Chondrocyte deformation within compressed agarose constructs at the cellular and sub-cellular levels

The differential effect of scaffold composition and architecture on chondrocyte response to mechanical stimulation

This study aims to explore the differential effect of scaffold composition and architecture on chondrogenic response to dynamic strain stimulation using encapsulating PEG-based hydrogels and primary bovine chondrocytes. Proteins and proteoglycans were conjugated to functionalized poly(ethylene glycol) (PEG) and immobilized in PEG hydrogels to create bio-synthetic materials to be used as scaffolds. Four different compositions were tested, including: PEG-Proteoglycan (PP), PEG-Fibrinogen (PF), PEG-Albumin (PA), and PEG only. Primary articular chondrocytes were encapsulated in the hydrogel scaffolds and subjected to 15% dynamic compressive strain stimulation at 1-Hz frequency for 28 days. Stimulation of PP, PF, PA and PEG constructs resulted in a respective increase in the unconfined true compressive modulus by 32%, 45.4%, 33.6%, and 28.2%, compared to their static controls. The PF showed a significantly larger relative increase in the modulus in comparison to all other scaffolds tested. These results support the hypothesis that mechanical stimulation and material bioactivity have a significant effect on the reported chondrocyte response. Similar trends were observed with the swelling ratio of the constructs. These findings indicate that while stimulation causes metabolic changes in chondrocytes seeded in PEG hydrogels, the matrix bioactivity has a significant role in enhancing chondrocyte mechanotransduction in encapsulating scaffolds subjected to physical deformations.

Synergistic effect of chondroitin sulfate and cyclic pressure on biochemical and morphological properties of chondrocytes from articular cartilage

OBJECTIVE

To investigate the synergistic effect of chondroitin sulfate (CS) and cyclic pressure on the biochemical and morphological properties of chondrocytes isolated from articular cartilage and cultured in alginate matrix.

METHODS

The chondrocytes obtained from articular cartilage of goat femoropatellar joint were isolated and cultured in alginate matrix. The cells were exposed to CS (100 microg/ml) along with cyclic pressure of 1.2 MPa and 2.4 MPa and biochemical analysis of DNA, proteoglycan, collagen and protease activity was carried out in different matrix fractions, i.e., cellular matrix (CM) and further removed matrix (FRM) and in culture medium. The morphological studies of chondrocytes were carried out using transmission electron microscopy (TEM).

RESULTS

The treatment of chondrocytes with CS along with cyclic pressure increased the rate of cell proliferation relative to control (without load and in the absence of CS) and CS alone (P<0.001). The proteoglycan content in CM increased in the presence of CS alone (P<0.05) as well as CS with cyclic pressure (P<0.001). The specific activity of protease in CM and FRM decreased in the presence of CS with cyclic pressure relative to control (P<0.001). The TEM images showed abundant CM, improved cell morphology and matrix organization in the presence of CS and cyclic load treatment.

CONCLUSIONS

The results of this study suggested that in the presence of CS along with cyclic loading, the cellular ability to utilize and incorporate exogenous CS as extracellular matrix improved, as compared to CS alone.

Interaction between the interstitial fluid and the extracellular matrix in confined indentation

The Movement of the interstitial fluid in extracellular matrices not only affects the mechanical properties of soft tissues, but also facilitates the transport of nutrients and the removal of waste products. In this study, we aim to quantify interstitial fluid movement and fluid-matrix interaction in a new loading configuration-confined tissue indentation, using a poroelastic theory. The tissue sample sits in a cylindrical chamber and loading is applied on the top central surface of the specimen by a porous indenter that is fixed on the specimen. The interaction between the solid and the fluid is examined using a finite element method under ramp and cyclic loads. Typical compression-relaxation responses of the specimen are observed in a ramp load. Under a cyclic load, the system reaches a dynamic equilibrium after a number of loading cycles. Fluid circulation, with opposite directions in the loading and unloading phases in the extracellular matrix, is observed. The most significant variation in the fluid pressure locates just beneath the indenter. Fluid pressurization arrives at equilibrium much faster than the solid matrix deformation. As the loading frequency increases, the location of the peak pressure oscillation moves closer to the indenter and the magnitude of the pressure oscillation increases. Concomitantly, the axial stress variation of the solid matrix is reduced. It is found that interstitial fluid movement helps to alleviate severe strain of the solid matrix beneath the indenter. This study quantifies the interaction between the interstitial fluid and the extracellular matrix by decomposing the loading response of the specimen into the "transient" and "dynamic equilibrium" phases. Confined indentation in this manuscript gives a better representation of some in vitro and in vivo loading configurations where the indenter covers part of the top surface of the tissue.

Biomechanical analysis of structural deformation in living cells

Most tissues are subject to some form of physiological mechanical loading which results in deformation of the cells triggering intracellular mechanotransduction pathways. This response to loading is generally essential for the health of the tissue, although more pronounced deformation may result in cell and tissue damage. In order to determine the biological response of cells to loading it is necessary to understand how cells and intracellular structures deform. This paper reviews the various loading systems that have been adopted for studying cell deformation both in situ within tissue explants and in isolated cell culture systems. In particular it describes loading systems which facilitate visualisation and subsequent quantification of cell deformation. The review also describes the associated microscopy and image analysis techniques. The review focuses on deformation of chondrocytes with additional information on a variety of other cell types including neurons, red blood cells, epithelial cells and skin and muscle cells.

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.

