Response of chondrocyte subpopulations cultured within unloaded and loaded agarose

Pericellular Matrix Formation and Atomic Force Microscopy of Single Primary Human Chondrocytes Cultured in Alginate Microgels

One of the main components of articular cartilage is the chondrocyte's pericellular matrix (PCM), which is critical for regulating mechanotransduction, biochemical cues, and healthy cartilage development. Here, individual primary human chondrocytes (PHC) are encapsulated and cultured in 50 µm diameter alginate microgels using drop-based microfluidics. This unique culturing method enables PCM formation and manipulation of individual cells. Over ten days, matrix formation is observed using autofluorescence imaging, and the elastic moduli of isolated cells are measured using AFM. Matrix production and elastic modulus increase are observed for the chondrons cultured in microgels. Furthermore, the elastic modulus of cells grown in microgels increases ≈ten-fold over ten days, nearly reaching the elastic modulus of in vivo PCM. The AFM data is further analyzed using a Gaussian mixture model and shows that the population of PHCs grown in microgels exhibit two distinct populations with elastic moduli averaging 9.0 and 38.0 kPa. Overall, this work shows that microgels provide an excellent culture platform for the growth and isolation of PHCs, enabling PCM formation that is mechanically similar to native PCM. The microgel culture platform presented here has the potential to revolutionize cartilage regeneration procedures through the inclusion of in vitro developed PCM.

Chasing Chimeras - The elusive stable chondrogenic phenotype

The choice of the best-suited cell population for the regeneration of damaged or diseased cartilage depends on the effectiveness of culture conditions (e.g. media supplements, three-dimensional scaffolds, mechanical stimulation, oxygen tension, co-culture systems) to induce stable chondrogenic phenotype. Herein, advances and shortfalls in in vitro, preclinical and clinical setting of various in vitro microenvironment modulators on maintaining chondrocyte phenotype or directing stem cells towards chondrogenic lineage are critically discussed. Chondrocytes possess low isolation efficiency, limited proliferative potential and rapid phenotypic drift in culture. Mesenchymal stem cells are relatively readily available, possess high proliferation potential, exhibit great chondrogenic differentiation capacity, but they tend to acquire a hypertrophic phenotype when exposed to chondrogenic stimuli. Embryonic and induced pluripotent stem cells, despite their promising in vitro and preclinical data, are still under-investigated. Although a stable chondrogenic phenotype remains elusive, recent advances in in vitro microenvironment modulators are likely to develop clinically- and commercially-relevant therapies in the years to come.

Altered spontaneous calcium signaling of in situ chondrocytes in human osteoarthritic cartilage

Intracellular calcium ([Ca²⁺]_i) signaling is an essential universal secondary messenger in articular chondrocytes. However, little is known about its spatiotemporal features in the context of osteoarthritis (OA). Herein, by examining the cartilage samples collected from patients undergoing knee arthroscopic surgery, we investigated the spatiotemporal features of spontaneous [Ca²⁺]_i signaling in in situ chondrocytes at different OA stages. Our data showed zonal dependent spontaneous [Ca²⁺]_i signaling in healthy cartilage samples under 4 mM calcium environment. This signal was significantly attenuated in healthy cartilage samples but increased in early-degenerated cartilage when cultured in 0 mM calcium environment. No significant difference was found in [Ca²⁺]_i intensity oscillation in chondrocytes located in middle zones among ICRS 1–3 samples under both 4 and 0 mM calcium environments. However, the correlation was found in deep zone chondrocytes incubated in 4 mM calcium environment. In addition, increased protein abundance of Ca_v3.3 T-type voltage dependent calcium channel and Nfatc2 activity were observed in early-degenerated cartilage samples. The present study exhibited OA severity dependent spatiotemporal features of spontaneous [Ca²⁺]_i oscillations of in situ chondrocytes, which might reflect the zonal specific role of chondrocytes during OA progression and provide new insight in articular cartilage degradation during OA progression.

In Vitro Engineering of High Modulus Cartilage-Like Constructs

To date, the outcomes of cartilage repair have been inconsistent and have frequently yielded mechanically inferior fibrocartilage, thereby increasing the chances of damage recurrence. Implantation of constructs with biochemical composition and mechanical properties comparable to natural cartilage could be advantageous for long-term repair. This study attempted to create such constructs, in vitro, using tissue engineering principles. Bovine synoviocytes were seeded on nonwoven polyethylene terephthalate fiber scaffolds and cultured in chondrogenic medium for 4 weeks, after which uniaxial compressive loading was applied using an in-house bioreactor for 1 h per day, at a frequency of 1 Hz, for a further 84 days. The initial loading conditions, determined from the mechanical properties of the immature constructs after 4 weeks in chondrogenic culture, were strains ranging between 13% and 23%. After 56 days (sustained at 84 days) of loading, the constructs were stained homogenously with Alcian blue and for type-II collagen. Dynamic compressive moduli were comparable to the high end values for native cartilage and proportional to Alcian blue staining intensity. We suggest that these high moduli values were attributable to the bioreactor setup, which caused the loading regime to change as the constructs developed, that is, the applied stress and strain increased with construct thickness and stiffness, providing continued sufficient cell stimulation as further matrix was deposited. Constructs containing cartilage-like matrix with response to load similar to that of native cartilage could produce long-term effective cartilage repair when implanted.

Shape of chondrocytes within articular cartilage affects the solid but not the fluid microenvironment under unconfined compression

Metabolic activity of the chondrocytes in articular cartilage is strongly related to their zone-specific shape and the composition and mechanical properties of their surrounding extracellular matrix (ECM). However the mechanisms by which cell shape influences the response of the ECM microenvironment to mechanical loading is yet to be elucidated. This relationship was studied using a biphasic multiscale finite element model of different shaped chondrocytes in the superficial and deep zones of the ECM during unconfined stress relaxation. For chondrocytes in the superficial zone, increasing the cell's initial aspect ratio (length/height) increased the deformation and solid stresses of the chondrocyte and pericellular matrix (PCM) during the loading phase; for chondrocytes in the deep zone the effect of the cell shape on the solid microenvironment was time and variable dependent. However, for superficial and deep zone chondrocytes the cell shape did not affect the fluid pressure and fluid shear stress. These results suggest that mechanotransduction of chondrocytes in articular cartilage may be regulated through the solid phase rather than the fluid phase, and that high stresses and deformations in the solid microenvironment in the superficial zone may be essential for the zone-specific biosynthetic activity of the chondrocyte. The biphasic multiscale computational analysis suggests that maintaining the cell shape is critical for regulating the microenvironment and metabolic activity of the chondrocyte in tissue engineering constructs.

STATEMENT OF SIGNIFICANCE

We investigated the effect of chondrocyte shape on the cellular microenvironment using a biphasic multiscale finite element analysis. Our study showed that cell shapes affects the solid but not the fluid microenvironment of the chondrocyte, and that maintaining the cell shape is critical for regulating the microenvironment and metabolic activity of the chondrocyte in native cartilage and tissue engineering constructs. As far as we know, this is the first study on the mechanotransduction mechanisms by which cell shape influences the response of the microenvironment to mechanical loading. This study is important for understanding cell mechanobiology, not only for regulation of cell phenotype in tissue engineered constructs but, as important, for understanding changes in normal chondrocyte function after post-traumatic injury and in the initiation and progression of osteoarthritis.

