Motile chondrocytes from newborn calf: migration properties and synthesis of collagen II

From polarity to pathology: Decoding the role of cell orientation in osteoarthritis

Cell polarity refers to the orientation of tissue and organelles within a cell and the direction of its function. It is one of the most critical characteristics of metazoans. The development, growth, and functional tissue distribution are closely related to holistic tissue or organ homeostasis. However, the connection between cell polarity and osteoarthritis (OA) is less well-known. In OA, multiple chondrocyte clusters and tissue disorganisation can be observed in the degraded cartilage tissue. The excessive upregulation of the planar cell polarity (PCP) signalling pathway leads to the loss of cell polarity and organisation in OA progression and aetiology. Recent research has become increasingly aware of the importance of cell polarity and its correlation with OA. Several cell polarity-related treatments have shed light on OA. A thorough understanding of cell polarity and OA would provide more insights for future investigations to treat this worldwide disease.

The translational potential of this article

Understanding cell polarity, associated signalling pathways, organelle changes, and cell movement in the development of OA could lead to advances in precision medicine and enhanced treatment strategies for OA patients.

Characterization of Migratory Cells From Bioengineered Bovine Cartilage in a 3D Co-culture Model

BACKGROUND

Chondrocyte migration in native cartilage is limited and has been implicated as one of the reasons for the poor integration of native implants. Through use of an in vitro integration model, it has previously been shown that cells from bioengineered cartilage can migrate into the native host cartilage during integration. Platelet-rich plasma (PRP) treatment further enhanced integration of bioengineered cartilage to native cartilage in vitro. However, it is not known how PRP treatment of the bioengineered construct promotes this.

HYPOTHESIS

PRP supports cell migration from bioengineered cartilage and these migratory cells have the ability to accumulate cartilage-like matrix.

STUDY DESIGN

Controlled laboratory study.

METHODS

Osteochondral-like constructs were generated by culturing primary bovine chondrocytes on the top surface of a porous bone substitute biomaterial composed of calcium polyphosphate. After 1 week in culture, the constructs were submerged in PRP and placed adjacent, but 2 mm distant, to a native bovine osteochondral plug in a co-culture model for 2 weeks. Cell migration was monitored using phase-contrast imaging. Cell phenotype was determined by evaluating the gene expression of matrix metalloprotease 13 (MMP-13), Ki67, and cartilage matrix molecules using quantitative polymerase chain reaction. When tissue formed, it was assessed by histology, immunohistochemistry, and quantification of matrix content.

RESULTS

PRP treatment resulted in the formation of a fiber network connecting the bioengineered cartilage and native osteochondral plug. Cells from both the bioengineered cartilage and the native osteochondral tissue migrated onto the PRP fibers and formed a tissue bridge after 2 weeks of culture. Migratory cells on the tissue bridge expressed higher levels of collagen types II and I (COL2, COL1), Ki67 and MMP-13 mRNA compared with nonmigratory cells in the bioengineered cartilage. Ki67 and MMP-13-positive cells were found on the edges of the tissue bridge. The tissue bridge accumulated COL1 and COL2 and aggrecan and contained comparable collagen and glycosaminoglycan content to the bioengineered cartilage matrix. The tissue bridge did not reliably develop in the absence of cells from the native osteochondral plug.

CONCLUSION

Bioengineered cartilage formed by bovine chondrocytes contains cells that can migrate on PRP fibers and form cartilaginous tissue.

CLINICAL RELEVANCE

Migratory cells from bioengineered cartilage may promote cartilage integration. Further studies are required to determine the role of migratory cells in integration in vivo.

Increased migratory activity and cartilage regeneration by superficial-zone chondrocytes in enzymatically treated cartilage explants

Background

Limited chondrocyte migration and impaired cartilage-to-cartilage healing is a barrier in cartilage regenerative therapy. Collagenase treatment and delivery of a chemotactic agent may play a positive role in chondrocyte repopulation at the site of cartilage damage. This study evaluated chondrocyte migratory activity after enzymatic treatment in cultured cartilage explant. Differential effects of platelet-derived growth factor (PDGF) dimeric isoforms on the migratory activity were investigated to define major chemotactic factors for cartilage.

Methods

Full-thickness cartilage (4-mm³ blocks) were harvested from porcine femoral condyles and subjected to explant culture. After 15 min or 60 min of actinase and collagenase treatments, chondrocyte migration and infiltration into a 0.5-mm cartilage gap was investigated. Cell morphology and lubricin, keratan sulfate, and chondroitin 4 sulfate expression in superficial- and deep-zone chondrocytes were assessed. The chemotactic activities of PDGF-AA, −AB, and -BB were measured in each zone of chondrocytes, using a modified Boyden chamber assay. The protein and mRNA expression and histological localization of PDGF-β were analyzed by western blot analysis, real-time reverse transcription polymerase chain reaction (RT-PCR), and immunohistochemistry, and results in each cartilage zone were compared.

Results

Superficial-zone chondrocytes had higher migratory activity than deep-zone chondrocytes and actively bridged the cartilage gap, while metachromatic staining by toluidine blue and immunoreactivities of keratan sulfate and chondroitin 4 sulfate were detected around the cells migrating from the superficial zone. These superficial-zone cells with weak immunoreactivity for lubricin tended to enter the cartilage gap and possessed higher migratory activity, while the deep-zone chondrocytes remained in the lacuna and exhibited less migratory activity. Among PDGF isoforms, PDGF-AB maximized the degree of chemotactic activity of superficial zone chondrocytes. Increased expression of PDGF receptor-β was associated with higher migratory activity of the superficial-zone chondrocytes.

Conclusions

In enzymatically treated cartilage explant culture, chondrocyte migration and infiltration into the cartilage gap was higher in the superficial zone than in the deep zone. Preferential expression of PDGF receptor-β combined with the PDGF-AB dimeric isoform may explain the increased migratory activity of the superficial-zone chondrocytes. Cells migrating from superficial zone may contribute to cartilage regeneration.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-022-05210-2.

Viscoelastic Chondroitin Sulfate and Hyaluronic Acid Double-Network Hydrogels with Reversible Cross-Links

Viscoelastic hydrogels are gaining interest as they possess necessary requirements for bioprinting and injectability. By means of reversible, dynamic covalent bonds, it is possible to achieve features that recapitulate the dynamic character of the extracellular matrix. Dually cross-linked and double-network (DN) hydrogels seem to be ideal for the design of novel biomaterials and bioinks, as a wide range of properties required for mimicking advanced and complex tissues can be achieved. In this study, we investigated the fabrication of chondroitin sulfate/hyaluronic acid (CS/HA)-based DN hydrogels, in which two networks are interpenetrated and cross-linked with the dynamic covalent bonds of very different lifetimes. Namely, Diels-Alder adducts (between methylfuran and maleimide) and hydrazone bonds (between aldehyde and hydrazide) were chosen as cross-links, leading to viscoelastic hydrogels. Furthermore, we show that viscoelasticity and the dynamic character of the resulting hydrogels could be tuned by changing the composition, that is, the ratio between the two types of cross-links. Also, due to a very dynamic nature and short lifetime of hydrazone cross-links (∼800 s), the DN hydrogel is easily processable (e.g., injectable) in the first stages of gelation, allowing the material to be used in extrusion-based 3D printing. The more long-lasting and robust Diels-Alder cross-links are responsible for giving the network enhanced mechanical strength and structural stability. Being highly charged and hydrophilic, the cross-linked CS and HA enable a high swelling capacity (maximum swelling ratio ranging from 6 to 12), which upon confinement results in osmotically stiffened constructs, able to mimic the mechanical properties of cartilage tissue, with the equilibrium moduli ranging from 0.3 to 0.5 MPa. Moreover, the mesenchymal stromal cells were viable in the presence of the hydrogels, and the effect of the degradation products on the macrophages suggests their safe use for further translational applications. The DN hydrogels with dynamic covalent cross-links hold great potential for the development of novel smart and tunable viscoelastic materials to be used as biomaterial inks or bioinks in bioprinting and regenerative medicine.

Chondroitinase ABC Enhances Integration of Self-Assembled Articular Cartilage, but Its Dosage Needs to Be Moderated Based on Neocartilage Maturity

OBJECTIVE

To enhance the in vitro integration of self-assembled articular cartilage to native articular cartilage using chondroitinase ABC.

