Visualization of living terminal hypertrophic chondrocytes of growth plate cartilage in situ by differential interference contrast microscopy and time-lapse cinematography

A Second Career for Chondrocytes-Transformation into Osteoblasts

PURPOSE OF REVIEW

The goal of the review is to summarize the current knowledge on the process of chondrocyte-to-osteoblast transdifferentiation during endochondral bone formation and its potential implications in fracture healing and disease.

RECENT FINDINGS

Lineage tracing experiments confirmed the transdifferentiation of chondrocytes into osteoblasts. More recent studies lead to the discovery of molecules involved in this process, as well as to the hypothesis that these cells may re-enter a stem cell-like phase prior to their osteoblastic differentiation. This review recapitulates the current knowledge regarding chondrocyte transdifferentiating into osteoblasts, the developmental and postnatal events where transdifferentiation appears to be relevant, and the molecules implicated in this process.

Therapeutic efficacy of intra-articular hyaluronan derivative and platelet-rich plasma in mice following axial tibial loading

Objective

To investigate the therapeutic potential of intra-articular hyaluronan-derivative HYADD^® 4-G and/or platelet-rich plasma (PRP) in a mouse model of non-invasive joint injury.

Methods

Non-invasive axial tibial loading was used to induce joint injury in 10-week-old C57BL/6J mice (n = 86). Mice underwent a single loading of either 6 Newton (N) or 9N axial tibial compression. HYADD^® 4-G was injected intra-articularly at 8 mg/mL or 15 mg/mL either before or after loading with or without PRP. Phosphate-buffered-saline was injected as control. Knee joints were harvested at 5 or 56 days post-loading and prepared for micro-computed tomography scanning and subsequently processed for histology. Immunostaining was performed for aggrecan to monitor its distribution, for CD44 to monitor chondrocyte reactive changes and for COMP (cartilage oligomeric matrix protein) as an index for cartilage matrix changes related to loading and cartilage injury. TUNEL assay was performed to identify chondrocyte apoptosis.

Results

Loading initiated cartilage proteoglycan loss and chondrocyte apoptosis within 5 days with slowly progressive post-traumatic osteoarthritis (no cartilage degeneration, but increased synovitis and ectopic calcification after 9N loading) at 56 days. Mice treated with repeated HYADD^® 4-G (15 mg/mL) or HYADD^® 4-G (8 mg/mL) ± PRP or PRP alone exhibited no significant improvement in the short-term (5 days) and long-term (56 days) consequences of joint loading except for a trend for improved bone changes compared to non-loaded joints.

Conclusion

While we failed to show an overall effect of intra-articular delivery of hyaluronan-derivative and/or PRP in reversing/protecting the pathological events in cartilage and synovium following joint injury, some bone alterations were relatively less severe with hyaluronan-derivative at higher concentration or in association with PRP.

Post-Traumatic Osteoarthritis in Mice Following Mechanical Injury to the Synovial Joint

We investigated the spectrum of lesions characteristic of post-traumatic osteoarthritis (PTOA) across the knee joint in response to mechanical injury. We hypothesized that alteration in knee joint stability in mice reproduces molecular and structural features of PTOA that would suggest potential therapeutic targets in humans. The right knees of eight-week old male mice from two recombinant inbred lines (LGXSM-6 and LGXSM-33) were subjected to axial tibial compression. Three separate loading magnitudes were applied: 6N, 9N, and 12N. Left knees served as non-loaded controls. Mice were sacrificed at 5, 9, 14, 28, and 56 days post-loading and whole knee joint changes were assessed by histology, immunostaining, micro-CT, and magnetic resonance imaging. We observed that tibial compression disrupted joint stability by rupturing the anterior cruciate ligament (except for 6N) and instigated a cascade of temporal and topographical features of PTOA. These features included cartilage extracellular matrix loss without proteoglycan replacement, chondrocyte apoptosis at day 5, synovitis present at day 14, osteophytes, ectopic calcification, and meniscus pathology. These findings provide a plausible model and a whole-joint approach for how joint injury in humans leads to PTOA. Chondrocyte apoptosis, synovitis, and ectopic calcification appear to be targets for potential therapeutic intervention.

Early changes in the knee of healer and non-healer mice following non-invasive mechanical injury

In this study, we examined early time-dependent changes in articular cartilage and synovium in response to tibial compression and sought the plausible origin of cells that respond to compression in the healer (LGXSM-6) and non-healer (LGXSM-33) recombinant inbred mouse strains. The right knee of 13-week old male mice was subjected to tibial compression using 9N axial loading. The contralateral left knee served as a control. Knees were harvested at 5, 9, and 14 days post-injury. Histological changes in cartilage and synovium, immunofluorescence pattern of CD44, aggrecan, type-II collagen, cartilage oligomeric matrix protein and the aggrecan neo-epitope NITEGE, and cell apoptosis (by TUNEL) were examined. We used a double nucleoside analog cell-labeling strategy to trace cells responsive to injury. We showed that tibial compression resulted in rupture of anterior cruciate ligament, cartilage matrix loss and chondrocyte apoptosis at the injury site. LGXSM-33 showed higher synovitis and ectopic synovial chondrogenesis than LGXSM-6 with no differences for articular cartilage lesions. With loading, an altered pattern of CD44 and NITEGE was observed: cells in the impacted area underwent apoptosis, cells closely surrounding the injured area expressed CD44, and cells in the intact area expressed NITEGE. Cells responding to injury were found in the synovium, subchondral bone marrow and the Groove of Ranvier. Taken together, we found no strain differences in chondrocytes in the early response to injury. However, the synovial response was greater in LGXSM-33 indicating that, at early time points, there is a genetic difference in synovial cell reaction to injury. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:524-536, 2017.

Fate of growth plate hypertrophic chondrocytes: death or lineage extension?

The vertebrate growth plate is an essential tissue that mediates and controls bone growth. It forms through a multistep differentiation process in which chondrocytes differentiate, proliferate, stop dividing and undergo hypertrophy, which entails a 20-fold increase in size. Hypertrophic chondrocytes are specialized cells considered to be the end state of the chondrocyte differentiation pathway, and are essential for bone growth. They are characterized by expression of type X collagen encoded by the Col10a1 gene, and synthesis of a calcified cartilage matrix. Whether hypertrophy marks a transition preceding osteogenesis, or it is the terminal differentiation stage of chondrocytes with cell death as the ultimate fate has been the subject of debate for over a century. In this review, we revisit this debate in the light of new findings arising from genetic-mediated lineage tracing studies showing that hypertrophic chondrocytes can survive at the chondro-osseous junction and further make the transition to become osteoblasts and osteocytes. The contribution of chondrocytes to the osteoblast lineage has important implications in bone development, disease and repair.

Automated Cell Detection and Morphometry on Growth Plate Images of Mouse Bone

Microscopy imaging of mouse growth plates is extensively used in biology to understand the effect of specific molecules on various stages of normal bone development and on bone disease. Until now, such image analysis has been conducted by manual detection. In fact, when existing automated detection techniques were applied, morphological variations across the growth plate and heterogeneity of image background color, including the faint presence of cells (chondrocytes) located deeper in tissue away from the image's plane of focus, and lack of cell-specific features, interfered with identification of cell. We propose the first method of automated detection and morphometry applicable to images of cells in the growth plate of long bone. Through ad hoc sequential application of the Retinex method, anisotropic diffusion and thresholding, our new cell detection algorithm (CDA) addresses these challenges on bright-field microscopy images of mouse growth plates. Five parameters, chosen by the user in respect of image characteristics, regulate our CDA. Our results demonstrate effectiveness of the proposed numerical method relative to manual methods. Our CDA confirms previously established results regarding chondrocytes' number, area, orientation, height and shape of normal growth plates. Our CDA also confirms differences previously found between the genetic mutated mouse Smad1/5^CKO and its control mouse on fluorescence images. The CDA aims to aid biomedical research by increasing efficiency and consistency of data collection regarding arrangement and characteristics of chondrocytes. Our results suggest that automated extraction of data from microscopy imaging of growth plates can assist in unlocking information on normal and pathological development, key to the underlying biological mechanisms of bone growth.

