Structure, formation and role of cartilage canals in the developing bone

Resident mesenchymal progenitors of articular cartilage

Articular cartilage has poor capacity of self-renewal and repair. Insufficient number and activity of resident mesenchymal (connective tissue) progenitors is likely one of the underlying reasons. Chondroprogenitors reside not only in the superficial zone of articular cartilage but also in other zones of articular cartilage and in the neighboring tissues, including perichondrium (groove of Ranvier), synovium and fat pad. These cells may respond to injury and contribute to articular cartilage healing. In addition, marrow stromal cells can migrate through subchondral bone when articular cartilage is damaged. We should develop drugs and methods that correctly stimulate resident progenitors for improvement of repair and inhibition of degenerative changes in articular cartilage.

Toward regeneration of articular cartilage

Articular cartilage is classified as permanent hyaline cartilage and has significant differences in structure, extracelluar matrix components, gene expression profile, and mechanical property from transient hyaline cartilage found in the epiphyseal growth plate. In the process of synovial joint development, articular cartilage originates from the interzone, developing at the edge of the cartilaginous anlagen, and establishes zonal structure over time and supports smooth movement of the synovial joint through life. The cascade actions of key regulators, such as Wnts, GDF5, Erg, and PTHLH, coordinate sequential steps of articular cartilage formation. Articular chondrocytes are restrictedly controlled not to differentiate into a hypertrophic stage by autocrine and paracrine factors and extracellular matrix microenvironment, but retain potential to undergo hypertrophy. The basal calcified zone of articular cartilage is connected with subchondral bone, but not invaded by blood vessels nor replaced by bone, which is highly contrasted with the growth plate. Articular cartilage has limited regenerative capacity, but likely possesses and potentially uses intrinsic stem cell source in the superficial layer, Ranvier's groove, the intra-articular tissues such as synovium and fat pad, and marrow below the subchondral bone. Considering the biological views on articular cartilage, several important points are raised for regeneration of articular cartilage. We should evaluate the nature of regenerated cartilage as permanent hyaline cartilage and not just hyaline cartilage. We should study how a hypertrophic phenotype of transplanted cells can be lastingly suppressed in regenerating tissue. Furthermore, we should develop the methods and reagents to activate recruitment of intrinsic stem/progenitor cells into the damaged site.

Palmitoyl acyltransferase, Zdhhc13, facilitates bone mass acquisition by regulating postnatal epiphyseal development and endochondral ossification: a mouse model

ZDHHC13 is a member of DHHC-containing palmitoyl acyltransferases (PATs) family of enzymes. It functions by post-translationally adding 16-carbon palmitate to proteins through a thioester linkage. We have previously shown that mice carrying a recessive Zdhhc13 nonsense mutation causing a Zdhcc13 deficiency develop alopecia, amyloidosis and osteoporosis. Our goal was to investigate the pathogenic mechanism of osteoporosis in the context of this mutation in mice. Body size, skeletal structure and trabecular bone were similar in Zdhhc13 WT and mutant mice at birth. Growth retardation and delayed secondary ossification center formation were first observed at day 10 and at 4 weeks of age, disorganization in growth plate structure and osteoporosis became evident in mutant mice. Serial microCT from 4-20 week-olds revealed that Zdhhc13 mutant mice had reduced bone mineral density. Through co-immunoprecipitation and acyl-biotin exchange, MT1-MMP was identified as a direct substrate of ZDHHC13. In cells, reduction of MT1-MMP palmitoylation affected its subcellular distribution and was associated with decreased VEGF and osteocalcin expression in chondrocytes and osteoblasts. In Zdhhc13 mutant mice epiphysis where MT1-MMP was under palmitoylated, VEGF in hypertrophic chondrocytes and osteocalcin at the cartilage-bone interface were reduced based on immunohistochemical analyses. Our results suggest that Zdhhc13 is a novel regulator of postnatal skeletal development and bone mass acquisition. To our knowledge, these are the first data to suggest that ZDHHC13-mediated MT1-MMP palmitoylation is a key modulator of bone homeostasis. These data may provide novel insights into the role of palmitoylation in the pathogenesis of human osteoporosis.

Altering the architecture of tissue engineered hypertrophic cartilaginous grafts facilitates vascularisation and accelerates mineralisation

Cartilaginous tissues engineered using mesenchymal stem cells (MSCs) can be leveraged to generate bone in vivo by executing an endochondral program, leading to increased interest in the use of such hypertrophic grafts for the regeneration of osseous defects. During normal skeletogenesis, canals within the developing hypertrophic cartilage play a key role in facilitating endochondral ossification. Inspired by this developmental feature, the objective of this study was to promote endochondral ossification of an engineered cartilaginous construct through modification of scaffold architecture. Our hypothesis was that the introduction of channels into MSC-seeded hydrogels would firstly facilitate the in vitro development of scaled-up hypertrophic cartilaginous tissues, and secondly would accelerate vascularisation and mineralisation of the graft in vivo. MSCs were encapsulated into hydrogels containing either an array of micro-channels, or into non-channelled ‘solid’ controls, and maintained in culture conditions known to promote a hypertrophic cartilaginous phenotype. Solid constructs accumulated significantly more sGAG and collagen in vitro, while channelled constructs accumulated significantly more calcium. In vivo, the channels acted as conduits for vascularisation and accelerated mineralisation of the engineered graft. Cartilaginous tissue within the channels underwent endochondral ossification, producing lamellar bone surrounding a hematopoietic marrow component. This study highlights the potential of utilising engineering methodologies, inspired by developmental skeletal processes, in order to enhance endochondral bone regeneration strategies.

