Oscillatory tension differentially modulates matrix metabolism and cytoskeletal organization in chondrocytes and fibrochondrocytes

Mechanical properties evaluation of metacarpophalangeal joint prosthesis with new titanium-nickel memory alloy: a cadaver study

OBJECTIVE

Ni-Ti memory alloys are unusual materials for hard-tissue replacement because of their unique superelasticity, good biocompatibility, high strength, low specific gravity, low magnetism, wear resistance, corrosion resistance and fatigue resistance. The current study aims to evaluate its mechanical properties and provide biomechanical basis for the clinical application of the prosthesis.

METHODS

Ten adult metacarpophalangeal joint specimens were randomly divided into a prosthesis group (n = 5, underwent metacarpophalangeal joint prosthesis) and a control group (n = 5, underwent sham operation). Firstly, the axial compression strength was tested with BOSE material testing machine to evaluate its biomechanical strength. Secondly, these specimens were tested for strain changes using BOSE material testing machine and GOM non-contact optical strain measurement system to evaluate the stress changes. Thirdly, fatigue test was performed between groups. Lastly, the mechanical wear of the metacarpophalangeal joint prosthesis was tested with ETK5510 material testing machine to study its mechanical properties.

RESULTS

Axial compression stiffness in the prosthesis group was greater than that in the control group in terms of 30 ° and 60 ° flexion positions (P < 0.05). There was no statistically significant difference between two groups with regards to axial compression stiffness and stress change test (P > 0.05). In the fatigue wear test, the mean mass loss in the prosthesis group's prosthesis was 17.2 mg and 17.619 mm3, respectively. The mean volume wear rate was 0.12%. There was no statistically significant difference in the maximum pull-out force of the metacarpal, phalangeal, and polymer polyethylene pads between the prosthesis group and the control group specimens.

CONCLUSIONS

Ni-Ti memory alloy metacarpophalangeal joint prosthesis conforms to the biomechanical characteristics of metacarpophalangeal joints without implants, and the fatigue strength can fully meet the needs of metacarpophalangeal joint activities after joint replacement.

Mechanical stimulation devices for mechanobiology studies: a market, literature, and patents review

Significant advancements in various research and technological fields have contributed to remarkable findings on the physiological dynamics of the human body. To more closely mimic the complex physiological environment, research has moved from two-dimensional (2D) culture systems to more sophisticated three-dimensional (3D) dynamic cultures. Unlike bioreactors or microfluidic-based culture models, cells are typically seeded on polymeric substrates or incorporated into 3D constructs which are mechanically stimulated to investigate cell response to mechanical stresses, such as tensile or compressive. This review focuses on the working principles of mechanical stimulation devices currently available on the market or custom-built by research groups or protected by patents and highlights the main features still open to improvement. These are the features which could be focused on to perform, in the future, more reliable and accurate mechanobiology studies.

Graphic abstract

A Novel In Vitro and In Silico System for Analyzing Complex Mechanobiological Behavior of Chondrocytes in Three-Dimensional Hydrogel Constructs

Physiological loading is essential for the maintenance of articular cartilage through the regulation of tissue remodeling. To correctly understand the behavior of chondrocytes in their native environment, cell stimulating devices and bioreactors have been developed to examine the effect of mechanical stimuli on chondrocytes. This study describes the design and validation of a novel system for analyzing chondrocyte deformation patterns. This involves an in vitro mechanical device for a controlled application of multi-axial-loading regimes to chondrocyte-seeded agarose constructs and in silico models for analyzing chondrocyte deformation patterns. The computer-controlled device precisely applies compressive, tensile, and shear strains to hydrogel constructs using a customizable macro-based program. The synchronization of the displacements is shown to be accurate with a 1.2% error and is highly reproducible. The device design allows housing for up to eight novel designed free-swelling three-dimensional hydrogel constructs. Constructs include mesh ends and are optimized to withstand the application of up to 7% mechanical tensile and 15% shear strains. Constructs were characterized through mapping the strain within as mechanical load was applied and was validated using light microscopy methods, chondrocyte viability using live/dead imaging, and cell deformation strains. Images were then analyzed to determine the complex deformation strain patterns of chondrocytes under a range of dynamic mechanical stimulations. This is one of the first systems that have characterized construct strains to cellular strains. The features in this device make the system ideally suited for a systematic approach for the investigation of the response of chondrocytes to a complex physiologically relevant deformation profile.

Explant models for meniscus metabolism, injury, repair, and healing

Purpose/Aim: Knee meniscus is a wedge-shaped fibrocartilaginous tissue, playing important roles in maintaining joint stability and function. Injuries to the meniscus, particularly with the avascular inner third zone, hardly heal and frequently progress into structural breakdown, followed by the initiation of osteoarthritis. As the importance of meniscus in joint function and diseases is being recognized, the field of meniscus research is growing. Not only development, biology, and metabolism but also injury, repair, and healing of meniscus are being actively investigated. As meniscus functions as an integrated unit of a knee joint, in vivo models with various species have been the predominant method for studying meniscus pathophysiology and for testing healing/regeneration strategies. However, in vivo models for meniscus studies suffer from low reproducibility and high cost. To complement the limitations of in vivo animal models, several types of meniscus explants have been applied as highly controlled, standardized in vitro models to investigate meniscus metabolism, pathophysiology, and repair or regeneration process. This review summarizes and compares the existing meniscus explant models. We also discuss the advantages and disadvantages of each explant model.Conclusion: Despite few outstanding challenges, meniscus explant models have potential to serve as an effective tool for investigations of meniscus metabolism, injury, repair and healing.

Screening of differentially expressed miRNAs in tensile strain‑treated HepG2 cells by miRNA microarray analysis

Cirrhosis and portal hypertension are associated with an increased risk of developing liver cancer. However, it is unknown how changes in the cellular mechanical microenvironment induced by portal hypertension affect the occurrence and development of liver cancer. The aim of this study was to determine the effect of tensile strain on the proliferation of a human liver cancer cell line (HepG2 cells) using methods such as flow cytometry, Cell Counting Kit-8 and 5-bromodeoxyuridine assays, and to examine the changes in microRNA (miRNA/miR) expression using microarray, reverse transcription-quantitative (RT-q)PCR and bioinformatics analyses. It was demonstrated that cyclic tensile force promoted the proliferation of HepG2 cells. The most suitable research conditions were as follows: Tensile strain force loading amplitude 15%; frequency 1 Hz; and time 24 h. After loading the HepG2 cells under such conditions, the differentially expressed miRNAs were screened out using an Agilent Human miRNA Microarray, identifying seven miRNAs with significant differences (expression difference >2 times and P<0.05). A total of five were upregulated, including hsa-miR-296-5p, hsa-miR-6752-5p, hsa-miR-6794-5p, hsa-miR-6889-5p and hsa-miR-7845-5p; and two were downregulated, hsa-miR-4428 and hsa-miR-503-5p. The results of RT-qPCR also further confirmed the expression changes of these miRNAs. Gene Ontology and pathway analyses showed the involvement of these miRNAs in numerous important physiological processes. These findings may provide novel miRNA-based information, thus enhancing the understanding of the pathophysiological processes leading to liver cancer.

