Influence of matricial molecules on growth and differentiation of entrapped chondrocytes

Glial Fibrillary Acidic Protein as Biomarker Indicates Purity and Property of Auricular Chondrocytes

Instead of the silicone implants previously used for repair and reconstruction of the auricle and nose lost due to accidents and disease, a new treatment method using tissue-engineered cartilage has been attracting attention. The quality of cultured cells is important in this method because it affects treatment outcomes. However, a marker of chondrocytes, particularly auricular chondrocytes, has not yet been established. The objective of this study was to establish an optimal marker to evaluate the quality of cultured auricular chondrocytes as a cell source of regenerative cartilage tissue. Gene expression levels were comprehensively compared using the microarray method between human undifferentiated and dedifferentiated auricular chondrocytes to investigate a candidate quality control index with an expression level that is high in differentiated cells, but markedly decreases in dedifferentiated cells. We identified glial fibrillary acidic protein (GFAP) as a marker that decreased with serial passages in auricular chondrocytes. GFAP was not detected in articular chondrocytes, costal chondrocytes, or fibroblasts, which need to be distinguished from auricular chondrocytes in cell cultures. GFAP mRNA expression was observed in cultured auricular chondrocytes, and GFAP protein levels were also measured in the cell lysates and culture supernatants of these cells. However, GFAP levels detected from mRNA and protein in cell lysates were significantly decreased by increases in the incubation period. In contrast, the amount of protein in the cell supernatant was not affected by the incubation period. Furthermore, the protein level of GFAP in the supernatants of cultured cells correlated with the in vitro and in vivo production of the cartilage matrix by these cells. The productivity of the cartilage matrix in cultured auricular chondrocytes may be predicted by measuring GFAP protein levels in the culture supernatants of these cells. Thus, GFAP is regarded as a marker of the purity and properties of cultured auricular chondrocytes.

A hydrophobically-modified alginate gel system: utility in the repair of articular cartilage defects

Alginate is a family of natural polysaccharides, widely used in industry and medicine for many applications, with its non toxic nature, gentle sol/gel transition procedure and low cost, alginate inferior biomechanical properties have limited its utility especially in tissue engineering. Additionally, ionically cross-linked alginate hydrogels generally lose most of their initial mechanical and swelling properties within a few hours in physiological solution. In order to overcome these limitations, the referenced alginate was treated by covalent fixation of octadecyl chains onto the polysaccharide backbone by esterification. In semi dilute solution, intermolecular hydrophobic interactions of long alkyl chains result in the formation of physical hydrogels, which can then be reinforced by the addition of calcium chloride. FTIR studies clearly showed the presence of ester bonds at 1612 and 1730 cm(-1) indicating that the alkyl groups are incorporated in the backbone of resulting polymer. The endothermic peak and exothermic peak present in the DSC thermogram of Alg-C18 had shifted to lower temperatures comparing to native alginate (from 106 to 83°C and from 250 to 245°C, respectively) due to the esterification reaction that leads to high hydrophobic nature of the modified sample. From rheological experiments, it can be inferred that the combination of both calcium bridges and intermolecular hydrophobic interaction in the treated alginate leads to enhanced gel strength accompanied by more stable structure in physiological solution comparing to native sodium alginate hydrogel. Finally, the modified alginate tended to have no toxic effects on mesenchymal stem cell culture, rather it supported MSC chondrogenic differentiation.

αvβ5 integrin promotes dedifferentiation of monolayer-cultured articular chondrocytes

OBJECTIVE

When cultured in monolayers, articular chondrocytes undergo an obvious phenotypic change. Although the involvement of integrins has been suggested, the exact mechanisms of the change have not been determined. This study was undertaken to clarify the mechanisms underlying the loss of chondrocyte phenotype early after plating.

METHODS

Primary cultured human articular chondrocytes were used for the experiments. Involvement of respective integrins in the phenotypic change was investigated in RNA interference (RNAi) experiments. A signaling pathway involved in the change was identified in experiments using specific inhibitors and adenoviruses encoding mutated genes involved in the pathway. Adenoviruses carrying mutated GTPases were used to determine the involvement of small GTPases in the process.

RESULTS

In monolayer-cultured chondrocytes, suppression of αv or β5 integrin expression by RNAi inhibited morphologic changes in the cells and increased (or prevented a reduction in) the expression of various cartilage matrix genes. Consistent results were obtained in experiments using a blocking antibody and a synthetic inhibitor of αvβ5 integrin. The decrease in cartilage matrix gene expression in chondrocytes after plating was mediated by ERK signaling, which was promoted primarily by αvβ5 integrin. In articular chondrocytes, the affinity of αvβ5 integrin for ligands was regulated by the small GTPase R-Ras. R-Ras was gradually activated in monolayer-cultured chondrocytes after plating, which caused a gradual decline in cartilage matrix gene expression through enhanced αvβ5 integrin activation and the subsequent increase in ERK signaling.

