Influence of feeding a soft diet on proteoglycan expression in rat temporomandibular joint discs.

OBJECTIVES: Extracellular matrix components play a significant role in maintaining tissue integrity and pathological processes of the temporomandibular joint (TMJ). This study aimed to evaluate the influence of a soft diet on the mRNA expression of proteoglycans and glycosaminoglycans (GAGs) linked to proteoglycan core proteins in rat TMJ discs. METHODS: Thirty 4-week-old male Wistar rats were assigned to one of two groups: a control group fed a regular pellet diet and a soft diet group fed a powdered diet for 4 weeks. The mRNA expression levels of 12 proteoglycans in TMJ discs were evaluated using real-time polymerase chain reaction (PCR). In addition, histomorphometric and biochemical analyses were performed to evaluate the thickness and deoxyribonucleic acid (DNA), GAG, and water content of the TMJ discs. RESULTS: The TMJ disc thickness in the anterior, intermediate, and posterior bands decreased significantly in the soft diet group. The GAG content decreased significantly in the soft-diet group, whereas no significant differences in DNA content or water content ratio were observed between the groups. Real-time PCR indicated that the expression levels of aggrecan, versican, biglycan, decorin, fibromodulin, lumican, and chondroadherin decreased in the soft diet group. The expression levels of all versican isoforms decreased in the soft diet group. CONCLUSIONS: These results indicate that the biomechanical environment of the TMJ caused by a soft diet is closely related to the expression of proteoglycans in TMJ discs, which may eventually increase the fragility of the TMJ discs.

Mesenchymal stem cells-derived exosomes alleviate temporomandibular joint disc degeneration in temporomandibular joint disorder.

Temporomandibular joint (TMJ) disorder (TMD) is a chronic progressive disease that is commonly seen in clinical settings. TMJ disc degeneration is an important manifestation of TMD, and further aggravates the progression of TMD. However, treatments on TMJ disc degeneration are very limited till now. In this study, we first observed the effects of bone marrow stem cells (BMSC) conditioned medium on functions of TMJ disc fibroblasts. Then BMSC-derived small extracellular vesicles (BMSC-EVs) were isolated and exposed to TMJ disc fibroblasts. RNA-sequencing was used to further investigate the mechanisms. BMSC-EVs were finally injected into a rat model with TMD. Results showed that in the transwell co-culture system, the medium derived from BMSC reduced inflammation and enhanced chondrogenesis in TMJ disc fibroblasts. BMSC-EVs promoted proliferation, migration, and chondrogenic differentiation of TMJ disc fibroblasts, and inhibited apoptosis and inflammatory responses. Local injection of BMSC-EVs into the TMD model alleviated TMJ disc degeneration. Therefore, BMSC-EVs were a potentially effective, sustainable and clinically translational-promising option for TMJ disc degeneration, and further reduce the progression of TMD.

A single-cell transcriptional atlas reveals resident progenitor cell niche functions in TMJ disc development and injury.

The biological characteristics of the temporomandibular joint disc involve complex cellular network in cell identity and extracellular matrix composition to modulate jaw function. The lack of a detailed characterization of the network severely limits the development of targeted therapies for temporomandibular joint-related diseases. Here we profiled single-cell transcriptomes of disc cells from mice at different postnatal stages, finding that the fibroblast population could be divided into chondrogenic and non-chondrogenic clusters. We also find that the resident mural cell population is the source of disc progenitors, characterized by ubiquitously active expression of the NOTCH3 and THY1 pathways. Lineage tracing reveals that Myh11+ mural cells coordinate angiogenesis during disc injury but lost their progenitor characteristics and ultimately become Sfrp2+ non-chondrogenic fibroblasts instead of Chad+ chondrogenic fibroblasts. Overall, we reveal multiple insights into the coordinated development of disc cells and are the first to describe the resident mural cell progenitor during disc injury.

Characterization of temporomandibular joint articular disc progenitor cell clones.

