Mechanics and mechanobiology of mesenchymal stem cell-based engineered cartilage

Scaffolds containing GAG-mimetic cellulose sulfate promote TGF-β interaction and MSC Chondrogenesis over native GAGs

Cartilage tissue engineering strategies seek to repair damaged tissue using approaches that include scaffolds containing components of the native extracellular matrix (ECM). Articular cartilage consists of glycosaminoglycans (GAGs) which are known to sequester growth factors. In order to more closely mimic the native ECM, this study evaluated the chondrogenic differentiation of mesenchymal stem cells (MSCs), a promising cell source for cartilage regeneration, on fibrous scaffolds that contained the GAG-mimetic cellulose sulfate. The degree of sulfation was evaluated, examining partially sulfated cellulose (pSC) and fully sulfated cellulose (NaCS). Comparisons were made with scaffolds containing native GAGs (chondroitin sulfate A, chondroitin sulfate C and heparin). Transforming growth factor-beta3 (TGF-β3) sequestration, as measured by rate of association, was higher for sulfated cellulose-containing scaffolds as compared to native GAGs. In addition, TGF-β3 sequestration and retention over time was highest for NaCS-containing scaffolds. Sulfated cellulose-containing scaffolds loaded with TGF-β3 showed enhanced chondrogenesis as indicated by a higher Collagen Type II:I ratio over native GAGs. NaCS-containing scaffolds loaded with TGF-β3 had the highest expression of chondrogenic markers and a reduction of hypertrophic markers in dynamic loading conditions, which more closely mimic in vivo conditions. Studies also demonstrated that TGF-β3 mediated its effect through the Smad2/3 signaling pathway where the specificity of TGF-β receptor (TGF- βRI)-phosphorylated SMAD2/3 was verified with a receptor inhibitor. Therefore, studies demonstrate that scaffolds containing cellulose sulfate enhance TGF-β3-induced MSC chondrogenic differentiation and show promise for promoting cartilage tissue regeneration.

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.

Biophysical Modulation of Mesenchymal Stem Cell Differentiation in the Context of Skeletal Repair

A prominent feature of the skeleton is its ability to remodel in response to biophysical stimuli and to repair under varied biophysical conditions. This allows the skeleton considerable adaptation to meet its physiological roles of stability and movement. Skeletal cells and their mesenchymal precursors exist in a native environment rich with biophysical signals, and they sense and respond to those signals to meet organismal demands of the skeleton. While mechanical strain is the most recognized of the skeletal biophysical stimuli, signaling phenomena also include fluid flow, hydrostatic pressure, shear stress, and ion-movement-related electrokinetic phenomena including, prominently, streaming potentials. Because of the complex interactions of these electromechanical signals, it is difficult to isolate the significance of each. The application of external electrical and electromagnetic fields allows an exploration of the effects of these stimuli on cell differentiation and extra-cellular matrix formation in the absence of mechanical strain. This review takes a distinctly translational approach to mechanistic and preclinical studies of differentiation and skeletal lineage commitment of mesenchymal cells under biophysical stimulation. In vitro studies facilitate the examination of isolated cellular responses while in vivo studies permit the observation of cell differentiation and extracellular matrix synthesis.

Chondrogenic Potential of Human Dental Pulp Stem Cells Cultured as Microtissues

Several tissue engineering stem cell-based procedures improve hyaline cartilage repair. In this work, the chondrogenic potential of dental pulp stem cell (DPSC) organoids or microtissues was studied. After several weeks of culture in proliferation or chondrogenic differentiation media, synthesis of aggrecan and type II and I collagen was immunodetected, and SOX9, ACAN, COL2A1, and COL1A1 gene expression was analysed by real-time RT-PCR. Whereas microtissues cultured in proliferation medium showed the synthesis of aggrecan and type II and I collagen at the 6^th week of culture, samples cultured in chondrogenic differentiation medium showed an earlier and important increase in the synthesis of these macromolecules after 4 weeks. Gene expression analysis showed a significant increase of COL2A1 after 3 days of culture in chondrogenic differentiation medium, while COL1A1 was highly expressed after 14 days. Cell-cell proximity promotes the chondrogenic differentiation of DPSCs and important synthesis of hyaline chondral macromolecules.

Mechanotransducive Biomimetic Systems for Chondrogenic Differentiation In Vitro

Osteoarthritis (OA) is a long-term chronic joint disease characterized by the deterioration of bones and cartilage, which results in rubbing of bones which causes joint stiffness, pain, and restriction of movement. Tissue engineering strategies for repairing damaged and diseased cartilage tissue have been widely studied with various types of stem cells, chondrocytes, and extracellular matrices being on the lead of new discoveries. The application of natural or synthetic compound-based scaffolds for the improvement of chondrogenic differentiation efficiency and cartilage tissue engineering is of great interest in regenerative medicine. However, the properties of such constructs under conditions of mechanical load, which is one of the most important factors for the successful cartilage regeneration and functioning in vivo is poorly understood. In this review, we have primarily focused on natural compounds, particularly extracellular matrix macromolecule-based scaffolds and their combinations for the chondrogenic differentiation of stem cells and chondrocytes. We also discuss different mechanical forces and compression models that are used for In Vitro studies to improve chondrogenic differentiation. Summary of provided mechanical stimulation models In Vitro reviews the current state of the cartilage tissue regeneration technologies and to the potential for more efficient application of cell- and scaffold-based technologies for osteoarthritis or other cartilage disorders.

Osteogenic differentiation of human mesenchymal stromal cells and fibroblasts differs depending on tissue origin and replicative senescence

The need for an autologous cell source for bone tissue engineering and medical applications has led researchers to explore multipotent mesenchymal stromal cells (MSC), which show stem cell plasticity, in various human tissues. However, MSC with different tissue origins vary in their biological properties and their capability for osteogenic differentiation. Furthermore, MSC-based therapies require large-scale ex vivo expansion, accompanied by cell type-specific replicative senescence, which affects osteogenic differentiation. To elucidate cell type-specific differences in the osteogenic differentiation potential and replicative senescence, we analysed the impact of BMP and TGF-β signaling in adipose-derived stromal cells (ASC), fibroblasts (FB), and dental pulp stromal cells (DSC). We used inhibitors of BMP and TGF-β signaling, such as SB431542, dorsomorphin and/or a supplemental addition of BMP-2. The expression of high-affinity binding receptors for BMP-2 and calcium deposition with alizarin red S were evaluated to assess osteogenic differentiation potential. Our study demonstrated that TGF-β signaling inhibits osteogenic differentiation of ASC, DSC and FB in the early cell culture passages. Moreover, DSC had the best osteogenic differentiation potential and an activation of BMP signaling with BMP-2 could further enhance this capacity. This phenomenon is likely due to an increased expression of activin receptor-like kinase-3 and -6. However, in DSC with replicative senescence (in cell culture passage 10), osteogenic differentiation sharply decreased, and the simultaneous use of BMP-2 and SB431542 did not result in further improvement of this process. In comparison, ASC retain a similar osteogenic differentiation potential regardless of whether they were in the early (cell culture passage 3) or later (cell culture passage 10) stages. Our study elucidated that ASC, DSC, and FB vary functionally in their osteogenic differentiation, depending on their tissue origin and replicative senescence. Therefore, our study provides important insights for cell-based therapies to optimize prospective bone tissue engineering strategies.