CONCLUSION

Our findings indicate that αvβ5 integrin may be involved in the change that occurs in monolayer-cultured chondrocytes after plating.

Mandibular appliance modulates condylar growth through integrins

Functional orthopedic therapy corrects growth discrepancies between the maxilla and mandible, possibly through postural changes in the musculature and modulation of the mandibular condylar cartilage growth. Using Wistar rats, we tested the hypothesis that chondrocytes respond to forces generated by a mandibular propulsor appliance by changes in gene expression, and that integrins are important mediators in this response. Immunohistochemical analyses demonstrated that the use of the appliance for different periods of time modulated the expression of fibronectin, alpha5 and alphav integrin subunits, as well as cell proliferation in the cartilage. In vitro, cyclic distension of condylar cartilage-derived cells increased fibronectin mRNA, as well as Insulin-like Growth Factor-I and II mRNA and cell proliferation. A peptide containing the Arginine-Glycine-Asparagine sequence (RGD), the main cell-binding sequence in fibronectin, blocked almost all these effects, confirming that force itself modulates the growth of the rat condylar cartilage, and that RGD-binding integrins participate in mechanotransduction.

Sonication-induced gelation of silk fibroin for cell encapsulation

Purified native silk fibroin forms beta-sheet-rich, physically cross-linked, hydrogels from aqueous solution, in a process influenced by environmental parameters. Previously we reported gelation times of days to weeks for aqueous native silk protein solutions, with high ionic strength and temperature and low pH responsible for increasing gelation kinetics. Here we report a novel method to accelerate the process and control silk fibroin gelation through ultrasonication. Depending on the sonication parameters, including power output and time, along with silk fibroin concentration, gelation could be controlled from minutes to hours, allowing the post-sonication addition of cells prior to final gel setting. Mechanistically, ultrasonication initiated the formation of beta-sheets by alteration in hydrophobic hydration, thus accelerating the formation of physical cross-links responsible for gel stabilization. K(+) at physiological concentrations and low pH promoted gelation, which was not observed in the presence of Ca(2+). The hydrogels were assessed for mechanical properties and proteolytic degradation; reported values matched or exceeded other cell-encapsulating gel material systems. Human bone marrow derived mesenchymal stem cells (hMSCs) were successfully incorporated into these silk fibroin hydrogels after sonication, followed by rapid gelation and sustained cell function. Sonicated silk fibroin solutions at 4%, 8%, and 12% (w/v), followed by mixing in hMSCs, gelled within 0.5-2 h. The cells grew and proliferated in the 4% gels over 21 days, while survival was lower in the gels with higher protein content. Thus, sonication provides a useful new tool with which to initiate rapid sol-gel transitions, such as for cell encapsulation.

Tissue-Engineered Cartilage Constructs Using Composite Hyaluronic Acid/Collagen I Hydrogels and Designed Poly(Propylene Fumarate) Scaffolds

Our approach to cartilage tissue-engineering scaffolds combines image-based design and solid free-form (SFF) fabrication to create load-bearing constructs with user-defined parameters. In this study, 3-dimensional scaffolds with cubic and ellipsoidal pore architecture were fabricated using poly(propylene fumarate) (PPF). To increase seeding efficiency and cellular retention, hydrogels were used to deliver cells into the scaffolds. The first objective of this study was to evaluate the concentrations of composite hyaluronic acid (HyA) and collagen I hydrogels best able to stimulate proteoglycan synthesis in porcine chondrocytes in vitro and in vivo. The second objective was to evaluate the differences in extracellular matrix production due to pore geometry and scaffold design. For the in vitro assessment, chondrocytes were encapsulated in collagen I hydrogels with varying concentrations of HyA. Hydrogels were cultured for 1 and 2 weeks, and then the sulfated glycosaminoglycan (sGAG) content was quantified using a dimethyl-methylene blue assay. The concentration of HyA best able to increase ECM synthesis was 5% HyA/collagen I, or 0.23 mg/mL HyA. The results from the in vitro experiment were used as culture parameters for the in vivo analysis. Composite 5% HyA/collagen I or collagen I-only hydrogels were used to seed chondrocytes into SFF-fabricated scaffolds made of PPF with designed cubic or ellipsoidal pore geometry. The scaffolds were implanted subcutaneously in immunocompromised mice for 4 weeks. Histomorphometric analyses of sections stained with Safranin O were used to quantify the amount of ECM deposited by cells in the scaffolds. Scaffolds seeded with 5% HyA/collagen hydrogels had significantly greater areas of positive Safranin O staining (approximately 60%, compared with 30% for scaffolds with collagen I hydrogels only), indicating that greater numbers of chondrocytes retained their metabolic activity in the ectopic environment. These scaffolds also had greater stain intensities (corresponding to greater amounts of sGAG in the ECM) than their counterparts seeded with collagen I hydrogels alone. Significant differences in matrix production were not found between the scaffold pore designs. Overall, these results indicate that a combination of composite HyA hydrogels and designed SFF scaffolds could provide a functional tissue-engineered construct for cartilage repair with enhanced tissue regeneration in a load-bearing scaffold.

