observed in the control group in response to HP was suppressed in the presence of all three inhibitors, suggesting that purinergic signaling is involved in the mechanoresponse of MSCs to HP. Interestingly, ATP was released from both soft and stiff hydrogels in response to HP, but HP only enhanced chondrogenesis in the stiff hydrogels, indicating that matrix stiffness may act downstream of purinergic signaling to regulate the mechanoresponse of MSCs to HP. Addition of exogenous ATP did not replicate the effects of HP on chondrogenesis, suggesting that mechanisms other than purinergic signaling also regulate the response of MSCs to HP.

Tensile force on human macrophage cells promotes osteoclastogenesis through receptor activator of nuclear factor κB ligand induction

Little is known about the effects of tensile forces on osteoclastogenesis by human monocytes in the absence of mechanosensitive cells, including osteoblasts and fibroblasts. In this study we consider the effects of tensile force on osteoclastogenesis in human monocytes. The cells were treated with receptor activator of nuclear factor κB ligand (RANKL) to promote osteoclastogenesis. Then,expression and secretion of cathepsin K were examined. RANKL and the formation of osteoclasts during the osteoclast differentiation process under continual tensile stress were evaluated by Western blot. It was also found that -100 kPa or lower induces RANKL-enhanced tartrate-resistant acid phosphatase activity in a dose-dependent manner. Furthermore, an increased tensile force raises the expression and secretion of cathepsin K elevated by RANKL, and is concurrent with the increase of TNF-receptor-associated factor 6 induction and nuclear factor κB activation. Overall, the current report demonstrates that tensile force reinforces RANKL-induced osteoclastogenesis by retarding osteoclast differentiation. The tensile force is able to modify every cell through dose-dependent in vitro RANKL-mediated osteoclastogenesis, affecting the fusion of preosteoclasts and function of osteoclasts. However, tensile force increased TNF-receptor-associated factor 6 expression. These results are in vitro findings and were obtained under a condition of tensile force. The current results help us to better understand the cellular roles of human macrophage populations in osteoclastogenesis as well as in alveolar bone remodeling when there is tensile stress.

Seaweed polysaccharide-based hydrogels used for the regeneration of articular cartilage

This manuscript provides an overview of the in vitro and in vivo studies reported in the literature focusing on seaweed polysaccharides based hydrogels that have been proposed for applications in regenerative medicine, particularly, in the field of cartilage tissue engineering. For a better understanding of the main requisites for these specific applications, the main aspects of the native cartilage structure, as well as recognized diseases that affect this tissue are briefly described. Current available treatments are also presented to emphasize the need for alternative techniques. The following part of this review is centered on the description of the general characteristics of algae polysaccharides, as well as relevant properties required for designing hydrogels for cartilage tissue engineering purposes. An in-depth overview of the most well known seaweed polysaccharide, namely agarose, alginate, carrageenan and ulvan biopolymeric gels, that have been proposed for engineering cartilage is also provided. Finally, this review describes and summarizes the translational aspect for the clinical application of alternative systems emphasizing the importance of cryopreservation and the commercial products currently available for cartilage treatment.

Differentiation of mesenchymal stem cells for cartilage tissue engineering: Individual and synergetic effects of three-dimensional environment and mechanical loading

Chondrogenesis of dedifferentiated chondrocytes and mesenchymal stem cells is influenced not only by soluble molecules like growth factors, but also by the cell environment itself. The latter is achieved through both mechanical cues - which act as stimulation factor and influences nutrient transport - and adhesion to extracellular matrix cues - which determine cell shape. Although the effects of soluble molecules and cell environment have been intensively addressed, few observations and conclusions about the interaction between the two have been achieved. In this work, we review the state of the art on the single effects between mechanical and biochemical cues, as well as on the combination of the two. Furthermore, we provide a discussion on the techniques currently used to determine the mechanical properties of materials and tissues generated in vitro, their limitations and the future research needs to properly address the identified problems.

