Localization of type X collagen in canine growth plate and adult canine articular cartilage

Passaged Articular Chondrocytes From the Superficial Zone and Deep Zone Can Regain Zone-Specific Properties After Redifferentiation

BACKGROUND

Bioengineered cartilage is a developing therapeutic to repair cartilage defects. The matrix must be rich in collagen type II and aggrecan and mechanically competent, withstanding compressive and shearing loads. Biomechanical properties in native articular cartilage depend on the zonal architecture consisting of 3 zones: superficial, middle, and deep. The superficial zone chondrocytes produce lubricating proteoglycan-4, whereas the deep zone chondrocytes produce collagen type X, which allows for integration into the subchondral bone. Zonal and chondrogenic expression is lost after cell number expansion. Current cell-based therapies have limited capacity to regenerate the zonal structure of native cartilage.

HYPOTHESIS

Both passaged superficial and deep zone chondrocytes at high density can form bioengineered cartilage that is rich in collagen type II and aggrecan; however, only passaged superficial zone-derived chondrocytes will express superficial zone-specific proteoglycan-4, and only passaged deep zone-derived chondrocytes will express deep zone-specific collagen type X.

STUDY DESIGN

Controlled laboratory study.

METHODS

Superficial and deep zone chondrocytes were isolated from bovine joints, and zonal subpopulations were separately expanded in 2-dimensional culture. At passage 2, superficial and deep zone chondrocytes were seeded, separately, in scaffold-free 3-dimensional culture within agarose wells and cultured in redifferentiation media.

RESULTS

Monolayer expansion resulted in loss of expression for proteoglycan-4 and collagen type X in passaged superficial and deep zone chondrocytes, respectively. By passage 2, superficial and deep zone chondrocytes had similar expression for dedifferentiated molecules collagen type I and tenascin C. Redifferentiation of both superficial and deep zone chondrocytes led to the expression of collagen type II and aggrecan in both passaged chondrocyte populations. However, only redifferentiated deep zone chondrocytes expressed collagen type X, and only redifferentiated superficial zone chondrocytes expressed and secreted proteoglycan-4. Additionally, redifferentiated deep zone chondrocytes produced a thicker and more robust tissue compared with superficial zone chondrocytes.

CONCLUSION

The recapitulation of the primary phenotype from passaged zonal chondrocytes introduces a novel method of functional bioengineering of cartilage that resembles the zone-specific biological properties of native cartilage.

CLINICAL RELEVANCE

The recapitulation of the primary phenotype in zonal chondrocytes could be a possible method to tailor bioengineered cartilage to have zone-specific expression.

Chondrocyte Homeostasis and Differentiation: Transcriptional Control and Signaling in Healthy and Osteoarthritic Conditions

Chondrocytes are the main cell type in articular cartilage. They are embedded in an avascular, abundant, and specialized extracellular matrix (ECM). Chondrocytes are responsible for the synthesis and turnover of the ECM, in which the major macromolecular components are collagen, proteoglycans, and non-collagen proteins. The crosstalk between chondrocytes and the ECM plays several relevant roles in the regulation of cell phenotype. Chondrocytes live in an avascular environment in healthy cartilage with a low oxygen supply. Although chondrocytes are adapted to anaerobic conditions, many of their metabolic functions are oxygen-dependent, and most cartilage oxygen is supplied by the synovial fluid. This review focuses on the transcription control and signaling responsible for chondrocyte differentiation, homeostasis, senescence, and cell death and the changes that occur in osteoarthritis. The effects of chondroitin sulfate and other molecules as anti-inflammatory agents are also approached and analyzed.

Osteochondral Interface: Regenerative Engineering and Challenges

Osteochondral (OC) defects are debilitating for patients and represent a significant clinical problem for orthopedic surgeons as well as regenerative engineers due to their potential complications, which are likely to lead to osteoarthritis and related diseases. If they remain untreated or are treated suboptimally, OC lesions are known to impact the articular cartilage and the transition from cartilage to bone, that is, the cartilage-bone interface. An important component of the OC interface, that is, a selectively permeable membrane, the tidemark, still remains unaddressed in more than 90% of the published research in the past decade. This review focuses on the structure, composition, and function of the OC interface, regenerative engineering attempts with different scaffolding strategies and challenges ahead of us in recapitulating the native OC interface. There are different schools of thought regarding the structure of the native OC interface: stratified and graded. The former assumes the cartilage-to-bone interface to be hierarchically divided into distinct yet continuous zones of uncalcified cartilage-calcified cartilage-subchondral bone. The latter assumes the interface is continuously graded, that is, formed by an infinite number of layers. The cellular composition of the interface, either in respective layers or continuously changing in a graded manner, is chondrocytes, hypertrophic chondrocytes, and osteoblasts as moved from cartilage to bone. Functionally, the interface is assumed to play a role in enabling a smooth transition of loads exerted on the cartilage surface to the bone underneath. Regenerative engineering involves, first, a characterization of the native OC interface in terms of the composition, structure, and function, and, then, proposes the appropriate biomaterials, cells, and biomolecules either alone or in combination to eventually form a structure that mimics and functionally behaves similar to the native interface. The major challenge regarding regeneration of the OC interface appears to lie, in addition to others, in the formation of tidemark, which is a thin membrane separating the OC interface into two distinct zones: the avascular OC interface and the vascular OC interface. There is a significant amount of literature on regenerative approaches to the OC interface; however, only a small portion of them consider the importance of tidemark. Therefore, this review aims at highlighting the significance of the structural organization of the components of the OC interface and increasing the awareness of the orthopedics community regarding the importance of tidemark formation after clinical interventions or regenerative engineering attempts.

Unusual Suspects: Bone and Cartilage ECM Proteins as Carcinoma Facilitators

The extracellular matrix (ECM) is the complex three-dimensional network of fibrous proteins and proteoglycans that constitutes an essential part of every tissue to provide support for normal tissue homeostasis. Tissue specificity of the ECM in its topology and structure supports unique biochemical and mechanical properties of each organ. Cancers, like normal tissues, require the ECM to maintain multiple processes governing tumor development, progression and spread. A large body of experimental and clinical evidence has now accumulated to demonstrate essential roles of numerous ECM components in all cancer types. Latest findings also suggest that multiple tumor types express, and use to their advantage, atypical ECM components that are not found in the cancer tissue of origin. However, the understanding of cancer-specific expression patterns of these ECM proteins and their exact roles in selected tumor types is still sketchy. In this review, we summarize the latest data on the aberrant expression of bone and cartilage ECM proteins in epithelial cancers and their specific functions in the pathogenesis of carcinomas and discuss future directions in exploring the utility of this selective group of ECM components as future drug targets.

3D Printing for Bone-Cartilage Interface Regeneration

Due to the vasculature defects and/or the avascular nature of cartilage, as well as the complex gradients for bone-cartilage interface regeneration and the layered zonal architecture, self-repair of cartilage and subchondral bone is challenging. Currently, the primary osteochondral defect treatment strategies, including artificial joint replacement and autologous and allogeneic bone graft, are limited by their ability to simply repair, rather than induce regeneration of tissues. Meanwhile, over the past two decades, three-dimension (3D) printing technology has achieved admirable advancements in bone and cartilage reconstruction, providing a new strategy for restoring joint function. The advantages of 3D printing hybrid materials include rapid and accurate molding, as well as personalized therapy. However, certain challenges also exist. For instance, 3D printing technology for osteochondral reconstruction must simulate the histological structure of cartilage and subchondral bone, thus, it is necessary to determine the optimal bioink concentrations to maintain mechanical strength and cell viability, while also identifying biomaterials with dual bioactivities capable of simultaneously regenerating cartilage. The study showed that the regeneration of bone-cartilage interface is crucial for the repair of osteochondral defect. In this review, we focus on the significant progress and application of 3D printing technology for bone-cartilage interface regeneration, while also expounding the potential prospects for 3D printing technology and highlighting some of the most significant challenges currently facing this field.