Effect of extracellular matrix protein on the rate of proteoglycan synthesis in rabbit intervertebral disc cells

OBJECTIVE

The extracellular matrix (ECM) is very important for fundamental cellular processes. However, the effects of ECM proteins on intervertebral disc (IVD) cell proliferation and metabolism have not been clarified. To verify the effects of ECM proteins on DNA and proteoglycan (PG) synthesis of IVD cells, PG synthesis rate was measured in IVD cells cultured in monolayer with or without ECM protein.

METHODS

Nucleus pulposus (NP) cells and anulus fibrosus (AF) cells isolated from adolescent rabbits were cultured in monolayer with or without ECM protein and at different concentrations of ECM protein for 4-6 days. [S]Sulfate incorporation into PG in the cell-associated matrix (CM) formed around cells and the further-removed matrix (FRM) in labeling medium was measured and standardized to DNA content.

CONCLUSIONS

NP cells in type I or type II collagen-coated plates significantly increased the rate of PG synthesis in both the CM and the FRM compared with those in uncoated plates and in fibronectin-coated plates; however, AF cells with ECM proteins did not increase the rate significantly. The rate of PG synthesis of nucleus cells was contra-dose dependent on both type I and type II collagen.

Human articular chondrocytes immortalized by HPV-16 E6 and E7 genes: Maintenance of differentiated phenotype under defined culture conditions

OBJECTIVE

To establish an immortalized normal human articular chondrocyte line which could be useful for a better understanding of cell molecular mechanisms relevant for the development of new therapeutic approaches in rheumatic diseases.

DESIGN

Chondrocytes from human adult articular healthy cartilage were transfected in primary culture with a plasmid containing two human papilloma virus type 16 (HPV-16) early function genes: E6 and E7, using the highly efficient cationic liposome-mediated (lipofection) procedure. The transfection was verified by reverse transcriptase-polymerase chain reaction analysis of E7 mRNA and by immunofluorence localization of the E7 protein in the cell cytoplasm. The established chondrocyte cell line was examined in monolayer and in two culture conditions that were described to re-induce differentiated characteristics: culturing in a serum-free defined medium supplemented with an insulin-containing serum substitute and seeding on a hyaluronan-based non-woven structured biomaterial. The expression of markers characteristic of cartilage was shown in the mRNA by reverse transcriptase-polymerase chain reaction. Immunohistological staining and Western blotting analysis were performed to evaluate type II collagen synthesis. Proteoglycans deposition was detected by Alcian Blue staining. A Field Emission In Lens Scanning Microscopy was used to look at the morphology of the immortalized cells at very high magnification.

RESULTS

Normal human articular chondrocytes were efficiently transfected leading to the establishment of an immortalized cell line as confirmed by HPV-16 E7 mRNA and protein detection. These cells were able to re-express type II collagen both at mRNA and protein levels under the two defined cultured conditions we used, still maintaining type I collagen expression. Collagen IX mRNA was present only in early primary culture while collagen type X and aggrecan transcripts were always detected. Alcian Blue staining showed a proteoglycan-rich matrix production. The ultrastructural analysis of the immortalized cells revealed that their morphology strictly resembled that of normal chondrocytes.

CONCLUSIONS

The cell line that we obtained may be a useful tool for increasing our knowledge of the genetic and biochemical events involved in the processes of cartilage growth and differentiation. Moreover, it appears to be a suitable model for pharmacological and toxicological studies related to rheumatic diseases relevant to humans.

