Mechanical Loading-Dependence of mRNA Expressions of Extracellular Matrices of Chondrocytes Inoculated into Elastomeric Microporous Poly(L-lactide-co-ε-caprolactone) Scaffold

Applied Compressive Strain Governs Hyaline-like Cartilage versus Fibrocartilage-like ECM Produced within Hydrogel Constructs

The goal of cartilage tissue engineering (CTE) is to regenerate new hyaline cartilage in joints and treat osteoarthritis (OA) using cell-impregnated hydrogel constructs. However, the production of an extracellular matrix (ECM) made of fibrocartilage is a potential outcome within hydrogel constructs when in vivo. Unfortunately, this fibrocartilage ECM has inferior biological and mechanical properties when compared to native hyaline cartilage. It was hypothesized that compressive forces stimulate fibrocartilage development by increasing production of collagen type 1 (Col1), an ECM protein found in fibrocartilage. To test the hypothesis, 3-dimensional (3D)-bioprinted hydrogel constructs were fabricated from alginate hydrogel impregnated with ATDC5 cells (a chondrogenic cell line). A bioreactor was used to simulate different in vivo joint movements by varying the magnitude of compressive strains and compare them with a control group that was not loaded. Chondrogenic differentiation of the cells in loaded and unloaded conditions was confirmed by deposition of cartilage specific molecules including glycosaminoglycans (GAGs) and collagen type 2 (Col2). By performing biochemical assays, the production of GAGs and total collagen was also confirmed, and their contents were quantitated in unloaded and loaded conditions. Furthermore, Col1 vs. Col2 depositions were assessed at different compressive strains, and hyaline-like cartilage vs. fibrocartilage-like ECM production was analyzed to investigate how applied compressive strain affects the type of cartilage formed. These assessments showed that fibrocartilage-like ECM production tended to reduce with increasing compressive strain, though its production peaked at a higher compressive strain. According to these results, the magnitude of applied compressive strain governs the production of hyaline-like cartilage vs. fibrocartilage-like ECM and a high compressive strain stimulates fibrocartilage-like ECM formation rather than hyaline cartilage, which needs to be addressed by CTE approaches.

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.

Dynamic Mechanical Compression of Chondrocytes for Tissue Engineering: A Critical Review

Articular cartilage functions to transmit and translate loads. In a classical structure-function relationship, the tissue resides in a dynamic mechanical environment that drives the formation of a highly organized tissue architecture suited to its biomechanical role. The dynamic mechanical environment includes multiaxial compressive and shear strains as well as hydrostatic and osmotic pressures. As the mechanical environment is known to modulate cell fate and influence tissue development toward a defined architecture in situ, dynamic mechanical loading has been hypothesized to induce the structure-function relationship during attempts at in vitro regeneration of articular cartilage. Researchers have designed increasingly sophisticated bioreactors with dynamic mechanical regimes, but the response of chondrocytes to dynamic compression and shear loading remains poorly characterized due to wide variation in study design, system variables, and outcome measurements. We assessed the literature pertaining to the use of dynamic compressive bioreactors for in vitro generation of cartilaginous tissue from primary and expanded chondrocytes. We used specific search terms to identify relevant publications from the PubMed database and manually sorted the data. It was very challenging to find consensus between studies because of species, age, cell source, and culture differences, coupled with the many loading regimes and the types of analyses used. Early studies that evaluated the response of primary bovine chondrocytes within hydrogels, and that employed dynamic single-axis compression with physiologic loading parameters, reported consistently favorable responses at the tissue level, with upregulation of biochemical synthesis and biomechanical properties. However, they rarely assessed the cellular response with gene expression or mechanotransduction pathway analyses. Later studies that employed increasingly sophisticated biomaterial-based systems, cells derived from different species, and complex loading regimes, did not necessarily corroborate prior positive results. These studies report positive results with respect to very specific conditions for cellular responses to dynamic load but fail to consistently achieve significant positive changes in relevant tissue engineering parameters, particularly collagen content and stiffness. There is a need for standardized methods and analyses of dynamic mechanical loading systems to guide the field of tissue engineering toward building cartilaginous implants that meet the goal of regenerating articular cartilage.