Identification of ALP+/CD73+ defining markers for enhanced osteogenic potential in human adipose-derived mesenchymal stromal cells by mass cytometry

BACKGROUND

The impressive progress in the field of stem cell research in the past decades has provided the ground for the development of cell-based therapy. Mesenchymal stromal cells obtained from adipose tissue (AD-MSCs) represent a viable source for the development of cell-based therapies. However, the heterogeneity and variable differentiation ability of AD-MSCs depend on the cellular composition and represent a strong limitation for their use in therapeutic applications. In order to fully understand the cellular composition of MSC preparations, it would be essential to analyze AD-MSCs at single-cell level.

METHOD

Recent advances in single-cell technologies have opened the way for high-dimensional, high-throughput, and high-resolution measurements of biological systems. We made use of the cytometry by time-of-flight (CyTOF) technology to explore the cellular composition of 17 human AD-MSCs, interrogating 31 markers at single-cell level. Subcellular composition of the AD-MSCs was investigated in their naïve state as well as during osteogenic commitment, via unsupervised dimensionality reduction as well as supervised representation learning approaches.

RESULT

This study showed a high heterogeneity and variability in the subcellular composition of AD-MSCs upon isolation and prolonged culture. Algorithm-guided identification of emerging subpopulations during osteogenic differentiation of AD-MSCs allowed the identification of an ALP+/CD73+ subpopulation of cells with enhanced osteogenic differentiation potential. We could demonstrate in vitro that the sorted ALP+/CD73+ subpopulation exhibited enhanced osteogenic potential and is moreover fundamental for osteogenic lineage commitment. We finally showed that this subpopulation was present in freshly isolated human adipose-derived stromal vascular fractions (SVFs) and that could ultimately be used for cell therapies.

CONCLUSION

The data obtained reveal, at single-cell level, the heterogeneity of AD-MSCs from several donors and highlight how cellular composition impacts the osteogenic differentiation capacity. The marker combination (ALP/CD73) can not only be used to assess the differentiation potential of undifferentiated AD-MSC preparations, but also could be employed to prospectively enrich AD-MSCs from the stromal vascular fraction of human adipose tissue for therapeutic applications.

Fabrication, maturation, and implantation of composite tissue-engineered total discs formed from native and mesenchymal stem cell combinations

Low back pain arising from disc degeneration is one of the most common causes of limited function in adults. A number of tissue engineering strategies have been used to develop composite tissue engineered total disc replacements to restore native tissue structure and function. In this study we fabricated a composite engineered disc based on the combination of a porous polycaprolactone (PCL) foam annulus fibrosus (AF) and a hyaluronic acid (HA) hydrogel nucleus pulposus (NP). To evaluate whether native tissue cells or mesenchymal stem cells (MSCs) would perform better, constructs were seeded with native AF/NP cells or with MSCs in the foam and/or gel region. Maturation of these composite engineered discs was evaluated for 9 weeks in vitro culture by biochemical content, histological analysis and mechanical properties. To evaluate the performance of these constructs in the in vivo space, engineered discs were implanted into the caudal spines of athymic rats for 5 weeks. Our findings show that engineered discs comprised of AF/NP cells and MSCs performed similarly and maintained their structure after 5 weeks in vivo. However, for both cell types, loss of proteoglycan was evident in the NP region. These data support the continued development of the more clinically relevant MSCs population for disc replacement applications. STATEMENT OF SIGNIFICANCE: A number of tissue engineering strategies have emerged that are focused on the creation of a composite disc replacement. We fabricated a composite engineered disc based on the combination of a porous foam AF and a HA gel NP. We used these constructs to determine whether the combination of AF/NP cells or MSCs would mature to a greater extent in vitro and which cell type would best retain their phenotype after implantation. Engineered discs comprised of AF/NP cells and MSCs performed similarly, maintaining their structure after 5 weeks in vivo. These data support the successful fabrication and in vivo function of an engineered disc composed of a PCL foam AF and a hydrogel NP using either disc cells or MSCs.

Replicating landmine blast loading in cellular in vitro models

Trauma arising from landmines and improvised explosive devices promotes heterotopic ossification, the formation of extra-skeletal bone in non-osseous tissue. To date, experimental platforms that can replicate the loading parameter space relevant to improvised explosive device and landmine blast wave exposure have not been available to study the effects of such non-physiological mechanical loading on cells. Here, we present the design and calibration of three distinct in vitro experimental loading platforms that allow us to replicate the spectrum of loading conditions recorded in near-field blast wave exposure. We subjected cells in suspension or in a three-dimensional hydrogel to strain rates up to 6000 s^-1 and pressure levels up to 45 MPa. Our results highlight that cellular activation is regulated in a non-linear fashion-not by a single mechanical parameter, it is the combined action of the applied mechanical pressure, rate of loading and loading impulse, along with the extracellular environment used to convey the pressure waves. Finally, our research indicates that PO MSCs are finely tuned to respond to mechanical stimuli that fall within defined ranges of loading.

Role of N-Cadherin in a Niche-Mimicking Microenvironment for Chondrogenesis of Mesenchymal Stem Cells In Vitro

During the development of natural cartilage, mesenchymal condensation is the starting event of chondrogenesis, and mesenchymal stem cells (MSCs) experienced a microenvironment transition from primarily cell-cell interactions to a later stage, where cell-extracellular matrix (ECM) interactions dominate. Although micromass pellet culture has been developed to mimic mesenchymal condensation in vitro, the molecular mechanism remains elusive, and the transition from cell-cell to cell-ECM interactions has been poorly recapitulated. In this study, we first constructed MSC microspheres (MMs) and investigated their chondrogenic differentiation with functional blocking of N-cadherin. The results showed that early cartilage differentiation and cartilage-specific matrix deposition of MSCs in the group with the N-cadherin antibody were significantly postponed. Next, poly(l-lysine) treatment was transiently applied to promote the expression of N-cadherin gene, CDH2, and the treatment-promoted MSC chondrogenesis. Upon one-day culture in MMs with established cell-cell adhesions, collagen hydrogel-encapsulated MMs (CMMs) were constructed to simulate the cell-ECM interactions, and the collagen microenvironment compensated the inhibitory effects from N-cadherin blocking. Surprisingly, chondrogenic-differentiated cell migration, which has important implications in cartilage repair and integration, was found in the CMMs without N-cadherin blocking. In conclusion, our study demonstrated that N-cadherin plays the critical role in early mesenchymal condensation, and the collagen hydrogel provides a supportive microenvironment for late chondrogenic differentiation. Therefore, sequential presentations of cell-cell adhesion and cell-ECM interaction in an engineered microenvironment seem to be a promising strategy to facilitate MSC chondrogenic differentiation.