A critical component of the temporomandibular joint (TMJ) is the fibrocartilage articular disc (AD). Researchers have attempted to regenerate the AD to alleviate TMJ osteoarthritis but alternative cell sources for use in AD regenerative approaches are needed due to insufficient extracellular matrix (ECM) production by total articular disc cells (TACs). Tissue-specific progenitor cells have been identified in many tissues. The aim of the present study was to identify adult multipotent progenitor cells within the AD suitable for regenerative medicine applications. A novel AD progenitor cell population was identified in rhesus macaques. Clonally derived articular disc progenitor cells (ADPs) were isolated using fibronectin differential cell adhesion. ADPs represent between 1 and 3 % of the TAC population and are capable of in vitro expansion beyond 60 population doublings. ADPs were characterized using osteogenic, adipogenic, and fibrochondrogenesis differentiation assays. Clones exhibited phenotypic plasticity, differentiating into osteocytes, adipocytes, and fibrochondrocytes. ECM secretion profiles following fibrochondrogenic differentiation were assessed using immunohistochemistry (IHC), fluorescently activated cell sorting (FACS), total collagen, and glycosaminoglycan (GAG) assays and compared with TACs, articular cartilage progenitor cells (ACPs), tendon progenitor cells (TPCs) and bone-marrow-derived mesenchymal stem cells (BMMSCs). ADP pellet cultures produced a biochemical phenotype similar to native AD tissue, with production of versican (VCAN) and collagen types I, II, III, and VI (COL1, COL2, COL3, COL6). However, clonally derived ADP cell lines produced different amounts of ECM and exhibited different expansion potentials. These findings indicated flexibility in clone selection for potential regenerative strategies to recapitulate native anisotropy.

COL5A1 RS12722 Is Associated with Temporomandibular Joint Anterior Disc Displacement without Reduction in Polish Caucasians.

Numerous reports describe the association between the single-nucleotide polymorphism (SNP) rs12722 and rs13946 in the COL5A1 gene and injuries, such as Achilles tendon pathology, anterior cruciate ligament (ACL) injuries, and tennis elbow. Hence, there were no studies investigating COL5A1 and temporomandibular joint (TMJ) pathology. The aim of this study is to evaluate the relationship between COL5A1 rs12722 and rs13946 SNPs and TMJ articular disc displacement without reduction (ADDwoR). In this case-control study, the study group consisted of 124 Caucasian patients of both sexes. Each patient had a history of ADDwoR no more than 3 months prior. The control group comprised 126 patients with no signs of TMD according to DC/TMD. Genotyping of the selected SNPs was performed by real-time PCR using TaqMan probes. The significance of the differences in the distribution of genotypes was analyzed using Pearson's chi-square test. Logistic regression modeling was performed to analyze the influence of the 164 investigated SNPs on ADDwoR. The COL5A1 marker rs12722 turned out to be statistically significant (p-value = 0.0119), implying that there is a difference in the frequencies of TMJ ADDwoR. The distribution of rs12722 SNPs in the study group TT(66), CC(27), CT(31) vs. control group TT(45), CC(26), CT(51) indicates that patients with CT had an almost 2.4 times higher likelihood of ADDwoR (OR = 2.41) than those with reference TT (OR = 1), while rs13946 genotypes were shown to be insignificant, with a p-value of 0.1713. The COL5A1 rs12722 polymorphism is a risk factor for ADDwoR in the Polish Caucasian population.

Acellular matrix hydrogel for repair of the temporomandibular joint disc.

Application of tissue-derived extracellular matrix (ECM) biomaterials in the repair of the temporomandibular joint (TMJ) disc is a promising approach for the treatment of disc abrasion and perforation, particularly for the young patient population. Although decellularized ECM (dECM) scaffolds preserve tissue-specific structures as well as biological and biomechanical properties, they require surgical implantation. To address this issue, we prepared porcine TMJ discs into decellularized ECM with serial detergent and enzyme treatments, and the TMJ disc-derived ECM was then processed into hydrogels via pepsin digestion. The decellularization efficiency was assessed by quantification of the DNA and matrix component contents. The fibrous ultrastructure of the hydrogel was observed by scanning electron microscopy (SEM). Rheological characterization and mechanical properties were measured. in vitro experiments with costal chondrocytes ensured the cellular proliferative capacity and compatibility in the injectable disc-derived ECM hydrogel. The results showed that a large amount of DNA (>95%) was removed after decellularization; but, the collagen was retained. SEM of the hydrogels demonstrated a multiaperture fiber ultrastructure. Rheological studies revealed a rapid gelation temperature (37°C) and injectable properties. The mechanical properties of the hydrogels were adjusted by changing the ECM concentration. The in vitro studies revealed that the hydrogels are not cytotoxic, but instead showed good cytocompatibility. The hydrogel also showed good injectability and degradability through an in vivo study. Overall, these results suggest the great potential of injectable disc-derived hydrogels for TMJ disc repair and regeneration applications.

MMP-7 and MMP-9 are overexpressed in the synovial tissue from severe temporomandibular joint dysfunction.