Fabrication of tissue engineered osteochondral grafts for restoring the articular surface of diarthrodial joints

Osteochondral allograft implantation is an effective cartilage restoration technique for large defects (>10 cm(2)), though the demand far exceeds the supply of available quality donor tissue. Large bilayered engineered cartilage tissue constructs with accurate anatomical features (i.e. contours, thickness, architecture) could be beneficial in replacing damaged tissue. When creating these osteochondral constructs, however, it is pertinent to maintain biofidelity to restore functionality. Here, we describe a step-by-step framework for the fabrication of a large osteochondral construct with correct anatomical architecture and topology through a combination of high-resolution imaging, rapid prototyping, impression molding, and injection molding.

The effect of oxygen tension on human articular chondrocyte matrix synthesis: integration of experimental and computational approaches

Significant oxygen gradients occur within tissue engineered cartilaginous constructs. Although oxygen tension is an important limiting parameter in the development of new cartilage matrix, its precise role in matrix formation by chondrocytes remains controversial, primarily due to discrepancies in the experimental setup applied in different studies. In this study, the specific effects of oxygen tension on the synthesis of cartilaginous matrix by human articular chondrocytes were studied using a combined experimental-computational approach in a “scaffold-free” 3D pellet culture model. Key parameters including cellular oxygen uptake rate were determined experimentally and used in conjunction with a mathematical model to estimate oxygen tension profiles in 21-day cartilaginous pellets. A threshold oxygen tension (pO₂ ≈ 8% atmospheric pressure) for human articular chondrocytes was estimated from these inferred oxygen profiles and histological analysis of pellet sections. Human articular chondrocytes that experienced oxygen tension below this threshold demonstrated enhanced proteoglycan deposition. Conversely, oxygen tension higher than the threshold favored collagen synthesis. This study has demonstrated a close relationship between oxygen tension and matrix synthesis by human articular chondrocytes in a “scaffold-free” 3D pellet culture model, providing valuable insight into the understanding and optimization of cartilage bioengineering approaches. Biotechnol. Bioeng. 2014;111: 1876–1885.

The effect of moving point of contact stimulation on chondrocyte gene expression and localization in tissue engineered constructs

Tissue engineering is a promising approach for articular cartilage repair. However, using current technologies, the developed engineered constructs generally do not possess an organized superficial layer, which contributes to the tissue's durability and unique mechanical properties. In this study, we investigated the efficacy of applying a moving point of contract-type stimulation (MPS) to stimulate the production of a superficial-like layer in the engineered constructs. MPS was applied to chondrocyte-agarose hydrogels at a frequency of 0.5, 1 or 2 Hz, under a constant compressive load of 10 mN for durations between 5 and 60 min over 3 consecutive days. Expression and localization of superficial zone constituents was conducted by qRT-PCR and in situ hybridization. Finite element modeling was also constructed to gain insight into the relationship between the applied stimulus and superficial zone constituent expression. Gene expression of superficial zone markers were affected in a frequency dependent manner with a physiologic frequency of 1 Hz producing maximal expression of PRG4, biglycan, decorin and collagen II. In situ hybridization revealed that localization of these markers predominantly occurred at 500-1000 μm below the construct surface which correlated to sub-surface strains between 10 and 25% as determined by finite element modeling. These results indicate that while mechanical stimuli can be used to enhance the expression of superficial zone constituents in engineered cartilage constructs, the resultant subsurface loading is a critical factor for localizing expression. Future studies will investigate altering the applied stimulus to further localize superficial zone constituent expression at the construct surface.

Cartilage regeneration using zonal chondrocyte subpopulations: a promising approach or an overcomplicated strategy?

Cartilage defects heal imperfectly and osteoarthritic changes develop frequently as a result. Although the existence of specific behaviours of chondrocytes derived from various depth-related zones in vitro has been known for over 20 years, only a relatively small body of in vitro studies has been performed with zonal chondrocytes and current clinical treatment strategies do not reflect these native depth-dependent (zonal) differences. This is surprising since mimicking the zonal organization of articular cartilage in neo-tissue by the use of zonal chondrocyte subpopulations could enhance the functionality of the graft. Although some research groups including our own have made considerable progress in tailoring culture conditions using specific growth factors and biomechanical loading protocols, we conclude that an optimal regime has not yet been determined. Other unmet challenges include the lack of specific zonal cell sorting protocols and limited amounts of cells harvested per zone. As a result, the engineering of functional tissue has not yet been realized and no long-term in vivo studies using zonal chondrocytes have been described. This paper critically reviews the research performed to date and outlines our view of the potential future significance of zonal chondrocyte populations in regenerative approaches for the treatment of cartilage defects. Secondly, we briefly discuss the capabilities of additive manufacturing technologies that can not only create patient-specific grafts directly from medical imaging data sets but could also more accurately reproduce the complex 3D zonal extracellular matrix architecture using techniques such as hydrogel-based cell printing.

Distinct patterns of gene expression in the superficial, middle and deep zones of bovine articular cartilage

Hyaline articular cartilage will not heal spontaneously, and lesions in hyaline articular cartilage often result in degenerative joint disease. Considerable progress has been made with respect to the responsive stem cells, inductive signals and extracellular scaffolding required for the optimal regeneration of cartilage. However, many challenges remain, such as topographic differences in the functional zones of articular cartilage. We hypothesized that a distinct set of differentially expressed genes define the surface, middle and deep zones of hyaline articular cartilage. Microarray analysis of bovine articular cartilage from the superficial and middle zones revealed 52 genes differentially expressed ≥ 10-fold and 114 additional genes differentially expressed ≥ five-fold. However, no genes were identified with a ≥ five-fold difference in expression when comparing articular cartilage from the middle and deep zones. There are distinct, differential gene expression patterns in the superficial and middle zones of hyaline articular cartilage that highlight the functional differences between these zones. This investigation has implications for the tissue engineering and regeneration of hyaline articular cartilage.

The participation of plasma membrane hemichannels to purinergic signaling

The field of hemichannels is closely related to the purinergic signaling and both areas have been growing in parallel. Hemichannels open in response to a wide range of stressful conditions, such as ischemia, pressure or swelling. Hemichannels represent an important mechanism for the cellular release of adenosine 5'-triphosphate (ATP), which is an agonist of the P2Y and P2X family of purinergic receptors. Therefore, hemichannels are key molecules in the regulation of purinergic receptor activation, during physiological and pathophysiological conditions. Furthermore, purinergic receptor activation can also lead to the opening of hemichannels and the subsequent amplification of purinergic signaling via a positive signaling feedback loop, giving rise to the concept of ATP-induced ATP release. Purinergic receptor signaling is involved in regulating many physiological and pathophysiological processes. P2Y receptors activate inositol trisphosphate and transiently increase intracellular calcium. This signaling opens both connexin and pannexin channels, therefore contributing to the expansion of calcium waves across astrocytes and epithelial cells. In addition, several of the P2X receptor subtypes, including the P2X2, P2X4 and P2X7 receptors, activate select cellular permeation pathways to large molecules, including the pannexin-1 channels, which are involved in the initiation of inflammatory responses and cell death. Consequently, the interplay between purinergic receptors and hemichannels could represent a novel target with substantial therapeutic implications in areas such as chronic pain, inflammation or atherosclerosis. This article is part of a Special Issue entitled: The communicating junctions, roles and dysfunctions.