DESIGN

To examine the hypothesis that chondroitinase ABC (C-ABC) integration treatment (C-ABC_int) would enhance integration of neocartilage of different maturity levels, this study was conducted in 2 phases. In phase I, the impact on integration of 2 treatments, TCL (TGF-β1, C-ABC, and lysyl oxidase like 2) and C-ABC_int, was examined via a 2-factor, full factorial design. In phase II, construct maturity (2 levels) and C-ABC_int concentration (3 levels) were the factors in a full factorial design to determine whether the effective C-ABC_int dose was dependent on neocartilage maturity level. Neocartilages formed or treated per the factors above were placed into native cartilage rings, cultured for 2 weeks, and, then, integration was studied histologically and mechanically. Prior to integration, in phase II, a set of treated constructs were also assayed to provide a baseline of properties.

RESULTS

In phase I, C-ABC_int and TCL treatments synergistically enhanced interface Young's modulus by 6.2-fold (P = 0.004) and increased interface tensile strength by 3.8-fold (P = 0.02) compared with control. In phase II, the interaction of the factors C-ABC_int and construct maturity was significant (P = 0.0004), indicating that the effective C-ABC_int dose to improve interface Young's modulus is dependent on construct maturity. Construct mechanical properties were preserved regardless of C-ABC_int dose.

CONCLUSIONS

Applying C-ABC_int to neocartilage is an effective integration strategy with translational potential, provided its dose is calibrated appropriately based on implant maturity, that also preserves implant biomechanical properties.

The Modified Hedgehog Technique to Repair Pure Chondral Shear-off Lesions in the Pediatric Knee

OBJECTIVE

The paediatric knee is prone to pure chondral shear-off lesions due to the developing osteochondral unit. Refixation of the chondral fragment is commonly done using metalwork or absorbable biomaterials. Both fixation methods come with biomaterial-related drawbacks. Earlier work on chondral allografts for cartilage repair in adults has shown successful osteochondral integration when the chondral allograft is treated with multiple incisions and then glued to the subchondral bone using fibrin glue. This is commonly referred to as the "hedgehog technique." This study investigates the feasibility of a modification of the hedgehog technique in autologous cartilage to repair shear-off lesions in children.

DESIGN

Three consecutive patients (aged 11, 12, and 14 years) with shear-off chondral fragments of 2, 5, and 8 cm² were treated using this modified hedgehog technique. The calcified side of the chondral fragments were multiply incised and trimmed obliquely for an interlocking fit in the defect site. Fibrin glue and, if indicated sutures, were applied to fix the fragment to the defect. In 1 patient, an anterior cruciate ligament (ACL) repair was also performed. Patients were evaluated clinically and by magnetic resonance imaging (MRI) up to 12 months postoperatively.

RESULTS

Twelve months after surgery, all patients reported no pain and showed complete return to sport and full range of motion. MRI showed no signs of fragment loosening.

CONCLUSIONS

The modified hedgehog technique is a feasible treatment option to repair pure chondral shear-off lesions in the paediatric knee. This was the first time this technique was used in autografting.

Probing Multicellular Tissue Fusion of Cocultured Spheroids-A 3D-Bioassembly Model

While decades of research have enriched the knowledge of how to grow cells into mature tissues, little is yet known about the next phase: fusing of these engineered tissues into larger functional structures. The specific effect of multicellular interfaces on tissue fusion remains largely unexplored. Here, a facile 3D-bioassembly platform is introduced to primarily study fusion of cartilage-cartilage interfaces using spheroids formed from human mesenchymal stromal cells (hMSCs) and articular chondrocytes (hACs). 3D-bioassembly of two adjacent hMSCs spheroids displays coordinated migration and noteworthy matrix deposition while the interface between two hAC tissues lacks both cells and type-II collagen. Cocultures contribute to increased phenotypic stability in the fusion region while close initial contact between hMSCs and hACs (mixed) yields superior hyaline differentiation over more distant, indirect cocultures. This reduced ability of potent hMSCs to fuse with mature hAC tissue further underlines the major clinical challenge that is integration. Together, this data offer the first proof of an in vitro 3D-model to reliably study lateral fusion mechanisms between multicellular spheroids and mature cartilage tissues. Ultimately, this high-throughput 3D-bioassembly model provides a bridge between understanding cellular differentiation and tissue fusion and offers the potential to probe fundamental biological mechanisms that underpin organogenesis.

Exploration of changes in spatial chondrocyte organisation in human osteoarthritic cartilage by means of 3D imaging

Using two-dimensional top-down view microscopy, researchers have recently described chondrocytes as being spatially arranged in distinct patterns such as strings, double strings, and small and large clusters. Because of the seeming association of these changes with tissue degeneration, they have been proposed as an image-based biomarker for early osteoarthritis (OA) staging. The aim of our study was to investigate the spatial arrangement of chondrocytes in human articular cartilage in a 3D fashion and to evaluate the 3D changes of these patterns in the context of local tissue destruction. Decalcified femoral condyle resections from the load-bearing area were analysed in 3D for their spatial chondrocyte organisation by means of fluorescence microscopy and synchrotron-radiation micro-computed tomography (SR-µCT). In intact cartilage chondrocyte strings can be found in the superficial, transitional and deep zones. The proposed pattern changes accompanying tissue destruction could be located not just along the surface but also through all layers of cartilage. Each spatial pattern was characterised by a different cellular density (the only exception being between single and double strings with p = 0.062), with cellular density significantly increasing alongside the increase in local tissue degeneration as defined by the chondrocyte patterns. We can thus corroborate that the proposed cellular spatial changes are a three-dimensional function of local tissue degeneration, underlining their relevance as an image-based biomarker for the early diagnosis and description of OA.

Clinical trial registration number: Project number of the ethics committee of the University of Tübingen:171/2014BO2.

Phylogeny and chemistry of biological mineral transport

Three physiologically mineralizing tissues - teeth, cartilage and bone - have critical common elements and important evolutionary relationships. Phylogenetically the most ancient densely mineralized tissue is teeth. In jawless fishes without skeletons, tooth formation included epithelial transport of phosphates, a process echoed later in bone physiology. Cartilage and mineralized cartilage are skeletal elements separate from bone, but with metabolic features common to bone. Cartilage mineralization is coordinated with high expression of tissue nonspecific alkaline phosphatase and PHOSPHO1 to harvest available phosphate esters and support mineralization of collagen secreted locally. Mineralization in true bone results from stochastic nucleation of hydroxyapatite crystals within the cross-linked collagen fibrils. Mineral accumulation in dense collagen is, at least in major part, mediated by amorphous aggregates - often called Posner clusters - of calcium and phosphate that are small enough to diffuse into collagen fibrils. Mineral accumulation in membrane vesicles is widely suggested, but does not correlate with a definitive stage of mineralization. Conversely mineral deposition at non-physiologic sites where calcium and phosphate are adequate has been shown to be regulated in large part by pyrophosphate. All of these elements are present in vertebrate bone metabolism. A key biological element of bone formation is an epithelial-like cellular organization which allows control of phosphate, calcium and pH during mineralization.

Construction and Evaluation of Osteochondral-Like Tissue Using Chondrocyte Sheet and Cancellous Bone

The manipulation of human chondrocyte sheets in target areas frequently results in their tearing because they are thin and fragile. In this study, human cancellous bones were used as a supporting material to create chondrocyte sheet-cancellous bone tissues, and their properties were evaluated. Using cell sheet technology, human chondrocytes were constructed into triple-layered chondrocyte sheets that displayed chondrogenic properties. After transferring the chondrocyte sheets onto cancellous bones, the top area of the chondrocyte sheet-cancellous bone tissues exhibited a smooth surface topography without cell sheet floating within 7 days of culture. The immunofluorescence staining of collagen type II (COL2A1) and fibronectin (FN1) was also performed and examined. Using the shotgun proteomic analysis, the proteins associated with cell adhesion, extracellular matrix (ECM) organization, cell-substrate junction assembly, and cell adhesion mediated by integrin were observed in the chondrocyte sheets, cancellous bones, and chondrocyte sheet-cancellous bone tissues. Three integrin members, including integrin β4 (ITGB4), ITGB6, and ITGB8, were found in the chondrocyte sheets. Only ITGB8 was found in the chondrocyte sheets and chondrocyte sheet-cancellous bone tissues. During 48 h, the mean velocity of the individual cell migration was low, which did not affect the structure and chondrogenic properties of the chondrocyte sheets. Staining of the filamentous actin (F-actin) cytoskeleton in the migratory cells also provided a better understanding of the dynamic communication between the cell cytoskeleton and adhesion molecules through ITGB8, which may play a key role in the attachment of the chondrocyte sheets and the synthesis of the cartilage ECM. Therefore, we suggest that cancellous bone could be used as a supporting material to construct chondrocyte sheet-cancellous bone tissues for potential treatment of osteochondral lesions. Impact Statement We proposed a method to construct an osteochondral-like tissue by placing human chondrocyte sheets onto cancellous bone. The stationary chondrocyte sheets and the low mean velocity of the individual cell migration on the cancellous bone with the expression of COL2A1 indicated that the cancellous bone served as an appropriate supporting material. Moreover, the cellular mechanism for the adhesion of the chondrocyte sheets on the cancellous bone based on ITGB8-mediated adhesion through the rearrangement of filamentous actin provided a better understanding to improve the construction of osteochondral-like tissues, and to predict the repair mechanism in osteoarthritis therapy.

