Chondrocyte ultrastructure in adult human articular cartilage

An ultrastructural report of human articular cartilage resident cells in correlation with their phenotypic characteristics

The recent discovery of progenitors based on their differential fibronectin-adhesion (FAA-CPs) and migratory-based (MCPs) assay has evoked interest due to their superiority in terms of their efficient chondrogenesis and reduced hypertrophic propensity. This study aims to isolate and enrich three articular cartilage subsets, chondrocytes, FAA-CPs, and MCPs, and compare their undifferentiated and chondrogenic differentiated status, using in-vitro phenotypical characterization in correlation with ultrastructural analysis using Transmission Electron Microscopy (TEM). Following informed consent, cartilage shavings were procured from a non-diseased human ankle joint and cultured to obtain the three subsets. Chondrocytes exhibited higher CD106 and lower CD49b and CD146 levels. Following chondrogenic differentiation, corroborative results were seen, with the MCP group showing the highest GAG/DNA ratio levels and uptake of extracellular matrix stain as compared to the FAA-CP group. TEM analysis of the chondrocytes revealed the presence of more autolytic cells with disintegrated cytoplasm and plasma membrane. The differentiated FAA-CPs and MCPs displayed higher collagen and rough endoplasmic reticulum. The results presented in this study provide novel information on the ultrastructural characteristics of cartilage resident cells, with the chondrocyte group displaying features of terminal differentiation. Both progenitor subtypes showed superiority in varied contexts, with greater collagen fibrils and greater GAG content in MCPs. The display of preferential and differentiation traits sheds insight on the necessity to enrich progenitors and coculturing them with the general pool of constituent cells to combine their advantages and reduce their drawbacks to achieve a regenerative tissue displaying genuine hyaline-like repair while limiting their terminal differentiation.

Bone and the Unfolded Protein Response: In Sickness and in Health

Differentiation and optimal function of osteoblasts and osteoclasts are contingent on synthesis and maintenance of a healthy proteome. Impaired and/or altered secretory capacity of these skeletal cells is a primary driver of most skeletal diseases. The endoplasmic reticulum (ER) orchestrates the folding and maturation of membrane as well as secreted proteins at high rates within a calcium rich and oxidative organellar niche. Three ER membrane proteins monitor fidelity of protein processing in the ER and initiate an intricate signaling cascade known as the Unfolded Protein Response (UPR) to remediate accumulation of misfolded proteins in its lumen, a condition referred to as ER stress. The UPR aids in fine-tuning, expanding and/or modifying  the cellular proteome, especially in specialized secretory cells, to match everchanging physiologic cues and metabolic demands. Sustained activation of the UPR due to chronic ER stress, however, is known to hasten cell death and drive pathophysiology of several diseases. A growing body of evidence suggests that ER stress and an aberrant UPR may contribute to poor skeletal health and the development of osteoporosis. Small molecule therapeutics that target distinct components of the UPR may therefore have implications for developing novel treatment modalities relevant to the skeleton. This review summarizes the complexity of UPR actions in bone cells in the context of skeletal physiology and osteoporotic bone loss, and highlights the need for future mechanistic studies to develop novel UPR therapeutics that mitigate adverse skeletal outcomes.

The osteoarthritis bone score (OABS): a new histological scoring system for the characterisation of bone marrow lesions in osteoarthritis

OBJECTIVES

Bone marrow lesions (BMLs) are associated with pain in osteoarthritis (OA), but histological scores for OA focus on cartilage pathology. We developed a new scoring system, the Osteoarthritis Bone Score (OABS), to characterise OA-related BMLs.

METHODS

BML/non-BML tissues identified by Magnetic Resonance Imaging (MRI) in 10 knee OA subjects were harvested at total knee replacement (TKR). Osteochondral tissue from a further 140 TKR and 23 post-mortem (PM) cases was assessed. Histological features distinguishing MRI-defined BML/non-BML tissues on qualitative analysis were classified as present (0) or absent (1), summated for the OABS, validated by Rasch analysis and sensitivity to distinguish between sample groups. Immunohistochemistry for PGP9.5 assessed innervation.

RESULTS

Subchondral characteristics associated with BML tissues were cysts, fibrosis, hypervascularity, cartilage islands, trabecular thickening, loss of tidemark integrity and inflammatory cell infiltration. PGP9.5 immunoreactive perivascular nerves were associated with BMLs. OABS performed well as a measurement tool, displayed good reliability (Cronbach alpha = 0.68), had a 2-factor structure (trabecular/non-trabecular), with moderate correlation between the two factors (r = 0.56, 95% CI 0.46, 0.65). OABS scores were higher in TKR than PM cases with chondropathy, median difference 1.5 (95% CI -2, 0). OABS and Mankin scores similarly distinguished TKR from non-OA controls, but only OABS was higher in BML than non-BML tissues, median difference -4 (95% CI -5 to -2).