Recombinant platelet-derived growth factor-BB alleviates osteoarthritis in a rat model by decreasing chondrocyte apoptosis in vitro and in vivo

Osteoarthritis (OA) is a common joint disease that mainly affects the diarthrodial joints. Treatments for OA include non‐pharmacological interventions, topical and oral therapies, intra‐articular therapies and joint surgery. However, all the treatments mentioned above mainly aim to control the symptoms instead of improving or reversing the joint condition. In this research, we observed the effect of recombinant platelet‐derived growth factor (PDGF)‐BB on OA in a monosodium iodoacetate (MIA)–induced rat model and revealed the possible mechanisms. In vitro, the level of inflammation in the chondrocytes was gradually alleviated, and the apoptosis rate was gradually decreased by PDGF‐BB at increasing concentrations. The levels of p‐p38, Bax and caspase‐3 decreased, and the level of p‐Erk increased with increasing PDGF‐BB concentration. In vivo, PDGF‐BB could significantly reverse chondrocyte and matrix loss. Furthermore, high concentrations of PDGF‐BB could alleviate cartilage hyperplasia to remodel the tissue. The level of collagen II was up‐regulated, and the levels of collagen X and apoptosis were down‐regulated by increasing concentrations of PDGF‐BB. In conclusion, recombinant PDGF‐BB alleviated OA by down‐regulating caspase‐3‐dependent apoptosis. The effects of PDGF‐BB on OA mainly include inhibiting chondrocyte loss, reducing cartilage hyperplasia and osteophyte formation, and regulating collagen anabolism.

Macro, Micro, and Molecular. Changes of the Osteochondral Interface in Osteoarthritis Development

Osteoarthritis (OA) is a long-term condition that causes joint pain and reduced movement. Notably, the same pathways governing cell growth, death, and differentiation during the growth and development of the body are also common drivers of OA. The osteochondral interface is a vital structure located between hyaline cartilage and subchondral bone. It plays a critical role in maintaining the physical and biological function, conveying joint mechanical stress, maintaining chondral microenvironment, as well as crosstalk and substance exchange through the osteochondral unit. In this review, we summarized the progress in research concerning the area of osteochondral junction, including its pathophysiological changes, molecular interactions, and signaling pathways that are related to the ultrastructure change. Multiple potential treatment options were also discussed in this review. A thorough understanding of these biological changes and molecular mechanisms in the pathologic process will advance our understanding of OA progression, and inform the development of effective therapeutics targeting OA.

Diverse and Productive Source of Biopolymer Inspiration: Marine Collagens

Marine biodiversity is expressed through the huge variety of vertebrate and invertebrate species inhabiting intertidal to deep-sea environments. The extraordinary variety of "forms and functions" exhibited by marine animals suggests they are a promising source of bioactive molecules and provides potential inspiration for different biomimetic approaches. This diversity is familiar to biologists and has led to intensive investigation of metabolites, polysaccharides, and other compounds. However, marine collagens are less well-known. This review will provide detailed insight into the diversity of collagens present in marine species in terms of their genetics, structure, properties, and physiology. In the last part of the review the focus will be on the most common marine collagen sources and on the latest advances in the development of innovative materials exploiting, or inspired by, marine collagens.

Nasal Chondrocyte-Derived Soluble Factors Affect Chondrogenesis of Cocultured Mesenchymal Stem Cells

To investigate the effect of soluble factors released from human nasal chondrocytes (NCs) on cocultured human bone marrow mesenchymal stem cells (MSCs) and NC tissue-engineered constructs. Cartilage engineered from pure NCs on a three-dimensional (3D) porous collagen scaffold was cultured indirectly in a Transwell system with cartilage engineered from a direct coculture of human bone marrow-derived MSCs and NCs on a 3D porous collagen scaffold. The soluble factors were measured in the conditioned media from the different chambers of the Transwell system. Engineered cartilage from cocultures exposed to the pure NC construct exhibited reduced chondrogenic potential relative to control constructs, shown by reduced extracellular matrix deposition and increased expression of hypertrophic markers. Analysis of the soluble factors within the conditioned media showed an increase in inflammatory cytokines in the coculture chamber exposed to the pure NC construct. Principal component analysis revealed that the majority of the data variance could be explained by proinflammatory factors and hypertrophic chondrogenesis. In conclusion, our data suggest that inflammatory cytokines derived from NCs reduce the chondrogenic potential of coculture engineered cartilage through the induction of hypertrophic chondrogenesis. Impact statement The use of engineered cartilage from cocultured nasal chondrocytes (NCs) and mesenchymal stem cells for nasal cartilage reconstruction may be problematic. Our data suggest that the soluble factors from surrounding native NCs in the cartilage to be fixed can compromise the quality of the engineered cartilage if used in reconstructive surgery.

Pathological calcification in osteoarthritis: an outcome or a disease initiator?

In the progression of osteoarthritis, pathological calcification in the affected joint is an important feature. The role of these crystallites in the pathogenesis and progression of osteoarthritis is controversial; it remains unclear whether they act as a disease initiator or are present as a result of joint damage. Recent studies reported that the molecular mechanisms regulating physiological calcification of skeletal tissues are similar to those regulating pathological or ectopic calcification of soft tissues. Pathological calcification takes place when the equilibrium is disrupted. Calcium phosphate crystallites are identified in most affected joints and the presence of these crystallites is closely correlated with the extent of joint destruction. These observations suggest that pathological calcification is most likely to be a disease initiator instead of an outcome of osteoarthritis progression. Inhibiting pathological crystallite deposition within joint tissues therefore represents a potential therapeutic target in the management of osteoarthritis.

Biochemical and Mechanical Gradients Synergize To Enhance Cartilage Zonal Organization in 3D

Articular cartilage is characterized by zonal organizations containing dual gradients of biochemical cues and mechanical cues. However, how biochemical gradient interacts with the mechanical gradient to drive the cartilage zonal development remains largely unknown. Here, we report the development of a dual-gradient hydrogel platform as a 3D niche to elucidate the relative contributions of biochemical and mechanical niche gradients in modulating zonal-specific chondrocyte responses and cartilage zonal organization. Chondroitin sulfate (CS), a major constituent of cartilage extracellular matrix, was chosen as the biochemical cue. Poly(ethylene glycol), a bioinert polymer, was used to create the stiffness gradient. Dual-gradient hydrogels upregulated cartilage marker expressions and increased chondrocyte proliferation and collagen deposition in a zonal-dependent manner. Hydrogels with CS gradient alone exhibited poor mechanical strength and degraded prematurely after 1 week of culture. While CS gradient alone did not support long-term culture, adding CS gradient to mechanical-gradient hydrogels substantially enhanced cell proliferation, glycosaminoglycan production, and collagen deposition compared to mechanical-gradient hydrogels alone. These results suggest that biochemical and mechanical gradient cues synergize to enhance cartilage zonal organization by chondrocytes in 3D. Together, our results validate the potential of dual-gradient hydrogels as a 3D cell niche for cartilage regeneration with zonal organization and may be used to recreate other tissue interfaces.