Extracellular Nucleotides Selectively Induce Migration of Chondrocytes and Expression of Type II Collagen

The migration of chondrocytes from healthy to injured tissues is one of the most important challenges during cartilage repair. Additionally, maintenance of the chondrogenic phenotype remains another limitation, especially during monolayer culture in vitro. Using both the differentiated and undifferentiated chondrogenic ATDC5 cell line, we showed that extracellular nucleotides are able to increase the migration rate of chondrocytes without affecting their chondrogenic phenotype. We checked the potency of natural nucleotides (ATP, ADP, UTP, and UDP) as well as their stable phosphorothioate analogs, containing a sulfur atom in the place of one nonbridging oxygen atom in a phosphate group. We also detected P2y1, P2y2, P2y4, P2y6, P2y12, P2y13, and P2y14 mRNA transcripts for nucleotide receptors, demonstrating that P2y1 and P2y13 are highly upregulated in differentiated ATDC5 cells. We showed that ADPβS, UDPβS, and ADP are the best stimulators of migration of differentiated chondrocytes. Additionally, ADP and ADPβS positively affected the expression of type II collagen, a structural component of the cartilage matrix.

Chondrocyte colonisation of a tissue-engineered cartilage substitute under a mechanical stimulus

Cell-free collagen scaffolds as cartilage substitute for small focal defects show promising results in first clinical studies. However, chondrocyte migration between scaffolds and the colonisation process of a cell-free implant is yet to be fully understood. We here focus on mechanobiological interdependencies between cell migration and mechanical stimulus in a 3D environment. We develop an in vitro model composed of a human chondrocyte-seeded collagen base and adjacent cell-free collagen type I scaffolds of varying collagen concentrations. Constructs are either cultured statically or dynamically under the influence of a physiological compression (0.5Hz, 0.5% initial strain). After 20 days we identify vital chondrocytes inside all collagen implants, proving that chondrocytes migrated from the underlying scaffold into the implants. Chondrocytes have not colonised the entire sample and are predominantly found in the bottom of the implant. In static culture conditions, a nearly equal cell number is found inside of all collagen scaffolds. In dynamic culture, the total amount of cells is increased by 30% to 320%, with the highest population in a commercial implant. Differences in cell population between the materials in dynamic culturing can be referred to differences in mechanical properties of the scaffolds, such as strain-rate insensitivity fostering the colonisation process.

Emerging therapies for cartilage regeneration in currently excluded 'red knee' populations

The field of articular cartilage repair has made significant advances in recent decades; yet current therapies are generally not evaluated or tested, at the time of pivotal trial, in patients with a variety of common comorbidities. To that end, we systematically reviewed cartilage repair clinical trials to identify common exclusion criteria and reviewed the literature to identify emerging regenerative approaches that are poised to overcome these current exclusion criteria. The term “knee cartilage repair” was searched on clinicaltrials.gov. Of the 60 trials identified on initial search, 33 were further examined to extract exclusion criteria. Criteria excluded by more than half of the trials were identified in order to focus discussion on emerging regenerative strategies that might address these concerns. These criteria included age (<18 or >55 years old), small defects (<1 cm²), large defects (>8 cm²), multiple defect (>2 lesions), BMI >35, meniscectomy (>50%), bilateral knee pathology, ligamentous instability, arthritis, malalignment, prior repair, kissing lesions, neurologic disease of lower extremities, inflammation, infection, endocrine or metabolic disease, drug or alcohol abuse, pregnancy, and history of cancer. Finally, we describe emerging tissue engineering and regenerative approaches that might foster cartilage repair in these challenging environments. The identified criteria exclude a majority of the affected population from treatment, and thus greater focus must be placed on these emerging cartilage regeneration techniques to treat patients with the challenging “red knee”.

Polymeric mesh and insulin-like growth factor 1 delivery enhance cell homing and graft-cartilage integration

Cartilage injury, such as full-thickness lesions, predisposes patients to the premature development of osteoarthritis, a degenerative joint disease. While surgical management of cartilage lesions has improved, long-term clinical efficacy has stagnated, owing to the lack of hyaline cartilage regeneration and inadequate graft-host integration. This study tests the hypothesis that integration of cartilage grafts with native cartilage can be improved by enhancing the migration of chondrocytes across the graft-host interface via the release of chemotactic factor from a degradable polymeric mesh. To this end, a polylactide-co-glycolide/poly-ε-caprolactone mesh was designed to localize the delivery of insulin-like growth factor 1 (IGF-1), a well-established chondrocyte attractant. The release of IGF-1 (100 ng/mg) enhanced cell migration from cartilage explants, and the mesh served as critical structural support for cell adhesion, growth, and production of a cartilaginous matrix in vitro, which resulted in increased integration strength compared with mesh-free repair. Further, this neocartilage matrix was structurally contiguous with native and grafted cartilage when tested in an osteochondral explant model in vivo. These results demonstrate that this combined approach of a cell homing factor and supportive matrix will promote cell-mediated integrative cartilage repair and improve clinical outcomes of cartilage grafts in the treatment of osteoarthritis.

Osteochondral Tissue Regeneration Using a Tyramine-Modified Bilayered PLGA Scaffold Combined with Articular Chondrocytes in a Porcine Model

Repairing damaged articular cartilage is challenging due to the limited regenerative capacity of hyaline cartilage. In this study, we fabricated a bilayered poly (lactic-co-glycolic acid) (PLGA) scaffold with small (200–300 μm) and large (200–500 μm) pores by salt leaching to stimulate chondrocyte differentiation, cartilage formation, and endochondral ossification. The scaffold surface was treated with tyramine to promote scaffold integration into native tissue. Porcine chondrocytes retained a round shape during differentiation when grown on the small pore size scaffold, and had a fibroblast-like morphology during transdifferentiation in the large pore size scaffold after five days of culture. Tyramine-treated scaffolds with mixed pore sizes seeded with chondrocytes were pressed into three-mm porcine osteochondral defects; tyramine treatment enhanced the adhesion of the small pore size scaffold to osteochondral tissue and increased glycosaminoglycan and collagen type II (Col II) contents, while reducing collagen type X (Col X) production in the cartilage layer. Col X content was higher for scaffolds with a large pore size, which was accompanied by the enhanced generation of subchondral bone. Thus, chondrocytes seeded in tyramine-treated bilayered scaffolds with small and large pores in the upper and lower parts, respectively, can promote osteochondral regeneration and integration for articular cartilage repair.

Stiffening by Osmotic Swelling Constraint in Cartilage-Like Cell Culture Scaffolds

Cartilage wounds result in chronic pain and degradation of the quality of life for millions of people. A synthetic cellular scaffold able to heal the damage by substituting the natural tissue is of great potential value. Here, it is shown for the first time that the unique interplay between the molecular components of cartilage can be reproduced in composite materials made of a polyelectrolyte hydrogel embedding a collagen scaffold. These composites possess a mechanical response determined by osmotic and electrostatic effects, comparable to articular cartilage in terms of elastic modulus, time-dependent response, and permeability to interstitial fluid flow. Made entirely from biocompatible materials, the cartilage-like composite materials developed permit 3D culture of chondrocyte-like cells through their microporosity. The biomimetic materials presented here constitute an entirely new class of osmotically stiffened composites, which may find use outside of biomedical applications.