CONCLUSIONS

OABS identifies and validly quantifies histopathological changes associated with OA BMLs. Histopathology underlying BMLs may represent 2 inter-related pathological processes affecting trabecular/non-trabecular structures. Increased vascularity/perivascular innervation in BMLs might contribute to pain.

Animal Models of Temporomandibular Joint Osteoarthritis: Classification and Selection

Temporomandibular joint osteoarthritis (TMJOA) is a common degenerative joint disease that can cause severe pain and dysfunction. It has a serious impact on the quality of lives of patients. Since mechanism underlying the pathogenesis of TMJOA is not fully understood, the development of effective tools for early diagnosis and disease-modifying therapies has been hindered. Animal models play a key role in understanding the pathological process of diseases and evaluating new therapeutic interventions. Although some similarities in disease processes between animals and humans are known, no one animal model is sufficient for studying all characteristics of TMJOA, as each model has different translatability to human clinical conditions. For the past 4 decades, TMJOA animal models have been studied by numerous researchers and can be broadly divided into induced, naturally occurring, and genetically modified models. The induced models can be divided into invasive models (intra-articular injection and surgical induction) or non-invasive models (mechanical loading, high-fat diet, and sleep deprivation). Different types of animal models simulate different pathological expressions of TMJOA and have their unique characteristics. Currently, mice, rats, and rabbits are commonly used in the study of TMJOA. This review sought to provide a general description of current experimental models of TMJOA and assist researchers in selecting the most appropriate models for different kinds of research.

Alterations in characteristics of canine articular chondrocytes in non-passaged long-term monolayer culture: Matter of differentiation, dedifferentiation and redifferentiation

This study investigated the effects of culture time on phenotype stability of canine articular chondrocytes (CACs) in non-passaged long-term monolayer culture. Third passage (P3) CACs isolated from four cartilage samples were seeded at three different initial seeding densities (0.2 × 10⁴, 1.0 × 10⁴ and 5.0 × 10⁴ cells/cm²) and maintained in monolayer condition up to 8 weeks without undergoing subculture after confluence. The characteristic changes of chondrocytes during the culture period were evaluated based on the cell morphology, cell proliferation, glycosaminoglycans (GAGs) content, DNA quantification, mRNA expression and ultrastructure of chondrocytes. Chondrocytes maintained under post-confluence condition exhibited a capability to grow and proliferate up to 4 weeks. Alcian blue staining and Dimethylmethylene blue (DMMB) assay revealed that the extracellular matrix (ECM) synthesis was increased in a time-dependent manner from 2 to 8 weeks. The chondrocyte mRNA expression profile was dramatically affected by prolonged culture time, with a significant downregulation of collagen type I, whereas the expression of collagen type II, aggrecan, Sox9 and matrix metalloproteinase 13 (MMP-13) were significantly upregulated. In addition, transmission electron microscopy (TEM) result indicated dilation of rough endoplasmic reticulum (RER) in these long-term monolayer cultured chondrocytes. These findings demonstrate that the chondrocytes phenotype could be partially redifferentiated through the spontaneous redifferentiation process in long-term cultures using standard culture medium without the addition of chondrogenic supplements or tissue-culture scaffolds.

Oxidative stress and chronic inflammation in osteoarthritis: can NRF2 counteract these partners in crime?

Osteoarthritis (OA) is an age-related joint degenerative disease associated with pain, joint deformity, and disability. The disease starts with cartilage damage but then progressively involves subchondral bone, causing an imbalance between osteoclast-driven bone resorption and osteoblast-driven remodeling. Here, we summarize the data for the role of oxidative stress and inflammation in OA pathology and discuss how these two processes are integrated during OA progression, as well as their contribution to abnormalities in cartilage/bone metabolism and integrity. At the cellular level, oxidative stress and inflammation are counteracted by transcription factor nuclear factor erythroid p45-related factor 2 (NRF2), and we describe the regulation of NRF2, highlighting its role in OA pathology. We also discuss the beneficial effect of some phytonutrients, including the therapeutic potential of NRF2 activation, in OA.

Microarray analysis of bone marrow lesions in osteoarthritis demonstrates upregulation of genes implicated in osteochondral turnover, neurogenesis and inflammation

Objective

Bone marrow lesions (BMLs) are well described in osteoarthritis (OA) using MRI and are associated with pain, but little is known about their pathological characteristics and gene expression. We evaluated BMLs using novel tissue analysis tools to gain a deeper understanding of their cellular and molecular expression.

