Pathogenesis of vitamin (A and D)-induced premature growth-plate closure in calves

Histological Analysis of Bone Repair in Mandibular Body Osteotomy Using Internal Fixation System in Three Different Gaps without Bone Graft in an Animal Model

The aim was to analyze histologically the bone repair in a mandibular osteotomy model with different gaps between the segments. Nine male rabbits who underwent osteotomies on the mandibular body were fixed with a 1.5 system plate and no bone graft; group 1 (2 mm gap between segments), group 2 (5 mm gap between segments), and group 3 (8 mm gap between segments) were included. After 8 weeks they were euthanized and the sample was processed for histological analysis. Group 1 showed advanced bone repair with cartilaginous tissue and cancellous bone, showing osteoblasts and type III collagenous fibers. In group 2, a more delayed ossification was observed, with an extensive area of peripheral ossifying cartilage and chondrocytes in greater number at the center of the defect; group 3 showed no evidence of ossification with fibrous tissue, a very low level of chondrocytes, and some bone sequestrate. We can conclude that, in this animal model, 2 or 5 mm gap in the osteotomy could be repaired as bone when fixation is used. The size of the gap is an important factor for the use of bone grafts considering endochondral ossification. This model can be used for graft analysis and related technologies.

Evaluating growth failure with diffusion tensor imaging in pediatric survivors of high-risk neuroblastoma treated with high-dose cis-retinoic acid

BACKGROUND

The survival of patients with high-risk neuroblastoma has increased with multimodal therapy, but most survivors demonstrate growth failure.

OBJECTIVE

To assess physeal abnormalities in children with high-risk neuroblastoma in comparison to normal controls by using diffusion tensor imaging (DTI) of the distal femoral physis and adjacent metaphysis.

MATERIALS AND METHODS

We prospectively obtained physeal DTI at 3.0 T in 20 subjects (mean age: 12.4 years, 7 females) with high-risk neuroblastoma treated with high-dose cis-retinoic acid, and 20 age- and gender-matched controls. We compared fractional anisotropy (FA), normalized tract volume (cm³/cm²) and tract concentration (tracts/cm²) between the groups, in relation to height Z-score and response to growth hormone therapy. Tractography images were evaluated qualitatively.

RESULTS

DTI parameters were significantly lower in high-risk neuroblastoma survivors compared to controls (P<0.01), particularly if the patients were exposed to both cis-retinoic acid and total body irradiation (P<0.05). For survivors and controls, DTI values were respectively [mean ± standard deviation]: tract concentration (tracts/cm²), 23.2±14.7 and 36.7±10.5; normalized tract volume (cm³/cm²), 0.44±0.27 and 0.70±0.21, and FA, 0.22±0.05 and 0.26±0.02. High-risk neuroblastoma survivors responding to growth hormone compared to non-responders had higher FA (0.25±0.04 and 0.18±0.03, respectively, P=0.02), and tract concentration (tracts/cm²) (31.4±13.7 and 14.8±7.9, respectively, P<0.05). FA, normalized tract volume and tract concentration were linearly related to height Z-score (R²>0.31; P<0.001). Qualitatively, tracts were nearly absent in all non-responders to growth hormone and abundant in all responders (P=0.02).

CONCLUSION

DTI shows physeal abnormalities that correlate with short stature in high-risk neuroblastoma survivors and demonstrates response to growth hormone treatment.

Bone physiology as inspiration for tissue regenerative therapies

The development, maintenance of healthy bone and regeneration of injured tissue in the human body comprise a set of intricate and finely coordinated processes. However, an analysis of current bone regeneration strategies shows that only a small fraction of well-reported bone biology aspects has been used as inspiration and transposed into the development of therapeutic products. Specific topics that include inter-scale bone structural organization, developmental aspects of bone morphogenesis, bone repair mechanisms, role of specific cells and heterotypic cell contact in the bone niche (including vascularization networks and immune system cells), cell-cell direct and soluble-mediated contact, extracellular matrix composition (with particular focus on the non-soluble fraction of proteins), as well as mechanical aspects of native bone will be the main reviewed topics. In this Review we suggest a systematic parallelization of (i) fundamental well-established biology of bone, (ii) updated and recent advances on the understanding of biological phenomena occurring in native and injured tissue, and (iii) critical discussion of how those individual aspects have been translated into tissue regeneration strategies using biomaterials and other tissue engineering approaches. We aim at presenting a perspective on unexplored aspects of bone physiology and how they could be translated into innovative regeneration-driven concepts.