Current strategies for integrative cartilage repair

Osteoarthritis (OA) is a degenerative joint condition characterized by painful cartilage lesions that impair joint mobility. Current treatments such as lavage, microfracture, and osteochondral implantation fail to integrate newly formed tissue with host tissues and establish a stable transition to subchondral bone. Similarly, tissue-engineered grafts that facilitate cartilage and bone regeneration are challenged by how to integrate the graft seamlessly with surrounding host cartilage and/or bone. This review centers on current approaches to promote cartilage graft integration. It begins with an overview of articular cartilage structure and function, as well as degenerative changes to this relationship attributed to aging, disease, and trauma. A discussion of the current progress in integrative cartilage repair follows, focusing on graft or scaffold design strategies targeting cartilage-cartilage and/or cartilage-bone integration. It is emphasized that integrative repair is required to ensure long-term success of the cartilage graft and preserve the integrity of the newly engineered articular cartilage. Studies involving the use of enzymes, choice of cell source, biomaterial selection, growth factor incorporation, and stratified versus gradient scaffolds are therefore highlighted. Moreover, models that accurately evaluate the ability of cartilage grafts to enhance tissue integrity and prevent ectopic calcification are also discussed. A summary and future directions section concludes the review.

Effect of Laminin-A4 inhibition on cluster formation of human osteoarthritic chondrocytes

Formation of chondrocyte clusters is not only a morphological sign of osteoarthritis but it is also observed in cell culture. Active locomotion of chondrocytes is controlled by integrins in vitro. Integrins bind to Laminin-A4 (LAMA4), a protein that is highly expressed in vivo in clusters of hypertrophic chondrocytes. We tested if LAMA4 is relevant for cluster formation. Human chondrocytes were cultured in a 2D matrigel model and treated with different concentrations of a monoclonal inhibitory anti-LAMA4-antibody. Migration and cluster formation was analysed using live cell imaging technique. Full genome gene expression analysis was performed to assess the effect of LAMA4 inhibition. The data set were screened for genes relevant to cell motility. F-actin staining was performed to document cytoskeletal changes. Anti-LAMA4 treatment significantly reduced the rate of cluster formation in human chondrocytes. Cells changed their surface morphology and exhibited fewer protrusions. Expression of genes associated with cellular motility and migration was affected by anti-LAMA4 treatment. LAMA4-integrin signalling affects chondrocyte morphology and gene expression in vitro, thereby contributing to cluster formation in human osteoarthritic chondrocytes.

Characterization of a migrative subpopulation of adult human nasoseptal chondrocytes with progenitor cell features and their potential for in vivo cartilage regeneration strategies

Background

Progenitor cells display interesting features for tissue repair and reconstruction. In the last years, such cells have been identified in different cartilage types. In this study, we isolated a migrative subpopulation of adult human nasoseptal chondrocytes with progenitor cell features by outgrowth from human nasal septum cartilage. These putative progenitor cells were comparatively characterized with mesenchymal stem cells (MSC) and human nasal septum chondrocytes with respect to their cellular characteristics as well as surface marker profile using flow cytometric analyses. Differentiation capacity was evaluated on protein and gene expression levels.

Results

The migrative subpopulation differentiated into osteogenic and chondrogenic lineages with distinct differences to chondrocytes and MSC. Cells of the migrative subpopulation showed an intermediate surface marker profile positioned between MSC and chondrocytes. Significant differences were found for CD9, CD29, CD44, CD90, CD105 and CD106. The cells possessed a high migratory ability in a Boyden chamber assay and responded to chemotactic stimulation. To evaluate their potential use in tissue engineering applications, a decellularized septal cartilage matrix was either seeded with cells from the migrative subpopulation or chondrocytes. Matrix production was demonstrated immunohistochemically and verified on gene expression level. Along with secretion of matrix metalloproteinases, cells of the migrative subpopulation migrated faster into the collagen matrix than chondrocytes, while synthesis of cartilage specific matrix was comparable.

Conclusions

Cells of the migrative subpopulation, due to their migratory characteristics, are a potential cell source for in vivo regeneration of nasal cartilage. The in vivo mobilization of nasal cartilage progenitor cells is envisioned to be the basis for in situ tissue engineering procedures, aiming at the use of unseeded biomaterials which are able to recruit local progenitor cells for cartilage regeneration.

Focal Adhesion Assembly Induces Phenotypic Changes and Dedifferentiation in Chondrocytes

The expansion of autologous chondrocytes in vitro is used to generate sufficient populations for cell-based therapies. However, during monolayer culture, chondrocytes lose inherent characteristics and shift to fibroblast-like cells as passage number increase. Here, we investigated passage-dependent changes in cellular physiology, including cellular morphology, motility, and gene and protein expression, as well as the role of focal adhesion and cytoskeletal regulation in the dedifferentiation process. We found that the gene and protein expression levels of both the focal adhesion complex and small Rho GTPases are upregulated with increasing passage number and are closely linked to chondrocyte dedifferentiation. The inhibition of focal adhesion kinase (FAK) but not small Rho GTPases induced the loss of fibroblastic traits and the recovery of collagen type II, aggrecan, and SOX9 expression levels in dedifferentiated chondrocytes. Based on these findings, we propose a strategy to suppress chondrogenic dedifferentiation by inhibiting the identified FAK or Src pathways while maintaining the expansion capability of chondrocytes in a 2D environment. These results highlight a potential therapeutic target for the treatment of skeletal diseases and the generation of cartilage in tissue-engineering approaches. J. Cell. Physiol. 231: 1822-1831, 2016. © 2015 Wiley Periodicals, Inc.

Peripheral blood derived mononuclear cells enhance osteoarthritic human chondrocyte migration

Introduction

A major problem in cartilage repair is the lack of chondrogenic cells migrating from healthy tissue into defects. Cartilage is essentially avascular and therefore its healing is not considered to involve mononuclear cells. Peripheral blood derived mononuclear cells (PBMC) offer a readily available autologous cell source for clinical use and therefore this study was designed to evaluate the effects of PBMCs on chondrocytes and cartilage.

Methods

Human primary chondrocytes and cartilage tissue explants were taken from patients undergoing total knee replacement (n = 17). Peripheral blood samples were obtained from healthy volunteers (n = 12) and mononuclear cells were isolated by density-gradient centrifugation. Cell migration and chemokinetic potential were measured using a scratch assay, xCELLigence and CyQuant assay. PCR array and quantitative PCR was used to evaluate mRNA expression of 87 cell motility and/or chondrogenic genes.

Results

The chondrocyte migration rate was 2.6 times higher at 3 hour time point (p < 0.0001) and total number of migrating chondrocytes was 9.7 times higher (p < 0.0001) after three day indirect PBMC stimulus and 8.2 times higher (p < 0.0001) after three day direct co-culture with PBMCs. A cartilage explant model confirmed that PBMCs also exert a chemokinetic role on ex vivo tissue. PBMC stimulation was found to significantly upregulate the mRNA levels of 2 chondrogenic genes; collagen type II (COL2A1 600–fold, p < 0.0001) and SRY box 9 (SOX9 30–fold, p < 0.0001) and the mRNA levels of 7 genes central in cell motility and migration were differentially regulated by 24h PBMC stimulation.

Conclusion

The results support the concept that PBMC treatment enhances chondrocyte migration without suppressing the chondrogenic phenotype possibly via mechanistic pathways involving MMP9 and IGF1. In the future, peripheral blood mononuclear cells could be used as an autologous point-ofcare treatment to attract native chondrocytes from the diseased tissue to aid in cartilage repair.

Menisectomized miniature Vietnamese pigs develop articular cartilage pathology resembling osteoarthritis

Animal models have been used to understand the basic biology of osteoarthritis (OA) and have helped to identify new candidate biomarkers for the early diagnosis and treatment of this condition. Small animals cannot sufficiently mimic human diseases; therefore, large animal models are needed. Pigs have been used as models for human diseases because they are similar to humans in terms of their anatomy, physiology and genome. Hence, we analyzed articular cartilage and synovial membrane pathology in miniature Vietnamese pigs after a unilateral partial menisectomy and 20-day exercise regimen to determine if the pigs developed pathological characteristics similar to human OA. Histological and protein expression analysis of articular cartilage from menisectomized pigs revealed the following pathologic changes resembling OA: fibrillation, fissures, chondrocyte cluster formation, decrease in proteoglycan content and upregulation of the OA-associated proteins MMP-3, MMP-13, procaspase-3 and IL-1β. Moreover, histological analysis of synovial membrane revealed mild synovitis, characterized by hyperplasia, cell infiltration and neoangiogenesis. Pathological changes were not observed in the contralateral joints or the joints of sham-operated pigs. Further studies are required to validate such an OA model; however, our results can encourage the use of pigs to study early stages of OA physiopathology. Based on their similarities to humans, pigs may be useful for preclinical studies to identify new candidate biomarkers and novel treatments for OA.