Investigation of multiphasic 3D-bioplotted scaffolds for site-specific chondrogenic and osteogenic differentiation of human adipose-derived stem cells for osteochondral tissue engineering applications

Osteoarthritis is a degenerative joint disease that limits mobility of the affected joint due to the degradation of articular cartilage and subchondral bone. The limited regenerative capacity of cartilage presents significant challenges when attempting to repair or reverse the effects of cartilage degradation. Tissue engineered medical products are a promising alternative to treat osteochondral degeneration due to their potential to integrate into the patient's existing tissue. The goal of this study was to create a scaffold that would induce site-specific osteogenic and chondrogenic differentiation of human adipose-derived stem cells (hASC) to generate a full osteochondral implant. Scaffolds were fabricated using 3D-bioplotting of biodegradable polycraprolactone (PCL) with either β-tricalcium phosphate (TCP) or decellularized bovine cartilage extracellular matrix (dECM) to drive site-specific hASC osteogenesis and chondrogenesis, respectively. PCL-dECM scaffolds demonstrated elevated matrix deposition and organization in scaffolds seeded with hASC as well as a reduction in collagen I gene expression. 3D-bioplotted PCL scaffolds with 20% TCP demonstrated elevated calcium deposition, endogenous alkaline phosphatase activity, and osteopontin gene expression. Osteochondral scaffolds comprised of hASC-seeded 3D-bioplotted PCL-TCP, electrospun PCL, and 3D-bioplotted PCL-dECM phases were evaluated and demonstrated site-specific osteochondral tissue characteristics. This technique holds great promise as cartilage morbidity is minimized since autologous cartilage harvest is not required, tissue rejection is minimized via use of an abundant and accessible source of autologous stem cells, and biofabrication techniques allow for a precise, customizable methodology to rapidly produce the scaffold.

Shape-dependent regulation of differentiation lineages of bone marrow-derived cells under cyclic stretch

Multipotent stem cells are considered as a key material in regenerative medicine, and the understanding of the heterogeneity in the differentiation potentials of bone marrow-derived cells is important in the successful regenerative tissue repair. Therefore, the present study has been performed to investigate how the differentiation of post-harvest, native bone marrow-derived cells is regulated by cyclic stretch in vitro. Bone marrow-derived cells were obtained from mouse femur of both hind limbs and categorized into the following five categories: amebocytes, round cells, spindle cells, stellate cells and others. The cells were seeded on a silicone-made stretch chamber, and subjected to cyclic stretch with an amplitude of 10% at a frequency of 1 Hz for 7 days for cell shape analysis and for 3 days for the analysis of the expression of marker proteins of osteogenic (osteocalcin), vascular smooth muscle (α-smooth muscle actin and smooth muscle myosin heavy chain) and neurogenic (neurofilament) differentiation. When disregarding the differences in the cell shapes, there was an overall trend that the application of 10% cyclic stretch inhibited osteogenic and neurogenic differentiation, but enhanced smooth muscle differentiation. Close examinations revealed that round cells were influenced the most by cyclic stretch (significant up- or down-regulation in all the four marker protein expressions) while amebocytes and spindle cells were only influenced by cyclic stretch for vascular smooth muscle and/or neurogenic differentiation. As far as the authors know, this is the first study reporting the shape-related differences in the fate decision criteria for mechanical strain in bone marrow-derived cells.

Control of Dissipation Sources: A Central Aspect for Enhancing the Mechanical and Mechanobiological Performances of Hydrogels

Development of mechanically durable and biologically inductive hydrogels is a major challenge for load-bearing applications such as engineered cartilage. Dissipative capacity of articular cartilage is central to its functional behavior when submitted to loading. While fluid frictional drag is playing a significant role in this phenomenon, the flow-dependent source of dissipation is mostly overlooked in the design of hydrogel scaffolds. Herein, we propose an original strategy based on the combination of fluidic and polymeric dissipation sources to simultaneously enhance hydrogel mechanical and mechanobiological performances. The nondestructive dissipation processes were carefully designed by hybrid cross-linking of the hydrogel network and low permeability of the porous structure. It was found that intrachain and pore water distribution in the porous hydrogels improves the mechanical properties in high water fractions. In contrast to widely reported tough hydrogels presenting limited load support capability at low strain values, we obtained stiff and dissipative hydrogels with unique fatigue behavior. We showed that the fatigue resistance capability is not a function of morphology, dissipation level, and stiffness of the viscoelastic hydrogels but rather depends on the origin of the dissipation. Moreover, the preserved dissipation source under mechanical stimulation maintained a mechanoinductive niche for enhancing chondrogenesis owing to fluid frictional drag contribution. The proposed strategy can be widely used to design functional scaffolds in high loading demands for enduring physiological stimuli and generating regulatory cues to cells.

N-Cadherin Mimetic Peptide Nanofiber System Induces Chondrogenic Differentiation of Mesenchymal Stem Cells

Cadherins are vital for cell-to-cell interactions during tissue growth, migration, and differentiation processes. Both biophysical and biochemical inputs are generated upon cell-to-cell adhesions, which determine the fate of the mesenchymal stem cells (MSCs). The effect of cadherin interactions on the MSC differentiation still remains elusive. Here we combined the N-Cadherin mimetic peptide (HAV-PA) with the self-assembling E-PA and the resultant N-cadherin mimetic peptide nanofibers promoted chondrogenic differentiation of MSCs in conjunction with chondrogenic factors as a synthetic extracellular matrix system. Self-assembly of the precursor peptide amphiphile molecules HAV-PA and E-PA enable the organization of HAV peptide residues in close proximity to the cell interaction site, forming a supramolecular N-cadherin-like system. These bioactive peptide nanofibers not only promoted viability and enhanced adhesion of MSCs but also augmented the expression of cartilage specific matrix components compared to the nonbioactive control nanofibers. Overall, the N-cadherin mimetic peptide nanofiber system facilitated MSC commitment into the chondrogenic lineage presenting an alternative bioactive platform for stem-cell-based cartilage regeneration.

Silk fibroin-poly(lactic acid) biocomposites: Effect of protein-synthetic polymer interactions and miscibility on material properties and biological responses

A protein-polymer blend system based on silkworm silk fibroin (SF) and polylactic acid (PLA) was systematically investigated to understand the interaction and miscibility of proteins and synthetic biocompatible polymers in the macro- and micro-meter scales, which can dramatically control the cell responses and enzyme biodegradation on the biomaterial interface. Silk fibroin, a semicrystalline protein with beta-sheet crystals, provides controllable crystal content and biodegradability; while noncrystallizable PDLLA provides hydrophobicity and thermal stability in the system. Differential scanning calorimetry (DSC) combined with scanning electron microscope (SEM) showed that the morphology of the blend films was uniform on a macroscopic scale, yet with tunable micro-phase patterns at different mixing ratios. Fourier transform infrared analysis (FTIR) revealed that structures of the blend system, such as beta-sheet crystal content, gradually changed with the mixing ratios. All blended samples have better stability than pure SF and PLA samples as evidenced by thermogravimetric analysis. Protease XIV enzymatic study showed that the biodegradability of the blend samples varied with their blending ratios and microscale morphologies. Significantly, the topology of the micro-phase patterns on the blends can promote cell attachment and manipulate the cell growth and proliferation. This study provided a useful platform for understanding the fabrication strategies of protein-synthetic polymer composites that have direct biomedical and green chemistry applications.