Stimulation of a calcified cartilage connecting zone by GDF-5-augmented fibrin hydrogel in a novel layered ectopic in vivo model

Tissue engineering approaches for reconstructing full-depth cartilage defects need to comprise a zone of calcified cartilage to tightly anchor cartilage constructs into the subchondral bone. Here, we investigated whether growth and differentiation factor-5-(GDF-5)-augmented fibrin hydrogel can induce a calcified cartilage-layer in vitro that seamlessly connects cartilage-relevant biomaterials with bone tissue. Human bone marrow stromal cells (BMSCs) were embedded in fibrin hydrogel and subjected to chondrogenesis with TGF-β with or without GDF-5 before constructs were implanted subcutaneously into SCID mice. A novel layered ectopic in vivo model was developed and GDF-5-augmented fibrin with BMSCs was used to glue hydrogel and collagen constructs onto bone disks to investigate formation of a calcified cartilage connecting zone. GDF-5 significantly enhanced ALP activity during in vitro chondrogenesis while ACAN and COL2A1 mRNA, proteoglycan-, collagen-type-II- and collagen-type-X-deposition remained similar to controls. Pellets pretreated with GDF-5 mineralized faster in vivo and formed more ectopic bone. In the novel layered ectopic model, GDF-5 strongly supported calcified cartilage formation that seamlessly connected with the bone. Pro-chondrogenic and pro-hypertrophic activity makes GDF-5-augmented fibrin an attractive bioactive hydrogel with high potential to stimulate a calcified cartilage connecting zone in situ that might promote integration of cartilage scaffolds with bone. Thus, GDF-5-augmented fibrin hydrogel promises to overcome poor fixation of biomaterials in cartilage defects facilitating their long-term regeneration. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 2214-2224, 2018.

Calcified cartilage or bone? Collagens in the tessellated endoskeletons of cartilaginous fish (sharks and rays)

The primary skeletal tissue in elasmobranchs -sharks, rays and relatives- is cartilage, forming both embryonic and adult endoskeletons. Only the skeletal surface calcifies, exhibiting mineralized tiles (tesserae) sandwiched between a cartilage core and overlying fibrous perichondrium. These two tissues are based on different collagens (Coll II and I, respectively), fueling a long-standing debate as to whether tesserae are more like calcified cartilage or bone (Coll 1-based) in their matrix composition. We demonstrate that stingray (Urobatis halleri) tesserae are bipartite, having an upper Coll I-based 'cap' that merges into a lower Coll II-based 'body' zone, although tesserae are surrounded by cartilage. We identify a 'supratesseral' unmineralized cartilage layer, between tesserae and perichondrium, distinguished from the cartilage core in containing Coll I and X (a common marker for mammalian mineralization), in addition to Coll II. Chondrocytes within tesserae appear intact and sit in lacunae filled with Coll II-based matrix, suggesting tesserae originate in cartilage, despite comprising a diversity of collagens. Intertesseral joints are also complex in their collagenous composition, being similar to supratesseral cartilage closer to the perichondrium, but containing unidentified fibrils nearer the cartilage core. Our results indicate a unique potential for tessellated cartilage in skeletal biology research, since it lacks features believed diagnostic for vertebrate cartilage mineralization (e.g. hypertrophic and apoptotic chondrocytes), while offering morphologies amenable for investigating the regulation of complex mineralized ultrastructure and tissues patterned on multiple collagens.

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.

The minor collagens in articular cartilage

Articular cartilage is a connective tissue consisting of a specialized extracellular matrix (ECM) that dominates the bulk of its wet and dry weight. Type II collagen and aggrecan are the main ECM proteins in cartilage. However, little attention has been paid to less abundant molecular components, especially minor collagens, including type IV, VI, IX, X, XI, XII, XIII, and XIV, etc. Although accounting for only a small fraction of the mature matrix, these minor collagens not only play essential structural roles in the mechanical properties, organization, and shape of articular cartilage, but also fulfil specific biological functions. Genetic studies of these minor collagens have revealed that they are associated with multiple connective tissue diseases, especially degenerative joint disease. The progressive destruction of cartilage involves the degradation of matrix constituents including these minor collagens. The generation and release of fragmented molecules could generate novel biochemical markers with the capacity to monitor disease progression, facilitate drug development and add to the existing toolbox for in vitro studies, preclinical research and clinical trials.

Temporally degradable collagen-mimetic hydrogels tuned to chondrogenesis of human mesenchymal stem cells

Tissue engineering strategies for repairing and regenerating articular cartilage face critical challenges to recapitulate the dynamic and complex biochemical microenvironment of native tissues. One approach to mimic the biochemical complexity of articular cartilage is through the use of recombinant bacterial collagens as they provide a well-defined biological 'blank template' that can be modified to incorporate bioactive and biodegradable peptide sequences within a precisely defined three-dimensional system. We customized the backbone of a Streptococcal collagen-like 2 (Scl2) protein with heparin-binding, integrin-binding, and hyaluronic acid-binding peptide sequences previously shown to modulate chondrogenesis and then cross-linked the recombinant Scl2 protein with a combination of matrix metalloproteinase 7 (MMP7)- and aggrecanase (ADAMTS4)-cleavable peptides at varying ratios to form biodegradable hydrogels with degradation characteristics matching the temporal expression pattern of these enzymes in human mesenchymal stem cells (hMSCs) during chondrogenesis. hMSCs encapsulated within the hydrogels cross-linked with both degradable peptides exhibited enhanced chondrogenic characteristics as demonstrated by gene expression and extracellular matrix deposition compared to the hydrogels cross-linked with a single peptide. Additionally, these combined peptide hydrogels displayed increased MMP7 and ADAMTS4 activities and yet increased compression moduli after 6 weeks, suggesting a positive correlation between the degradation of the hydrogels and the accumulation of matrix by hMSCs undergoing chondrogenesis. Our results suggest that including dual degradation motifs designed to respond to enzymatic activity of hMSCs going through chondrogenic differentiation led to improvements in chondrogenesis. Our hydrogel system demonstrates a bimodal enzymatically degradable biological platform that can mimic native cellular processes in a temporal manner. As such, this novel collagen-mimetic protein, cross-linked via multiple enzymatically degradable peptides, provides a highly adaptable and well defined platform to recapitulate a high degree of biological complexity, which could be applicable to numerous tissue engineering and regenerative medicine applications.

Harnessing the Versatility of Bacterial Collagen to Improve the Chondrogenic Potential of Porous Collagen Scaffolds

Collagen I foams are used in the clinic as scaffolds to promote articular cartilage repair as they provide a bioactive environment for cells with chondrogenic potential. However, collagen I as a base material does not allow for precise control over bioactivity. Alternatively, recombinant bacterial collagens can be used as "blank slate" collagen molecules to offer a versatile platform for incorporation of selected bioactive sequences and fabrication into 3D scaffolds. Here, we show the potential of Streptococcal collagen-like 2 (Scl2) protein foams modified with peptides designed to specifically and noncovalently bind hyaluronic acid and chondroitin sulfate to improve chondrogenesis of human mesenchymal stem cells (hMSCs) compared to collagen I foams. Specific compositions of functionalized Scl2 foams lead to improved chondrogenesis compared to both nonfunctionalized Scl2 and collagen I foams, as indicated by gene expression, extracellular matrix accumulation, and compression moduli. hMSCs cultured in functionalized Scl2 foams exhibit decreased collagens I and X gene and protein expression, suggesting an advantage over collagen I foams in promoting a chondrocytic phenotype. These highly modular foams can be further modified to improve specific aspects chondrogenesis. As such, these scaffolds also have the potential to be tailored for other regenerative medicine applications.

Chondroinduction Is the Main Cartilage Repair Response to Microfracture and Microfracture With BST-CarGel: Results as Shown by ICRS-II Histological Scoring and a Novel Zonal Collagen Type Scoring Method of Human Clinical Biopsy Specimens

BACKGROUND

Current cartilage repair histological scoring systems are unable to explain the relationship between collagen type II deposition and overall repair quality.

