Compressive force promotes sox9, type II collagen and aggrecan and inhibits IL-1beta expression resulting in chondrogenesis in mouse embryonic limb bud mesenchymal cells

Muscle loading and endochondral ossification are involved in the regeneration of a fibrocartilaginous enthesis during tendon to bone healing in rabbits

PURPOSE

The purposes of the present study are to investigate the effects of reduced muscle loading by prolonged immobilization on the regeneration of fibrocartilaginous enthesis through endochondral ossification in rabbits.

METHODS

Forty-eight rabbits underwent standard partial patellectomy were randomly divided into the control group and the prolonged immobilization (PIM) group. The immobilized cast was only maintained for the first 4 weeks in the control group, while for the first 12 weeks or until euthanization in the PIM group. The Patella-patella tendon complexes were harvested for Micro-CT and histology at week 6, 12 and 18.

RESULTS

There was significantly lower bone volume in the PIM group than the control group at week 12, but not at week 6 and 18. At week 6, new bone was formed at the osteotomy site of the residual patella through endochondral ossification. At week 12, the chondrocytes in the tendon to bone interface were ordered and arranged in longitudinal rows separated by collagen fibres in the control group. While there were no visible fibers running continuously from tendon into bone in the PIM group. At week 18, a nearly normal fibrocartilaginous enthesis were regenerated in the control group. A similar fibrocartilaginous enthesis were also formed at the tendon to bone interface in the PIM group, but the four zones were not as distinct as that in the control group.

CONCLUSION

Muscle loading and endochondral ossification are involved in the regeneration of a fibrocartilaginous enthesis during tendon to bone healing in this partial patellectomy model.

Comparison of the structural features between chicken quadratomandibular joint and rat temporomandibular joint with reference to ligament and articular meniscus/disc

OBJECTIVES

The new jaw joint of mammals evolved between the squamosal and the dentary. We investigated the structural features of the chicken quadratomandibular joint (QMJ) (primary jaw joint) and the rat temporomandibular joint (TMJ) (new jaw joint) to determine whether these structures reflect their putative movements from macroscopic and histological perspectives.

METHODS

Chicken QMJ and rat TMJ were dissected, and their joint apparatuses, including ligaments and meniscus/disc, were analyzed macroscopically. Microscopic analysis, incorporating immunohistochemistry and in situ hybridization, was conducted to investigate protein localization and gene expression within the extracellular matrix of cartilage and bone.

RESULTS

The chicken QMJ contained the jugomandibular ligament, running laterally and posteriorly to the QMJ, tightly bound to the articular meniscus. These features suggest a role in supporting and controlling quadrate bone movements. The rat TMJ lacked a lateral ligament, and the mandibular head had an elongated oval shape along the anterior-posterior axis. This morphology indicates that lateral mandibular movements are rare, with jaw motion predominantly occurring in the anterior-posterior direction. The chicken QMJ meniscus contained distinct cartilaginous tissues, whereas the rat TMJ articular disc acquired cartilaginous characteristics after occlusion was established (5 weeks postnatally), suggesting that the meniscus/disc properties are closely associated with mastication.

CONCLUSIONS

The structural features of chicken and rat jaw joints, including their joint apparatuses, appropriately reflect their respective functions, such as movement dynamics and resistance to mastication pressure.

Mature murine supraspinatus tendons demonstrate regional differences in multiscale structure, function and gene expression

The hierarchical structure of tendon dictates its ability to effectively transmit loads from muscle to bone. Tendon- and site-specific differences in mechanical loading result in the establishment and remodeling of structure, as well as associated changes in composition throughout development and healing. Previous work has demonstrated region-specific differences in the response of collagen fibrils to mechanical loading within the insertion region and midsubstance regions of mouse supraspinatus tendons using atomic force microscopy. However, multiscale structure, function, and gene expression differences between the insertion and midsubstance of the supraspinatus tendon have not yet been linked together in a comprehensive study. Therefore, the purpose of this study was to elucidate site-specific hierarchical structure, function, and gene expression differences in mouse supraspinatus tendons. Supraspinatus tendons from day 150 wild-type C57BL/6 mice were harvested for regional mechanics, histology, transmission electron microscopy (TEM), and quantitative polymerase chain reaction (qPCR). Mechanical testing revealed that the midsubstance region demonstrated a greater modulus and increased collagen fiber realignment compared to the insertion region. Histological scoring demonstrated greater cellularity and more rounded cells in the insertion region. TEM analysis showed differences in collagen fibril diameter distributions between the two regions, with a shift towards smaller diameters observed at the insertion region. Gene expression analysis identified several genes that were differentially expressed between regions, with principal component analysis revealing distinct clustering based on region. These findings provide insight into the regional heterogeneity of the supraspinatus tendon and underscore the importance of considering these differences in the context of tendon injury and repair, contributing to a better understanding of tendon structure-function and guiding future studies aimed at elucidating the mechanisms underlying tendon pathology.

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 forces across compartments coordinate cell shape and fate transitions to generate tissue architecture

Morphogenesis and cell state transitions must be coordinated in time and space to produce a functional tissue. An excellent paradigm to understand the coupling of these processes is mammalian hair follicle development, which is initiated by the formation of an epithelial invagination-termed placode-that coincides with the emergence of a designated hair follicle stem cell population. The mechanisms directing the deformation of the epithelium, cell state transitions and physical compartmentalization of the placode are unknown. Here we identify a key role for coordinated mechanical forces stemming from contractile, proliferative and proteolytic activities across the epithelial and mesenchymal compartments in generating the placode structure. A ring of fibroblast cells gradually wraps around the placode cells to generate centripetal contractile forces, which, in collaboration with polarized epithelial myosin activity, promote elongation and local tissue thickening. These mechanical stresses further enhance compartmentalization of Sox9 expression to promote stem cell positioning. Subsequently, proteolytic remodelling locally softens the basement membrane to facilitate a release of pressure on the placode, enabling localized cell divisions, tissue fluidification and epithelial invagination into the underlying mesenchyme. Together, our experiments and modelling identify dynamic cell shape transformations and tissue-scale mechanical cooperation as key factors for orchestrating organ formation.

Combining biomechanical stimulation and chronobiology: a novel approach for augmented chondrogenesis?