Epidermal growth factor receptor (EGFR) signaling regulates epiphyseal cartilage development through β-catenin-dependent and -independent pathways

The epidermal growth factor receptor (EGFR) is an essential player in the development of multiple organs during embryonic and postnatal stages. To understand its role in epiphyseal cartilage development, we generated transgenic mice with conditionally inactivated EGFR in chondrocytes. Postnatally, these mice exhibited a normal initiation of cartilage canals at the perichondrium, but the excavation of these canals into the cartilage was strongly suppressed, resulting in a delay in the formation of the secondary ossification center (SOC). This delay was accompanied by normal chondrocyte hypertrophy but decreased mineralization and apoptosis of hypertrophic chondrocytes and reduced osteoclast number at the border of marrow space. Immunohistochemical analyses demonstrated that inactivation of chondrocyte-specific EGFR signaling reduced the amounts of matrix metalloproteinases (MMP9, -13, and -14) and RANKL (receptor activator of NF-κB ligand) in the hypertrophic chondrocytes close to the marrow space and decreased the cartilage matrix degradation in the SOC. Analyses of EGFR downstream signaling pathways in primary epiphyseal chondrocytes revealed that up-regulation of MMP9 and RANKL by EGFR signaling was partially mediated by the canonical Wnt/β-catenin pathway, whereas EGFR-enhanced MMP13 expression was not. Further biochemical studies suggested that EGFR signaling stimulates the phosphorylation of LRP6, increases active β-catenin level, and induces its nuclear translocation. In line with these in vitro studies, deficiency in chondrocyte-specific EGFR activity reduced β-catenin amount in hypertrophic chondrocytes in vivo. In conclusion, our work demonstrates that chondrocyte-specific EGFR signaling is an important regulator of cartilage matrix degradation during SOC formation and epiphyseal cartilage development and that its actions are partially mediated by activating the β-catenin pathway.

Defective postnatal endochondral bone development by chondrocyte-specific targeted expression of parathyroid hormone type 2 receptor

The human parathyroid hormone type 2 receptor (PTH2R) is activated by PTH and by tuberoinfundibular peptide of 39 residues (TIP39), the latter likely acting as its natural ligand. Although the receptor is expressed at highest levels in the nervous system, we have observed that both PTH2R and TIP39 are expressed in the newborn mouse growth plate, with the receptor localizing in the resting zone and the ligand TIP39 localizing exclusively in prehypertrophic and hypertrophic chondrocytes. To address the role of PTH2R in postnatal skeletal growth and development, Col2a1-hPTH2R (PTH2R-Tg) transgenic mice were generated. The mice were viable and of nearly normal size at birth. Expression of the transgene in the growth plate was limited to chondrocytes. We found that chondrocyte proliferation was decreased, as determined by in vivo BrdU labeling of proliferating chondrocytes and CDK4 and p21 expression in the growth plate of Col2a1-hPTH2R transgenic mice. Similarly, the differentiation and maturation of chondrocytes was delayed, as characterized by decreased Sox9 expression and weaker immunostaining for the chondrocyte differentiation markers collagen type II and type X and proteoglycans. As well, there was altered expression of Gdf5, Wdr5, and β-catenin, factors implicated in chondrocyte maturation, proliferation, and differentiation.These effects impacted on the process of endochondral ossification, resulting in delayed formation of the secondary ossification center, and diminished trabecular bone volume. The findings substantiate a role for PTH2R signaling in postnatal growth plate development and subsequent bone mass acquisition.

Retinol-binding protein 4 is expressed in chondrocytes of developing mouse long bones: implications for a local role in formation of the secondary ossification center

Retinol-binding protein 4 (Rbp4) is the major carrier of retinol in the bloodstream, a retinoid whose metabolites influence osteogenesis, chondrogenesis and adipogenesis. Rbp4 is mainly produced in the liver where it mobilizes hepatic retinol stores to supply other tissues. However, Rbp4 is also expressed in several extrahepatic tissues, including limbs, where its role is largely unknown. This study aimed to identify the cellular localization of Rbp4 to gain insight into its involvement in limb development and bone growth. Using immunohistochemistry, we discovered that Rbp4 was present in a variety of locations in developing embryonic and postnatal mouse hindlimbs. Rbp4 was present in a restricted population of epiphyseal chondrocytes and perichondral cells correlating to the future region of secondary ossification. With the onset of secondary ossification, Rbp4 was detected in chondrocytes of the resting zone and in chondrocytes that bordered invading cartilage canals and the expanding front of ossification. Rbp4 was less abundant in proliferating chondrocytes involved in primary ossification. Our data implicate the involvement of chondrocytic Rbp4 in bone growth, particularly in the formation of the secondary ossification center of the limb.

