Biology Of Articular Cartilage Research Articles

Comparison of Kindlin-2 deficiency-stimulated osteoarthritis-like lesions induced by Prg4CreERT2 versus AggrecanCreERT2 transgene in mice

BackgroundGenetically modified mice are the most useful tools for investigating the gene functions in articular cartilage biology and the pathogenesis of osteoarthritis. The AggrecanCreERT2 mice are one of the most reported mouse lines used for this purpose. The Prg4 (proteoglycan 4) gene encodes the lubricin protein and is expressed selectively in chondrocytes located at the superficial layer of the articular cartilage. While the Prg4GFPCreERT2 knock-in inducible-Cre transgenic mice were generated a while ago, so far, few studies have used this mouse line to perform gene functional studies in cartilage biology. MethodsWe have recently reported that deleting the Fermt2 gene, which encodes the key focal adhesion protein Kindlin-2, in articular chondrocytes by using the AggrecanCreERT2 transgenic mice, results in spontaneous osteoarthritis (OA) lesions, which highly mimics the human OA pathologies. In this study, we have compared the Kindlin-2 deficiency-caused OA phenotypes induced by Prg4GFPCreERT2 with those caused by AggrecanCreERT2 using imaging and histological analyses. ResultsWe find that Kindlin-2 protein is deleted in about 75% of the superficial articular chondrocytes in the tamoxifen (TAM)-treated Prg4GFPCreERt2/+; Fermt2fl/fl mice compared to controls. At 6 months after TAM injections, the OARSI scores of AggrecanCreERT2/+; Fermt2fl/fl and Prg4GFPCreERt2/+; Fermt2fl/fl mice were 5 and 3, respectively. The knee joints histological osteophyte and synovitis scores were also significantly decreased in Prg4GFPCreERT2/+; Fermt2fl/fl mice compared to those in AggrecanCreERT2/+; Fermt2fl/fl mice. Furthermore, magnitudes of upregulation of the extracellular matrix-degrading enzymes Mmp13 and hypertrophic chondrocyte markers Col10a1 and Runx2 were decreased in Prg4GFPCreERT2/+; Fermt2fl/fl versus AggrecanCreERT2/+; Fermt2fl/fl mice. We finally examined the susceptibility of Prg4GFPCreERT2/+; Fermt2fl/fl mouse model to surgically induce OA lesions. The pathological features of OA in the TAM-DMM model exhibited significant enhancement in cartilage erosion, proteoglycan loss, osteophyte, and synovitis and an increase in OARSI score in articular cartilage compared with those in corn-oil DMM mice. ConclusionKindlin-2 loss causes milder OA-like lesions in Prg4GFPCreERT2/+;Fermt2fl/fl than in AggrecanCreERT2/+; Fermt2fl/fl mice. In contrast, Kindlin-2 loss similarly accelerates the destabilization of the medial meniscus-induced OA lesions in both mice.Translational Potential of this Article: Our study demonstrates that Prg4GFPCreERT2 is a useful tool for gene functional study in OA research. This study provides useful information for investigators to choose appropriate Cre mouse lines for their research in cartilage biology.

Differential expression and methylation patterns of NFATC1, NADSYN1 and JAK3 gene in equine chondrocytes expanded in monolayer culture

Ex vivo expansion of chondrocytes in monolayer (ML) culture for therapeutic purposes is burdened with difficulties related to the loss of cartilaginous phenotype. Epigenetic mechanisms responsible for regulation of gene expression are believed to underlie chondrocyte dedifferentiation. We have inspected the relevance of DNA methylation alterations for passage-related differential expression of NFATC1 gene involved in hard connective tissue turnover and development, NADSYN1 influencing redox metabolism, and JAK3 - an important driver of inflammation. We have assessed relative amount of transcript abundance and performed DNA bisulfite sequencing of upstream located elements. It seems that anabolic-like effects of chondrogenic differentiation were observed in form of NFATC1 and NADSYN1 upregulation in chondrocytes at the earlier stages of passaging whereas JAK3 upregulation at the 11th passage was the sign of chondrocytes dedifferentiation. Summarizing the inversely correlated DNA methylation and expression patterns in NFATC1 and JAK3 locus might be relevant for cellular dedifferentiation during chondrocyte expansion in monolayer. Obtained results are supportive for further studies on the role of encoded proteins in regenerative biology of articular cartilage using in vitro expanded chondrocytes.