Formation of cartilage matrix proteins by BMP-transfected murine mesenchymal stem cells encapsulated in a novel class of alginates

Proliferation and differentiation of wild-type, BMP-2 and BMP-4 transfected cells of C3H10T1/2, a mouse mesenchymal stem cell line that can differentiate into chondrocytes, were studied under monolayer (2D-) and encapsulation (3D-) conditions. Cells were encapsulated in a novel class of alginate. The alginate was of clinical grade (CG) because of complete removal of mitogenic and cytotoxic contaminants by chemical means. Compared to commercial alginates used so far for encapsulation it was characterized by ultra-high viscosity (UHV; viscosity of a 0.1% w/v solution of about 20 cP). In contrast to monolayer cultures, proliferation of cells was prevented when the cells were encapsulated in UHV/CG alginate at the same suspension density. As revealed by immunohistochemistry and quantitative RT-PCR, transfected and wild-type monolayer cells showed synthesis of type I collagen after transfer into differentiation medium, while culture in an alginate scaffold resulted in an upregulation of type II collagen and other hyaline cartilage proteins. BMP-4 transfected cells produced considerably more type II collagen than BMP-2 transfected and wild-type cells. BMP-4 transfected cells were also characterized by type I collagen production up to Day 10 and exhibited transient alkaline phosphatase activity levels that were much higher than the peak values observed for the other two cell lines. The coincidence of the ALP peak values with downregulation of type I collagen in BMP-4 transfected cells suggested that C3H10T1/2 cells differentiate into chondrocytes via a chondroprogenitor-like cell.

The role of the extracellular matrix in articular chondrocyte regulation

The in vivo role of the extracellular matrix and the manner in which it interfaces with soluble regulators remains largely unknown. The current study reports the extracellular Type II collagen modulation of transforming growth factor-beta 1-stimulated proliferation, proteoglycan synthesis, messenger ribonucleic acid expression for transforming growth factor-beta 1, and integrin messenger ribonucleic acid expression in articular chondrocytes from adults. This study shows that this cytokine modulation occurs through a mechanism initiated by the attachment of Type II collagen to the beta1-integrin. Transforming growth factor-beta 1 stimulated deoxyribonucleic acid and proteoglycan synthesis in a bimodal fashion. Extracellular Type II collagen increased transforming growth factor-beta 1-stimulated deoxyribonucleic acid and proteoglycan synthesis, aggrecan gene expression as much as 400%, and alpha1(II) procollagen gene expression as much as 180% in a dose-dependent fashion. Heat inactivation of the Type II collagen abrogated the observed effects on deoxyribonucleic acid and proteoglycan synthesis. In contrast to Type II collagen, heat-denatured collagen and bovine serum albumin showed none of the observed effects. The presence of Type II collagen in the alginate bead cultures was found to diminish the messenger ribonucleic acid expression for alpha2 integrin and alter the cellular distribution pattern of the beta1 integrin receptors. Blocking of the beta1-integrin with cyclic-peptides containing the Arg-Gly-Asp sequences and antibodies reduced chondrocyte attachment to Type II collagen by 93%. The physiologic effects shown by the chondrocyte as a result of blocking this attachment to Type II collagen were a significant reduction in transforming growth factor-beta 1-stimulated deoxyribonucleic acid and proteoglycan synthesis. The conclusions elucidate the role played by the extracellular matrix in cytokine-specific regulation of the articular chondrocyte. The authors have shown that extracellular Type II collagen acts through a beta1-integrin mediated mechanism to modulate the chondrocyte response to transforming growth factor-beta 1.

Physiological and cell biological aspects of perfusion culture technique employed to generate differentiated tissues for long term biomaterial testing and tissue engineering

Optimal results in biomaterial testing and tissue engineering under in vitro conditions can only be expected when the tissue generated resembles the original tissue as closely as possible. However, most of the presently used stagnant cell culture models do not produce the necessary degree of cellular differentiation, since important morphological, physiological, and biochemical characteristics disappear, while atypical features arise. To reach a high degree of cellular differentiation and to optimize the cellular environment, an advanced culture technology allowing the regulation of differentiation on different cellular levels was developed. By the use of tissue carriers, a variety of biomaterials or individually selected scaffolds could be tested for optimal tissue development. The tissue carriers are to be placed in perfusion culture containers, which are constantly supplied with fresh medium to avoid an accumulation of harmful metabolic products. The perfusion of medium creates a constant microenvironment with serum-containing or serum-free media. By this technique, tissues could be used for biomaterial or scaffold testing either in a proliferative or in a postmitotic phase, as is observed during natural development. The present paper summarizes technical developments, physiological parameters, cell biological reactions, and theoretical considerations for an optimal tissue development in the field of perfusion culture.