STATEMENT OF SIGNIFICANCE

The importance of biomechanical cues in chondrogenesis is well known. This paper reviews the existing literature on the effect of mechanical stimulation on chondrogenic differentiation of mesenchymal stem cells in order to regenerate hyaline cartilage. Contradictory results found with respect to the effect of different modes of external loading can be explained by the different properties of the scaffolding system that holds the cells, which determine cell adhesion and morphology and spatial distribution of cells, as well as the stress transmission to the cells. Thus, this review seeks to provide an insight into the interplay between external loading program and scaffold properties during chondrogenic differentiation. The review of the literature reveals an important gap in the knowledge in this field and encourages new experimental studies. The main issue is that in each of the few cases in which the interplay is investigated, just two groups of scaffolds are compared, leaving intermediate adhesion conditions out of study. The authors propose broader studies implementing new high-throughput techniques for mechanical characterization of tissue engineering constructs and the inclusion of fatigue analysis as support methodology to more exhaustive mechanical characterization.

Stem Cells for Bone Regeneration: From Cell-Based Therapies to Decellularised Engineered Extracellular Matrices

Currently, autologous bone grafting represents the clinical gold standard in orthopaedic surgery. In certain cases, however, alternative techniques are required. The clinical utility of stem and stromal cells has been demonstrated for the repair and regeneration of craniomaxillofacial and long bone defects although clinical adoption of bone tissue engineering protocols has been very limited. Initial tissue engineering studies focused on the bone marrow as a source of cells for bone regeneration, and while a number of promising results continue to emerge, limitations to this technique have prompted the exploration of alternative cell sources, including adipose and muscle tissue. In this review paper we discuss the advantages and disadvantages of cell sources with a focus on adipose tissue and the bone marrow. Additionally, we highlight the relatively recent paradigm of developmental engineering, which promotes the recapitulation of naturally occurring developmental processes to allow the implant to optimally respond to endogenous cues. Finally we examine efforts to apply lessons from studies into different cell sources and developmental approaches to stimulate bone growth by use of decellularised hypertrophic cartilage templates.

The effects of dynamic compression on the development of cartilage grafts engineered using bone marrow and infrapatellar fat pad derived stem cells

Bioreactors that subject cell seeded scaffolds or hydrogels to biophysical stimulation have been used to improve the functionality of tissue engineered cartilage and to explore how such constructs might respond to the application of joint specific mechanical loading. Whether a particular cell type responds appropriately to physiological levels of biophysical stimulation could be considered a key determinant of its suitability for cartilage tissue engineering applications. The objective of this study was to determine the effects of dynamic compression on chondrogenesis of stem cells isolated from different tissue sources. Porcine bone marrow (BM) and infrapatellar fat pad (FP) derived stem cells were encapsulated in agarose hydrogels and cultured in a chondrogenic medium in free swelling (FS) conditions for 21 d, after which samples were subjected to dynamic compression (DC) of 10% strain (1 Hz, 1 h d(-1)) for a further 21 d. Both BM derived stem cells (BMSCs) and FP derived stem cells (FPSCs) were capable of generating cartilaginous tissues with near native levels of sulfated glycosaminoglycan (sGAG) content, although the spatial development of the engineered grafts strongly depended on the stem cell source. The mechanical properties of cartilage grafts generated from both stem cell sources also approached that observed in skeletally immature animals. Depending on the stem cell source and the donor, the application of DC either enhanced or had no significant effect on the functional development of cartilaginous grafts engineered using either BMSCs or FPSCs. BMSC seeded constructs subjected to DC stained less intensely for collagen type I. Furthermore, histological and micro-computed tomography analysis showed mineral deposition within BMSC seeded constructs was suppressed by the application of DC. Therefore, while the application of DC in vitro may only lead to modest improvements in the mechanical functionality of cartilaginous grafts, it may play an important role in the development of phenotypically stable constructs.