Hypertrophic chondrocytes can become osteoblasts and osteocytes in endochondral bone formation

According to current dogma, chondrocytes and osteoblasts are considered independent lineages derived from a common osteochondroprogenitor. In endochondral bone formation, chondrocytes undergo a series of differentiation steps to form the growth plate, and it generally is accepted that death is the ultimate fate of terminally differentiated hypertrophic chondrocytes (HCs). Osteoblasts, accompanying vascular invasion, lay down endochondral bone to replace cartilage. However, whether an HC can become an osteoblast and contribute to the full osteogenic lineage has been the subject of a century-long debate. Here we use a cell-specific tamoxifen-inducible genetic recombination approach to track the fate of murine HCs and show that they can survive the cartilage-to-bone transition and become osteogenic cells in fetal and postnatal endochondral bones and persist into adulthood. This discovery of a chondrocyte-to-osteoblast lineage continuum revises concepts of the ontogeny of osteoblasts, with implications for the control of bone homeostasis and the interpretation of the underlying pathological bases of bone disorders.

Defective endochondral ossification-derived matrix and bone cells alter the lymphopoietic niche in collagen X mouse models

Despite the appreciated interdependence of skeletal and hematopoietic development, the cell and matrix components of the hematopoietic niche remain to be fully defined. Utilizing mice with disrupted function of collagen X (ColX), a major hypertrophic cartilage matrix protein associated with endochondral ossification, our data identified a cytokine defect in trabecular bone cells at the chondro-osseous hematopoietic niche as a cause for aberrant B lymphopoiesis in these mice. Specifically, analysis of ColX transgenic and null mouse chondro-osseous regions via micro-computed tomography revealed an altered trabecular bone environment. Additionally, cocultures with hematopoietic and chondro-osseous cell types highlighted impaired hematopoietic support by ColX transgenic and null mouse derived trabecular bone cells. Further, cytokine arrays with conditioned media from the trabecular osteoblast cocultures suggested an aberrant hematopoietic cytokine milieu within the chondro-osseous niche of the ColX deficient mice. Accordingly, B lymphopoiesis was rescued in the ColX mouse derived trabecular osteoblast cocultures with interlukin-7, stem cell factor, and stromal derived factor-1 supplementation. Moreover, B cell development was restored in vivo after injections of interlukin-7. These data support our hypothesis that endrochondrally-derived trabecular bone cells and matrix constituents provide cytokine-rich niches for hematopoiesis. Furthermore, this study contributes to the emerging concept that niche defects may underlie certain immuno-osseous and hematopoietic disorders.

Transient exposure to TGF-β3 improves the functional chondrogenesis of MSC-laden hyaluronic acid hydrogels

Tissue engineering with adult stem cells is a promising approach for the restoration of focal defects in articular cartilage. For this, progenitor cells would ideally be delivered to (and maintained within) the defect site via a biocompatible material and in combination with soluble factors to promote initial cell differentiation and subsequent tissue maturation in vivo. While growth factor delivery methods are continually being optimized, most offer only a short (days to weeks) delivery profile at high doses. To address this issue, we investigated mesenchymal stem cell (MSC) differentiation and maturation in photocrosslinkable hyaluronic acid (HA) hydrogels with transient exposure to the pro-chondrogenic molecule transforming growth factor-beta3 (TGF-β3), at varying doses (10, 50 and 100 ng/mL) and durations (3, 7, 21 and 63 days). Mechanical, biochemical, and histological outcomes were evaluated through 9 weeks of culture. Results showed that a brief exposure (7 days) to a very high level (100 ng/mL) of TGF-β3 was sufficient to both induce and maintain cartilage formation in these 3D constructs. Indeed, this short delivery resulted in constructs with mechanical and biochemical properties that exceeded that of continuous exposure to a lower level (10 ng/mL) of TGF-β3 over the entire 9-week time course. Of important note, the total TGF delivery in these two scenarios was roughly equivalent (200 vs. 180 ng), but the timing of delivery differed markedly. These data support the idea that acute exposure to a high dose of TGF will induce functional and long-term differentiation of stem cell populations, and further our efforts to improve cartilage repair in vivo.

Evidence for lysosomal exocytosis and release of aggrecan-degrading hydrolases from hypertrophic chondrocytes, in vitro and in vivo

The abundant proteoglycan, aggrecan, is resorbed from growth plate cartilage during endochondral bone ossification, yet mice with genetically-ablated aggrecan-degrading activity have no defects in bone formation. To account for this apparent anomaly, we propose that lysosomal hydrolases degrade extracellular, hyaluronan-bound aggrecan aggregates in growth plate cartilage, and that lysosomal hydrolases are released from hypertrophic chondrocytes into growth plate cartilage via Ca²⁺-dependent lysosomal exocytosis. In this study we confirm that hypertrophic chondrocytes release hydrolases via lysosomal exocytosis in vitro and we show in vivo evidence for lysosomal exocytosis in hypertrophic chondrocytes during skeletal development. We show that lysosome-associated membrane protein 1 (LAMP1) is detected at the cell surface following in vitro treatment of epiphyseal chondrocytes with the calcium ionophore, ionomycin. Furthermore, we show that in addition to the lysosomal exocytosis markers, cathepsin D and β-hexosaminidase, ionomycin induces release of aggrecan- and hyaluronan-degrading activity from cultured epiphyseal chondrocytes. We identify VAMP-8 and VAMP7 as v-SNARE proteins with potential roles in lysosomal exocytosis in hypertrophic chondrocytes, based on their colocalisation with LAMP1 at the cell surface in secondary ossification centers in mouse tibiae. We propose that resorbing growth plate cartilage involves release of destructive hydrolases from hypertrophic chondrocytes, via lysosomal exocytosis.

Interaction of human mesenchymal stem cells with osteopontin coated hydroxyapatite surfaces

In vitro studies of the initial attachment, spreading and motility of human bone mesenchymal stem cells have been carried out on bovine osteopontin (OPN) coated hydroxyapatite (HA) and gold (Au) model surfaces. The adsorption of OPN extracted from bovine milk was monitored by the quartz crystal microbalance with dissipation (QCM-D) and the ellipsometry techniques, and the OPN coated surfaces were further investigated by antigen-antibody interaction. It is shown that the OPN surface mass density is significantly lower and that the number of antibodies binding to the resulting OPN layers is significantly higher on the HA as compared to the Au surfaces. The initial attachment, spreading and motility of human mesenchymal stem cells show a larger cell area, a faster arrangement of vinculin in the basal cell membrane and more motile cells on the OPN coated HA surfaces as compared to the OPN coated Au surfaces and to the uncoated Au and HA surfaces. These in vitro results indicate that there may be great potential for OPN coated biomaterials, for instance as functional protein coatings or drug delivery systems on orthopaedic implants or scaffolds for tissue-engineering.