Comparing Single Cell Versus Pellet Encapsulation of Mesenchymal Stem Cells in Three-Dimensional Hydrogels for Cartilage Regeneration

While the gold standard for inducing mesenchymal stem cell (MSC) chondrogenesis utilizes pellet culture, most tissue engineering strategies for cartilage regeneration encapsulate MSCs as single cells, partially due to the technical challenge to homogeneously encapsulate cell pellets in three-dimensional (3D) hydrogels. It remains unclear whether encapsulating MSCs as single cell suspension or cell aggregates in 3D hydrogels would enhance MSC-based cartilage formation. In this study, we determined that the optimal size of MSC micropellets (μPellets) that can be homogeneously encapsulated in hydrogels with high cell viability is 100 cells/pellet. Using optimized μPellet size, MSCs were encapsulated either as single cell suspension or μPellets in four soft hydrogel formulations with stiffness ranging 3-6 kPa. Regardless of hydrogel formulations, single cell encapsulation resulted in more neocartilage deposition with improved mechanical functions over μPellet encapsulation. For single cell encapsulation, polyethylene glycol (PEG) hydrogels containing chondroitin sulfate led to the most cartilage matrix deposition, with compressive modulus reaching 211 kPa after only 21 days, a range approaching the stiffness of native cartilage. The findings from this study offer valuable insights on guiding optimal method design for MSCs and hydrogel-based cartilage regeneration. The optimized μPellet encapsulation method may be broadly applicable to encapsulate other stem cell types or cancer cells as aggregates in hydrogels. Impact Statement While the gold standard for inducing mesenchymal stem cell (MSC) chondrogenesis utilizes pellet culture, it remains unclear whether encapsulating MSCs as cell pellets in three-dimensional hydrogels would enhance MSC-based cartilage formation. In this study, we determined the optimal size of MSC micropellet (μPellet) that can be homogeneously encapsulated in hydrogels with high cell viability. Unexpectedly, single cell encapsulation resulted in more robust new cartilage formation than μPellet encapsulation. Furthermore, tuning hydrogel formulation led to rapid cartilage regeneration with stiffness approaching that of native cartilage. The findings from this study would facilitate clinical translation of MSCs and hydrogel-based therapies for cartilage regeneration with optimized parameters.

Dynamic observation and quantification of type I/II collagen in chondrogenesis of mesenchymal stem cells by second-order susceptibility microscopy

Second-order susceptibility (SOS) microscopy is used to image and characterize chondrogenesis in cultured human mesenchymal stem cells. SOS analysis shows that the SOS tensor ratios can be used to characterize type I and II collagens in living tissues and that both collagen types are produced at the onset of chondrogenesis. Time-lapse analysis shows a modulation of extracellular matrix results in a higher rate in increase of type II collagen, as compared to type I collagen. With time, type II collagen content stabilizes at the composition of 70% of total collagen content. SOS microscopy can be used to continuously and noninvasively monitor the production of collagens I and II. With additional development, this technique can be developed into an effective quality control tool for monitoring extracellular matrix production in engineered tissues.

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.

Cell number in mesenchymal stem cell aggregates dictates cell stiffness and chondrogenesis

Background

Although mesenchymal stem/stromal cell (MSC) chondrogenic differentiation has been thoroughly investigated, the rudiments for enhancing chondrogenesis have remained largely dependent on external cues. Focus to date has been on extrinsic variables such as soluble signals, culture conditions (bioreactors), and mechanical stimulation. However, the role of intrinsic mechanisms of MSC programming-based mechanobiology remains to be explored. Since aggregation of MSCs, a prerequisite for chondrogenesis, generates tension within the cell agglomerate, we inquired if the initial number of cells forming the aggregate (aggregate cell number (ACN)) can impact chondrogenesis.

Methods

Aggregates of varying ACN were formed using well-established centrifugation approach. Progression of chondrogenic differentiation in the aggregates was assessed over 3 weeks in presence and absence of transforming growth factor-beta 1 (TGF-β1). Mechanical properties of the cells were characterized using high-throughput real-time deformability cytometry (RT-DC), and gene expression was analyzed using Affymetrix gene array. Expression of molecular markers linked to chondrogenesis was assessed using western blot and immunofluorescence.

Results

Reducing ACN from 500 k to 70 k lead to activation and acceleration of the chondrogenic differentiation, independent of soluble chondro-inductive factors, which involves changes to β-catenin-dependent TCF/LEF transcriptional activity and expression of anti-apoptotic protein survivin. RT-DC analysis revealed that stiffness and size of cells within aggregates are modulated by ACN. A direct correlation between progression of chondrogenesis and emergence of stiffer cell phenotype was found. Affymetrix gene array analysis revealed a downregulation of genes associated with lipid synthesis and regulation, which could account for observed changes in cell stiffness. Immunofluorescence and western blot analysis revealed that increasing ACN upregulates the expression of lipid raft protein caveolin-1, a β-catenin binding partner, and downregulates the expression of N-cadherin. As a demonstration of the relevance of these findings in MSC-based strategies for skeletal repair, it is shown that implanting aggregates within collagenous matrix not only decreases the necessity for high cell numbers but also leads to marked improvement in the quality of the deposited tissue.

Conclusions

This study presents a simple and donor-independent strategy to enhance the efficiency of MSC chondrogenic differentiation and identifies changes in cell mechanics coincident with MSC chondrogenesis with potential translational applications.

Electronic supplementary material

The online version of this article (10.1186/s13287-018-1103-y) contains supplementary material, which is available to authorized users.

Chondrocyte and mesenchymal stem cell derived engineered cartilage exhibits differential sensitivity to pro-inflammatory cytokines

Tissue engineering is a promising approach for the repair of articular cartilage defects, with engineered constructs emerging that match native tissue properties. However, the inflammatory environment of the damaged joint might compromise outcomes, and this may be impacted by the choice of cell source in terms of their ability to operate anabolically in an inflamed environment. Here, we compared the response of engineered cartilage derived from native chondrocytes and mesenchymal stem cells (MSCs) to challenge by TNFα and IL-1β in order to determine if either cell type possessed an inherent advantage. Compositional (extracellular matrix) and functional (mechanical) characteristics, as well as the release of catabolic mediators (matrix metalloproteinases [MMPs], nitric oxide [NO]) were assessed to determine cell- and tissue-level changes following exposure to IL-1β or TNF-α. Results demonstrated that MSC-derived constructs were more sensitive to inflammatory mediators than chondrocyte-derived constructs, exhibiting a greater loss of proteoglycans and functional properties at lower cytokine concentrations. While MSCs and chondrocytes both have the capacity to form functional engineered cartilage in vitro, this study suggests that the presence of an inflammatory environment is more likely to impair the in vivo success of MSC-derived cartilage repair. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2901-2910, 2018.

Composite Cellularized Structures Created from an Interpenetrating Polymer Network Hydrogel Reinforced by a 3D Woven Scaffold

Biomaterial scaffolds play multiple roles in cartilage tissue engineering, including controlling architecture of newly formed tissue while facilitating growth of embedded cells and simultaneously providing functional properties to withstand the mechanical environment within the native joint. In particular, hydrogels-with high water content and desirable transport properties-while highly conducive to chondrogenesis, often lack functional mechanical properties. In this regard, interpenetrating polymer network (IPN) hydrogels can provide mechanical toughness greatly exceeding that of individual components; however, many IPN materials are not biocompatible for cell encapsulation. In this study, an agarose and poly(ethylene) glycol IPN hydrogel is seeded with human mesenchymal stem cells (MSCs). Results show high viability of MSCs within the IPN hydrogel, with improved mechanical properties compared to constructs comprised of individual components. These properties are further strengthened by integrating the hydrogel with a 3D woven structure. The resulting fiber-reinforced hydrogels display functional macroscopic mechanical properties mimicking those of native articular cartilage, while providing a local microenvironment that supports cellular viability and function. These findings suggest that a fiber-reinforced IPN hydrogel can support stem cell chondrogenesis while allowing for significantly enhanced, complex mechanical properties at multiple scales as compared to individual hydrogel or fiber components.