The unique structure and composition of articular cartilage is critical for its physiological function. However, this architecture may get disrupted by degeneration or trauma. Due to the low intrinsic regeneration properties of the tissue, the healing response is generally poor. Low-grade inflammation in patients with osteoarthritis advances cartilage degradation, resulting in pain, immobility, and reduced quality of life. Generating neocartilage using advanced tissue engineering approaches may address these limitations. The biocompatible microenvironment that is suitable for cartilage regeneration may not only rely on cells and scaffolds, but also on the spatial and temporal features of biomechanics. Cell-autonomous biological clocks that generate circadian rhythms in chondrocytes are generally accepted to be indispensable for normal cartilage homeostasis. While the molecular details of the circadian clockwork are increasingly well understood at the cellular level, the mechanisms that enable clock entrainment by biomechanical signals, which are highly relevant in cartilage, are still largely unknown. This narrative review outlines the role of the biomechanical microenvironment to advance cartilage tissue engineering via entraining the molecular circadian clockwork, and highlights how application of this concept may enhance the development and successful translation of biomechanically relevant tissue engineering interventions.

Chondromodulin is necessary for cartilage callus distraction in mice

Chondromodulin (Cnmd) is a glycoprotein known to stimulate chondrocyte growth. We examined in this study the expression and functional role of Cnmd during distraction osteogenesis that is modulated by mechanical forces. The right tibiae of the mice were separated by osteotomy and subjected to slow progressive distraction using an external fixator. In situ hybridization and immunohistochemical analyses of the lengthened segment revealed that Cnmd mRNA and its protein in wild-type mice were localized in the cartilage callus, which was initially generated in the lag phase and was lengthened gradually during the distraction phase. In Cnmd null (Cnmd-/-) mice, less cartilage callus was observed, and the distraction gap was filled by fibrous tissues. Additionally, radiological and histological investigations demonstrated delayed bone consolidation and remodeling of the lengthened segment in Cnmd-/- mice. Eventually, Cnmd deficiency caused a one-week delay in the peak expression of VEGF, MMP2, and MMP9 genes and the subsequent angiogenesis and osteoclastogenesis. We conclude that Cnmd is necessary for cartilage callus distraction.

Biomechanical, biophysical and biochemical modulators of cytoskeletal remodelling and emergent stem cell lineage commitment

Across complex, multi-time and -length scale biological systems, redundancy confers robustness and resilience, enabling adaptation and increasing survival under dynamic environmental conditions; this review addresses ubiquitous effects of cytoskeletal remodelling, triggered by biomechanical, biophysical and biochemical cues, on stem cell mechanoadaptation and emergent lineage commitment. The cytoskeleton provides an adaptive structural scaffold to the cell, regulating the emergence of stem cell structure-function relationships during tissue neogenesis, both in prenatal development as well as postnatal healing. Identification and mapping of the mechanical cues conducive to cytoskeletal remodelling and cell adaptation may help to establish environmental contexts that can be used prospectively as translational design specifications to target tissue neogenesis for regenerative medicine. In this review, we summarize findings on cytoskeletal remodelling in the context of tissue neogenesis during early development and postnatal healing, and its relevance in guiding lineage commitment for targeted tissue regeneration. We highlight how cytoskeleton-targeting chemical agents modulate stem cell differentiation and govern responses to mechanical cues in stem cells' emerging form and function. We further review methods for spatiotemporal visualization and measurement of cytoskeletal remodelling, as well as its effects on the mechanical properties of cells, as a function of adaptation. Research in these areas may facilitate translation of stem cells' own healing potential and improve the design of materials, therapies, and devices for regenerative medicine.

Non-hypertrophic chondrogenesis of mesenchymal stem cells through mechano-hypoxia programing

Cartilage tissue engineering aims to generate functional replacements to treat cartilage defects from damage and osteoarthritis. Human bone marrow-derived mesenchymal stem cells (hBM-MSC) are a promising cell source for making cartilage, but current differentiation protocols require the supplementation of growth factors like TGF-β1 or -β3. This can lead to undesirable hypertrophic differentiation of hBM-MSC that progress to bone. We have found previously that exposing engineered human meniscus tissues to physiologically relevant conditions of the knee (mechanical loading and hypoxia; hence, mechano-hypoxia conditioning) increased the gene expression of hyaline cartilage markers, SOX9 and COL2A1, inhibited hypertrophic marker COL10A1, and promoted bulk mechanical property development. Adding further to this protocol, we hypothesize that combined mechano-hypoxia conditioning with TGF-β3 growth factor withdrawal will promote stable, non-hypertrophic chondrogenesis of hBM-MSC embedded in an HA-hydrogel. We found that the combined treatment upregulated many cartilage matrix- and development-related markers while suppressing many hypertrophic- and bone development-related markers. Tissue level assessments with biochemical assays, immunofluorescence, and histochemical staining confirmed the gene expression data. Further, mechanical property development in the dynamic compression treatment shows promise toward generating functional engineered cartilage through more optimized and longer culture conditions. In summary, this study introduced a novel protocol to differentiate hBM-MSC into stable, cartilage-forming cells.

Cyclic uniaxial mechanical load enhances chondrogenesis through entraining the molecular circadian clock

The biomechanical environment plays a key role in regulating cartilage formation, but the current understanding of mechanotransduction pathways in chondrogenic cells is incomplete. Among the combination of external factors that control chondrogenesis are temporal cues that are governed by the cell-autonomous circadian clock. However, mechanical stimulation has not yet directly been proven to modulate chondrogenesis via entraining the circadian clock in chondroprogenitor cells. The purpose of this study was to establish whether mechanical stimuli entrain the core clock in chondrogenic cells, and whether augmented chondrogenesis caused by mechanical loading was at least partially mediated by the synchronised, rhythmic expression of the core circadian clock genes, chondrogenic transcription factors, and cartilage matrix constituents at both transcript and protein levels. We report here, for the first time, that cyclic uniaxial mechanical load applied for 1 h for a period of 6 days entrains the molecular clockwork in chondroprogenitor cells during chondrogenesis in limb bud-derived micromass cultures. In addition to the several core clock genes and proteins, the chondrogenic markers SOX9 and ACAN also followed a robust sinusoidal rhythmic expression pattern. These rhythmic conditions significantly enhanced cartilage matrix production and upregulated marker gene expression. The observed chondrogenesis-promoting effect of the mechanical environment was at least partially attributable to its entraining effect on the molecular clockwork, as co-application of the small molecule clock modulator longdaysin attenuated the stimulatory effects of mechanical load. This study suggests that an optimal biomechanical environment enhances tissue homoeostasis and histogenesis during chondrogenesis at least partially through entraining the molecular clockwork.