Analysis of the orientation of primary cilia in growth plate cartilage: a mathematical method based on multiphoton microscopical images

The chondrocytic primary cilium has been hypothesized to act as a mechano-sensor, analogously to primary cilium of cells in epithelial tissues. We hypothesize that mechanical inputs during growth, sensed through the primary cilium, result in directed secretion of the extracellular matrix, thereby establishing tissue anisotropy in growth plate cartilage. The cilium, through its orientation in three-dimensional space, is hypothesized to transmit to the chondrocyte the preferential direction for matrix secretion. This paper reports on the application of classical mathematical methods to develop an algorithm that addresses the particular challenges relative to the assessment of the orientation of the primary cilium in growth plate cartilage, based on image analysis of optical sections visualized by multiphoton microscopy. Specimens are prepared by rapid cold precipitation-based fixation to minimize possible artifactual post-mortem alterations of ciliary orientation. The ciliary axoneme is localized by immunocytochemistry with antibody acetylated-alpha-tubulin. The method is applicable to investigation of ciliary orientation in different zones of the growth plate, under either normal or altered biomechanical environments. The methodology is highly flexible and adaptable to other connective tissues where tissue anisotropy and directed secretion of extracellular matrix components are hypothesized to depend on the tissue's biomechanical environment during development and growth.

Metabolic consideration of epiphyseal growth: survival responses in a taxing environment

The goal of this review is to examine some of the metabolic features of the maturing chondrocyte within the epiphyseal growth plate. The energy status of the tissue is examined in light of the energy needs of the tissue and the availability of oxygen. The role of HIF, PHDs and other proteins concerned with transduction of the oxemic response is considered and related to chondrocyte survival in a complex extracellular matrix.

Nonlinear optical microscopy of articular cartilage

OBJECTIVE

To assess the ability of nonlinear optical microscopy (NLOM) to image ex vivo healthy and degenerative bovine articular cartilage.

METHOD

Fresh bovine femoral-tibial joints were obtained from an abattoir. Articular cartilage specimens were harvested from the tibial plateau. Normal and degenerative specimens were imaged by NLOM and subsequently fixed and processed for histological examination.

RESULTS

NLOM provided high resolution images of articular cartilage at varying depths with high sensitivity to tissue morphology and high specificity to tissue components without fixing, sectioning or staining. Spectroscopic segmentation of nonlinear optical signals isolated the collagen matrix from the chondron (chondrocyte and non-collagen pericellular matrix). Images from the superficial zone were consistent with the presence of a matrix composed of both elastin-like and collagen fibers distributed in a depth-dependent morphological arrangement, whereas only collagen was demonstrated in the middle and deep zones. Alterations of collagen matrix associated with advanced degenerative joint disease (fibrocartilage) were observed with NLOM. Individual chondrocytes were imaged and demonstrated intracellular fluorescence consistent with the presence of products of intracellular biochemical processes.

CONCLUSION

Thin images of living articular cartilage using NLOM may be obtained with (sub-)cellular resolution at varying depths without fixing, sectioning or staining. Extracellular matrical collagen and chondron may be imaged separately in native tissue using spectrally distinct, endogenous, nonlinear optical signals. NLOM was sensitive to macromolecular composition and pathologic changes in articular cartilage matrix. Advances in instrumentation may lead to the application of NLOM to study articular cartilage in vivo.

Apoptosis of terminal hypertrophic chondrocytes in an in vitro model of endochondral ossification

It is widely accepted that growth plate chondrocytes undergo apoptosis when they reach the terminal hypertrophic stage of their differentiation during the process of endochondral ossification in vivo. In this report, an established chondrocyte cell culture model of mammalian endochondral ossification was utilized to investigate the fate of chondrocytes after they had entered hypertrophy in vitro. Fetal bovine epiphyseal chondrocytes were treated with the demethylating agent, 5-azacytidine, for 48 h and then cultured under azacytidine-depleted conditions. There was evidence for apoptosis in azacytidine-treated cells, as demonstrated by nuclear condensation and fragmentation (days 27 and 35) using transmission electron microscopy, and the detection of exposed phosphatidylserine on the plasma membrane surface of apoptotic chondrocytes (day 27) using fluorescence-labelled annexin V. Treated cultures on days 10 and 20 and untreated cultures at all corresponding time-points showed no morphological characteristics of apoptosis. In situ hybridization studies of treated cultures revealed that expression of the apoptotic suppressor, bcl-2, remained consistently high throughout the culture period, whilst the apoptotic inducer, bax, was not expressed until day 23. Quantification of these data showed a gradual shift in the ratio of the expression level of bcl-2 and bax in favour of bax with time in culture, particularly from day 23 onwards. Taken together, the results indicate that azacytidine-treated epiphyseal chondrocytes entered terminal hypertrophy from day 23 onwards in culture and died by apoptosis. This study confirms this culture system as a successful recapitulation of the entire mammalian chondrocyte differentiation pathway, including apoptosis. The culture model will prove valuable for studies of the apoptotic fate of terminally differentiated chondrocytes in the growth plate with a view to providing a better understanding of the underlying mechanisms of skeletal malformations and other pathological disorders such as osteoarthritis.

Developmental expression patterns of Beta-ig (betaIG-H3) and its function as a cell adhesion protein

Beta-ig is a secretory protein embodied by fasciclin I-like repeats containing sequences that might bind integrins and glycosaminoglycans in vivo. Expression of Beta-ig is responsive to Transforming Growth Factor-beta and the protein is found to be associated with extracellular matrix (ECM) molecules, implicating Beta-ig as an ECM adhesive protein of developmental processes. The spatiotemporal distribution of Beta-ig during various stages of murine development was examined and its ability to support adhesion of various cell types assessed. In situ hybridization of mouse embryos (E12.5-E18.5) indicated a prominent, distinct expression pattern for Beta-ig message in connective tissue. Beta-ig transcripts were abundantly expressed during mesenchymal cell condensation in areas of axial, craniofacial and appendicular primordial cartilage from E12.5-E14.5. Beginning at E15.5, Beta-ig transcripts appeared in collagen-rich tissues, including dura mater and corneal stroma. During E16.5-E18.5, Beta-ig transcripts were observed in proliferating chondrocytes and areas of endochondral ossification in joint and articular cartilage formation. Connective tissues expressed Beta-ig transcripts within the nasal septum and surrounding cartilage primordia, and in the pericardium, optic cup, kidney, ovary, esophagus, diaphragm, bronchi, trachea and corneal epithelium, and during cardiac valve formation. These patterns of expression indicate that Beta-ig may be involved in tissue morphogenesis. Cells derived from mesenchyme attached onto a substratum comprised of purified recombinant Beta-ig. Taken together, the results indicate that Beta-ig is expressed principally in collagen-rich tissues where it may interact with cells and ECM molecules, perhaps playing a role in tissue morphogenesis.

The fate of the terminally differentiated chondrocyte: evidence for microenvironmental regulation of chondrocyte apoptosis

Chondrocytes contained within the epiphyseal growth plate promote rapid bone growth. To achieve growth, cells activate a maturation program that results in an increase in chondrocyte number and volume and elaboration of a mineralized matrix; subsequently, the matrix is resorbed and the terminally differentiated cells are deleted from the bone. The major objective of this review is to examine the fate of the epiphyseal chondrocytes in the growing bone. Current studies strongly suggest that the terminally differentiated epiphyseal cells are deleted from the cartilage by apoptosis. Indeed, morphological, biochemical, and end-labeling techniques confirm that death is through the apoptotic pathway. Since the induction of apoptosis is spatially and temporally linked to the removal of the cartilage matrix, current studies have examined the apoptogenic activity of Ca(2+)-, Pi-, and RGD-containing peptides of extracellular matrix proteins. It is observed that all of these molecules are powerful apoptogens. With respect to the molecular mechanism of apoptosis, studies of cell death with Pi as an apoptogen indicate that the anion is transported into the cytosol via a Na(+/)Pi transporter. Subsequently, there is activation of caspases, generation of NO, and a decrease in the thiol reserve. Finally, we examine the notion that chondrocytes transdifferentiate into osteoblasts, and briefly review evidence for, and the rationale of, the transdifferentiation process. It is concluded that specific microenvironments exist in cartilage that can serve to direct chondrocyte apoptosis.