Cartilage Differentiation of Bone Marrow-Derived Mesenchymal Stem Cells in Three-Dimensional Silica Nonwoven Fabrics

In cartilage tissue engineering, three-dimensional (3D) scaffolds provide native extracellular matrix (ECM) environments that induce tissue ingrowth and ECM deposition for in vitro and in vivo tissue regeneration. In this report, we investigated 3D silica nonwoven fabrics (Cellbed®) as a scaffold for mesenchymal stem cells (MSCs) in cartilage tissue engineering applications. The unique, highly porous microstructure of 3D silica fabrics allows for immediate cell infiltration for tissue repair and orientation of cell–cell interaction. It is expected that the morphological similarity of silica fibers to that of fibrillar ECM contributes to the functionalization of cells. Human bone marrow-derived MSCs were cultured in 3D silica fabrics, and chondrogenic differentiation was induced by culture in chondrogenic differentiation medium. The characteristics of chondrogenic differentiation including cellular growth, ECM deposition of glycosaminoglycan and collagen, and gene expression were evaluated. Because of the highly interconnected network structure, stiffness, and permeability of the 3D silica fabrics, the level of chondrogenesis observed in MSCs seeded within was comparable to that observed in MSCs maintained on atelocollagen gels, which are widely used to study the chondrogenesis of MSCs in vitro and in vivo. These results indicated that 3D silica nonwoven fabrics are a promising scaffold for the regeneration of articular cartilage defects using MSCs, showing the particular importance of high elasticity.

Sheep as a model for evaluating mesenchymal stem/stromal cell (MSC)-based chondral defect repair

Osteoarthritis results from the degradation of articular cartilage and is one of the leading global causes of pain and immobility. Cartilage has a limited capacity for self-repair. While repair can be enhanced through surgical intervention, current methods often generate inferior fibrocartilage and repair is transient. The development of tissue engineering strategies to improve repair outcomes is an active area of research. While small animal models such as rodents and rabbits are often used in early pre-clinical work, larger animals that better recapitulate the anatomy and loading of the human joint are required for late-stage preclinical evaluation. Because of their physiological similarities to humans, and low cost relative to other large animals, sheep are routinely used in orthopedic research, including cartilage repair studies. In recent years, there has been considerable research investment into the development of cartilage repair strategies that utilize mesenchymal stem/stromal cells (MSC). In contrast to autologous chondrocytes derived from biopsies of articular cartilage, MSC offer some benefits including greater expansion capacity and elimination of the risk of morbidity at the cartilage biopsy site. The disadvantages of MSC are related to the challenges of inducing and maintaining a stable chondrocyte-like cell population capable of generating hyaline cartilage. Ovine MSC (oMSC) biology and their utility in sheep cartilage repair models have not been reviewed. Herein, we review the biological properties of MSC derived from sheep tissues, and the use of these cells to study articular cartilage repair in this large animal model.

Hydrogel scaffolds for differentiation of adipose-derived stem cells

Natural extracellular matrices (ECMs) have been widely used as a support for the adhesion, migration, differentiation, and proliferation of adipose-derived stem cells (ADSCs). However, poor mechanical behavior and unpredictable biodegradation properties of natural ECMs considerably limit their potential for bioapplications and raise the need for different, synthetic scaffolds. Hydrogels are regarded as the most promising alternative materials as a consequence of their excellent swelling properties and their resemblance to soft tissues. A variety of strategies have been applied to create synthetic biomimetic hydrogels, and their biophysical and biochemical properties have been modulated to be suitable for cell differentiation. In this review, we first give an overview of common methods for hydrogel preparation with a focus on those strategies that provide potential advantages for ADSC encapsulation, before summarizing the physical properties of hydrogel scaffolds that can act as biological cues. Finally, the challenges in the preparation and application of hydrogels with ADSCs are explored and the perspectives are proposed for the next generation of scaffolds.

A Comparison of Bone Marrow and Cord Blood Mesenchymal Stem Cells for Cartilage Self-Assembly

Joint injury is a common cause of premature retirement for the human and equine athlete alike. Implantation of engineered cartilage offers the potential to increase the success rate of surgical intervention and hasten recovery times. Mesenchymal stem cells (MSCs) are a particularly attractive cell source for cartilage engineering. While bone marrow-derived MSCs (BM-MSCs) have been most extensively characterized for musculoskeletal tissue engineering, studies suggest that cord blood MSCs (CB-MSCs) may elicit a more robust chondrogenic phenotype. The objective of this study was to determine a superior equine MSC source for cartilage engineering. MSCs derived from bone marrow or cord blood were stimulated to undergo chondrogenesis through aggregate redifferentiation and used to generate cartilage through the self-assembling process. The resulting neocartilage produced from either BM-MSCs or CB-MSCs was compared by measuring mechanical, biochemical, and histological properties. We found that while BM constructs possessed higher tensile properties and collagen content, CB constructs had superior compressive properties comparable to that of native tissue and higher GAG content. Moreover, CB constructs had alkaline phosphatase activity, collagen type X, and collagen type II on par with native tissue suggesting a more hyaline cartilage-like phenotype. In conclusion, while both BM-MSCs and CB-MSCs were able to form neocartilage, CB-MSCs resulted in tissue more closely resembling native equine articular cartilage as determined by a quantitative functionality index. Therefore, CB-MSCs are deemed a superior source for the purpose of articular cartilage self-assembly.

Towards the scale up of tissue engineered intervertebral discs for clinical application

Replacement of the intervertebral disc with a viable, tissue-engineered construct that mimics native tissue structure and function is an attractive alternative to fusion or mechanical arthroplasty for the treatment of disc pathology. While a number of engineered discs have been developed, the average size of these constructs remains a fraction of the size of human intervertebral discs. In this study, we fabricated medium (3 mm height × 10 mm diameter) and large (6 mm height × 20 mm diameter) sized disc-like angle ply structures (DAPS), encompassing size scales from the rabbit lumbar spine to the human cervical spine. Maturation of these engineered discs was evaluated over 15 weeks in culture by quantifying cell viability and metabolic activity, construct biochemical content, MRI T2 values, and mechanical properties. To assess the performance of the DAPS in the in vivo space, pre-cultured DAPS were implanted subcutaneously in athymic rats for 5 weeks. Our findings show that both sized DAPS matured functionally and compositionally during in vitro culture, as evidenced by increases in mechanical properties and biochemical content over time, yet large DAPS under-performed compared to medium DAPS. Subcutaneous implantation resulted in reductions in NP cell viability and GAG content at both size scales, with little effect on AF biochemistry or metabolic activity. These findings demonstrate that engineered discs at large size scales will mature during in vitro culture, however, future work will need to address the challenges of reduced cell viability and heterogeneous matrix distribution throughout the construct.

STATEMENT OF SIGNIFICANCE

This work establishes, for the first time, tissue-engineered intervertebral discs for total disc replacement at large, clinically relevant length scales. Clinical translation of tissue-engineered discs will offer an alternative to mechanical disc arthroplasty and fusion procedures, and may contribute to a paradigm shift in the clinical care for patients with disc pathology and associated axial spine and neurogenic extremity pain.

Strain distribution of repaired articular cartilage defects by tissue engineering under compression loading

Background

It is difficult to repair cartilage damage when cartilage undergoes trauma or degeneration. Cartilage tissue engineering is an ideal treatment method to repair cartilage defects, but at present, there are still some uncertainties to be researched in cartilage tissue engineering including the mechanical properties of the repaired region.