Spongiosa primary development: a biochemical hypothesis by Turing patterns formations

We propose a biochemical model describing the formation of primary spongiosa architecture through a bioregulatory model by metalloproteinase 13 (MMP13) and vascular endothelial growth factor (VEGF). It is assumed that MMP13 regulates cartilage degradation and the VEGF allows vascularization and advances in the ossification front through the presence of osteoblasts. The coupling of this set of molecules is represented by reaction-diffusion equations with parameters in the Turing space, creating a stable spatiotemporal pattern that leads to the formation of the trabeculae present in the spongy tissue. Experimental evidence has shown that the MMP13 regulates VEGF formation, and it is assumed that VEGF negatively regulates MMP13 formation. Thus, the patterns obtained by ossification may represent the primary spongiosa formation during endochondral ossification. Moreover, for the numerical solution, we used the finite element method with the Newton-Raphson method to approximate partial differential nonlinear equations. Ossification patterns obtained may represent the primary spongiosa formation during endochondral ossification.

Double Muscling in Cattle: Genes, Husbandry, Carcasses and Meat

Molecular biology has enabled the identification of the mechanisms whereby inactive myostatin increases skeletal muscle growth in double-muscled (DM) animals. Myostatin is a secreted growth differentiation factor belonging to the transforming growth factor-β superfamily. Mutations make the myostatin gene inactive, resulting in muscle hypertrophy. The relationship between the different characteristics of DM cattle are defined with possible consequences for livestock husbandry. The extremely high carcass yield of DM animals coincides with a reduction in the size of most vital organs. As a consequence, DM animals may be more susceptible to respiratory disease, urolithiasis, lameness, nutritional stress, heat stress and dystocia, resulting in a lower robustness. Their feed intake capacity is reduced, necessitating a diet with a greater nutrient density. The modified myofiber type is responsible for a lower capillary density, and it induces a more glycolytic metabolism. There are associated changes for the living animal and post-mortem metabolism alterations, requiring appropriate slaughter conditions to maintain a high meat quality. Intramuscular fat content is low, and it is characterized by more unsaturated fatty acids, providing healthier meat for the consumer. It may not always be easy to find a balance between the different disciplines underlying the livestock husbandry of DM animals to realize a good performance and health and meat quality.

Cartilage-specific β-catenin signaling regulates chondrocyte maturation, generation of ossification centers, and perichondrial bone formation during skeletal development

The WNT/β-catenin signaling pathway is a critical regulator of chondrocyte and osteoblast differentiation during multiple phases of cartilage and bone development. Although the importance of β-catenin signaling during the process of endochondral bone development has been previously appreciated using a variety of genetic models that manipulate β-catenin in skeletal progenitors and osteoblasts, genetic evidence demonstrating a specific role for β-catenin in committed growth-plate chondrocytes has been less robust. To identify the specific role of cartilage-derived β-catenin in regulating cartilage and bone development, we studied chondrocyte-specific gain- and loss-of-function genetic mouse models using the tamoxifen-inducible Col2Cre(ERT2) transgene in combination with β-catenin(fx(exon3)/wt) or β-catenin(fx/fx) floxed alleles, respectively. From these genetic models and biochemical data, three significant and novel findings were uncovered. First, cartilage-specific β-catenin signaling promotes chondrocyte maturation, possibly involving a bone morphogenic protein 2 (BMP2)-mediated mechanism. Second, cartilage-specific β-catenin facilitates primary and secondary ossification center formation via the induction of chondrocyte hypertrophy, possibly through enhanced matrix metalloproteinase (MMP) expression at sites of cartilage degradation, and potentially by enhancing Indian hedgehog (IHH) signaling activity to recruit vascular tissues. Finally, cartilage-specific β-catenin signaling promotes perichondrial bone formation possibly via a mechanism in which BMP2 and IHH paracrine signals synergize to accelerate perichondrial osteoblastic differentiation. The work presented here supports the concept that the cartilage-derived β-catenin signal is a central mediator for major events during endochondral bone formation, including chondrocyte maturation, primary and secondary ossification center development, vascularization, and perichondrial bone formation.

Role of tartrate-resistant acid phosphatase (TRAP) in long bone development

Tartrate resistant acid phosphatase (TRAP) was shown to be critical for skeleton development, and TRAP deficiency leads to a reduced resorptive activity during endochondral ossification resulting in an osteopetrotic phenotype and shortened long bones in adult mice. A proper longitudinal growth depends on a timely, well-coordinated vascularization and formation of the secondary ossification center (SOC) of the long bones epiphysis. Our results demonstrate that TRAP is not essential for the formation of the epiphyseal vascular network. Therefore, in wild type (Wt) controls as well as TRAP deficient (TRAP^−/−) mutants vascularised cartilage canals are present from postnatal day (P) five. However, in the epiphysis of the TRAP^−/− mice cartilage mineralization, formation of the marrow cavity and the SOC occur prematurely compared with the controls. In the mutant mice the entire growth plate is widened due to an expansion of the hypertrophic zone. This is not seen in younger animals but first detected at week (W) three and during further development. Moreover, an enhanced number of thickened trabeculae, indicative of the osteopetrotic phenotype, are observed in the metaphysis beginning with W three. Epiphyseal excavation was proposed as an important function of TRAP, and we examined whether TRAP deficiency affects this process. We therefore evaluated the marrow cavity volume (MCV) and the epiphyseal volume (EV) and computed the MCV to EV ratio (MCV/EV). We investigated developmental stages until W 12. Our results indicate that both epiphyseal excavation and establishment of the SOC are hardly impaired in the knockouts. Furthermore, no differences in the morphology of the epiphyseal bone trabeculae and remodeling of the articular cartilage layers are noted between Wt and TRAP^−/− mice. We conclude that in long bones, TRAP is critical for the development of the growth plate and the metaphysis but apparently not for the epiphyseal vascularization, excavation, and establishment of the SOC.