Alterations in the vimentin cytoskeleton in response to single impact load in an in vitro model of cartilage damage in the rat

Background

Animal models have provided much information on molecular and cellular changes in joint disease, particularly OA. However there are limitations to in vivo work and single tissue in vitro studies can provide more specific information on individual events. The rat is a commonly used laboratory species but at the current time only in vivo models of rat OA are available to study. The purpose of this study was to investigate the damage that single impact load (SIL) of 0.16J causes in a rat cartilage in vitro model and assess whether this load alters the arrangement of vimentin.

Methods

Rat cartilage was single impact loaded (200 g from 8 cm) and cultured for up to 48 hours (n = 72 joints). Histological changes were measured using a semi-quantitative modified Mankin score. Immunolocalisation was used to identify changes in vimentin distribution.

Results

SIL caused damage in 32/36 cartilage samples. Damage included surface fibrillation, fissures, fragmentation, changes in cellularity and loss of proteoglycan. SIL caused a statistically significant increase in modified Mankin score and chondrocyte clusters over time. SIL caused vimentin disassembly (as evidenced by collapse of vimentin around the nucleus).

Conclusion

This study describes a model of SIL damage to rat cartilage. SIL causes changes in histological/chemical parameters which have been measured using a semi-quantitative modified Mankin score. Single impact load also causes changes in the pattern of vimentin immunoreactivity, indicating vimentin dissassembley. Using a semi-quantitative scoring system the disassembly was shown to be statistically significant in SIL damaged cartilage.

The changes described in this paper suggest that this novel single tissue rat model of joint damage is a possible candidate model to replace in vivo models.

Differential effects of cyclic and static pressure on biochemical and morphological properties of chondrocytes from articular cartilage

BACKGROUND

Mechanical stresses are known to play important role on articular cartilage functions in vivo and also on cartilage explants and chondrocytes monolayer culture. This study examined the differential effect of cyclic and static pressures on chondrocytes cultured in alginate matrix, which is physiologically closer to the in vivo environment of cells in cartilage.

METHODS

Goat knee joint articular cartilage chondrocytes cultured in alginate beads were exposed to 1.2 and 2.4 MPa cyclic and static loadings via a custom-made cam/follower based machine. Biochemical contents (glycosaminoglycan, collagen, DNA) and protease activity of cells were analyzed separately in cellular matrix, further removed matrix and in culture medium. Morphology of chondrocytes was studied under transmission electron microscopy.

FINDINGS

Compared with controls (unloaded cells), cyclic loading increased the glycosaminoglycan content of cells at 1.2 and 2.4 MPa in cellular matrix and further removed matrix (P<0.001) whereas it decreased at similar static loads (P<0.001). In alginate matrix, chondrocytes released a metalloprotease, which required Mn(2+) for activity. Both cyclic load levels inhibited its specific activity in cellular matrix but increased it at static loading (P<0.001). The protease specific activity in further removed matrix increased at both cyclic and static loadings (P<0.001). Transmission electron microscopy data showed improved cells ultrastructure and cell-matrix interactions under cyclic load whereas these deteriorated under static loadings.

INTERPRETATION

The study suggests that cyclic load has a positive effect on chondrocytes metabolism and morphology whereas static load has a degenerative effect.

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.

Articular cartilage and growth plate defects are associated with chondrocyte cytoskeletal abnormalities in Tg737orpk mice lacking the primary cilia protein polaris

Primary cilia are highly conserved organelles found on almost all eukaryotic cells. Tg737(orpk) (orpk) mice carry a hypomorphic mutation in the Tg737 gene resulting in the loss of polaris, a protein essential for ciliogenesis. Orpk mice have an array of skeletal patterning defects and show stunted growth after birth, suggesting defects in appositional and endochondral development. This study investigated the association between orpk tibial long bone growth and chondrocyte primary cilia expression using histomorphometric and immunohistochemical analysis. Wild-type chondrocytes throughout the developing epiphysis and growth plate expressed primary cilia, which showed a specific orientation away from the articular surface in the first 7-10 cell layers. In orpk mice, primary cilia were identified on very few cells and were significantly shorter. Orpk chondrocytes also showed significant increases in cytoplasmic tubulin, a likely result of failed ciliary assembly. The growth plates of orpk mice were significantly smaller in length and width, with marked changes in cellular organization in the presumptive articular cartilage, proliferative and hypertrophic zones. Cell density at the articular surface and in the hypertrophic zone was significantly altered, suggesting defects in both appositional and endochondral growth. In addition, orpk hypertrophic chondrocytes showed re-organization of the F-actin network into stress fibres and failed to fully undergo hypertrophy, while there was a marked reduction in type X collagen sequestration. These data suggest that failure to form a functional primary cilium affects chondrocyte differentiation and results in delayed chondrocyte hypertrophy within the orpk growth plate.