Biological basis of exercise-based treatments for musculoskeletal conditions

Exercise-based therapies are the cornerstone of rehabilitation programs. While the benefits of exercise on systemic and tissue function are generally accepted, mechanisms underlying these benefits are sometimes poorly understood. An improved understanding of the effects of mechanical loading on molecular and cellular processes has the potential to lead to more disease-specific and efficacious exercise-based therapies. The purpose of this paper is to review the current literature examining the role of mechanical signaling on muscle and cartilage biology.

APPLICATION OF BIOMECHANICS TO TISSUE ENGINEERING: A PERSONAL VIEW

Cellular biomechanics is an area of study that is receiving more attention as time progresses. The response of cells to their mechanical environment, including biomechanical stimuli, has far-reaching ramifications for the area of tissue engineering, especially for tissues designed to withstand mechanical loading (e.g. bone, cartilage, tendons and ligaments, and arteries). The effects of mechanical stimuli on cells are only recently being examined, and the potential role of mechanical stimuli in tissue engineering is still one that is largely ignored in the design of tissue engineering scaffolds. The relationship of mechanical properties of scaffolds or of mechanical stimuli to cell behavior is complex, but vital to the development of the field. Also, understanding the complex interplay of form and environment on cells involves an increase in our knowledge of how cells react to their total environment including mechanical stimuli and material properties. In order to improve tissue engineering outcomes, a nexus must be developed between the mechanical, biochemical, and biological studies of cellular behavior, in the context of extremely complex systems.

Finite element method (FEM), mechanobiology and biomimetic scaffolds in bone tissue engineering

Techniques of bone reconstructive surgery are largely based on conventional, non-cell-based therapies that rely on the use of durable materials from outside the patient's body. In contrast to conventional materials, bone tissue engineering is an interdisciplinary field that applies the principles of engineering and life sciences towards the development of biological substitutes that restore, maintain, or improve bone tissue function. Bone tissue engineering has led to great expectations for clinical surgery or various diseases that cannot be solved with traditional devices. For example, critical-sized defects in bone, whether induced by primary tumor resection, trauma, or selective surgery have in many cases presented insurmountable challenges to the current gold standard treatment for bone repair. The primary purpose of bone tissue engineering is to apply engineering principles to incite and promote the natural healing process of bone which does not occur in critical-sized defects. The total market for bone tissue regeneration and repair was valued at $1.1 billion in 2007 and is projected to increase to nearly $1.6 billion by 2014.Usually, temporary biomimetic scaffolds are utilized for accommodating cell growth and bone tissue genesis. The scaffold has to promote biological processes such as the production of extra-cellular matrix and vascularisation, furthermore the scaffold has to withstand the mechanical loads acting on it and to transfer them to the natural tissues located in the vicinity. The design of a scaffold for the guided regeneration of a bony tissue requires a multidisciplinary approach. Finite element method and mechanobiology can be used in an integrated approach to find the optimal parameters governing bone scaffold performance.In this paper, a review of the studies that through a combined use of finite element method and mechano-regulation algorithms described the possible patterns of tissue differentiation in biomimetic scaffolds for bone tissue engineering is given. Firstly, the generalities of the finite element method of structural analysis are outlined; second, the issues related to the generation of a finite element model of a given anatomical site or of a bone scaffold are discussed; thirdly, the principles on which mechanobiology is based, the principal theories as well as the main applications of mechano-regulation models in bone tissue engineering are described; finally, the limitations of the mechanobiological models and the future perspectives are indicated.

Distribution and role of tenascin-C in human osteoarthritic cartilage

BACKGROUND

Tenascin-C (TN-C) is expressed in the cartilage of osteoarthritis (OA). We examined whether TN-C was involved in cartilage repair of the diseased joints. Human articular cartilage samples were obtained from patients with OA and those with normal joints.

METHODS

Immunohistochemistry testing of TN-C, chondroitin sulfate (CS), and proliferating cell nuclear antigen (PCNA) was performed. Chondrocytes were isolated from human cartilage and cultured. After treatment with TN-C, chondrocyte proliferation s was analyzed by bromodeoxyuridine (BrdU) incorporation assay using an enzyme-linked immunosorbent assay kit. Glycosaminoglycan content was determined by dimethylmethylene blue (DMMB) assay. The mRNA expression of aggrecan was also analyzed, by quantitative real-time polymerase chain reaction (PCR).

RESULTS

In osteoarthritic cartilage, increased TN-C staining was observed with the degeneration of articular cartilage in comparison with normal cartilage. TN-C staining was shown in the cartilage surface overlying CS-positive areas. In addition, the expression of PCNA in the positive areas for TN-C was significantly higher than that in the negative areas. Treatment of human articular chondrocytes with 10 μg/ml TN-C accelerated chondrocyte proliferation, increased the proteoglycan amount in culture, and increased the expression of aggrecan mRNA.

CONCLUSIONS

Our findings indicate that the distribution of TN-C is related to CS production and chondrocyte proliferation in osteoarthritic cartilage and that TN-C has effects on DNA synthesis, proteoglycan content, and aggrecan mRNA expression in vitro. TN-C may be responsible for repair in human osteoarthritic cartilage.

Gene expression profiles in 3D tumor analogs indicate compressive strain differentially enhances metastatic potential

Non-physiological mechanobiological stimuli typically occur in tumors and are considered to promote cancer spreading. Non-fluid related pressure (solid stress), which arises as tumors grow against adjacent tissues, is among the least studied endogenous stimuli due to challenges in replicating the in vivo environment. To this end, the novel devices well-pressor and the videomicroscopy-compatible optic-pressor were developed to exert precise compressive strain on cells in 3D gels in absence of other mechanical stimuli and soluble gradients. Glioblastoma (U87, HGL21) and breast cancer (MDA-MB-231) cells in 1% agarose hydrogels were exposed to 50% compressive strain for 3 h (0.25-0.05 kPa). Live imaging showed that cells elongate and deflect vertically to the load. This stimulation is shown for the first time to differentially regulate metastasis-associated genes. Furthermore, a group of differentially expressed genes was identified in all cell types, both by microarrays and confirmed by RT-PCR for select genes (caveolin-1, integrin-β1, Rac1), indicating shared response mechanisms. These genes are functionally linked and involved in decreasing cell-cell contact, increasing ECM degradation, and ultimately promoting invasion. Caveolin could orchestrate these responses while the uPA and PI3K/Akt systems could play major roles. Future work will focus on specific molecular partnerships under compression and their impact on cancer progression.