A mechanobiological model of epiphysis structures formation

Developing bone consists of epiphysis, metaphysis and diaphysis. The secondary ossification centre (SOC) appears and grows within the epiphysis, involving two histological stages. Firstly, cartilage canals appear; they carry hypertrophy factors towards the central area of the epiphysis. Canal growth and expansion is modulated by stress on the epiphysis. Secondly, the diffusion of hypertrophy factors causes SOC growth. Hypertrophy is regulated by biological and mechanical factors present within the epiphysis. The finite element method has been used for solving a coupled system of differential equations for modelling these histological stages of epiphyseal development. Cartilage canal spatial-temporal growth patterns were obtained as well as the SOC formation pattern. This model qualitatively agreed with experimental results reported by other authors.

Don't hedge your bets: hedgehog signaling as a central mediator of endochondral bone development and cartilage diseases

Cell differentiation and patterning are vital processes in the development of the appendicular skeleton. The hedgehog (Hh) signaling pathway plays a central role in regulating the anterior-posterior axis of the distal limb as well as the length of endochondral bones. Ligand-induced Hh signaling inhibits the processing of the Gli transcription factors from activator to repressor isoforms. In the growth plate, Indian hedgehog inhibits Gli processing, resulting in accumulation of Gli activators that induce chondrocyte maturation and hypertrophic differentiation. Parathyroid hormone-like hormone promote and Gli processing to repressor forms, thus regulating the rate of hypertrophic differentiation. In cartilage diseases such as osteoarthritis and cartilage tumors, there is a recapitulation of developmental processes that involve increased Hh signaling. Studies have shown that pharmacological inhibitors of Hh signaling can attenuate the progression osteoarthritis and cartilage tumor growth. Thus, Hh blockade can serve as a potential therapy for the treatment of various cartilage diseases.

Origin and growth of otosclerosis

CONCLUSION

The external layer of the otic capsule arises from periosteal osteoblasts, which produce large numbers of Volkmann's canals as well as lamellar bone. The main plaque of otosclerosis is a histologic replica of the external layer and seems to arise from similar cells in the periosteum and to follow a defined invasive course into the footplate of the stapes, the basal coil of the cochlea and the saccule.

OBJECTIVES

To determine by histologic study of the developing otic capsule and temporal bones with otosclerosis the site, tissue of origin, and pathways of growth of the disease.

METHODS

Step sections of 60 celloidin-embedded temporal bones from fetuses and 24 from patients aged between 1 and 52 years were examined in the study of the development of the otic capsule. Step sections of 65 temporal bones each with 2 or more deposits of otosclerosis were surveyed to obtain data on the site, tissue of origin, and pathways of its growth.

RESULTS

The otic capsule differs from other bones in that the formation of the ultimate lamellar bone tissue is accompanied by very numerous intercommunicating channels. In the middle (cartilage origin) layer these are chondro-osseous canals and Volkmann's canals (like Haversian canals, but multidirectional). In the external (periosteal origin) layer these are Volkmann's canals only. In all, 63 of the 65 temporal bones with otosclerosis that were studied showed a prominent posterior otic capsule plaque. Evidence that this is derived from the periosteum of the external canal is as follows. (a) The otosclerotic tissue of the plaque--like all otosclerotic tissue--is composed of Volkmann's canals and lamellar bone only, as does external layer tissue. (b) All posterior plaques have an edge at the periosteum bordering the processus cochleariformis and tensor tympani muscle. The presumed invasive edge of the plaque on the opposite (cochlear) side shows a variable level of its front. (c) The tissue on the cochlear side of the plaque has a darkly stained appearance with large numbers of osteoblasts and poorly differentiated Volkmann's canals, suggesting that this is an invasive front. The otosclerosis becomes progressively better differentiated away from the darkly stained zone, indicating increasing maturation, which is greatest in the suggested origin of the plaque at the processus/tensor tympani muscle region because this would be the oldest region of the plaque. The pathway of the growth indicated by this study suggests a possible time sequence in the symptomatology of otosclerosis as it moves first to stapes footplate and then through the spiral ligament of cochlea to the saccule. An anterior plaque was seen in 42 of the 65 temporal bones with multiple sites of otosclerosis examined. These showed features similar to those listed above for the posterior plaque, with a base on the periosteum bordering the canal for the internal carotid artery, dark zonation at the invasive front near the cochlea, and increasing differentiation towards the base.