Effects of medium perfusion on matrix production by bovine chondrocytes in three-dimensional collagen sponges

Various culture systems have been used for examining the anabolic and catabolic functions of isolated chondrocytes as well as for tissue engineering purposes. Perfusion or frequent medium change is beneficial for three-dimensional (3D) cultures of many cell types. In this study, bovine articular chondrocytes (bACs) were grown in 3D collagen sponges with or without medium perfusion (0.33 mL/min) for up to 15 days. The influence of medium perfusion was evaluated using markers of cartilage matrix accumulation, synthesis, and gene expression. Metachromatic matrix, collagen type II, and hyaluronan accumulated around the cells within the collagen sponges. Sulfated glycosaminoglycans (S-GAGs) that accumulated in the sponge exposed to nonperfused control were 130% of that in the perfused sponge at day 7. S-GAG accumulation after 15 days in the nonperfused control was 230% more than at day 7 (p < 0.01). (35)S-sulfate incorporation during the final 18 h of culture in the sponge exposed to nonperfusion was 180% greater than that in the perfused sponge (p < 0.01). Quantitative analyses show that at day 7, aggrecan and collagen type II gene expression were 350% and 240% greater, respectively, in the nonperfused culture than in the perfused one. These results indicate that perfused conditions that are beneficial for other cell types inhibit chondrogenesis by articular chondrocytes in 3D culture.

Bovine articular chondrocyte function in vitro depends upon oxygen tension

Articular cartilage is a physiologically hypoxic tissue with a proposed gradient of oxygen tension ranging from about 10% oxygen at the cartilage surface to less than 1% in the deepest layers. The position of the chondrocyte within this gradient may modulate the cell's behavior and phenotype. Moreover, the oxygen gradient is likely to be disturbed during joint diseases in which the pO(2)of the synovial fluid declines which may cause changes in chondrocyte behavior and gene expression. Thus, there is a need to understand the chondrocyte's response to different oxygen tensions. We compared the behavior of bovine articular chondrocytes cultured in alginate beads for 7 days in medium maintained at <0.1, 5, 10 or 20% oxygen. The chondrocytes' survival, differentiation, cell division, viability and matrix production were assessed at each oxygen tension and rRNA and mRNA abundance was measured. Chondrocytes were able to survive under all oxygen tensions for at least 7 days but cells cultured under anoxic conditions were metabolically less active than cells maintained in higher oxygen tensions; this was associated with a decrease in matrix production. In <0.1% oxygen there was a marked decrease in rRNA and mRNA abundance in the cells. There were no differences in cell division or differentiation between any oxygen tensions. These findings indicate that articular chondrocytes can be cultured successfully in the pO(2)range in which they are thought to exist in vivo (5-10% pO(2)) and are fully active under these conditions. Under anoxic conditions (<0.1% pO(2)) function is severely compromised.

Col2-GFP reporter marks chondrocyte lineage and chondrogenesis during mouse skeletal development

Mice were generated in which a Col2-GFP transgene serves as a reporter for the chondrocyte lineage and for chondrogenesis in live embryos and newborn pups. Cells actively engaged in chondrogenesis were identified by confocal optical sectioning within their native environments in embryos and in thick tissue slices. Chondrocytes exhibiting GFP fluorescence were purified from rib cages by high-speed cell sorting of crude cell suspensions. Intensity of fluorescence correlated with biosynthesis of procollagen II in these cells. The use of these mice and their cells provides a novel approach for studying chondrocyte differentiation and chondrogenesis during skeletal development.

Matrix-mixed culture: new methodology for chondrocyte culture and preparation of cartilage transplants

For cartilage engineering a variety of biomaterials were applied for 3-dimensional chondrocyte embedding and transplantation. In order to find a suitable carrier for the in vitro culture of chondrocytes and the subsequent preparation of cartilage transplants we investigated the feasibility of a combination of the well-established matrices fibrin and alginate. In this work human articular chondrocytes were embedded and cultured either in alginate, a mixture of alginate and fibrin, or in a fibrin gel after the extraction of the alginate component (porous fibrin gel) over a period of 30 days. Histomorphological analysis, electron microscopy, and immunohistochemistry were performed to evaluate the phenotypic changes of the chondrocytes, as well as the quality of the newly formed cartilaginous matrix. Our experiments showed that a mixture of 0.6% alginate with 4.5% fibrin promoted sufficient chondrocyte proliferation and differentiation, resulting in the formation of a specific cartilage matrix. Alginate served as a temporary supportive matrix component during in vitro culture and can be easily removed prior to transplantation. The presented tissue engineering method on the basis of a mixed alginate-fibrin carrier offers the opportunity to create stable cartilage transplants for reconstructive surgery.

