Phenotypic switching of in vitro mandibular condylar cartilage during matrix mineralization

Age-related alterations of articular cartilage in pituitary adenylate cyclase-activating polypeptide (PACAP) gene-deficient mice

Pituitary adenylate cyclase activating polypeptide (PACAP) is an evolutionarly conserved neuropeptide which is produced by various neuronal and non-neuronal cells, including cartilage and bone cells. PACAP has trophic functions in tissue development, and it also plays a role in cellular and tissue aging. PACAP takes part in the regulation of chondrogenesis, which prevents insufficient cartilage formation caused by oxidative and mechanical stress. PACAP knockout (KO) mice have been shown to display early aging signs affecting several organs. In the present work, we investigated articular cartilage of knee joints in young and aged wild-type (WT) and PACAP KO mice. A significant increase in the thickness of articular cartilage was detected in aged PACAP gene-deficient mice. Amongst PACAP receptors, dominantly PAC1 receptor was expressed in WT knee joints and a remarkable decrease was found in aged PACAP KO mice. Expression of PKA-regulated transcription factors, Sox5, Sox9 and CREB, decreased both in young and aged gene deficient mice, while Sox6, collagen type II and aggrecan expressions were elevated in young but were reduced in aged PACAP KO animals. Increased expression of hyaluronan (HA) synthases and HA-binding proteins was detected parallel with an elevated presence of HA in aged PACAP KO mice. Expression of bone related collagens (I and X) was augmented in young and aged animals. These results suggest that loss of PACAP signaling results in dysregulation of cartilage matrix composition and may transform articular cartilage in a way that it becomes more prone to degenerate.

The Adaptive Remodeling of Condylar Cartilage— A Transition from Chondrogenesis to Osteogenesis

Mandibular condylar cartilage is categorized as articular cartilage but markedly distinguishes itself in many biological aspects, such as its embryonic origin, ontogenetic development, post-natal growth mode, and histological structures. The most marked uniqueness of condylar cartilage lies in its capability of adaptive remodeling in response to external stimuli during or after natural growth. The adaptation of condylar cartilage to mandibular forward positioning constitutes the fundamental rationale for orthodontic functional therapy, which partially contributes to the correction of jaw discrepancies by achieving mandibular growth modification. The adaptive remodeling of condylar cartilage proceeds with the biomolecular pathway initiating from chondrogenesis and finalizing with osteogenesis. During condylar adaptation, chondrogenesis is activated when the external stimuli, e.g., condylar repositioning, generate the differentiation of mesenchymal cells in the articular layer of cartilage into chondrocytes, which proliferate and then progressively mature into hypertrophic cells. The expression of regulatory growth factors, which govern and control phenotypic conversions of chondrocytes during chondrogenesis, increases during adaptive remodeling to enhance the transition from chondrogenesis into osteogenesis, a process in which hypertrophic chondrocytes and matrices degrade and are replaced by bone. The transition is also sustained by increased neovascularization, which brings in osteoblasts that finally result in new bone formation beneath the degraded cartilage.

Imaging analysis of osteogenic transformation of Meckel's chondrocytes from green fluorescent protein-transgenic mice during intrasplenic transplantation

Our previous studies demonstrated that Meckel's chondrocytes, which are derived from ectomesenchyme, have the potential to transform into osteogenic phenotypes. The present study aimed to clarify the role of cell origin in the phenotypic transformation of chondrocytes. Cell pellets from ectomesenchyme-derived Meckel's cartilage and mesoderm-derived costal cartilage from green fluorescent protein (GFP)-transgenic mice were transplanted into the spleen for up to 4 weeks. Chondrocyte pellets from both cartilages adapted well to the splenic tissues and formed an alizarin red-positive calcified matrix, with increasing duration of transplantation. Following the production of cartilage-specific type II and type X collagens, newly-formed type I collagen appeared in the chondrocyte pellets from Meckel's cartilage during the late stage of transplantation. Although the bone-marker proteins: osteocalcin, osteopontin, osteonectin and bone morphogenetic protein-2, were detected in pellets from both Meckel's and costal cartilage, only type I collagen in Meckel's cartilage was a significant marker protein for detecting transformation. These bone-type protein-producing cells represented osteogenic cells transformed from GFP-expressing cells, rather than from recipient cells. These results indicate that neural crest-derived Meckel's cartilage displays a higher potential for phenotypic switching than mesoderm-derived costal chondrocytes under in vivo conditions.