Dynamic loading of immature epiphyseal cartilage pumps nutrients out of vascular canals

The potential influence of mechanical loading on transvascular transport in vascularized soft tissues has not been explored extensively. This experimental investigation introduced and explored the hypothesis that dynamic mechanical loading can pump solutes out of blood vessels and into the surrounding tissue, leading to faster uptake and higher solute concentrations than could otherwise be achieved under unloaded conditions. Immature epiphyseal cartilage was used as a model tissue system, with fluorescein (332 Da), dextran (3, 10, and 70 kDa) and transferrin (80 kDa) as model solutes. Cartilage disks were either dynamically loaded (± 10% compression over a 10% static offset strain, at 0.2 Hz) or maintained unloaded in solution for up to 20 h. Results demonstrated statistically significant solute uptake in dynamically loaded (DL) explants relative to passive diffusion (PD) controls for all solutes except unbound fluorescein, as evidenced by the DL:PD concentration ratios after 20 h (1.0 ± 0.2, 2.4 ± 1.1, 6.1 ± 3.3, 9.0 ± 4.0, and 5.5 ± 1.6 for fluorescein, 3, 10, and 70 kDa dextran, and transferrin). Significant uptake enhancements were also observed within the first 30s of loading. Termination of dynamic loading produced dissipation of enhanced solute uptake back to PD control values. Confocal images confirmed that solute uptake occurred from cartilage canals into their surrounding extracellular matrix. The incidence of this loading-induced transvascular solute pumping mechanism may significantly alter our understanding of the interaction of mechanical loading and tissue metabolism.

Modulating endochondral ossification of multipotent stromal cells for bone regeneration

For years it has been recognized that engineering of large bone constructs will be feasible only if the hurdle of vascularization is overcome. Attempts to engineer bone tissue have predominantly focused on intramembranous (direct) bone formation. A relatively new and most likely more physiological approach in this line is endochondral bone formation, comprising an intermediate cartilaginous stage. Cartilage in nature is an avascular tissue and its cells are equipped to survive the poor oxygenation and nutritional conditions inherent to implanted tissues. Subsequent terminal differentiation (hypertrophy) of the chondrocytes initiates the formation of a mineralized matrix that will then be converted into bone. Through this mechanism, our long bones grow and most fractures heal through the process of secondary fracture healing. The feasibility of the attractive concept of endochondral bone tissue engineering has already been shown. Most emphasis has gone to the multipotent stromal cells because of their great potential for expansion and differentiation and immunoprivileged nature. This review will focus on the promises and current status of this new field. Further, potent modulators of endochondral bone tissue engineering, including oxygen tension and mechanical stimuli, will be discussed.

Development of articular cartilage and the metaphyseal growth plate: the localization of TRAP cells, VEGF, and endostatin

During long bone development the original cartilaginous model in mammals is replaced by bone, but at the long bone endings the avascular articular cartilage remains. Before the articular cartilage attains structural maturity it undergoes reorganization, and molecules such as vascular endothelial growth factor (VEGF) and endostatin could be involved in this process. VEGF attracts blood vessels, whereas endostatin blocks their formation. The present study therefore focused on the spatio-temporal localization of these two molecules during the development of the articular cartilage. Furthermore, we investigated the distribution of the chondro/osteoclasts at the chondro-osseous junction of the articular cartilage with the subchondral bone. Mice served as our animal model, and we examined several postnatal stages of the femur starting with week (W) 4. Our results indicated that during the formation of the articular cartilage, VEGF and endostatin had an overlapping localization. The former molecule was, however, down-regulated, whereas the latter was uniformly intensely localized until W12. At the chondro-osseous junction, the number of tartrate-resistant acid phosphatase (TRAP)-positive chondro/osteoclasts declined with increasing age. Until W3 the articular cartilage was not well organized but at W8 it appeared structurally mature. At that time only a few TRAP cells were present, indicative of a low resorptive activity at the chondro-osseous junction. Subsequently, we examined the metaphyseal growth plate that is closed when skeletal maturity is attained. Within its hypertrophic zone, localization of endostatin and VEGF was observed until W6 and W8, respectively. At the chondro-osseous junction of the growth plate, chondro/osteoclasts remained numerous until W12 to allow for its complete resorption. According to former findings, VEGF is critical for a normal skeleton development, whereas endostatin has almost no effect on this process. In conclusion, our findings suggest that both VEGF and endostatin play a role in the structural reorganization of the articular cartilage and endostatin may be involved in the maintenance of its avascularity. In the growth plate, however, endostatin does not appear to counteract VEGF, allowing vascular invasion of hypertrophic cartilage and bone growth.

VEGF and its role in the early development of the long bone epiphysis

In long bones of murine species, undisturbed development of the epiphysis depends on the generation of vascularized cartilage canals shortly after birth. Despite its importance, it is still under discussion how this event is exactly regulated. It was suggested previously that, following increased hypoxia in the epiphyseal core, angiogenic factors are expressed and hence stimulate the ingrowth of the vascularized canals. In the present study, we tested this model and examined the spatio-temporal distribution of two angiogenic molecules during early development in mice. In addition, we investigated the onset of cartilage hypertrophy and mineralization. Our results provide evidence that the vascular endothelial growth factor is expressed in the epiphyseal resting cartilage prior to the moment of canal formation and is continuously expressed until the establishment of a large secondary ossification centre. Interestingly, we found no expression of secretoneurin before the establishment of the canals although this factor attracts blood vessels under hypoxic conditions. Epiphyseal development further involves maturation of the resting chondrocytes into hypertrophic ones, associated with the mineralization of the cartilage matrix and eventual death of the latter cells. Our results suggest that vascular endothelial growth factor is the critical molecule for the generation of the epiphyseal vascular network in mice long bones. Secretoneurin, however, does not appear to be a player in this event. Hypertrophic chondrocytes undergo cell death by a mechanism interpreted as chondroptosis.