Cartilaginous constructs using primary chondrocytes from continuous expansion culture seeded in dense collagen gels

Cell-based therapies such as autologous chondrocyte implantation require in vitro cell expansion. However, standard culture techniques require cell passaging, leading to dedifferentiation into a fibroblast-like cell type. Primary chondrocytes grown on continuously expanding culture dishes (CE culture) limits passaging and protects against dedifferentiation. The authors tested whether CE culture chondrocytes were advantageous for producing mechanically competent cartilage matrix when three-dimensionally seeded in dense collagen gels. Primary chondrocytes, grown either in CE culture or passaged twice on static silicone dishes (SS culture; comparable to standard methods), were seeded in dense collagen gels and cultured for 3 weeks in the absence of exogenous chondrogenic growth factors. Compared with gels seeded with SS culture chondrocytes, CE chondrocyte-seeded gels had significantly higher chondrogenic gene expression after 2 and 3 weeks in culture, correlating with significantly higher aggrecan and type II collagen protein accumulation. There was no obvious difference in glycosaminoglycan content from either culture condition, yet CE chondrocyte-seeded gels were significantly thicker and had a significantly higher dynamic compressive modulus than SS chondrocyte-seeded gels after 3 weeks. Chondrocytes grown in CE culture and seeded in dense collagen gels produce more cartilaginous matrix with superior mechanical properties, making them more suitable than SS cultured cells for tissue engineering applications.

Bioreactor-Induced Chondrocyte Maturation Is Dependent on Cell Passage and Onset of Loading

OBJECTIVE

To explore the effect of shifting in vitro culture conditions regarding cellular passage and onset of loading within matrix-associated bovine articular chondrocytes cultured under free-swelling and/or dynamical loading conditions on general chondrocyte maturation.

METHODS

Primary or passage 3 bovine chondrocytes were seeded in fibrin-polyurethane scaffolds. Constructs were cultured either free-swelling for 2 or 4 weeks, under direct mechanical loading for 2 or 4 weeks, or free swelling for 2 weeks followed by 2 weeks of loading. Samples were collected for glycosaminoglycan (GAG) quantification, mRNA expression of chondrogenic genes, immunohistochemistry, and histology.

RESULTS

Mechanical loading generally stimulated GAG synthesis, up-regulated chondrogenic genes, and improved the accumulation of matrix in cell-laden constructs when compared with free-swelling controls. Primary chondrocytes underwent more effective cartilage maturation when compared with passaged chondrocytes. Constructs of primary chondrocytes that were initially free-swelling followed by 2 weeks of mechanical load (delayed) had overall highest GAG with strongest responsiveness to load regarding matrix synthesis. Constructs that experienced the delayed loading regime also demonstrated most favorable chondrogenic gene expression profiles in both primary and third passage cells. Furthermore, most intense matrix staining and immunostaining of collagen type II and aggrecan were visualized in these constructs.

CONCLUSIONS

Primary chondrocytes were more effective than passage 3 chondrocytes when chondrogenesis was concerned. The most efficient chondrogenesis resulted from primary articular chondrocytes, which were initially free-swelling followed by a standardized loading protocol.

Modulation of the proliferation and matrix synthesis of chondrocytes by dynamic compression on genipin-crosslinked chitosan/collagen scaffolds

Dynamic compression is an important physical stimulus for the physiology of chondrocyte and articular cartilage tissue engineering. In this study, modulation of chondrocyte behaviors in chitosan/collagen scaffolds with different mechanical properties under free-swelling or dynamic compression conditions was investigated. Rabbit chondrocytes were seeded in chitosan/collagen scaffolds crosslinked by genipin (GP) with different concentrations, and then cultured for 3 days prior to cyclic compression of 40% strain, 0.1 Hz, and 30 min/day for 2 weeks. The results showed that the cell proliferation was increased with increasing genipin concentrations and dynamic compression. On the other hand, although total glycosaminoglycans (GAGs) deposition was enhanced by dynamic compression under certain conditions, e.g. the GP0.5 chitosan/collagen scaffolds for 1 week of compression culture, normalized GAGs deposition per cell was decreased by dynamic compression. Our results suggest that while several studies suggest that dynamic compression benefits articular cartilage tissue engineering, many factors including scaffold types and compression conditions determine the outcome of dynamic compression culture.