Dynamic compression regulates the expression and synthesis of chondrocyte-specific matrix molecules in bone marrow stromal cells

The overall objective of the present study was to investigate the mechanotransduction of bovine bone marrow stromal cells (BMSCs) through the interactions between transforming growth factor beta1 (TGF-beta1), dexamethasone, and dynamic compressive loading. Overall, the addition of TGF-beta1 increased cell viability, extracellular matrix (ECM) gene expression, matrix synthesis, and sulfated glycosaminoglycan content over basal construct medium. The addition of dexamethasone further enhanced extracellular matrix gene expression and protein synthesis. There was little stimulation of ECM gene expression or matrix synthesis in any medium group by mechanical loading introduced on day 8. In contrast, there was significant stimulation of ECM gene expression and matrix synthesis in chondrogenic media by dynamic loading introduced on day 16. The level of stimulation was also dependent on the medium supplements, with the samples treated with basal medium being the least responsive and the samples treated with TGF-beta1 and dexamethasone being the most responsive at day 16. Both collagen I and collagen II gene expressions were more responsive to dynamic loading than aggrecan gene expression. Dynamic compression upregulated Smad2/3 phosphorylation in samples treated with basal and TGF-beta1 media. These findings suggest that interactions between mechanical stimuli and TGF-beta signaling may be an important mechanotransduction pathway for BMSCs, and they indicate that mechanosensitivity may vary during the process of chondrogenesis.

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.

Regulation of Cartilaginous ECM Gene Transcription by Chondrocytes and MSCs in 3D Culture in Response to Dynamic Loading

This study explored the biologic response of chondrocytes and mesenchymal stem cells (MSCs) to a dynamic mechanical loading regime. We developed a time-efficient methodology for monitoring regional changes in extracellular matrix gene transcription using reporter promoter constructs. Specifically, transfected cells were homogenously distributed throughout agarose hydrogel constructs, and spatial and temporal gene expression and the ability to form functional ECM were analyzed in response to dynamic mechanical stimuli. Theoretical analyses were used to predict the physical signals generated within the gel in response to these loading regimes. Using a custom compression bioreactor system, changes in aggrecan and type II collagen promoter activity in transfected chondrocyte-laden cylindrical constructs were evaluated in response to a range of loading frequencies and durations. In general, aggrecan promoter activity increased with increasing duration of loading, particularly in the outer annulus region. Interestingly, type II collagen promoter activity decreased in this annular region under identical loading conditions. In addition, we explored the role of mechanical compression in directing chondrogenic differentiation of MSCs by monitoring short-term aggrecan promoter activity. As an example of long-term utility, a specific loading protocol was applied to MSC-laden constructs for 5 days, and the resultant changes in glycosaminoglycan (GAG) production were evaluated over a 4-week period. This dynamic loading regime increased not only short-term aggrecan transcriptional activity but also GAG deposition in long-term culture. These results demonstrate the utility of a new reporter promoter system for optimizing loading protocols to improve the outcome of engineered chondrocyte- and MSC-laden cartilaginous constructs.

Chondrocyte deformation induces mitochondrial distortion and heterogeneous intracellular strain fields

Chondrocyte mechanotransduction is poorly understood but may involve cell deformation and associated distortion of intracellular structures and organelles. This study quantifies the intracellular displacement and strain fields associated with chondrocyte deformation and in particular the distortion of the mitochondria network, which may have a role in mechanotransduction. Isolated articular chondrocytes were compressed in agarose constructs and simultaneously visualised using confocal microscopy. An optimised digital image correlation technique was developed to calculate the local intracellular displacement and strain fields using confocal images of fluorescently labelled mitochondria. The mitochondria formed a dynamic fibrous network or reticulum, which co-localised with microtubules and vimentin intermediate filaments. Cell deformation induced distortion of the mitochondria, which collapsed in the axis of compression with a resulting loss of volume. Compression generated heterogeneous intracellular strain fields indicating mechanical heterogeneity within the cytoplasm. The study provides evidence supporting the potential involvement of mitochondrial deformation in chondrocyte mechanotransduction, possibly involving strain-mediated release of reactive oxygen species. Furthermore the heterogeneous strain fields, which appear to be influenced by intracellular structure and organisation, may generate significant heterogeneity in mechanotransduction behaviour for cells subjected to identical levels of deformation.

Activation of chondrocytes calcium signalling by dynamic compression is independent of number of cycles

Mechanical loading is necessary for the development and maintenance of healthy articular cartilage through the control of extracellular matrix synthesis and catabolism. However, the underlying process of chondrocyte mechanotransduction remains unclear. This study examined the influence of cyclic compression on intracellular calcium (Ca(2+)) signalling within isolated articular chondrocytes cultured in agarose constructs. A validated experimental system was developed for applying controlled cyclic cell deformation. Cell-agarose constructs were subjected to 1Hz cyclic compression between 0 and 10% gross strain for 1, 10, 100 or 300 cycles. The cells were subsequently visualised for 300s in the unstrained state using confocal microscopy and the Ca(2+) indicator, Fluo-4 AM. Within unloaded control constructs, a sub-population of approximately 50% of chondrocytes exhibited characteristic spontaneous Ca(2+) transients each lasting approximately 40-60s. Cyclic compression, for only 1 cycle, significantly up-regulated the percentage of cells exhibiting Ca(2+) transients in the subsequent 5min period (p<0.05). Increasing the number of cycles to 10 or 100 had no additional effect. The up-regulated Ca(2+) signalling was maintained for up to 5min before returning to basal levels. By contrast, 300 cycles were followed by Ca(2+) signalling that was not significantly different from that in unloaded controls. However, this response was shown to be due to the increased time following the start of compression. In conclusion, this study indicates that chondrocyte Ca(2+) signalling is stimulated by dynamic compression, probably mediated by cyclic cell deformation. The overall response appears to be independent of the number of cycles or duration of cyclic compression. The sustained up-regulation of Ca(2+) signalling after 1, 10 or 100 cycles suggests the involvement of an autocrine-paracrine signalling mechanism. Furthermore, the reduced response following 300 cycles indicates a possible receptor desensitisation mechanism. Therefore, Ca(2+) signalling may be part of a mechanotransduction pathway through which chondrocyte populations can modulate their metabolic activity in response to changing mechanical stimuli.