PURPOSE/HYPOTHESIS

The purpose of this study was to develop a novel zonal collagen type (ZCT) 5-point scoring system to measure chondroinduction in human clinical biopsy specimens collected after marrow stimulation. The hypothesis was that the ZCT scores would correlate with the International Cartilage Repair Society-II (ICRS-II) overall histological repair assessment score and glycosaminoglycan (GAG) content.

STUDY DESIGN

Descriptive laboratory study.

METHODS

After optimizing safranin O staining for GAG and immunostaining for human collagen type II and type I (Col2 and Col1, respectively), serial sections from clinical osteochondral repair biopsy specimens (13 months after microfracture or microfracture with BST-CarGel; n = 39 patients) were stained and 3 blinded readers performed histomorphometry for percentage of staining, ICRS-II histological scoring, polarized light microscopy (PLM) scoring, and 5-point ZCT scoring based on tidemark morphology, zonal distribution of Col2 and Col1, and Col1 percentage stain. Because 1 biopsy specimen was missing bone, 38 biopsy specimens were evaluated for ICRS-II, PLM, and ZCT scores.

RESULTS

Chondroinduction was identified in 21 biopsy specimens as a Col2 matrix fused to bone that spanned the deep-middle-superficial zones ("full-thickness hyaline repair"), deep-middle zones, or deep zone ("stalled hyaline") that was covered with a variable-thickness Col1-positive matrix, and was scored, respectively, as ZCT = 1 (n = 4 biopsy specimens), ZCT = 2 (n = 6) and ZCT = 3 (n = 11). Other biopsy specimens (n = 17) were fibrocartilage (n = 9; ZCT = 4), fibrous tissue (n = 4, ZCT = 5), or non-marrow derived (n = 4; ZCT = 0). Non-marrow derived tissue had a mean mature tidemark score of 84 out of 100 versus a regenerating tidemark score of 24 for all other biopsy specimens (P = .005). Both "stalled hyaline" repair and fibrocartilage had the same mean Col2 percentage stain; however, fibrocartilage was distinguished by heavy Col1 deposits in the deep zone, a 2-fold higher mean Col1 percentage stain (P = .001), and lower surface integrity (P = .03). ZCT scores correlated with GAG content and the ICRS-II overall assessment score, especially when combined with the PLM score for collagen organization (R = 0.82). Histological scores of the deep zone strongly predicted the ICRS-II overall assessment score (R = 0.99).

CONCLUSION

The ICRS-II overall repair assessment score and GAG content correlated with the extent of Col2 deposition free of fibrosis in the deep/middle zone rather than bulk accumulation of Col2.

CLINICAL RELEVANCE

Biopsy tissue from the BST-CarGel randomized clinical trial (microfracture without and with BST-CarGel, as treatment groups were not unblinded) showed regenerated tissue consistent with a chondroinduction mechanism in at least half of the treated lesions.

Activating enhancer binding protein 2 epsilon (AP-2ε)-deficient mice exhibit increased matrix metalloproteinase 13 expression and progressive osteoarthritis development

Introduction

The transcription factor activating enhancer binding protein 2 epsilon (AP-2ε) was recently shown to be expressed during chondrogenesis as well as in articular chondrocytes of humans and mice. Furthermore, expression of AP-2ε was found to be upregulated in affected cartilage of patients with osteoarthritis (OA). Despite these findings, adult mice deficient for AP-2ε (Tfap2e^−/−) do not exhibit an obviously abnormal cartilaginous phenotype. We therefore analyzed embryogenesis of Tfap2e^−/− mice to elucidate potential transient abnormalities that provide information on the influence of AP-2ε on skeletal development. In a second part, we aimed to define potential influences of AP-2ε on articular cartilage function and gene expression, as well as on OA progression, in adult mice.

Methods

Murine embryonic development was accessed via in situ hybridization, measurement of skeletal parameters and micromass differentiation of mesenchymal cells. To reveal discrepancies in articular cartilage of adult wild-type (WT) and Tfap2e^−/− mice, light and electron microscopy, in vitro culture of cartilage explants, and quantification of gene expression via real-time PCR were performed. OA was induced via surgical destabilization of the medial meniscus in both genotypes, and disease progression was monitored on histological and molecular levels.

Results

Only minor differences between WT and embryos deficient for AP-2ε were observed, suggesting that redundancy mechanisms effectively compensate for the loss of AP-2ε during skeletal development. Surprisingly, though, we found matrix metalloproteinase 13 (Mmp13), a major mediator of cartilage destruction, to be significantly upregulated in articular cartilage of adult Tfap2e^−/− mice. This finding was further confirmed by increased Mmp13 activity and extracellular matrix degradation in Tfap2e^−/− cartilage explants. OA progression was significantly enhanced in the Tfap2e^−/− mice, which provided evidence for in vivo relevance. This finding is most likely attributable to the increased basal Mmp13 expression level in Tfap2e^−/− articular chondrocytes that results in a significantly higher total Mmp13 expression rate during OA as compared with the WT.

Conclusions

We reveal a novel role of AP-2ε in the regulation of gene expression in articular chondrocytes, as well as in OA development, through modulation of Mmp13 expression and activity.

Optical clearing in collagen- and proteoglycan-rich osteochondral tissues

OBJECTIVE

Recent developments in optical clearing and microscopy technology have enabled the imaging of intact tissues at the millimeter scale to characterize cells via fluorescence labeling. While these techniques have facilitated the three-dimensional (3D) cellular characterization within brain and heart, study of dense connective tissues of the musculoskeletal system have been largely unexplored. Here, we quantify how optical clearing impacted the cell and tissue morphology of collagen-, proteoglycan-, and mineral-rich cartilage and bone from the articulating knee joint.

METHODS

Water-based fructose solutions were used for optical clearing of bovine osteochondral tissues, followed by imaging with transmission and confocal microscopy. To confirm preservation of tissue structure during the clearing process, samples were mechanically tested in unconfined compression and visualized by cryo-SEM.

RESULTS

Optical clearing enhanced light transmission through cartilage, but not subchondral bone regions. Fluorescent staining and immunolabeling was preserved through sample preparations, enabling imaging to cartilage depths five times deeper than previously reported, limited only by the working distance of the microscope objective. Chondrocyte volume remained unchanged in response to, and upon the reversal, of clearing. Equilibrium modulus increased in cleared samples, and was attributed to exchange of interstitial fluid with the more viscous fructose solution, but returned to control levels upon unclearing. In addition, cryo-SEM-based analysis of cartilage showed no ultrastructural changes.

CONCLUSION

We anticipate large-scale microscopy of diverse connective tissues will enable the study of intact, 3D interfaces (e.g., osteochondral) and cellular connectivity as a function of development, disease, and regeneration, which have been previously hindered by specimen opacity.

Cartilage to bone transitions in health and disease

Aberrant redeployment of the 'transient' events responsible for bone development and postnatal longitudinal growth has been reported in some diseases in what is otherwise inherently 'stable' cartilage. Lessons may be learnt from the molecular mechanisms underpinning transient chondrocyte differentiation and function, and their application may better identify disease aetiology. Here, we review the current evidence supporting this possibility. We firstly outline endochondral ossification and the cellular and physiological mechanisms by which it is controlled in the postnatal growth plate. We then compare the biology of these transient cartilaginous structures to the inherently stable articular cartilage. Finally, we highlight specific scenarios in which the redeployment of these embryonic processes may contribute to disease development, with the foresight that deciphering those mechanisms regulating pathological changes and loss of cartilage stability will aid future research into effective disease-modifying therapies.