Effects of hydrostatic pressure and deviatoric stress on human articular chondrocytes for designing neo-cartilage construct

Autologous chondrocyte implantation is a promising therapy for the treatment of the articular cartilage defects. Recently, we have developed a three-dimensional chondrocyte construct manufactured with a collagen gel/sponge scaffold and cyclic hydrostatic pressure. However, the roles of various mechanical stresses, specifically hydrostatic pressure and deviatoric stress, as well as poststress loading, were unclear on metabolic function in chondrocytes. We hypothesized that hydrostatic pressure and deviatoric stresses each alter individual metabolic characteristics of chondrocytes. We embedded human articular chondrocytes within an agarose hydrogel and applied hydrostatic pressure and/or deviatoric stress individually or simultaneously for 4 days. Subsequently, we kept the cell constructs without stress for an additional 3 days. With hydrostatic pressure and/or deviatoric stress, more cells proliferated significantly than no stress (p < .05) and more cells proliferated near the inner side of the construct than the outer (p < .05). Cartilage specific aggrecan core protein and collagen type II were upregulated significantly after off-loading hydrostatic pressure alone at Day 7 (p < .05). On the other hand, these molecules were upregulated significantly immediately after deviatoric stress alone and combined with hydrostatic pressure at Day 4 (p < .05). Tissue inhibitor of metalloproteinase-2 was upregulated significantly after off-loading hydrostatic pressure alone and combined deviatoric stress at Day 7 (p < .05). Metalloproteinnase-13 was upregulated significantly with deviatoric stress at Day 4 (p < .05) and combined with hydrostatic pressure at Day 4. These results suggest that metabolic functions are regulated by the combination of hydrostatic pressure and deviatoric stress and by the timing of stress loading.

A computational simulation of cyclic stretch of an individual stem cell using a nonlinear model

Many experiments have shown that mechanical stimuli like cyclic strains might be helpful in stem cell differentiation. To maximize such differentiations efficiency, it is imperative to detect the cellular mechanical responses to these stimuli. The purpose of this research was to show that a newly presented hyper-viscoelastic model could correctly predict the level of stresses required to obtain a different response from a single mesenchymal stem cell cultured in a fibrin hydrogel block under a 10% cyclic strain at a frequency of 1 Hz, employing finite element method. One of the novelties of the research was the use of a model based on Simo's model. Another important feature of the research was the proposition of a multiscale model considering a layer of integrins. It was concluded that the forces exerted on the biological molecules had the maximum values of 24, 45, and 15 pN for the circumferential, radial, and shear forces, respectively. According to the results, the exerted forces within the cytoskeleton can lead to a different cellular response. These results might be a premise for interpreting events that lead to differentiation of stem cells into fibrochondrocytes. In addition, they can be beneficial in effective design of biological experiments as regards to this issue.

A Guide for Using Mechanical Stimulation to Enhance Tissue-Engineered Articular Cartilage Properties

The use of tissue-engineered articular cartilage (TEAC) constructs has the potential to become a powerful treatment option for cartilage lesions resulting from trauma or early stages of pathology. Although fundamental tissue-engineering strategies based on the use of scaffolds, cells, and signals have been developed, techniques that lead to biomimetic AC constructs that can be translated to in vivo use are yet to be fully confirmed. Mechanical stimulation during tissue culture can be an effective strategy to enhance the mechanical, structural, and cellular properties of tissue-engineered constructs toward mimicking those of native AC. This review focuses on the use of mechanical stimulation to attain and enhance the properties of AC constructs needed to translate these implants to the clinic. In vivo, mechanical loading at maximal and supramaximal physiological levels has been shown to be detrimental to AC through the development of degenerative changes. In contrast, multiple studies have revealed that during culture, mechanical stimulation within narrow ranges of magnitude and duration can produce anisotropic, mechanically robust AC constructs with high cellular viability. Significant progress has been made in evaluating a variety of mechanical stimulation techniques on TEAC, either alone or in combination with other stimuli. These advancements include determining and optimizing efficacious loading parameters (e.g., duration and frequency) to yield improvements in construct design criteria, such as collagen II content, compressive stiffness, cell viability, and fiber organization. With the advancement of mechanical stimulation as a potent strategy in AC tissue engineering, a compendium detailing the results achievable by various stimulus regimens would be of great use for researchers in academia and industry. The objective is to list the qualitative and quantitative effects that can be attained when direct compression, hydrostatic pressure, shear, and tensile loading are used to tissue-engineer AC. Our goal is to provide a practical guide to their use and optimization of loading parameters. For each loading condition, we will also present and discuss benefits and limitations of bioreactor configurations that have been used. The intent is for this review to serve as a reference for including mechanical stimulation strategies as part of AC construct culture regimens.

[Mechanism research progress of tendon-derived stem cells in reconstruction of fibrocartilage zone at bone-tendon junction]

Objective

To summarize the mechanism research progress of tendon-derived stem cells (TDSCs) in the reconstruction of fibrocartilage zone at bone-tendon junction (BTJ).

Methods

The domestic and abroad related literature about TDSCs in the reconstruction of fibrocartilage zone at BTJ was summarized and analyzed.

Results

TDSCs can be induced to osteocytes, fibrochondrocytes, and tenocytes in vitro. Therefore, TDSCs have potential to reconstruct fibrocartilage zone at BTJ. Factors, such as mechanical stimulation, bioactive factor, extracelluar matrix, inflammatory factors, and so on, may influence osteogenic or chondrogenic differentiation of TDSCs.

Conclusion

Because of the specificity of origin and location of TDSCs, TDSCs have the potential to be the seed cells for BTJ fibrocartilage zone repair. By applying external stimuli, TDSCs can be induced to form structures which are similar to fibrocartilage zone.