Methods

We recruited 98 participants, 72 with advanced OA requiring total knee replacement (TKR), 12 with mild OA and 14 non-OA controls. Participants were assessed for pain (using Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC)) and with a knee MRI (using MOAKS). Tissue was then harvested at TKR for BML analysis using histology and tissue microarray.

Results

The mean (SD) WOMAC pain scores were significantly increased in advanced OA 59.4 (21.3) and mild OA 30.9 (20.3) compared with controls 0.5 (1.28) (p<0.0001). MOAKS showed all TKR tissue analysed had BMLs, and within these lesions, bone marrow volume was starkly reduced being replaced by dense fibrous connective tissue, new blood vessels, hyaline cartilage and fibrocartilage. Microarray comparing OA BML and normal bone found a significant difference in expression of 218 genes (p<0.05). The most upregulated genes included stathmin 2, thrombospondin 4, matrix metalloproteinase 13 and Wnt/Notch/catenin/chemokine signalling molecules that are known to constitute neuronal, osteogenic and chondrogenic pathways.

Conclusion

Our study is the first to employ detailed histological analysis and microarray techniques to investigate knee OA BMLs. BMLs demonstrated areas of high metabolic activity expressing pain sensitisation, neuronal, extracellular matrix and proinflammatory signalling genes that may explain their strong association with pain.

Zoledronic Acid Effect on Chondrocyte Apoptosis in Degenerative Osteoarthritis Model

Apoptosis refers to cell death without inflammatory process, and is related to degenerative changes in joints. We hypothesized that zoledronic acid (ZA) would have a positive effect on chondrocyte viability and decreases in chondrocyte loss, which are important for the progression of degeneration. This study aimed to reveal the difference in time-dependent apoptotic changes in cartilage tissue in the anterior cruciate ligament (ACL) transection model of osteoarthritis (OA) in rat knees after treatment with zoledronic acid. We randomly divided 48 male Wistar albino rats into 6 groups. The knees of all rats except those in the control group underwent the operation for ACL transection. ZA for half of the rats and saline solution for the others was injected weekly into knees. Animals were killed at 0, 3, and 6 weeks after surgery. Apoptosis of chondrocytes were analyzed using the terminal deoxynucleotidyl transferase dUTP nick end labeling method. Comparison of groups was performed using Kruskal Wallis analysis and the Mann Whitney U test. Significant differences were observed between the groups treated with ZA and saline. ZA treatment significantly decreased the number of apoptotic cells in chondral tissue. ZA prevents time-dependent degenerative changes in chondral tissue by decreasing chondrocyte death. Intra-articular ZA may have the potential to treat and conserve chondral viability. ZA prevents chondrocyte loss and may play a therapeutic role in OA and conserving joint health. Further studies are needed to evaluate the potential of intra-articular ZA for the prevention or treatment of age-related degenerative changes.

Ultra-structural changes and expression of chondrogenic and hypertrophic genes during chondrogenic differentiation of mesenchymal stromal cells in alginate beads

Chondrogenic differentiation of mesenchymal stromal cells (MSCs) in the form of pellet culture and encapsulation in alginate beads has been widely used as conventional model for in vitro chondrogenesis. However, comparative characterization between differentiation, hypertrophic markers, cell adhesion molecule and ultrastructural changes during alginate and pellet culture has not been described. Hence, the present study was conducted comparing MSCs cultured in pellet and alginate beads with monolayer culture. qPCR was performed to assess the expression of chondrogenic, hypertrophic, and cell adhesion molecule genes, whereas transmission electron microscopy (TEM) was used to assess the ultrastructural changes. In addition, immunocytochemistry for Collagen type II and aggrecan and glycosaminoglycan (GAG) analysis were performed. Our results indicate that pellet and alginate bead cultures were necessary for chondrogenic differentiation of MSC. It also indicates that cultures using alginate bead demonstrated significantly higher (p < 0.05) chondrogenic but lower hypertrophic (p < 0.05) gene expressions as compared with pellet cultures. N-cadherin and N-CAM1 expression were up-regulated in second and third weeks of culture and were comparable between the alginate bead and pellet culture groups, respectively. TEM images demonstrated ultrastructural changes resembling cell death in pellet cultures. Our results indicate that using alginate beads, MSCs express higher chondrogenic but lower hypertrophic gene expression. Enhanced production of extracellular matrix and cell adhesion molecules was also observed in this group. These findings suggest that alginate bead culture may serve as a superior chondrogenic model, whereas pellet culture is more appropriate as a hypertrophic model of chondrogenesis.