Corroboration of in vivo cartilage pressures with implications for synovial joint tribology and osteoarthritis causation

Pressures on normal human acetabular cartilage have been collected from two implanted instrumented femoral head hemiprostheses. Despite significant differences in subjects' gender, morphology, mobility, and coordination, in vivo pressure measurements from both subjects covered similar ranges, with maximums of 5-6 MPa in gait, and as high as 18 MPa in other movements. Normalized for subject weight and height (nMPa), for free-speed walking the maximum pressure values were 25.2 for the female subject and 24.5 for the male subject. The overall maximum nMPa values were 76.2 for the female subject during rising from a chair at 11 months postoperative and 82.3 for the male subject while descending steps at 9 months postoperative. These unique in vivo data are consistent with corresponding cadaver experiments and model analyses. The collective results, in vitro data, model studies, and now corroborating in vivo data support the self-pressurizing "weeping" theory of synovial joint lubrication and provide unique information to evaluate the influence of in vivo pressure regimes on osteoarthritis causation and the efficacy of augmentations to, and substitutions for, natural cartilage.

The local matrix distribution and the functional development of tissue engineered cartilage, a finite element study

Assessment of the functionality of tissue engineered cartilage constructs is hampered by the lack of correlation between global measurements of extra cellular matrix constituents and the global mechanical properties. Based on patterns of matrix deposition around individual cells, it has been hypothesized previously, that mechanical functionality arises when contact occurs between zones of matrix associated with individual cells. The objective of this study is to determine whether the local distribution of newly synthesized extracellular matrix components contributes to the evolution of the mechanical properties of tissue engineered cartilage constructs. A computational homogenization approach was adopted, based on the concept of a periodic representative volume element. Local transport and immobilization of newly synthesized matrix components were described. Mechanical properties were taken dependent on the local matrix concentration and subsequently the global aggregate modulus and hydraulic permeability were derived. The transport parameters were varied to assess the effect of the evolving matrix distribution during culture. The results indicate that the overall stiffness and permeability are to a large extent insensitive to differences in local matrix distribution. This emphasizes the need for caution in the visual interpretation of tissue functionality from histology and underlines the importance of complementary measurements of the matrix's intrinsic molecular organization.

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.

Viscoelastic properties of single attached cells under compression

The viscoelastic properties of single, attached C2C12 myoblasts were measured using a recently developed cell loading device. The device allows global compression of an attached cell, while simultaneously measuring the associated forces. The viscoelastic properties were examined by performing a series of dynamic experiments over two frequency decades (0.1-10 Hz) and at a range of axial strains (approximately 10-40%). Confocal laser scanning microscopy was used to visualize the cell during these experiments. To analyze the experimentally obtained force-deformation curves, a nonlinear viscoelastic model was developed. The nonlinear viscoelastic model was able to describe the complete series of dynamic experiments using only a single set of parameters, yielding an elastic modulus of 2120 +/- 900 Pa for the elastic spring, an elastic modulus of 1960 +/- 1350 for the nonlinear spring, and a relaxation time constant of 0.3 +/- 0.12 s. To our knowledge, it is the first time that the global viscoelastic properties of attached cells have been quantified over such a wide range of strains. Furthermore, the experiments were performed under optimal environmental conditions and the results are, therefore, believed to reflect the viscoelastic mechanical behavior of cells, such as would be present in vivo.

Mechanical compression and hydrostatic pressure induce reversible changes in actin cytoskeletal organisation in chondrocytes in agarose

In numerous cell types, the cytoskeleton has been widely implicated in mechanotransduction pathways involving stretch-activated ion channels, integrins and deformation of intracellular organelles. Studies have also demonstrated that the cytoskeleton can undergo remodelling in response to mechanical stimuli such as tensile strain or fluid flow. In articular chondrocytes, the mechanotransduction pathways are complex, inter-related and as yet, poorly understood. Furthermore, little is known of how the chondrocyte cytoskeleton responds to physiological mechanical loading. This study utilises the well-characterised chondrocyte-agarose model and an established confocal image-analysis technique to demonstrate that both static and cyclic, compressive strain and hydrostatic pressure all induce remodelling of actin microfilaments. This remodelling was characterised by a change from a uniform to a more punctate distribution of cortical actin around the cell periphery. For some loading regimes, this remodelling was reversed over a subsequent 1h unloaded period. This reversible remodelling of actin cytoskeleton may therefore represent a mechanism through which the chondrocyte alters its mechanical properties and mechanosensitivity in response to physiological mechanical loading.

Depth-varying density and organization of chondrocytes in immature and mature bovine articular cartilage assessed by 3d imaging and analysis

Articular cartilage is a heterogeneous tissue, with cell density and organization varying with depth from the surface. The objectives of the present study were to establish a method for localizing individual cells in three-dimensional (3D) images of cartilage and quantifying depth-associated variation in cellularity and cell organization at different stages of growth. Accuracy of nucleus localization was high, with 99% sensitivity relative to manual localization. Cellularity (million cells per cm3) decreased from 290, 310, and 150 near the articular surface in fetal, calf, and adult samples, respectively, to 120, 110, and 50 at a depth of 1.0 mm. The distance/angle to the nearest neighboring cell was 7.9 microm/31 degrees , 7.1 microm/31 degrees , and 9.1 microm/31 degrees for cells at the articular surface of fetal, calf, and adult samples, respectively, and increased/decreased to 11.6 microm/31 degrees , 12.0 microm/30 degrees , and 19.2 microm/25 degrees at a depth of 0.7 mm. The methodologies described here may be useful for analyzing the 3D cellular organization of cartilage during growth, maturation, aging, degeneration, and regeneration.