Sub-toxic levels of Co2+ are anti-inflammatory and protect cartilage from degradation caused by IL-1β

BackgroundCobalt ions from some orthopaedic implants induce a dose-dependent cytotoxic and pro-inflammatory response. Recent studies show that sub-toxic levels of cobalt influence actin organisation regulating fibroblasts and macrophages behaviour. However little is known about the influence of sub-toxic levels of cobalt on articular cartilage biology and biomechanics. Previously, we have reported that IL-1β signalling in chondrocytes, is regulated by primary cilia and associated intraflagellar transport. Since primary cilia expression is modulated by actin organisation, we set out to test the hypothesis that sub-toxic levels of cobalt regulate cilia expression and IL-1β signalling thereby influencing articular cartilage degradation. MethodsIsolated chondrocytes and bovine cartilage explants were subjected to Co2+ in the presence and absence of IL-1β. Primary cilia were monitored by confocal immunofluorescence. Nitric oxide and PGE2 release were used to monitor IL-1β signalling. Degradation of cartilage matrix was assessed by the release of sGAG and the biomechanical properties of the tissue in uniaxial unconfined compression. FindingsSub-toxic levels of Co2+ (50 μM) blocked IL-1β-induced primary cilia elongation in isolated chondrocytes. This was associated with disruption of pro-inflammatory signalling in both isolated chondrocytes and cartilage explants, and inhibition of cartilage matrix degradation and loss of biomechanical properties. InterpretationThis study reveals that low levels of cobalt ions are anti-inflammatory, preventing cartilage degradation in response to IL-1β. This mechanism is associated with regulation of primary cilia elongation. These observations provide new insight into the potential beneficial role of cobalt and may lead to novel mechanisms for controlling cartilage inflammation.

Musculoskeletal health from the “One Medicine” perspective – what can we learn from large and small animal models (with emphasis on articular cartilage)?

In human medicine musculoskeletal diseases rank (together with mental disorders) first in reasons for occupational disability and have a huge impact on both quality of life and overall healthcare costs[1]. The current increase in life expectancy, together with decreasing societal acceptance of impaired mobility, have strongly pushed musculoskeletal research in recent years. The classic animal models for research into musculoskeletal disease are small rodents, especially mice and rats. As larger species, goats and to a lesser extent sheep have been the species of choice. This choice was largely based on practical and logistical considerations such as the required size, availability, costs and ease of handling, rather than on biomedical criteria. The growing acceptance of the “One Health, One Medicine” concept has, together with better knowledge of fundamental differences between mammalian species in articular cartilage biology and the increasing pressure to reduce, refine and replace (the three “Rs”) animal experimentation, led to a change in attitude towards the use of animal models in musculoskeletal research[2]. Whereas small rodents may still be a logical step after in vitro research, the fundamental differences between articular cartilage composition of smaller species and those heavier than about 1Kg[3], together with the increasing recognition of the role of biomechanics within the joint, cast severe doubts on the validity of these species for anything but very basic work in musculoskeletal research. In contrast, within the “One Medicine” concept it is clear that in veterinary medicine there are several species featuring a high prevalence of musculoskeletal disorders that are very similar to those seen in humans. This applies to dogs with intervertebral disc disease[4] and chronic joint disorders (especially osteoarthritis (OA)) in both horses and dogs[4]. These developments have led to a gradual shift in the use of animals in musculoskeletal research. Also, regulatory bodies are making this shift of mind with the US Food and Drug Administration (FDA) now requiring preparatory work in horses before approval for certain orthopaedic devices is granted. There is one other important aspect to this development. Whereas the classic animal models were solely used to the benefit of human research, research in dogs and horses will forcibly lead to medical improvements for these species, as they are patients too and hence not only experimental animals, but target species as well. This is an important asset for the ethical justification for the use of animals for scientific research.