A Simplified Method for the Aspiration of Bone Marrow from Patients Undergoing Hip and Knee Joint Replacement for Isolating Mesenchymal Stem Cells and In Vitro Chondrogenesis

The procedure for aspiration of bone marrow from the femur of patients undergoing total knee arthroplasty (TKA) or total hip arthroplasty (THA) may vary from an OR (operating room) to OR based on the surgeon's skill and may lead to varied extent of clotting of the marrow and this, in turn, presents difficulty in the isolation of mesenchymal stem cells (MSCs) from such clotted bone marrow. We present a simple detailed protocol for aspirating bone marrow from such patients, isolation, and characterization of MSCs from the aspirated bone marrow specimens and show that the bone marrow presented no clotting or exhibited minimal clotting. This represents an economical source and convenient source of MSCs from bone marrow for use in regenerative medicine. Also, we presented the detailed protocol and showed that the MSCs derived from such bone marrow specimens exhibited MSCs characteristics and generated micromass cartilages, the recipe for regenerative medicine for osteoarthritis. The protocols we presented can be used as standard operating procedures (SOPs) by researchers and clinicians.

Autophagy modulates articular cartilage vesicle formation in primary articular chondrocytes

Chondrocyte-derived extracellular organelles known as articular cartilage vesicles (ACVs) participate in non-classical protein secretion, intercellular communication, and pathologic calcification. Factors affecting ACV formation and release remain poorly characterized; although in some cell types, the generation of extracellular vesicles is associated with up-regulation of autophagy. We sought to determine the role of autophagy in ACV production by primary articular chondrocytes. Using an innovative dynamic model with a light scatter nanoparticle counting apparatus, we determined the effects of autophagy modulators on ACV number and content in conditioned medium from normal adult porcine and human osteoarthritic chondrocytes. Healthy articular chondrocytes release ACVs into conditioned medium and show significant levels of ongoing autophagy. Rapamycin, which promotes autophagy, increased ACV numbers in a dose- and time-dependent manner associated with increased levels of autophagy markers and autophagosome formation. These effects were suppressed by pharmacologic autophagy inhibitors and short interfering RNA for ATG5. Caspase-3 inhibition and a Rho/ROCK inhibitor prevented rapamycin-induced increases in ACV number. Osteoarthritic chondrocytes, which are deficient in autophagy, did not increase ACV number in response to rapamycin. SMER28, which induces autophagy via an mTOR-independent mechanism, also increased ACV number. ACVs induced under all conditions had similar ecto-enzyme specific activities and types of RNA, and all ACVs contained LC3, an autophagosome-resident protein. These findings identify autophagy as a critical participant in ACV formation, and augment our understanding of ACVs in cartilage disease and repair.

Utility of transmission electron microscopy in small round cell tumors

Small round cell tumors (SRCTs) are a heterogeneous group of neoplasms composed of small, primitive, and undifferentiated cells sharing similar histology under light microscopy. SRCTs include Ewing sarcoma/peripheral neuroectodermal tumor family tumors, neuroblastoma, desmoplastic SRCT, rhabdomyosarcoma, poorly differentiated round cell synovial sarcoma, mesenchymal chondrosarcoma, small cell osteosarcoma, small cell malignant peripheral nerve sheath tumor, and small cell schwannoma. Non-Hodgkin's malignant lymphoma, myeloid sarcoma, malignant melanoma, and gastrointestinal stromal tumor may also present as SRCT. The current shift towards immunohistochemistry and cytogenetic molecular techniques for SRCT may be inappropriate because of antigenic overlapping or inconclusive molecular results due to the lack of differentiation of primitive cells and unavailable genetic service or limited moleculocytogenetic experience. Although usage has declined, electron microscopy (EM) remains very useful and shows salient features for the diagnosis of SRCTs. Although EM is not always required, it provides reliability and validity in the diagnosis of SRCT. Here, the ultrastructural characteristics of SRCTs are reviewed and we suggest that EM would be utilized as one of the reliable modalities for the diagnosis of undifferentiated and poorly differentiated SRCTs.

Apoptosis in the traumatic and degenerative tears of human meniscus

The apoptotic bodies were observed aging in osteoarthritic cartilage and meniscus but there is still a lack of knowledge in the literature about the relationship between meniscal injury and apoptosis. The aim of this study is to investigate the apoptotic changes on meniscal tears caused by degeneration and trauma. Thirty-eight specimens of human meniscal tissues harvested from cadavers and patients were selected from groups below the age of 40. There were 14 normal meniscal tissues from cadavers in the control group. There were 24 meniscal tissues harvested from patients who had had arthroscopic operations for meniscal tear due to degeneration or trauma. The torn meniscal samples were divided into two groups according to their etiologies, either degenerative or traumatic. Apoptotic cells were investigated in each of the three groups in the histopathological examination. There was statistically significant difference among the groups according to apoptotic index (p < 0.01). The mean indexes of apoptosis in both groups of torn meniscus (group 2 = 0.497 and group 3 = 0.725) were significantly higher than that in the normal tissue (group 1 = 0.237) (p < 0.01) but the difference between groups 2 and 3 was not significant (p > 0.001). This study indicates that an increase in the amount of apoptosis appeared to have a close relationship with meniscal tear caused by either trauma or degenerative changes.