Three-dimensional cortical bone microstructure in a rat model of hypoxia-induced growth retardation

Little is known about hypoxia-induced modification of the canal network in the cortical bone despite its involvement in intracortical vascularity and bone blood supply. In this study, we examined the effect of chronic hypoxia on the canal network in postnatal bone. Tibiae were harvested from 4- and 8-week-old rats (hyp-4 and -8, n = 8 each), whose growth was retarded owing to postnatal exposure to hypoxia (12-14% O₂), and from 3- and 4-week-old normoxic rats (cnt-4 and -5, n = 8 each), which were similar in tibial length and cortical cross-sectional area to hyp-4 and -8, respectively. The diaphyseal canals were detected by monochromatic synchrotron radiation CT with a 3.1-μm voxel resolution. The anatomical properties of the canal network were compared between age- or size-matched hypoxic and normoxic groups. The canals were larger in diameter, were more densely distributed and connected, and opened into the marrow cavity with a higher density in hyp-4 than in cnt-4. The canal density and connectivity were also higher in hyp-4 than in cnt-3. The canal diameter, density, and connectivity were smaller in hyp-8 than in cnt-4; however, the densities of endocortical and periosteal canal openings did not differ between hyp-8 and cnt-4. We concluded that chronic hypoxia enhanced the formation of cortical canal networks at the postnatal developmental stage, probably facilitating intra- and transcortical vascularization and bone perfusion accordingly.

Delayed bone age due to a dual effect of FGFR3 mutation in Achondroplasia

Achondroplasia (ACH), the most common form of human dwarfism is caused by a mutation in the Fibroblast Growth Factor Receptor 3 (FGFR3) gene, resulting in constitutive activation of the receptor. Typical radiological features include shortening of the tubular bones and macrocephaly, due to disruption of endochondral ossification. Consequently, FGFR3 has been described as a negative regulator of bone growth. Studying a large cohort of ACH patients, a delay in bone age was observed shortly after birth (for boys p=2.6×10(-9) and for girls p=1.2×10(-8)). This delay was no longer apparent during adolescence. In order to gain further insight into bone formation, bone development was studied in a murine model of chondrodysplasia (Fgfr3(Y367C/+)) from birth to 6weeks of age. Delayed bone age was also observed in Fgfr3(Y367C/+) mice at 1week of age followed by an accelerated secondary ossification center formation. A low level of chondrocyte proliferation was observed in the normal growth plate at birth, which increased with bone growth. In the pathological condition, a significantly high level of proliferative cells was present at birth, but exhibited a transient decrease only to rise again subsequently. Histological and in situ analyses suggested the altered endochondral ossification process may result from delayed chondrocyte differentiation, disruption of vascularization and osteoblast invasion of the femur. All these data provide evidence that FGFR3 regulates normal chondrocyte proliferation and differentiation during bone growth and suggest that constitutive activation of the receptor disrupts both processes. Therefore, the consequences of FGFR3 activation on the physiological process of bone development appear to be dependent on spatial and temporal occurrence. In conclusion, these observations support the notion that FGFR3 has a dual effect, as both a negative and a positive regulator of the endochondral ossification process during post-natal bone development.

Phenotypic diversity in chondromyxoid fibroma reveals differentiation pattern of tumor mimicking fetal cartilage canals development: an immunohistochemical study

Chondromyxoid fibroma represents a rare benign cartilaginous tumor of young patients occurring in a subcortical metaphyseal location. The histogenesis of chondromyxoid fibroma has not yet been postulated, even though the conventional histology and recent immunohistochemical studies on phenotype of the mesenchymal cells and extracellular matrix components suggested its origin in immature cartilage. Therefore, we wished to compare the morphological pattern of immature cartilage tissue with chondromyxoid fibroma to investigate a possible developmental counterpart of chondromyxoid fibroma. Archival paraffin-embedded tissues from 4 fetal femora and 10 cases of chondromyxoid fibroma were analyzed simultaneously using histochemistry (safranin O) and established immunohistochemical antibodies (CD34, CD163, and smooth muscle actin). Vascularized cartilage canals growing into the fetal cartilage from the perichondrium displayed characteristic glomeruloid structures with central arterioles within the immature mesenchymal stroma and numerous superficial sinusoidal blood vessels accompanied by macrophage infiltration. Similarly, each case of chondromyxoid fibroma demonstrated admixture of two characteristic components: immature fibrous tissue of vascularized stroma with accumulation of macrophages in areas of superficial sinusoidal proliferation, and variable amounts of lobulated chondroid tissue. Based on the observed substantial morphological similarity between the cartilage canals and chondromyxoid fibroma, we suggest that the chondromyxoid fibroma represents a neoplasm originating from or mimicking the fetal cartilage canals within the immature cartilage.