Using Costal Chondrocytes to Engineer Articular Cartilage with Applications of Passive Axial Compression and Bioactive Stimuli

Generating neocartilage with suitable mechanical integrity from a cell source that can circumvent chondrocyte scarcity is indispensable for articular cartilage regeneration strategies. Costal chondrocytes of the rib eliminate donor site morbidity in the articular joint, but it remains unclear how neocartilage formed from these cells responds to mechanical loading, especially if the intent is to use it in a load-bearing joint. In a series of three experiments, this study sought to determine efficacious parameters of passive axial compressive stimulation that would enable costal chondrocytes to synthesize mechanically robust cartilage. Experiment 1 determined a suitable time window for stimulation by its application during either the matrix synthesis phase, the maturation phase, or during both phases of the self-assembling process. The results showed that compressive stimulation at either time was effective in increasing instantaneous moduli by 92% and 87% in the synthesis and maturation phases, respectively. Compressive stimulation during both phases did not further improve properties beyond a one-time stimulation. The magnitude of passive axial compression was examined in Experiment 2 by applying 0, 3.3, 5.0, or 6.7 kPa stresses to the neocartilage. Unlike 6.7 kPa, both 3.3 and 5.0 kPa significantly increased neocartilage compressive properties by 42% and 48% over untreated controls, respectively. Experiment 3 examined how the passive axial compression regimen developed from the previous phases interacted with a bioactive regimen (transforming growth factor [TGF]-β1, chondroitinase ABC, and lysyl oxidase-like 2). Passive axial compression significantly improved the relaxation modulus compared with bioactive treatment alone. Furthermore, a combined treatment of compressive and bioactive stimulation improved the tensile properties of neocartilage 2.6-fold compared with untreated control. The ability to create robust articular cartilage from passaged costal chondrocytes through appropriate mechanical and bioactive stimuli will greatly extend the clinical applicability of tissue-engineered products to a wider patient population.

Cartilage Strain Distributions Are Different Under the Same Load in the Central and Peripheral Tibial Plateau Regions

There is increasing evidence that the regional spatial variations in the biological and mechanical properties of articular cartilage are an important consideration in the pathogenesis of knee osteoarthritis (OA) following kinematic changes at the knee due to joint destabilizing events (such as an anterior cruciate ligament (ACL) injury). Thus, given the sensitivity of chondrocytes to the mechanical environment, understanding the internal mechanical strains in knee articular cartilage under macroscopic loads is an important element in understanding knee OA. The purpose of this study was to test the hypothesis that cartilage from the central and peripheral regions of the tibial plateau has different internal strain distributions under the same applied load. The internal matrix strain distribution for each specimen was measured on osteochondral blocks from the tibial plateau of mature ovine stifle joints. Each specimen was loaded cyclically for 20 min, after which the specimen was cryofixed in its deformed position and freeze fractured. The internal matrix was viewed in a scanning electron microscope (SEM) and internal strains were measured by quantifying the deformation of the collagen fiber network. The peak surface tensile strain, maximum principal strain, and maximum shear strain were compared between the regions. The results demonstrated significantly different internal mechanical strain distributions between the central and peripheral regions of tibial plateau articular cartilage under both the same applied load and same applied nominal strain. These differences in the above strain measures were due to differences in the deformation patterns of the collagen network between the central and peripheral regions. Taken together with previous studies demonstrating differences in the biochemical response of chondrocytes from the central and peripheral regions of the tibial plateau to mechanical load, the differences in collagen network deformation observed in this study help to provide a fundamental basis for understanding the association between altered knee joint kinematics and premature knee OA.

Physiologically Distributed Loading Patterns Drive the Formation of Zonally Organized Collagen Structures in Tissue-Engineered Meniscus

The meniscus is a dense fibrocartilage tissue that withstands the complex loads of the knee via a unique organization of collagen fibers. Attempts to condition engineered menisci with compression or tensile loading alone have failed to reproduce complex structure on the microscale or anatomic scale. Here we show that axial loading of anatomically shaped tissue-engineered meniscus constructs produced spatial distributions of local strain similar to those seen in the meniscus when the knee is loaded at full extension. Such loading drove formation of tissue with large organized collagen fibers, levels of mechanical anisotropy, and compressive moduli that match native tissue. Loading accelerated the development of native-sized and aligned circumferential and radial collagen fibers. These loading patterns contained both tensile and compressive components that enhanced the major biochemical and functional properties of the meniscus, with loading significantly improved glycosaminoglycan (GAG) accumulation 200-250%, collagen accumulation 40-55%, equilibrium modulus 1000-1800%, and tensile moduli 500-1200% (radial and circumferential). Furthermore, this study demonstrates local changes in mechanical environment drive heterogeneous tissue development and organization within individual constructs, highlighting the importance of recapitulating native loading environments. Loaded menisci developed cartilage-like tissue with rounded cells, a dense collagen matrix, and increased GAG accumulation in the more compressively loaded horns, and fibrous collagen-rich tissue in the more tensile loaded outer 2/3, similar to native menisci. Loaded constructs reached a level of organization not seen in any previous engineered menisci and demonstrate great promise as meniscal replacements.

Passive strain-induced matrix synthesis and organization in shape-specific, cartilaginous neotissues

Tissue-engineered musculoskeletal soft tissues typically lack the appropriate mechanical robustness of their native counterparts, hindering their clinical applicability. With structure and function being intimately linked, efforts to capture the anatomical shape and matrix organization of native tissues are imperative to engineer functionally robust and anisotropic tissues capable of withstanding the biomechanically complex in vivo joint environment. The present study sought to tailor the use of passive axial compressive loading to drive matrix synthesis and reorganization within self-assembled, shape-specific fibrocartilaginous constructs, with the goal of developing functionally anisotropic neotissues. Specifically, shape-specific fibrocartilaginous neotissues were subjected to 0, 0.01, 0.05, or 0.1 N axial loads early during tissue culture. Results found the 0.1-N load to significantly increase both collagen and glycosaminoglycan synthesis by 27% and 67%, respectively, and to concurrently reorganize the matrix by promoting greater matrix alignment, compaction, and collagen crosslinking compared with all other loading levels. These structural enhancements translated into improved functional properties, with the 0.1-N load significantly increasing both the relaxation modulus and Young's modulus by 96% and 255%, respectively, over controls. Finite element analysis further revealed the 0.1-N uniaxial load to induce multiaxial tensile and compressive strain gradients within the shape-specific neotissues, with maxima of 10.1%, 18.3%, and -21.8% in the XX-, YY-, and ZZ-directions, respectively. This indicates that strains created in different directions in response to a single axis load drove the observed anisotropic functional properties. Together, results of this study suggest that strain thresholds exist within each axis to promote matrix synthesis, alignment, and compaction within the shape-specific neotissues. Tailoring of passive axial loading, thus, presents as a simple, yet effective way to drive in vitro matrix development in shape-specific neotissues toward more closely achieving native structural and functional properties.