Phenotypic variations in chondrocyte subpopulations and their response to in vitro culture and external stimuli

Articular cartilage defects have limited capacity to self-repair, and cost society up to 60 billion dollars annually in both medical treatments and loss of working days. Recent developments in cartilage tissue engineering have resulted in many new products coming to market or entering clinical trials. However, there is a distinct lack of treatments which aim to recreate the complex zonal organization of articular cartilage. Cartilage tissue withstands repetitive strains throughout an individual's lifetime and provides frictionless movement between joints. The structure and composition of its intricately organized extracellular matrix varies with tissue depth to provide optimal resistance to loading, ensure ease of movement, and integrate with the subchondral bone. Each tissue zone is specially designed to resist the load it experiences, and maximize the tissue properties needed for its location. It is unlikely that a homogenous solution to tissue repair will be able to optimally restore the function of such a heterogeneous tissue. For zonal engineering of articular cartilage to become practical, maintenance of phenotypically stable zonal cell populations must be achieved. The chondrocyte phenotype varies considerably by zone, and it is the activity of these cells that help achieve the structural organization of the tissue. This review provides an examination of literature which has studied variations in cellular phenotype between cartilage zones. By doing so, we have identified critical differences between cell populations and highlighted areas of research which show potential in the field. Current research has made the morphological and metabolic variations between these cell populations clear, but an ideal way of maintaining these differences in vitro culture is yet to be established. Combinations of delivered growth factors, mechanical loading, and layered three-dimensional culture systems all show potential for achieving this goal. Furthermore, differentiation of progenitor cell populations into chondrocyte subpopulations may also hold promise for achieving large numbers of zonal chondrocytes. Success of the field lies in establishing methods of retaining phenotypically stable cell populations for in vitro culture.

Both superficial and deep zone articular chondrocyte subpopulations exhibit the Crabtree effect but have different basal oxygen consumption rates

In the absence of in vivo measurements, the oxygen concentration within articular cartilage is calculated from the balance between cellular oxygen consumption and mass transfer. Current estimates of the oxygen tension within articular cartilage are based on oxygen consumption data from full-depth tissue samples. However, superficial and deep cell subpopulations of articular cartilage express intrinsic metabolic differences. We test the hypothesis that the subpopulations differ with respect to their intrinsic oxygen consumption rate. Chondrocytes from the full cartilage thickness demonstrate enhanced oxygen consumption when deprived of glucose, consistent with the Crabtree phenomena. Chondrocyte subpopulations differ in the prevailing availability of oxygen and glucose, which decrease with distance from the cartilage-synovial fluid interface. Thus, we tested the hypothesis that the oxygen consumption of each subpopulation is modulated by nutrient availability, by examining the expression of the Crabtree effect. The deep cells had a greater oxygen consumption than the superficial cells (V(max) of 6.6 compared to 3.2 fmol/cell/h), consistent with our observations of mitochondrial volume (mean values 52.0 vs. 36.4 microm(3)/cell). Both populations expressed the Crabtree phenomena, with oxygen consumption increasing approximately 2.5-fold in response to glycolytic inhibition by glucose deprivation or 2-deoxyglucose. Over 90% of this increase was oligomycin-sensitive and thus accounted for by oxidative phosphorylation. The data contributes towards our understanding of chondrocyte energy metabolism and provides information valuable for the accurate calculation of the oxygen concentration that the cells experience in vivo. The work has further application to the optimisation of bioreactor design and engineered tissues.

Contribution of the cytoskeleton to the compressive properties and recovery behavior of single cells

The cytoskeleton is known to play an important role in the biomechanical nature and structure of cells, but its particular function in compressive characteristics has not yet been fully examined. This study focused on the contribution of the main three cytoskeletal elements to the bulk compressive stiffness (as measured by the compressive modulus), volumetric or apparent compressibility changes (as further indicated by apparent Poisson's ratio), and recovery behavior of individual chondrocytes. Before mechanical testing, cytochalasin D, acrylamide, or colchicine was used to disrupt actin microfilaments, intermediate filaments, or microtubules, respectively. Cells were subjected to a range of compressive strains and allowed to recover to equilibrium. Analysis of the video recording for each mechanical event yielded relevant compressive properties and recovery characteristics related to the specific cytoskeletal disrupting agent and as a function of applied axial strain. Inhibition of actin microfilaments had the greatest effect on bulk compressive stiffness ( approximately 50% decrease compared to control). Meanwhile, intermediate filaments and microtubules were each found to play an integral role in either the diminution (compressibility) or retention (incompressibility) of original cell volume during compression. In addition, microtubule disruption had the largest effect on the "critical strain threshold" in cellular mechanical behavior (33% decrease compared to control), as well as the characteristic time for recovery ( approximately 100% increase compared to control). Elucidating the role of the cytoskeleton in the compressive biomechanical behavior of single cells is an important step toward understanding the basis of mechanotransduction and the etiology of cellular disease processes.

Mechanical properties and gene expression of chondrocytes on micropatterned substrates following dedifferentiation in monolayer

Chondrocytes in articular cartilage normally exhibit high expression of collagen II and aggrecan but rapidly dedifferentiate to a fibroblastic phenotype if passaged in culture. Previous studies have suggested that the loss of chondrocyte phenotype is associated with changes in the structure of the F-actin cytoskeleton, which also controls cell mechanical properties. In this study, we examined how dedifferentiation in monolayer influences the mechanical properties of chondrocytes isolated from different zones of articular cartilage. Atomic force microscopy was used to measure the mechanical properties of superficial and middle/deep zone chondrocytes as they underwent serial passaging and subsequent growth on fibronectin-coated, micropatterned self-assembled monolayers (MSAMs) that restored a rounded cell shape in 2D culture. Chondrocytes exhibited significant increases in elastic and viscoelastic moduli with dedifferentiation in culture. These changes were only partially ameliorated by the restoration of a rounded shape on micropatterned surfaces. Furthermore, intrinsic zonal differences in cell mechanical properties were rapidly lost with passage. These findings indicate that cell mechanical properties may provide additional measures of phenotypic expression of chondrocytes as they undergo dedifferentiation and possibly redifferentiation in culture.

Articular chondrocytes express connexin 43 hemichannels and P2 receptors - a putative mechanoreceptor complex involving the primary cilium?

Mechanical loading is essential for the health and homeostasis of articular cartilage, although the fundamental mechanotransduction pathways are unclear. Previous studies have demonstrated that cyclic compression up-regulates proteoglycan synthesis via an intracellular Ca(2+) signalling pathway, mediated by the release of ATP. However, the mechanism(s) of ATP release has not been elucidated. The present study examines expression of the putative mechanosensitive ATP-release channel, connexin 43 and whether it is expressed on the chondrocyte primary cilium, which acts as a mechanosensor in a variety of other cell types. In addition the study characterized the expression of a range of purine receptors through which ATP may activate downstream signalling events controlling cell function. Bovine articular chondrocytes were isolated by sequential enzyme digestion and seeded in agarose constructs. To verify the presence of functional hemichannels, Lucifer yellow (LY) uptake into viable cells was quantified following treatment with a hemichannel agonist (EGTA) and antagonist (flufenamic acid). LY uptake was observed in 45% of chondrocytes, increasing to 83% following EGTA treatment (P < 0.001). Treatment with the hemichannel blocker, flufenamic acid, significantly decreased LY uptake to less than 5% with and without EGTA. Immunofluorescence and confocal microscopy confirmed the presence of primary cilia and the expression of connexin 43. Approximately 50% of bovine chondrocyte primary cilia were decorated with connexin 43. Human chondrocytes in situ within cartilage explants also expressed connexin 43 hemichannels. However, expression was confined to the upper 200 microm of the tissue closest to the articular surface. Immunofluorescence revealed the expression of a range of P2X and P2Y receptor subtypes within human articular cartilage. In conclusion, the expression of functional hemichannels by articular chondrocytes may represent the mechanism through which mechanical loading activates ATP release as part of a purinergic mechanotransduction pathway. Furthermore, the expression of connexin 43 on the chondrocyte primary cilium suggests the possible involvement of the cilium in this pathway.