Volume increase in growth plate chondrocytes during hypertrophy: the contribution of organic osmolytes

During the differentiation cascade of growth plate chondrocytes, cells undergo as much as a 10-15-fold increase in volume. This volume increase, which occurs to different extents in growth plates growing at different rates, has been demonstrated to be the single most significant variable in understanding the quantitative aspects of the cellular kinetics of long bone growth. Our hypothesis is that this volume increase, which occurs through cell swelling by water imbibition, requires intracellular accumulation of osmolytes through activation or upregulation of membrane transport mechanisms. Significant intracellular accumulation of inorganic osmolytes, such as Na+, K+, and Cl-, is potentially disruptive to normal cellular metabolism, whereas intracellular accumulation of organic osmolytes is considered to be more compatible with metabolic function. Thus, we concentrated on determining the contributions of organic osmolytes--betaine, amino acids, inositol, and sorbitol--to volume increase. Pooled cryostat sections of young bovine growth plates were extracted followed by automated analysis for their content of amino acids. Analysis for betaine and the sugar alcohols was done by extraction and derivatization, followed by high-performance liquid chromatography (HPLC). Parallel stereological analyses correlated osmolyte changes to stages of chondrocytic differentiation, specifically comparing intracellular concentration and amount in proliferative vs. hypertrophic chondrocytes. Calculations demonstrated that, maximally, these organic osmolytes, in total, account for 6%-7% of the intracellular osmolytes required to sustain the volume increase, and that the most significant contribution is from betaine. This suggests that intracellular accumulation of organic osmolytes is not a primary strategy used by growth plate chondrocytes during volume increase of their terminal differentiation. The data also suggest that there is a differential regulation of transporters of these osmolytes such that intracellular concentrations are constantly modified as cells proceed through the differentiation cascade.

Apoptotic cell death is not a widespread phenomenon in normal aging and osteoarthritis human articular knee cartilage: a study of proliferation, programmed cell death (apoptosis), and viability of chondrocytes in normal and osteoarthritic human knee cartilage

OBJECTIVE

Chondrocytes are crucial for adequate matrix balance and function. Cell proliferation and, recently, extensive apoptotic cell death have been reported in osteoarthritic (OA) cartilage. Apoptotic cell death would be an obvious central factor in the initiation and progression of OA, since there is no potential for replacing articular chondrocytes in the adult. Therefore, we studied the occurrence of apoptotic cell disintegration and cell proliferation in OA and normal articular cartilage obtained from the knees of adult donors of all ages.

METHODS

Following immunostaining for cellular proteins as well as staining for nuclear DNA, we performed triple-channel confocal laser scanning microscopy on thick cartilage slices to evaluate lacunar emptying and cell viability. Cell proliferation and apoptotic cell death were evaluated morphologically, by immunodetection of the proliferation-associated Ki-67 antigen, and by the TUNEL reaction.

RESULTS

With the exception of the calcified layer, we were not able to detect any major (apoptotic or nonapoptotic) cell disintegration in normal young or aged articular knee cartilage. Single apoptotic cells were detected in OA articular knee cartilage. A significant increase in lacunar emptying was observed in late-stage specimens with higher Mankin scores compared with age-matched normal control cartilage specimens, but not in low-grade lesions. A significant (but lesser) increase in empty lacunae was also observed with age in normal cartilage. Cell proliferation was rarely detected in OA cartilage samples and was not detected at all in normal cartilage samples.

CONCLUSION

Our results confirm the findings of previous studies showing that cell proliferation occurs in OA cartilage. They also show that, contrary to previous suggestions, apoptotic cell death is not a widespread phenomenon in aging or OA cartilage.

Bcl-2 and Bax expression in cartilage and bone cells after high-dose corticosterone treatment in rats

The expression of Bcl-2 and Bax has been evaluated by immunohistochemistry in normal rats, and in rats after treatment with high-dose corticosterone (CORT). Proliferative (PC) and maturative/hypertrophic (MaHC) chondrocytes of the growth plate have been examined, as well as osteoblasts (Obs), osteocytes (Ots) and osteoclasts (Ocs) of the metaphyseal secondary spongiosa. For each cell type, the Bcl-2 and Bax immunopositive cells were expressed as a percentage of the total number of cells. Bcl-2 and Bax expression was considered to be enhanced when the percentage of positive cells rose. Bcl-2 and Bax were expressed in all cell types, and two main kinds of labeling distribution, both suggestive of association with intracellular organelles, were observed in the cytoplasm: scarce and spotty labeling (type 1) or abundant, granular and diffuse labeling (type 2). In some cases, nuclear membranes could also be seen to be positive. Positive PCs and Obs generally showed a labeling of type 1, MaHCs and Ocs of type 2, while Ots varied with labeling of type 1 or type 2. CORT administration induced a fall in the percentage of Bcl-2 immunopositive cells, and a rise in that of Bax immunopositive cells, in PCs and Ots. The same trend was observed in MaHCs, although the Bcl-2 decrease was not significant. The percentage of Bcl-2 and Bax immunopositive Obs rose, and their labeling distribution shifted from type 1- to type 2-labeled cells. Ocs showed the highest immunopositivity for both Bcl-2 and Bax, which did not change after CORT administration. These data suggest that CORT treatment, by lowering Bcl-2, and raising Bax expression, may promote the apoptotic process in PCs, MaHCs and Ots. Obs, however, do not undergo the same variations. This finding, together with the results of a previous study showing that CORT administration raises the frequency of apoptotic Obs, does not support a direct relationship between apoptosis and Bax overexpression, at least in Obs. The CORT effect might be related to cell types and their state of differentiation, so that Bcl-2 and Bax might regulate not only the machinery of cell death, but also cell proliferation and differentiation.

Abl expression, tumour grade, and apoptosis in chondrosarcoma

AIMS

To determine whether Abl immunoreactivity correlates with grade and cell kinetics (apoptosis and mitosis) in chondrosarcoma.

METHODS

Sections from 16 chondrosarcomas were stained immunohistochemically using a polyclonal antibody to the c-Abl/Bcr-Abl oncoprotein. Apoptotic indices and mitotic indices were assessed in all tumours. Sections from 24 paraffin wax blocks of human fetal rib (gestational ages, 15-42 weeks) were also stained to determine whether the Abl protein is synthesised consistently throughout endochondral ossification.

RESULTS

Abl staining in immature fetal rib chondrocytes at all stages of development was predominantly nuclear, and 70% of cells showed moderate to strong staining. Abl immunoreactivity was minimal or absent in hypertrophic chondrocytes about to undergo apoptosis at the growth plate. There was strong Abl staining in grade 1 and grade 2 chondrosarcomas but staining was greatly reduced or absent in grade 3 chondrosarcomas. There was a very significant linear correlation between apoptotic index (mean, 0.68%; range, 0-3.2%) and mitotic index (mean, 0.23%; range, 0-0.9%), and both indices were significantly lower in grade 1 than in grade 2 and grade 3 chondrosarcomas.

CONCLUSIONS

These data suggest that abl gene expression is associated with differentiation and apoptosis inhibition in fetal and neoplastic chondrocytes. However, these putative effects cannot be ascribed solely to the Abl protein, because several additional factors contribute to the regulation of both differentiation and apoptosis.