Exogenous Vitamin D signaling alters skeletal patterning, differentiation, and tissue integration during limb regeneration in the axolotl

Urodele amphibians such as the axolotl regenerate complete limbs as adults, and understanding how the "blueprint", or pattern, of the regenerate is established and manipulated are areas of intense interest. Nutrient signaling plays an important role in pattern formation during regeneration. Retinoic acid signaling is the most characterized pathway during this process. Exogenous retinoic acid (RA) reprograms the pattern information in regenerating cells to a more posterior, ventral, and proximal identity. Vitamin D signaling shares several molecular similarities with RA and has been shown to alter pattern formation during zebrafish pectoral fin regeneration. To determine if exogenous Vitamin D signaling is capable of reprograming pattern in the axolotl limb blastema, we treated regenerating limbs with a potent Vitamin D agonist. Under the studied conditions, exogenous Vitamin D did not act in a manner similar to RA and failed to proximalize the pattern of the resulting regenerates. The Vitamin D treatment did result in several skeletal defects during regeneration, including carpal fusions along the A/P axis; failure to integrate the newly regenerated tissue with the existing tissue, formation of ectopic nodules of cartilage at the site of amputation, and altered bone morphology in uninjured skeletal tissue.

Approach to Investigating Congenital Skeletal Abnormalities in Livestock

Congenital skeletal abnormalities may be genetic, teratogenic, or nutritional in origin; distinguishing among these different causes is essential in the management of the disease but may be challenging. In some cases, teratogenic or nutritional causes of skeletal abnormalities may appear very similar to genetic causes. For example, chondrodysplasia associated with intrauterine zinc or manganese deficiency and mild forms of hereditary chondrodysplasia have very similar clinical features and histologic lesions. Therefore, historical data are essential in any attempt to distinguish genetic and acquired causes of skeletal lesions; as many animals as possible should be examined; and samples should be collected for future analysis, such as genetic testing. Acquired causes of defects often show substantial variation in presentation and may improve with time, while genetic causes frequently have a consistent presentation. If a disease is determined to be of genetic origin, a number of approaches may be used to detect mutations, each with advantages and disadvantages. These approaches include sequencing candidate genes, single-nucleotide polymorphism array with genomewide association studies, and exome or whole genome sequencing. Despite advances in technology and increased cost-effectiveness of these techniques, a good clinical history and description of the pathology and a reliable diagnosis are still key components of any investigation.

Accelerated growth plate mineralization and foreshortened proximal limb bones in fetuin-A knockout mice

The plasma protein fetuin-A/alpha2-HS-glycoprotein (genetic symbol Ahsg) is a systemic inhibitor of extraskeletal mineralization, which is best underscored by the excessive mineral deposition found in various tissues of fetuin-A deficient mice on the calcification-prone genetic background DBA/2. Fetuin-A is known to accumulate in the bone matrix thus an effect of fetuin-A on skeletal mineralization is expected. We examined the bones of fetuin-A deficient mice maintained on a C57BL/6 genetic background to avoid bone disease secondary to renal calcification. Here, we show that fetuin-A deficient mice display normal trabecular bone mass in the spine, but increased cortical thickness in the femur. Bone material properties, as well as mineral and collagen characteristics of cortical bone were unaffected by the absence of fetuin-A. In contrast, the long bones especially proximal limb bones were severely stunted in fetuin-A deficient mice compared to wildtype littermates, resulting in increased biomechanical stability of fetuin-A deficient femora in three-point-bending tests. Elevated backscattered electron signal intensities reflected an increased mineral content in the growth plates of fetuin-A deficient long bones, corroborating its physiological role as an inhibitor of excessive mineralization in the growth plate cartilage matrix - a site of vigorous physiological mineralization. We show that in the case of fetuin-A deficiency, active mineralization inhibition is a necessity for proper long bone growth.