Matrix metalloproteinases (MMPs) are tissue-enzymes that play a key role during the remodeling process, such as in inflammatory diseases. MMP-7 and MMP-9 have been shown to be implicated in extracellular matrix homeostasis and in joint disc remodeling. The objective of this study was to determine the relation of MMP-7 and MMP-9 expression with severe temporomandibular joint dysfunction, in particular with anterior disk displacement without reduction (ADDwoR), using an immunohistochemical approach. Therefore, twenty human temporomandibular synovia in the test group and ten in the control group were collected. The results showed there was a statistically significant difference (P<0.001) for morphometric and densitometric analysis of both detected MMPs in control and test groups. In conclusion, MMP-7 and MMP-9 were overexpressed in the synovial tissue of patients with ADDwoR.

Effect of Sustained Joint Loading on TMJ Disc Nutrient Environment.

The temporomandibular joint (TMJ) disc nutrient environment profoundly affects cell energy metabolism, proliferation, and biosynthesis. Due to technical challenges of in vivo measurements, the human TMJ disc extracellular nutrient environment under load, which depends on metabolic rates, solute diffusion, and disc morphometry, remains unknown. Therefore, the study objective was to predict the TMJ disc nutrient environment under loading conditions using combined experimental and computational modeling approaches. Specifically, glucose consumption and lactate production rates of porcine TMJ discs were measured under varying tissue culture conditions (n = 40 discs), and mechanical strain-dependent glucose and lactate diffusivities were measured using a custom diffusion chamber (n = 6 discs). TMJ anatomy and loading area were obtained from magnetic resonance imaging of healthy human volunteers (n = 11, male, 30 ± 9 y). Using experimentally determined nutrient metabolic rates, solute diffusivities, TMJ anatomy, and loading areas, subject-specific finite element (FE) models were developed to predict the 3-dimensional nutrient profiles in unloaded and loaded TMJ discs (unloaded, 0% strain, 20% strain). From the FE models, glucose, lactate, and oxygen concentration ranges for unloaded healthy human TMJ discs were 0.6 to 4.0 mM, 0.9 to 5.0 mM, and 0% to 6%, respectively, with steep gradients in the anterior and posterior bands. Sustained mechanical loading significantly reduced nutrient levels (P < 0.001), with a critical zone in which cells may die representing approximately 13.5% of the total disc volume. In conclusion, this study experimentally determined TMJ disc metabolic rates, solute diffusivities, and disc morphometry, and through subject-specific FE modeling, revealed critical interactions between mechanical loading and nutrient supply and metabolism for the in vivo human TMJ disc. The results suggest that TMJ disc homeostasis may be vulnerable to pathological loading (e.g., clenching, bruxism), which impedes nutrient supply. Given difficulties associated with direct in vivo measurements, this study provides a new approach to systematically investigate homeostatic and degenerative mechanisms associated with the TMJ disc.

Catabolic changes of rat temporomandibular joint discs induced by unilateral anterior crossbite.

BACKGROUND: The temporomandibular joint (TMJ) disc plays a role in joint movement and in load absorbance and distribution. An experimental unilateral anterior crossbite (UAC) prosthesis induces mandibular condylar cartilage degeneration in rats. However, the changes in the articular disc are still unknown. OBJECTIVE: To describe changes in the TMJ discs of UAC rats. METHODS: The discs of fifty-four Sprague-Dawley rats, equally distributed into a UAC group and an age-matched sham-operated control group at 4, 12 and 20 weeks (n = 9), were evaluated by gross and histomorphological observation and by detection at the mRNA or protein expression levels of the markers related to the matrix elements. RESULTS: No macro- or micro-morphological differences were observed between groups. However, there were catabolic degradative changes at the molecular level in the UAC group, showing a significant reduction in the mRNA and/or protein expression levels of many molecules. The reduction became worse with time (P < 0.05). The reduced molecules included: (a) those related to the extracellular matrix, such as type I collagen, decorin and fibromodulin; (b) those related to chondrogenesis, such as type II collagen and aggrecan; and (c) those related to osteogenesis, such as alkaline phosphatase and runt-related transcription factor 2. The mRNA expression of vascular endothelial growth factor did not change. In contrast, fibronectin, which can promote wound healing, and its N-terminal fragment, which can induce cartilage degradation, were accumulated (P < 0.05). CONCLUSION: TMJ discs were stimulated to catabolic changes by the aberrant dental occlusion and seemed to go to inanimate with time.