Characterization of mature vs aged rabbit articular cartilage: analysis of cell density, apoptosis-related gene expression and mechanisms controlling chondrocyte apoptosis

OBJECTIVE

The prevalence of osteoarthritis (OA) is increased in aged individuals and a direct correlation between chondrocyte apoptosis and cartilage degradation secondary to OA has been demonstrated. To address the question of whether age predisposes articular cartilage to apoptosis, the objective of the present study was to characterize and compare in aged and mature non-OA rabbit articular cartilage, cell density and expression levels of specific genes associated with apoptosis. Mechanistic studies on the inhibition of induced apoptosis were also carried out.

METHODS

Grade I (non-OA) femoral condyles and tibial plateaus from mature and aged rabbits were taken for assessment of viable cell density (VCD) and mRNA (reverse transcription-polymerase chain reaction) expression levels of the pro-apoptotic genes, Fas, Fas ligand (FasL), caspase-8, inducible nitric oxide synthase (iNOS) and p53. In vitro insulin-like growth factor (IGF-1)-mediated inhibition of nitric oxide (NO)-induced apoptosis was also examined using sodium nitroprusside (SNP) as NO donor.

RESULTS

VCD was decreased 50-70% in aged articular cartilage relative to mature cartilage. mRNA expression levels of Fas, FasL, caspase-8 and p53 were higher in aged cartilage than in mature cartilage. iNOS expression was unchanged. IGF-1-mediated inhibition of NO-induced apoptosis was dose-dependent and reversed with addition of phosphatidylinositol-3 kinase inhibitor.

CONCLUSIONS

This controlled animal model study demonstrates that age predisposes articular cartilage to changes in VCD and expression levels of specific pro-apoptotic genes. It is significant that these findings were demonstrated on cartilage that showed no prior signs of OA; it is also possible that such changes are a prelude to the age-related development of OA.

Evidence of chondrocyte repopulation in adult ovine articular cartilage following cryoinjury and long-term transplantation

OBJECTIVE

To characterize the response of articular chondrocytes to a specific cryoinjury that leads to cluster formation following long-term transplantation.

DESIGN

Osteochondral dowels from 20 adult sheep were cryopreserved to optimize the recovery of chondrocytes immediately after thawing. The dowels were transplanted as allografts and observed at 3 and 12 months. Chondrocyte distribution and viability was assessed using paravital dyes after transplantation. Chondrocyte phenotype was assessed by in situ hybridization and immunohistochemistry to detect type II collagen. An anticentrosome antibody was used to identify cells undergoing cell cycle progression towards mitosis.

RESULTS

All cryopreserved grafts showed the presence of spheroidal clusters of chondrocytes 1 year after transplantation while the host cartilage adjacent to the graft appeared morphologically normal. The average size of the clusters increased from four cells at 3 months to 12 cells at 1 year. The chondrocytes in the clusters displayed newly formed type II collagen protein and mRNA. Some cells within clusters were observed with two centrosomes, indicative of cells progressing through the S phase of the cell cycle.

CONCLUSION

Adult articular chondrocytes retain the ability to repopulate the matrix, an ability which is demonstrated with this specific cryoinjury. This may be an initial stage of cartilage regeneration.

Morphological and histochemical ageing changes in patellar articular cartilage of the rat

The aim of this study was to investigate the variation in cartilage characteristics with age. Fresh-frozen cryostat sections of the patellar articular cartilage of the rat were used to demonstrate the enzyme activity of succinate dehydrogenase, lactate dehydrogenase, alkaline phosphatase, and acid phosphatase in the different layers and at different ages. Light microscopic techniques were used to analyse quantitative features such as thickness, cell density and the histological characteristics of the articular cartilage. The results indicate that cell density is significantly affected by age. Furthermore, it depends on the distance from the surface. The most marked decline in cell density occurred between months 3 and 6. The thickness of the articular cartilage also varies with age. The reduction in cartilage thickness was most striking between months 3 and 6. Differentiation into the histological layers is obvious after 3 months. Glycolytic enzymes were strongly reactive in all regions and at all ages, whereas aerobic activity declines with age. The metabolic and morphological changes in ageing cartilage contribute to trophic disorders and deterioration of the functional cartilaginous situation in adult cartilage.

