Intracytoplasmic filaments in the cells of adult human articular cartilage

Tissue Engineering of Cartilage Using a Random Positioning Machine

Articular cartilage is a skeletal tissue of avascular nature and limited self-repair capacity. Cartilage-degenerative diseases, such as osteoarthritis (OA), are difficult to treat and often necessitate joint replacement surgery. Cartilage is a tough but flexible material and relatively easy to damage. It is, therefore, of high interest to develop methods allowing chondrocytes to recolonize, to rebuild the cartilage and to restore joint functionality. Here we studied the in vitro production of cartilage-like tissue using human articular chondrocytes exposed to the Random Positioning Machine (RPM), a device to simulate certain aspects of microgravity on Earth. To screen early adoption reactions of chondrocytes exposed to the RPM, we performed quantitative real-time PCR analyses after 24 h on chondrocytes cultured in DMEM/F-12. A significant up-regulation in the gene expression of IL6, RUNX2, RUNX3, SPP1, SOX6, SOX9, and MMP13 was detected, while the levels of IL8, ACAN, PRG4, ITGB1, TGFB1, COL1A1, COL2A1, COL10A1, SOD3, SOX5, MMP1, and MMP2 mRNAs remained unchanged. The STRING (Search Tool for the Retrieval of Interacting Genes/Proteins) analysis demonstrated among others the importance of these differentially regulated genes for cartilage formation. Chondrocytes grown in DMEM/F-12 medium produced three-dimensional (3D) spheroids after five days without the addition of scaffolds. On day 28, the produced tissue constructs reached up to 2 mm in diameter. Using specific chondrocyte growth medium, similar results were achieved within 14 days. Spheroids from both types of culture media showed the typical cartilage morphology with aggrecan positivity. Intermediate filaments form clusters under RPM conditions as detected by vimentin staining after 7 d and 14 d. Larger meshes appear in the network in 28-day samples. Furthermore, they were able to form a confluent chondrocyte monolayer after being transferred back into cell culture flasks in 1 g conditions showing their suitability for transplantation into joints. Our results demonstrate that the cultivation medium has a direct influence on the velocity of tissue formation and tissue composition. The spheroids show properties that make them interesting candidates for cellular cartilage regeneration approaches in trauma and OA therapy.

The long-term effects of articular impaction

Ultimately, the long-term effects of articular impact injury remain unknown. Impact injury has long been considered an explanation for posttraumatic arthritis. It is an attractive hypothesis because often it is the only explanation we can put forth for why arthritis develops in an injured joint. There has been recent interest in the role of chondrocyte apoptosis in the pathogenesis of osteoarthritis. Chondrocyte apoptosis has also been observed with cartilage impact injury and may provide an explanation for how impact injury leads to osteoarthrosis. This paper reviews impact injury and its association with apoptosis as a potential etiology for posttraumatic arthritis.

Articular cartilage chondrocytes in type I and type II collagen-GAG matrices exhibit contractile behavior in vitro

Natural healing of articular cartilage defects generally does not occur, and untreated lesions may predispose the joint to osteoarthritis. To promote healing of cartilage defects, many researchers are turning toward a tissue engineering approach involving cultured cells and/or porous, resorbable matrices. This study investigated the contractile behavior of cultured canine chondrocytes seeded in a porous collagen-glycosaminoglycan (GAG) scaffold. Chondrocytes isolated from the knee joints of adult canines and expanded in monolayer culture were seeded into porous collagen-GAG scaffolds. Scaffolds were of two different compositions, with the predominant collagen being either type I or type II collagen, and of varying pore diameters. Over the 4-week culture period, the seeded cells contracted all of the type I and type II collagen-based matrices, despite a wide range of stiffness (145 +/- 23 Pa, for the type I scaffold, to 732 +/- 35 Pa, for the type II material). Pore diameter (25-85 microm, type I; and 53-257 microm, type II) did not affect cell-mediated contraction. Immunohistochemical staining revealed the presence of alpha-smooth muscle actin, an isoform responsible for contraction of smooth muscle cells and myofibroblasts, in the cytoplasm of the seeded cells and in chondrocytes in normal adult canine articular cartilage.