Regional and disease-related differences in properties of the equine temporomandibular joint disc.

Temporomandibular joint (TMJ) disorders affect up to 12% of the human population, and naturally occurring TMJ diseases are increasingly recognized in animals. The TMJ disc plays a major role in TMJ disorders in people, but little is known about its role in TMJ pathology in animals. This study characterizes differences in properties of equine TMJ discs associated with age, disc region, and presence of TMJ osteoarthritis (OA). Discs were dissected from both TMJ's of sixteen horses euthanized for reasons unrelated to this study. Each joint was grossly evaluated and scored as normal, mild OA, or severe OA. Samples from the rostral, caudal, lateral, central, and medial regions of the disc were subject to compressive testing, quantitative biochemistry, and histology. Samples from the lateral, central, and medial region were tested for tensile properties in the rostrocaudal and mediolateral directions. We found that the equine TMJ disc is highly anisotropic, and its glycosaminoglycan (GAG) content and compressive stiffness vary between disc regions. The disc also exhibits increasing GAG content and compressive stiffness with increasing age. While equine TMJ disc properties are generally similar to other herbivores, greater compressive stiffness throughout the disc and greater GAG content in its rostral region suggest that mechanical demands on the TMJ disc differ between horses and other species. Importantly, a region-specific decrease in compressive stiffness was observed associated with joint disease and corresponded to cartilage erosions in the underlying condylar surface.

Part I: Development and Physiology of the Temporomandibular Joint.

PURPOSE OF REVIEW: Investigate the developmental physiology of the temporomandibular joint (TMJ), a unique articulation between the cranium and the mandible. RECENT FINDINGS: Principal regulatory factors for TMJ and disc development are Indian hedgehog (IHH) and bone morphogenetic protein (BMP-2). The mechanism is closely associated with ear morphogenesis. Secondary condylar cartilage emerges as a subperiosteal blastema on the medial surface of the posterior mandible. The condylar articular surface is immunoreactive for tenascin-C, so it is a modified fibrous periosteum with an underlying proliferative zone (cambrium layer) that differentiates into fibrocartilage. The latter cushions high loads and subsequently produces endochondral bone. The TMJ is a heavily loaded joint with three cushioning layers of fibrocartilage in the disc, as well as in subarticular zones in the fossa and mandibular condyle. The periosteal articular surface produces fibrocartilage to resist heavy loads, and has unique healing and adaptive properties for maintaining life support functions under adverse environmental conditions.

Layering Poly (lactic-co-glycolic acid)-based electrospun membranes and co-culture cell sheets for engineering temporomandibular joint disc.

The temporomandibular joint disk (TMJD) lacks blood vessels and is characterized by slow self-repair. Qualitative lesions in TMJD are difficult to repair. In this study, electrospun poly (lactic-co-glycolic acid) (PLGA) scaffolds were used to reconstruct temporomandibular joint discs by tissue engineering. Rabbit temporomandibular joint disc cells (TMJDCs) and rabbit synovium-derived mesenchymal stem cells (SMSCs) were co-cultured in 1:1 ratios. Cell sheets were induced by ascorbic acid incubated with electrospun PLGA scaffolds for 14 days in the presence (10 ng/ml in culture medium) or absence of TGF-ß3. Dimethylmethylene Blue Assay (DMMB) was used to determine the content of glycosaminoglycans in the extracellular matrix. The expression of Col1a1, Col2a1, Sox-9 and Runx-2 was quantified by RT-PCR, and the expression of type II collagen was observed by immunofluorescent staining. After 14 days of cultivation, the electrospun PLGA scaffold-loaded cell sheets could form an articular disc tissue with certain morphological characteristics. The expression of chondrogenic-related genes (Col2a1, Sox-9) and the secretion of extracellular matrix (GAG, type II collagen) in the co-culture group were close to those in the TMJDC group alone. The results suggest that PLGA electrospun scaffold-loaded co-cultured cell membrane could be used in the tissue engineering reconstruction of the temporomandibular joint disc.

Effect of mechanical loading on the metabolic activity of cells in the temporomandibular joint: a systematic review.