Ultrastructural studies of the epiphyseal plate of chicks fed a vitamin D-deficient and low-calcium diet

The epiphyseal plates of rachitic chicks fed a vitamin D-deficient and low-calcium diet were examined ultrastructurally at 4, 7, 14, 18 and 21 days old. On and after 14 days, changes were observed. Chondrocytes in resting, proliferating and maturing zones commonly showed a reduction in cell organelles, suggestive of a decrease in synthetic activity of matrix components. In addition, the resting zone cells had numerous intracytoplasmic microfilaments. Mitotic figures were present but not increased in number in the proliferating zone. Autolysosome-like dense bodies in the chondrocytes and clusters of degenerative and necrotic chondrocytes were observed in the proliferating and maturing zones. In the calcifying zone of the rachitic epiphyseal plate, initial calcification was observed characterized by the deposition of apatite crystals in matrix vesicles and the formation of spherical crystal clusters. The crystal clusters were separated from each other by collagen fibrils on which only small amounts of apatite crystals had been deposited. In this zone, hypertrophic chondrocytes responsible for the initial calcification were seen, but not the stellate chondrocytes responsible for the progression of matrix calcification after its initiation. These findings suggest that accumulation of the proliferating and maturing zone cells as a characteristic lesion of the epiphyseal plate in rachitic chicks is caused by a disturbance of cell maturation. Such disturbance in the calcifying zone may explain the progression of the defect in matrix calcification.

Age-related changes to the surface ultrastructure of the rabbit temporomandibular disc

The ultrastructural characteristics of the temporomandibular disc were investigated by transmission electron microscopy in rabbits, aged from 7 d to 6 y. The disc is interposed between the head of the mandibular condyle below, and the squamous temporal bone above, within the temporomandibular joint. A single layer of flattened mesenchymal cells covered the articular surface of the disc during early postnatal development. Gradual disappearance of this cell layer was succeeded by the development of an articular surface lamina. The first appearance of the lamina coincided with the transition from suckling to adult masticatory function. With advancing age, the lamina increased in thickness from 0.3 to 1.0 micron in adulthood, and was well demarcated from the subsurface tissue. The lamina was a continuous, smooth, electron-dense layer, that reacted biochemically as proteoglycan complex. The surface lamina may be responsible for the low frictional properties associated with articular movement and load, and the selective passage of molecules bidirectionally across the cartilage-fluid interface. Variations in lamina thickness occurred topographically across the disc and were attributed to site concentration of shearing stress. Beneath the articular surface lamina were cells surrounded by collagenous intercellular stroma. Chondrocytic, fibrocytic and fibrochondrocytic cellular phenotypes were present. Subsurface cellular debris accumulated within the rabbit disc from the commencement of mastication. Mature adult disc contained expanses of debris, attributed to cell death in situ. Foci of mineral precipitates were found within ageing discal tissue.

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.

The ultrastructure of articular cartilage of the chicken's knee joint

The articular cartilage and synovial membrane of immature and mature chicken knee joints were studied by light, scanning and transmission microscopy. The findings differed from human articular cartilage and we conclude that the chicken knee joint is not suitable as a model for human joint degeneration.

Activation of collagen type II expression in osteoarthritic and rheumatoid cartilage

In situ hybridization and immunohistochemical techniques were applied to investigate gene expression and extracellular deposition of collagen type II in normal, osteoarthritic and rheumatoid human articular cartilage. Normal cartilage showed an essentially even extracellular distribution of type II collagen with poly- and monoclonal antibodies, while only a few cells were positive for alpha 1(II) collagen mRNA. In situ hybridization of osteoarthritic and rheumatoid cartilage, however, showed strong enhancement of type II collagen gene expression; transcripts were observed predominantly in the upper middle zone of the articular cartilage while the upper layer was mostly negative and correlated with a zone of reduced proteoglycan staining. The elevated mRNA levels frequently coincided with pericellular immunostaining for type II collagen, indicative for enhanced synthesis of the protein. In two samples, however, pericellular loss of collagen type II staining was found despite positive cytoplasmic signals with the alpha 1(II) RNA probe, suggesting enhanced collagen destruction. Control hybridization with a probe for 18S rRNA revealed very few negative cells throughout both normal and arthritic cartilage samples, ruling out major cell necrosis in the specimens investigated. Thus, our observations identify sites of activated type II collagen synthesis in osteoarthritic cartilage that were predicted by previous biochemical studies and support the notion that damaged cartilage attempts to restore matrix by enhanced synthesis of its components.

The surface lamina of the articular cartilage of human zygapophyseal joints

Literature referring to the conflicting results of investigations into the possible existence and composition of the lamina splendens is reviewed. Two hundred micrometer thick histological sections from 80 human cadaveric lower lumbar zygapophyseal joint articular cartilages were examined by ordinary light and darkfield microscopy. The findings illustrate what appears to be an acellular surface lamina on the opposing cartilaginous surfaces. No speculation is made regarding the possible physiological significance of the lamina based on this anatomical study.