Cells and biomaterials in cartilage tissue engineering

Cartilage defects are notoriously difficult to repair and owing to the long-term prognosis of osteoarthritis, and a rapidly aging population, a need for new therapies is pressing. Cell-based therapies for cartilage regeneration were introduced into patients in the early 1990s. Since that time the technology has developed from a simple cell suspension to more complex 3D structures. Cells, both chondrocytes and stem cells, have been incorporated into scaffold material with the aim to better recreate the natural environment of the cell, while providing more structural support to withstand the large forces applied on the de novo tissue. This review aims to provide an overview of potential cell sources and different scaffold materials, which are in development for cartilage tissue engineering.

Articular chondrocytes derived from distinct tissue zones differentially respond to in vitro oscillatory tensile loading

OBJECTIVE

The cell morphology, gene expression, and matrix synthesis of articular chondrocytes are known to vary with depth from the tissue surface. The objective of this study was to investigate if chondrocytes from different zones respond to in vitro oscillatory tensile loading in distinct ways and whether tensile strain, which is most prevalent near the articular surface, would preferentially stimulate superficial zone chondrocytes.

DESIGN

Chondrocytes were separately isolated from the superficial, middle, and deep zones of articular cartilage and seeded into three-dimensional fibrin hydrogel constructs. An intermittent protocol of oscillatory tensile loading was applied for 3 days, and the effects on extracellular matrix (ECM) synthesis were assessed by measuring the incorporation of radiolabed precursors, size exclusion gel chromatography, and western blotting.

RESULTS

Tensile loading was found to be a potent stimulus for proteoglycan synthesis only in superficial zone chondrocytes. Although overall biosynthesis rates by deep zone chondrocytes were unaffected by tensile loading, the molecular characteristics of proteins and proteoglycans released to the culture medium were significantly altered so as to resemble those of superficial zone chondrocytes.

CONCLUSIONS

Oscillatory tensile loading differentially affected subpopulations of articular chondrocytes in three-dimensional fibrin hydrogel constructs. Cells isolated from deeper regions of the tissue developed some characteristics of superficial zone chondrocytes after exposure to tensile loading, which may indicate an adaptive response to the new mechanical environment. Understanding how exogenous mechanical stimuli can differentially influence chondrocytes from distinct tissue zones will yield important insights into mechanobiological processes involved in cartilage tissue development, maintenance, disease, and repair.

Static and dynamic compressive strains influence nitric oxide production and chondrocyte bioactivity when encapsulated in PEG hydrogels of different crosslinking densities

OBJECTIVE

Mechanical loading is an important regulator of chondrocytes; however, many of the mechanisms involved in chondrocyte mechanotransduction still remain unclear. Here, poly(ethylene glycol) (PEG) hydrogels are proposed as a model system to elucidate chondrocyte response due to cell deformation, which is controlled by gel crosslinking (rho(x)).

METHODS

Bovine articular chondrocytes (50 x 10(6)cells/mL) were encapsulated in gels with three rho(x)s and subjected to static (15% strain) or dynamic (0.3 Hz or 1 Hz, 15% amplitude strain) loading for 48 h. Cell deformation was examined by confocal microscopy. Cell response was assessed by total nitric oxide (NO) production, proteoglycan (PG) synthesis ((35)SO(4)(2-)-incorporation) and cell proliferation (CP) ([(3)H]-thymidine incorporation). Oxygen consumption was assessed using an oxygen biosensor.

RESULTS

An increase in rho(x) led to lower water contents, higher compressive moduli, and higher cell deformations. Chondrocyte response was dependent on both loading regime and rho(x). For example, under a static strain, NO was not affected, while CP and PG synthesis were inhibited in low rho(x) and stimulated in high rho(x). Dynamic loading resulted in either no effect or an inhibitory effect on NO, CP, and PG synthesis. Overall, our results showed correlations between NO and CP and/or PG synthesis under static and dynamic (0.3 Hz) loading. This finding was attributed to the hypoxic environment that resulted from the high cell-seeding density.

CONCLUSION

This study demonstrates gel rho(x) and loading condition influence NO, CP, and PG synthesis. Under a hypoxic environment and certain loading conditions, NO appears to have a positive effect on chondrocyte bioactivity.

Passage and reversal effects on gene expression of bovine meniscal fibrochondrocytes

The knee meniscus contains a mixed population of cells that exhibit fibroblastic as well as chondrocytic characteristics. Tissue engineering studies and future therapies for the meniscus require a large population of cells that are seeded on scaffolds. To achieve this, monolayer expansion is often used as a technique to increase cell number. However, the phenotype of these cells may be significantly different from that of the primary population. The objective of this study was to investigate changes in meniscal fibrochondrocytes at the gene expression level over four passages using quantitative real-time reverse transcriptase polymerase chain reaction. Cells from the inner two-thirds of bovine medial menisci were used. Four extracellular matrix (ECM) molecules, commonly found in the meniscus, were investigated, namely collagen I, collagen II, aggrecan and cartilage oligomeric matrix protein (COMP). In addition, primary and passaged meniscus fibrochondrocytes were placed on surfaces coated with collagen I or aggrecan protein to investigate whether any gene expression changes resulting from passage could be reversed. Collagen I expression was found to increase with the number of passages, whereas collagen II and COMP expression decreased. Collagen I and aggrecan surface coatings were shown to downregulate and upregulate collagen I and COMP expression levels, respectively, in passaged cells. However, decreases in collagen II expression could not be reversed by either protein coating. These results indicate that although monolayer expansion results in significant changes in gene expression in meniscal fibrochondrocytes, protein coatings may be used to regain the primary cell expression of several ECM molecules.

Low-serum media and dynamic deformational loading in tissue engineering of articular cartilage

High-serum media have been shown to produce significant improvement in the properties of tissue-engineered articular cartilage when applied in combination with dynamic deformational loading. To mitigate concerns regarding the culture variability introduced by serum, we examined the interplay between low-serum/ITS-supplemented media and dynamic deformational loading. Our results show that low serum/ITS supplementation does not support the same level of tissue formation as compared to high serum controls. In free-swelling culture, using a combination of ITS with concentrations of FBS above 2% negated the beneficial effects of ITS. Although there were beneficial effects with loading and 0.2%FBS + ITS, these constructs significantly underperformed relative to 20%FBS constructs. At 2%FBS + ITS, the free-swelling construct stiffness and composition approached or exceeded that of 20%FBS constructs. With dynamic loading, the properties of 2%FBS + ITS constructs were significantly lower than free-swelling controls and 20%FBS constructs by day 42. By priming the chondrocytes in 20%FBS prior to exposure to low-serum/ITS media, we observed that low-serum/ITS media produced significant enhancement in tissue properties compared to constructs grown continuously in 20%FBS.