Differential effects of interleukin-1 and transforming growth factor beta on the synthesis of small proteoglycans by rabbit articular chondrocytes cultured in alginate beads as compared to monolayers

Small proteoglycans (PGs) are supposed to play great roles in the assembly of cartilage matrix but the influence of cytokines and growth factors on their synthesis by articular chondrocytes is largely unknown. We investigated whether IL-1 and TGFbeta1 influence the production of small leucine-rich proteoglycans by chondrocytes cultured in a three-dimensional gel, as compared to the common monolayer system. Rabbit articular chondrocytes were cultured in alginate beads for 14 days or as monolayers for 7 days. The effect of 2 ng/ml IL-1beta or TGFbeta1 during the last two days in culture was determined, after [35S]methionine labeling over the last 24 h. Cell-associated and further-removed matrix compartments were separated by centrifugation after sodium citrate/EDTA treatment of alginate beads whereas medium and cell-layer fractions were isolated from monolayer cultures. Total newly synthesized PGs were first isolated by anion-exchange chromatography and the small PGs were further separated from aggrecans by gel-filtration (Sepharose CL-4B) and analyzed by SDS-polyacrylamide gel electrophoresis (SDS-PAGE). Addition of TGFbeta1 resulted in an overall rise in neosynthesized small PG content in both culture systems. However, TGFbeta1 significantly increased to the same extent the percentage of small PGs laid down in the cell-associated and the further-removed matrix compartments of the beads culture (+60%) whereas it augmented the content of small PGs in the medium (+40%) and reduced that of the cell fraction (-35%) in the monolayer culture. By adding IL-1, the amount of total newly synthesized small PGs was decreased in monolayers while it increased in alginate beads. IL-1 was also shown to change the relative distribution of these molecules in the monolayer system in contrast to the alginate beads culture where the proportions were not significantly altered. Electrophoretic analysis of the 35S-labeled small PGs-containing fractions confirmed these effects at the level of the 45-50 kDa-related core proteins. This study demonstrates that TGFbeta and IL-1 differently influence small PG synthesis of rabbit articular chondrocytes depending on whether they are cultured in alginate beads or in monolayers. Moreover, the regulation of small PG expression appears to be different from that of high-molecular weight aggrecans. As these small molecules are playing major roles in matrix assembly and growth factor regulation, the data may have great relevance to the pathogenesis of osteoarthritis and repair of articular cartilage lesions.

Re-expression of differentiated proteoglycan phenotype by dedifferentiated human chondrocytes during culture in alginate beads

The proteoglycans (PGs) synthesised by normal human articular chondrocytes and a chondrocyte cell line cultured in monolayer and alginate beads were compared. Chondrocytes became dedifferentiated after serial subcultures in monolayer, exhibited a fibroblastic morphology and synthesised a large proportion of lower molecular weight, dermatan sulphate containing PGs. When transferred into alginate beads, the cells quickly regained their spherical shape and actively incorporated [3H]thymidine and [35S]sulphate during 70 days of culture. This resulted in a continuous increase in their DNA content and a rapid deposition of PGs for the first 25 days of culture, which then remained stable. Immediately after dedifferentiated chondrocytes were encapsulated into alginate beads, they began to synthesise a population of PGs with normal monomer size and an increased ability to form aggregates. The monomer size of newly synthesised PGs remained unchanged during extended periods of culture, but their ability to form aggregates and the ratios of chondroitin-6-sulphate to chondroitin-4-sulphate in their glycosaminoglycan chains gradually increased for the first 25 days before reaching normal values. Parallel experiments with HCS-2/8 cells, derived from a human chondrosarcoma, showed that they followed a similar pattern of development in alginate culture. The ability of their newly synthesised PGs to form aggregates increased with time and their sulphation pattern also gradually became normal. These results showed that culture in alginate promoted redifferentiation of dedifferentiated articular chondrocytes and assisted differentiation of HCS-2/8 chondrocytes. However, complete redifferentiation took a period of several weeks, after which synthesis of normal aggregating PGs was maintained.

Cartilage formation by fetal rat chondrocytes cultured in alginate beads: a proposed model for investigating tissue-biomaterial interactions

Chondrocytes from 21-day-old rat fetal nasal cartilage were cultured in alginate beads for up to 20 days. It was found that chondrocytes retained their spherical shape and typical chondrocytic appearance. During the culture time, chondrocytes underwent differentiation, as demonstrated by the alkaline phosphatase-specific activity and rate of proteoglycan synthesis. Morphological data confirmed chondrocyte differentiation with the appearance of hypertrophic chondrocytes scattered in the alginate gel and a dense extracellular matrix containing filamentous structures and matrix vesicles. In addition, Northern blot analysis performed on day 8 of culture showed that chondrocytes cultured in alginate beads expressed type II collagen mRNA. The alginate bead method also appeared to be suitable for testing biomaterials, and the ready dissolution of the alginate beads by chelating agents provided a simple means for the rapid recovery of encapsulated chondrocytes. Powdered glass-ceramic particles entrapped in the alginate gel were colonized by chondrocytes, which then proliferated and formed a tissue similar to a true calcified cartilaginous structure. These results indicate that the alginate system represents a relevant model for studies of chondrogenesis and endochondral ossification. Furthermore, the encapsulation method could prove useful for studies of tissue-biomaterial interactions in an in vitro environment which more closely mirrors the cartilage matrix than other culture methods.