Confocal laser scanning microscopy in orthopaedic research

Confocal laser scanning microscopy (CLSM) is a type of high-resolution fluorescence microscopy that overcomes the limitations of conventional widefield microscopy and facilitates the generation of high-resolution 3D images from relatively thick sections of tissue. As a comparatively non-destructive imaging technique, CLSM facilitates the in situ characterization of tissue microstructure. Images generated by CLSM have been utilized for the study of articular cartilage, bone, muscle, tendon, ligament and menisci by the foremost research groups in the field of orthopaedics including those teams headed by Bush, Errington, Guilak, Hall, Hunziker, Knight, Mow, Poole, Ratcliffe and White. Recent evolutions in techniques and technologies have facilitated a relatively widespread adoption of this imaging modality, with increased "user friendliness" and flexibility. Applications of CLSM also exist in the rapidly advancing field of orthopaedic implants and in the investigation of joint lubrication.

Low-density cultures of bovine chondrocytes: effects of scaffold material and culture system

Chondrocytes were seeded on either agarose or polyglycolic acid (PGA) unwoven meshes at 10 million cells/ml of scaffold volume to evaluate the effect that these two biomaterials have on the low-density culture of chondrocytes in a rotating-wall bioreactor. For both static and bioreactor culture, agarose constructs contained more glycosaminoglycan than their PGA counterparts. However, the PGA constructs contained more collagen for both culture conditions when compared to agarose. For the low seeding density of this study, PGA constructs cultured in the bioreactor did not outperform static cultures when comparing collagen content after 8 weeks. The mechanical properties of the PGA constructs also did not improve with culture time. Similar results were observed with the agarose culture, though both static- and bioreactor-culture agarose constructs exhibited increases in aggregate modulus at the end of the culture period. As in PGA culture, chondrocytes cultured in agarose may require a higher density to reap the benefits of the bioreactor environment.

Local, Three-Dimensional Strain Measurements Within Largely Deformed Extracellular Matrix Constructs

The ability to create extracellular matrix (ECM) constructs that are mechanically and biochemically similar to those found in vivo and to understand how their properties affect cellular responses will drive the next generation of tissue engineering strategies. To date, many mechanisms by which cells biochemically communicate with the ECM are known. However, the mechanisms by which mechanical information is transmitted between cells and their ECM remain to be elucidated. “Self-assembled” collagen matrices provide an in vitro-model system to study the mechanical behavior of ECM. To begin to understand how the ECM and the cells interact mechanically, the three-dimensional (3D) mechanical properties of the ECM must be quantified at the micro-(local) level in addition to information measured at the macro-(global) level. Here we describe an incremental digital volume correlation (IDVC) algorithm to quantify large (>0.05) 3D mechanical strains in the microstructure of 3D collagen matrices in response to applied mechanical loads. Strain measurements from the IDVC algorithm rely on 3D confocal images acquired from collagen matrices under applied mechanical loads. The accuracy and the precision of the IDVC algorithm was verified by comparing both image volumes collected in succession when no deformation was applied to the ECM (zero strain) and image volumes to which simulated deformations were applied in both 1D and 3D (simulated strains). Results indicate that the IDVC algorithm can accurately and precisely determine the 3D strain state inside largely deformed collagen ECMs. Finally, the usefulness of the algorithm was demonstrated by measuring the microlevel 3D strain response of a collagen ECM loaded in tension.

Crosslinking density influences the morphology of chondrocytes photoencapsulated in PEG hydrogels during the application of compressive strain

Chondrocyte deformation, which occurs during mechanical loading, is thought to play an important role in the mechanotransduction pathway. In designing a scaffold that can be gelled in situ for cartilage tissue engineering, an important consideration is the influence of mechanical loading. This study tested the hypothesis that changes in the crosslinking density of a hydrogel scaffold influence the morphology of encapsulated chondrocytes in response to an applied load. Chondrocytes were entrapped in photo-crosslinkable hydrogel scaffolds based on poly(ethylene glycol) (PEG) with two crosslinking densities, 0.119 and 0.376 mol/l, with the higher density having a 11-fold higher compressive modulus. The cell-embedded hydrogels were subjected to static compressive strains between 0% and 20% after 1 and 6 days of culture. Using confocal laser scanning microscopy, chondrocytes in the highly crosslinked gel at day 1 deformed more than gels in the more loosely crosslinked gel. By day 6, this finding was reversed. When single cells within a region were followed, heterogeneities in cell deformation were observed on both a macroscopic and microscopic scale. These heterogeneities were greater in the highly crosslinked gel. These findings demonstrate that different levels of cell deformation and heterogeneity may be obtained by varying the crosslinking density in PEG hydrogels.