Articular cartilage tissue engineering based on a mechano-active scaffold made of poly(L-lactide-co-epsilon-caprolactone): In vivo performance in adult rabbits

Our previous studies showed that a mechano-active scaffold made of poly(L-lactide-co-epsilon-caprolactone) (PLCL) exhibited a high potential to realize the formation of a functional, engineered cartilage in vitro. This animal study therefore was designed to investigate the feasibility of repairing on osteochondral defect with the use of bone marrow-derived mesenchymal stem cells (BMSCs) incorporated with a PLCL scaffold. Rabbit BMSCs, isolated and subsequently cultured in monolayer, were seeded into a porous PLCL scaffold sponge following an implantation onto a full-thickness osteochondral defect (diameter of 4.5 mm, depth of 5 mm) that was artificially created on the medial femoral condyles at a high load-bearing site on a rabbit's knee joint. Time-dependent healing of the defect was evaluated by macroscopic, histological examinations at both 3- and 6-month-implantations, respectively. A PLCL sponge incorporated with BMSCs exhibited sufficient structural support, resulting in new osteochondral tissue regeneration: a physiologically well-integrated subchondral bone formation, a hyaline cartilage-like morphology containing chondrocytes surrounded by abundant cartilaginous matrices. In addition, quantitative biochemical assays also demonstrated high potential for the synthesis of sulfated glycosaminoglycan and collagen, both of which are biomolecules essential to extracelluar matrix in normal cartilage tissue. In contrast, defects filled with cell-free PLCL scaffold or left empty showed a very limited potential for regeneration. Our findings suggest that a composite of PLCL-based sponge scaffold and BMSCs promote the repair of osteochondral defects at high load-bearing sites in adult rabbits.

Three-dimensional in vitro effects of compression and time in culture on aggregate modulus and on gene expression and protein content of collagen type II in murine chondrocytes

The objectives of this study were to determine how culture time and dynamic compression, applied to murine chondrocyte-agarose constructs, influence construct stiffness, expression of col2 and type II collagen. Chondrocytes were harvested from the ribs of six newborn double transgenic mice carrying transgenes that use enhanced cyan fluorescent protein (ECFP) and green fluorescent protein (GFP-T) as reporters for expression from the col2a1 and col1a1 promoters, respectively. Sixty-three constructs (8 mm diameter x 3 mm thick) per animal were created by seeding chondrocytes (10 x 10(6) per mL) in agarose gel (2% w/v). Twenty-eight constructs from each animal were stimulated for 7, 14, 21, or 28 days in a custom bioreactor housed in an electromagnetic system. Twenty-eight constructs exposed to identical culture conditions but without mechanical stimulation served as nonstimulated controls for 7, 14, 21, and 28 days. The remaining seven constructs served as day 0 controls. Fluorescing cells with rounded morphology were present in all constructs at all five time points. Seven, 14, 21, and 28 days of stimulation significantly increased col2 expression according to ECFP fluorescence and messenger RNA expression according to quantitative reverse transcriptase polymerase chain reaction. Col2 gene expression in stimulated and nonstimulated constructs showed initial increases up to day 14 and then showed decreases by day 28. Stimulation significantly increased type II collagen content at 21 and 28 days and aggregate modulus only at 28 days. There was a significant increase in aggregate modulus in stimulated constructs between day 0 and 7 and between day 21 and day 28. This study reveals that compressive mechanical stimulation is a potent stimulator of col2 gene expression that leads to measurable but delayed increases in protein (type II collagen) and then biomechanical stiffness. Future studies will examine the effects of components of the mechanical signal in culture and address the question of whether such in vitro improvements in tissue-engineered constructs enhance repair outcomes after surgery.