Distinct horizontal patterns in the spatial organization of superficial zone chondrocytes of human joints

A better understanding of the unique cellular and functional properties of the superficial zone of articular cartilage may aid current strategies in tissue engineering which attempts a layered design for the repair of cartilage lesions to avert or postpone the onset of osteoarthritis. However, data pertaining to the cellular organization of non-degenerated superficial zone of articular cartilage is not available for most human joints. The present study analyzed the arrangement of chondrocytes of non-degenerated human joints (shoulder, elbow, knee, and ankle) by using fluorescence microscopy of the superficial zone in a top-down view. The resulting horizontal chondrocyte arrangements were tested for randomness, homogeneity or a significant grouping via point pattern analysis and were correlated with the joint type in which they occurred. The present study demonstrated that human superficial chondrocytes occurred in four distinct patterns of strings, clusters, pairs or single chondrocytes. Those patterns represented a significant grouping (p < 0.0001) with horizontal alignment. Each articular joint surface was dominated by only one of these four patterns (p < 0.001). Specific patterns correlated with specific diarthrodial joint types (p < 0.001). Further studies need to establish whether these organizational patterns are a consequence of their surrounding environment or whether they are linked to a functional purpose.

Different response of articular chondrocyte subpopulations to surface motion

OBJECTIVE

To investigate the effect of surface motion on the gene expression of proteoglycan 4 (PRG4), hyaluronan synthases (HAS1, HAS2) and on the hyaluronan (HA) and proteoglycan 4 (PRG4) release of chondrocytes from different zones of bovine articular cartilage.

DESIGN

Superficial zone, deep zone, full thickness, and superficial/deep 1:1 mixed chondrocytes were seeded into 3D polyurethane scaffolds and stimulated using our bioreactor that approximates kinematics and surface motion characteristics of natural joints. One hour of surface motion superimposed on cyclic compression was applied twice a day over 3 consecutive days. Scaffolds were cut into top and bottom sections and analyzed for gene expression of PRG4, HAS1, and HAS2.

RESULTS

Depending on the cell population, the gene expression levels increased within 8 days of culture in unloaded scaffolds, with a stronger increase in the top compared to the bottom sections. Mechanical loading further enhanced the messenger RNA (mRNA) levels in all cell types, with most pronounced up-regulations observed for the PRG4 expression in deep zone and the HAS2 expression in superficial zone cells. The effect of the biochemical and biomechanical environment appeared to be additive, resulting in highest mRNA levels in the top sections of loaded constructs. Bioreactor stimulation also enhanced the HA release in all cell populations. Full thickness chondrocytes experienced the greatest effect on HAS1 mRNA expression and HA release, indicating that the interaction between cell populations may promote HA synthesis compared to subpopulations alone.

CONCLUSIONS

Reciprocating sliding can be an efficient tool for generating tissue-engineered constructs from various chondrocyte populations by providing a functional cartilage-synovial interface.

Immobilized fibrinogen in PEG hydrogels does not improve chondrocyte-mediated matrix deposition in response to mechanical stimulation

The present investigation aims to explore the role of cell-scaffold interactions and whole cell compression in chondrocyte mechanotransduction using encapsulating poly(ethylene glycol) (PEG) hydrogel scaffolds and primary bovine chondrocytes. Scaffolds made from PEG hydrogels with immobilized fibrinogen molecules were seeded with chondrocytes and subjected to 15% dynamic compressive strain at 1-Hz frequency. Dynamic strain stimulation resulted in a 37% increase in the levels of sulfated glycosaminoglycan (sGAG) after 2 weeks of stimulation, when compared to static controls. Comparing results of the PEG-fibrinogen scaffolds with their respective PEG control group did not show significant differences between the two, even following 2 weeks of dynamic mechanical stimulation. Accordingly, these findings indicate that while cell deformations cause metabolic changes in chondrocytes seeded in PEG hydrogels, it is difficult to ascertain the role of matrix bioactivity in enhancing chondrocyte mechanotransduction in encapsulating scaffolds subjected to physical deformations. This study shows how interactions between mechanical stimulation and scaffold composition are evaluated using an experimental approach and customized biomaterial scaffolds.

Cyclic compression of chondrocytes modulates a purinergic calcium signalling pathway in a strain rate- and frequency-dependent manner

Mechanical loading modulates cartilage homeostasis through the control of matrix synthesis and catabolism. However, the mechanotransduction pathways through which chondrocytes detect different loading conditions remain unclear. The present study investigated the influence of cyclic compression on intracellular Ca2+ signalling using the well-characterised chondrocyte-agarose model. Cells labelled with Fluo4 were visualised using confocal microscopy following a period of 10 cycles of compression between 0% and 10% strain. In unstrained agarose constructs, not subjected to cyclic compression, a subpopulation of approximately 45% of chondrocytes exhibited spontaneous global Ca2+ transients with mean transient rise and fall times of 19.4 and 29.4 sec, respectively. Cyclic compression modulated global Ca2+ signalling by increasing the percentage of cells exhibiting Ca2+ transients (population modulation) and/or reducing the rise and fall times of these transients (transient shape modulation). The frequency and strain rate of compression differentially modulated these Ca2+ signalling characteristics providing a potential mechanism through which chondrocytes may distinguish between different loading conditions. Treatment with apyrase, gadolinium and the P2 receptor blockers, suramin and basilen blue, significantly reduced the percentage of cells exhibiting Ca2+ transients following cyclic compression, such that the mechanically induced upregulation of Ca2+ signalling was completely abolished. Thus cyclic compression appears to activate a purinergic pathway involving the release of ATP followed by the activation of P2 receptors causing a combination of extracellular Ca2+ influx and intracellular Ca2+ release. Knowledge of this fundamental cartilage mechanotransduction pathway may lead to improved therapeutic strategies for the treatment of cartilage damage and disease.

Tailoring secretion of proteoglycan 4 (PRG4) in tissue-engineered cartilage

Articular cartilage provides a low-friction surface for joint articulation, with boundary lubrication facilitated by proteoglycan 4 (PRG4), which is secreted by chondrocytes of the superficial zone. Chondrocytes from different zones are phenotypically distinct, and their phenotypes in vitro are influenced by the system in which they are cultured. We hypothesized that culturing cells from the superficial (S) zone in two-dimensional monolayer or three-dimensional alginate would affect their synthesis of PRG4, and that subsequently seeding them atop alginate-recovered cells from the middle/ deep (M) zone in various proportions would result in tissue-engineered constructs with varying levels of PRG4 secretion and matrix accumulation. During monolayer culture, S cells retained their PRG4-secreting phenotype, whereas in alginate culture the percentage of cells secreting PRG4 decreased with time. Constructs formed with increasing percentages of S cells decreased in thickness and matrix accumulation, depending on both the culture conditions before construct formation and the S-cell density. PRG4-secreting cells were localized to the S-cell seeded construct surface, with secretion rates of 0.1-4 pg/cell/day or 0.1-1 pg/cell/day for constructs formed with monolayer-recovered or alginate-recovered S cells, respectively. Tailoring secretion of PRG4 in cartilage constructs may be useful for enhancing low-friction properties at the articular surface, while maintaining other surfaces free of PRG4 for enhancing integration with surrounding tissues.