A mathematical model of epiphyseal development: hypothesis on the cartilage canals growth

The role of cartilage canals is to transport nutrients and biological factors that cause the appearance of the secondary ossification centre (SOC). The SOC appears in the centre of the epiphysis of long bones. The canal development is a complex interaction between mechanical and biological factors that guide its expansion into the centre of the epiphysis. This article introduces the 'Hypothesis on the growth of cartilage canals'. Here, we have considered that the development of these canals is an essential event for the appearance of SOC. Moreover, it is also considered to be important for the transport of molecular factors (RUNX2 and MMP9) at the ends of such canals. Once the canals are merged in the centre of the epiphysis, these factors are released causing hypertrophy of adjacent cells. This RUNX2 and MMP9 release occurs due to the action of mechanical loads that supports the epiphysis. In order to test this hypothesis, we use a hybrid approach using the finite element method to simulate the mechanical stresses present in the epiphysis and the cellular automata to simulate the expansion of the canals and the hypertrophy factors pathway. By using this hybrid approach, we have obtained as a result the spatial-temporal patterns for the growth of cartilage canals and hypertrophy factors within the epiphysis. The model is in qualitative agreement with experimental results previously reported by other authors. Thus, we conclude that this model may be used as a methodological basis to present a complete mathematical model of the processes involved in epiphyseal development.

The intravestibular source of the vestibular aqueduct. II: its structure and function clarified by a developmental study of the intra-skeletal channels of the otic capsule

CONCLUSION

A developmental histologic study of the otic capsule indicates that it grows a system of lamellar bone with abundant interconnecting intraosseous channels. These include the 'cartilage canals' in the cartilage model, the chondro-osseous and Haversian-like (Volkmann's) canals in the ossified otic capsule, the fissula ante fenestram, which seems to function as a lifelong manufacturer of the latter two channels, and the inner layer (vestibular arch) of the vestibular aqueduct, which is a complex series of Volkmann's canals and microcanals. Chemical changes, possibly produced by breakdown of cells within the channels, may provide a homeostatic environment for the functions of hearing and balance that take place in the endolymphatic fluid.

OBJECTIVES

We studied the development of the otic capsule to clarify the cellular appearances that we had previously described in the normal vestibular arch and the changes in that structure in Ménière's disease.

METHODS

Step sections from 84 temporal bones, including those from fetuses, children and adults from a variety of ages were examined histologically.

RESULTS

Cartilage canals, bringing blood vessels and mesenchymal cells from perichondrium to the depths of the cartilage model to mediate ossification, are found early in fetal life and disappear when ossification is complete at about 24 weeks. The otic capsule is formed of chondro-osseous canals, which are composed of trabeculae of mineralized cartilage lacunae containing mesenchymal cells that undergo ossification (globuli ossei); also Volkmann's canals (like Haversian canals in long bones but multidirectional), which are produced from osteoblasts. The lumina of the latter frequently link up with chondro-osseous canals. Lamellar bone forms the background of the otic capsule. The fissula ante fenestram is present from early in the cartilage model and then throughout life. It appears to mediate bone production and the new formation of chondro-osseous channels and Volkmann's canals. The internal layer of the vestibular aqueduct (vestibular arch) is seen in the cartilage model of the otic capsule (present in early fetal life) as a vascular layer of perichondrally derived connective tissue (not cartilage) surrounding the endolymphatic duct. When endochondral ossification starts, the bone from the adjoining cochlear and vestibular sides embrace this connective tissue layer to form the outer bony layer of the vestibular aqueduct. Osteoblasts then fill the inner layer with lamellar bone and macro- and mini-Volkmann's canals. At 1 year osteoblasts in the walls of macro-Volkmann's canals, proliferating thereafter throughout life, produce large numbers of microcanals. It is possible that slow breakdown of these osteoblasts and of similar cells in the canals of the otic capsule proper may contribute to the homeostasis of the endolymphatic duct and that of the rest of the membranous labyrinth, respectively.

The role of Akt1 in terminal stages of endochondral bone formation: angiogenesis and ossification

Longitudinal bone growth is the result of endochondral bone formation which takes place in the growth plate. The rate of chondrocyte proliferation and hypertrophy, vascular invasion with the formation of primary ossification centers and cartilage replacement by bone tissue are all important processes required for normal growth. We have shown a role for the PI3K signaling pathway in chondrocyte hypertrophy and bone growth in tibia explant cultures. In this current study, we aimed to investigate the role of Akt1, an important target of PI3K, in endochondral ossification. Akt1 KO mice showed reduced size compared to their littermates throughout life, but the largest difference in body size was observed around 1 week of age. Focusing on this specific developmental stage, we discovered delayed secondary ossification in the long bones of Akt1 KO mice. A delay in formation of a structure resembling a secondary ossification center was also seen in tibia organ cultures treated with the PI3K inhibitor LY294002. The expression of matrix metalloproteinase-14 (MMP-14), the main protease responsible for development of secondary ossification centers, was decreased in the epiphysis of Akt1 KO mice, possibly explaining the delay in secondary ossification centers seen in the Akt1 KO mice. Bone mineral density (BMD) and bone mineral content (BMC) measured in the proximal tibia of 1-year-old mice were decreased in Akt1 KO mice, suggesting that the original delay in ossification might affect bone quality in older animals.