Culture of organized cell communities

Cells cultured in vitro will tend to retain their differentiated phenotype under conditions that resemble their natural in vivo environment, for example, when cultured on polymer scaffolds in tissue culture bioreactors. In this chapter, we define organized cell communities as three-dimensional in vitro grown cell-polymer constructs that display important structural and functional features of the natural tissue. We review representative studies in which the research goal was to culture organized cell communities resembling cartilage, bone, skeletal muscle or cardiac-like tissue. These constructs can potentially serve as tissue equivalents for in vivo transplantation or as a model system for the in vitro testing of cell and tissue-level responses to molecular, mechanical or genetic manipulations.

Expression of decorin and biglycan by rabbit articular chondrocytes. Effects of cytokines and phenotypic modulation

In this study, the levels of mRNAs coding for aggrecan, decorin and biglycan in rabbit articular chondrocytes were investigated, using both monolayer and 3D-alginate cultures treated with TGF-beta 1 and IL-1 beta. The cells were shown to express higher amounts of proteoglycan messages, specially the aggrecan, in gels than in monolayers. TGF-beta 1 increased aggrecan mRNA in both systems, whereas biglycan message was elevated only in alginate. It markedly decreased decorin expression in monolayer, either in primary or passaged cultures. In contrast, IL-1 beta had a weak inhibitory effect on both decorin and biglycan expression. Subculturing induced a dramatic decrease of aggrecan mRNA, while that of decorin augmented. Biglycan expression transiently increased after two passages, whereas it declined in further subcultures. Passaged chondrocytes transferred to alginate re-expressed high levels of aggrecan, decorin and biglycan. The data point to the influence of morphology, proliferative state and environment of the articular chondrocytes on their biosynthetic responses to cytokines. Although these immature cells do not fully reflect the adult chondrocytes present in the cartilage, this study may help in understanding the behaviour of these cells in osteoarticular diseases, where the surrounding extracellular matrix is profoundly altered.

Effect of type II collagen in chondrocyte response to TGF-beta 1 regulation

The in vivo role of the extracellular matrix and the manner in which it interfaces with soluble regulators remains unknown. This study reports the modulation by extracellular type II collagen of TGF-beta 1-stimulated DNA synthesis, proteoglycan synthesis, and mRNA expression for alpha 1(II) procollagen and aggrecan core protein in the adult articular chondrocyte. Bovine chondrocytes were isolated and resuspended in alginate beads which contained increasing amounts of type II collagen from 0 to 1.5% (w/v). Cultures were maintained for 7 days in basal, DMEM, TGF-beta 1 (10 ng/ml), or FBS (10%) supplemented medium. DNA and proteoglycan synthesis were determined by radiotracer incorporation. The relative amounts of mRNA were analyzed by Northern blot analysis. Exogenous collagen increased DNA synthesis in all culture conditions beginning at concentrations of 0.75% (w/v). We observed that extracellular type II collagen augments both TGF-beta 1 stimulated increases of aggrecan gene expression up to 400% and alpha 1(II) procollagen gene expression up to 180% in a dose-dependent fashion. This is distinct from cultures which were either basal or FBS supplemented medium which lacked a dose-dependent change in aggrecan gene expression and demonstrated a decrease in alpha 1(II) procollagen gene expression. Exogenous collagen above 0.75% (w/v) increased proteoglycan synthesis significantly in FBS and TGF-beta 1-stimulated cultures but not in basal cultures. We have demonstrated that the alterations in gene expression that occur in response to TGF-beta 1 are modulated by extracellular type II collagen. This modulation is possible through both transcriptional and posttranscriptional regulatory mechanisms.

Culture of chondrocytes in alginate gel: variations in conditions of gelation influence the structure of the alginate gel, and the arrangement and morphology of proliferating chondrocytes

Sodium alginate, which gels in the presence of calcium ions, is commonly used for culture of anchorage-independent cells, such as chondrocytes. Normally, the gel appears microscopically homogeneous but, depending on the conditions of gelation, it may contain a varying number of small channels that extend inward from the surface. We have examined the influence of these channels on the morphology of cultured chondrocytes entrapped in alginate beads. Growth-plate or articular chondrocytes cultured in alginate normally proliferate and form rounded cell clusters but, in alginate beads containing numerous channels, many chondrocytes become aligned and form columns similar to those in the growth plate in vivo. As the pattern of cellular growth and morphology in alginate is profoundly influenced by the presence of channels in the gel, further studies were conducted to determine what specific conditions of gelation affect their formation. The channels are especially numerous when both the alginate and the gelling solutions lack sodium ions or other monovalent cations. The channels are cavities in the gel formed by particulate blocking of the rapid diffusion of calcium ions from the gelling solution into the boundary of the calcium alginate solution, and hence they extend inward from cells at the surface of the alginate gel. An understanding of the conditions under which these channels develop makes it possible either to avoid their formation or, alternatively, to enhance the number of channels in order to encourage proliferating cells to grow in radial columns, rather than in a less organized pattern characteristic of most culture systems.