Preclinical good laboratory practice-compliant safety study to evaluate biodistribution and tumorigenicity of a cartilage advanced therapy medicinal product (ATMP)

BACKGROUND

The clinical development of advanced therapy medicinal products (ATMPs), a new class of drugs, requires initial safety studies that deviate from standard non-clinical safety protocols. The study provides a strategy to address the safety aspects of biodistribution and tumorigenicity of ATMPs under good laboratory practice (GLP) conditions avoiding cell product manipulation. Moreover, the strategy was applied on a human ATMP for cartilage repair.

METHODS

The testing strategy addresses biodistribution and tumorigenicity using a multi-step analysis without any cell manipulation to exclude changes of test item characteristics. As a safeguard measurement for meeting regulatory expectations, the project design and goals were discussed continuously with the regulatory authority using a staggered scientific advice concept. Subsequently, the strategy was applied to co.don chondrosphere® (huChon spheroid), a tissue-engineered matrix-free ATMP of human normal chondrocytes. In both the biodistribution and tumorigenicity studies, huChon spheroids were implanted subcutaneously into 40 immunodeficient mice. Biodistribution was studied 1 month after implantation. A skin disc containing the huChon spheroid, two surrounding skin rings and selected organs were analyzed by validated, gender-specific, highly-sensitive triplex qPCR and by immunohistochemistry (IHC).

RESULTS

No human DNA was detected in distant skin rings and analyzed organs. IHC revealed no direct or indirect indications of cell migration. Tumorigenicity was assessed 6 months after huChon spheroid implantation by palpation, macroscopic inspection, histology and IHC. No mice from the huChon spheroid group developed a tumor at the implantation site. In two mice, benign tumors were detected that were negative for HLA-ABC, suggesting that they were of spontaneous murine origin.

CONCLUSIONS

In summary, the presented strategy using a multi-step analysis was confirmed to be suitable for safety studies of ATMPs.

Chondrocyte clusters adjacent to sites of cartilage degeneration have characteristics of progenitor cells

The purpose of this study was to investigate the site-specific characteristics and roles of chondrocyte clusters in human knee osteoarthritis. Cartilage explants were obtained from 45 knees undergoing total knee replacement surgery. The explants were taken from 4 locations in the knee: the medial femoral condyle, the medial posterior femoral condyle (MPC), the lateral femoral condyle, and the lateral posterior femoral condyle (LPC). Cartilage degeneration, cell density, and cell arrangement were compared histologically. A live/dead cell viability assay and immunohistochemical analyses using antibodies against STRO-1, FGF2, and Ki-67 were performed. Cell proliferation and cartilaginous nodule production in MPC and LPC explants in monolayer culture were compared. Finally, MPC cartilage explants were cultured to observe histological changes. The cell density of the MPC explants was higher than that of the LPC because of clustering. MPC explants contained more live cells than the LPC did, and the expression of IHC markers in MPC explants was higher than that in LPC. Chondrocytes from MPC proliferated faster and produced more nodules in monolayer culture than those from the LPC and MPC explants were repaired during organ culture. In conclusion, chondrocyte clusters adjacent to severe cartilage degeneration have specific characteristics, with progenitor and proliferative potential.

Peripheral blood derived mononuclear cells enhance the migration and chondrogenic differentiation of multipotent mesenchymal stromal cells

A major challenge in cartilage repair is the lack of chondrogenic cells migrating from healthy tissue into damaged areas and strategies to promote this should be developed. The aim of this study was to evaluate the effect of peripheral blood derived mononuclear cell (PBMC) stimulation on mesenchymal stromal cells (MSCs) derived from the infrapatellar fat pad of human OA knee. Cell migration was measured using an xCELLigence electronic migration chamber system in combination with scratch assays. Gene expression was quantified with stem cell PCR arrays and validated using quantitative real-time PCR (rtPCR). In both migration assays PBMCs increased MSC migration by comparison to control. In scratch assay the wound closure was 55% higher after 3 hours in the PBMC stimulated test group (P = 0.002), migration rate was 9 times faster (P = 0.008), and total MSC migration was 25 times higher after 24 hours (P = 0.014). Analysis of MSCs by PCR array demonstrated that PBMCs induced the upregulation of genes associated with chondrogenic differentiation over 15-fold. In conclusion, PBMCs increase both MSC migration and differentiation suggesting that they are an ideal candidate for inclusion in regenerative medicine therapies aimed at cartilage repair.

A novel in vitro bovine cartilage punch model for assessing the regeneration of focal cartilage defects with biocompatible bacterial nanocellulose

Introduction

Current therapies for articular cartilage defects fail to achieve qualitatively sufficient tissue regeneration, possibly because of a mismatch between the speed of cartilage rebuilding and the resorption of degradable implant polymers. The present study focused on the self-healing capacity of resident cartilage cells in conjunction with cell-free and biocompatible (but non-resorbable) bacterial nanocellulose (BNC). This was tested in a novel in vitro bovine cartilage punch model.

Methods

Standardized bovine cartilage discs with a central defect filled with BNC were cultured for up to eight weeks with/without stimulation with transforming growth factor-β1 (TGF-β1. Cartilage formation and integrity were analyzed by histology, immunohistochemistry and electron microscopy. Content, release and neosynthesis of the matrix molecules proteoglycan/aggrecan, collagen II and collagen I were also quantified. Finally, gene expression of these molecules was profiled in resident chondrocytes and chondrocytes migrated onto the cartilage surface or the implant material.

Results

Non-stimulated and especially TGF-β1-stimulated cartilage discs displayed a preserved structural and functional integrity of the chondrocytes and surrounding matrix, remained vital in long-term culture (eight weeks) without signs of degeneration and showed substantial synthesis of cartilage-specific molecules at the protein and mRNA level. Whereas mobilization of chondrocytes from the matrix onto the surface of cartilage and implant was pivotal for successful seeding of cell-free BNC, chondrocytes did not immigrate into the central BNC area, possibly due to the relatively small diameter of its pores (2 to 5 μm). Chondrocytes on the BNC surface showed signs of successful redifferentiation over time, including increase of aggrecan/collagen type II mRNA, decrease of collagen type I mRNA and initial deposition of proteoglycan and collagen type II in long-term high-density pellet cultures. Although TGF-β1 stimulation showed protective effects on matrix integrity, effects on other parameters were limited.

Conclusions

The present bovine cartilage punch model represents a robust, reproducible and highly suitable tool for the long-term culture of cartilage, maintaining matrix integrity and homoeostasis. As an alternative to animal studies, this model may closely reflect early stages of cartilage regeneration, allowing the evaluation of promising biomaterials with/without chondrogenic factors.

[The bovine cartilage punch model: a tool for the in vitro analysis of biomaterials and cartilage regeneration]

BACKGROUND

The limited regeneration capacity of hyaline articular cartilage requires detailed studies concerning the tissue integration of cartilage transplants with meaningful but time and/or resource-consuming and in part ethically problematic animal models or, alternatively, with in vitro test systems for implant materials.

MATERIAL AND METHODS

The present study describes a regeneration model with bovine cartilage rings (outer Ø 6 mm, central defect Ø 2 mm) for insertion, cultivation and biomechanical or histological testing of cartilage replacement materials (HE and safranin O staining). In this study, resorbable polymers composed of polyglycolic acid (PGA) were analyzed.

RESULTS

Biomechanical testing showed a continuous decrease of the push-out force for the PGA inserts from the cartilage rings, probably due to the resorbability of the material. Histologically, clear immigration of cells into cell-free PGA was observed even after 4 weeks of culture, but in particular after 10 weeks. In addition, storage of proteoglycans was interpreted as an initial sign of the formation of new matrix.

CONCLUSION

Thus, the new regeneration model is in principle suitable for the testing of biomaterials, but shows limitations in assessing the "lateral bonding" of resorbable materials.

The identification of CD163 expressing phagocytic chondrocytes in joint cartilage and its novel scavenger role in cartilage degradation

Background

Cartilage degradation is a typical characteristic of arthritis. This study examined whether there was a subset of phagocytic chondrocytes that expressed the specific macrophage marker, CD163, and investigated their role in cartilage degradation.