Expression of Ten-m/Odz3 in the fibrous layer of mandibular condylar cartilage during postnatal growth in mice

It has been speculated that the mandibular condyle develops via the differentiation of the fibroblast-like cells covering the condyle into chondrocytes; however, the developmental mechanisms behind this process have not been revealed. We used laser-capture microdissection and cDNA microarray analysis to elucidate the genes that are highly expressed in these fibroblast-like cells. Among these genes, the transcription of Ten-m/Odz3 was significantly increased in the fibroblast-like cells compared with other cartilage tissues. For the first time, we describe the temporal and spatial expression of Ten-m/Odz3 mRNA in relation to the expression of type I, II, and X collagen mRNA, as determined by in-situ hybridization in mouse mandibular condylar cartilage and mouse femoral cartilage during the early stages of development. Ten-m/Odz3 was expressed in the fibrous layer and the proliferating and mature chondrocyte layers, which expressed type I and II collagen, respectively, but was not detected in the hypertrophic chondrocyte layer. Furthermore, we evaluated the in-vitro expression of Ten-m/Odz3 using ATDC5 cells, a mouse chondrogenic cell line. Ten-m/Odz3 was expressed during the early stage of the differentiation of mesenchymal cells into chondrocytes. These findings suggest that Ten-m/Odz3 is involved in the differentiation of chondrocytes and that it acts as a regulatory factor in the early stages of the development of mandibular condylar cartilage.

Histochemical evidences on the chronological alterations of the hypertrophic zone of mandibular condylar cartilage

The hypertrophic chondrocytes lack the ability to proliferate, thus permitting matrix mineralization as well as vascular invasion from the bone in both the mandibular condyle and the epiphyseal cartilage. This study attempted to verify whether the histological appearance of the hypertrophic chondrocytes is in a steady state during postnatal development of the mouse mandibular condyle. Type X collagen immunohistochemistry apparently distinguished the fibrous layer described previously as the "articular zone," "articular layer," and "resting zone" from the hypertrophic zone. Interestingly, the ratio of the type X collagen-positive hypertrophic zone in the entire condyle seemed higher in the early stages but decreased in the later stages. Some apparently compacted cells in the hypertrophic zone showed proliferating cell nuclear antigen (PCNA) immunoreaction, indicating the potential for cell proliferation at the early stages. As the mice matured, in contrast, they further enlarged and assumed typical features of hypertrophic chondrocytes. Apoptotic cells were also discernible in the hypertrophic zone at the early but not later stages. Consistent with morphological configurations of hypertrophic chondrocytes, immunoreactions for alkaline phosphatase, osteopontin, and type I collagen were prominent at the later stage, but not the early stage. Cartilaginous matrices demonstrated scattered patches of mineralization at the early stage, but increased in their volume and connectivity at the later stage. Thus, the spatial and temporal occurrence of these immunoreactions as well as apoptosis likely reflect the prematurity of hypertrophying cells at the early stage, and imply a physiological relevance during the early development of the mandibular condyles.

The role of type X collagen in facilitating and regulating endochondral ossification of articular cartilage

AUTHOR: Shen G Objective -This review was compiled to explore the role of type X collagen in growth, development and remodeling of articular cartilage by elucidating the linkage between the synthesis of this protein and the phenotypic changes in chondrogenesis and the onset of endochondral ossification.

DESIGN

The current studies closely dedicated to elucidating the role of type X collagen incorporating into chondrogenesis and endochondral ossification of articular cartilage were assessed and analyzed to allow for obtaining the mainstream consensus on the bio-molecular mechanism with which type X collagen functions in articular cartilage.