Moderate cyclic tensile strain alters the assembly of cartilage extracellular matrix proteins in vitro

Mechanical loading influences the structural and mechanical properties of articular cartilage. The cartilage matrix protein collagen II essentially determines the tensile properties of the tissue and is adapted in response to loading. The collagen II network is stabilized by the collagen II-binding cartilage oligomeric matrix protein (COMP), collagen IX, and matrilin-3. However, the effect of mechanical loading on these extracellular matrix proteins is not yet understood. Therefore, the aim of this study was to investigate if and how chondrocytes assemble the extracellular matrix proteins collagen II, COMP, collagen IX, and matrilin-3 in response to mechanical loading. Primary murine chondrocytes were applied to cyclic tensile strain (6%, 0.5 Hz, 30 min per day at three consecutive days). The localization of collagen II, COMP, collagen IX, and matrilin-3 in loaded and unloaded cells was determined by immunofluorescence staining. The messenger ribo nucleic acid (mRNA) expression levels and synthesis of the proteins were analyzed using reverse transcription-polymerase chain reaction (RT-PCR) and western blots. Immunofluorescence staining demonstrated that the pattern of collagen II distribution was altered by loading. In loaded chondrocytes, collagen II containing fibrils appeared thicker and strongly co-stained for COMP and collagen IX, whereas the collagen network from unloaded cells was more diffuse and showed minor costaining. Further, the applied load led to a higher amount of COMP in the matrix, determined by western blot analysis. Our results show that moderate cyclic tensile strain altered the assembly of the extracellular collagen network. However, changes in protein amount were only observed for COMP, but not for collagen II, collagen IX, or matrilin-3. The data suggest that the adaptation to mechanical loading is not always the result of changes in RNA and/or protein expression but might also be the result of changes in matrix assembly and structure.

Mechanobiology of the meniscus

The meniscus plays a critical biomechanical role in the knee, providing load support, joint stability, and congruity. Importantly, growing evidence indicates that the mechanobiologic response of meniscal cells plays a critical role in the physiologic, pathologic, and repair responses of the meniscus. Here we review experimental and theoretical studies that have begun to directly measure the biomechanical effects of joint loading on the meniscus under physiologic and pathologic conditions, showing that the menisci are exposed to high contact stresses, resulting in a complex and nonuniform stress-strain environment within the tissue. By combining microscale measurements of the mechanical properties of meniscal cells and their pericellular and extracellular matrix regions, theoretical and experimental models indicate that the cells in the meniscus are exposed to a complex and inhomogeneous environment of stress, strain, fluid pressure, fluid flow, and a variety of physicochemical factors. Studies across a range of culture systems from isolated cells to tissues have revealed that the biological response of meniscal cells is directly influenced by physical factors, such as tension, compression, and hydrostatic pressure. In addition, these studies have provided new insights into the mechanotransduction mechanisms by which physical signals are converted into metabolic or pro/anti-inflammatory responses. Taken together, these in vivo and in vitro studies show that mechanical factors play an important role in the health, degeneration, and regeneration of the meniscus. A more thorough understanding of the mechanobiologic responses of the meniscus will hopefully lead to therapeutic approaches to prevent degeneration and enhance repair of the meniscus.

Modulation of annulus fibrosus cell alignment and function on oriented nanofibrous polyurethane scaffolds under tension

BACKGROUND CONTEXT

Annulus fibrosus (AF), a component of the intervertebral disc (IVD), is always under tension in vivo, a condition that must be taken into consideration when tissue engineering an IVD. Loss of the tensile forces has been implicated in the pathogenesis of disc degeneration characterized by mechanical and structural breakdown of the AF.

PURPOSE

In this study, we hypothesize that tensile forces modulate cellular and molecular behavior of AF cells grown on nanofibrous scaffolds in vitro.

STUDY DESIGN/SETTING

Bovine AF cells were seeded onto strained electrospun-aligned nanofibrous polycarbonate urethane (PU) scaffolds. Tension was either maintained throughout the culture duration (monotonic) or removed after 24 hours (relaxed).

METHODS

The effect of tension on AF cells cultured on PU scaffolds was evaluated over 7 days by scanning electron microscopy, biochemical assays, immunofluorescence microscopy, and quantitative polymerase chain reaction.

RESULTS

Cells grown on the relaxed scaffold were significantly more proliferative, synthesized more collagen and had increased collagen type I and TGFβ-1 gene expression; however these cells were not as aligned as were the cells and matrix on monotonic strained scaffolds. The alignment of AF cells grown on monotonic scaffolds correlated with significantly greater scaffold elastic modulus on day 7. Additionally, the cellular response to the change in strain was delayed by 3 to 5 days after tension release, which correlated with the time at which changes in scaffold length were detected.

CONCLUSIONS

This study demonstrated that AF cells respond at the molecular and cellular level to the changes in matrix/scaffold tension. This suggests that it may be necessary to determine the optimal elastic modulus and applied tensile forces to tissue engineer an AF that mimics the native tissue. Furthermore, this study provides insight into how changes in tensile forces may lead to changes in the AF cell function.

Mechanical loading promotes mast cell degranulation via RGD-integrin dependent pathways

Mast cells are known to respond to a number of stimuli, such as IgE antibody-antigen complexes, pathogens, chemical compounds, and physical stimulation, resulting in the activation of these cells and subsequent release of cytokines, inflammatory mediators and granules which can influence the pathophysiology of neighboring cells. Although different forms of physical stimulation (i.e. shear stress and acupuncture) have been investigated, the effect of cyclic tensile loading on mast cell activation has not. To characterize the response of mast cells to tensile loading, RBL-2H3 cells were embedded in a 3-dimensional fibrin construct and subjected to 24h of cyclic loading at 0%, 5% or 10% peak tensile strain. Mechanical loading significantly increased RBL-2H3 cell secretion of β-hexosaminidase (2.1- to 2.3-fold, respectively) in a load- and time-dependent manner when compared to the controls. Furthermore, no evidence of load-induced cell death or alterations in cell proliferation was observed. To determine if RGD-dependent integrins mediated the degranulation of mast cells during mechanical loading, cell-matrix interactions were inhibited by treating the cells with echistatin, a disintegrin that binds RGD-dependent integrins. Treatment with echistatin significantly attenuated load-induced degranulation without compromising cell viability. These results suggest a novel mechanism through which mechanical loading induces mast cell activation via RGD binding integrins.