Chondrocyte distribution in the articular cartilage of human femoral condyles

The distribution of chondrocytes throughout the total thickness of articular cartilage from the femoral condyles of infants, children and adults has been studied using serial sections cut parallel as well as perpendicular to the articular surface. The thickness of the articular cartilage was estimated in fixed sections. In one of the adult specimens, the thickness of the articular cartilage was estimated firstly by direct measurement of the cut surfaces of a series of blocks cut from both condyles and then from the number of parallel sections of the cartilage prepared from those blocks. Cell density was highest in the superficial zone of all specimens examined, declining to lower values in the deep zone of the cartilage. Within this pattern the infant specimens had the highest values for cell density and the adults the lowest, with values for children in an intermediate range. There was no significant variation in cell density across the condyles of the selected adult specimen. The absolute values for cartilage thickness depended on the method used, but in general total thickness was found to approximately double from late gestation to maturity. In the selected adult specimen, the cartilage was thickest just anterior and posterior to the main weight-bearing area of the condyles.

Early histopathologic and ultrastructural alterations in femorotibial joints of partial medial meniscectomized guinea pigs

The articular cartilage from femorotibial joints of partial medial meniscectomized male guinea pigs was evaluated at 24, 48, 72, and 96 hours post-surgery to determine the sequential histopathologic and ultrastructural alterations. At 24 hours post-surgery, histopathologic alterations were in the superficial and middle layers and consisted of degeneration and necrosis of chondrocytes and minimal decreased intensity of toluidine blue matrix staining. Changes in chondrocytes and matrix became progressively more extensive 48 hours after surgery. Ultrastructurally, the changes in the superficial matrix appeared to be the result of loss of the fine granular material interspersed between collagen fibers. At 72 and 96 hours post-surgery, chondrocyte loss was extensive and surface fibrillation was seen. These findings suggested that chondrocyte death was the initial important event which led to progressive severe cartilage degeneration in this model.

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.

Effect of postnatal age on the ultrastructure of six anatomical areas of canine flexor digitorum profundus tendon

We report findings of a transmission electron microscopic study comparing the morphological appearance of cells and extracellular matrix of two fibrocartilaginous regions of canine flexor digitorum profundus (FDP) tendon with that for typical tendinous regions. In addition, we determined the size distribution of collagen fibrils in six anatomical areas of the tendon from animals of three different ages. Average collagen fibril diameters for each of the six anatomical sites of 11-week-old FDP tendon were consistently different from that for older tissue. As growth proceeds, fibrils in tendinous regions almost double in size and take on a broad bimodal distribution. Collagen diameters in fibrocartilaginous areas do not increase, but rather decrease in size with age. Finally, the cells and associated pericellular matrix of fibrocartilaginous areas of adolescent and mature FDP tendon are ultrastructurally distinct from those of typical tendinous regions. On the contrary, the cellular morphology of 11-week-old tendon was invariant regardless of the anatomical region examined. In summary, fibrocartilage of canine FDP tendon, although not evident at 11 weeks of age, is well established by 6-12 months after birth and is the result of cellular and extracellular matrix specialization.

Ultrastructure of the articular cartilage of the mandibular condyle: aging and degeneration

To obtain more insight into the pathogenesis of osteoarthrosis of the temporomandibular joint, we examined the ultrastructure of articular cartilage of six healthy and sixteen osteoarthrotic human mandibular condyles. Ultrastructural changes due to aging and osteoarthrosis are described and compared with the findings of other ultrastructural studies of articular cartilage of synovial joints. Aging was accompanied by some slight degenerative signs. Osteoarthrotic hyaline cartilage and fibrocartilage showed a striking similarity. The only ultrastructural difference was the presence of elastic fibers in the latter. Therefore, both seem to have the same pathogenesis. Several current statements on the pathogenesis of osteoarthrosis are discussed.

The effects of immobilization on the rabbit temporomandibular joint

While the effects of immobilization of joints covered with hyaline cartilage have been widely studied, the effects on the fibrous tissue-covered temporomandibular joint have not been studied as extensively. This study was designed to determine the short-term effects of immobilization on the rabbit temporomandibular joint. Nineteen rabbits were placed in maxillomandibular fixation. The temporomandibular joints were studied histologically after periods of from ten to 28 days. Significant thinning was observed as early as after ten days, as was degeneration of the cartilage. Degeneration became progressively more severe as the duration of the immobilization increased. Reparative events began appearing after 28 days. These findings suggest that although initially immobilization produces destructive changes, the changes may well be reversible.

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.