Methods

Cartilage from the knee and temporomandibular joints of Sprague-Dawley rats was harvested. Cartilage degradation was experimentally-induced in rat temporomandibular joints, using published biomechanical dental methods. The expression levels of CD163 and inflammatory factors within cartilage, and the ability of CD163⁺ chondrocytes to conduct phagocytosis were investigated. Cartilage from the knees of patients with osteoarthritis and normal cartilage from knee amputations was also investigated.

Results

In the experimentally-induced degrading cartilage from temporomandibular joints, phagocytes were capable of engulfing neighboring apoptotic and necrotic cells, and the levels of CD163, TNF-α and MMPs were all increased (P<0.05). However, the levels of ACP-1, NO and ROS, which relate to cellular digestion capability were unchanged (P>0.05). CD163⁺ chondrocytes were found in the cartilage mid-zone of temporomandibular joints and knee from healthy, three-week old rats. Furthermore, an increased number of CD163⁺ chondrocytes with enhanced phagocytic activity were present in Col-II⁺ chondrocytes isolated from the degraded cartilage of temporomandibular joints in the eight-week experimental group compared with their age-matched controls. Increased number with enhanced phagocytic activity of CD163⁺ chondrocytes were also found in isolated Col-II⁺ chondrocytes stimulated with TNF-α (P<0.05). Mid-zone distribution of CD163⁺ cells accompanied with increased expression of CD163 and TNF-α were further confirmed in the isolated Col-II⁺ chondrocytes from the knee cartilage of human patients with osteoarthritis, in contrast to the controls (both P<0.05).

Conclusions

An increased number of CD163⁺ chondrocytes with enhanced phagocytic activity were discovered within degraded joint cartilage, indicating a role in eliminating degraded tissues. Targeting these cells provides a new strategy for the treatment of arthritis.

Musculoskeletal tissue engineering by endogenous stem/progenitor cells

From its inception, tissue engineering has had three tenets: cells, biomaterial scaffolds and signaling molecules. Among the triad, cells are the center piece, because cells are the building blocks of tissues. For decades, cell therapies have focused on the procurement, manipulation and delivery of healthy cells for the treatment of diseases or trauma. Given the complexity and potential high cost of cell delivery, there is recent and surging interest to orchestrate endogenous cells for tissue regeneration. Biomaterial scaffolds are vital for many but not all, tissue-engineering applications and serve to accommodate or promote multiple cellular functions. Signaling molecules can be produced by transplanted cells or endogenous cells, or delivered specifically to regulate cell functions. This review highlights recent work in tissue engineering and cell therapies, with a focus on harnessing the capacity of endogenous cells as an alternative or adjunctive approach for tissue regeneration.

Response of Chondrocytes to Local Mechanical Injury in an Ex Vivo Model

BACKGROUND

Our goal was to set up an ex vivo culture system to assess whether cartilage wounding (partial-thickness defects) can induce morphological changes in neighboring chondrocytes and whether these cells can translocate to the surface of the defect.

METHODS

Two-millimeter partial-depth defects were created in human osteochondral explants followed by culture for up to 4 weeks. Frozen sections of defects and defect-free regions were labeled using immunofluorescence for a plasma membrane protein, CD44, and actin with TRITC-phalloidin. Viable nuclei were detected with Hoechst 33342. Differential interference contrast (DIC), confocal, and transmission electron microscopy (TEM) were used to examine process extension.

RESULTS

Significant changes in cell morphology occurred in response to wounding in the superficial and deep cartilage zones. These included cell flattening, polarization of the actin cytoskeleton, extension of pseudopods projecting towards the edge of the defect, and interactions of these filopodia with collagen fibers. Cell density decreased progressively in the 300-µm zone adjacent to the defect to an average of approximately 25% to 35% after 3 weeks. Concomitant increases in cell density in the defect margin were observed. By contrast, minimal changes were seen in the middle cartilage zone.

CONCLUSIONS

These novel observations strongly suggest active cartilage cell responses and movements in response to wounding. It is proposed that cartilage cells use contact guidance on fibrillated collagen to move into and populate defect areas in the superficial and deep zones.

Axonemal positioning and orientation in three-dimensional space for primary cilia: what is known, what is assumed, and what needs clarification

Two positional characteristics of the ciliary axoneme--its location on the plasma membrane as it emerges from the cell, and its orientation in three-dimensional (3D) space--are known to be critical for optimal function of actively motile cilia (including nodal cilia), as well as for modified cilia associated with special senses. However, these positional characteristics have not been analyzed to any significant extent for primary cilia. This review briefly summarizes the history of knowledge of these two positional characteristics across a wide spectrum of cilia, emphasizing their importance for proper function. Then the review focuses what is known about these same positional characteristics for primary cilia in all major tissue types where they have been reported. The review emphasizes major areas that would be productive for future research for understanding how positioning and 3D orientation of primary cilia may be related to their hypothesized signaling roles within different cellular populations.

Hydrogels for the repair of articular cartilage defects

The repair of articular cartilage defects remains a significant challenge in orthopedic medicine. Hydrogels, three-dimensional polymer networks swollen in water, offer a unique opportunity to generate a functional cartilage substitute. Hydrogels can exhibit similar mechanical, swelling, and lubricating behavior to articular cartilage, and promote the chondrogenic phenotype by encapsulated cells. Hydrogels have been prepared from naturally derived and synthetic polymers, as cell-free implants and as tissue engineering scaffolds, and with controlled degradation profiles and release of stimulatory growth factors. Using hydrogels, cartilage tissue has been engineered in vitro that has similar mechanical properties to native cartilage. This review summarizes the advancements that have been made in determining the potential of hydrogels to replace damaged cartilage or support new tissue formation as a function of specific design parameters, such as the type of polymer, degradation profile, mechanical properties and loading regimen, source of cells, cell-seeding density, controlled release of growth factors, and strategies to cause integration with surrounding tissue. Some key challenges for clinical translation remain, including limited information on the mechanical properties of hydrogel implants or engineered tissue that are necessary to restore joint function, and the lack of emphasis on the ability of an implant to integrate in a stable way with the surrounding tissue. Future studies should address the factors that affect these issues, while using clinically relevant cell sources and rigorous models of repair.

A semi-degradable composite scaffold for articular cartilage defects

Few options exist to replace or repair damaged articular cartilage. The optimal solution that has been suggested is a scaffold that can carry load and integrate with surrounding tissues; but such a construct has thus far been elusive. The objectives of this study were to manufacture and characterize a nondegradable hydrated scaffold. Our hypothesis was that the polymer content of the scaffold can be used to control its mechanical properties, while an internal porous network augmented with biological agents can facilitate integration with the host tissue. Using a two-step water-in-oil emulsion process a porous polyvinyl alcohol (PVA) hydrogel scaffold combined with alginate microspheres was manufactured. The scaffold had a porosity of 11-30% with pore diameters of 107-187 μm, which readily allowed for movement of cells through the scaffold. Alginate microparticles were evenly distributed through the scaffold and allowed for the slow release of biological factors. The elastic modulus (Es ) and Poisson's ratio (υ), Aggregate modulus (Ha ) and dynamic modulus (ED ) of the scaffold were significantly affected by % PVA, as it varied from 10 to 20% wt/vol. Es and υ were similar to that of articular cartilage for both polymer concentrations, while Ha and ED were similar to that of cartilage only at 20% PVA. The ability to control scaffold mechanical properties, while facilitating cellular migration suggest that this scaffold is a potentially viable candidate for the functional replacement of cartilage defects.

Chondrocyte repopulation of the zone of death induced by osteochondral harvest

OBJECTIVE

Harvesting osteochondral grafts results in a zone of chondrocyte death (ZCD) in and around the periphery of the graft, creating a barrier for chondrocytes to migrate to the graft periphery, thus limiting cartilage-to-cartilage healing. The purpose of this study was to repopulate the induced ZCD through the combined effects of collagenase treatment and delivery of a chemotactic agent.

DESIGN

In bovine cartilage, the ZCD induced by the OATS™ osteochondral harvesting system was determined, followed by a corresponding collagenase treatment to penetrate the developed ZCD. The chemotactic potential of platelet derived growth factor (PDGF-bb), insulin-like growth factor I (IGF-I), and basic fibroblast growth factor (bFGF) (2.5-100 ng/mL) was then assessed using a modified Boyden chamber assay to select an appropriate agent to induce chondrocyte migration. Afterwards, the combined effects of collagenase treatment and chondrocyte chemotaxis on the repopulation of an induced ZCD were examined in cartilage explants over a 4-week-period.