Influence of external uniaxial cyclic strain on oriented fibroblast-seeded collagen gels

This study investigates the influence of cyclic tensile strain, applied to fully contracted fibroblast-seeded collagen constructs. The constructs were preloaded to either 2 or 10 mN. The preloaded constructs were subsequently subjected to a further 10% cyclic strain (0-10%) at 1 Hz, using a triangular waveform, or were cultured in the preloaded state. In all cases cellular viability was maintained during the conditioning period. Cell proliferation was enhanced by the application of cyclic strain within constructs preloaded to both 2 and 10 mN. Collagen synthesis was enhanced by cyclic strain within constructs preloaded at 2 mN only. The profile of matrix metalloproteinase (MMP) expression, determined by zymography, was broadly similar in constructs preloaded at 2 mN with or without the application of cyclic strain. By contrast, constructs preloaded at 10 mN and subjected to cyclic strain expressed enhanced levels of staining for latent MMP-1, latent MMP-9, and both latent and active MMP-2, when compared with the other conditioning regimens. The structural stiffness of constructs preloaded at 2 mN and subjected to cyclic strain was enhanced compared with control specimens, reflecting the increase in collagen synthesis. By contrast, the initial failure loads for cyclically strained constructs preloaded at 10 mN were reduced, potentially because of enhanced catabolic activity.

Dynamic compression counteracts IL-1 beta-induced release of nitric oxide and PGE2 by superficial zone chondrocytes cultured in agarose constructs

OBJECTIVE

To examine the effect of IL-1 beta-induced *NO and PGE(2)release by stimulated superficial and deep chondrocyte/agarose constructs subjected to mechanical compression.

DESIGN

Chondrocyte sub-populations were seeded separately in agarose constructs and cultured unstrained, within a 24-well tissue culture plate, for 48 h in medium supplemented with IL-1 beta and/or L-N-(1-iminoethyl)-ornithine (L-NIO). In a separate experiment, superficial and deep cell containing constructs were subjected to 15% dynamic compressive strain at 1 Hz, for 48 h, in the presence or absence of IL-1 beta and/or L-NIO. Nitrite was measured using the Griess assay, PGE(2)release was determined using an EIA kit and [3H]-thymidine and 35SO(4)incorporation were assessed by TCA and alcian blue precipitation, respectively.

RESULTS

The current data reveal that IL-1 beta significantly enhanced *NO and PGE(2)release for superficial chondrocytes, an effect reversed with L-NIO. *NO and PGE(2)levels did not significantly change by deep cells in the presence of IL-1 beta and/or L-NIO. For both cell sub-populations, IL-1 beta inhibited cell proliferation whereas proteoglycan synthesis was not affected. Dynamic compression inhibited the release of *NO and PGE(2)in the presence and absence of IL-1 beta, for cells from both sub-populations. L-NIO reduced *NO and enhanced PGE(2)release for superficial zone chondrocytes, an effect not observed for deep cells in response to dynamic compression. The magnitude of stimulation of [3H]-thymidine incorporation was similar for both cell sub-populations and was not influenced by L-NIO, indicating an z.rad;NO-independent pathway. The dynamic compression-induced stimulation of 35SO(4)incorporation was enhanced with L-NIO for IL-1 beta-stimulated deep cells, indicating an *NO-dependent pathway.

CONCLUSION

The present findings suggest that dynamic compression inhibits *NO and PGE(2)release in IL-1 beta-stimulated superficial cells via distinct pathways, a significant finding that may contribute to the development of intervention strategies for the treatment of inflammatory joint disorders.

Temporal regulation of chondrocyte metabolism in agarose constructs subjected to dynamic compression

The temporal response of chondrocyte metabolism in agarose constructs subjected to different dynamic compression regimes was investigated. The current study explored the effects of continuous or intermittent compression using various duty cycles of dynamic compressive loading, over a 48 h culture period. For the continuous compression experiments, duty cycles ranged from 5400 to 172,800 and intermittent compression delivered a total of 86,400 cycles. Large numbers of duty cycles significantly stimulated proteoglycan synthesis with maximal levels obtained for constructs subjected to 12h of intermittent compression. The shortest duration of intermittent compression suggested that further cycles are inhibitory for cell proliferation. Nitrite release was independent of the length or type of compressive regime applied. The uncoupled nature of the metabolic response determined in this study suggests that mechanical conditioning regimes may be fine tuned to selectively stimulate key metabolic parameters of relevance to cartilage tissue engineering.

Conditioned medium of mechanically compressed chick limb bud cells promotes chondrocyte differentiation

Previous studies have demonstrated that chondrocyte differentiation can be stimulated by cyclic mechanical compression of chick limb bud cell-agarose constructs. This study reveals that medium collected from these compressed cultures promotes chondrocyte differentiation of non-compressed cells to approximately the same extent as compression alone. In micromass cultures of chick limb bud cells, incubation with medium from compressed cells significantly enhanced cell proliferation and the average rate of proteoglycan synthesis in a dose-dependent manner. These findings indicate that the response of undifferentiated chick limb bud cells to mechanical loading involves the secretion of one or more soluble differentiation factors. The chondrogenic activity of the conditioned medium was substantially inhibited by passage through a filter with a nominal molecular-weight cutoff (MWCO) of 10 kDa but not inhibited when using a MWCO of 30 kDa, suggesting that at least one of these factors has a molecular mass between 10 and 30 kDa.

