Cellular turnover at the chondro-osseous junction of growth plate cartilage: analysis by serial sections at the light microscopical level

Hypertrophic chondrocytes at the junction of musculoskeletal structures

Hypertrophic chondrocytes are found at unique locations at the junction of skeletal tissues, cartilage growth plate, articular cartilage, enthesis and intervertebral discs. Their role in the skeleton is best understood in the process of endochondral ossification in development and bone fracture healing. Chondrocyte hypertrophy occurs in degenerative conditions such as osteoarthritis. Thus, the role of hypertrophic chondrocytes in skeletal biology and pathology is context dependent. This review will focus on hypertrophic chondrocytes in endochondral ossification, in which they exist in a transient state, but acting as a central regulator of differentiation, mineralization, vascularization and conversion to bone. The amazing journey of a chondrocyte from being entrapped in the extracellular matrix environment to becoming proliferative then hypertrophic will be discussed. Recent studies on the dynamic changes and plasticity of hypertrophic chondrocytes have provided new insights into how we view these cells, not as terminally differentiated but as cells that can dedifferentiate to more progenitor-like cells in a transition to osteoblasts and adipocytes, as well as a source of skeletal stem and progenitor cells residing in the bone marrow. This will provide a foundation for studies of hypertrophic chondrocytes at other skeletal sites in development, tissue maintenance, pathology and therapy.

New morphological evidence of the 'fate' of growth plate hypertrophic chondrocytes in the general context of endochondral ossification

The 'fate' of growth plate hypertrophic chondrocytes has been long debated with two opposing theories: cell apoptosis or survival with transformation into osteogenic cells. This study was carried out on the proximal tibial growth plate of rabbits using light microscopy, scanning and transmission electron microscopy. We focused particularly on the orientation of the specimens included in order to define the mineral deposition and the vascular invasion lines and obtain histological and ultrastructural images at the corresponding height of the plate. Chondrocyte morphology transformation through the maturation process (characterized by vesicles and then large cytoplasmic lacunae before condensation, fragmentation and disappearance of the nuclear chromatin) did not correspond to that observed in the 'in vitro' apoptosis models. These findings rather suggested the passage of free water from the cartilage matrix into a still live cell (swelling). The level of these changes suggested a close relationship with the mineral deposition line. Furthermore, the study provided evidence that the metaphyseal capillaries could advance inside the columns of stacked hypertrophic chondrocytes (delimited by the intercolumnar septa) without the need for calcified matrix resorption because the thin transverse septa between the stacked chondrocyte (below the mineral deposition line) were not calcified. The zonal distribution of cell types (hypertrophic chondrocytes, osteoblasts, osteoclasts and macrophages) did not reveal osteoclasts or chondroclasts at this level. Morphological and morphometric analysis recorded globular masses of an amorphous, necrotic material in a zone 0-70 μm below the vascular invasion line occasionally surrounded by a membrane (indicated as 'hypertrophic chondrocyte ghosts'). These masses and the same material not bound by a membrane were surrounded by a large number of macrophages and other blood cell precursors, suggesting this could be the cause of macrophage recall and activation. The most recent hypotheses based on genetic and lineage tracing studies stating that hypertrophic chondrocytes can survive and transform into osteoblasts and osteocytes (trans-differentiation) were not confirmed by the ultrastructural morphology or by the zonal comparative counting and distribution of cell types below the vascular invasion line.

Disrupted type II collagenolysis impairs angiogenesis, delays endochondral ossification and initiates aberrant ossification in mouse limbs

Cartilage remodelling and chondrocyte differentiation are tightly linked to angiogenesis during bone development and endochondral ossification. To investigate whether collagenase-mediated cleavage of the major cartilage collagen (collagen II) plays a role in this process, we generated a knockin mouse in which the mandatory collagenase cleavage site at PQG⁷⁷⁵↓⁷⁷⁶LAG, was mutated to PPG⁷⁷⁵↓⁷⁷⁶MPG (Col2a1^Bailey). This approach blocked collagen II cleavage, and the production of putative collagen II matrikines derived from this site, without modifying matrix metalloproteinase expression or activity. We report here that this mouse (Bailey) is viable. It has a significantly expanded growth plate and exhibits delayed and abnormal angiogenic invasion into the growth plate. Deeper electron microscopy analyses revealed that, at around five weeks of age, a small number of blood vessel(s) penetrate into the growth plate, leading to its abrupt shrinking and the formation of a bony bridge. Our results from in vitro and ex vivo studies suggest that collagen II matrikines stimulate the normal branching of endothelial cells and promote blood vessel invasion at the chondro-osseous junction. The results further suggest that failed collagenolysis in Bailey leads to expansion of the hypertrophic zone and formation of a unique post-hypertrophic zone populated with chondrocytes that re-enter the cell cycle and proliferate. The biological rescue of this in vivo phenotype features the loss of a substantial portion of the growth plate through aberrant ossification, and narrowing of the remaining portion that leads to limb deformation. Together, these data suggest that collagen II matrikines stimulate angiogenesis in skeletal growth and development, revealing novel strategies for stimulating angiogenesis in other contexts such as fracture healing and surgical applications.