TGFβ signaling promotes matrix assembly during mechanosensitive embryonic salivary gland restoration

Mechanical properties of the microenvironment regulate cell morphology and differentiation within complex organs. However, methods to restore morphogenesis and differentiation in organs in which compliance is suboptimal are poorly understood. We used mechanosensitive mouse salivary gland organ explants grown at different compliance levels together with deoxycholate extraction and immunocytochemistry of the intact, assembled matrices to examine the compliance-dependent assembly and distribution of the extracellular matrix and basement membrane in explants grown at permissive or non-permissive compliance. Extracellular matrix and basement membrane assembly were disrupted in the glands grown at low compliance compared to those grown at high compliance, correlating with defective morphogenesis and decreased myoepithelial cell differentiation. Extracellular matrix and basement membrane assembly as well as myoepithelial differentiation were restored by addition of TGFβ1 and by mechanical rescue, and mechanical rescue was prevented by inhibition of TGFβ signaling during the rescue. We detected a basal accumulation of active integrin β1 in the differentiating myoepithelial cells that formed a continuous peripheral localization around the proacini and in clefts within active sites of morphogenesis in explants that were grown at high compliance. The pattern and levels of integrin β1 activation together with myoepithelial differentiation were interrupted in explants grown at low compliance but were restored upon mechanical rescue or with application of exogenous TGFβ1. These data suggest that therapeutic application of TGFβ1 to tissues disrupted by mechanical signaling should be examined as a method to promote organ remodeling and regeneration.

Mechanical regulation of mesenchymal stem cell differentiation

Biophysical cues play a key role in directing the lineage commitment of mesenchymal stem cells or multipotent stromal cells (MSCs), but the mechanotransductive mechanisms at play are still not fully understood. This review article first describes the roles of both substrate mechanics (e.g. stiffness and topography) and extrinsic mechanical cues (e.g. fluid flow, compression, hydrostatic pressure, tension) on the differentiation of MSCs. A specific focus is placed on the role of such factors in regulating the osteogenic, chondrogenic, myogenic and adipogenic differentiation of MSCs. Next, the article focuses on the cellular components, specifically integrins, ion channels, focal adhesions and the cytoskeleton, hypothesized to be involved in MSC mechanotransduction. This review aims to illustrate the strides that have been made in elucidating how MSCs sense and respond to their mechanical environment, and also to identify areas where further research is needed.

Introduction to cell-hydrogel mechanosensing

The development of hydrogel-based biomaterials represents a promising approach to generating new strategies for tissue engineering and regenerative medicine. In order to develop more sophisticated cell-seeded hydrogel constructs, it is important to understand how cells mechanically interact with hydrogels. In this paper, we review the mechanisms by which cells remodel hydrogels, the influence that the hydrogel mechanical and structural properties have on cell behaviour and the role of mechanical stimulation in cell-seeded hydrogels. Cell-mediated remodelling of hydrogels is directed by several cellular processes, including adhesion, migration, contraction, degradation and extracellular matrix deposition. Variations in hydrogel stiffness, density, composition, orientation and viscoelastic characteristics all affect cell activity and phenotype. The application of mechanical force on cells encapsulated in hydrogels can also instigate changes in cell behaviour. By improving our understanding of cell-material mechano-interactions in hydrogels, this should enable a new generation of regenerative medical therapies to be developed.

Matrix as an interstitial transport system

The extracellular matrix (ECM) is best known for its function as a structural scaffold for the tissue and more recently as a microenvironment to sequester growth factors and cytokines allowing for rapid and localized changes in their activity in the absence of new protein synthesis. In this review, we explore this and additional new aspects of ECM function in mediating cell-to-cell communications. Fibrillar and nonfibrillar components of ECM can limit and facilitate the transport of molecules through the extracellular space while also regulating interstitial hydrostatic pressure. In turn, transmembrane communications via molecules, such as ECM metalloproteinase inducer, thrombospondins, and integrins, can further mediate cell response to extracellular cues and affect ECM composition and tissue remodeling. Other means of cell-to-cell communication include extracellular microRNA transport and its contribution to gene expression in target cells and the nanotube formation between distant cells, which has recently emerged as a novel conduit for intercellular organelle sharing thereby influencing cell survival and function. The information summarized and discussed here are not limited to the cardiovascular ECM but encompass ECM in general with specific references to the cardiovascular system.