Methods

In this study, using an agarose gel as artificial cartilage implanted into the cartilage defect and gluing the agarose gel to cartilage by using the medical bio-adhesive, the full-thickness and half-thickness defects models of articular cartilage in vitro repaired by tissue engineering were constructed. Strain behaviors of the repaired region were analyzed by the digital correlation technology under 5, 10, 15, and 20% compressive load.

Results

The axial normal strain (Ex) perpendicular to the surface of the cartilage and lateral normal strain (Ey) as well as shear strain (Exy) appeared obviously heterogeneous in the repaired region. In the full-defect model, Ex showed depth-dependent strain profiles where maximum Ex occurs at the low middle zone while in the half-defect mode, Ex showed heterogeneous strain profiles where maximum Ex occurs at the near deep zone. Ey and Exy at the interface site of both models present significantly differed from the host cartilage site. Ey and Exy exhibited region-specific change at the host, interface, and artificial cartilage sites in the superficial, middle, and deep zones due to the artificial cartilage implantation.

Conclusion

Both defect models of cartilage exhibited a heterogeneous strain field due to the engineered cartilage tissue implant. The abnormal strain field can cause the cells within the repaired area to enter complex mechanical states which will affect the restoration of cartilage defects.

The role of Sox9 in collagen hydrogel-mediated chondrogenic differentiation of adult mesenchymal stem cells (MSCs)

Sox9 is a transcription factor that regulates chondrogenesis, but its role in the chondrogenic differentiation of mesenchymal stem cells (MSCs) triggered by materials is poorly understood.

On the origin and impact of mesenchymal stem cell heterogeneity: new insights and emerging tools for single cell analysis

Mesenchymal stem cells (MSCs) display substantial cell-to-cell variation. This heterogeneity manifests among donors, among tissue sources, and within cell populations. Such pervasive variability complicates the use of MSCs in regenerative applications and may limit their therapeutic efficacy. Most conventional assays measure MSC properties in bulk and, as a consequence, mask this cell-to-cell variation. Recent studies have identified extensive variability amongst and within clonal MSC populations, in dimensions including functional differentiation capacity, molecular state (e.g. epigenetic, transcriptomic, and proteomic status), and biophysical properties. While the origins of these variations remain to be elucidated, potential mechanisms include in vivo micro-anatomical heterogeneity, epigenetic bistability, and transcriptional fluctuations. Emerging tools for single cell analysis of MSC gene and protein expression may yield further insight into the mechanisms and implications of single cell variation amongst these cells, and ultimately improve the clinical utility of MSCs in tissue engineering and regenerative medicine applications. This review outlines the dimensions across which MSC heterogeneity is present, defines some of the known mechanisms that govern this heterogeneity, and highlights emerging technologies that may further refine our understanding and improve our clinical application of this unique cell type.

Micrometer scale guidance of mesenchymal stem cells to form structurally oriented large-scale tissue engineered cartilage

Current clinical methods to treat articular cartilage lesions provide temporary relief of the symptoms but fail to permanently restore the damaged tissue. Tissue engineering, using mesenchymal stem cells (MSCs) combined with scaffolds and bioactive factors, is viewed as a promising method for repairing cartilage injuries. However, current tissue engineered constructs display inferior mechanical properties compared to native articular cartilage, which could be attributed to the lack of structural organization of the extracellular matrix (ECM) of these engineered constructs in comparison to the highly oriented structure of articular cartilage ECM. We previously showed that we can guide MSCs undergoing chondrogenesis to align using microscale guidance channels on the surface of a two-dimensional (2-D) collagen scaffold, which resulted in the deposition of aligned ECM within the channels and enhanced mechanical properties of the constructs. In this study, we developed a technique to roll 2-D collagen scaffolds containing MSCs within guidance channels in order to produce a large-scale, three-dimensional (3-D) tissue engineered cartilage constructs with enhanced mechanical properties compared to current constructs. After rolling the MSC-scaffold constructs into a 3-D cylindrical structure, the constructs were cultured for 21days under chondrogenic culture conditions. The microstructure architecture and mechanical properties of the constructs were evaluated using imaging and compressive testing. Histology and immunohistochemistry of the constructs showed extensive glycosaminoglycan (GAG) and collagen type II deposition. Second harmonic generation imaging and Picrosirius red staining indicated alignment of neo-collagen fibers within the guidance channels of the constructs. Mechanical testing indicated that constructs containing the guidance channels displayed enhanced compressive properties compared to control constructs without these channels. In conclusion, using a novel roll-up method, we have developed large scale MSC based tissue-engineered cartilage that shows microscale structural organization and enhanced compressive properties compared to current tissue engineered constructs.

STATEMENT OF SIGNIFICANCE

Tissue engineered cartilage constructs made with human mesenchymal stem cells (hMSCs), scaffolds and bioactive factors are a promising solution to treat cartilage defects. A major disadvantage of these constructs is their inferior mechanical properties compared to the native tissue, which is likely due to the lack of structural organization of the extracellular matrix of the engineered constructs. In this study, we developed three-dimensional (3-D) cartilage constructs from rectangular scaffold sheets containing hMSCs in micro-guidance channels and characterized their mechanical properties and metabolic requirements. The work led to a novel roll-up method to embed 2-D microscale structures in 3-D constructs. Further, micro-guidance channels incorporated within the 3-D cartilage constructs led to the production of aligned cell-produced matrix and enhanced mechanical function.

Effects of Type I Collagen Concentration in Hydrogel on the Growth and Phenotypic Expression of Rat Chondrocytes

It is controversial whether type I collagen itself can maintain and improve chondrogenic phenotype of chondrocytes in a three-dimensional (3D) environment. In this study, we examined the effect of type I collagen concentration in hydrogel (0.5, 1, and 2 mg/ml) on the growth and phenotype expression of rat chondrocytes in vitro. All collagen hydrogels showed substantial contractions during culture, in a concentration-dependent manner, which was due to the cell proliferation. The cell viability was shown to be the highest in 2 mg/ml collagen gel. The mRNA expression of chondrogenic phenotypes, including SOX9, type II collagen, and aggrecan, was significantly up-regulated, particularly in 1 mg/ml collagen gel. Furthermore, the production of type II collagen and glycosaminoglycan (GAG) content was also enhanced. The results suggest that type I collagen hydrogel is not detrimental to, but may be useful for, the chondrocyte culture for cartilage tissue engineering.

Effects of mechanical loading on human mesenchymal stem cells for cartilage tissue engineering

Today, articular cartilage damage is a major health problem, affecting people of all ages. The existing conventional articular cartilage repair techniques, such as autologous chondrocyte implantation (ACI), microfracture, and mosaicplasty, have many shortcomings which negatively affect their clinical outcomes. Therefore, it is essential to develop an alternative and efficient articular repair technique that can address those shortcomings. Cartilage tissue engineering, which aims to create a tissue-engineered cartilage derived from human mesenchymal stem cells (MSCs), shows great promise for improving articular cartilage defect therapy. However, the use of tissue-engineered cartilage for the clinical therapy of articular cartilage defect still remains challenging. Despite the importance of mechanical loading to create a functional cartilage has been well demonstrated, the specific type of mechanical loading and its optimal loading regime is still under investigation. This review summarizes the most recent advances in the effects of mechanical loading on human MSCs. First, the existing conventional articular repair techniques and their shortcomings are highlighted. The important parameters for the evaluation of the tissue-engineered cartilage, including chondrogenic and hypertrophic differentiation of human MSCs are briefly discussed. The influence of mechanical loading on human MSCs is subsequently reviewed and the possible mechanotransduction signaling is highlighted. The development of non-hypertrophic chondrogenesis in response to the changing mechanical microenvironment will aid in the establishment of a tissue-engineered cartilage for efficient articular cartilage repair.