Variations in chondrogenesis of human bone marrow-derived mesenchymal stem cells in fibrin/alginate blended hydrogels

Fibrin and alginate hydrogels have been widely used to support chondrogenesis of bone marrow-derived mesenchymal stem cells (BM-MSCs) for articular cartilage and fibrocartilage tissue engineering, with each material offering distinct advantages and disadvantages. Attempting to produce a gel scaffold exhibiting beneficial characteristics of both materials, we fabricated fibrin/alginate blended hydrogels at various blend ratios and evaluated the gel morphology, mechanical properties and their support for BM-MSC chondrogenesis. Results show that when the fibrin/alginate ratio decreased, the fibrin architecture transitioned from uniform to interconnected fibrous and finally to disconnected islands against an alginate background, with opposing trends in the alginate architecture. Fibrin maintained gel extensibility and promoted cell proliferation, while alginate improved the gel biostability and better supported glycosaminoglycan and collagen II production and chondrogenic gene expression. Blended gels had physical and biological characteristics intermediate between fibrin and alginate. Of the blends examined, FA 40:8 (40 mg ml(-1) fibrinogen blended with 8 mg ml(-1) alginate) was found to be the most appropriate group for future studies on tension-driven BM-MSC fibrochondrogenesis. As BM-MSC differentiation appeared to vary between fibrin and alginate regions of blended scaffolds, this study also highlighted the potential to develop spatially heterogeneous tissues through manipulating the heterogeneity of scaffold composition.

The attachments of the temporomandibular joint disc: a biochemical and histological investigation

OBJECTIVE

The complex movement of the temporomandibular joint (TMJ) disc during mastication is controlled in large part by the disc's attachments to the surrounding tissues. This study seeks to address the lack of available quantitative data characterizing the extracellular matrix composition of the discal attachments and how these properties compare to the disc.

DESIGN

Porcine TMJ disc-attachment complexes were carefully dissected into six discal attachments and five TMJ disc regions. All samples were assayed biochemically for total collagen, glycosaminoglycan (GAG), DNA, and hydration. Additionally, histology was performed on the whole joint to investigate the anatomy of the disc-attachment complex, and to verify the regional distribution of matrix components.

RESULTS

Quantitative biochemical assays showed that overall water content was fairly constant in all disc and attachment regions. Disc regions generally showed higher sulfated GAG and collagen content than the attachments. In contrast, the attachments contained greater DNA content than the disc. Histological staining supported the quantitative results and also indicated more elastic fibres to be present in the attachments than the disc.

CONCLUSIONS

Although macroscopically the TMJ disc and its attachments form a seamless complex within the joint, a closer look at regional biochemical constituents reveals that these two components are distinct. Whilst the disc and attachments both contain the same major constituents, the relative amounts of these components vary based on the functional requirements of the tissue. These results can further understanding of both TMJ biology and pathology.

Tension-compression loading with chemical stimulation results in additive increases to functional properties of anatomic meniscal constructs

OBJECTIVE

This study aimed to improve the functional properties of anatomically-shaped meniscus constructs through simultaneous tension and compression mechanical stimulation in conjunction with chemical stimulation.

METHODS

Scaffoldless meniscal constructs were subjected to simultaneous tension and compressive stimulation and chemical stimulation. The temporal aspect of mechanical loading was studied by employing two separate five day stimulation periods. Chemical stimulation consisted of the application of a catabolic GAG-depleting enzyme, chondroitinase ABC (C-ABC), and an anabolic growth factor, TGF-β1. Mechanical and chemical stimulation combinations were studied through a full-factorial experimental design and assessed for histological, biochemical, and biomechanical properties following 4 wks of culture.

RESULTS

Mechanical loading applied from days 10-14 resulted in significant increases in compressive, tensile, and biochemical properties of meniscal constructs. When mechanical and chemical stimuli were combined significant additive increases in collagen per wet weight (4-fold), compressive instantaneous (3-fold) and relaxation (2-fold) moduli, and tensile moduli in the circumferential (4-fold) and radial (6-fold) directions were obtained.

CONCLUSIONS

This study demonstrates that a stimulation regimen of simultaneous tension and compression mechanical stimulation, C-ABC, and TGF-β1 is able to create anatomic meniscus constructs replicating the compressive mechanical properties, and collagen and GAG content of native tissue. In addition, this study significantly advances meniscus tissue engineering by being the first to apply simultaneous tension and compression mechanical stimulation and observe enhancement of tensile and compressive properties following mechanical stimulation.

Tensile loading modulates bone marrow stromal cell differentiation and the development of engineered fibrocartilage constructs

Mesenchymal progenitors such as bone marrow stromal cells (BMSCs) are an attractive cell source for fibrocartilage tissue engineering, but the types or combinations of signals required to promote fibrochondrocyte-specific differentiation remain unclear. The present study investigated the influences of cyclic tensile loading on the chondrogenesis of BMSCs and the development of engineered fibrocartilage. Cyclic tensile displacements (10%, 1 Hz) were applied to BMSC-seeded fibrin constructs for short (24 h) or extended (1-2 weeks) periods using a custom loading system. At early stages of chondrogenesis, 24 h of cyclic tension stimulated both protein and proteoglycan synthesis, but at later stages, tension increased protein synthesis only. One week of intermittent cyclic tension significantly increased the total sulfated glycosaminoglycan and collagen contents in the constructs, but these differences were lost after 2 weeks of loading. Constraining the gels during the extended culture periods prevented contraction of the fibrin matrix, induced collagen fiber alignment, and increased sulfated glycosaminoglycan release to the media. Cyclic tension specifically stimulated collagen I mRNA expression and protein synthesis, but had no effect on collagen II, aggrecan, or osteocalcin mRNA levels. Overall, these studies suggest that the combination of chondrogenic stimuli and tensile loading promotes fibrochondrocyte-like differentiation of BMSCs and has the potential to direct fibrocartilage development in vitro.

Effects of hydrostatic pressure on leporine meniscus cell-seeded PLLA scaffolds

Hydrostatic pressure (HP) is an important component of the loading environment of the knee joint. Studies with articular chondrocytes and TMJ disc fibrochondrocytes have identified certain benefits of HP for tissue engineering purposes. However, similar studies with meniscus cells are lacking. Thus, in this experiment, the effects of applying 10 MPa of HP at three different frequencies (0, 0.1, and 1 Hz) to leporine meniscus cell-seeded PLLA scaffolds were examined. HP was applied once every 3 days for 1 h for a period of 28 days. Constructs were analyzed for cellular, biochemical, and biomechanical properties. At t = 4 weeks, total collagen/scaffold was found to be significantly higher in the 10 MPa, 0 Hz group when compared with other groups. This despite the fact that the cell numbers/scaffold were found to be lower in all HP groups when compared with the culture control. Additionally, the total GAG/scaffold, instantaneous modulus, and relaxation modulus were significantly increased in the 10 MPa, 0 Hz group when compared with the culture control. In summary, this experiment provides evidence for the benefit of a 10 MPa, 0 Hz stimulus, on both biochemical and biomechanical aspects, for the purposes of meniscus tissue engineering using PLLA scaffolds.