Agrin: A new player in joint biology and a potent growth factor for cartilage regeneration

Purpose: Osteoarthritis is a chronic disabling disease characterized by cartilage breakdown for which there is no cure. Disruption of the gene encoding for the heparan sulphate proteoglycan Agrin results in embryonic skeletal dysplasia suggesting a role for Agrin in cartilage biology and prompting us to study its role in articular cartilage biology and osteoarthritis. The aims of this study are to establish the expression pattern of Agrin in healthy/normal cartilage and compare it to the expression pattern in osteoarthritic or injured cartilage; determine the effects of knockdown and over-expression of Agrin within cartilage in vitro and in vivo and to determine the epistasis of Agrin in articular chondrocytes. Methods: Agrin expression was determined by immunohistochemistry and qPCR. Osteoathritis was induced in 8 week old 129sv mice by destabilisation of the medial meniscus (DMM) and Agrin expression was evaluated by immunofluorescence 8 weeks post-surgery. Paired human samples of preserved cartilage vs severely osteoarthritic cartilage were compared by immunofluorescence. Gain and loss of function experiments were performed using an expression plasmid encoding mammalian Agrin and siRNAs in C28/I2 and bovine chondrocytes in micromass culture. In vivo cartilage formation was assessed using an ectopic implantation model in nude mice; growth-arrested COS7 cells overexpressing Agrin or GFP were combined with bovine chondrocytes (ratio 1:10) and implanted ectopically into nude mice for two weeks. Retrieved implants were characterised by histology and qPCR. Results: Agrin and its known receptors were expressed in healthy adult human articular cartilage and downregulated in human and experimental murine osteoarthritis. Silencing of Agrin by siRNA resulted in reduced GAG production in C28/I2 and in chondrocyte de-differentiation characterised by decreased expression of SOX9, COL2A1 and ACAN mRNA. Overexpression of Agrin in the human chondrocyte cell line C28/I2 and bovine primary articular chondrocytes resulted in enhanced GAG production and SOX9 upregulation. Importantly, in contrast to BMP-2, Agrin over-expression did not induce markers of cartilage hypertrophy including COL10A1 and MMP-13. Delivering Agrin to primary bovine chondrocytes transplanted in the muscle of nude mice resulted in enhanced formation of ectopic cartilage, which did not display signs of hypertrophy, vascular invasion, or endochondral bone formation. Conclusions: Our data show that Agrin is essential for the maintenance of the chondrocytic phenotype and extracellular matrix production whilst exogenous Agrin enhances chondrocyte differentiation and cartilage formation in vitro and in vivo; and therefore may be a valuable chondrogenic molecule in tissue engineering technologies.

Activin receptor-like kinase ALK5 and ALK1 are both required for TGFβ-initiated chondrogenic differentiation of mesenchymal stem cells

Activin receptor-like kinase ALK5 and ALK1 are both required for TGFβ-initiated chondrogenic differentiation of mesenchymal stem cells

Biochemical evidence for gap junctions and Cx43 expression in immortalized human chondrocyte cell line: a potential model in the study of cell communication in human chondrocytes

The development of chondrocytic cell lines has enabled the investigation of the role of cellular phenotype and mechanisms in articular cartilage biology and physiopathology of several rheumatic diseases. Among them, the T/C-28a2 cell line has become a common tool in cartilage research. Recent results from our group have revealed that primary human chondrocytes in tissue and in monolayer culture contain high levels of connexin 43 (Cx43) and are able to directly communicate through gap junction (GJ) channels. These results challenge the existing thesis of cartilage physiology, that chondrocytes do not have the capacity to physically communicate with each other. Established cell lines offer the advantage of convenience and uniformity; however, the establishment process may cause a disruption of GJ. This study was performed to investigate if T/C-28a2 cells contain Cx43 protein and form functional channels. Cx43 was characterized by RT-qPCR, Western blotting, and immunohistochemistry (IHC). Electrophysiology experiments, Lucifer Yellow (LY) uptake, electroporation in situ and scrape loading assay were performed to test the functionality of GJs. T/C-28a2 cells express Cx43. Electrophysiology experiments and LY uptake confirmed the capacity of these cells to communicate through GJ channels, although these cells contain significant levels of active c-Src kinase, presumably due to their immortalization with the Simian Virus 40 large T antigen. The results were validated using primary chondrocytes (PC). These results reveal that the T/C-28a2 line may provide a useful invitro model for the study of Cx43 function and cell communication to understand the physiology of chondrocytes and cartilage.