The long-term effect of a short course of transforming growth factor-beta1 on rat articular cartilage

The long-term effect of 3 days' topical administration of TGF-beta1 on unloaded articular cartilage in growing rats was investigated. Three to five rats were sacrificed on days 15, 30, 45, 60 and 90 after the last injection and the patellas were subjected to light and electron microscopic evaluation. The patellas showed age-related changes when entering the maturation phase. Absence of a hypertrophic zone and formation of a subchondral bone plate were observed in both treated rats and controls from that time point. Reduction of total cell amount was observed between older and younger patellas in both treated rats and controls. TGF-beta1 treatment resulted in accelerated maturation and ageing. Furthermore, the administration of TGF-beta1 resulted in a higher unmineralized cartilage with an increased total amount of cells in the intermediate zone. Matrix areas showing ultrastructural features of disturbed matrix composition were seen in the deeper part of this zone at all time points, but in the treated patellas only. The present results indicate that TGF-beta1 treatment may induce changes in articular cartilage in some respects similar to those seen in the early stages of degenerative and inflammatory joint diseases.

Development of vimentin filaments in the cells of the articular disc of the rat squamosomandibular joint with age

Age-related changes in the vimentin filaments were studied by immunohistochemistry and electron microscopy. In 2-week-old rats, cells showed only weak labelling for vimentin. A few intermediate filaments but well-developed rough endoplasmic reticulum and Golgi apparatus were observed in the cytoplasm. As the age of the rats advanced from 4-weeks to adulthood, labelling intensity increased and the cytoplasm of articular-disc cells was gradually filled with intermediate filaments surrounding degraded cytoplasmic organelles in the cytosol. Articular-disc cells in adult rats showed intense vimentin labelling and their cytoplasm was filled with closely packed intermediate filaments aligned in alternating layers. This age-related accumulation of vimentin filaments in articular-disc cells may be an adaptive response to compressive forces.

Ultrastructure of adult human articular cartilage matrix after cryotechnical processing

The ultrastructure of adult human articular cartilage matrix is reexamined in tissue processed according to recently improved cryotechniques [Studer et al. (1995) J. Microsc., 179:321-332]. In truely vitrified tissue, a network of fine cross-banded filaments (10-15 nm in diameter) with a periodicity characteristic of collagen fibrils is seen throughout the extracellular substance, even within the pericellular compartment, which has hitherto been deemed free of such components. Proteoglycans fill the interstices between these entities as a homogeneously distributed granular mass; they do not manifest a morphologically identifiable reticular structure. Longitudinally sectioned collagen fibrils exhibit variations in thickness and kinking; they tend to align with their periodic banding in register and are frequently seen to split or fuse along their longitudinal course. The tendency of fibrils to form bundles is greater in deeper zones than in more superficial ones. A duality in the orientation of fibrils and fibril bundles is observed within the interterritorial matrix compartment: superimposed upon the well-characterized arcade-like structure formed by one subpopulation is another, more randomly arranged one. The classical concepts of matrix organization thus need to be modified and refined to encompass these findings.

Cytoskeleton of cartilage cells

The cytoskeleton of chondrocytes consists of microfilaments made of actin, microtubules made of tubulin, and intermediate filaments made of a variety of subunits. Actin filaments are not prominent in vivo but may form in vitro. In culture, changes in filament polymerisation are important in determining cell shape, initiating chondrogenesis, and maintaining the chondrogenic phenotype. Microtubules, besides their role in cell division, organise the distribution of organelles and are involved in secretory transport mechanisms in collagen and proteoglycan synthesis. A variety of intermediate filaments may be present, frequently forming large whorled aggregates. The filaments include vimentin, cytokeratins, and glial fibrillary acidic protein. These may occur at different depths in articular cartilage. Vimentin accumulates during development of some fibrocartilages with increased mechanical loading. Together with other elements of the cytoskeleton, intermediate filaments could form part of a mechanotransduction system by which cells respond to external forces and sense changes in their external environment.