Mechanical feedback defines organizing centers to drive digit emergence

During embryonic development, digits gradually emerge in a periodic pattern. Although genetic evidence indicates that digit formation results from a self-organizing process, the underlying mechanisms are still unclear. Here, we find that convergent-extension tissue flows driven by active stresses underlie digit formation. These active stresses simultaneously shape cartilage condensations and lead to the emergence of a compressive stress region that promotes high activin/p-SMAD/SOX9 expression, thereby defining digit-organizing centers via a mechanical feedback. In Wnt5a mutants, such mechanical feedback is disrupted due to the loss of active stresses, organizing centers do not emerge, and digit formation is precluded. Thus, digit emergence does not result solely from molecular interactions, as was previously thought, but requires a mechanical feedback that ensures continuous coupling between phalanx specification and elongation. Our work, which links mechanical and molecular signals, provides a mechanistic context for the emergence of organizing centers that may underlie various developmental processes.

Mechanical Regulation of Limb Bud Formation

Early limb bud development has been of considerable interest for the study of embryological development and especially morphogenesis. The focus has long been on biochemical signalling and less on cell biomechanics and mechanobiology. However, their importance cannot be understated since tissue shape changes are ultimately controlled by active forces and bulk tissue rheological properties that in turn depend on cell-cell interactions as well as extracellular matrix composition. Moreover, the feedback between gene regulation and the biomechanical environment is still poorly understood. In recent years, novel experimental techniques and computational models have reinvigorated research on this biomechanical and mechanobiological side of embryological development. In this review, we consider three stages of early limb development, namely: outgrowth, elongation, and condensation. For each of these stages, we summarize basic biological regulation and examine the role of cellular and tissue mechanics in the morphogenetic process.

Tissue Engineering: An Alternative to Repair Cartilage

Herein we review the state-of-the-art in tissue engineering for repair of articular cartilage. First, we describe the molecular, cellular, and histologic structure and function of endogenous cartilage, focusing on chondrocytes, collagens, extracellular matrix, and proteoglycans. We then explore in vitro cell culture on scaffolds, discussing the difficulties involved in maintaining or obtaining a chondrocytic phenotype. Next, we discuss the diverse compounds and designs used for these scaffolds, including natural and synthetic biomaterials and porous, fibrous, and multilayer architectures. We then report on the mechanical properties of different cell-loaded scaffolds, and the success of these scaffolds following in vivo implantation in small animals, in terms of generating tissue that structurally and functionally resembles native tissue. Last, we highlight future trends in this field. We conclude that despite major technical advances made over the past 15 years, and continually improving results in cartilage repair experiments in animals, the development of clinically useful implants for regeneration of articular cartilage remains a challenge

Primary cilia and autophagy interaction is involved in mechanical stress mediated cartilage development via ERK/mTOR axis

AIMS

Biomechanical reactivity is a special property of chondrocytes and mechanical stress can affect the development of cartilage. Primary cilia have been proved a cellular sensory which can detect physical and chemical stimuli extracellular and initiate multiple signaling transduction. Autophagy is an important environmental adaptive mechanism for cells maintenance of homeostasis. The aims of this study were to detect whether there is an interaction between primary cilia and autophagy in the regulation of mechanical stress-mediated cartilage development and to explore the underlying mechanism.

MAIN METHODS

In this study, chondrocytes were treated with cyclic tensile strain (CTS) by the four-point bending system. Chondrocytes viability, proliferation and differentiation capacities were analyzed by western blot and live/dead assays after CTS of different intensities. Meanwhile, primary cilia incidence and length changes, and autophagy expression were detected by immunofluorescence staining. The primary cilia and autophagy interaction regulation and the underlying mechanism were detected by immunofluorescence double staining and western blot.

KEY FINDINGS

Mechanical stress could affect chondrocytes proliferation, phenotype and viability in an intensity dependent manner. The incidence and length of primary cilia as well as autophagy expression could be regulated by CTS. The integrity of primary cilia structure is vital for mechanical stress regulated ERK/mTOR signaling transduction and autophagy expression in chondrocyte.

SIGNIFICANCE

These findings indicate that mechanical stress could affect the interaction between primary cilia and autophagy and help to reveal the underlying mechanism of stress regulated cartilage development.

Into-Tunnel Repair Versus Onto-Surface Repair for Rotator Cuff Tears in a Rabbit Model

BACKGROUND

Clinically, onto-surface repair is commonly used for rotator cuff tears. The retear rate after rotator cuff repair (RCR) is relatively high, with failure occurring mostly at the tendon-bone connection site. For anterior cruciate ligament (ACL) reconstruction, into-tunnel reconstruction is commonly employed. The retear rate after ACL reconstruction is relatively low, with retears seldom occurring at the tendon-bone interface. No study on into-tunnel RCR has been conducted.

HYPOTHESIS

Into-tunnel RCR could promote fibrocartilage regeneration at the tendon-bone interface and has biomechanical advantage over onto-surface repair in a rabbit rotator cuff tear model.

STUDY DESIGN

Controlled laboratory study.

METHODS

Thirty-six New Zealand White rabbits were used in this study. The supraspinatus tendons were cut from the footprint to create a rotator cuff tear on both shoulders. On one side, the supraspinatus was cut longitudinally into 2 halves, sutured, and pulled into 2 tunnels through the greater tuberosity (into-tunnel repair). On the other side, the tendon was reattached to the surface of the footprint with transosseous sutures (onto-surface repair). Twelve animals were sacrificed, of which 6 were used for a histological examination and the other 6 for biomechanical testing, at 4, 8, and 12 weeks, respectively.

RESULTS

The tendon-bone interface in the into-tunnel group showed a different healing pattern from that in the onto-surface group. In the former, most of the tendon tissue in the tunnel was replaced with newly generated fibrocartilage; the rest of the tendon fibers appeared in large bundles with direct connection to the bone. In the latter, fibrocartilage regeneration was seldom found at the tendon-bone interface; the tendon near the bone surface appeared as small fibrils. The biomechanical evaluation revealed a higher ultimate load ( P < .001) and stiffness ( P < .001) at the tendon-bone junction in the into-tunnel group than those in the onto-surface group at 12 weeks.