Chondrocytes and meniscal fibrochondrocytes differentially process aggrecan during de novo extracellular matrix assembly

Aggrecan is an extracellular matrix molecule that contributes to the mechanical properties of articular cartilage and meniscal fibrocartilage, but the abundance and processing of aggrecan in these tissues are different. The objective of this study was to compare patterns of aggrecan processing by chondrocytes and meniscal fibrochondrocytes in tissue explants and cell-agarose constructs. The effects of transforming growth factor-beta 1 (TGF-beta1) stimulation on aggrecan deposition and processing were examined, and construct mechanical properties were measured. Fibrochondrocytes synthesized and retained less proteoglycans than did chondrocytes in tissue explants and agarose constructs. In chondrocyte constructs, TGF-beta1 induced the accumulation of a 120-kDa aggrecan species previously detected in mature bovine cartilage. Fibrochondrocyte-seeded constructs contained high-molecular-weight aggrecan but lacked aggrecanase-generated fragments found in native, immature meniscus. In addition, reflecting the lesser matrix accumulation, fibrochondrocyte constructs had significantly lower compression moduli than did chondrocyte constructs. These cell type-specific differences in aggrecan synthesis, retention, and processing may have implications for the development of functional engineered tissue grafts.

Biomechanical regulation of cell orientation and fate

Biomechanical regulation of tumor phenotypes have been noted for several decades, yet the function of mechanics in the co-evolution of the tumor epithelium and altered cancer extracellular matrix has not been appreciated until fairly recently. In this review, we examine the dynamic interaction between the developing epithelia and the extracellular matrix, and discuss how similar interactions are exploited by the genetically modified epithelium during tumor progression. We emphasize the process of mechanoreciprocity, which is a phenomenon observed during epithelial transformation, in which tension generated within the extracellular microenvironment induce and cooperate with opposing reactive forces within transformed epithelium to drive tumor progression and metastasis. We highlight the importance of matrix remodeling, and present a new, emerging paradigm that underscores the importance of tissue morphology as a key regulator of epithelial cell invasion and metastasis.

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.

Mechanoactive scaffold induces tendon remodeling and expression of fibrocartilage markers

Biological fixation of soft tissue-based grafts for anterior cruciate ligament (ACL) reconstruction poses a major clinical challenge. The ACL integrates with subchondral bone through a fibrocartilage enthesis, which serves to minimize stress concentrations and enables load transfer between two distinct tissue types. Functional integration thus requires the reestablishment of this fibrocartilage interface on reconstructed ACL grafts. We designed and characterized a novel mechanoactive scaffold based on a composite of poly-alpha-hydroxyester nanofibers and sintered microspheres; we then used the scaffold to test the hypothesis that scaffold-induced compression of tendon grafts would result in matrix remodeling and the expression of fibrocartilage interface-related markers. Histology coupled with confocal microscopy and biochemical assays were used to evaluate the effects of scaffold-induced compression on tendon matrix collagen distribution, cellularity, proteoglycan content, and gene expression over a 2-week period. Scaffold contraction resulted in over 15% compression of the patellar tendon graft and upregulated the expression of fibrocartilage-related markers such as Type II collagen, aggrecan, and transforming growth factor-beta3 (TGF-beta3). Additionally, proteoglycan content was higher in the compressed tendon group after 1 day. The data suggest the potential of a mechanoactive scaffold to promote the formation of an anatomic fibrocartilage enthesis on tendon-based ACL reconstruction grafts.

Creating a spectrum of fibrocartilages through different cell sources and biochemical stimuli

In this study a scaffoldless approach was employed with two different cell sources and biochemical stimuli to engineer a spectrum of fibrocartilages representative of the different regions of the knee meniscus. Constructs composed of 100% fibrochondrocytes (FC) or a 50:50 co-culture of fibrochondrocytes and chondrocytes (CC) were cultured in 10% fetal bovine serum medium or serum-free "chondrogenic" medium, each +/-10 ng/mL TGF-beta1 (+T). Constructs from these two cell groups and four culture conditions were cultured for 6 weeks. By varying the cell type and presence of the growth factor, GAG per dry weight of the constructs spanned that of native tissue, ranging 16-45% and 1-7% in the CC and FC groups, respectively. Collagen density was most dependent on cell type and was significantly lower than tissue values. The collagen I/II ratio could be manipulated by cell type and serum presence to span the native range, from 3.5 in the serum-free CC group to over 1,000 in the FC groups treated with serum-containing medium. Using the CC cell group in the presence of serum-free medium dramatically increased the compressive stiffness to 128 +/- 34 kPa, similar to native tissue. Similarly, serum-free medium or TGF-beta1 treatment enhanced the tensile modulus by an order of magnitude, up to 3,000 kPa. Using two cell sources and manipulating biochemical stimuli, a range of fibrocartilaginous neotissues was engineered. Fibrocartilages such as the knee meniscus are characterized by heterogeneity in matrix and functional properties, and this work demonstrates a strategy for recreating these heterogeneous tissues.

Changes in surface topologies of chondrocytes subjected to mechanical forces: an AFM analysis

The cartilage is composed of chondrocytes embedded in a matrix of collagen fibrils interspersed within a network of proteoglycans and is constantly exposed to biomechanical forces during normal joint movement. Characterization of the surface morphology, cytoskeletal structure, adherance and elastic properties of these mechanosensitive cells are crucial in understanding the effects of mechanical forces around a cell and how a cell responds to changes in its physical environment. In this work, we employed the atomic force microscope (AFM) to image cultured chondrocytes before and after subjecting them to mechanical forces in the presence or absence of interleukin-1beta to mimic inflammatory conditions. Nanoscale imaging and quantitative measurements from AFM data revealed that there are distinct changes in cell-surface topology and cytoskeleton arrangement in the cells following treatment with mechanical forces, IL-1beta or both. Our findings for the first time demonstrate that cultured chondrocytes are amenable to high-resolution AFM imaging and dynamic tensile forces may help overcome the effect of inflammatory factors on chondrocyte response.