An improved collagen scaffold for skeletal regeneration

Bone repair and regeneration is one of the most extensively studied areas in the field of tissue engineering. All of the current tissue engineering approaches to create bone focus on intramembranous ossification, ignoring the other mechanism of bone formation, endochondral ossification. We propose to create a transient cartilage template in vitro, which could serve as an intermediate for bone formation by the endochondral mechanism once implanted in vivo. The goals of the study are (1) to prepare and characterize type I collagen sponges as a scaffold for the cartilage template, and (2) to establish a method of culturing chondrocytes in type I collagen sponges and induce cell maturation. Collagen sponges were generated from a 1% solution of type I collagen using a freeze/dry technique followed by UV light crosslinking. Chondrocytes isolated from two locations in chick embryo sterna were cultured in these sponges and treated with retinoic acid to induce chondrocyte maturation and extracellular matrix deposition. Material strength testing as well as microscopic and biochemical analyzes were conducted to evaluate the properties of sponges and cell behavior during the culture period. We found that our collagen sponges presented improved stiffness and supported chondrocyte attachment and proliferation. Cells underwent maturation, depositing an abundant extracellular matrix throughout the scaffold, expressing high levels of type X collagen, type I collagen and alkaline phosphatase. These results demonstrate that we have created a transient cartilage template with potential to direct endochondral bone formation after implantation.

Biglycan regulates the expression of EGF receptors through EGF signaling pathways in human articular chondrocytes

Biglycan is a member of the family of small leucine-rich proteoglycans. It is an important structural component of articular cartilage and participates in the assembly of the chondrocyte extracellular matrix through formation of protein interactions with collagen type VI and large proteoglycan aggregates. Biglycan also possesses signaling properties. In articular chondrocytes, short-term activation of epidermal growth factor receptors (EGFR) with biglycan initiated mitogen-activated protein kinase and phosphatidylinositol 3-kinase (PI3K) signaling events, similar to the effect of epidermal growth factor (EGF) observed in other cell types. The extent and duration of intracellular signaling resolves biological effects initiated by EGFR stimulation, thus, establishing cell fate. In this study, we elucidate a novel regulatory mechanism of EGFR expression in human articular chondrocytes that is modulated by prolonged biglycan treatment and is in contrast to changes detected in the expression of EGFR following EGF stimulation. Treatment of chondrocytes for 24 hr with biglycan upregulated EGFR mRNA and protein expression, whereas treatment with EGF downregulated EGFR message and protein levels. Biglycan and EGF treatment protracted extracellular signal-regulated kinases (ERK1/2) and Akt phosphorylation, albeit to different extents. Mechanistic studies with mitogen-activated protein kinase and phosphatidylinositol 3-kinase pathway-specific inhibitors revealed that biglycan and EGF distinctly modulate the expression of EGFR in chondrocytes. Biglycan promoted the coactivation of ERK1/2 and Akt, however, phosphorylated Akt induced a prolonged inhibition of ERK1/2. Consequently, total EGFR mRNA and protein expression was increased. This regulatory mechanism contrasts the modulation of EGFR expression by exogenous EGF, which strongly protracts ERK1/2 activation, therefore, inducing a decrease of EGFR message and protein levels. Thus, biglycan might impinge on the expression of total EGFR and possibly, on the cell-surface expression of the receptors. These observations suggest that biglycan might play a critical role in the regulation of chondrocyte and pericellular matrix homeostasis.

Hypervitaminosis A-induced premature closure of epiphyses (physeal obliteration) in humans and calves (hyena disease): a historical review of the human and veterinary literature

Vitamin A toxicity in the infant, which now occurs rarely from dietary overdosage, was recognized in the 1940s as painful periostitis with rare progression to premature closure of the lower limb epiphyses. Decades later, most cases of vitamin A-induced premature epiphyseal closure (physeal obliteration) occur in pediatric dermatologic patients given vitamin A analogues. This phenomenon resembles a strange disease discovered in more recent years in calves with closed epiphyses of the hind limbs, known as hyena disease. This was a mystery until proved to be caused by vitamin A toxicity from enriched grain that causes the calves to have short hind limbs that resemble those of a hyena and gait disturbance. This historical review links the human and veterinary literature in terms of vitamin A-induced epiphyseal closure using a case report format of a 16-month-old human infant with closed knee epiphyses and gait disturbance that is reminiscent of hyena disease seen in calves.