Potential of Raloxifene in reversing osteoarthritis-like alterations in rat chondrocytes: an in vitro model study

The aim of this study was to investigate the effects of Raloxifene (Ral) on degeneration-related changes in osteoarthritis (OA)-like chondrocytes using two- and three-dimensional models. Five-azacytidine (Aza-C) was used to induce OA-like alterations in rat articular chondrocytes and the model was verified at molecular and macrolevels. Chondrocytes were treated with Ral (1, 5 and 10 mu M) for 10 days. Caspase-3 activity, gene expressions of aggrecan, collagen II, alkaline phosphatase (ALP), collagen X, matrix metalloproteinases (MMP-13, MMP-3 and MMP-2), and MMP-13, MMP-3 and MMP-2 protein expressions were studied in two-dimensional model. Matrix deposition and mechanical properties of agarose-chondrocyte discs were evaluated in three-dimensional model. One mu M Ral reduced expression of OA-related genes, decreased apoptosis, and MMP-13 and MMP-3 protein expressions. It also increased aggrecan and collagen II gene expressions relative to untreated OA-like chondrocytes. In three-dimensional model, 1 mu M Ral treatment resulted in increased collagen deposition and improved mechanical properties, although a significant increase for sGAG was not observed. In summation, 1 mu M Ral improved matrix-related activities, whereas dose increment reversed these effects except ALP gene expression and sGAG deposition. These results provide evidence that low-dose Ral has the potential to cease or reduce the matrix degeneration in OA.

Zone-specific gene expression patterns in articular cartilage

OBJECTIVE

To identify novel genes and pathways specific to the superficial zone (SZ), middle zone (MZ), and deep zone (DZ) of normal articular cartilage.

METHODS

Articular cartilage was obtained from the knees of 4 normal human donors. The cartilage zones were dissected on a microtome. RNA was analyzed on human genome arrays. The zone-specific DNA array data obtained from human tissue were compared to array data obtained from bovine cartilage. Genes differentially expressed between zones were evaluated using direct annotation for structural or functional features, and by enrichment analysis for integrated pathways or functions.

RESULTS

The greatest differences in genome-wide RNA expression data were between the SZ and DZ in both human and bovine cartilage. The MZ, being a transitional zone between the SZ and DZ, thereby shared some of the same pathways as well as structural/functional features of the adjacent zones. Cellular functions and biologic processes that were enriched in the SZ relative to the DZ included, most prominently, extracellular matrix-receptor interactions, cell adhesion molecule functions, regulation of actin cytoskeleton, ribosome-related functions, and signaling aspects such as the IFN, IL4, Cdc42/Rac, and JAK/STAT signaling pathways. Two pathways were enriched in the DZ relative to the SZ, including PPARG and EGFR/SMRTE.

CONCLUSION

These differences in cartilage zonal gene expression identify new markers and pathways that govern the unique differentiation status of chondrocyte subpopulations.

A functional agarose-hydroxyapatite scaffold for osteochondral interface regeneration

Regeneration of the osteochondral interface is critical for integrative and functional cartilage repair. This study focuses on the design and optimization of a hydrogel-ceramic composite scaffold of agarose and hydroxyapatite (HA) for calcified cartilage formation. The first study objective was to compare the effects of HA on non-hypertrophic and hypertrophic chondrocytes cultured in the composite scaffold. Specifically, cell growth, biosynthesis, hypertrophy, and scaffold mechanical properties were evaluated. Next, the ceramic phase of the scaffold was optimized in terms of particle size (200 nm vs. 25 μm) and dose (0-6 w/v%). It was observed that while deep zone chondrocyte (DZC) biosynthesis and hypertrophy remained unaffected, hypertrophic chondrocytes measured higher matrix deposition and mineralization potential with the addition of HA. Most importantly, higher matrix content translated into significant increases in both compressive and shear mechanical properties. While cell hypertrophy was independent of ceramic size, matrix deposition was higher only with the addition of micron-sized ceramic particles. In addition, the highest matrix content, mechanical properties and mineralization potential were found in scaffolds with 3% micro-HA, which approximates both the mineral aggregate size and content of the native interface. These results demonstrate that the biomimetic hydrogel-ceramic composite is optimal for calcified cartilage formation and is a promising design strategy for osteochondral interface regeneration.

Effect of exercise on articular cartilage

This review primarily focuses on how the macromolecular composition and architecture of articular cartilage and its unique biomechanical properties play a pivotal role in the ability of articular cartilage to withstand mechanical loads several magnitudes higher than the weight of the individual. Current findings on short-term and long-term effects of exercise on human articular cartilage are reviewed, and the importance of appropriate exercises for individuals with normal and diseased or aberrated cartilage is discussed.

International Cartilage Repair Society (ICRS) Recommended Guidelines for Histological Endpoints for Cartilage Repair Studies in Animal Models and Clinical Trials

Cartilage repair strategies aim to resurface a lesion with osteochondral tissue resembling native cartilage, but a variety of repair tissues are usually observed. Histology is an important structural outcome that could serve as an interim measure of efficacy in randomized controlled clinical studies. The purpose of this article is to propose guidelines for standardized histoprocessing and unbiased evaluation of animal tissues and human biopsies. Methods were compiled from a literature review, and illustrative data were added. In animal models, treatments are usually administered to acute defects created in healthy tissues, and the entire joint can be analyzed at multiple postoperative time points. In human clinical therapy, treatments are applied to developed lesions, and biopsies are obtained, usually from a subset of patients, at a specific time point. In striving to standardize evaluation of structural endpoints in cartilage repair studies, 5 variables should be controlled: 1) location of biopsy/sample section, 2) timing of biopsy/sample recovery, 3) histoprocessing, 4) staining, and 5) blinded evaluation with a proper control group. Histological scores, quantitative histomorphometry of repair tissue thickness, percentage of tissue staining for collagens and glycosaminoglycan, polarized light microscopy for collagen fibril organization, and subchondral bone integration/structure are all relevant outcome measures that can be collected and used to assess the efficacy of novel therapeutics. Standardized histology methods could improve statistical analyses, help interpret and validate noninvasive imaging outcomes, and permit cross-comparison between studies. Currently, there are no suitable substitutes for histology in evaluating repair tissue quality and cartilaginous character.

Modulation of cartilage differentiation by melanoma inhibiting activity/cartilage-derived retinoic acid-sensitive protein (MIA/CD-RAP)

Melanoma inhibiting activity/cartilage-derived retinoic acid-sensitive protein (MIA/CD-RAP) is a small soluble protein secreted from malignant melanoma cells and from chondrocytes. Recently, we revealed that MIA/CD-RAP can modulate bone morphogenetic protein (BMP)2-induced osteogenic differentiation into a chondrogenic direction. In the current study we aimed to find the molecular details of this MIA/CD-RAP function. Direct influence of MIA on BMP2 by protein-protein-interaction or modulating SMAD signaling was ruled out experimentally. Instead, we revealed inhibition of ERK signaling by MIA/CD-RAP. This inhibition is regulated via binding of MIA/CD-RAP to integrin alpha5 and abolishing its activity. Active ERK signaling is known to block chondrogenic differentiation and we revealed induction of aggrecan expression in chondrocytes by treatment with MIA/CD-RAP or PD098059, an ERK inhibitor. In in vivo models we could support the role of MIA/CD-RAP in influencing osteogenic differentiation negatively. Further, MIA/CD-RAP-deficient mice revealed an enhanced calcified cartilage layer of the articular cartilage of the knee joint and disordered arrangement of chondrocytes. Taken together, our data indicate that MIA/CD-RAP stabilizes cartilage differentiation and inhibits differentiation into bone potentially by regulating signaling processes during differentiation.

Tissue engineering of articular cartilage with biomimetic zones

Articular cartilage damage is a persistent and increasing problem with the aging population, and treatments to achieve biological repair or restoration remain a challenge. Cartilage tissue engineering approaches have been investigated for over 20 years, but have yet to achieve the consistency and effectiveness for widespread clinical use. One of the potential reasons for this is that the engineered tissues do not have or establish the normal zonal organization of cells and extracellular matrix that appears critical for normal tissue function. A number of approaches are being taken currently to engineer tissue that more closely mimics the organization of native articular cartilage. This review focuses on the zonal organization of native articular cartilage, strategies being used to develop such organization, the reorganization that occurs after culture or implantation, and future prospects for the tissue engineering of articular cartilage with biomimetic zones.