CONCLUSION

In a rabbit rotator cuff tear model, into-tunnel RCR could result in a different tendon-bone healing pattern, with obvious fibrocartilage regeneration at the interface and higher tendon-bone healing strength than that in onto-surface repair.

CLINICAL RELEVANCE

New RCR patterns may be developed to improve the tendon-bone healing pattern and obtain better tendon-bone healing strength.

Time-dependent regulation of morphological changes and cartilage differentiation markers in the mouse pubic symphysis during pregnancy and postpartum recovery

Animal models commonly serve as a bridge between in vitro experiments and clinical applications; however, few physiological processes in adult animals are sufficient to serve as proof-of-concept models for cartilage regeneration. Intriguingly, some rodents, such as young adult mice, undergo physiological connective tissue modifications to birth canal elements such as the pubic symphysis during pregnancy; therefore, we investigated whether the differential expression of cartilage differentiation markers is associated with cartilaginous tissue morphological modifications during these changes. Our results showed that osteochondral progenitor cells expressing Runx2, Sox9, Col2a1 and Dcx at the non-pregnant pubic symphysis proliferated and differentiated throughout pregnancy, giving rise to a complex osteoligamentous junction that attached the interpubic ligament to the pubic bones until labour occurred. After delivery, the recovery of pubic symphysis cartilaginous tissues was improved by the time-dependent expression of these chondrocytic lineage markers at the osteoligamentous junction. This process potentially recapitulates embryologic chondrocytic differentiation to successfully recover hyaline cartilaginous pads at 10 days postpartum. Therefore, we propose that this physiological phenomenon represents a proof-of-concept model for investigating the mechanisms involved in cartilage restoration in adult animals.

Mechanical Activation of Adipose Tissue and Derived Mesenchymal Stem Cells: Novel Anti-Inflammatory Properties

The adipose tissue is a source of inflammatory proteins, such as TNF, IL-6, and CXCL8. Most of their production occurs in macrophages that act as scavengers of dying adipocytes. The application of an orbital mechanical force for 6-10 min at 97 g to the adipose tissue, lipoaspirated and treated according to Coleman procedures, abolishes the expression of TNF-α and stimulates the expression of the anti-inflammatory protein TNF-stimulated gene-6 (TSG-6). This protein had protective and anti-inflammatory effects when applied to animal models of rheumatic diseases. We examined biopsy, lipoaspirate, and mechanically activated fat and observed that in addition to the increased TSG-6, Sox2, Nanog, and Oct4 were also strongly augmented by mechanical activation, suggesting an effect on stromal cell stemness. Human adipose tissue-derived mesenchymal stem cells (hADSCs), produced from activated fat, grow and differentiate normally with proper cell surface markers and chromosomal integrity, but their anti-inflammatory action is far superior compared to those mesenchymal stem cells (MSCs) obtained from lipoaspirate. The expression and release of inflammatory cytokines from THP-1 cells was totally abolished in mechanically activated adipose tissue-derived hADSCs. In conclusion, we report that the orbital shaking of adipose tissue enhances its anti-inflammatory properties, and derived MSCs maintain such enhanced activity.

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.

Paracrine Potential of the Human Adipose Tissue-Derived Stem Cells to Modulate Balance between Matrix Metalloproteinases and Their Inhibitors in the Osteoarthritic Cartilage In Vitro

Adipose tissue represents an abundant source of stem cells. Along with anti-inflammatory effects, ASC secrete various factors that may modulate metabolism of extracellular matrix in osteoarthritic (OA) cartilage, suggesting that the presence of ASC could be advantageous for OA cartilage due to the recovery of homeostasis between matrix metalloproteinases (MMPs) and their tissue inhibitors of metalloproteinases (TIMPs). To evaluate these effects, cartilage explants (CE) were cocultured with ASC for 3 and 7 days under stimulation with or without IL-1β. The pattern of gene expression in CE was modified by ASC, including the upregulation of COL1A1 and COL3A1 and the downregulation of MMP13 and COL10A1. The production of MMP-1, MMP-3, and MMP-13 by ASC was not significant; moreover, cocultures with ASC reduced MMP-13 production in CE. In conclusion, active production of TIMP-1, TIMP-2, TIMP-3, IL-6, IL-8, and gelatinases MMP-2 and MMP-9 by ASC may be involved in the extracellular matrix remodelling, as indicated by the altered expression of collagens, the downregulated production of MMP-13, and the reduced chondrocyte apoptosis in the cocultured CE. These data suggest that ASC modulated homeostasis of MMPs/TIMPs in degenerated OA cartilage in vitro and might be favourable in case of the intra-articular application of ASC therapy for the treatment of OA.

The Adaptive Remodeling of Condylar Cartilage— A Transition from Chondrogenesis to Osteogenesis

Mandibular condylar cartilage is categorized as articular cartilage but markedly distinguishes itself in many biological aspects, such as its embryonic origin, ontogenetic development, post-natal growth mode, and histological structures. The most marked uniqueness of condylar cartilage lies in its capability of adaptive remodeling in response to external stimuli during or after natural growth. The adaptation of condylar cartilage to mandibular forward positioning constitutes the fundamental rationale for orthodontic functional therapy, which partially contributes to the correction of jaw discrepancies by achieving mandibular growth modification. The adaptive remodeling of condylar cartilage proceeds with the biomolecular pathway initiating from chondrogenesis and finalizing with osteogenesis. During condylar adaptation, chondrogenesis is activated when the external stimuli, e.g., condylar repositioning, generate the differentiation of mesenchymal cells in the articular layer of cartilage into chondrocytes, which proliferate and then progressively mature into hypertrophic cells. The expression of regulatory growth factors, which govern and control phenotypic conversions of chondrocytes during chondrogenesis, increases during adaptive remodeling to enhance the transition from chondrogenesis into osteogenesis, a process in which hypertrophic chondrocytes and matrices degrade and are replaced by bone. The transition is also sustained by increased neovascularization, which brings in osteoblasts that finally result in new bone formation beneath the degraded cartilage.