Effects of tensile and compressive strains on response of a chondrocytic cell line embedded in type I collagen gel

Tensile and compressive strains are commonly used in mechanobiological models. Here we report on the development of a novel three-dimensional cell-culture method, which allows both tensile and compressive loads to be applied. Preliminary results were obtained using HCS2/8 chondrocytic cells embedded in type I collagen gel. This construct was subjected to either 16% tension or 14% compression. Confocal laser scanning microscopy showed that both tension and compression caused significant cell deformation. The collagen gel-embedded HCS2/8 cells were subjected to static tension, dynamic tension, static compression or dynamic compression for 24h. Dynamic compression led to significantly decreased 5-bromo-2'-deoxyuridine incorporation compared with the control group. PCR analysis revealed upregulation of type II collagen caused by dynamic tension, upregulation of aggrecan caused by static compression, and downregulation of type II collagen and aggrecan caused by dynamic compression. Nitric oxide production was significantly increased by static tension and static compression compared with the control group. Our experimental system effectively applied several types of strain to HCS2/8 cells embedded in collagen gel. Our results suggest that the mode of mechanical strain affects the response of HCS2/8 cells.

Fibrochondrogenesis in two embryonic stem cell lines: effects of differentiation timelines

Human embryonic stem cells (hESCs) are an exciting cell source for fibrocartilage engineering. In this study, the effects of differentiation time and cell line, H9 versus BG01V, were examined. Embryoid bodies (EBs) were fibrochondrogenically differentiated for 1, 3, or 6 weeks and then used to engineer tissue constructs that were grown for an additional 4 weeks. Construct matrix was fibrocartilaginous, containing glycosaminoglycans (GAGs) and collagens I, II, and VI. A differentiation time of 3 or 6 weeks produced homogeneous constructs, with matrix composition varying greatly with cell line and differentiation time: from 2.6 to 17.4 microg of GAG per 10(6) cells and from 22.3 to 238.4 microg of collagen per 10(6) cells. Differentiation for 1 week resulted in small constructs with poor structural integrity that could not be mechanically tested. The compressive stiffness of the constructs obtained from EBs differentiated for 3 or 6 weeks did not vary significantly as a function of either differentiation time or cell line. In contrast, the tensile properties were markedly greater with the H9 cell line, 1,562-1,940 versus 32-80 kPa in the BG01V constructs. These results demonstrate the dramatic effects of hESC line and differentiation time on the biochemical and functional properties of tissue-engineered constructs and show progress in fibrocartilage tissue engineering with an exciting new cell source.

Cyclic equibiaxial tensile strain induces both anabolic and catabolic responses in articular chondrocytes

Mechanical disturbance is directly implicated in the development of osteoarthritis (OA) but the precise mode for degenerative changes is still largely unknown because of the complexity of the biomechanical and biochemical milieu in the articular joint. To investigate the effects of tensile strain on articular cartilage, cyclic equibiaxial tensile strain (CTS, 0.5 Hz, 10% strain) was applied to monolayer cultures of porcine articular chondrocytes by using a Flexercell strain unit. Overproduction of proinflammatory mediators and imbalanced expression of anabolic and catabolic genes were induced. The cellular secretion of nitric oxide (NO) and prostaglandin E(2) (PGE(2)), as well as the mRNA level of cyclooxygenase-2 (COX-2) were up-regulated in response to mechanical stimuli. Additionally, CTS resulted in an initial peak of anabolic response at 3 h of stretch with respect to the expression of type II collagen and aggrecan. After 12 h of CTS, the expression for these two cartilage-specific matrix proteins fell to control levels. A distinct catabolic response developed after 24 h of stretch with an increase in matrix metalloproteinase-1 (MMP-1). Interestingly, a parallel increase in transforming growth factor (TGF) beta3 was associated with the anabolic changes while an increase in expression of TGF beta1, the predominant isoform of the TGF family, appeared at 24 h. The expression at 24 h of MMP-1, an enzyme that degrades interstitial collagens as well as other cartilage matrix proteins and TGF beta1, may signify a shift towards matrix remodeling and potentially a change in matrix composition as a consequence of continuous CTS.

Techniques for biological characterization of tissue-engineered tendon and ligament

Injuries to tendons and ligaments are prevalent and result in a significant decrease in quality of patient life. Tissue-engineering strategies hold promise as alternatives to current treatments for these injuries, which often fail to fully restore proper joint biomechanics and produce significant donor site morbidity. Commonly, tissue engineering involves the use of a three-dimensional scaffold seeded with cells that can be directed to form tendon/ligament tissue. When determining the success of such approaches, the viability and proliferation of the cells in the construct, as well as extracellular matrix production and structure should be taken into account. Histology and histochemistry, microscopy, colorimetric assays, and real-time reverse transcriptase-polymerase chain reaction (RT-PCR) are techniques that are employed to assess these biological characteristics. This review provides an overview of each of these methods, including specific examples of how they have been used in evaluation of tissue-engineered tendon and ligament tissue. Basic physical principles underlying each method and advantages and disadvantages of the various techniques are summarized.

Inhibition of in vitro chondrogenesis in RGD-modified three-dimensional alginate gels

The goal of this study was to investigate the effects of adhesion to the arginine-glycine-aspartic acid (RGD) sequence on the chondrogenesis of bone marrow stromal cells (BMSCs). Synthetic RGE- and RGD-containing peptides were conjugated to sodium alginate, and bovine BMSCs were seeded onto 2D alginate surfaces or encapsulated in 3D gels. BMSCs spread specifically on RGD-modified surfaces, and spreading was inhibited by a soluble RGD peptide and by anti-beta1 and anti-alpha(v)beta3 integrin blocking antibodies. After 7 days in 3D gel culture, the chondrogenic supplements (TGF-beta1 and dexamethasone) significantly stimulated chondrocytic gene expression (collagen II, aggrecan, and Sox-9) and matrix accumulation (collagen II and sGAG) in RGE-modified gels, but this response was inhibited in the RGD-modified gels. Inhibition of sGAG synthesis increased with increasing RGD density, and synthesis was partially rescued by adding a soluble RGD peptide. Addition of an anti-alpha(v)beta3 integrin blocking antibody had no effect on chondrogenesis, while an anti-alpha5 antibody reduced sGAG accumulation. Overall, this study demonstrates that interaction with the RGD motif significantly inhibits the initial chondrogenesis of BMSCs within 3D alginate gels. These results provide new insights into the role of cell-matrix interactions in regulating chondrogenesis and highlight the importance of choosing appropriate biomaterials for tissue engineering therapies.