In vivo incidence of apoptosis evaluated with the TdT FragEL DNA fragmentation detection kit in cartilage and bone cells of the rat tibia

Previous studies have shown the occurrence of cell death by apoptosis in cartilage and bone cells, and have suggested a functional relationship between bone growth and remodelling on one hand, and numbers of apoptotic cells on the other. At present, no in vivo studies are available on the frequency of the apoptotic process measured at one time and in one place using the cartilage and bone cells of single specimens. The aim of the present investigation was to measure the in vivo incidence of apoptosis in cartilage and bone cells of the upper epiphysis and secondary ossification metaphyseal bone of the tibia in normal young adult rats. Apoptotic cells were visualized with the terminal deoxynucleotidyl transferase (TdT) FragEL DNA fragmentation detection kit, which is analogous to the TdT-mediated nick end-labelling (TUNEL) method. In the growth cartilage, only a few TUNEL-positive terminal hypertrophic chondrocytes were found; they were 1.32 +/- 0.70% of the total hypertrophic chondrocytes counted along the chondro-osseous junction. There were only a few apoptotic osteoblastic cells and osteocytes (0.22 +/- 0.22% and 0.15 +/- 0.16% of total osteoblasts and osteocytes respectively). TUNEL-positive osteoclasts were 1.03 +/- 0.57% of the total of osteoclastic cells; they usually showed only one or two apoptotic nuclei. The total number of TUNEL-positive bone marrow cells were also counted (56.78 +/- 10.29/mm2 of bone marrow spaces). Our results confirm that apoptosis does occur in hypertrophic chondrocytes and bone cells, and show that its frequency is very low. However, chiefly because of its short lifespan, the frequency of apoptosis in cartilage and bone may be higher than that shown by the TUNEL method. The static estimate that can be obtained with this method might lead to misleading conclusions on the physiological significance of such a dynamic, rapid and asynchronous process, whose precise importance in bone growth and remodelling remains to be determined.

Changes in cell, matrix compartment, and fibrillar collagen volumes between growth-plate zones

To define the contributions of changes in cell, matrix compartment, and fibrillar collagen volumes to longitudinal bone growth, we measured the differences in cell, pericellular/territorial matrix and interterritorial matrix volumes, and fibrillar collagen concentrations between the upper proliferative and lower hypertrophic zones of the proximal tibial physes of six miniature pigs. The mean numerical density of cells decreased from 110,000 cells/mm3 in the upper proliferative zone to 59,900 cells/mm3 in the lower hypertrophic zone. The mean cell volume increased nearly 5-fold (from 1,174 to 5,530 microm3), and the total matrix volume per cell increased 46% (from 8,040 to 11,760 microm3/cell) between the upper proliferative and lower hypertrophic zones. Both the pericellular/territorial matrix volume per cell and the interterritorial matrix volume per cell increased between the upper proliferative and lower hypertrophic zones; the pericellular/territorial matrix volume per cell increased 61% (from 4,580 to 7,390 microm3/cell), whereas the interterritorial matrix volume per cell increased 26% (from 3,460 to 4,370 microm3/cell). The total increase in mean cell volume of 4,356 microm3 exceeded the total increase in mean matrix volume per cell of 3,720 microm3; the total mean pericellular/territorial matrix volume per cell increased more than the total mean interterritorial matrix volume per cell (2,810 compared with 910 microm3/cell). Fibrillar collagen concentration was greater in the interterritorial matrix than in the pericellular/territorial matrix in both zones and increased in both matrix compartments between the upper proliferative and lower hypertrophic zones. The amount of fibrillar collagen per cell also increased in both matrix compartments between the upper proliferative and lower hypertrophic zones (from 1,720 to 3,100 microm3/cell in the pericellular/territorial matrix and from 1,490 to 2,230 microm3/cell in the interterritorial matrix; thus, the total amount of fibrillar collagen per cell increased from 3,210 to 5,530 microm3/cell). Growth rate was inversely related to the cell numerical density in the upper proliferative and lower hypertrophic zones and was directly related to interterritorial matrix volume per cell in the upper proliferative zone and to pericellular/territorial matrix volume per cell in the lower hypertrophic zone. These results show that cell enlargement contributes more to longitudinal bone growth than does increased matrix volume, that increased pericellular/territorial matrix volume makes a greater contribution to growth than does increased interterritorial matrix volume, and that the total amount of fibrillar collagen per cell increases between the upper proliferative and lower hypertrophic zones. The differences between the two matrix compartments in increase in volume, fibrillar collagen concentration, and amount of fibrillar collagen per cell strongly suggest that they differ not only in matrix organization but in rate of matrix accumulation and assembly and that these differences give the two compartments different roles in skeletal growth.

Odontoblast differentiation: a response to environmental calcium?

The response of the dental pulp to calcium hydroxide has been well described but the process of pulpal repair leading to dentinal bridge formation appears complex and the mechanisms remain incompletely understood. Through the precise regulation of the free calcium ion in the cytosol, cells have been able to utilize anions such as phosphates for a wide range of activities such as energy production (oxidative phosphorylation). As anions are abundant in the cytosol, intracellular levels of calcium ions are kept low, several orders of magnitude less than that of the surrounding extracellular matrix. Consequently, cells are able to use calcium ions for the regulation of many cellular events. The binding of extracellular molecules such as cytokines, hormones or antibodies, with receptors on the plasma membrane may result in short- or long-term modifications to cellular metabolism, including the mechanisms of intracellular calcium homeostasis. Cell survival depends upon the ability to adapt to changes in the cell's micro-environment. Adaptation in turn results in altered cellular activity that may be interpreted as showing that the cell has become more or less specialised. In some instances this may include the resumption of mitotic activity. If the rate or magnitude of change exceeds a cell's adaptive capacity, the cell dies. Responses of cells to alterations in their environment are reviewed as they may provide an explanation for the success of calcium hydroxide in facilitating pulpal repair and the differentiation of odontoblasts.

Pharmacological effect of tetracyclines on proteoglycanases from interleukin-1-treated articular cartilage

Based on previous in vivo and in situ studies showing that tetracyclines possess antidegenerative effects on cartilage in conjunction with a reduced proteoglycan (PG) loss from the extracellular matrix, we investigated the effects of doxycycline, minocycline and tetracycline on the degradation and biosynthesis of PGs by bovine articular cartilage explants, both in vitro and in situ. Doxycycline, minocycline and tetracycline dose dependently, although weakly, inhibited PG degrading matrix metalloproteinases (MMPs) in vitro, when tested at concentrations ranging from 1 to 100 microM. Ro 31-4724 proved to be a potent inhibitor of MMP proteoglycanases (IC50 value 1.5 nM). Only at a concentration of 100 microM did doxycycline and minocycline significantly inhibit the interleukin-1 (IL-1)-induced augmentation of PG loss from cartilage explants into the nutrient media. The tetracyclines did not modulate the IL-1-mediated reduced aggregability of PGs, whereas 10 microM Ro 31-4724 partially restored the aggregability of PGs ex vivo. Tetracycline even at this high concentration was ineffective. Compared to the effects of the MMP inhibitor Ro 31-4724, treatment with tetracyclines at therapeutic serum levels of 1 or 10 microM was minimal, with little or no effect on cartilage proteoglycanases and PG biosynthesis. In our experiments, tetracyclines and Ro 31-4724 at doses evaluated had no cytotoxic effects on chondrocytes.