Matrix and gene expression in the rat cranial base growth plate

Recent data have shown that the proliferation and differentiation of the cranial base growth plate (CBGP) chondrocytes are modulated by mechanical stresses. However, little is known about the expression of genes and matrix molecules in the CBGP during development or under mechanical stresses. The objective of the present study was to determine whether several cartilage- and bone-related molecules are expressed in the CBGP and whether their expression is modulated by cyclic loading. The CBGP of normal 8-day-old rats (n=8) were isolated immediately after death, followed by extraction of total RNA and reverse transcription/polymerase chain reaction (RT-PCR) analysis. All studied genes, including type II and X collagens, biglycan, versican, osteocalcin, osteopontin, and fetal liver kinase 1, were expressed in the CBGP with a reproducible absence of decorin mRNA. In age- and sex-matched rats (n=10), exogenous cyclic forces were applied to the maxilla at 500 mN and 4 Hz for 20 min/day over 2 days, followed by RNA isolation and RT-PCR analysis. This exogenous cyclic loading consistently induced the expression of the decorin gene, which was non-detectable, by the current RT-PCR approach, in control neonatal CBGPs without loading. Immunolocalization of several of the above-studied gene products demonstrated their remarkable site-specific expression. Decorin proteoglycan was primarily expressed in the perichondrium instead of various cartilage growth zones, especially upon mechanical loading. These findings serve as baseline data for the expression of several genes and gene products in the neonatal CBGP. Mechanical modulation of decorin expression is consistent with recent reports of its susceptibility to mechanical loading in several connective tissues.

Discovery of sonic hedgehog expression in postnatal growth plate chondrocytes: differential regulation of sonic and Indian hedgehog by retinoic acid

Sonic hedgehog (Shh) is a key signal protein in early embryological patterning of limb bud development. Its analog, Indian hedgehog (Ihh), primarily expressed during early cartilage development in prehypertrophic chondrocytes, regulates proliferation and suppresses terminal differentiation of postnatal growth plate (GP) chondrocytes. We report here for the first time that both Shh and Ihh mRNA are expressed in the GP of rapidly growing 6-week-old broiler-strain chickens. They are also expressed in other tissues such as articular chondrocytes, kidney, and bone. In situ hybridization and RT-PCR analyses reveal Shh in all zones of the GP, with peak expression in late hypertrophy. Using primary cultures of GP chondrocytes in serum-containing medium, we followed the patterns of Shh and Ihh mRNA expression as the cultures matured and mineralized. We find a cyclical expression of both hedgehog genes during the early period of culture development between day 10 and 14; when one is elevated, the other tended to be suppressed, suggesting that the two hedgehogs may play complementary roles during GP development. Retinoic acid (RA), a powerful modulator of gene expression in cell differentiation, stimulates GP chondrocytes toward terminal differentiation, enhancing mineral formation. We find that RA strongly suppresses Ihh, but enhances expression of Shh in this system. While Ihh suppresses maturation of GP chondrocytes to hypertrophy, we hypothesize that Shh acts to push these cells toward hypertrophy.

Extracellular Ca(2+)-sensing receptors modulate matrix production and mineralization in chondrogenic RCJ3.1C5.18 cells

Previous studies in chondrogenic RCJ3.1C5.18 (C5.18) cells showed that growth of these cells at high extracellular Ca(2+) concentrations ([Ca(2+)](o)) reduced the expression of markers of early chondrocyte differentiation. These studies addressed whether raising [Ca(2+)](o) accelerates C5.18 cell differentiation and whether Ca(2+) receptors (CaRs) are involved in coupling changes in [Ca(2+)](o) to cellular responses. We found that high [Ca(2+)](o) increased expression of osteopontin (OP), osteonectin, and osteocalcin, all markers of terminal differentiation, in C5.18 cells and increased the production of matrix mineral. Overexpression of wild-type CaR cDNA in C5.18 cells suppressed proteoglycan synthesis and aggrecan RNA, two early differentiation markers, and increased OP expression. The sensitivity of these parameters to changes in [Ca(2+)](o) was significantly increased, as indicated by left-shifted dose-responses. In contrast, stable expression of a signaling-defective CaR mutant (Phe707Trp CaR) in C5.18 cells, presumably through dominant-negative inhibition of endogenous CaRs, blocked the suppression of aggrecan RNA levels and proteoglycan accumulation and the enhancement of OP expression by high [Ca(2+)](o). These data support a role for CaRs in mediating high [Ca(2+)](o)-induced differentiation of C5.18 cells.