Concise Review: Mesenchymal Stem Cells for Functional Cartilage Tissue Engineering: Taking Cues from Chondrocyte-Based Constructs

Osteoarthritis, the most prevalent form of joint disease, afflicts 9% of the U.S. population over the age of 30 and costs the economy nearly $100 billion annually in healthcare and socioeconomic costs. It is characterized by joint pain and dysfunction, though the pathophysiology remains largely unknown. Due to its avascular nature and limited cellularity, articular cartilage exhibits a poor intrinsic healing response following injury. As such, significant research efforts are aimed at producing engineered cartilage as a cell-based approach for articular cartilage repair. However, the knee joint is mechanically demanding, and during injury, also a milieu of harsh inflammatory agents. The unforgiving mechano-chemical environment requires tissue replacements that are capable of bearing such burdens. The use of mesenchymal stem cells (MSCs) for cartilage tissue engineering has emerged as a promising cell source due to their ease of isolation, capacity to readily expand in culture, and ability to undergo lineage-specific differentiation into chondrocytes. However, to date, very few studies utilizing MSCs have successfully recapitulated the structural and functional properties of native cartilage, exposing the difficult process of uniformly differentiating stem cells into desired cell fates and maintaining the phenotype during in vitro culture and after in vivo implantation. To address these shortcomings, here, we present a concise review on modulating stem cell behavior, tissue development and function using well-developed techniques from chondrocyte-based cartilage tissue engineering. Stem Cells Translational Medicine 2017;6:1295-1303.

Influence of oxygen levels on chondrogenesis of porcine mesenchymal stem cells cultured in polycaprolactone scaffolds

Chondrogenesis of mesenchymal stem cells (MSCs) is known to be regulated by a number of environmental factors, including local oxygen levels. The hypothesis of this study is that the response of MSCs to hypoxia is dependent on the physical and chemical characteristics of the substrate used. The objective of this study was to explore how different modifications to polycaprolactone (PCL) scaffolds influenced the response of MSCs to hypoxia. PCL, PCL-hyaluronic acid (HA), and PCL-Bioglass^® (BG) scaffolds were seeded with MSCs derived from bone marrow and cultured for 35 days under normoxic or low oxygen conditions, and the resulting biochemical properties of the MSC laden construct were assessed. Low oxygen tension has a positive effect over cell proliferation and macromolecules biosynthesis. Furthermore, hypoxia enhanced the distribution of collagen and glycosaminoglycans (GAGs) deposition through the scaffold. On the other hand, MSCs displayed certain material dependent responses to hypoxia. Low oxygen tension had a positive effect on cell proliferation in BG and HA scaffolds, but only a positive effect on GAGs synthesis in PCL and HA scaffolds. In conclusion, hypoxia increased cell viability and expression of chondrogenic markers but the cell response was modulated by the type of scaffold used. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 1684-1691, 2017.

Comparative assessment of intrinsic mechanical stimuli on knee cartilage and compressed agarose constructs

A well-established cue for improving the properties of tissue-engineered cartilage is mechanical stimulation. However, the explicit ranges of mechanical stimuli that correspond to favorable metabolic outcomes are elusive. Usually, these outcomes have only been associated with the applied strain and frequency, an oversimplification that can hide the fundamental relationship between the intrinsic mechanical stimuli and the metabolic outcomes. This highlights two important key issues: the firstly is related to the evaluation of the intrinsic mechanical stimuli of native cartilage; the second, assuming that the intrinsic mechanical stimuli will be important, deals with the ability to replicate them on the tissue-engineered constructs. This study quantifies and compares the volume of cartilage and agarose subjected to a given magnitude range of each intrinsic mechanical stimulus, through a numerical simulation of a patient-specific knee model coupled with experimental data of contact during the stance phase of gait, and agarose constructs under direct-dynamic compression. The results suggest that direct compression loading needs to be parameterized with time-dependence during the initial culture period in order to better reproduce each one of the intrinsic mechanical stimuli developed in the patient-specific cartilage. A loading regime which combines time periods of low compressive strain (5%) and frequency (0.5Hz), in order to approach the maximal principal strain and fluid velocity stimulus of the patient-specific cartilage, with time periods of high compressive strain (20%) and frequency (3Hz), in order to approach the pore pressure values, may be advantageous relatively to a single loading regime throughout the full culture period.

[Biology and mechanobiology of the intervertebral disc]

The intervertebral disc (IVD) is noted for its low cell content, and being the largest avascular structure of human body. The low amount of cells in the disc have to adapt to an anaerobic metabolism with low oxygen pressure and acidic pH. Apart from surviving in an adverse microenvironment, they are exposed to a high level of mechanical stress. The biological adaptation of cells to acidosis and hyperosmolarity conditions are regulated by mechanoproteins, which are responsible for converting a mechanical signal into a cellular response, thus modifying its gene expression. Mechanobiology helps us to better understand the pathophysiology of IVD and its potential biological repair.

Non-surgical Management of Congenital Auricular Deformities

BACKGROUND

Unlike congenital auricular malformations which are identified by underdevelopment of dermal and cartilaginous tissues, deformed ears are less sever congenital anomalies characterized only by a misshaped pinna structure and can be improved with acceptable cosmetic results and minimal cost through ear molding if treated in early neonatal period. In this study, authors present the first report of using splinting techniques for treatment of deformational auricular anomalies in Iranian children.

METHODS

Our case load consisted of a series of 29 patients (Male=16, Female=13) who were referred to Plastic Surgery Unit of Shiraz University of Medical Sciences from September 2011 to December 2014. Children aged more than 6 moths were excluded. Twenty-nine children affected by various deformities including prominent ears (n=11), lop ears (n=8) and constricted ears (n=10) were treated by splintage as a nonsurgical technique. The mean time of treatment was 13.33±2 weeks.

RESULTS

Eight (27.6%) patients did not complete the treatment. Splinting resulted in excellent or satisfactory results in 12 (57.14%) of treated cases. No improvement was observed at the end of the molding treatment in 9 patients. No complication was observed during the treatment in any of the patients.

CONCLUSION

The nonsurgical molding can be used as an effective approach for achieving natural outcomes and correcting cosmetic abnormalities. Rate of satisfaction is dependent on type of deformity, the neonatal age in which treatment started and also parents' adherence to treatment methods and principals. Concerning the low rate of complications and high satisfactory results the method can be used instead of surgery in appropriate cases.