Sustained hypoxia enhances chondrocyte matrix synthesis

Articular cartilage is an avascular tissue with chondrocytes in the deeper zones existing under conditions of sustained hypoxia. Using a hypoxic chamber to provide controlled hypoxia, this study was performed to determine whether sustained hypoxia enhances the production of cartilage matrix proteins. Freshly isolated primary bovine articular chondrocytes were encapsulated in three-dimensional alginate beads and maintained at 2% oxygen with media changes using media pre-equilibrated to 2% oxygen. Immunolocalization of HIF-1alpha was performed to verify hypoxic conditions. Sustained hypoxia resulted in an increase in proteoglycan synthesis after only 1 day, as measured by 35S-sulfate incorporation. This increase was maintained for the duration of the 17 day study. After 17 days of hypoxic culture, increases in total type II collagen and COL2A1 gene expression were probed by indirect immunofluorescence, type II collagen ELISA, and real-time qPCR; in addition, increased glycosaminoglycan deposition was observed as determined by chemical analysis. These studies show that sustained hypoxia enhances articular chondrocyte matrix synthesis and viability in three-dimensional alginate culture.

Development of the supraspinatus tendon-to-bone insertion: localized expression of extracellular matrix and growth factor genes

The adult healing response of the rotator cuff tendon-to-bone insertion site differs from the ordered process of insertion site development. Healing is characterized by disorganized scar and a lack of fibrocartilage formation, in contrast to the well organized fibrocartilaginous transition which forms during the normal development of the tendon-to-bone insertion. The purpose of this study was to localize the expression of a number of extracellular matrix and growth factor genes during insertion site development in order to guide future strategies for augmenting adult rotator cuff healing. The rotator cuff was morphologically distinct at 13.5 dpc (days postconception). Neo-tendon was evident as a condensation of cells adjacent to bone. The interface between tendon and bone did not form into a mature fibrocartilaginous insertion until 21-days postnatally, based upon the appearance of four distinct zones with a mineralized humeral head. Fibroblasts of the supraspinatus tendon expressed type I collagen at all timepoints. Type II collagen was first expressed by chondrocytes in the fibrocartilage and mineralized fibrocartilage at 7 days and persisted in the mineralized fibrocartilage at 56 days. Type X collagen was first expressed by the chondrocytes in the mineralized fibrocartilage at 14 days and persisted in the mineralized fibrocartilage at 56 days. A shift from TGF-beta3 to TGF-beta1 expression occurred at 15.5 dpc.

Chitosan-glycerol phosphate/blood implants improve hyaline cartilage repair in ovine microfracture defects

BACKGROUND

Microfracture is a surgical procedure that is used to treat focal articular cartilage defects. Although joint function improves following microfracture, the procedure elicits incomplete repair. As blood clot formation in the microfracture defect is an essential initiating event in microfracture therapy, we hypothesized that the repair would be improved if the microfracture defect were filled with a blood clot that was stabilized by the incorporation of a thrombogenic and adhesive polymer, specifically, chitosan. The objectives of the present study were to evaluate (1) blood clot adhesion in fresh microfracture defects and (2) the quality of the repair, at six months postoperatively, of microfracture defects that had been treated with or without chitosan-glycerol phosphate/blood clot implants, using a sheep model.

METHODS

In eighteen sheep, two 1-cm2 full-thickness chondral defects were created in the distal part of the femur and treated with microfracture; one defect was made in the medial femoral condyle, and the other defect was made in the trochlea. In four sheep, microfracture defects were created bilaterally; the microfracture defects in one knee received no further treatment, and the microfracture defects in the contralateral knee were filled with chitosan-glycerol phosphate/autologous whole blood and the implants were allowed to solidify. Fresh defects in these four sheep were collected at one hour postoperatively to compare the retention of the chitosan-glycerol phosphate/blood clot with that of the normal clot and to define the histologic characteristics of these fresh defects. In the other fourteen sheep, microfracture defects were made in only one knee and either were left untreated (control group; six sheep) or were treated with chitosan-glycerol phosphate/blood implant (treatment group; eight sheep), and the quality of repair was assessed histologically, histomorphometrically, and biochemically at six months postoperatively.

RESULTS

In the defects that were examined one hour postoperatively, chitosan-glycerol phosphate/blood clots showed increased adhesion to the walls of the defects as compared with the blood clots in the untreated microfracture defects. After histological processing, all blood clots in the control microfracture defects had been lost, whereas chitosanglycerol phosphate/blood clot adhered to and was partly retained on the surfaces of the defect. At six months, defects that had been treated with chitosan-glycerol phosphate/blood were filled with significantly more hyaline repair tissue (p < 0.05) compared with control defects. Repair tissue from medial femoral condyle defects that had been treated with chitosan-glycerol phosphate/blood contained more cells and more collagen compared with control defects and showed complete restoration of glycosaminoglycan levels.

CONCLUSIONS

Solidification of a chitosan-glycerol phosphate/blood implant in microfracture defects improved cartilage repair compared with microfracture alone by increasing the amount of tissue and improving its biochemical composition and cellular organization.

The tidemark of the chondro-osseous junction of the normal human knee joint

The chondro-osseous junction includes the junction between calcified and non-calcified cartilage matrices often referred to as the tidemark. A detailed knowledge of the structure, function and pathophysiology of the chondro-osseous junction is essential for an understanding both of the normal elongation of bones and of the pathogenesis of osteoarthrosis. In this study the molecular anatomy of the tidemark was studied using histochemical techniques, including lectin histochemistry, on blocks of normal cartilage from human knee joints. The tidemark stained with H and E, picro-sirius red, toluidine blue, safranin O and methyl green, but not with alcian blue in the presence of magnesium chloride at 0.05 M or above. It stained with only four lectins, those from Datura stramonium, Maclura pomifera, Erythrina crystagalli and Helix pomatia, out of the 19 used. Therefore, it is rich in collagen and contains hyaluronan, but appears to lack the glycosaminoglycans of 'conventional' proteoglycans and it expresses a very limited and distinctive lectin staining glycoprofile, which is probably attributable to specific glycoproteins. In addition, the tidemark had a distinct microanatomical trilaminate appearance. From all of these results it is clear that this part of the chondro-osseous junctional region is chemically more complex and distinctive than has previously been described.

Articular cartilage functional histomorphology and mechanobiology: a research perspective

The histomorphogenesis of articular cartilage is regulated during skeletal development by the intermittent forces and motions imposed at diarthrodial joints. A key feature in this development is the formation of the superficial, transitional, radial, and calcified cartilage zones through the cartilage thickness. The histomorphological, biological, and mechanical characteristics of these zones can be correlated with the distributions of pressures, deformations, and pressure-induced fluid flow that are created in vivo. In a mature joint, cyclic loads produce cyclic hydrostatic fluid pressure through the entire cartilage thickness that is comparable in magnitude to the applied joint pressure. Prolonged physical activity can cause the total cartilage thickness to decrease about 5%, although the consolidation strains vary tremendously in the different zones. The superficial zone can experience significant fluid exudation and consolidation (compressive strains) in the range of 60% while the radial zone experiences relatively little fluid flow and consolidation. The topological variation in the histomorphologic appearance of articular cartilage is influenced by the local mechanical loading of chondrocytes in the different zones. Patterns of stress, strain, and fluid flow created in the joint result in spatial and temporal changes in the rates of synthesis and degradation of matrix proteins. When viewed over the course of a lifetime, even subtle difference in these cellular processes can affect the micro- and macro-morphology of articular cartilage. This hypothesis is supported by in vivo and ex vivo experiments where load-induced changes in matrix synthesis and catabolism, gene expression, and signal transduction pathways have been observed.