The effect of finite compressive strain on chondrocyte viability in statically loaded bovine articular cartilage

Recent studies have reported that certain regimes of compressive loading of articular cartilage result in increased cell death in the superficial tangential zone (STZ). The objectives of this study were (1) to test the prevalent hypothesis that preferential cell death in the STZ results from excessive compressive strain in that zone, relative to the middle and deep zones, by determining whether cell death correlates with the magnitude of compressive strain and (2) to test the corollary hypothesis that the viability response of cells is uniform through the thickness of the articular layer when exposed to the same loading environment. Live cartilage explants were statically compressed by approximately 65% of their original thickness, either normal to the articular surface (axial loading) or parallel to it (transverse loading). Cell viability after 12 h was compared to the local strain distribution measured by digital image correlation. Results showed that the strain distribution in the axially loaded samples was highest in the STZ (77%) and lowest in the deep zone (55%), whereas the strain was uniformly distributed in the transversely loaded samples (64%). In contrast, axially and transversely loaded samples exhibited very similar profiles of cell death through the depth, with a preferential distribution in the STZ. Unloaded control samples showed negligible cell death. Thus, under prolonged static loading, depth-dependent variations in chondrocyte death did not correlate with the local depth-dependent compressive strain, and the prevalent hypothesis must be rejected. An alternative hypothesis, suggested by these results, is that superficial zone chondrocytes are more vulnerable to prolonged static loading than chondrocytes in the middle and deep zones.

Viscoelastic properties of zonal articular chondrocytes measured by atomic force microscopy

OBJECTIVE

Articular chondrocytes respond to chemical and mechanical signals depending on their zone of origin with respect to distance from the tissue surface. However, little is known of the zonal variations in cellular mechanical properties in cartilage. The goal of this study was to determine the zonal variations in the elastic and viscoelastic properties of porcine chondrocytes using atomic force microscopy (AFM), and to validate this method against micropipette aspiration.

METHODS

A theoretical solution for stress relaxation of a viscoelastic, incompressible, isotropic surface indented with a hard, spherical indenter (5 microm diameter) was derived and fit to experimental stress-relaxation data for AFM indentation of chondrocytes isolated from the superficial or middle/deep zones of cartilage.

RESULTS

The instantaneous moduli of chondrocytes were 0.55+/-0.23 kPa for superficial cells (S) and 0.29+/-0.14 kPa for middle/deep cells (M/D) (P<0.0001), and the relaxed moduli were 0.31+/-0.15 kPa (S) and 0.17+/-0.09 kPa (M/D) (P<0.0001). The apparent viscosities were 1.15+/-0.66 kPas (S) and 0.61+/-0.69 kPa-s (M/D) (P<0.0001). Results from the micropipette aspiration test showed similar cell moduli but higher apparent viscosities, indicating that mechanical properties measured by these two techniques are similar.

CONCLUSION

Our findings suggest that chondrocyte biomechanical properties differ significantly with the zone of origin, consistent with previous studies showing zonal differences in chondrocyte biosynthetic activity and gene expression. Given the versatility and dynamic testing capabilities of AFM, the ability to conduct stress-relaxation measurements using this technique may provide further insight into the viscoelastic properties of isolated cells.

Localization of extracellular matrix receptors on the chondrocyte primary cilium

A single primary cilium is found in chondrocytes and other connective tissue cells. We have previously shown that extracellular matrix (ECM) macromolecules such as collagen fibers closely associate with chondrocyte primary cilia, and their points of contact are characterized by electron-opaque plaques suggesting a direct link between the ECM and the cilium. This study examines the expression of receptors for ECM molecules on chondrocyte primary cilia. Embryonic chick sterna were fluorescently labeled with antibodies against alpha and beta integrins, NG2, CD44, and annexin V. Primary cilia were labeled using acetylated alpha-tubulin antibody. Expression of ECM receptors was examined on chondrocyte plasma membranes and their primary cilia using immunofluorescence and confocal microscopy. All receptors examined showed a punctate distribution on the plasma membrane. alpha2, alpha3, and beta1 integrins and NG2 were also present on primary cilia, whereas annexin V and CD44 were excluded. The number of receptor-positive cilia varied from 8/50 for NG2 to 43/50 for beta1 integrin. This is the first study to demonstrate the expression of integrins and NG2 on chondrocyte primary cilia. The data strongly suggest that chondrocyte primary cilia have the necessary machinery to act as mechanosensors, linking the ECM to cytoplasmic organelles responsible for matrix production and secretion.

Computational Modeling to Predict the Temporal Regulation of Chondrocyte Metabolism in Response to Various Dynamic Compression Regimens

Based on previously published experimental work, computational models were developed to simulate the effect of different dynamic compression regimens on the activity of chondrocytes seeded in agarose constructs. In particular, the balance between proliferation and matrix synthesis can be adjusted by applying different intervals of continuous or intermittent mechanical compression. A phenomenological compartment based-modeling approach was used as first model. A more mechanistic cell cycle model was used as the second model. The compartment-based modeling approach was found to be useful in representing a balance between proliferation and proteoglycan synthesis, when the effect of a certain stimulation protocol is known. In order to predict the response to different intervals of mechanical stimulation, however, a more mechanistic cell cycle-based approach is required. The cell cycle model supports an important role of the onset of loading. In addition, an inhibitory effect of further loading is required, which is more likely to be related to cell cycle progression velocity than to a decreased probability of commitment to the cell cycle. The mechanisms behind this inhibitory effect and the computational implementation, however, require further investigation.

Heterogeneity in patellofemoral cartilage adaptation to anterior cruciate ligament transection; chondrocyte shape and deformation with compression

OBJECTIVE

The purpose of this study was to determine if the opposing cartilages of the feline patellofemoral joint adapted differently to short-term anterior cruciate ligament transection (ACL-T) and if the magnitude of chondrocyte deformation upon tissue loading was altered under ACL-T conditions compared to contralateral controls. In situ static compression of physiological magnitude was applied to the feline patellofemoral cartilage 16 weeks post-ACL-T and cartilage and chondrocyte deformation were evaluated by histomorphometry.

DESIGN

Six adult cats were euthanized 16 weeks after unilateral ACL-T. A peak surface pressure of 9 MPa was applied to the fully intact patella and femoral groove cartilages. After in situ fixation under compression, sections from the centre of the indent and from an adjacent unloaded area of the cartilages were analysed. Chondrocyte shape, size, clustering and volumetric fraction were quantified.

RESULTS

Experimental patellar articular cartilage was thicker, contained larger chondrocytes that were more frequently arranged in clusters and had, on average, a larger chondrocyte volumetric fraction compared to contralateral controls. In contrast, the experimental femoral groove cartilage demonstrated little adaptation to ACL-T.

CONCLUSIONS

The patellar articular cartilage adapts to short-term ACL-T to a greater extent than femoral groove cartilage. We speculate that differences in the histological parameters of control tissues, such as cartilage thickness and the magnitude and depth distribution of chondrocyte shape, size and volumetric fraction may contribute to predisposing patellar cartilage, and not femoral groove cartilage, to adaptation after ACL-T.