Articular Cartilage Changes in Maturing Athletes

Context:Articular cartilage has a unique functional architecture capable of providing a lifetime of pain-free joint motion. This tissue, however, undergoes substantial age-related physiologic, mechanical, biochemical, and functional changes that reduce its ability to overcome the effects of mechanical stress and injury. Many factors affect joint function in the maturing athlete—from chondrocyte survival and metabolism to structural composition and genetic/epigenetic factors governing cartilage and synovium. An evaluation of age-related changes for joint homeostasis and risk for osteoarthritis is important to the development of new strategies to rejuvenate aging joints.Objective:This review summarizes the current literature on the biochemical, cellular, and physiologic changes occurring in aging articular cartilage.Data Sources:PubMed (1969-2013) and published books in sports health, cartilage biology, and aging.Study Selection:Keywords included aging, athlete, articular cartilage, epigenetics, and functional performance with age.Study Design:Systematic review.Level of Evidence:Level 3.Data Extraction:To be included, research questions addressed the effect of age-related changes on performance, articular cartilage biology, molecular mechanism, and morphology.Results:The mature athlete faces challenges in maintaining cartilage health and joint function due to age-related changes to articular cartilage biology, morphology, and physiology. These changes include chondrocyte loss and a decline in metabolic response, alterations to matrix and synovial tissue composition, and dysregulation of reparative responses.Conclusion:Although physical decline has been regarded as a normal part of aging, many individuals maintain overall fitness and enjoy targeted improvement to their athletic capacity throughout life. Healthy articular cartilage and joints are needed to maintain athletic performance and general activities. Genetic and potentially reversible epigenetic factors influence cartilage physiology and its response to mechanical and injurious stimuli. Improved understandings of the physical and molecular changes to articular cartilage with aging are important to develop successful strategies for joint rejuvenation.

THE USE OF DELAYED GADOLINIUM ENHANCED MAGNETIC RESONANCE IMAGING OF CARTILAGE AND T2 MAPPING TO EVALUATE ARTICULAR CARTILAGE IN THE NORMAL CANINE ELBOW

Commonly used diagnostic tools used to evaluate articular cartilage lack the sensitivity, specificity, and objectivity to measure early changes associated with osteoarthritis. Two techniques using magnetic resonance (MR) imaging have been developed to detect the biology of articular cartilage are delayed gadolinium-enhanced MR imaging of cartilage (dGEMRIC) and T2 mapping. Both techniques have been validated and are used to study the degenerative and adaptive nature of articular cartilage in people. The use of these techniques as a diagnostic tool in dogs has not been well described. We evaluated articular cartilage in the region of the medial coronoid process (MCP) of six healthy dogs free of detectable orthopedic disease using both MR imaging techniques. Histology and proteoglycan (PG) content of the MCP were used to confirm normal articular cartilage. All dogs had ground reaction forces consistent with normal function. Mean dGEMRIC index (T1 value) was 400 +/- 47 ms and mean T2 value was 56 +/- 8 ms. Intra- and interobserver variability was low. dGEMRIC and T2 values for normal cartilage in the elbow of the dog can be generated reproducibly using 3T MR imaging. Using these techniques as objective outcome measures for clinical studies in dogs with OA conditions should help delineate the efficacy of some disease interventions.

Applications of Mesenchymal Stem Cells in Cartilage Tissue Engineering- Part 1

Arthritic diseases such as osteoarthritis (OA) and rheumatoid arthritis (RA) cause considerable pain, reduced mobility and significant disability among affected patients and present a major challenge to clinicians and basic scientists due to the limited inherent repair capacity of articular cartilage. The poor capacity of articular cartilage for self-repair is largely due to its avascular nature and has resulted in the development of a variety of surgical treatments including Autologous Chondrocyte Implantation (ACI) or Autologous Chondrocyte Transplantation (ACT), microfracture and mosaicplasty. Mesenchymal stem cells (MSCs) are multipotent progenitor cells with significant potential for chondrogenesis and new cartilage formation. Novel approaches using MSCs derived from bone marrow and adipose tissue have been proposed as alternatives to patient derived chondrocytes. In this paper we provide a scientific background to the biology of articular cartilage biology and its degeneration in arthritis. We also summarize some of the recent patents on applications of MSCs in articular cartilage tissue engineering and regenerative medicine for OA, RA and other joint diseases that involve cartilage degradation. Keywords: Osteoarthritis (OA), articular cartilage, mesenchymal stem cell, tissue engineering, regenerative medicine, patent, invention, rheumatoid arthritis, Autologous Chondrocyte Implantation, Autologous Chondrocyte Transplantation, microfracture, mosaicplasty, musculoskeletal, disorders, synovial joints, psoriatic arthritis, autoimmune diseases, subchondral bone, pathogenesis, extracellular matrix (ECM), Chondrocytes, proteoglycans, glycolytic cells, Hypoxia, normoxia, fibrilassociated collagens with interrupted helices (FACIT), decorin, biglycan, cartilage oligomeric matrix protein (COMP), physicochemical signals, osteophyte formation, obese juveniles, Pluripotent adult stem cells, hematopoietic stem cells, periosteum, trabecular bone, adipose tissue, synovium, density-gradient, centrifugation, nucleus pulposus (NP), biosynthesis, thrombin-like enzyme, venom, periodontal ligament, mineralization, thrombospondin, osteoblast, Cartilage-Bone Interface, tissue progenitor cells, biocompatible scaffold, hypoxiainducible factors (HIFs), organogenesis, reinfusion chamber, articular cartilage regeneration