Evaluation of insulin medium or chondrogenic medium on proliferation and chondrogenesis of ATDC5 cells

Background. The ATDC5 cell line is regarded as an excellent cell model for chondrogenesis. In most studies with ATDC5 cells, insulin medium (IM) was used to induce chondrogenesis while chondrogenic medium (CM), which was usually applied in chondrogenesis of mesenchymal stem cells (MSCs), was rarely used for ATDC5 cells. This study was mainly designed to investigate the effect of IM, CM, and growth medium (GM) on chondrogenesis of ATDC5 cells. Methods. ATDC5 cells were, respectively, cultured in IM, CM, and GM for a certain time. Then the proliferation and the chondrogenesis progress of cells in these groups were analyzed. Results. Compared with CM and GM, IM promoted the proliferation of cells significantly. CM was effective for enhancement of cartilage specific markers, while IM induced the cells to express endochondral ossification related genes. Although GAG deposition per cell in CM group was significantly higher than that in IM and GM groups, the total GAG contents in IM group were the most. Conclusion. This study demonstrated that CM focused on induction of chondrogenic differentiation while IM was in favor of promoting proliferation and expression of endochondral ossification related genes. Combinational use of these two media would be more beneficial to bone/cartilage repair.

Deciphering mechanical regulation of chondrogenesis in fibrin-polyurethane composite scaffolds enriched with human mesenchymal stem cells: a dual computational and experimental approach

Fibrin-polyurethane composite scaffolds support chondrogenesis of human mesenchymal stem cells (hMSCs) derived from bone marrow and due to their robust mechanical properties allow mechanical loading in dynamic bioreactors, which has been shown to increase the chondrogenic differentiation of MSCs through the transforming growth factor beta pathway. The aim of this study was to use the finite element method, mechanical testing, and dynamic in vitro cell culture experiments on hMSC-enriched fibrin-polyurethane composite scaffolds to quantitatively decipher the mechanoregulation of chondrogenesis within these constructs. The study identified compressive principal strains as the key regulator of chondrogenesis in the constructs. Although dynamic uniaxial compression did not induce chondrogenesis, multiaxial loading by combined application of dynamic compression and interfacial shear induced significant chondrogenesis at locations where all the three principal strains were compressive and had a minimum magnitude of 10%. In contrast, no direct correlation was identified between the level of pore fluid velocity and chondrogenesis. Due to the high permeability of the constructs, the pore fluid pressures could not be increased sufficiently by mechanical loading, and instead, chondrogenesis was induced by triaxial compressive deformations of the matrix with a minimum magnitude of 10%. Thus, it can be concluded that dynamic triaxial compressive deformations of the matrix is sufficient to induce chondrogenesis in a threshold-dependent manner, even where the pore fluid pressure is negligible.

Cyclic hydrostatic pressure promotes a stable cartilage phenotype and enhances the functional development of cartilaginous grafts engineered using multipotent stromal cells isolated from bone marrow and infrapatellar fat pad

The objective of this study was to investigate how joint specific biomechanical loading influences the functional development and phenotypic stability of cartilage grafts engineered in vitro using stem/progenitor cells isolated from different source tissues. Porcine bone marrow derived multipotent stromal cells (BMSCs) and infrapatellar fat pad derived multipotent stromal cells (FPSCs) were seeded in agarose hydrogels and cultured in chondrogenic medium, while simultaneously subjected to 10MPa of cyclic hydrostatic pressure (HP). To mimic the endochondral phenotype observed in vivo with cartilaginous tissues engineered using BMSCs, the culture media was additionally supplemented with hypertrophic factors, while the loss of phenotype observed in vivo with FPSCs was induced by withdrawing transforming growth factor (TGF)-β3 from the media. The application of HP was found to enhance the functional development of cartilaginous tissues engineered using both BMSCs and FPSCs. In addition, HP was found to suppress calcification of tissues engineered using BMSCs cultured in chondrogenic conditions and acted to maintain a chondrogenic phenotype in cartilaginous grafts engineered using FPSCs. The results of this study point to the importance of in vivo specific mechanical cues for determining the terminal phenotype of chondrogenically primed multipotent stromal cells. Furthermore, demonstrating that stem or progenitor cells will appropriately differentiate in response to such biophysical cues might also be considered as an additional functional assay for evaluating their therapeutic potential.