Multifunctional Quantum Dot Nanoparticles for Effective Differentiation and Long-Term Tracking of Human Mesenchymal Stem Cells In Vitro and In Vivo

Human mesenchymal stem cells (hMSCs) hold great potential for regenerative medicine. Efficient induction of hMSC differentiation and better understanding of hMSCs behaviors in vitro and in vivo are essential to the clinical translation of stem cell therapy. Here a quantum dots (QDs)-based multifunctional nanoparticle (RGD-β-CD-QDs) is developed for effective enhancing differentiation and long-term tracking of hMSCs in vitro and in vivo. The RGD-β-CD-QDs are modified with β-cyclodextrin (β-CD) and Cys-Lys-Lys-Arg-Gly-Asp (CKKRGD) peptide on the surface. The β-CD can harbor hydrophobic osteogenic small molecule dexamethasone (Dex) and the RGD peptide not only facilitates the complexation of siRNA and delivers siRNA into hMSCs but also leads to cellular uptake of nanoparticles by RGD receptor. Co-delivery of Dex and siRNA by RGD-β-CD-QDs nanocarrier significantly expedites and enhances the osteogenesis differentiation of hMSCs in vitro and in vivo by combined effect of small molecule and RNAi. Furthermore, the RGD-β-CD-QDs can be labeled with hMSCs for a long-term tracking (3 weeks) in vivo to observe the behaviors of implanted hMSCs in animal level. These findings demonstrate that the RGD-β-CD-QDs nanocarrier provides a powerful tool to simultaneously enhance differentiation and long-term tracking of hMSCs in vitro and in vivo for regenerative medicine.

Single Cell Imaging to Probe Mesenchymal Stem Cell N-Cadherin Mediated Signaling within Hydrogels

N-cadherin (Ncad) mediates cell-cell interactions, regulates β-catenin (βcat) signaling, and promotes the chondrogenic differentiation of mesenchymal lineage cells. Here, we utilized confocal imaging to investigate the influence of Ncad interactions on single mesenchymal stem cell (MSC) behavior within 3-dimensional hydrogel environments under conditions that promote chondrogenic differentiation. Human MSCs were photoencapsulated in hyaluronic acid hydrogels functionalized with Ncad mimetic peptides and compared to cells in environments with control non-active peptides (Ctrl). Using single-cell imaging, we observed a significant increase in membrane βcat, nuclear βcat, and cell roundness after 3 days in Ncad hydrogels compared to Ctrl hydrogels. The extent of membrane and nuclear βcat localization and MSC roundness decreased to Ctrl hydrogel levels via pre-treatment with Ncad-specific antibodies prior to encapsulation in the Ncad hydrogels, confirming the activity of the peptide. Interestingly, there was a pronounced (>80%) increase in βcat nuclear localization in two-cell clusters within the Ctrl hydrogels, which was much greater than the increase (~30%) in βcat nuclear localization in two-cell clusters within the Ncad hydrogels. In summary, we utilized fluorescent imaging to demonstrate Ncad-mediated single cell responses to developmental cues within hydrogels towards chondrogenesis.

Cartilage tissue engineering: From biomaterials and stem cells to osteoarthritis treatments

Articular cartilage is a non-vascularized and poorly cellularized connective tissue that is frequently damaged as a result of trauma and degenerative joint diseases such as osteoarthrtis. Because of the absence of vascularization, articular cartilage has low capacity for spontaneous repair. Today, and despite a large number of preclinical data, no therapy capable of restoring the healthy structure and function of damaged articular cartilage is clinically available. Tissue-engineering strategies involving the combination of cells, scaffolding biomaterials and bioactive agents have been of interest notably for the repair of damaged articular cartilage. During the last 30 years, cartilage tissue engineering has evolved from the treatment of focal lesions of articular cartilage to the development of strategies targeting the osteoarthritis process. In this review, we focus on the different aspects of tissue engineering applied to cartilage engineering. We first discuss cells, biomaterials and biological or environmental factors instrumental to the development of cartilage tissue engineering, then review the potential development of cartilage engineering strategies targeting new emerging pathogenic mechanisms of osteoarthritis.

Single-cell differences in matrix gene expression do not predict matrix deposition

Mesenchymal stem cells (MSCs) display substantial cell-to-cell heterogeneity, complicating their use in regenerative medicine. However, conventional bulk assays mask this variability. Here we show that both chondrocytes and chondrogenically induced MSCs exhibit substantial mRNA expression heterogeneity. Single-molecule RNA FISH to measure mRNA expression of differentiation markers in single cells reveals that sister cell pairs have high levels of mRNA variability, suggesting that marker expression is not heritable. Surprisingly, this variability does not correlate with cell-to-cell differences in cartilage-like matrix production. Transcriptome-wide analysis suggests that no combination of markers can predict functional potential. De-differentiating chondrocytes also show a disconnect between mRNA expression of the cartilage marker aggrecan and cartilage-like matrix accumulation. Altogether, these quantitative analyses suggest that sorting subpopulations based on these markers would only marginally enrich the progenitor population for ‘superior' MSCs. Our results suggest that instantaneous mRNA abundance of canonical markers is tenuously linked to the chondrogenic phenotype at the single-cell level.

Regenerative tissue engineering with mesenchymal stem cells is hampered by bulk methods of assessing differentiation status and a general assumption that expression of individual markers of stem cell differentiation correlate with functional capacity. Here the authors debunk this assumption by applying single-cell techniques to disassociate aggrecan mRNA abundance and matrix deposition.

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.

Mechanobiology of TGFβ signaling in the skeleton

Physical and biochemical cues play fundamental roles in the skeleton at both the tissue and cellular levels. The precise coordination of these cues is essential for skeletal development and homeostasis, and disruption of this coordination can drive disease progression. The growth factor TGFβ is involved in both the regulation of and cellular response to the physical microenvironment. It is essential to summarize the current findings regarding the mechanisms by which skeletal cells integrate physical and biochemical cues so that we can identify and address remaining gaps that could ultimately improve skeletal health. In this review, we describe the role of TGFβ in mechanobiological signaling in bone and cartilage at the tissue and cellular levels. We provide detail on how static and dynamic physical cues at the macro-level are transmitted to the micro-level, ultimately leading to regulation at each level of the TGFβ pathway and to cell differentiation. The continued integration of engineering and biological approaches is needed to answer many remaining questions, such as the mechanisms by which cells generate a coordinated response to physical and biochemical cues. We propose one such mechanism, through which the combination of TGFβ and an optimal physical microenvironment leads to synergistic induction of downstream TGFβ signaling.

The development of a mature collagen network in cartilage from human bone marrow stem cells in Transwell culture

Damaged hyaline cartilage shows a limited capacity for innate repair. Potential sources of cells to augment the clinical repair of cartilage defects include autologous chondrocytes and mesenchymal stem cells. We have reported that culture of human bone marrow mesenchymal stem cells with specific growth and differentiation factors as shallow multilayers on Transwell permeable membranes provided ideal conditions for chondrogenesis. Rigid translucent cartilaginous disks formed and expressed cartilage-specific structural proteins aggrecan and type II collagen. We report here the analysis of the collagen network assembled in these cartilage constructs and identify key features of the network as it became mature during 28 days of culture. The type II collagen was co-polymerized with types XI and IX collagens in a fibrillar network stabilized by hydroxylysyl pyridinoline cross-links as in epiphyseal and hyaline cartilages. Tandem ion-trap mass-spectrometry identified 3-hydroxylation of Proline 986 and Proline 944 of the α1(II) chains, a post-translational feature of human epiphyseal cartilage type II collagen. The formation of a type II collagen based hydroxy-lysyl pyridinoline cross-linked network typical of cartilage in 28 days shows that the Transwell system not only produces, secretes and assembles cartilage collagens, but also provides all the extracellular mechanisms to modify and generate covalent cross-links that determine a robust collagen network. This organized assembly explains the stiff, flexible nature of the cartilage constructs developed from hMSCs in this culture system.