Morphogenetic and regulatory mechanisms during developmental chondrogenesis: new paradigms for cartilage tissue engineering

Cartilage is the first skeletal tissue to be formed during embryogenesis leading to the creation of all mature cartilages and bones, with the exception of the flat bones in the skull. Therefore, errors occurring during the process of chondrogenesis, the formation of cartilage, often lead to severe skeletal malformations such as dysplasias. There are hundreds of skeletal dysplasias, and the molecular genetic etiology of some remains more elusive than of others. Many efforts have aimed at understanding the morphogenetic event of chondrogenesis in normal individuals, of which the main morphogenetic and regulatory mechanisms will be reviewed here. For instance, many signaling molecules that guide chondrogenesis--for example, transforming growth factor-beta, bone morphogenetic proteins, fibroblast growth factors, and Wnts, as well as transcriptional regulators such as the Sox family--have already been identified. Moreover, extracellular matrix components also play an important role in this developmental event, as evidenced by the promotion of the chondrogenic potential of chondroprogenitor cells caused by collagen II and proteoglycans like versican. The growing evidence of the elements that control chondrogenesis and the increasing number of different sources of progenitor cells will, hopefully, help to create tissue engineering platforms that could overcome many developmental or degenerative diseases associated with cartilage defects.

Novel electrospun scaffolds for the molecular analysis of chondrocytes under dynamic compression

Mechanical training of engineered tissue constructs is believed necessary to improve regeneration of cartilaginous grafts. Nevertheless, molecular mechanisms underlying mechanical activation are not clear. This is partly due to unavailability of appropriate scaffolds allowing exposure of cells to dynamic compressive strains (DCS) in vitro while permitting subsequent molecular analyses. We demonstrate that three-dimensional macroporous electrospun poly(epsilon-caprolactone) scaffolds can be fabricated that are suitable for the functional and molecular analysis of dynamically loaded chondrocytes. These scaffolds encourage chondrocytic proliferation promoting expression of collagen type II, aggrecan, and Sox9 while retaining mechanical strength after prolonged dynamic compression. Further, they exhibit superior infiltration of exogenous agents into the cells and permit easy retrieval of cellular components postcompression to allow exploration of molecular mechanisms of DCS. Using these scaffolds, we observed that chondrocytes responded to DCS in a magnitude-dependent manner exhibiting antiinflammatory and proanabolic responses at low physiological magnitudes. Proinflammatory responses and decreased cellular viability were observed at hyperphysiological magnitudes. These scaffolds provide a means of unraveling the mechanotransduction-induced transcriptional and posttranslational activities involved in cartilage regeneration and repair.

Modulation of gene expression of rabbit chondrocytes by dynamic compression in polyurethane scaffolds with collagen gel encapsulation

Chondrocytes have been demonstrated to be sensitive to mechanical stimuli, such as compression, tension, shear force, and hydrostatic pressure. The responses of chondrocytes to mechanical compression have been often studied in vitro with cartilage and chondrocyte/hydrogel systems. The aim of this study was to investigate the effects of dynamic compression on gene expression of rabbit chondrocytes which were seeded in elastic polyurethane scaffolds with or without collagen gel encapsulation. Dynamic compression of 20% or 30% strain with 0.1 Hz frequency was applied to the cell-seeded scaffolds for 4, 8, 12, or 24 h, and then the expression of the three genes related to chondrogenic phenotype, type I and II collagens and aggrecan, was analyzed by RT-PCR. We also investigated the gene expression of the compressed chondrocytes, which had experienced 12-h 30% strain dynamic loading, during the post-compression resting period. We found that the expression of type II collagen did not seem to respond to cyclic compression. On the other hand, aggrecan gene was stimulated by dynamic compression. The stimulatory effect disappeared gradually after the dynamic compression was ceased. Furthermore, the mechano-response of the chondrocytes to aggrecan expression was delayed by collagen gel encapsulation. The expression of type I collagen was enhanced by collagen gel. We found that collagen gel encapsulation prolonged the expression of aggrecan and type I collagen during post-compression resting period. We demonstrated that mechanical and biochemical stimuli modulate the gene expression of chondrocytes.