Successful chondrogenesis within scaffolds, using magnetic stem cell confinement and bioreactor maturation

Tissue engineering strategies, such as cellularized scaffolds approaches, have been explored for cartilage replacement. The challenge, however, remains to produce a cartilaginous tissue incorporating functional chondrocytes and being large and thick enough to be compatible with the replacement of articular defects. Here, we achieved unprecedented cartilage tissue production into a porous polysaccharide scaffold by combining of efficient magnetic condensation of mesenchymal stem cells, and dynamic maturation in a bioreactor. In optimal conditions, all the hallmarks of chondrogenesis were enhanced with a 50-fold increase in collagen II expression compared to negative control, an overexpression of aggrecan and collagen XI, and a very low expression of collagen I and RUNX2. Histological staining showed a large number of cellular aggregates, as well as an increased proteoglycan synthesis by chondrocytes. Interestingly, electron microscopy showed larger chondrocytes and a more abundant extracellular matrix. In addition, the periodicity of the neosynthesized collagen fibers matched that of collagen II. These results represent a major step forward in replacement tissue for cartilage defects.

STATEMENT OF SIGNIFICANCE

A combination of several innovative technologies (magnetic cell seeding, polysaccharide porous scaffolds, and dynamic maturation in bioreactor) enabled unprecedented successful chondrogenesis within scaffolds.

Phenotypic Novelty in EvoDevo: The Distinction Between Continuous and Discontinuous Variation and Its Importance in Evolutionary Theory

The introduction of novel phenotypic structures is one of the most significant aspects of organismal evolution. Yet the concept of evolutionary novelty is used with drastically different connotations in various fields of research, and debate exists about whether novelties represent features that are distinct from standard forms of phenotypic variation. This article contrasts four separate uses for novelty in genetics, population genetics, morphology, and behavioral science, before establishing how novelties are used in evolutionary developmental biology (EvoDevo). In particular, it is detailed how an EvoDevo-specific research approach to novelty produces insight distinct from other fields, gives the concept explanatory power with predictive capacities, and brings new consequences to evolutionary theory. This includes the outlining of research strategies that draw attention to productive areas of inquiry, such as threshold dynamics in development. It is argued that an EvoDevo-based approach to novelty is inherently mechanistic, treats the phenotype as an agent with generative potential, and prompts a distinction between continuous and discontinuous variation in evolutionary theory.

Compression regulates gene expression of chondrocytes through HDAC4 nuclear relocation via PP2A-dependent HDAC4 dephosphorylation

Biomechanics plays a critical role in the modulation of chondrocyte function. The mechanisms by which mechanical loading is transduced into intracellular signals that regulate chondrocyte gene expression remain largely unknown. Histone deacetylase 4 (HDAC4) is specifically expressed in chondrocytes. Mice lacking HDAC4 display chondrocyte hypertrophy, ectopic and premature ossification, and die early during the perinatal period. HDAC4 has a remarkable ability to translocate between the cell's cytoplasm and nucleus. It has been established that subcellular relocation of HDAC4 plays a critical role in chondrocyte differentiation and proliferation. However, it remains unclear whether subcellular relocation of HDAC4 in chondrocytes can be induced by mechanical loading. In this study, we first report that compressive loading induces HDAC4 relocation from the cytoplasm to the nucleus of chondrocytes via stimulation of Ser/Thr-phosphoprotein phosphatases 2A (PP2A) activity, which results in dephosphorylation of HDAC4. Dephosphorylated HDAC4 relocates to the nucleus to achieve transcriptional repression of Runx2 and regulates chondrocyte gene expression in response to compression. Our results elucidate the mechanism by which mechanical compression regulates chondrocyte gene expression through HDAC4 relocation from the cell's cytoplasm to the nucleus via PP2A-dependent HDAC4 dephosphorylation.

Multifaceted signaling regulators of chondrogenesis: Implications in cartilage regeneration and tissue engineering

Defects of articular cartilage present a unique clinical challenge due to its poor self-healing capacity and avascular nature. Current surgical treatment options do not ensure consistent regeneration of hyaline cartilage in favor of fibrous tissue. Here, we review the current understanding of the most important biological regulators of chondrogenesis and their interactions, to provide insight into potential applications for cartilage tissue engineering. These include various signaling pathways, including: fibroblast growth factors (FGFs), transforming growth factor β (TGF-β)/bone morphogenic proteins (BMPs), Wnt/β-catenin, Hedgehog, Notch, hypoxia, and angiogenic signaling pathways. Transcriptional and epigenetic regulation of chondrogenesis will also be discussed. Advances in our understanding of these signaling pathways have led to promising advances in cartilage regeneration and tissue engineering.

Effect of Dynamic Culture and Periodic Compression on Human Mesenchymal Stem Cell Proliferation and Chondrogenesis

We have recently developed a bioreactor that can apply both shear and compressive forces to engineered tissues in dynamic culture. In our system, alginate hydrogel beads with encapsulated human mesenchymal stem cells (hMSCs) were cultured under different dynamic conditions while subjected to periodic, compressive force. A customized pressure sensor was developed to track the pressure fluctuations when shear forces and compressive forces were applied. Compared to static culture, dynamic culture can maintain a higher cell population throughout the study. With the application of only shear stress, qRT-PCR and immunohistochemistry revealed that hMSCs experienced less chondrogenic differentiation than the static group. The second study showed that chondrogenic differentiation was enhanced by additional mechanical compression. After 14 days, alcian blue staining showed more extracellular matrix formed in the compression group. The upregulation of the positive chondrogenic markers such as Sox 9, aggrecan, and type II collagen were demonstrated by qPCR. Our bioreactor provides a novel approach to apply mechanical forces to engineered cartilage. Results suggest that a combination of dynamic culture with proper mechanical stimulation may promote efficient progenitor cell expansion in vitro, thereby allowing the culture of clinically relevant articular chondrocytes for the treatment of articular cartilage defects.

Strategies to engineer tendon/ligament-to-bone interface: Biomaterials, cells and growth factors

Integration between tendon/ligament and bone occurs through a specialized tissue interface called enthesis. The complex and heterogeneous structure of the enthesis is essential to ensure smooth mechanical stress transfer between bone and soft tissues. Following injury, the interface is not regenerated, resulting in high rupture recurrence rates. Tissue engineering is a promising strategy for the regeneration of a functional enthesis. However, the complex structural and cellular composition of the native interface makes enthesis tissue engineering particularly challenging. Thus, it is likely that a combination of biomaterials and cells stimulated with appropriate biochemical and mechanical cues will be needed. The objective of this review is to describe the current state-of-the-art, challenges and future directions in the field of enthesis tissue engineering focusing on four key parameters: (1) scaffold and biomaterials, (2) cells, (3) growth factors and (4) mechanical stimuli.