RESULTS

There are spatial and temporal correlations between synthesis of type X collagen and occurrence of endochondral ossification. The expression of type X collagen is confined within hypertrophic condrocytes and precedes the embark of endochondral bone formation. Type X collagen facilitates endochondral ossification by regulating matrix mineralization and compartmentalizing matrix components.

CONCLUSION

Type X collagen is a reliable marker for new bone formation in articular cartilage. The future clinical application of this collagen in inducing or mediating endochondral ossification is perceived, e.g. the fracture healing of synovial joints and adaptive remodeling of madibular condyle.

Primary culture of rat growth plate chondrocytes: an in vitro model of growth plate histotype, matrix vesicle biogenesis and mineralization

During endochondral ossification (EO), cartilage is replaced by bone. Chondrocytes of growth plate undergo proliferation, maturation, hypertrophy, matrix vesicle (MV) biogenesis and programmed cell death (PCD, apoptosis). The in vitro system presented here provides a potential experimental model for studying in vitro differentiation and MV biogenesis in chondrocyte cultures. Chondrocytes were obtained from collagenase-digested tibial and femoral growth plate cartilage of 7-week-old rachitic rats. The isolated chondrocytes were plated as monolayers at a density of 0.5 x 10(6) cells per 35-mm plate and grown for 17 days in BGJ(b) medium supplemented with 10% fetal bovine serum, 50 microg/ml ascorbic acid. Light microscopy revealed Sirius red-positive, apparent bone matrix in layers at the surfaces of cartilaginous nodules that developed in the cultures. The central matrix was largely alcian blue staining thus resembling cartilage matrix. Electron microscopy revealed superficial areas of bone like matrix with large banded collagen fibrils, consistent with type I collagen. Most of the central matrix was cartilaginous, with small fibrils, randomly arranged consistent with type II collagen. The presence of peripheral type I and central type II and type X collagen was confirmed by immunohistochemical staining. Immunohistochemistry with anti-Bone morphogenetic proteins 2, 4 and 6 showed that BMP expression is associated with maturing hypertrophic central chondrocytes, many of which were TUNEL positive and undergoing cell death with plasma membrane breaks, hydropic swelling and cell fragmentation. During early mineralization, small radial clusters of hydroxyapatite-like mineral were associated with matrix vesicles. Collagenase digestion-released MVs from the cultures showed a high specific activity for alkaline phosphatase and demonstrated a pattern of AMP-stimulated nonradioactive (40)Calcium deposition comparable to that observed with native MVs. These studies confirm that primary cultures of rat growth plate chondrocytes are a reasonable in vitro model of growth plate histotype, MV biogenesis and programmed cell death.

Origin-associated features of chondrocytes in mouse Meckel's cartilage and costal cartilage: an in vitro study

Using a cell culture method, we histochemically and immunohistochemically investigated whether chondrocytes deriving from different origins, such as Meckel's or costal cartilages, express similar phenotypic characteristics. Chondrocytes isolated enzymatically from Meckel's and costal cartilages of 17-day embryonic mice both actively proliferated and formed cartilage nodules consisting of toluidine blue-positive proteoglycans and type II collagen. Both deposited calcified cartilaginous matrix as revealed by alkaline phosphatase (ALPase) activity and alizarin red staining throughout 3 weeks in culture. Immunostaining for osteopontin (OP), osteocalcin (OC), and osteonectin (ON) revealed that chondrocytes from both cartilages were positive for their proteins, but type I collagen was detected only in cells transforming from Meckel's chondrocytes late in the culture. Electron microscopy demonstrated that although costal and Meckel's chondrocytes had typical chondrocytic features during 2 weeks in culture, Meckel's chondrocytes transformed into osteocytic cells that produced thick, banded type I collagen fibrils. In contrast, costal chondrocytes maintained typical hypertrophic morphology throughout the final stage of culture. The present study suggests that Meckel's chondrocytes derived from neural crest-ectomesenchyme retain osteogenic potential, and differ from costal chondrocytes originating from mesoderm.