Both retinoic acid and 1,25(OH)2 vitamin D3 inhibit thyroid hormone-induced terminal differentiaton of growth plate chondrocytes

Thyroid hormone has been known for over 50 years to be a potent regulator of skeletal maturation at the growth plate. The receptor for thyroid hormone has been discovered to be a member of the nuclear hormone receptor superfamily. Retinoic acid and 1,25(OH)2 vitamin D3, whose receptors also belong to this nuclear hormone receptor family, have been implicated in the control of chondrocyte proliferation and differentiation at the growth plate. Recent studies demonstrate that the receptors for thyroid hormone, retinoic acid, and vitamin D bind to a similar DNA response element in the promoter region of target genes and may form heterodimers to regulate gene transcription in target cells. These observations led us to hypothesize that the retinoic acid and/or vitamin D signaling pathways may interact with thyroid hormone signaling at the molecular level to modulate growth plate chondrocyte differentiation. Using a chemically defined, serum-free model of growth plate chondrocyte maturation, both all-trans retinoic acid and 1,25(OH)2 vitamin D3 markedly inhibited thyroid hormone-induced terminal differentiation in a dose-dependent manner. In the absence of thyroid hormone, retinoic acid stimulated alkaline phosphatase activity modestly at the highest dose used, however neither retinoic acid nor 1,25(OH)2 vitamin D3 induced expression of type X collagen mRNA. We conclude that retinoic acid and vitamin D are likely to be antagonists of thyroid hormone signaling in the growth plate.

Structural basis for isotype selectivity of the human retinoic acid nuclear receptor

The human retinoic acid receptor (hRAR) belongs to the family of nuclear receptors that regulate transcription in a ligand-dependent way. The isotypes RARalpha,beta and gamma are distinct pharmacological targets for retinoids that are involved in the treatment of various skin diseases and cancers, in particular breast cancer and acute promyelocytic leukemia. Therefore, synthetic retinoids have been developed aiming at isotype selectivity and reduced side-effects. We report the crystal structures of three complexes of the hRARgamma ligand-binding domain (LBD) bound to agonist retinoids that possess selectivity either for RARgamma (BMS184394) or for RARbeta/gamma (CD564), or that are potent for all RAR-isotypes (panagonist BMS181156). The high resolution data (1.3-1. 5 A) provide a description at the atomic level of the ligand pocket revealing the molecular determinants for the different degrees of ligand selectivity. The comparison of the complexes of the chemically closely related retinoids BMS184394 and CD564 shows that the side-chain of Met272 adopts different conformations depending on the presence of a hydrogen bond between its sulfur atom and the ligand. This accounts for their different isotype selectivity. On the other hand, the difference between the pan- and the RARbeta, gamma-selective agonist is probably due to a steric discrimination at the level of the 2-naphthoic acid moiety of CD564. Based on this study, we propose a model for a complex with the RARgamma-specific agonist CD666 that shows the possible applications for structure-based drug design of RAR isotype-selective retinoids.

Retinoid signaling is required for chondrocyte maturation and endochondral bone formation during limb skeletogenesis

Retinoids have long been known to influence skeletogenesis but the specific roles played by these effectors and their nuclear receptors remain unclear. Thus, it is not known whether endogenous retinoids are present in developing skeletal elements, whether expression of the retinoic acid receptor (RAR) genes alpha, beta, and gamma changes during chondrocyte maturation, or how interference with retinoid signaling affects skeletogenesis. We found that immature chondrocytes present in stage 27 (Day 5.5) chick embryo humerus exhibited low and diffuse expression of RARalpha and gamma, while RARbeta expression was strong in perichondrium. Emergence of hypertrophic chondrocytes in Day 8-10 embryo limbs was accompanied by a marked and selective up-regulation of RARgamma gene expression. The RARgamma-rich type X collagen-expressing hypertrophic chondrocytes lay below metaphyseal prehypertrophic chondrocytes expressing Indian hedgehog (Ihh) and were followed by mineralizing chondrocytes undergoing endochondral ossification. Bioassays revealed that cartilaginous elements in Day 5.5, 8.5, and 10 chick embryo limbs all contained endogenous retinoids; strikingly, the perichondrial tissues surrounding the cartilages contained very large amounts of retinoids. Implantation of beads filled with retinoid antagonist Ro 41-5253 or AGN 193109 near the humeral anlagens in stage 21 (Day 3.5) or stage 27 chick embryos severely affected humerus development. In comparison to their normal counterparts, antagonist-treated humeri in Day 8.5-10 chick embryos were significantly shorter and abnormally bent; their diaphyseal chondrocytes had remained prehypertrophic Ihh-expressing cells, did not express RARgamma, and were not undergoing endochondral ossification. Interestingly, formation of an intramembranous bony collar around the diaphysis was not affected by antagonist treatment. Using chondrocyte cultures, we found that the antagonists effectively interfered with the ability of all-trans-retinoic acid to induce terminal cell maturation. The results provide clear evidence that retinoid-dependent and RAR-mediated mechanisms are required for completion of the chondrocyte maturation process and endochondral ossification in the developing limb. These mechanisms may be positively influenced by cooperative interactions between the chondrocytes and their retinoid-rich perichondrial tissues.