Changes in collagens and chondrocytes in the temporomandibular joint cartilage in growing rats fed a liquid diet

The temporomandibular joint (TMJ) of growing rats fed a soft diet is reported to be smaller in size and to have thinner condyle and glenoid fossa cartilage than rats fed a solid diet. The aim of this study was to determine the effect of a soft diet on the collagens and chondrocytes in the growing TMJ cartilage. Forty-eight male Wistar rats were divided into a control group fed a solid diet and an experimental group fed a liquid diet for 1-8 weeks. After the experimental period, the TMJs were harvested and examined histologically, immunohistochemically for collagen types I, II, and X, and with transmission electron microscopy. The condylar cartilage in the experimental rats showed weak immunoreactions for three types of collagens compared with the controls. The ultrastructure had fewer fine collagen fibrils in the experimental rats compared with that of the controls. The glenoid fossa cartilage in the experimental rats showed narrower Alcian blue-positive areas than the control staining. The immunoreactions for three types of collagen in the experimental rats were also weaker than those of the controls. The chondrocytes in the experimental rats appeared dark, had extended thin cytoplasmic processes, and had formed gap junctions, as assessed by transmission electron microscopy. Fewer fine collagen fibrils, but thick bands of collagen fibrils were observed in the glenoid fossa of the experimental cartilage. The results of the present study showed that a liquid diet had deleterious effects on the quality and quantity of collagens and chondrocytes in the TMJ cartilage in growing rats.

Physical, Spatial, and Molecular Aspects of Extracellular Matrix of In Vivo Niches and Artificial Scaffolds Relevant to Stem Cells Research

Extracellular matrix can influence stem cell choices, such as self-renewal, quiescence, migration, proliferation, phenotype maintenance, differentiation, or apoptosis. Three aspects of extracellular matrix were extensively studied during the last decade: physical properties, spatial presentation of adhesive epitopes, and molecular complexity. Over 15 different parameters have been shown to influence stem cell choices. Physical aspects include stiffness (or elasticity), viscoelasticity, pore size, porosity, amplitude and frequency of static and dynamic deformations applied to the matrix. Spatial aspects include scaffold dimensionality (2D or 3D) and thickness; cell polarity; area, shape, and microscale topography of cell adhesion surface; epitope concentration, epitope clustering characteristics (number of epitopes per cluster, spacing between epitopes within cluster, spacing between separate clusters, cluster patterns, and level of disorder in epitope arrangement), and nanotopography. Biochemical characteristics of natural extracellular matrix molecules regard diversity and structural complexity of matrix molecules, affinity and specificity of epitope interaction with cell receptors, role of non-affinity domains, complexity of supramolecular organization, and co-signaling by growth factors or matrix epitopes. Synergy between several matrix aspects enables stem cells to retain their function in vivo and may be a key to generation of long-term, robust, and effective in vitro stem cell culture systems.

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.

Cyclic compressive loading on 3D tissue of human synovial fibroblasts upregulates prostaglandin E2 via COX-2 production without IL-1β and TNF-α

Objective

Excessive mechanical stress on synovial joints causes osteoarthritis (OA) and results in the production of prostaglandin E2 (PGE2), a key molecule in arthritis, by synovial fibroblasts. However, the relationship between arthritis-related molecules and mechanical stress is still unclear. The purpose of this study was to examine the synovial fibroblast response to cyclic mechanical stress using an in vitro osteoarthritis model.

Method

Human synovial fibroblasts were cultured on collagen scaffolds to produce three-dimensional constructs. A cyclic compressive loading of 40 kPa at 0.5 Hz was applied to the constructs, with or without the administration of a cyclooxygenase-2 (COX-2) selective inhibitor or dexamethasone, and then the concentrations of PGE2, interleukin-1β (IL-1β), tumour necrosis factor-α (TNF-α), IL-6, IL-8 and COX-2 were measured.

Results

The concentrations of PGE2, IL-6 and IL-8 in the loaded samples were significantly higher than those of unloaded samples; however, the concentrations of IL-1β and TNF-α were the same as the unloaded samples. After the administration of a COX-2 selective inhibitor, the increased concentration of PGE2 by cyclic compressive loading was impeded, but the concentrations of IL-6 and IL-8 remained high. With dexamethasone, upregulation of PGE2, IL-6 and IL-8 was suppressed.

Conclusion

These results could be useful in revealing the molecular mechanism of mechanical stress in vivo for a better understanding of the pathology and therapy of OA.

Cite this article: Bone Joint Res 2014;3:280–8.

EFFECTS OF SHORT-TERM CYCLIC HYDROSTATIC PRESSURE ON INITIATING AND ENHANCING THE EXPRESSION OF CHONDROGENIC GENES IN HUMAN ADIPOSE-DERIVED MESENCHYMAL STEM CELLS

Cartilage tissue engineering is a promising treatment for damaged or diseased cartilage that requires thorough understanding of influential parameters involved in chondrogenic differentiation. This study examined how 4-h application of cyclic hydrostatic pressure (CHP) of 5 MPa at 0.5 Hz could modulate chondroinduction of human adipose-derived mesenchymal stem cells (hAMSCs) in vitro. Four groups were examined including a negative control group, a chemical group treated by growth factor for 10 days, a mechanical group exposed to 4-h loading on the 10th day of pellet culture without any chondrogenic stimulator, and finally a chemical-mechanical group subjected to both growth factor and loading. Application of cyclic hydrostatic pressure increased the expression of chondrogenic genes, including sox9 and aggrecan to higher levels than those of the chemical group. This study indicates that cyclic hydrostatic pressure initiates and enhances the chondrogenic differentiation of mesenchymal stem cells with or without growth factors in vitro and confirms the important role of hydrostatic pressure during chondrogenesis in vivo.