On the Horizon From the ORS

On the Horizon From the ORS

Topographic variation in redifferentiation capacity of chondrocytes in the adult human knee joint

The aim of this study was to investigate the topographic variation in matrix production and cell density in the adult human knee joint. Additionally, we have examined the redifferentiation potential of chondrocytes expanded in vitro from the different locations. Full thickness cartilage-bone biopsies were harvested from seven separate anatomical locations of healthy knee joints from deceased adult human donors. Chondrocytes were isolated, expanded in vitro and redifferentiated in a pellet mass culture. Biochemical analysis of total collagen, proteoglycans and cellular content as well as histology and immunohistochemistry were performed on biopsies and pellets. In the biochemical analysis of the biopsies, we found lower proteoglycan to collagen (GAG/HP) ratio in the non-weight bearing (NWB) areas compared to the weight bearing (WB) areas. The chondrocytes harvested from different locations in femur showed a significantly better attachment and proliferation ability as well as good post-expansion chondrogenic capacity in pellet mass culture compared with the cells harvested from tibia. These results demonstrate that there are differences in extra cellular content within the adult human knee in respect to GAG/HP ratio. Additionally, the data show that clear differences between chondrocytes harvested from femur and tibia from healthy human knee joints exist and that the differences are not completely abolished during the process of de- and redifferentiation. These findings emphasize the importance of the understanding of topographic variation in articular cartilage biology when approaching new cartilage repair strategies.

Ultrastructural Insights into the World of Cartilage: Electron Microscopy of Articular Cartilage

Articular cartilage has a hierarchical organization, which is based on the mechanical stress to which it is exposed. It functions as a gliding surface, as well as in force distribution and varies according to the kind and duration of mechanical demand. In order to understand articular cartilage biology and developmental or pathological variations, tissue morphology is an important and versatile tool. In the present overview, the fine structure of cartilage is presented and related to functional aspects.

JNK/ERK–AP-1/Runx2 induction “paves the way” to cartilage load-ignited chondroblastic differentiation

Chondro-osteogenesis and subsequently skeletal morphology are greatly influenced by mechanical loads. The exact mechanism(s) by which mechanical stimuli are transduced in chondrocytes remains obscure and appears to be equally complex with similar signal transducing systems. Here we investigated whether and to what extent the MAPK (JNK/ERK)-AP-1/Runx2 signaling pathways are engaged in this phenomenon, and assessed their involvement in the functional biology of articular cartilage. For this purpose, 14-day-old female Wistar rats were divided into 2 groups: the first group was fed hard diet (simulating physiologic temporomandibular joint (TMJ) loading), while the second group was fed soft diet (reduced TMJ loading). On day 21 (experiment initiation day - weaning day), biopsies from condyles of both groups were obtained after 6, 12 and 48 h of functional TMJ loading. Immunohistochemical methodology was employed to evaluate the expression levels of pc-Jun, c-Fos, JNK2, p-JNK, p-ERK and Runx2 due to alteration in functional load. Our data demsonstrate that the protein levels of all the aforementioned molecules were markedly increased in animals fed with the hard diet, throughout the experimental procedure. These results indicate that functional cartilage loading induces the AP-1 and Runx2 transcription factors through the JNK and ERK MAPK cascades. In as much as the above signaling mediators/effectors are considered to be crucial in the differentiation/maturation process of cartilage tissue, we pose that functional mechanical loading of condylar cartilage serves to "fine tune" chondroblastic differentiation/maturation.