RESULTS

The OATS™ osteochondral harvesting system induced a significant ZCD (173 μm, 95% CI: [72-274 μm]) in the grafts. Chondrocyte chemotaxis was induced by all agents investigated at concentrations greater than 25 ng/mL. After 4 weeks in culture, collagenase treatment alone reduced the ZCD by approximately 40% relative to untreated explants. Coupling the collagenase treatment with 25 ng/mL IGF-I reduced the ZCD by approximately 80% relative to untreated explants, and 65% relative to explants treated only with collagenase.

CONCLUSION

Treating cartilage explants with collagenase and 25 ng/mL IGF-I resulted in a decreased ZCD after a 4-week-period, and increased chondrocyte density within the induced ZCD.

Orientation of primary cilia of articular chondrocytes in three-dimensional space

Primary cilia have functions as sensory organelles integral to signal transduction and establishment of cell polarity. In articular cartilage the primary cilium has been hypothesized to function as an antenna to sense the biomechanical environment, regulate the secretion of extracellular matrix components, and maintain cellular positional information, leading to high tissue anisotropy. We used analysis of electron microscopy serial sections to demonstrate positional attributes of the primary cilium of adult equine articular chondrocytes in situ. Data for ~500 axonemes, comparing superficial to radiate chondrocytes from both load-bearing and non-load-bearing regions, were graphed using spherical co-ordinates θ, φ. The data demonstrate the axoneme has a definable orientation in 3D space differing in superficial and radiate zone chondrocytes, cells that differ by 90° in the orientation of their major axes to the articular surface. Axonemal orientation is more definable in load-bearing than in non-load-bearing areas. The position of emergence of the axoneme from the cell also is variable. In load-bearing regions of the superficial zone, extension of the axoneme is from the cellular side facing the subchondral bone. In radiate zone cells, axonemes extend from either face of the chondrocyte, that is, both toward the articular surface or toward the subchondral bone. In non-load-bearing regions this consistency is lost. These observations relate to current hypotheses concerning establishment of tissue anisotropy in articular cartilage during development, involving both migration of cells from the joint periphery and a restricted zone of division within the tissue resulting in the columnar arrangement of radiate zone cells.

Comparison of articular cartilage repair by autologous chondrocytes with and without in vitro cultivation

OBJECTIVE

autologous chondrocyte implantation usually requires in vitro cell expansion before implantation. We compared the efficacy of cartilage regeneration by in vitro-expanded chondrocytes at high density and freshly harvested chondrocytes at low density.

DESIGN

surgically created osteochondral defects at weight-bearing surface of femoral condyles of domestic pigs were repaired by biphasic cylindrical porous plugs of DL-poly-lactide-co-glycolide and beta-tricalcium phosphate. Plugs were seeded with autologous chondrocytes in its chondral phase, and press-fit to defects. Seeded cells were (1) in vitro-expanded chondrocytes harvested from stifle joint 3 weeks before implantation and (2) freshly harvested chondrocytes from recipient knee. Seeding densities were 70 x 10(6) and 7 x 10(6) cells/mL, respectively. Cell-free plugs served as control and defects remained untreated as null control. Outcome was examined at 6 months with International Cartilage Repair Society Scale.

RESULTS

the two experimental groups were repaired by hyaline cartilage with collagen type II and Safranin-O. Tissue in control group was primarily fibrocartilage. No regeneration was found in null control. Experimental groups had higher mean International Cartilage Repair Society scores than control in surface, matrix, and cell distribution, but were comparable with control in cell viability, subchondral bone, and mineralization. No significant difference existed between two experimental groups in any of the six categories. Uni-axial indentation test revealed similar creeping stress-relaxation property as native cartilage on experimental, but not control, specimen.

CONCLUSIONS

cartilage could regenerate in both experimental models, in comparable quality. Culture of chondrocytes before implantation is not necessary.

A nanofibrous cell-seeded hydrogel promotes integration in a cartilage gap model

The presence of a defect in mature articular cartilage can lead to degenerative changes of the joint. This is in part caused by the inability of cartilage to regenerate tissue that is capable of spanning a fissure or crack. In this study, we hypothesized that introduction of a biodegradable cell-seeded nanofibrous hydrogel, Puramatrix(), into a cartilage gap would facilitate the generation of a mechanically stable interface. The effects of chondrocyte incorporation within the hydrogel and supplementation with transforming growth factor-beta3 (TGFbeta3), a known regulator of cell growth and differentiation, on cartilage integration were examined mechanically and histologically as a function of cell density and incubation time. When supplemented with TGFbeta3, the cell-seeded hydrogel exhibited abundant matrix generation within the hydrogel and a corresponding increase in maximum push-out stress as compared to all other groups. Furthermore, initial cell seeding density affected interfacial strength in a time-dependent manner. This study suggests that a cell-seeded TGFbeta3-supplemented hydrogel can encourage integration between two opposing pieces of articular cartilage.

Effects of introducing cultured human chondrocytes into a human articular cartilage explant model

Articular cartilage (AC) heals poorly and effective host-tissue integration after reconstruction is a concern. We have investigated the ability of implanted chondrocytes to attach at the site of injury and to be incorporated into the decellularized host matrix adjacent to a defect in an in vitro human explant model. Human osteochondral dowels received a standardized injury, were seeded with passage 3 chondrocytes labelled with PKH 26 and compared with two control groups. All dowels were cultured in vitro, harvested at 0, 7, 14 and 28 days and assessed for chondrocyte adherence and migration into the region of decellularized tissue adjacent to the defects. Additional evaluation included cell viability, general morphology and collagen II production. Seeded chondrocytes adhered to the standardized defect and areas of lamina splendens disruption but did not migrate into the adjacent acellular region. A difference was noted in viable-cell density between the experimental group and one control group. A thin lattice-like network of matrix surrounded the seeded chondrocytes and collagen II was present. The results indicate that cultured human chondrocytes do indeed adhere to regions of AC matrix injury but do not migrate into the host tissue, despite the presence of viable cells. This human explant model is thus an effective tool for studying the interaction of implanted cells and host tissue.

Osteochondral integration of multiply incised pure cartilage allograft: repair method of focal chondral defects in a porcine model

BACKGROUND

A focal cartilage lesion has limited capacity to heal, and the repair modalities used at present are still unable to provide a universal solution. Pure cartilage graft implantation appears to be a simple option, but it has not been applied widely as cartilage will not reattach easily to the subchondral bone.

HYPOTHESIS

We used a multiple-incision technique (processed chondrograft) to increase cartilage graft surface. We hypothesized that pure cartilage graft with augmented osteochondral fusion capacity may be used for cartilage repair and we compared this method with other repair techniques.

STUDY DESIGN

Controlled laboratory study.

METHODS

Full-thickness focal cartilage defects were created on the medial femoral condyle of 9-month-old pigs; defects were repaired using various methods including bone marrow stimulation, autologous chondrocyte implantation, and processed chondrograft. After the repair, at weeks 6 and 24, macroscopic and histologic evaluation was carried out.

RESULTS

Compared with other methods, processed chondrograft was found to be similarly effective in cartilage repair. Defects without repair and defects treated with bone marrow stimulation appeared slightly irregular with fibrocartilage filling. Autologous chondrocyte implantation produced hyalinelike cartilage, although its cellular organization was distinguishable from the surrounding articular cartilage. Processed chondrograft demonstrated good osteochondral integration, and the resulting tissue appeared to be hyaline cartilage.

CONCLUSION

The applied cartilage surface processing method allows acceptable osteochondral integration, and the repair tissue appears to have good macroscopic and histologic characteristics.

CLINICAL RELEVANCE

If further studies confirm its efficacy, this technique could be considered for human application in the future.

Hybrid cellular automaton modeling of nutrient modulated cell growth in tissue engineering constructs

Mathematic models help interpret experimental results and accelerate tissue engineering developments. We develop in this paper a hybrid cellular automata model that combines the differential nutrient transport equation to investigate the nutrient limited cell construct development for cartilage tissue engineering. Individual cell behaviors of migration, contact inhibition and cell collision, coupled with the cell proliferation regulated by oxygen concentration were carefully studied. Simplified two-dimensional simulations were performed. Using this model, we investigated the influence of cell migration speed on the overall cell growth within in vitro cell scaffolds. It was found that intense cell motility can enhance initial cell growth rates. However, since cell growth is also significantly modulated by the nutrient contents, intense cell motility with conventional uniform cell seeding method may lead to declined cell growth in the final time because concentrated cell population has been growing around the scaffold periphery to block the nutrient transport from outside culture media. Therefore, homogeneous cell seeding may not be a good way of gaining large and uniform cell densities for the final results. We then compared cell growth in scaffolds with various seeding modes, and proposed a seeding mode with cells initially residing in the middle area of the scaffold that may efficiently reduce the nutrient blockage and result in a better cell amount and uniform cell distribution for tissue engineering construct developments.