Mesenchymal Stem Cells for Treating Articular Cartilage Defects and Osteoarthritis

Articular cartilage damage and osteoarthritis are the most common joint diseases. Joints are prone to damage caused by sports injuries or aging, and such damage regularly progresses to more serious joint disorders, including osteoarthritis, which is a degenerative disease characterized by the thinning and eventual wearing out of articular cartilage, ultimately leading to joint destruction. Osteoarthritis affects millions of people worldwide. Current approaches to repair of articular cartilage damage include mosaicplasty, microfracture, and injection of autologous chondrocytes. These treatments relieve pain and improve joint function, but the long-term results are unsatisfactory. The long-term success of cartilage repair depends on development of regenerative methodologies that restore articular cartilage to a near-native state. Two promising approaches are (i) implantation of engineered constructs of mesenchymal stem cell (MSC)-seeded scaffolds, and (ii) delivery of an appropriate population of MSCs by direct intra-articular injection. MSCs may be used as trophic producers of bioactive factors initiating regenerative activities in a defective joint. Current challenges in MSC therapy are the need to overcome current limitations in cartilage cell purity and to in vitro engineer tissue structures exhibiting the required biomechanical properties. This review outlines the current status of MSCs used in cartilage tissue engineering and in cell therapy seeking to repair articular cartilage defects and related problems. MSC-based technologies show promise when used to repair cartilage defects in joints.

The influence of a spatiotemporal 3D environment on endothelial cell differentiation of human induced pluripotent stem cells

Current EC differentiation protocols are inefficient, and the phenotypes of the differentiated ECs are only briefly stable, which significantly inhibits their utility for basic science research. Here, a remarkably more efficient hiPSC-EC differentiation protocol that incorporates a three-dimensional (3D) fibrin scaffold is presented. With this protocol, up to 45% of the differentiated hiPSCs assumed an EC phenotype, and after purification, greater than 95% of the cells displayed the EC phenotype (based on CD31 expression). The hiPSC-ECs continued to display EC characteristics for 4 weeks in vitro. Gene and protein expression levels of CD31, CD144 and von Willebrand factor-8 (vWF-8) were significantly up-regulated in differentiated hiPSC-ECs. hiPSC-ECs also have biological function to up-take Dil-conjugated acetylated LDL (Dil-ac-LDL) and form tubular structures on Matrigel. Collectively, these data demonstrate that a 3D differentiation protocol can efficiently generate ECs from hiPSCs and, furthermore, the differentiated hiPSC-ECs are functional and can maintain EC fate up to 4 weeks in vitro.

Differences in Cartilage Repair between Loading and Unloading Environments in the Rat Knee

We investigated the histopathological and immunohistochemical effects of loading on cartilage repair in rat full-thickness articular cartilage defects. A total of 40 male 9-week-old Wistar rats were studied. Full-thickness articular cartilage defects were created over the capsule at the loading portion in the medial condyle of the femur. Twenty rats were randomly allocated into each of the 2 groups: a loading group and a unloading group. Twenty rats from these 2 groups were later randomly allocated to each of the 2 groups for evaluation at 1 and 2 weeks after surgery. At the end of each period, knee joints were examined histopathologically and immunohistochemically. In both groups at 1 and 2 weeks, the defects were filled with a mixture of granulation tissue and some remnants of hyaline cartilage. The repair tissue was not stained with toluidine blue in both groups. Strong staining of type I collagen was observed in the repair tissue of both groups. The area stained with type I collagen was smaller in the unloading group than in the loading groups, and the stained area was smaller at 2 weeks than at 1 week. In the staining for type II collagen, apparent staining of type II collagen was observed in the repair tissue of both groups at 1 week. At 2 weeks, there was a tendency toward a higher degree of apparent staining in the loading group than in the unloading group. Accordingly, these results indicated that loading and unloading in the early phase of cartilage repair have both merits and demerits.

Cell sources for nucleus pulposus regeneration

PURPOSE

There is increasing interest in the development of cell therapy as a possible approach for the treatment of degenerative disc disease. To regenerate nucleus pulposus tissue, the cells must produce an appropriate proteoglycan-rich matrix, as this is essential for the functioning of the intervertebral disc. The natural environment within the disc is very challenging to implanted cells, particularly if they have been subcultured in normal laboratory conditions. The purpose of this work is to discuss parameters relevant to translating different proposed cell therapies of IVD into clinical use.

RESULTS

Several sources of cells have been proposed, including nucleus pulposus cells, chondrocytes and mesenchymal stem cells derived from bone marrow or adipose tissue. There are some clinical trials and reports of attempts to regenerate nucleus pulposus utilising either autologous or allogenic cells. While the published results of clinical applications of these cell therapies do not indicate any safety issues, additional evidence will be needed to prove their long-term efficacy.

CONCLUSION

This article discusses parameters relevant for successful translation of research on different cell sources into clinically applicable cell therapies: the influence of the intervertebral disc microenvironment on the cell phenotype, issues associated with cell culture and technical preparation of cell products, as well as discussing current regulatory requirements. There are advantages and disadvantages of each proposed cell type, but no strong evidence to favour any one particular cell source at the moment.

Cell adhesion and mechanical stimulation in the regulation of mesenchymal stem cell differentiation

Stem cells have been shown to have the potential to provide a source of cells for applications to tissue engineering and organ repair. The mechanisms that regulate stem cell fate, however, mostly remain unclear. Mesenchymal stem cells (MSCs) are multipotent progenitor cells that are isolated from bone marrow and other adult tissues, and can be differentiated into multiple cell lineages, such as bone, cartilage, fat, muscles and neurons. Although previous studies have focused intensively on the effects of chemical signals that regulate MSC commitment, the effects of physical/mechanical cues of the microenvironment on MSC fate determination have long been neglected. However, several studies provided evidence that mechanical signals, both direct and indirect, played important roles in regulating a stem cell fate. In this review, we summarize a number of recent studies on how cell adhesion and mechanical cues influence the differentiation of MSCs into specific lineages. Understanding how chemical and mechanical cues in the microenvironment orchestrate stem cell differentiation may provide new insights into ways to improve our techniques in cell therapy and organ repair.

Osterix regulates calcification and degradation of chondrogenic matrices through matrix metalloproteinase 13 (MMP13) expression in association with transcription factor Runx2 during endochondral ossification

Endochondral ossification is temporally and spatially regulated by several critical transcription factors, including Sox9, Runx2, and Runx3. Although the molecular mechanisms that control the late stages of endochondral ossification (e.g. calcification) are physiologically and pathologically important, these precise regulatory mechanisms remain unclear. Here, we demonstrate that Osterix is an essential transcription factor for endochondral ossification that functions downstream of Runx2. The global and conditional Osterix-deficient mice studied here exhibited a defect of cartilage-matrix ossification and matrix vesicle formation. Importantly, Osterix deficiencies caused the arrest of endochondral ossification at the hypertrophic stage. Microarray analysis revealed that matrix metallopeptidase 13 (MMP13) is an important target of Osterix. We also showed that there exists a physical interaction between Osterix and Runx2 and that these proteins function cooperatively to induce MMP13 during chondrocyte differentiation. Most interestingly, the introduction of MMP13 stimulated the calcification of matrices in Osterix-deficient mouse limb bud cells. Our results demonstrated that Osterix was essential to endochondral ossification and revealed that the physical and functional interaction between Osterix and Runx2 were necessary for the induction of MMP13 during endochondral ossification.