Human osteogenic sarcoma: fine structure of the chondroblastic type

The fine structure of representative regions of four chondroblastic osteogenic sarcomas was studied. These regions contained four morphologically distinguishable subtypes of chondroblastlike cells. In addition, multinucleated giant cells, fibroblastlike cells, and macrophagelike cells were present, along with small populations of unclassifiable cells forming at least two subgroups of cells likely to be of a neoplastic nature. With only one exception, all types of chondroblastlike cells were separated by wide zones of extracellular matrix. The large multinucleated cells showed a fine structure that differed from that seen in multinucleated giant cells of other tissues. The evidence suggested that the multinucleated cells in the chondroblastic osteogenic sarcomas were active in phagocytic functions. It is not clear whether or not they are neoplastic in nature. Osteoblastlike cells were not encountered in the chondroid areas of the osteogenic sarcomas studied. On the basis of the findings it is concluded that the observed fine structural polymorphism of the chondroblastlike cells may reflect differences in maturation and differentiation among these cells. The most well-differentiated cells (type 1) appear to be able to exert secretory functions.

The effects of immobilization on the primate temporomandibular joint: a histologic and histochemical study

The effects of prolonged immobilization on the primate temporomandibular joint (TMJ) condylar cartilage are reported. After eight weeks of immobilization, changes were assayed histochemically using the specific glycosaminoglycan (GAG) stain safranin O. Following the immobilization period, the articulating cartilage showed marked thinning, disorganization, and loss of the superficial GAG stain. After two weeks of remobilization, further thinning had occurred and GAG staining was nearly absent. Sections from condyles of animals sacrificed serially from two weeks to eight months after immobilization showed a gradual return to the thickness and GAG content of the control animals' condyles. But even at this stage, GAG staining was less marked than in the controls, and zonal reorganization was incomplete. These results suggest that TMJ immobilization is not wholly benign procedure.

Contribution to the knowledge of the fine structure of chondrosarcoma of bone. With a note on the localization of alkaline phosphatase and "ATPase"

Seven well differentiated chondrosarcomas of bone have been analyzed by electron microscopy, and the fine structural localization of adenosine triphosphatase and nonspecific alkaline phosphatase has been elucidated. On the basis of the fine structural appearance, two distinct cell types were shown to constitute the tumor tissue: chondrocyte-like cells and large "mitochondria-rich cells". Large, multinucleated cells in the tumor did not seem to correspond to osteoclasts but rather were likely to represent true neoplastic cells. Some chondrocyte-like cells appeared to be binucleated by virtue of deep, groove-like nuclear indentations. Adenosine triphosphatase and alkaline phosphatase were associated with the plasma membrane of both chondrocyte-like and mitochondria-rich cells suggesting that they might be of common origin. Normal chondroblasts and chondrocytes lack histochemically demonstrable adenosine triphosphatase on their plasma membrane. Presence of this enzyme in the tumor cells may indicate that they are histogenetically related to immature non-chondroid matrix forming cells (known to carry the enzymes).

The fine structure of proliferating cartilage cells: structural changes in an experimental model

Extensive development of the rough endoplasmic reticulum and comparatively less developed Golgi membranes were seen in the cells of chondrocyte clusters which had formed in the vicinity of cartilage defects produced by intra-articular papain and cortisone injections. These cells frequently showed myelin-like membranous figures. These figures were seen either close to the cell membrane, or between cells, or connected to the nuclear envelope or the endoplasmic reticulum. The hypothesis is advanced that the myelin figures are made from redundant material which was not used for membrane formation. Cortisone, which is known to interfere with mitochondrial function and general metabolic processes, could have been responsible for the deficiency in membrane differentiation and transfer. This investigation was supported by grants from the Orthopaedic Research and Education Foundation and from the Research Evaluation and Allocation Committee of the School of Medicine, Univeristy of California, San Francisco.

Malignant mixed osteogenic tumours of the breast. An ultrastructural study of two cases

Two cases of malignant mesodermic tumours of the human mammary gland with osteogenic and chondrogenic structures were analysed by optical and electronic microscopical means. One of them was associated with an adenocarcinoma of the gland. The histological pattern was similar to that of those cases previously examined under the optical microscope in the mammary gland and in extraskeletal osteogenic sarcomas of soft tissues. When investigated under the electron microscope, the chondroblasts possessed a highly developed RER in active synthesis with an amorphous material which contributed to the building up of the ground substance matrix of the tumorous cartilage. Osteoid fields with scattered osteoblasts appear throughout the tumorous stroma and were associated with calcium deposits. They were continuous with fibroblasts and mesenchymal undifferentiated cells of a very immature character. Giant cells of osteoclastic type were included within the mononucleated mesenchymal cells mimicking bone osteoclastoma. The presence of all these cell types suggests the existence of a common malignant origin, the stem cell being differentiated into epithelial carcinomatous and mesenchymal sarcomatous chondral and osteogenic tissues.

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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