Site-specific immunostaining for type X collagen in noncalcified articular cartilage of canine stifle knee joint

Type X collagen is a short-chain collagen that is strongly expressed in hypertrophic chondrocytes. In this study, we used an immunohistochemical technique exploiting a prolonged hyaluronidase unmasking of type X collagen epitopes to show that type X collagen is not restricted to calcified cartilage, but is also present in normal canine noncalcified articular cartilage. A 30 degrees valgus angulation procedure of the right tibia was performed in 15 dogs at the age of 3 months, whereas their nonoperated sister dogs served as controls. Samples were collected 7 and 18 months after the surgery and immunostained for type X collagen. The deposition of type X collagen increased during maturation from age 43 weeks to 91 weeks. In the patella, most of the noncalcified cartilage stained for type X collagen, whereas, in the patellar surface of the femur, it was present mainly in the femoral groove close to cartilage surface. In femoral condyles, the staining localized mostly in the superficial cartilage on the lateral and medial sides, but not in the central weight-bearing area. In tibial condyles, type X collagen was often observed close to the cartilage surface in medial parts of the condyles, although staining could also be seen in the deep zone of the cartilage. Staining for type X collagen appeared strongest at sites where the birefringence of polarized light was lowest, suggesting a colocalization of type X collagen with the collagen fibril arcades in the intermediate zone. No significant difference in type X collagen immunostaining was observed in lesion-free articular cartilage between controls and dogs that underwent a 30 degrees valgus osteotomy. In osteoarthritic lesions, however, there was strong immunostaining for both type X collagen and collagenase-induced collagen cleavage products. The presence of type X collagen in the transitional zone of cartilage in the patella, femoropatellar groove, and in tibial cartilage uncovered by menisci suggests that it may involve a modification of collagen fibril arrangement at the site of collagen fibril arcades, perhaps providing additional support to the collagen network.

Cellular immunolocalization of gastric and pancreatic lipase in various tissues obtained from dogs

OBJECTIVE

To determine cellular immunolocalization of canine gastric lipase (cGL) and canine pancreatic lipase (cPL) in various tissues obtained from clinically healthy dogs.

SAMPLE POPULATION

Samples of 38 tissues collected from 2 climically healthy dogs.

PROCEDURES

The cGL and cPL were purified from gastric and pancreatic tissue, respectively, obtained from dogs. Antisera against both proteins were developed, using rabbits, and polyclonal antibodies were purified by use of affinity chromatography. Various tissues were collected from 2 healthy dogs. Primary antibodies were used to evaluate histologic specificity. Replicate sections from the collected tissues were immunolabeled for cGL and cPL and examined by use of light microscopy.

RESULTS

Mucous neck cells and mucous pit cells of gastric glands had positive labeling for cGL, whereas other tissues did not immunoreact with cGL. Pancreatic acinar cells had positive labeling for cPL, whereas other tissues did not immunoreact with cPL.

CONCLUSIONS AND CLINICAL RELEVANCE

We concluded that cGL and cPL are exclusively expressed in gastric glands and pancreatic acinar cells, respectively. Also, evidence for cross-immunoreactivity with other lipases or related proteins expressed by other tissues was not found for either protein. Analysis of these data suggests that gastric lipase is a specific marker for gastric glands and that pancreatic lipase is a specific marker for pancreatic acinar cells. These markers may have clinical use in the diagnosis of gastric and exocrine pancreatic disorders, respectively.

Collagen of articular cartilage

The extracellular framework and two-thirds of the dry mass of adult articular cartilage are polymeric collagen. Type II collagen is the principal molecular component in mammals, but collagens III, VI, IX, X, XI, XII and XIV all contribute to the mature matrix. In developing cartilage, the core fibrillar network is a cross-linked copolymer of collagens II, IX and XI. The functions of collagens IX and XI in this heteropolymer are not yet fully defined but, evidently, they are critically important since mutations in COLIX and COLXI genes result in chondrodysplasia phenotypes that feature precocious osteoarthritis. Collagens XII and XIV are thought also to be bound to fibril surfaces but not covalently attached. Collagen VI polymerizes into its own type of filamentous network that has multiple adhesion domains for cells and other matrix components. Collagen X is normally restricted to the thin layer of calcified cartilage that interfaces articular cartilage with bone.

Culture of chondrocytes in alginate surrounded by fibrin gel: characteristics of the cells over a period of eight weeks

OBJECTIVE

To produce tissue engineered cartilage by human articular chondrocytes in vitro for further use in in vivo manipulations for the treatment of cartilage defects.

METHODS

Human articular chondrocytes were cultured in 0.5%, 1.0%, and 2.0% of alginate for up to four weeks. The optimal concentration of an alginate matrix for cell replication and for aggrecan synthesis by chondrocytes was determined. DNA content in the different culture conditions was measured after two and four weeks. Aggrecan synthesis rates and accumulation in the surrounding extracellular matrix were assessed by [(35)S]sulphate incorporation after the same periods of culture. To follow the outgrowth of chondrocytes from the alginate beads, chondrocytes were cultured for four weeks in 0.5 or 1.0% alginate surrounded by 0.25 or 0.5% fibrin gel. DNA content of each culture was measured after different culture periods. Finally, human chondrocytes in 1.0% alginate beads were embedded in 0.5% fibrin gel for eight weeks. Immunohistochemical analysis for aggrecan, type I and II collagen was performed weekly.

RESULTS

At two weeks the DNA content in each culture significantly increased in 0.5 and 1.0% alginate cultures in comparison with baseline values. This increase continued until week 4 at the three alginate concentrations. Aggrecan synthesis at two weeks was highest in 0.5 and 1.0% alginate cell cultures. At four weeks aggrecan synthesis rates decreased independently of the alginate concentrations. Aggrecan mainly accumulated in the interterritorial matrix. Proliferation of chondrocytes in alginate and outgrowth of these cells in the surrounding fibrin gel were evident throughout the culture period. The accumulation of aggrecan and type II collagen around the cells, in alginate as well as in fibrin gel, gradually increased over the culture period. Type I collagen appeared after six weeks in alginate and in the surrounding fibrin.

CONCLUSION

Human chondrocytes proliferate in this culture system, show an outgrowth into the surrounding fibrin, and synthesise a cartilage-like matrix for up to eight weeks.

Type II collagen degradation in articular cartilage fibrillation after anterior cruciate ligament transection in rats

OBJECTIVE

To investigate the kinetics of early cartilage changes in mechanically induced osteoarthritis (OA) and the association of these changes with damage to the type II collagen network.

METHODS

Experimental OA was induced by anterior cruciate ligament transsection in the rat knee joint (ACLT-OA). Animals were sacrificed after 2, 7, 14, 28 and 70 days. Knee joints were evaluated using routine histology and immunohistochemistry for denatured (unwound) type II collagen to detect collagen damage. An antibody recognizing the collagenase cleavage site in type II collagen was used to study the role of collagenase in this process.

RESULTS

The first changes of the articular cartilage after anterior cruciate ligament transection occurred in the superficial zone. These changes included loss of superficial chondrocytes, swelling of the remaining chondrocytes and superficial fibrillation. The swelling of the chondrocytes did not result from a change towards the hypertrophic phenotype, since these cells did not stain for type X collagen. A marked increase in denatured type II collagen staining was present in the fibrillated areas. Staining of the collagenase cleavage site showed the same distribution as denatured collagen but was clearly less intense. Collagen damage could never be detected before fibrillation occurred and was not present in non-fibrillated areas.