Mesenchymal stem cells and cartilage in situ regeneration

Cartilage repair is a very successful pioneering area of regenerative medicine in which techniques of in situ regeneration and cell and tissue transplantation dominate over cell-free approaches to generate durable neocartilage. This review concentrates on advantages and limitations of mesenchymal stem cell (MSC)-based cartilage repair strategies induced by marrow stimulation. Detailed knowledge on the biology of MSC will be discussed in light of the requirements for MSC recruitment, retention, proliferation and chondrogenic differentiation. An improved microenvironment with timely correlated signals from biomaterials, growth factors, proteases, adjacent cartilage and subchondral bone may be key to a third generation of techniques to regenerate hyaline cartilage.

A 3D hybrid model for tissue growth: the interplay between cell population and mass transport dynamics

To provide theoretical guidance for the design and in vitro cultivation of bioartificial tissues, we have developed a multiscale computational model that can describe the complex interplay between cell population and mass transport dynamics that governs the growth of tissues in three-dimensional scaffolds. The model has three components: a transient partial differential equation for the simultaneous diffusion and consumption of a limiting nutrient; a cellular automaton describing cell migration, proliferation, and collision; and equations that quantify how the varying nutrient concentration modulates cell division and migration. The hybrid discrete-continuous model was parallelized and solved on a distributed-memory multicomputer to study how transport limitations affect tissue regeneration rates under conditions encountered in typical bioreactors. Simulation results show that the severity of transport limitations can be estimated by the magnitude of two dimensionless groups: the Thiele modulus and the Biot number. Key parameters including the initial seeding mode, cell migration speed, and the hydrodynamic conditions in the bioreactor are shown to affect not only the overall rate, but also the pattern of tissue growth. This study lays the groundwork for more comprehensive models that can handle mixed cell cultures, multiple nutrients and growth factors, and other cellular processes, such as cell death.

Notochordal cells stimulate migration of cartilage end plate chondrocytes of the intervertebral disc in in vitro cell migration assays

BACKGROUND CONTEXT

It was recently demonstrated that the postnatal transition from a notochordal to a fibrocartilaginous nucleus pulposus (NP) is accomplished exogenously by chondrocytes migrating from hyaline cartilage end plates (CEs) into the ectopic notochordal NP region. Although our previous in vivo studies showed evidences for the migration of CE chondrocyte from hyaline CEs into the notochordal NP, it is unknown whether CE chondrocytes of the intervertebral disc (IVD) really have a motile property. In addition, the effect of notochordal cells on this property has not been elucidated.

PURPOSE

The purpose of this in vitro study was to demonstrate whether CE chondrocytes of the IVD are capable of migration, and whether there is any biological link between notochordal cells and CE chondrocytes that may regulate the CE chondrocyte migration.

STUDY DESIGN/SETTING

In vitro cell migration assays were performed using rat IVDs.

METHODS

Notochordal cells and chondrocytes were obtained from the NP and CE tissues, respectively, and were cultured separately. The different numbers of notochordal cells and the supernatant (conditioned medium) that contained soluble factors produced by notochordal cells were used to demonstrate their effects on the migration of CE chondrocytes. Bovine serum albumin (BSA) and lysophosphatidic acid (LPA) were used as negative and positive controls, respectively.

RESULTS

Compared with BSA, LPA, notochordal cells (N=4x, 2x, 1x, and 0.5 x 10(5)), and its conditioned media (unconcentrated and fivefold concentrated) significantly increased migration of CE chondrocytes (p<.05 in all comparisons). Particularly, notochordal cells and its conditioned media increased migration in a number- and concentration-dependent manner, respectively.

CONCLUSIONS

This study demonstrates that CE chondrocytes of the IVD are capable of migration and that soluble factors produced by notochordal cells stimulate the migration. These results provide a plausible explanation to the question of why CE chondrocytes of the IVD migrate into the ectopic NP region during the natural transition from the notochordal to fibrocartilaginous NP.

Extracellular matrix-induced gene expression in human breast cancer cells

Extracellular matrix (ECM) molecules modify gene expression through attachment-dependent (focal adhesion-related) integrin receptor signaling. It was previously unknown whether the same molecules acting as soluble peptides could generate signal cascades without the associated mechanical anchoring, a condition that may be encountered during matrix remodeling and degradation and relevant to invasion and metastatic processes. In the current study, the role of ECM ligand-regulated gene expression through this attachment-independent process was examined. It was observed that fibronectin, laminin, and collagen type I and II induce Smad2 activation in MCF-10A and MCF-7 cells. This activation is not caused by transforming growth factor (TGF)-beta ligand contamination or autocrine TGF involvement and is 3- to 5-fold less robust than the TGF-beta1 ligand. The resulting nuclear translocation of Smad4 in response to ECM ligand indicates downstream transcriptional responses occurring. Coimmunoprecipitation experiments determined that collagen type II and laminin act through interaction with integrin alpha(2)beta(1) receptor complex. The ECM ligand-induced Smad activation (termed signaling crosstalk) resulted in cell type and ligand-specific transcriptional changes, which are distinct from the TGF-beta ligand-induced responses. These findings show that cell-matrix communication is more complex than previously thought. Soluble ECM peptides drive transcriptional regulation through corresponding adhesion and non-attachment-related processes. The resultant gene expressional patterns correlate with pathway activity and not by the extent of Smad activation. These results extend the complexity and the existing paradigms of ECM-cell communication to ECM ligand regulation without the necessity of mechanical coupling.

Subcellular distribution of the insulin-like growth factor (IGF) binding proteins (IGFBPs) 2 and 3 in articular chondrocytes

The insulin-like growth factor (IGF) is a major anabolic regulator in articular cartilage. The IGF-binding proteins (IGFBPs) are increased during osteoarthritis (OA), but the function of the later proteins remains unknown. In general, the IGFBPs are pluripotential effectors capable of IGF regulation and of acting on their own to control key cell functions, including survival and proliferation. The independent functions are often associated with their cell location, and therefore this study explores the distribution of IGFBP-2 and IGFBP-3 in articular chondrocytes. Immunohistochemistry was used to localize IGFBP-2 in normal human articular cartilage. Bovine chondrocytes were used for subcellular fractionation (hypotonic cell lysis) under nonreducing conditions and nuclear purification (centrifugation on sucrose cushions). Cell fraction markers and IGFBPs were assayed in the subcellular fractions by Western immunoblot. The IHC results showed association of IGFBP-2 with chondrocytes, but not with the nuclei. Subcellular fractionation of isolated chondrocytes yielded intact nuclei as assessed at the light microscopic level; the nuclear marker histone H1 was exclusively associated with this fraction. More than 90% of the cytoplasmic marker GAPDH and all the detectable IGFBP-2 were in the cytoplasmic fraction. Immunoreactive IGFBP-3 was found in the cytoplasmic and peri-nuclear/nuclear fractions. Chondrocytes contain intracellular IGFBP-2 and IGFBP-3 but only IGFBP-3 is associated with nuclei. This suggests the hypothesis that the actions of these IGFBPs in articular cartilage extend beyond the classic modulation of IGF receptor action.

Chemotaxis of human articular chondrocytes and mesenchymal stem cells

Migration of chondrocytes and mesenchymal stem cells (MSCs) may be important in cartilage development, tissue response to injury, and in tissue engineering. This study analyzed growth factors and cytokines for their ability to induce migration of human articular chondrocytes and bone marrow-derived mesenchymal stem cells in Boyden chamber assays. In human articular chondrocytes serum induced dose- and time-dependent increases in cell migration. Among a series of growth factors and cytokines tested only PDGF induced a significant increase in cell migration. The PDGF isoforms AB and BB were more potent than AA. There was an aging-related decline in the ability of chondrocytes to migrate in response to serum and PDGF. Human bone marrow MSC showed significant chemotaxis responses to several factors, including FBS, PDGF, VEGF, IGF-1, IL-8, BMP-4, and BMP-7. In summary, these results demonstrate that directed cell migration is inducible in human articular chondrocytes and MSC. PDGF is the most potent factor analyzed, and may be useful to promote tissue integration during cartilage repair or tissue engineering.