An axisymmetric boundary integral model for assessing elastic cell properties in the micropipette aspiration contact problem

The micropipette aspiration technique has been used extensively in recent years to measure the mechanical properties of living cells. In the present study, a boundary integral formulation with quadratic elements is used to predict the elastic equilibrium response in the micropipette aspiration contact problem for a three-dimensional incompressible spherical continuum cell model (Young's modulus E). In contrast to the halfspace model, the spherical cell model accounts for nonlinearities in the cell response which result from a consideration of geometric factors including the finite cell dimension (radius R), curvature of the cell boundary, evolution of the cell-micropipette contact region and curvature of the edges of the micropipette (inner radius a, edge curvature radius epsilon). The efficiency of the boundary element method facilitates the quantification of cell response as a function of the scaled pressure p/E, for the range of parameters a/R = 0.4-0.7, epsilon/a = 0.02-0.08, in terms of two measures that can be quantified using video microscopy. These are the aspiration length, which measures projection of the cell into the micropipette, and a characteristic strain, which measures stretching along the symmetry axis. For both measures of cell response, the resistance to aspiration is found to decrease with increasing values of the aspect ratio a/R and curvature parameter epsilon/a, and the nonlinearities in the cell response are most pronounced in the earlier portion of the aspiration test. The aspiration length is found to exhibit less sensitivity to the aspect ratio a/R than to the curvature parameter epsilon/a, whereas the characteristic strain, which provides a more realistic measure of overall cell stiffness, exhibits sensitivity to the aspect ratio a/R. The resistance to aspiration in the spherical cell model is initially less than that of the half space model but eventually exceeds the halfspace prediction and the deviation between the two models increases as the parameter epsilon/a decreases. Adjustment factors for the Young's modulus E, as predicted by the halfspace model, are presented and the deviation from the spherical cell model is found to be as large as 35%, when measured locally on the response curve. In practice, the deviation will be less than the maximum figure but its precise value will depend on the number of data points available in the experiment and the specific curve-fitting procedure. The spherical cell model allows for efficient and more realistic simulations of the micropipette aspiration contact problem and quantifies two observable measures of cell response that, using video microscopy, can facilitate the determination of Young's modulus for various cell populations while, simultaneously, providing a means of evaluating the validity of continuum cell models. Furthermore, this numerical model may be readily extended to account for more complex geometries, inhomogeneities in cellular properties, or more complex constitutive descriptions of the cell.

Regional variations in the cellular matrix of the annulus fibrosus of the intervertebral disc

The three-dimensional architecture of cells in the annulus fibrosus was studied by a systematic, histological examination using antibodies to cytoskeletal components, in conjunction with confocal microscopy. Variations in cell shape, arrangement of cellular processes and cytoskeletal architecture were found both within and between the defined zones of the outer and inner annulus. The morphology of three, novel annulus fibrosus cells is described: extended cordlike cells that form an interconnected network at the periphery of the disc; cells with extensive, sinuous processes in the inner region of the annulus fibrosus; and cells with broad, branching processes specific to the interlamellar septae of the outer annulus. The complex, yet seemingly deliberate arrangement of various cell shapes and their processes suggests multiple functional roles. Regional variations in the organization of the actin and vimentin cytoskeletal networks is reported across all regions of the annulus. Most notable is the continuous, strand arrangement of the actin label at the disc's periphery in contrast to its punctate appearance in all other regions. The gap junction protein connexin 43 was found within cells from all regions of the annulus, including those which did not form physical connections with surrounding cells. These observations of the cellular matrix in the healthy intervertebral disc should contribute to a better understanding of site-specific changes in tissue architecture, biochemistry and mechanical properties during degeneration, injury and healing.

Cell and nucleus deformation in compressed chondrocyte-alginate constructs: temporal changes and calculation of cell modulus

Mechanical loading is essential for the homeostasis of articular cartilage and may be necessary for achieving functional tissue engineered cartilage repair using isolated cells seeded in scaffolds such as alginate. Chondrocyte mechanotransduction is poorly understood, but may involve cell deformation and associated distortion of intracellular organelles. The present study used confocal microscopy to examine cell and nucleus morphology in isolated chondrocytes compressed in alginate constructs. Compression of 2% alginate resulted in cell deformation from a spherical to an oblate ellipsoid morphology with conservation of cell volume. Cell deformation was associated with deformation, to a lesser degree, of the nucleus. Despite constant cell deformation over a 25 min period of static compression, the nucleus deformation reduced significantly, particularly in the axis perpendicular to the applied compression. Constructs made of a lower alginate concentration exhibited a reduced compressive modulus with an altered cellular response to compression. In 1.2% alginate, compression resulted in cell deformation which was initially of a similar magnitude to that in 2% alginate but subsequently reduced over a 60 min period reflecting the viscoelastic behaviour of the gel. This phenomenon enabled the calculation of a stress-strain relationship for the cell with an estimated Young's modulus value of approx. 3 kPa.