Differential expression of vascular endothelial growth factor in glucocorticoid-related osteonecrosis of the femoral head

VEGF plays a role in bone remodeling. Ingrowth of reparative arterioles can be observed in late-stage osteonecrosis. To explore the reparative processes, we quantified the most important angiogenesis factor (VEGF) in different zones of late-stage glucocorticoid-induced osteonecrosis. We treated primary nonosteonecrosis osteoblasts with glucocorticoids in vitro as a model for the bone cells in early-stage steroid-related osteonecrosis. We obtained six late-stage (ARCO Stage IV) osteonecrosis femoral heads and six osteoarthritic femoral heads during THA. The expression of vascular endothelial growth factor was analyzed by reverse-transcription PCR, ELISA, or immunohistochemistry. Osteoblasts from the reactive interface (penumbra) of osteonecrosis femoral heads exhibited increased immunoreactivity to VEGF compared to those from the periphery. ELISA confirmed VEGF upregulation in the penumbra from osteonecrosis femoral heads. Primary osteoblasts derived from osteoarthritic femoral heads exhibited downregulation of VEGF after 24 hours of coincubation with glucocorticoids. The increase in VEGF in the reactive interface (penumbra) of osteonecrosis in late-stage femoral head may reflect a secondary phenomenon. The observed high amount of VEGF in later-stage osteonecrosis might stimulate the ingrowth of reparative arterioles into the femoral head.

TIP39/parathyroid hormone type 2 receptor signaling is a potent inhibitor of chondrocyte proliferation and differentiation

Tuberoinfundibular peptide of 39 residues (TIP39) is a member of the parathyroid hormone (PTH) family of peptide hormones that exerts its function by interacting with the PTH type 2 receptor (PTH2R). Presently, no known function has been attributed to this signaling pathway in the developing skeleton. We observed that TIP39 and PTH2R were present in the newborn mouse growth plate, with the receptor localizing in the resting zone whereas ligand expression was restricted exclusively in prehypertrophic and hypertrophic chondrocytes. By 8 wk of life, PTH2R, and to a lesser degree TIP39, immunoreactivity was present in articular chondrocytes. We therefore sought to investigate the role of TIP39/PTH2R signaling in chondrocytes by generating stably transfected CFK2 chondrocytic cells overexpressing PTH2R (CFK2R). TIP39 treatment of CFK2R clones in culture inhibited their proliferation by restricting cells at the G(0)/G(1) phase of the cell cycle, coupled with decreased expression and activity of cyclin-dependent kinases Cdk2 and Cdk4, while p21, an inhibitor of Cdks, was upregulated. In addition, TIP39 treatment decreased expression of differentiation markers in these cells associated with marked alterations in extracellular matrix and metalloproteinase expression. Transcription of Sox9, the master regulator of cartilage differentiation, was reduced in TIP39-treated CFK2R clones. Moreover, Sox9 promoter activity, as measured by luciferase reporter assay, was markedly diminished after TIP39 treatment. In summary, our results show that TIP39/PTH2R signaling inhibits proliferation and alters differentiation of chondrocytes by modulating SOX9 expression, thereby substantiating the functional significance of this signaling pathway in chondrocyte biology.

Localization of tartrate-resistant acid phosphatase (TRAP), membrane type-1 matrix metalloproteinases (MT1-MMP) and macrophages during early endochondral bone formation

Endochondral bone formation, the process by which most parts of our skeleton evolve, leads to the establishment of the diaphyseal primary (POC) and epiphyseal secondary ossification centre (SOC) in long bones. An essential event for the development of the SOC is the early generation of vascularized cartilage canals that requires the proteolytic cleavage of the cartilaginous matrix. This in turn will allow the canals to grow into the epiphysis. In the present study we therefore initially investigated which enzymes and types of cells are involved in this process. We have chosen the mouse as an animal model and focused our studies on the distal part of the femur during early stages after birth. The formation of the cartilage canals was promoted by tartrate-resistant acid phosphatase (TRAP) and membrane type-1 matrix metalloproteinases (MT1-MMP). In addition, macrophages and cells containing numerous lysosomes contributed to the establishment of the canals and enabled their further advancement into the epiphysis. As development continued, the SOC was formed, and in mice aged 10 days a distinct layer of type I collagen (= osteoid) was laid down onto the cartilage scaffold. The events leading to the establishment of the SOC were compared with those of the POC. Basically these processes were quite similar, and in both ossification centers, TRAP-positive chondroclasts resorbed the cartilage matrix. However, occasionally co-expression of TRAP and MT1-MMP was noted in a small subpopulation of this cell type. Furthermore, numerous osteoblasts expressed MT1-MMP from the start of endochondral ossification, whereas others did not. In osteocytogenesis, MT1-MMP has been shown to be critical for the establishment of the cytoplasmic processes mediating the communication between osteocytes and bone-lining cells. Considering the well-known fact that not all osteoblasts transform into osteocytes, and in accordance with the present data, we suggest that MT1-MMP is needed at the very beginning of osteocytogenesis and may additionally determine whether an osteoblast further differentiates into an osteocyte.