CURRENT CONCEPTS OF GENE THERAPY AND CARTILAGE REPAIR

Lesions of the articular cartilage are commonly encountered in orthopaedics and sports medicine. 1,2 Damage to articular cartilage leads to premature osteoarthritis and may cause a decrease in the quality of life with long-term costs of health care. The management of chondral and osteochondral lesions represents a challenge to clinicians and scientists. 3,4 Although current concepts of treatment for these injuries are promising, no treatment has yet resulted in complete restoration of the hyaline cartilage and the subchondral bone to a normal state. 5 The advent of gene therapy provides new possibilities for the treatment of injuries to cartilage. 6 Extended scientific contributions have been made towards the understanding of the biology of articular cartilage and to new approaches to treatment. The delivery of growth factors, cells, scaffolds, and therapeutic genes into lesions of articular cartilage promises to change the treatment of an aspect of medicine which historically has been limited to frustrating outcomes. Despite significant progress in the treatment of osteochondral lesions, many unsolved problems remain, especially those of the safety of gene transfer. 7 This review focuses on the basic premises underlying gene therapy and describes the current state of genetic engineering for defects of cartilage.

Primary repair of osteochondral and chondral injury

The treatment of osteochondral and chondral injuries of the knee continues to be a challenging problem. Despite significant advances in our understanding of the unique structure and biology of articular cartilage, modern medicine has still not overcome the limited intrinsic capacity of hyaline cartilage to repair or regenerate. Furthermore, the mechanical properties of fibrocartilage, as a repair tissue, are inferior to articular cartilage and deteriorate with time. The exact incidence of articular cartilage lesions in the knee remains unknown. Furthermore, the cause of these injuries remains somewhat hypothetical, but is usually related to direct knee trauma (ie, patellar dislocation/subluxation, violent twist or impaction) and is dependent on patient age and activity level. Chondral fractures generally occur in skeletally mature individuals, whereas osteochondral injuries typically occur in skeletally immature people or young adults. Symptoms tend to be nonspecific (pain and swelling) and associated ligamentous and meniscal injuries are common. The diagnosis of these injuries can be made quite readily with the aid of radiographs and magnetic resonance imaging. Although there have been exciting developments in arthroscopic instrumentation and cartilage restoration techniques, primary repair of osteochondral and chondral lesions, when possible, remains the preferred method of treatment. Successful repair is dependent on many factors, including the size and location of the lesion, its chronicity, and any associated injuries.

Differential expression of multiple genes during articular chondrocyte redifferentiation.

Articular chondrocytes undergo a rapid change in phenotype and gene expression, termed dedifferentiation, when isolated from cartilage tissue and cultured on tissue culture plastic. On the other hand, "redifferentiation" of articular chondrocytes in suspension culture is characterized by decreased cellular proliferation and the reinitiation of synthesis of hyaline articular cartilage extracellular matrix molecules. The molecular triggers for these events have yet to be defined. Subtracted cDNA libraries representing genes involved in the early events of adult human articular chondrocyte redifferentiation were generated from human articular chondrocytes that were first cultured in monolayer, and subsequently transferred to suspension culture at 10(6) cells/ml for redifferentiation. Differential regulation of genes involved in cellular organization, nuclear structure, cellular growth regulation, and extracellular matrix deposition and remodeling were observed within 48 hr of this transfer. Many of these genes had not been previously identified in the chondrocyte differentiation pathway and a number of the isolated cDNAs did not have homologies to sequences in the public data banks. Genes involved in IL-6 signal transduction including acute phase response factor (APRF), Mn superoxide dismutase, and IL-6 itself were up-regulated in suspension culture. Membrane glycoprotein gp130, a component of the IL-6 receptor, was down-regulated. Other genes involved in cell polarity, cell adherence, apoptosis, and possibly TGF-beta signaling were differentially regulated. The differential regulation of the cytokine connective tissue growth factor (CTGF) during the early stages of articular chondrocyte redifferentiation, decreasing within 48 hours of transfer to suspension culture, was particularly interesting given its reported role in the stimulation of cellular proliferation. CTGF was highly expressed in proliferative monolayer culture, and then greatly reduced by redifferentiation in standard high-density suspension culture. When articular chondrocytes were seeded in suspension at low-density (10(4) cells/ml), however, high levels of CTGF were observed along with increased levels of mature articular cartilage extracellular matrix protein RNAs, such as type II collagen and aggrecan. Although the role of CTGF in articular cartilage biology remains to be elucidated, the results described here demonstrate the potential utility of subtractive hybridization in understanding the process of articular chondrocyte redifferentiation.