Cell and matrix biology of the suprapatella in the rat: a structural and immunocytochemical study of fibrocartilage in a tendon subject to compression

The structure, ultrastructure, histochemistry, and immunohistochemistry of the suprapatella have been described in the rat. The suprapatella is a fibrocartilaginous sesamoid within the tendon of quadriceps femoris that articulates with the femoral condyles during flexion of the knee joint and reduces the amount of bending required at the tendon-bone junction. The cells of the suprapatella were much larger and more numerous than those in the associated tendon and were packed with vimentin-containing, intermediate filaments. The tendon cells contained far fewer filaments. The cells of both regions contained actin and tubulin. Histochemical and immunohistochemical studies showed that the suprapatellar cells were embedded in a matrix that is rich in chondroitin sulphate, but does not contain keratan or heparan sulphate. The fibrocartilage of the adjacent attachment zone of the quadriceps tendon also contained chondroitin sulphate, but in addition was rich in type II collagen. The structure of the suprapatella was similar to that of the fibrocartilaginous regions of tendons that pass around bony pulleys. However, there were differences in matrix composition that could reflect functional differences between the fibrocartilages.

An investigation of vanishing bone disease

Vanishing bone disease is a rare condition producing local deformity and instability. Fibrovascular tissue replaces bone completely but the mechanism of bone destruction and resorption is unknown and there is controversy regarding the presence or absence of osteoclasts in the disease. Radiography, clinical chemistry, light microscopy, transmission electron microscopy (TEM) and cytochemistry were used to investigate the condition of a young woman presenting early in the disease process. We detected atypical ultrastructure in osteoblasts and endothelial cells. The rare osteoclasts, numerous mononuclear phagocytes and vascular endothelium found in the condition reacted positively for the enzyme acid phosphatase. Aggressive local excision of diseased tissue and insertion of a free vascularized bone graft at an advanced stage of the disease, accompanied by subsequent radiotherapy for residual disease only were successful in rehabilitating the affected forearm and hand.

Quantitation of structural features characterizing weight- and less-weight-bearing regions in articular cartilage: a stereological analysis of medial femoral condyles in young adult rabbits

The structural organization of articular cartilage from the medial femoral condyle of young adult rabbits has been examined after processing according to an improved fixation procedure. By using recently developed stereological methods, a quantitative analysis of chondrocyte number, surface area, volume, and matrix volume per cell was carried out in the light microscope; at the electron microscopic level, quantities of cytoplasmic components within chondrocytes (including organelles) were estimated. These measurements were made for each of the four zones from the (articular cartilage) surface down to the tidemark, and the results (for each zone) were compared between weight- and less-weight-bearing regions. In general, articular cartilage revealed considerable heterogeneity in structure throughout its depth. The number of cells per unit volume is maximal beneath the surface and decreases toward the tidemark. The size of chondrocytes, and the mean matrix volume surrounding each, increases from the surface toward the deeper zones. Comparison between weight- and less-weight-bearing regions reveals striking differences. The numerical volume density of cells in the superficial zone of regions bearing high physiological load is less than half of that in less-weight-bearing regions, chondrocyte death being principally responsible for this reduction. A comparison between the midzones of weight- and less-weight-bearing areas reveals that the former is characterized by a decrease in cell density and an approximately threefold increase in cell size in relation to the latter. The increase in cell volume is attributable principally to an accumulation of intermediate filaments and glycogen particles, and represents an adaptation to increased functional requirements. Near the tidemark, numerical volume densities of chondrocytes in both weight- and less-weight-bearing locations are similar, but the larger cell size in the former still persists.

Rough endoplasmic reticulum and fine intracytoplasmic filaments in articular cartilage chondrocytes of young rabbits; a stereological morphometric study using transmission electron microscopy

Articular cartilage of eight young NZW rabbits was investigated by electron microscopy. A simple and rapid stereological method was designed for quantifying the synthetic activity of the chondrocytes in the different zones of uncalcified articular cartilage by measuring the amount of rough endoplasmic reticulum (RER) on electron micrographs. The surface area of RER per unit volume of chondrocyte cytoplasm, of cartilage, and per chondrocyte, were determined. In addition the volume density, and mean diameter, of RER cisternae were computed. The surface area of RER was least in the superficial zone (402 micron 2) and largest in the deep zone (879 micron 2) chondrocytes. The RER surface area per unit volume of tissue was, however, significantly (P less than 0.05) greater in the superficial zone (12.8 X 10(-2) micron2/micron3) than in the deep zone (8.5 X 10(-2) micron2/micron3) of the articular cartilage. Percentages of chondrocytes displaying masses of intracytoplasmic fine filaments were also registered. 8.5% of the chondrocytes in the superficial, 48.8% in the middle, and 53.5% in the deep zone contained intracytoplasmic fine filaments. Chondrocytes containing filaments are probably degenerating cells. They contained diminished amounts of RER. The altered nutritional status in the deeper zones of the cartilage might have caused these changes in the cells. The formation of the calcified zone of the cartilage in the young rabbits, contributing to the cessation of the nutritional supply of chondrocytes from the subchondral bone marrow spaces, may have affected the process. Since the proportion of RER can be regarded as indicating the synthetic activity, and that of filaments as indicating the degree of degeneration, these parameters can be used in the evaluation of the functional status of the uncalcified articular cartilage chondrocytes.