Meniscal Repair With Fibrocartilage Engineering

Injuries to the knee meniscus, particularly those in the avascular region, pose a complex problem and a possible solution is tissue engineering of a replacement tissue. Tissue engineering of the meniscus involves scaffold selection, addition of cells, and stimulation of the construct to synthesize, maintain, or enhance matrix production. An acellular collagen implant is currently in clinical trials and there are promising results with other scaffolds, composed of both polymeric and natural materials. The addition of cells to these constructs may promote good matrix production in vitro, but has been studied in a limited manner in animal studies. Cell sources ranging from fibroblasts to stem cells could be used to overcome challenges in cell procurement, expansion, and synthetic capacity currently encountered in studies with fibrochondrocytes. Manipulation of construct culture with exogenous growth factors and mechanical stimulation will also likely play a role in these strategies.

Cyclic tensile stretch load and oxidized low density lipoprotein synergistically induce lectin-like oxidized ldl receptor-1 in cultured bovine chondrocytes, resulting in decreased cell viability and proteoglycan synthesis

Mechanical stimulation is known to be an essential factor in the regulation of cartilage metabolism. We tested the hypothesis that expression of lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) can be modulated by cyclic tensile stretch load in chondrocytes. Cyclic loading of repeated stretch stress at 10 cycles per minute with 10 kPa of stress for 6 h induced expression of LOX-1 to 2.6 times control in cultured bovine articular chondrocytes, equivalent to the addition of 10 microg/mL oxidized low density lipoprotein (ox-LDL) (2.4 times control). Application of the cyclic load to the chondrocytes along with 10 microg/mL ox-LDL resulted in synergistically increased LOX-1 expression to 6.3 times control. Individual application of cyclic loading and 10 microg/mL ox-LDL significantly suppressed chondrocytes viability (84.6% +/- 3.4% and 80.9% +/- 3.2% of control at 24 h, respectively; n = 3; p < 0.05) and proteoglycan synthesis [81.0% +/- 7.1% and 85.7% +/- 5.2% of control at 24 h, respectively; p < 0.05 when compared with 94.6% +/- 4.6% for native-LDL (n = 3)]. Cyclic loading and 10 microg/mL ox-LDL synergistically affected cell viability and proteoglycan synthesis, which were significantly suppressed to 45.6% +/- 4.9% and 48.7% +/- 6.7% of control at 24 h, respectively (n = 3; p < 0.01 when compared with individual application of cyclic loading or 10 microg/mL ox-LDL). In this study, we demonstrated synergistic effects of cyclic tensile stretch load and ox-LDL on cell viability and proteoglycan synthesis in chondrocytes, which may be mediated through enhanced expression of LOX-1 and which has important implications in the progression of cartilage degeneration in osteoarthritis.

Cartilage tissue engineering: its potential and uses

PURPOSE OF REVIEW

The prevalent nature of osteoarthritis, a cartilage degenerative disease that results in the erosion of joint surfaces and loss of mobility, underscores the importance of developing functional articular cartilage replacement. Recent research efforts have focused on tissue engineering as a promising approach for cartilage regeneration and repair. Tissue engineering is a multidisciplinary research area that incorporates both biological and engineering principles for the purpose of generating new, living tissues to replace the diseased/damaged tissue and restore tissue/organ function. This review surveys and highlights the current concepts and recent progress in cartilage tissue engineering, and discusses the challenges and potential of this rapidly advancing field of biomedical research.

RECENT FINDINGS

Cartilage tissue engineering is critically dependent on selection of appropriate cells (differentiated or progenitor cells); fabrication and utilization of biocompatible and mechanically suitable scaffolds for cell delivery; stimulation with chondrogenically bioactive molecules introduced in the form of recombinant proteins or via gene transfer; and application of dynamic, mechanical loading regimens for conditioning of the engineered tissue constructs, including the design of specialized biomechanically active bioreactors.

SUMMARY

Cell selection, scaffold design and biological stimulation remain the challenges of function tissue engineering. Successful regeneration or replacement of damaged or diseased cartilage will depend on future advances in our understanding of the biology of cartilage and stem cells and technological development in engineering.

Biaxial strain effects on cells from the inner and outer regions of the meniscus

During knee joint loading, the fibrocartilaginous menisci experience significant spatial variations in mechanical stimuli. Meniscus cells also exhibit significant variations in biosynthesis and gene expression depending on their location within the tissue. These metabolic patterns are consistent with a more chondrocytic phenotype for cells located within the avascular inner two-thirds compared with a more fibroblastic phenotype for cells within the vascularized outer periphery. The spatial distribution of cell biosynthesis and gene expression patterns within the meniscus suggest that cells may exhibit intrinsically different responses to mechanical stimuli. The objective of our study was to test for intrinsic differences in the responsiveness of these meniscus cell populations to an equivalent mechanical stimulus. Cellular biosynthesis and gene expression for extracellular matrix proteins in isolated inner and outer meniscus cells in monolayer were quantified following cyclic biaxial stretch. The results demonstrate that inner and outer meniscus cells exhibit significant differences in matrix biosynthesis and gene expression regardless of stretching condition. Both inner and outer meniscus cells responded to stretch with increased nitric oxide production and total protein synthesis. The results suggest that inner and outer meniscus cells may respond similarly to biaxial stretch in vitro with measures of biosynthesis and gene expression.

Variations in matrix composition and GAG fine structure among scaffolds for cartilage tissue engineering

OBJECTIVE

To compare matrix composition and glycosaminoglycan (GAG) fine structure among five scaffolds commonly used for in vitro chondrocyte culture and cartilage tissue engineering.

DESIGN

Bovine articular chondrocytes were seeded into agarose, alginate, collagen I, fibrin and polyglycolic acid (PGA) constructs and cultured for 20 or 40 days. In addition to construct DNA and sulfated GAG (sGAG) contents, the delta-disaccharide compositions of the chondroitin/dermatan sulfate GAGs were determined for each scaffold group via fluorophore-assisted carbohydrate electrophoresis (FACE).

RESULTS

Significant differences were found in cell proliferation and extracellular matrix accumulation among the five scaffold groups. Significant cell proliferation was observed for all scaffold types but occurred later (20-40 days) in PGA constructs compared to the other groups (0-20 days). By 40 days, agarose constructs had the highest sGAG to DNA ratio, while alginate and collagen I had the lowest levels. Quantitative differences in the Delta-disaccharide composition of the GAGs accumulated in the different scaffolds were also found, with the most striking variations in unsulfated and disulfated delta-disaccharides. Agarose constructs had the highest fraction of disulfated residues and the lowest fraction of unsulfated residues, with a 6-sulfated/4-sulfated disaccharide ratio most similar to that of native articular cartilage.

CONCLUSIONS

The similarities and differences among scaffolds in proteoglycan accumulation and GAG composition suggest that the scaffold material directly or indirectly influences chondrocyte proteoglycan metabolism and may have an influence on the quality of tissue engineered cartilage.