VISCOSUPPLEMENTATION THERAPY WITH INTRA-ARTICULAR HYALURONIC ACID: Fact or Fantasy?

Pharmacologic therapy for osteoarthritis is presently only palliative and is based on the use of analgesic or anti-inflammatory agents. Simple analgesics, however, do not provide enough of an effect to satisfy the needs of many OA patients, and anti-inflammatory drugs that are currently available do not have favorable risk-to-benefit ratio in typical patients with OA. A need remains, therefore, for therapies that will be analgesic, appropriately anti-inflammatory when necessary, and that may favorably alter the natural history of the disease. The development of intra-articular hyaluronic acid (HA) supplementation was thought to fulfill these criteria. This therapy has been shown to modulate pain in OA of the knee and investigators have attempted to show that it have positive effects on articular cartilage biology. This article considers these claims for HA supplementation as an important therapy for the treatment of OA.

Isolated chondrons: a concept comes of age!

The chondron concept was first introduced in 1925 by Benninghoff who used polarized light microscopy to show that the chondrocyte in hyaline cartilages was surrounded by a specialized region of matrix. However, this concept was not widely accepted at the time, and it was until 1969 that Szirmai rekindled interest in the chondron concept. Using new histochemical interpretations coupled with microchemical analysis, Szirmai showed the micro-world of the chondron consisted of a fine collagenous perilacunar rim surrounding a proteoglycan rich lacunar space. He also showed that chondrons could be extracted from horse nasal septum by high speed homogenization and concluded that the chondron was mechanically robust and represented the primary functional and metabolic unit of hyaline cartilages. Again the chondron concept was not widely embraced despite the rapid accumulation of data indicating common specializations in the vicinity of hyaline cartilage chondrocytes. In 1985, our own research on the structure and function of adult articular cartilage matrices led to the discovery of chondrons in the homogenate produced by slow speed homogenization of canine tibial cartilage. The isolation of intact chondrons conclusively established the chondron as a microanatomical unit of adult articular cartilage. Each chondron consisted of a chondrocyte intimately associated with a proteoglycan rich pericellular matrix which was surrounded by a densely compacted pericellular capsule composed of fine collagen species. We have now developed the isolated chondron as a model to study the natural interrelationship between the chondrocyte and its pericellular microenvironment. Using morphological, histochemical, ultrastructural, immunohistochemical, biochemical and metabolic techniques, we have successfully shown the isolated chondron is rich in chondroitin sulfate, keratan sulfate and hyaluronan glycosaminoglycans, collagen types II, VI, IX, and XI, and glycoproteins such as fibronectin and possibly laminin. Further studies of chondrons extracted from osteoarthritic cartilage suggest the chondron is remodeled during the degenerative process causing the pericellular microenvironment to expand and the chondrocyte to proliferate into the large clusters typical of osteoarthritic pathology. The data presented in this short review indicates that the chondron is a highly specialized microanatomical unit of adult articular cartilage and plays a fundamental role in the initiation and progression of degenerative arthritis. Future studies on the isolated chondron model will ensure a highly greater understanding of the role of the cellular microenvironment in articular cartilage biology and pathology. Biomedical Reviews 1992; 1: 53-62.

Quinolone arthropathy--acute toxicity to immature articular cartilage.

A class effect of quinolone antibacterial agents observed during animal toxicity testing is a specific arthropathy (QAP). Despite the growing list of laboratory animals susceptible to QAP and reports of arthralgia in patients treated with quinolones, the potential for QAP development in humans remains unknown. This review discusses current concepts in the biology of articular cartilage and how these concepts elucidate QAP pathogenesis. Biomechanical forces within synovial joints and toxicokinetic properties of quinolones contribute to QAP induction. Since a limited number of mechanistic pathways exist for acute articular damage, QAP may serve as a research tool to probe the pathobiology of injury to articular cartilage.