The proteoglycan metabolism, morphology and viability of articular cartilage treated with a synthetic matrix metalloproteinase inhibitor

Matrix metalloproteinases (MMP) are among the key enzymes responsible for the proteolytic destruction of articular cartilage during chronic rheumatic diseases. Articular cartilage is one potential target for drugs designed to inhibit the activity of MMPs in order to stop or to slow down the proteolytic destruction of the extracellular matrix of cartilage. The purpose of this study was to investigate the effect of the synthetic inhibitor of MMPs U-24522 for its ability (1) to inhibit in vitro the activity of MMP-proteoglycanases; (2) to modulate the morphology and viability of cartilage explants; and (3) to modify the biosynthesis and release of proteoglycans from articular cartilage explants. U-24522 dose-dependently inhibited the activity of MMP-proteoglycanases and significantly reduced the release of proteoglycans from interleukin-1 treated bovine articular cartilage explants when tested at concentrations ranging from 10(-4) to 10(-9) M. This hydroxamic acid derivative proved not to be harmful to chondrocyte viability and cartilage morphology. In addition, U-24522 had no effect on the rate of proteoglycan biosynthesis of interleukin-1 treated cartilage explants and increased the percentage of newly synthesized proteoglycans to form macromolecular aggregates. Thus U-24522 combines direct inhibitory potential on the activity of MMP-proteoglycanases with the inhibition of interleukin-1 stimulated proteoglycan loss from articular cartilage explants without affecting the morphology, viability and biosynthesis of proteoglycans of bovine articular cartilage explants.

Intermittent cyclic loading of cartilage explants modulates fibronectin metabolism

OBJECTIVE

The aim of this study was to evaluate systematically the effect of tissue load, its amplitude, time of intermittence and duration of loading on the biosynthesis and release of fibronectin by intermittently loaded mature bovine articular cartilage explants.

METHODS

Cyclic compressive pressure was introduced using a sinusoidal waveform of 0.5 Hz-frequency with a peak stress of 0.1, 0.5 or 1.0 MPa for a period of 10 s followed by an unloaded period lasting 10, 100 or 1000 s. Fibronectin and total proteins were radiolabeled with 10 microCi/ml [3H]-phenylalanine during the final 18 h of the 1, 3 or 6 day experiments. The content of endogenous fibronectin was determined using enzyme-linked immunosorbant assay (ELISA), whereas the viability of explants was measured using sections of cartilage explants stained with fluorescein diacetate and propidium iodide. The deformation of loaded explants was determined using a load-displacement transducer system.

RESULTS

The mechanical factor time of intermittence significantly altered the synthesis and release of fibronectin by cartilage explants, whereas the tested range of load magnitudes, as well as the duration of loading, seemed to be of subordinate importance. Loading affected the viability of the superficial zone in the cartilage, whereas the chondrocytes of the intermediate and deep zone remained viable. The compression of loaded explants was dependent on the magnitude of stress, as well as on the duration of unloading between each loading cycle. Synthesis of fibronectin, the retention of newly synthesized fibronectin within the extracellular matrix, and the portion of newly synthesized proteins that were fibronectin was significantly increased in cartilage explants which were cyclically compressed with 0.5 MPa for 10 s followed by a period of unloading lasting 100 s.

CONCLUSIONS

Previous studies reporting that cartilage explants of human and animal osteoarthritic joints synthesize and retain elevated amounts of fibronectin imply that in our experiments mechanical stimuli can induce a fibronectin metabolism in vitro which mimics some of the osteoarthritic characteristics.

A computer-controlled mechanical culture system for biological testing of articular cartilage explants

A computer-controlled mechanical culture system was designed to investigate the interaction between mechanical stimuli and the metabolism of articular cartilage. The main features of this system include the following capabilities: (1) Accurately controlled static, permanent cyclic or intermittent cyclic mechanical loads can be applied; (2) a great variety of different functions to load cartilage explants can be chosen; (3) a wide range of selectable forces (1.0-500 N) and frequencies (up to 5.0 Hz) can be used to load explants; (4) cartilage explants can be cultured and loaded within a standard CO2-incubator for extended time periods; (5) similar culture conditions are provided within the loading chambers as in standard tissue culture plates; (6) simultaneously the applied load and the resulting displacement of specimens is measured, and (7) the load chambers are biocompatible, sterilizable, and non-corrosive. We expect that the newly designed mechanical culture system will increase our understanding on the regulatory role of direct mechanical pressure on the metabolic activities of chondrocytes.

Pharmacological influence of antirheumatic drugs on proteoglycans from interleukin-1 treated articular cartilage

The purpose of this study was to examine whether drugs used in the treatment of arthritic disorders possess any inhibitory potential on the proteoglycanolytic activities of matrix metalloproteinases (MMPs), and to determine whether drugs which inhibit these enzymes also modulate the biosynthesis and release of proteoglycans (PGs) from interleukin-1-(IL-1) treated articular cartilage explants. The cartilage-bone marrow extract and the glycosaminoglycan-peptide complex (DAK-16) dose-dependently inhibited MMP proteoglycanases in vitro when tested at concentrations ranging from 0.5 to 55 mg/mL, displaying an IC50 value of 31.78 mg/mL and 10.64 mg/mL (1.9 x 10[-4] M) respectively. (R,S)-N-[2-[2-(hydroxyamino)-2-oxoethyl]-4-methyl-1-oxopentyl++ +]-L-leucyl-L-phenylalaninamide (U-24522) proved to be a potent inhibitor of MMP proteoglycanases (IC50 value 1.8 x 10[-9] M). None of the other tested drugs, such as possible chondroprotective drugs, nonsteroidal anti-inflammatory drugs (NSAIDs), disease modifying antirheumatic drugs (DMARDs), glucocorticoids and angiotensin-converting enzyme inhibitors tested at a concentration of 10(-4) M displayed any significant inhibition. Only U-24522, tested at a concentration ranging from 10(-4) to 10(-6) M, significantly inhibited the IL-1-induced augmentation of PG loss from cartilage explants into the nutrient media, whereas DAK-16 and the cartilage-bone marrow extract were ineffective. DAK-16 and the cartilage-bone marrow extract did not modulate the IL-1-mediated reduced biosynthesis and aggregability of PGs by the cartilage explants. The addition of 10(-5) M U-24522, however, partially maintained the aggregability of PGs ex vivo. In our experiments, both possible chondroprotective drugs as well as U-24522 demonstrated no cytotoxic effects on chondrocytes.

Four-dimensional imaging of living chondrocytes in cartilage using confocal microscopy: a pragmatic approach

Regulation of cell volume is a fundamental cellular homeostatic mechanism in the face of osmotic stress. In normal articular cartilage, chondrocytes are exposed to a changing osmotic environment. We present a comprehensive protocol for studying the volume regulatory behavior of chondrocytes within intact cartilage tissue using confocal laser-scanning microscopy. Our data acquisition regime optimizes both signal-to-noise and cell viability during time-lapsed three-dimensional (3-D) (x, y, z, t) imaging. The porcine cartilage is treated as an integrated component of the imaging system, and we demonstrate methods for the direct assessment of tissue-induced axial attenuation and image distortion. Parameterized functions describing these two components of image degradation are used to correct experimental data. The current study also highlights the problems associated with the analysis and visualization of four-dimensional (4-D) images. We have devised two new types of data reconstruction. The first compresses each 3-D time point into a single quantitative view, termed a coordinate view. From these reconstructions we are able to simultaneously view and extract cell measurements. A second type, a 4-D reconstruction, uses color to represent relative changes in cell volume, again while maintaining the morphological and spatial information. Both these approaches of image analysis and visualization have been implemented to study the morphology, spatial distribution, and dynamic volume behavior of chondrocytes after osmotic perturbation. We have mapped chondrocyte shape, arrangement, and absolute volume in situ, which vary significantly from the tissue surface through to the underlying bone. Despite the rigid nature of the extracellular matrix, cartilage cells are osmotically sensitive and respond to stimulation of volume regulatory mechanisms. The combined techniques of confocal laser-scanning microscopy and vital cell labeling have enabled us to study, for the first time, the response of chondrocytes in situ to changes in interstitial osmotic pressure.