Chondrons in cartilage: ultrastructural analysis of the pericellular microenvironment in adult human articular cartilages

A combination of scanning and transmission electron microscopy was used to investigate the morphology and ultrastructure of normal human articular cartilage sampled from adult amputation specimens. This study confirms our previous observations on canine articular cartilage, which showed middle and deep layer chondrocytes surrounded by a pericellular matrix and enclosed within a pericellular capsule composed of filamentous and fine fibrillar materials. Pores in the "felt-like" organization of the capsular weave progressively decreased in size from the inner to the outer border of the capsule. Matrix vesicles were found embedded within the capsular weave and distributed throughout the territorial matrix. It is suggested that the chondrocyte, its pericellular matrix, and capsule together constitute the "chondron," a primary functional and metabolic unit of cartilage that acts hydrodynamically to protect the integrity of the chondrocyte and its pericellular microenvironment during compressive loading.

Cytochemical localization of alkaline and acid phosphatase in human vanishing bone disease

This report is the first cytochemical investigation of vanishing bone disease "Gorham's Disease" (Gorham and Stout 1955). The ultrastructural localization of non-specific alkaline phosphatase and of specific and non-specific acid phosphatase activity was studied in slices of tissue removed from a patient with this rare disorder. Sodium beta-glycerophosphate and phosphorylcholine chloride were used as substrates. Alkaline phosphatase was present around the plasma membranes of osteoblasts and associated with extracellular matrix vesicles in new woven bone. This is consistent with the proposed role for this enzyme (Robison 1923) and for matrix vesicles (Bonucci 1967) in the mineralization of bone (Bernard and Marvaso 1981). Concentrations of specific secretory acid phosphatase reaction product in the cytoplasm of degenerating osteoblasts may contribute to the imbalance between bone formation and resorption. Osteoclasts, while few in number, showed non-specific and specific acid phosphatase activity. The Golgi apparatus and heterophagic lysosomes of mononuclear phagocytes were rich in non-specific acid phosphatase. This was also present in the Golgi lamellae and lysosomes of endothelial cells. Acid phosphatase cytochemistry suggests that mononuclear phagocytes, multinuclear osteoclasts and the vascular endothelium are involved in bone resorption in this disease.

Meckel's cartilage in Xenopus laevis during metamorphosis: a light and electron microscope study

Meckel's cartilage, in Xenopus laevis prior to metamorphosis, is a tissue exhibiting very large lacunae, separated by thin rims of matrix, presenting a net-like appearance, similar to that of cartilage in invertebrates. The cells on the periphery of the tissue are rather more flattened, and more closely packed. On the lateral aspects of the cartilage distinct columns of apparently dividing cells are evident. During metamorphic climax, the amount of matrix separating the lacunae increases, with an associated decrease in lacunar size, and some of the deeper cells develop cilia, which are not seen either before or after climax. By the end of metamorphic climax there is a considerable increase in the amount of matrix present in the tissue, while many cells at all depths in the cartilage show the presence of lysosome-like structures, possibly associated with the changing shape of the cartilage. Intramembranous ossification is proceeding around Meckel's cartilage, but there is no evidence of endochondral ossification up to the end of metamorphosis.