OBJECTIVES: The purpose of this systematic review was to elucidate how different modalities and intensities of mechanical loading affect the metabolic activity of cells within the fibro-cartilage of the temporomandibular joint (TMJ). MATERIALS AND METHODS: A systematic review was conducted according to PRISMA guidelines using PubMed, Embase, and Web of Science databases. The articles were selected following a priori formulated inclusion criteria (viz., in vivo and in vitro studies, mechanical loading experiments on TMJ, and the response of the TMJ). A total of 254 records were identified. After removal of duplicates, 234 records were screened by assessing eligibility criteria for inclusion. Forty-nine articles were selected for full-text assessment. Of those, 23 were excluded because they presented high risk of bias or were reviews. Twenty-six experimental studies were included in this systematic review: 15 in vivo studies and 11 in vitro ones. CONCLUSION: The studies showed that dynamic mechanical loading is an important stimulus for mandibular growth and for the homeostasis of TMJ cartilage. When this loading is applied at a low intensity, it prevents breakdown of inflamed cartilage. Yet, frequent overloading at excessive levels induces accelerated cell death and an increased cartilage degradation. CLINICAL SIGNIFICANCE: Knowledge about the way temporomandibular joint (TMJ) fibrocartilage responds to different types and intensities of mechanical loading is important to improve existing treatment protocols of degenerative joint disease of the TMJ, and also to better understand the regenerative pathway of this particular type of cartilage.

A review of in-vitro fibrocartilage tissue engineered therapies with a focus on the temporomandibular joint.

The inability of fibrocartilage, specifically the temporomandibular joint (TMJ) disc, to regenerate and remodel following injury presents a unique problem for clinicians. Tissue engineering then offers a potential regenerative therapy. In vitro testing provides a valuable screening tool for potential tissue engineered solutions. The conclusions drawn for TMJ in vitro research were compared against state of the art fibrocartilage studies in the knee meniscus, and annulus fibrosus of the intervertebral disc (IVD). For TMJ disc regeneration, in vitro tissue engineered approaches, focused on cellular therapies with fibrochondrocytes, have displayed an inability to produce enough collagen, as well as an inability to recapitulate native mechanical properties. Biomaterial approaches have recapitulated the native properties of the TMJ disc, but their in vivo efficacy has yet to be determined. By comparison, the knee meniscus field is the most progressive in the use of stem cells as a cell source. The knee meniscus field has moved away from measuring mechanical properties, and are instead more focused on biochemistry and gene expression. IVD studies mainly use electrospun scaffolds, and have produced the best success in mechanical properties. The TMJ field, in comparison to knee meniscus and IVD, needs to employ stem cell therapies, new biomaterials and manufacturing techniques, and cutting edge molecular assays, in future in vitro approaches to screen for viable technologies to move to in vivo studies.

Interleukin-6 expression in disc derangement of human temporomandibular joint and association with osteoarthrosis.

The inflammatory process is a coordinated response that protects host after infection or trauma, involving several molecular reactions. Once the inflammation is closely linked to the process of destruction of the temporomandibular joint, this study aims to examine, by immunohistochemistry, the expression of interleukin-6 (IL-6), an important inflammatory marker, in temporomandibular articular discs of patients with anterior disc displacement with (ADDwR) and without reduction (ADDwoR) and its association with osteoarthrosis (OA). Thirty-eight (n = 38) articular discs were divided into two cutoffs: 1) analysis 1: 4 control (acute pathology), 17 ADDwR, 17 ADDwoR; and 2) analysis 2: without OA (n = 21) and with OA (n = 17). The area of immunostaining was compared statistically between groups (p < 0.05). In the disc samples, no significant differences were observed between the groups ADDwR and ADDwoR, and with and without OA, in respect to the expression of IL-6 by immunohistochemical examination. Future studies should be conducted with a larger sample size, which could clarify the association of the inflammatory mediator IL-6 with temporomandibular joint dysfunction.

Immunohistochemical analysis of IL-1 beta in the discs of patients with temporomandibular joint dysfunction.

PURPOSE: Interleukin-1 beta (IL-1ß) is a cytokine that participates in the regulation of immune responses and inflammatory reactions. It is hypothesized that IL-1 levels may be elevated in patients suffering from temporomandibular joint dysfunction. The purpose of this study was to determine the association of IL-1ß expression with TMD using an immunohistochemical approach to evaluate the joint disc. MATERIALS AND METHODS: A total of 39 human temporomandibular joint disc samples were collected, with 31 samples in the test group. Nineteen of the test group samples were from discs of patients with anterior disc displacement with reduction, and 12 of the samples were from patients with anterior disc displacement without reduction. Eight control samples were used in the control group. The samples were immunostained and evaluated on both quantity and intensity of staining. RESULTS: There was a statistically significant difference (p < 0.05) between the control and test groups for both quantity and intensity of staining. CONCLUSION: IL-1ß plays a role in the inflammatory process and degradation of TMJ discs in patients with TMJ dysfunctions.