Morphological and biochemical evidence for apoptosis in the terminal hypertrophic chondrocytes of the growth plate

The purpose of this study was to investigate the mechanism of cell death in chondrocytes of the growth plate. In the degenerative chondrocyte zone of the growth plate, apoptotic chondrocytes were defeated by the in situ nick end labelling method, by DNA analysis in agarose gel, and by electron microscopy. The results of the in situ nick end labelling method and the occurrence of a ladder pattern of DNA in agarose gel analysis indicated the activation of endogenous endonucleases, resulting in DNA fragmentation. Electron micrographs showed the early morphological changes associated with apoptosis. This report presents both morphological and biochemical evidence for apoptosis in the terminal hypertrophic chondrocytes of the growth plate. These data suggest that apoptosis of degenerative chondrocytes may play an important role in the control of normal and pathological endochondral ossification.

Fourier transform infrared microspectroscopic analysis identifies alterations in mineral properties in bones from mice transgenic for type X collagen

Type X collagen has been implicated in the morphogenetic events of endochondral ossification (EO), including the calcification of hypertrophic cartilage and trabeculae prior to their replacement by bone and marrow. Recently, transgenic mice, which expressed a truncated collagen X protein, were reported to exhibit morphologic alterations in all tissues arising through EO. Specifically, the growth plates were compressed within the zone of cartilage hypertrophy, and the number and size of calcified trabeculae were reduced. The condition in the mouse is comparable to Schmid metaphyseal chondrodysplasia in humans for which, to date, 20 defined type X collagen mutations have been reported. The transgenic mouse showed no alterations in mineralization by conventional histology, however, it did show a decrease in newly formed bony trabeculae, and a thinning of periosteal bones. Fourier transform infrared (FTIR) spectroscopy has previously been shown to provide quantitative and qualitative information about the relative amount of mineral and carbonate present, mineral composition, and crystal perfection. To determine whether the expression of abnormal collagen X molecules had an effect on mineral properties, the "quality" of mineral crystals was analyzed in thin sections of tibia from day 17 and day 25 genotypically negative (normal) and positive (mutant) mice from several independent transgenic mouse lines showing varying degrees of the mutant phenotype, by means of Fourier transform infrared microscopic analysis (FTIRM). The results indicate definite differences between normal and transgenic mice calcified cartilage mineral, both in the amount present and the "quality" of the crystals. Calcified cartilage mineral from transgenic mice exhibited less crystallinity and higher acidic phosphate content than the corresponding mineral from normal specimens.

The role of type X collagen in endochondral ossification as deduced by Fourier transform infrared microscopy analysis

Type X collagen has been implicated in the morphogenetic events of endochondral ossification (EO), including the calcification of hypertrophic cartilage and trabeculae prior to their replacement by bone and marrow. Recently, transgenic mice which expressed a truncated collagen X protein were reported to exhibit morphologic alterations in all tissues arising through EO. Fourier Transform InfraRed (FTIR) spectroscopy has previously been shown to provide quantitative and qualitative information about the relative amount of mineral and carbonate present, mineral composition, and crystal perfection. To determine the role of collagen X in mineralization, the "quality" of mineral crystals was analyzed in thin sections of calcified cartilage from tibia obtained from several independent transgenic mouse lines showing varying degrees of the mutant phenotype and mice without type X collagen expression, by means of Fourier Transform InfraRed microscopy (FTIRM). In the present paper, the term "mineral quality" is employed to describe crystallinity/crystal maturation, and acid phosphate content. The results indicate significant differences between normal and transgenic mice bone mineral, both in the amount present and the "quality" of the crystals. In contrast, the analysis of the mineral in mice without type X collagen expression was not different from their age/sex-matched controls.

Osteogenic differentiation of hypertrophic chondrocytes involves asymmetric cell divisions and apoptosis

We have investigated the early cellular events that take place during the change in lineage commitment from hypertrophic chondrocytes to osteoblast-like cells. We have induced this osteogenic differentiation by cutting through the hypertrophic cartilage of embryonic chick femurs and culturing the explants. Immunocytochemical characterization, [3H]thymidine pulse-chase labeling, in situ nick translation or end labeling of DNA breaks were combined with ultrastructural studies to characterize the changing pattern of differentiation. The first responses to the cutting, seen after 2 d, were upregulation of alkaline phosphatase activity, synthesis of type I collagen and single-stranded DNA breaks, probably indicating a metastable state. Associated with the change from chondrogenic to osteogenic commitment was an asymmetric cell division with diverging fates of the two daughter cells, where one daughter cell remained viable and the other one died. The available evidence suggests that the viable daughter cell then divided and generated osteogenic cells, while the other daughter cell died by apoptosis. The results suggest a new concept of how changes in lineage commitment of differentiated cells may occur. The concepts also reconcile previously opposing views of the fate of the hypertrophic chondrocyte.

Ultrastructural and histochemical studies of the epiphyseal plate in normal chicks

BACKGROUND

Chondrocytes in the epiphyseal plate undergo a series of well-defined stages, each stage containing a morphologically homogeneous cell population. However, biochemical studies show that there are some functionally heterogeneous cell types in the calcifying zone of the chick epiphyseal plate.

METHODS

We studied the sequence of chondrocytic maturation in the normal chick epiphyseal plate ultrastructurally and histochemically. Chondrocytes in the calcifying zone were of three distinct types, the appearance of each cell type being closely related to the stage of matrix calcification.

RESULTS

Clear cells were observed in the upper calcifying region, stellate cells appeared in the middle calcifying region, and hypertrophic clear cells appeared in the lower calcifying region. Rough endoplasmic reticulum (RER) and lysosome-rich cells were found, these being limited to the outermost layers of the calcifying zone and containing ACPase-positive products. Osteoclasts were attached to the matrix near the RER and lysosome-rich cells in the poorly calcified regions.

CONCLUSION

We hypothesized that each cell type played a different role in the initiation, progression, and maintenance of cartilage calcification. RER and lysosome-rich cells may be responsible for the resorption of uncalcified cartilage matrix, this resulting in induction of the osteoclastic resorption of the calcified matrix. In addition, the fate of the chondrocytes was twofold: hypertrophic clear cells died, while the RER and lysosome-rich cells survived, suggesting that these cells were transformed into osteogenic cells.

Mechanism of longitudinal bone growth and its regulation by growth plate chondrocytes

Growth plate chondrocytes play a pivotal role in promoting longitudinal bone growth. The current review represents a brief survey of the phenomena involved in this process at the cellular level; it delineates the contributions made by various activities during the course of the chondrocyte life cycle, notably proliferation and hypertrophy, and illustrates how the relative contributions may be modulated according to the particular needs of an organism at critical phases of growth. The cellular mechanisms by which a few well characterized growth-promoting substances exert their influences are discussed in the light of recent findings pertaining to epiphyseal plate chondrocytes in vivo.

Stereological and serial section analysis of chondrocytic enlargement in the proximal tibial growth plate of the rat

BACKGROUND

It has been suggested that within the growth plate, the final volume and shape of hypertrophic chondrocytes are important variables in determining the rate of longitudinal bone growth. To better understand the organization and regulation of chondrocytic hypertrophy as related to longitudinal bone growth, the beginning and end, and the location and magnitude of chondrocytic volume and shape changes during the hypertrophic process were defined in the proximal tibial growth plate of 35-day-old rats.