The effects of osmotic stress on the viscoelastic and physical properties of articular chondrocytes

The metabolic activity of chondrocytes in articular cartilage is influenced by alterations in the osmotic environment of the tissue, which occur secondary to mechanical compression. The mechanism by which osmotic stress modulates cell physiology is not fully understood and may involve changes in the physical properties of the membrane or the cytoskeleton. The goal of this study was to determine the effect of the osmotic environment on the mechanical and physical properties of chondrocytes. In isoosmotic medium, chondrocytes exhibited a spherical shape with numerous membrane ruffles. Normalized cell volume was found to be linearly related to the reciprocal of the extracellular osmolality (Boyle van't Hoff relationship) with an osmotically active intracellular water fraction of 61%. In deionized water, chondrocytes swelled monotonically until lysis at a mean apparent membrane area 234 +/- 49% of the initial area. Biomechanically, chondrocytes exhibited viscoelastic solid behavior. The instantaneous and equilibrium elastic moduli and the apparent viscosity of the cell were significantly decreased by hypoosmotic stress, but were unchanged by hyperosmotic stress. Changes in the viscoelastic properties were paralleled by the rapid dissociation and remodeling of cortical actin in response to hypoosmotic stress. These findings indicate that the physicochemical environment has a strong influence on the viscoelastic and physical properties of the chondrocyte, potentially through alterations in the actin cytoskeleton.

Hydrogel properties influence ECM production by chondrocytes photoencapsulated in poly(ethylene glycol) hydrogels

When using hydrogel scaffolds for cartilage tissue engineering, two gel properties are particularly important: the equilibrium water content (q, equilibrium swelling ratio) and the compressive modulus, K. In this work, chondrocytes were photoencapsulated in degrading and nondegrading poly(ethylene glycol)-based hydrogels to assess extracellular matrix (ECM) formation as a function of these gel properties. In nondegrading gels, the glycosaminoglycan (GAG) content was not significantly different in gels when q was varied from 4.2 to 9.3 after 2 and 4 weeks in vitro. However, gels with a q of 9.3 allowed GAGs to diffuse throughout the gels homogenously, but a q < or = 5.2 resulted in localization of GAGs pericellularly. Interestingly, in the moderately crosslinked gels with a K of 360 kPa, an increase in type II collagen synthesis was observed compared with gels with a higher (960 kPa) and lower (30 kPa) K after 4 weeks. With the incorporation of degradable linkages into the network, gel properties with an initially high K (350 kPa) and final high q (7.9) were obtained, which allowed for increased type II collagen synthesis coupled with a homogenous distribution of GAGs. Thus, a critical balance exists between gel swelling, mechanics, and degradation in forming a functional ECM.

Chondrocyte deformation within mechanically and enzymatically extracted chondrons compressed in agarose

Within articular cartilage, the chondron microenvironment will influence chondrocyte behaviour and response to loading. Chondrons were extracted from intact cartilage using either mechanical homogenisation (MC) or enzymatic digestion (EC) and cell and matrix morphology in unstrained and compressed agarose constructs was examined. Isolated chondrocytes (IC) were used for comparison. Immunolocalisation of type VI collagen and keratan sulphate revealed differences in the structure of the pericellular microenvironment such that MC most closely resembled chondrons in situ. The unstrained cell diameters of IC and EC were larger than MC at day 1 and increased significantly over a 7 day culture period. In contrast, cell diameters for MC remained constant. Compression of constructs at day 1 resulted in cell deformation for IC and EC but not MC. The two chondron extraction methods yielded chondrons of differing matrix morphology and associated differences in cell size and cellular response to load. The results indicate that the pericellular microenvironment of MC initially possessed a greater mechanical integrity than that of EC. Although these differences may be reduced with time in culture, characterisation of mechanically isolated chondrons suggests that the stiffness of the chondrons in situ may be greater than previous estimates.

Temporal changes in cytoskeletal organisation within isolated chondrocytes quantified using a novel image analysis technique

This paper examines temporal changes in the organisation of the cytoskeleton within isolated articular chondrocytes cultured for up to 7 days in agarose constructs. Fluorescent labelling and confocal microscopy were employed to visualise microtubules (MT), vimentin intermediate filaments (VIF) and actin microfilaments (AMF). To quantify the degree of cytoskeletal organisation within populations of cells, a novel image analysis technique has been developed and fully characterised. Organisation was quantified in terms of an Edge Index, which reflects the density of 'edges' present within the confocal images as defined by a Sobel digital filter. This parameter was shown to be independent of image intensity and, for all three cytoskeletal components, was validated statistically against a visual assessment of organisation. Both MT and VIF exhibited fibrous networks extending throughout the cytoplasm, while AMF appeared as punctate units associated with the cell membrane. The use of the Edge Index parameter revealed statistical significant temporal variation, in particular associated with VIF and AMF. These findings indicate the possibility of cytoskeletal mediated temporal variation in many aspects of cell behaviour following isolation from the intact tissue. Furthermore, the image analysis techniques are likely to be useful for future studies aiming to quantify changes in cytoskeletal organisation.

Mechanical compression influences intracellular Ca2+ signaling in chondrocytes seeded in agarose constructs

Ca2+ signaling forms part of a possible mechanotransduction pathway by which chondrocytes may alter their metabolism in response to mechanical loading. In this study, a well-characterized model system utilizing bovine articular chondrocytes embedded in 4% agarose constructs was used to investigate the effect of physiological mechanical compressive strain applied after 1 and 3 days in culture. The intracellular Ca2+ concentration was measured by use of the ratiometric Ca2+ indicator indo 1-AM and confocal microscopy. A positive Ca2+ response was defined as a percent increase in Ca2+ ratio above a preset threshold. A significantly greater percentage of cells exhibited a positive Ca2+ response in strained constructs compared with unstrained controls at both time points. In strained constructs, treatment with either Ga3+ or EGTA significantly reduced the number of positive Ca2+ responders compared with untreated controls. These results represent an important step in understanding the physiological role of intracellular Ca2+ in chondrocytes under mechanical compression.