Zonal analysis of cytoplasmic components of articular cartilage chondrocytes

The cytoplasmic components of chondrocytes in the various zones of articular cartilage of the medial femoral condyle of adult male New Zealand white rabbits were quantitated from electron micrographs. A progressive increase in the content of endoplasmic reticulum, Golgi apparatus, mitochondria, and electrondense bodies (presumed to be lysosomes) from the tangential zone through the top half of the calcified zone was observed. These organelles then showed a decrease in content in the bottom half of the calcified zone. This was associated with an even greater increase in content of holes in the cytoplasm at that level, providing clear evidence of cellular swelling. Based on these quantitative morphologic studies, it is concluded that the chondrocytes in all zones of articular cartilage are metabolically active, but those cells in the calcified zone show signs of degeneration. The ultimate fate of the chondrocytes in the bottom of the calcified zone is not clear from this study.

beta-Fibrillary bodies in low-grade adenocarcinoma of parotid gland: a histochemical and ultrastructural study

Rounded or polyhedral, acellular, dense lamellated structures were seen within the cystic and glandular spaces in a case of low-grade papillary adenocarcinoma of the parotid gland. These structures displayed the histochemical characteristics of amyloid, namely, pink color with apple-green birefringence and bright red fluorescence with alkaline congo red, orthochromasia and red birefringence with standardized toluidine blue, positive dimethylaminobenzaldehyde nitrite and diazotization coupling reactions, and spontaneous autofluorescence. At the ultrastructural level the tumor consisted of cuboid, flattened, or elongated epithelial cells with varying degrees of differentiation, but most mature tumor cells had numerous intracellular intermediate filaments. These filaments were randomly oriented and had no lateral fasciation; they were 10 nm wide and similar to those constituting the intraluminal corpora amylacea. These bodies displayed increased fibrillary density. Cellular organelles and pyknotic nuclei were frequently present at the periphery of the fibrillary masses. Amyloid masses, histochemically and ultrastructurally indistinguishable from those in the lumens, were present in the stroma as well. It is postulated that both stromal and intraluminal beta-fibrillary bodies result from apoptosis and desquamation of neoplastic cells, with progressive release and confluency of intracellular intermediate filaments within closed cystic spaces. This additional evidence of beta-fibrillosis associated with a slow-growing non-hormone-secreting salivary gland tumor suggests that these tumors may be associated with amyloidosis more frequently than was previously suspected.

Antigen-induced arthritis. Studies on the inhibition of proteoglycan synthesis observed in articular cartilage during short-term joint inflammation

During the acute phase of antigen-induced arthritis, cartilage was obtained from five different sites within the joint, and chondrocyte activity was assessed by autoradiography of sections labeled with 35S-sulfate. There was a marked inhibition of chondrocyte proteoglycan synthesis in all weight-bearing areas; in addition, the complete superficial layer of cells and many mid-zone cells in these areas were completely inactive. Electron microscopy showed that the inactive surface cells had degenerated completely, and that many mid-zone cells contained an accumulation of intracytoplasmic filaments and were depleted in biosynthetic organelles. A biochemical study of the inhibition showed that: 1) the incorporation of 35S-sulfate and 3H-acetate into glycosaminoglycans was inhibited to a similar extent; 2) the inhibition of glycosaminoglycan synthesis could not be reversed either by the addition of benzyl-beta-D-xyloside to incubations or by maintenance of the cartilage in organ culture for 6 days; 3) the inhibited chondrocytes exhibited a decreased ability to secrete proteoglycans into the extracellular fraction of the cartilage.

Long term effects of myochrysine in articular cartilage

Intra-articularly administered sodium aurothiomalate (Myochrysine) produced aurosomes containing characteristic electron dense contents (indicating the presence of gold), in the chondrocytes of rabbit articular cartilage. At first the aurosomes were bounded by a membrane but later the electron dense contents were seen lying free in the cytoplasmic matrix. Such deposits were detectable up to 14 months after injection of Myochrysine but none were found at later time intervals (18 months and 2 years). There was a reduction in the population of superficial chondrocytes (Zone I) while those in deeper zones (Zones II and III) showed an increased content of intracytoplasmic filaments. It is thought that these are regressive or degenerative changes produced by gold.

Chondrocytes

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Fine structure of rabbit articular chondrocytes in tissue culture during logarithmic and confluent stages of growth

Ultrastructural changes in cultured articular cartilage chondrocytes from rabbit, during two growth phases were examined by transmission and scanning electron microscopy. Cells in logarithmic growth are characterized by an abundance of intracellular lipoid bodies, little development of rough endoplasmic reticulum (RER), and few cytoplasmic microfilaments. As the cells reach confluency there is a concomitant development of RER, organization and abundance of microfilaments, loss of lipoid bodies, and increase in the number of mitochondria. The fine structure of cultured chondrocytes is very similar to that of rabbit cartilage cells in situ, in that numerous lipoid bodies and microfilaments are prominent features in both cases.