Effect of collagen type I or type II on chondrogenesis by cultured human articular chondrocytes

INTRODUCTION

Current cartilage repair procedures using autologous chondrocytes rely on a variety of carriers for implantation. Collagen types I and II are frequently used and valuable properties of both were shown earlier in vitro, although a preference for either was not demonstrated. Recently, however, fibrillar collagens were shown to promote cartilage degradation. The goal of this study was to evaluate the effects of collagen type I and type II coating on chondrogenic properties of in vitro cultured human chondrocytes, and to investigate if collagen-mediated cartilage degradation occurs.

METHODS

Human chondrocytes of eight healthy cartilage donors were isolated, expanded, and cultured on culture well inserts coated with either collagen type I, type II, or no coating (control). After 28 days of redifferentiation culture, safranin O and immunohistochemical staining for collagen types I, II, X, and Runx2/Cbfa1 were performed and glycosaminoglycan (GAG) and DNA content and release were examined. Further, expression of collagen type I, type II, type X, MMP13, Runx2/Cbfa1, DDR2, α2 and β1 integrin were examined by reverse transcriptase-polymerase chain reaction.

RESULTS

The matrix, created by chondrocytes grown on collagen type I- and II-coated membranes, resembled cartilage more than when grown on noncoated membranes as reflected by histological scoring. Immunohistochemical staining did not differ between the conditions. GAG content as well as GAG/DNA were higher for collagen type II-coated cartilage constructs than control. GAG release was also higher on collagen type I- and II-coated constructs. Expression of collagen type X was higher of chondrocytes grown on collagen type II compared to controls, but no collagen X protein could be demonstrated by immunohistochemistry. No effects of collagen coating on DDR2 nor MMP-13 gene expression were found. No differences were observed between collagen types I and II.

CONCLUSION

Chondrocyte culture on collagen type I or II promotes more active matrix production and turnover. No significant differences between collagen types I and II were observed, nor were hypertrophic changes more evident in either condition. The use of collagen type I or II coating for in vitro models, thus, seems a sound basis for in vivo repair procedures.

Biomechanics-driven chondrogenesis: from embryo to adult

Biomechanics plays a pivotal role in articular cartilage development, pathophysiology, and regeneration. During embryogenesis and cartilage maturation, mechanical stimuli promote chondrogenesis and limb formation. Mechanical loading, which has been characterized using computer modeling and in vivo studies, is crucial for maintaining the phenotype of cartilage. However, excessive or insufficient loading has deleterious effects and promotes the onset of cartilage degeneration. Informed by the prominent role of biomechanics, mechanical stimuli have been harnessed to enhance redifferentiation of chondrocytes and chondroinduction of other cell types, thus providing new chondrocyte cell sources. Biomechanical stimuli, such as hydrostatic pressure or compression, have been used to enhance the functional properties of neocartilage. By identifying pathways involved in mechanical stimulation, chemical equivalents that mimic mechanical signaling are beginning to offer exciting new methods for improving neocartilage. Harnessing biomechanics to improve differentiation, maintenance, and regeneration is emerging as pivotal toward producing functional neocartilage that could eventually be used to treat cartilage degeneration.

Functional biomaterials for cartilage regeneration

The injury and degeneration of articular cartilage and associated arthritis are leading causes of disability worldwide. Cartilage tissue engineering as a treatment modality for cartilage defects has been investigated for over 20 years. Various scaffold materials have been developed for this purpose, but has yet to achieve feasibility and effectiveness for widespread clinical use. Currently, the regeneration of articular cartilage remains a formidable challenge, due to the complex physiology of cartilage tissue and its poor healing capacity. Although intensive research has been focused on the developmental biology and regeneration of cartilage tissue and a diverse plethora of biomaterials have been developed for this purpose, cartilage regeneration is still suboptimal, such as lacking a layered structure, mechanical mismatch with native cartilage and inadequate integration between native tissue and implanted scaffold. The ideal scaffold material should have versatile properties that actively contribute to cartilage regeneration. Functional scaffold materials may overcome the various challenges faced in cartilage tissue engineering by providing essential biological, mechanical, and physical/chemical signaling cues through innovative design. This review thus focuses on the complex structure of native articular cartilage, the critical properties of scaffolds required for cartilage regeneration, present strategies for scaffold design, and future directions for cartilage regeneration with functional scaffold materials.

Development of methods for studying the differentiation of human mesenchymal stem cells under cyclic compressive strain

Human mesenchymal stem cells (hMSC) have numerous potential advantages over terminally differentiated cells and embryonic stem cells for use in tissue engineering applications. The aims of this study were to develop methods to test the hypothesis that hMSC could be differentiated using cyclic compressive strain alone. hMSC were successfully isolated, purified using D7-FIB antibody, cloned, and characterized. The cells were subsequently analyzed using fluorescence-activated cell sorting using a panel of antibodies and differentiation into multiple cell lineages. D7FIB-positive cells were then seeded into collagen-alginate scaffolds and subjected to 10% or 15% cyclic compressive strain for 4 out of 24 hours for up to 21 days in a bespoke servo-assisted displacement-controlled device. Cells were analyzed using adenosine triphosphate assay to determine cell number, live-dead cell assay, and quantitative real-time polymerase chain reaction at 7 and 21 days. Cloned D7-FIB-positive hMSCs showed evidence of differentiation to an osteogenic lineage under 10% cyclic compressive strain alone (core binding factor alpha 1 (CBFA-1) was significantly upregulated at 7 and 21 days by a factor of 18.3 and 32.2, respectively) and to an osteo-chondrogenic lineage under 15% cyclic compressive strain alone (increased expression of CBFA-1, Sox9, and aggrecan). A combination of a composite viscoelastic scaffold and controlled cyclic compressive strain may be useful for study of the differentiation of MSC.