Clonal populations of chondrocytes with progenitor properties identified within human articular cartilage

The aim of the present study was to identify and characterize progenitor properties of human articular chondrocytes selected by using agarose suspension culture. In this chondrogenic selective culture condition, about 3.6% of seeded surplus chondrocytes from patients undergoing articular chondrocyte transplantation proliferated and formed cell clusters after 6 weeks. Phase-contrast microscopy and transmission electron microscopy revealed four different types of cell clusters differing in cellular content and matrix production. Based on their morphological features, they were named the homogenous (H), the homogenous matrix (HM), the differentiated matrix (DM) and the differentiated (D) cell clusters. All cell clusters showed positive safranin O staining, and matrix was positive for antibodies detecting type II collagen and aggrecan. The clusters were further demonstrated to express the genes for fibroblast growth factor receptor 3, type IIA collagen and type IIB collagen, while type X collagen was not expressed. After subcloning, the H and HM clusters demonstrated the best proliferative capacity. Chondrocytes from these two cell clusters also showed phenotypic plasticity in chondrogenic, adipogenic as well as osteogenic assays. This study demonstrates that existing subpopulations of cells with chondroprogenitor properties can be isolated from human adult articular cartilage using agarose suspension cultures.

Regenerated soft tissue survival using repulsive force of magnetized devices: preliminary report

BACKGROUND

Large full-thickness cartilage defects in the weight-bearing area are difficult to treat. A new therapeutic strategy called the total joint regeneration (TJR) system is proposed for such large defects. The purpose of this study was to evaluate the effects of the magnet-type TJR device using a rabbit model.

METHODS

The magnetized devices were implanted in full-thickness chondral defects on the patellofemoral joints of rabbits. The specimens and surrounding tissue were harvested 4 weeks after the surgery and observed macroscopically and histologically. The thickness of the regenerated soft tissue on the femur joint surface was measured and compared. The difference between the two groups (magnetized and nonmagnetized) was significant at P < 0.05.

RESULTS

Some cartilaginous regeneration was seen in the repair tissue. However, about half of the experimental knees were omitted from the study because of some trouble, such as loosening of the device or patella fracture.

CONCLUSIONS

This study suggested that magnetized devices were useful for regenerating soft tissue by maintaining the joint space. Some hyaline cartilage-like tissue was regenerated partially on the magnetized devices. It was suggested that these devices might be useful for cartilage regeneration if the devices are improved.

Biomechanics of single zonal chondrocytes

Articular cartilage has a distinct zonal architecture, and previous work has shown that chondrocytes from different zones exhibit variations in gene expression and biosynthesis. In this study, the material properties of single chondrocytes from the superficial and middle/deep zones of bovine distal metatarsal articular cartilage were determined using unconfined compression and digital videocapture. To determine the viscoelastic properties of zonal chondrocytes, unconfined creep compression experiments were performed and the resulting creep curves of individual cells were fit using a standard linear viscoelastic solid model. In the model, a fixed value of the Poisson's ratio was used, determined optically from direct compression of middle/deep chondrocytes. The two approaches used in this study yielded the following average material properties of single chondrocytes: Poisson's ratio of 0.26+/-0.08, instantaneous modulus of 1.06+/-0.82 kPa, relaxed modulus of 0.78+/-0.58 kPa, and apparent viscosity of 4.08+/-7.20 kPa s. Superficial zone chondrocytes were found to be significantly stiffer than middle/deep zone chondrocytes. Attachment time did not affect the stiffness of the cells. The zonal variation in viscoelastic properties may result from the distinct mechanical environments experienced by the cells in vivo. Identifying intrinsic differences in the biomechanics of superficial and middle/deep zone chondrocytes is an important component in understanding how biomechanics influence articular cartilage health and disease.

Chondrocytes from different zones exhibit characteristic differences in high density culture

Superficial and middle/deep zone chondrocytes were isolated from goat femoral cartilage by a zonal abrasion method. The cells were expanded 100-fold through two passages, then seeded into agarose wells to form high-density constructs through a self-assembling process. After 4 weeks in culture, the superficial zone constructs contracted into a dense cell mass, while middle/deep zone chondrocytes formed constructs with four distinct regions. Middle/deep zone chondrocytes produced 250% more glycosaminoglycans per dry weight and more collagen per dry weight than superficial zone chondrocytes. The superficial and middle/deep zone chondrocytes were found to retain characteristic differences even after 100-fold expansion, as evidenced by construct morphology and extracellular matrix content. This study uniquely demonstrated the ability of expanded superficial and middle/deep zone chondrocytes to form constructs of distinct characteristics without a scaffold. The goal of tissue engineering different zones of cartilage is to eventually replicate the specific function of each zone.

Down-regulation of basic fibroblast growth factor production from cartilage by excessive mechanical stress

BACKGROUND

We examined the effect of excessive stress on the production of basic fibroblast growth factor (bFGF) by bovine cartilage.

METHODS

Bovine articular carpal bone was harvested and cut into 1 cm thick slices horizontally. Continuous compressive stress (0, 2, 20 MPa) was loaded on the cartilage of the carpal bone using a stainless-steel cylinder with a diameter of 8 mm for 1 h. The slices were cultured immediately after compression, and the supernatant of the culture medium was collected for bFGF and nitric oxide (NO) measurement. Interleukin-4 (IL-4) or N(G)-monoethyl-L: -arginine (L: -NMEA) was added to the culture medium in some experiments.

RESULTS

Basic FGF production was significantly increased after 36 h of cultivation without mechanical stress (0.40 +/- 0.03 microg/ml). In contrast, the bFGF concentration was not increased by compressive stress of 20 MPa after 36 h of cultivation. An NO inhibitor, L: -NMEA, did not alter the effect of compressive stress on the production of bFGF. IL-4 reduced the production of bFGF by cartilage with or without mechanical stress.

CONCLUSIONS

Excessive stress on cartilage inhibits the production of bFGF in an NO-independent manner, and IL-4 plays an important role in the reduction of bFGF.

Effects of cyclic compressive loading on chondrogenesis of rabbit bone-marrow derived mesenchymal stem cells

The objective of this study was to examine the effects of cyclic compressive loading on chondrogenic differentiation of rabbit bone-marrow mesenchymal stem cells (BM-MSCs) in agarose cultures. Rabbit BM-MSCs were obtained from the tibias and femurs of New Zealand white rabbits. After the chondrogenic potential of BM-MSCs was verified by pellet cultures, cell-agarose constructs were made by suspending BM-MSCs in 2% agarose (10(7) cells/ml) for a cyclic, unconfined compression test performed in a custom-made bioreactor. Specimens were divided into four groups: control; transforming growth factor (TGF-beta) (with TGF-beta1 treatment); loading (with stimulation of cyclic, unconfined compressive loading); and TGF-beta loading (with TGF-beta1 treatment and loading stimulation) groups. In the loading experiment, specimens were subjected to sinusoidal loading with a 10% strain magnitude at a frequency of 1 Hz for 4 hours a day. Experiments were conducted for 3, 7, and 14 consecutive days. While the experimental groups (TGF-beta, loading, and TGF-beta loading) exhibited significantly higher levels of expressions of chondrogenic markers (collagen II and aggrecan) at three time periods, there were no differences among the experimental groups after an extra 5-day culture. This suggests that compressive loading alone induces chondrogenic differentiation of rabbit BM-MSCs as effectively as TGF-beta or TGF-beta plus loading treatment. Moreover, both the compressive loading and the TGF-beta1 treatment were found to promote the TGF-beta1 gene expression of rabbit BM-MSCs. These findings suggest that cyclic compressive loading can promote the chondrogenesis of rabbit BM-MSCs by inducing the synthesis of TGF-beta1, which can stimulate the BM-MSCs to differentiate into chondrocytes.