CONCLUSIONS

These results indicate that in this model cartilage degeneration starts at the articular surface and that this degeneration is associated with a localized expression of type II collagen degradation products.

Expression of type X collagen in young and old C57Bl/6 and Balb/c mice. Relation with articular cartilage degeneration

OBJECTIVE

To investigate whether the development of osteoarthritic lesions in the knee joints of mice is associated with increased immunostaining of type X collagen.

METHODS

Sections of total knee joints in combination with immunohistochemistry were used to study the distribution of type X collagen in the cartilage of young and old mice of two mouse strains, Balb/c and C57Bl/6, known to develop osteoarthritic lesions at different locations. Expression of type X collagen and PTH/PTHrP-receptor mRNA were studied by RT-PCR.

RESULTS

Young adult Balb/c and C57Bl/6 mice both expressed type X collagen in the non-calcified cartilage of the tibia-femoral joint. Old mice of both strains had a strongly increased deposition of type X collagen in the patella-femoral but not in the tibia-femoral joint. The locations in the murine knee joints prone to develop osteoarthritis (OA) did not preferentially express increased amounts of type X collagen. Thus, whereas increased type X was observed in both strains in the patella-femoral joints, only Balb/c mice preferentially developed osteoarthritic lesions in these joints. Also cartilage degeneration was usually seen only in the lateral compartment of the knee joints of C57Bl/6 mice but this was not accompanied by increased type X collagen immunostaining. Increased deposition of type X collagen was not associated with elevated levels of type X collagen mRNA or with decreased levels of PTH/PTHrP-receptor mRNA.

CONCLUSION

Type X collagen expression and spontaneous OA in mice are not necessarily related since OA prone locations in the murine knee joint do not preferentially express type X collagen.

Activation of annexin II and V expression, terminal differentiation, mineralization and apoptosis in human osteoarthritic cartilage

OBJECTIVE

To test the hypothesis that terminal differentiation of chondrocytes in human osteoarthritic cartilage might lead to the failure of repair mechanisms and might cause progressive loss of structure and function of articular cartilage.

DESIGN

Markers for terminally differentiated chondrocytes, such as alkaline phosphatase, annexin II, annexin V and type X collagen, were detected by immunohistochemical analysis of human normal and osteoarthritic knee cartilage from medial and lateral femoral condyles. Apoptosis in these specimens was detected using the TUNEL labeling. Mineralization and matrix vesicles were detected by alizarin red S staining and electron microscopic analysis.

RESULTS

Alkaline phosphatase, annexin II, annexin V and type X collagen were expressed by chondrocytes in the upper zone of early stage and late stage human osteoarthritic cartilage. However, these proteins, which are typically expressed in hypertrophic and calcifying growth plate cartilage, were not detectable in the upper, middle and deep zones of healthy human articular cartilage. TUNEL labeling of normal and osteoarthritic human cartilage sections provided evidence that chondrocytes in the upper zone of late stage osteoarthritic cartilage undergo apoptotic changes. In addition, mineral deposits were detected in the upper zone of late stage osteoarthritic cartilage. Needle-like mineral crystals were often associated with matrix vesicles in these areas, as seen in calcifying growth plate cartilage.

CONCLUSION

Human osteoarthritic chondrocytes adjacent to the joint space undergo terminal differentiation, release alkaline phosphatase-, annexin II- and annexin V-containing matrix vesicles, which initiate mineral formation, and eventually die by apoptosis. Thus, these cells resume phenotypic changes similar to terminal differentiation of chondrocytes in growth plate cartilage culminating in the destruction of articular cartilage in osteoarthritis.

Comparison of surgically attached and non-attached repair of the rat Achilles tendon-bone interface. Cellular organization and type X collagen expression

The effects of surgical repair versus non-repair on cell morphology and type X collagen expression were investigated using a rat model of Achilles tendon avulsion. The animals were divided into four groups. In Group 1, tendon was reattached to the original attachment site by suturing through a drill hole in the calcaneus; in Group II, tendon was not reattached and a drill hole was not made; in Group III, tendon was not reattached but a drill hole was made; and the animals in Group IV were sham operated. In Group I (tendon reattached), at 2 weeks postoperatively, many hypertrophic chondrocytes appeared at the reattachment site adjacent to bone and type X collagen was detected immunologically both in the cells and in the extracellular matrix. After 4 weeks, the cells at the original site of attachment were arranged in rows along the newly formed tendon fibers and were stained with type X collagen antibody. By contrast, when tendon was not reattached (Groups II and III), a gap between the original attachment site and the tendon stump was observed through the entire postoperative period. At 8 weeks, the original attachment site was covered by fibrocartilaginous tissue and tendon became attached to the calcaneal fibrocartilage area, which is proximal to the original attachment site. Type X collagen was detected in the cells which were adjacent to bone. In Group IV (sham operation), there were no changes in histology or type X collagen distribution, either at the attachment site or in tendon and bone, compared with the non-operated control rats. These results suggest that surgical reattachment of tendon to the original site is important to help reorganize cells during the repair process. Type X collagen was identified immunohistochemically in the cells adjacent to bone in all the groups, suggesting that it may play a role in maintaining distinct areas of calcified and non-calcified fibrocartilage.

Type X collagen, a natural component of mouse articular cartilage: association with growth, aging, and osteoarthritis

OBJECTIVE

To perform a systematic study on the production and deposition of type X collagen in developing, aging, and osteoarthritic (OA) mouse articular cartilage.

METHODS

Immunohistochemistry was employed to define the distribution of type X collagen and Northern analyses to determine the messenger RNA levels as an indicator of the synthetic activity of the protein.

RESULTS

Type X collagen was observed in the epiphyseal and articular cartilage of mouse knee joints throughout development and growth. Type X collagen deposition in the transitional zone of articular cartilage became evident toward cessation of growth, at the age of 2-3 months. The most intense staining for type X collagen was limited to the tidemark, the border between uncalcified and calcified cartilage. Northern analysis confirmed that the type X collagen gene is also transcribed by articular cartilage chondrocytes. Intense immunostaining was observed in the areas of OA lesions, specifically, at sites of osteophyte formation and surface fibrillation. Type X collagen deposition was also seen in degenerating menisci.

CONCLUSION

This study demonstrates that type X collagen is a natural component of mouse articular cartilage throughout development, growth, and aging. This finding and the deposition of type X collagen at sites of OA lesions suggest that type X collagen may have a role in providing structural support for articular cartilage.

Identification of types II, IX and X collagens at the insertion site of the bovine achilles tendon

Achilles tendinous collagen fibrils insert into the calcaneus by first passing through a zone that is defined histologically as fibrocartilaginous. This zone consists of four regions: tendon proper, non-mineralized and mineralized fibrocartilage and bone. The function of this zone has not yet been clearly defined. To gain more insight into the role of this fibrocartilaginous zone, collagens present in the zone of the Achilles tendon-calcaneus interface were isolated and characterized. Types II, IX and X collagens were identified in the pepsin digests of the tissue harvested from the bovine Achilles tendon-calcaneus interface. Western blotting using specific antisera to types II, IX and X collagens confirmed the identity of these collagens. Immunofluorescence localization placed type X collagen predominantly in the mineralized zone of the tendon-calcaneus junction, while type IX collagen was distributed throughout the the insertion site. The presence of the cartilage-specific collagens at the Achilles tendon-calcaneus-interface suggests that this zone is cartilaginous in nature. The presence of type X collagen at this junction is not clear, but our present findings go along with the previous report which showed that type X collagen is present in the mineralized zone of the medial collateral ligament femoral insertion site. These data suggest that type X collagen may be a resident of mineralized fibrocartilaginous zones of tendon or ligament-bone junctions and may participate in anchoring ligament or tendon to bone.