Retinol-binding protein is produced by rabbit chondrocytes and responds to parathyroid hormone (PTH)/PTH-related peptide-cyclic adenosine monophosphate pathway

PTH and dibutyryl cAMP [(Bu)2cAMP] induced the expression of a 19-kDa protein in the conditioned media of rabbit growth plate chondrocyte cultures. The 19-kDa protein was identified as plasma retinol-binding protein (RBP) by aminoterminal sequence analysis and immunoblot analysis with an anti-RBP monoclonal antibody. Northern blot analysis showed that PTH, PTH-related peptide (PTHrP), and (Bu)2cAMP increased the RBP messenger RNA (mRNA) level in chondrocyte cultures. Further, both PTH and (Bu)2cAMP markedly induced the expression of RBP mRNA by about 10-fold at 3 h and by about 40-fold at 24 h, indicating a pretranslational regulation. The level of the mRNA expression induced by PTH, PTHrP, and (Bu)2cAMP was as high as that by retinoic acid (RA), known as a potent inducer of RBP in hepatoma cells. RBP mRNA was also detected in cartilage tissues at higher levels than in the other tissues examined except liver. Both RBP and PTH/PTHrP inhibited the dedifferentiative activity of RA on growth plate chondrocytes when added to the culture medium. These results demonstrate that chondrocytes synthesize and secrete RBP in vivo and in vitro and suggest that PTH/PTHrP modulates the effect of RA by means of RBP production in chondrocytes.

Bone formation via cartilage models: the "borderline" chondrocyte

Increasing evidence substantiates the view that death is not necessarily the only fate of hypertrophic chondrocytes and that, when exposed to the right microenvironment, these cells can further differentiate to osteoblast-like cells and contribute to initial bone formation. In vitro, when replated as adherent cells in the presence of ascorbic acid, hypertrophic chondrocytes resume cell proliferation, switch from the synthesis of the cartilage-characteristic type II and X collagens to the synthesis of type I collagen, and organize a mineralizing bone matrix. In vivo, expression of bone specific markers by growth plate chondrocytes occurs initially in early hypertrophic cells located at the mid-diaphysis and directly facing the osteogenic perichondrium. In bones formed via cartilage models, the first mineralized bone matrix (the earliest bony collar preceding vascular invasion and the onset of endochondral bone formation) is deposited at the outer aspect of the mid-diaphysis between rows of early hypertrophic chondrocytes and osteoblasts, which are arranged in a peculiar "vis à vis" fashion. The "vis à vis" organization of perichondrial osteogenic cells and peripheral early hypertrophic chondrocytes suggests that the latter cells are exposed -- compared to their cognate, the central hypertrophic chondrocytes -- to a specific microenvironment composed of unique matrix-originating signals and cellular cross-talks. A major role in the differentiation control of, and interaction between, hypertrophic chondrocytes and osteogenic perichondrial cells is certainly played by the Indian Hedgehog/PTHrP signalling system. We propose that all early hypertrophic chondrocytes have the inherent potential to differentiate to osteoblast-like cells and to contribute to initial bone formation, but that only chondrocytes positioned at the "borderland" between cartilage and (non-cartilage) osteogenic tissues undergo further differentiation to bone producing cells. We call these hypertrophic chondrocytes "borderline chondrocytes" to emphasize both their specific location and their dual differentiation potential. Hypertrophic chondrocytes located in different cartilage areas are exposed to an inappropriate matrix and endocrine/paracrine environment, cannot differentiate to osteoblast-like cells and therefore undergo apoptosis.