Changes in articular cartilage following intraarticular injection of tritiated glyceryl trioleate

The effects of intra-articular injections of non-radioactive and tritium-labelled glyceryl trioleate into the mandibular and knee joints of adult rabbits have been investigated using autoradiographic and histochemical techniques and electron microscopy. As observed at the fourth day after operation, fat droplets accumulate in cells of the fibrous, intermediate and cartilaginous layers of mandibular condylar, and in the superficial and upper middle (rather than the deeper) zones of femoral condylar cartilage. Autoradiography of frozen sections shows that numerous silver grains are located over these fat-laden cells following injection of trioleate which has been labelled in the fatty acid moiety of the molecule. In the knee joint the number of grains is directly related to the amount of lipid in the cell. Following injection of glyceryl-labelled trioleate no such result is obtained; it seems doubtful whether or not there is any uptake of this label. However, synovial membrane from the knee joint appears to take up both kinds of trioleate. Results of histochemical methods of NADH diaphorase, lactic dehydrogenase, acid phosphatase and beta-glucosaminidase are consistent with ultrastructural evidence of degeneration in some chondrocytes and of loss of ground substance from the matrix. A raised level of alpha-glycerophosphate dehydrogenase activity is probably associated with synthesis of endogenous glycerol for re-esterification of absorbed fatty acids, and enhanced activity of UDPglucose dehydrogenase with the chondrocytic reaction to matrix depletion. Apart from the increase in fat content, ultrastructural features in injected knee joints include flattening of cell processes against the chondrocyte surface and more abundant intracytoplasmic filaments. Injected mandibular joints show little evidence of these changes although the number of cells in the cartilage appears to be greatly reduced. No extracellular fat droplets occur in femoral cartilage, but material similar in electron density to intracellular fat is observed at the external aspect of some mandibular chondrocytes. The findings indicate that the fatty acid portion of triglyceride injected intra-articularly is taken up by the chondrocytes and retained until at least the fourth day after injection. It is suggested that prior lipolysis takes place either in the synovial cavity (or membrane) or at the chondrocyte surface, but it is uncertain how or in what form fat traverses the matrix. Lipoarthrosis appears to produce changes in the chondrocytes which are thought to be pathological; a number of cell deaths occur. The possibility that gross degeneration of the articular cartilage may ensue is subject to further investigation.

The development of the elastic cartilage of the mouse pinna

The development of the elastic cartilage in the mouse pinna was studied with the electron microscope. For descriptive purposes, the developmental process was divided into five stages. These begin with the onset of cartilage matrix production by chondroblasts. The second stage is the major mitotic stage although the cells now resemble chondrocytes and cartilage matrix production evidently continues. This is followed by a period during which cartilage matrix production appears to be the major activity of the chondrocytes. In Stage IV, the cells continue to produce cartilage matrix but at the same time they also produce lipid which comes to occupy most of the cell as a large lipid droplet. In the adult, Stage V, matrix production appears to be at an end and each cell contains a lipid droplet which is so large that the cells bear some resemblance to white fat cells and might be designated as lipochondrocytes. The adult cartilage matrix contains elastic plates in a three-dimensional honeycomb pattern. Between the elastic plates and the cells, the matrix contains elements similar to those seen in hyaline cartilage but it does not contain matrix vesicles such as those which have been associated with the calcification of cartilage. Two types of filaments differing in appearance, size, and location are seen within the chondrocytes. They have been designated cytofilaments and cortical filaments. Cytofilaments are dispersed throughout the cell and also contact the substance of the lipid droplet; cortical filaments (sub-plasmalemmal microfilaments) are located directly under the plasma membrane. They are thought to be involved in movement of the plasma membrane as a concomitant to extracellular fibrogenesis.

Articular cartilage: a review and scanning electron microscope study. II. The territorial fibrillar architecture

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Ultrastructure of synovial cells in vitro

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Meshwork patterns in the ground substance of articular cartilage and nucleus pulposus

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Multifunctional mesenchymal cells resembling smooth muscle cells in ganglia of the wrist

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Ultrastructure of articular cartilage in experimental immobilization

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Fine structure of juvenile human nucleus pulposus

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Ultrastructure of synovial membrane in rheumatoid arthritis

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