Isolation and characterization of mesenchymal stromal progenitors from the temporomandibular joint disc.

Disorders of the temporomandibular joint (TMJ) complex affect 6-12% of the population; the joint's disc is usually involved. Tissue engineering and regenerative medicine may constitute a promising therapeutic approach, with resident stromal progenitor cells a key factor in the process. We hypothesized that the TMJ disc (TMJD) contains multipotent stromal progenitors that may play an important role in regeneration of the disc. TMJD cells were cultured and evaluated for growth kinetics and colony-forming units (CFUs). Single cell-derived clones were isolated and induced to differentiate toward the osteogenic, adipogenic and chondrogenic lineages by culturing in various induction media. Flow cytometry was used to identify multipotent stromal cell surface markers in additional cell samples, and reverse transcription-polymerase chain reaction (RT-PCR) was used to determine gene expression patterns within isolated cells. High numbers of CFUs were observed, indicating cell self-renewal. Biochemical assays showed significantly higher alkaline phosphatase (ALP) activity, lipid droplet concentration and glycosaminoglycan levels in cells cultured in osteogenic, adipogenic and chondrogenic induction medium, respectively. Approximately 1% of the total cell population demonstrated the capability to differentiate into all three mesenchymal lineages. Chondrogenic gene levels within TMJD-derived cells were significantly reduced in passaged culture. Our results support the hypothesis that multipotent stromal progenitor cells populate the TMJD and possess proliferation and differentiation capabilities. These cells may contribute to the regeneration potential of dysfunctional tissue and become the primary component in future attempts at tissue engineering or regeneration of this complex. Copyright © 2015 John Wiley & Sons, Ltd.

Differential gene expression in the condylar cartilage of growing rabbits with temporomandibular joint anterior disk displacement-A transcriptomic study.

Internal derangement (ID) in the temporomandibular joint (TMJ) comprises a group of clinical problems with relatively high prevalence. However, the temporal changes in gene expression of condylar cartilage during continuous ID remain unclear. The aim of the current study is to investigate by microarray analysis, the differentially-expressed gene pattern in condylar cartilage of rabbits with ID from one to eight weeks of ID progression. Histological results (hematoxylin and eosin staining) indicated that abnormal collagen fiber arrangements, fragmentation of fibrils, and inflammatory cell-infiltration were detected from one to four weeks in joint disc specimens, while newly formed vessels, mucoid degeneration, meniscal tears, and the presence of osteoclasts and osteoblasts were observed at later time points. The microarray analysis revealed 6478 genes among all tested transcripts, to have a greater than two-fold expression change compared to controls. The inflammation-associated gene group including ace and il1ß increased rapidly in the early stage of disease and decreased later. In contrast, bone construction-related genes showed low expression levels at first and increased at later period in the ID progression. The current study also found some genes such as hla2g, which have not been previously reported, to be potentially relevant within ID. Our findings provide useful insights into the pathological mechanism of ID in TMJ.

Juvenile porcine temporomandibular joint: Three different cartilaginous structures?

OBJECTIVE: The temporomandibular joint (TMJ) consists of three cartilaginous structures: the fossa, disc, and condyle. In juvenile idiopathic arthritis (JIA), inflammation of the TMJ leads to destruction of the condyle, but not of the fossa or the disc. Such a different effect of inflammation might be related to differences in matrix composition of the cartilaginous structures. METHODS: The matrix composition of the three TMJ structures was analyzed in juvenile porcine samples and in an in vitro system of cells isolated from each anatomical structure embedded in 3% agarose gels. RESULTS: The matrix of all three anatomical structures of the TMJ contained collagen type I and its gene expression was maintained after isolation. The condyle and the fossa stained positive for collagen type II and proteoglycans, but the condyle contained considerably more collagen type II and proteoglycans than the fossa. The disc contained neither collagen type II protein nor expression of its gene, and the disc did not stain positive for proteoglycans. Aggrecan gene expression was lower in the disc compared to condyle and fossa cell-isolates. In general, the cell-isolates in vitro closely mimicked the characteristic features found in the tissue. CONCLUSION: The collagen type II content of the condyle clearly distinguished this cartilaginous structure from the disc and fossa. Since autoimmunity against collagen type II is associated with JIA, the relatively abundant presence of this type of collagen in the condyle might provide an explanation why primarily this cartilaginous structure of the TMJ is affected in JIA patients.