METHODS

In this study we used two different approaches, a stereological analysis of chondrocytes in unbiasedly defined, narrow growth plate strata, and a serial section reconstruction and measurement of individual cells. In both experiments chondrocytes were preserved using optimal chemical fixation. Proliferating chondrocytes were identified using bromodeoxyuridine labelling, and the rate of longitudinal bone growth was determined using oxytetracycline labelling.

RESULTS

In both studies, immediately following cell division in the proliferative zone, chondrocytic volume gradually increased toward the mid-point of the growth plate. During this phase of about 30 hours, approximately 20% of the final cell volume was obtained. During the following 20 hours the remaining 80% was acquired. The estimated rate of cell volume increased changed from approximately 50 microns 3/hr during the first 30 hours to about 800 microns 3/hr during the last 20 hours. The increase in cell volume resulted in an increase in both the vertical and the horizontal chondrocytic diameters. Cell parameters did not change during the final five hours of the maturation process.

CONCLUSIONS

In this study we demonstrated that chondrocytic enlargement starts immediately following cell division in the proliferative zone, and that chondrocytic enlargement consists of two morphologically distinguishable phases. The transition point between the first and the second phase of chondrocytic enlargement corresponded with the junction between the proliferative zone and the maturation zone.

Effects of calcium deficiency on chondrocyte hypertrophy and type X collagen expression in chick embryonic sternum

Maintenance of chick embryos in long-term culture without their calcareous egg-shell is a useful method for studying the relationship between calcium homeostasis and cell differentiation during skeletogenesis. Previously, we have shown that in shell-less (SL) embryos, calcium deficiency induces a cartilage-like phenotype in osteogenic tissues, such as calvaria (Jacenko and Tuan [1986] Dev. Biol. 115:215). In this investigation, we have studied the relationship between cartilage calcification and hypertrophy, and the expression of type X collagen, a specific product of hypertrophic chondrocytes. For this study, the cephalic (calcifying) and caudal (permanently cartilaginous) regions of sterna from day 18 and day 20 normal (NL) and SL embryos were metabolically labeled with [14C]-proline. Analysis of the biosynthetic products revealed significant differences in type X collagen expression in the cephalic region of sternal cartilage. In NL tissues, type X collagen production increased from 13.1% of total collagen at day 18 to 43.7% at day 20. In contrast, in SL embryos, type X collagen was not detectable until day 20, when it represented only 1% of total collagen. Comparison of the NL and SL embryos with respect to their serum calcium level and sternal calcium content and histology revealed a direct relationship between low systemic calcium and limited cartilage hypertrophy, undermineralization, and decreased type X collagen production in the sternal cephalic cartilage. Supplementation of CaCO3 to SL embryos increased their serum and sternal calcium, and restored cartilage hypertrophy, mineralization, and type X collagen synthesis in the cephalic portion of the sterna. These findings confirm that a critical relationship exists between calcium homeostasis, chondrocyte hypertrophy, mineralization, and type X collagen synthesis in the cephalic region of sternal cartilage. These results further demonstrate the importance of calcium in the morphogenetic events of endochondral ossification, in particular the transition from hyaline cartilage to hypertrophic cartilage, and eventually to bone.

Effects of distraction and compression on proliferation of growth plate chondrocytes. A study in rabbits

An external fixation device was applied across the distal femoral physis in 30 rabbits, and distraction or compression was performed for 3-21 days; either no operation or a sham-operation was performed on the contralateral side. Proliferating cells were labeled with 5-bromo-2'-deoxyuridine, a thymidine analogue, and subsequently localized in decalcified histologic sections using a specific monoclonal antibody. The height of the proliferative and hypertrophic zones was increased after distraction; additionally, fracture-separation through the hypertrophic zone or at the junction of the physis and metaphysis was seen in 13 of the 15 specimens. Labeled cells were encountered only in the proliferative zone in all specimens except after early distraction, where labeled chondrocytes were seen close to the separation gap in the hypertrophic zone, too. Distraction had no effect on the number of labeled cells. A reduction in the height of the proliferative and hypertrophic zones occurred after compression, and the number of proliferating chondrocytes decreased.

Trans-differentiation of hypertrophic chondrocytes into cells capable of producing a mineralized bone matrix

Trans-differentiation of hypertrophic chondrocytes into bone-forming cells was observed when femurs from 14-day-old chick embryos were cut through the region of hypertrophic cartilage and the separated pieces were cultured for 2-18 days. Inside many chondrocytic lacunae a new matrix was present which had the staining characteristics of bone matrix including birefringence and the capacity to mineralize. The cells within the lacunae had the characteristics of osteoblasts, such as alkaline phosphatase activity and positive immunocytochemical staining for osteocalcin, osteonectin, osteopontin and type I collagen. Chondrocyte necrosis and empty lacunae were only observed immediately at the cut edge, and in that region no bone-forming cells were present inside the lacunae. Where bone-matrix was present, the lacunae had remained intact, the cells were viable and no evidence of cell migration was observed. This suggested that the bone-forming cells had originated from the hypertrophic chondrocytes. The temporal sequence of events was followed closely. Two days following the cut only a few chondrocytes showed a positive reaction for osteocalcin, osteonectin, osteopontin and the type I collagen. At that time no such reaction product was observed in the chondrocytes of uncut femurs. Many hypertrophic chondrocytes divided, as shown by tritiated thymidine incorporation. The rate of cell division increased between 2-6 days, when several smaller basophilic cells were present inside the lacuna instead of the single hypertrophic chondrocyte. These cells expressed alkaline phosphatase activity, were positive for fibronectin, the above non-collagenous bone proteins and type I collagen. The bone matrix that was observed after 6-18 days was initially confined to the inside of the chondrocytic lacunae, but later spread beyond the lacunar confines. The bone proteins were still associated with the bone-forming cells, but fibronectin was absent when matrix formation was evident. Mineralization of the intra-lacunar osteoid took place after 12-18 days. It is speculated that the trans-differentiation was initiated by disruptions of the normal cell-cell associations.

Ocular-chondrodysplasia in labrador retriever dogs: a morphometric and electron microscopical analysis

Ocular-chondrodysplasia in Labrador Retriever dogs is characterized by short-limbed dwarfism and ocular abnormalities. The purposes of the present study were to develop morphological criteria to define the matrix and/or chondrocytic abnormalities associated with this chondrodysplasia, and to test the hypothesis that ineffective matrix-directed cellular swelling was associated with the decreased longitudinal bone growth in these animals. The proximal and distal radial growth plates were collected from four affected animals of the same litter. Stereological techniques were used to analyze both cellular shapes and cellular volume changes in the hypertrophic zone. The pathological changes seen in these growth plates varied between animals and included disorganization of cellular columns with abnormal extent of calcification. Chondrocytes of all zones contained two types of abnormal cellular inclusions classified as light and dark, based on the intensity of eosinophilic staining. Both types of inclusions contained material that resembled the surrounding extracellular matrix, varying only in the apparent hydration of the contents. It could be demonstrated that light inclusions were located in the peripheral cytoplasm and connected to the extracellular matrix through narrow channels. By contrast, dark inclusions were membrane bound and perinuclear. Chondrocytes with multiple, large inclusions appeared to be undergoing degenerative changes. Although the final volume achieved by hypertrophic chondrocytes was consistent with that of normal growth plates, there was a high level of variability of chondrocytic shape and evidence of premature cellular condensation in the maturation zone. The severity of the dwarfism correlated both with the extent of chondrocytic changes and the severity of the ocular lesions.(ABSTRACT TRUNCATED AT 250 WORDS)