Extracellular collagen modulates the regulation of chondrocytes by transforming growth factor-beta 1

This article describes the modulation, by extracellular collagen, of DNA and proteoglycan synthesis in articular chondrocytes stimulated with transforming growth factor-beta 1. Type-I and type-II collagen, heat-denatured type-II collagen, and bovine serum albumin were each incorporated into alginate in increasing concentrations. Bovine articular chondrocytes were isolated and were resuspended in the alginate, yielding alginate beads with final extracellular protein concentrations of 0-1.5% (wt/vol) for the collagens and 0-2.5% (wt/vol) for bovine serum albumin. Cultures of beads were maintained for 7 days in basal Dulbecco's modified Eagle medium or in medium supplemented with 10 ng/ml transforming growth factor-beta 1. Subsequently, the synthesis of DNA and proteoglycan was measured by radiolabel-incorporation methods with [35S]sulfate and [3H]thymidine, and the values were normalized to the DNA content. Transforming growth factor-beta 1 stimulated the synthesis of both DNA and proteoglycan in a bimodal fashion. The presence of extracellular type-II collagen increased the rate of DNA and proteoglycan synthesis in a dose-dependent fashion in cultures stimulated by transforming growth factor-beta 1, whereas heat-inactivated type-II collagen abrogated the effects observed with type-II collagen for synthesis of both DNA and proteoglycan. In contrast, the presence of extracellular type-I collagen caused a dose-dependent inhibition of synthesis of both DNA and proteoglycan in cultures stimulated with transforming growth factor-beta 1. Extracellular bovine serum albumin brought about a limited increase in synthesis rates, presumably by blocking nonspecific cytokine binding. These results suggest that type-II collagen has a specific role in chondrocyte regulation and serves to mediate the response of chondrocytes to transforming growth factor-beta 1.

Differential expression of membrane-anchored proteoglycans in rabbit articular chondrocytes cultured in monolayers and in alginate beads. Effect of transforming growth factor-beta 1

Cell-surface proteoglycans (PGs) were extracted with Triton X-100 from rabbit articular chondrocytes cultured in monolayers and in alginate beads. They were first purified on DEAE-Trisacryl columns and the proportion of hydrophobic PGs was determined by both Octyl-Sepharose chromatography and partitioning in Triton X-114. These two methods revealed that the proportion of hydrophobic PGs was higher in monolayer culture system as compared to alginate beads (24 and 15%, respectively). Characterization of the PGs by Sepharose CL 6B gel filtration followed by electrophoresis indicated that the PGs isolated from monolayers were composed of three chondroitin sulfate (CS) PGs (core proteins of 180, 100 and 50 kDa) and a heparan sulfate (HS) PG (core protein of 60 kDa). In the alginate system. CSPGs with core proteins of 180, 45 and 32 kDa were observed, but no HSPG was present. In parallel, the effect of TGF-beta on the distribution of membrane-associated PGs was studied. The results showed that the synthesis of cell-surface PGs was stimulated by TGF-beta in monolayers whereas it was inhibited in alginate beads, but the amount of hydrophobic PGs was not altered by the growth factor. These data clearly indicate that TGF-beta induces a differential expression of the PG families present at the cell surface. Taken together, the results reveal the complex regulation of cell-surface PG distribution, which obviously depends on the culture method used and suggest that rabbit articular chondrocytes may differentially respond to extracellular ligands according to their morphological state and environment.

Chondrocyte differentiation

Data obtained while investigating growth plate chondrocyte differentiation during endochondral bone formation both in vivo and in vitro indicate that initial chondrogenesis depends on positional signaling mediated by selected homeobox-containing genes and soluble mediators. Continuation of the process strongly relies on interactions of the differentiating cells with the microenvironment, that is, other cells and extracellular matrix. Production of and response to different hormones and growth factors are observed at all times and autocrine and paracrine cell stimulations are key elements of the process. Particularly relevant is the role of the TGF-beta superfamily, and more specifically of the BMP subfamily. Other factors include retinoids, FGFs, GH, and IGFs, and perhaps transferrin. The influence of local microenvironment might also offer an acceptable settlement to the debate about whether hypertrophic chondrocytes convert to bone cells and live, or remain chondrocytes and die. We suggest that the ultimate fate of hypertrophic chondrocytes may be different at different microanatomical sites.