The role of BMP6 in the proliferation and differentiation of chicken cartilage cells

Previous studies have indicated that bone morphogenetic protein (BMP) 6 may play an important role in skeletal system development and progression. However, the mechanism underlying the effects of BMP6 in cartilage cell proliferation and differentiation remains unknown. In this study, cartilage cells were isolated from shanks of chicken embryos and treated with different concentrations of Growth Hormone. Cell proliferation potential was assessed using real-time polymerase chain reaction (RT-PCR), western blotting and CCK-8 assays in vitro. The results showed that at 48 h, the Collagen II and BMP6 expression levels in 50 ng/μl GH-treated cartilage cells were significantly higher than in groups treated with 100 ng/μl or 200 ng/μl GH. We further observed that knockdown of BMP6 in cartilage cells led to significantly decreased expression mRNAs and proteins of Collagen II and Collagen X. Moreover, the suppression of BMP6 expression by a specific siRNA led to significantly decreased expression mRNA levels of IGF1R, JAK2, PKC, PTH, IHH and PTHrP and decreased protein levels of PKC, IHH and PTHrP. Taken together, our data suggest that BMP6 may play a critical role in chicken cartilage cell proliferation and differentiation through the regulation of IGF1, JAK2, PKC, PTH, and IHH-PTHrP signaling pathways.

Physeal fracture in the wrist and hand due to stress injury in a child climber: A case report

RATIONALE

In competitive athletes, the upper extremity is subject to tremendous torsional forces with axial loading due to repetitive weight bearing. Approximately 25% of injuries in sports are related to the hand or wrist. Skeletal deformity on the wrist physis is common in athletes due to repetitive loading and presents at early ages between 6 and 13 years. Additionally, it is more common in female than in male athletes.

PATIENT CONCERNS

An 11-year-old girl who was a climber complained of pain on her left wrist without direct trauma. She had participated in climbing exercise for several years and had no medical history. Thorough radiological evaluation, we diagnosed physeal injury of the left radius. After healing of the physeal injury of the radius, she complained of pain on fourth finger of right hand and radiographs revealed physeal injury of the right fourth finger.

DIAGNOSIS

Radiographs revealed physeal injury of the left radius. Magnetic resonance imaging revealed epiphyseal widening of the radial aspect of the wrist and bone marrow signal increase on T2-weighted imaging. Likewise, radiographs showed physeal injury of the right fourth finger INTERVENTIONS:: No surgery was performed and we applied wrist brace and finger splint for conservative treatment.

OUTCOMES

The patient's pain was immediately relieved. The patient had no complications or recurrence of symptoms and was undergoing regular check-ups every 6 months.

LESSONS

During climbing exercise, repeated high pressure causes damage of the hand and wrist joints in young patients. Chronic pain in this group must be carefully evaluated, and radiographs should be obtained for diagnosis and early treatment. Conservative treatment of these injuries has good results, and avoiding intensive power training avoids the risk of this injury.

Growth and the Growth Hormone-Insulin Like Growth Factor 1 Axis in Children With Chronic Inflammation: Current Evidence, Gaps in Knowledge, and Future Directions

Growth failure is frequently encountered in children with chronic inflammatory conditions like juvenile idiopathic arthritis, inflammatory bowel disease, and cystic fibrosis. Delayed puberty and attenuated pubertal growth spurt are often seen during adolescence. The underlying inflammatory state mediated by proinflammatory cytokines, prolonged use of glucocorticoid, and suboptimal nutrition contribute to growth failure and pubertal abnormalities. These factors can impair growth by their effects on the GH-IGF axis and also directly at the level of the growth plate via alterations in chondrogenesis and local growth factor signaling. Recent studies on the impact of cytokines and glucocorticoid on the growth plate further advanced our understanding of growth failure in chronic disease and provided a biological rationale of growth promotion. Targeting cytokines using biological therapy may lead to improvement of growth in some of these children, but approximately one-third continue to grow slowly. There is increasing evidence that the use of relatively high-dose recombinant human GH may lead to partial catch-up growth in chronic inflammatory conditions, although long-term follow-up data are currently limited. In this review, we comprehensively review the growth abnormalities in children with juvenile idiopathic arthritis, inflammatory bowel disease, and cystic fibrosis, systemic abnormalities of the GH-IGF axis, and growth plate perturbations. We also systematically reviewed all the current published studies of recombinant human GH in these conditions and discussed the role of recombinant human IGF-1.

Engineering of hyaline cartilage with a calcified zone using bone marrow stromal cells

OBJECTIVE

In healthy joints, a zone of calcified cartilage (ZCC) provides the mechanical integration between articular cartilage and subchondral bone. Recapitulation of this architectural feature should serve to resist the constant shear force from the movement of the joint and prevent the delamination of tissue-engineered cartilage. Previous approaches to create the ZCC at the cartilage-substrate interface have relied on strategic use of exogenous scaffolds and adhesives, which are susceptible to failure by degradation and wear. In contrast, we report a successful scaffold-free engineering of ZCC to integrate tissue-engineered cartilage and a porous biodegradable bone substitute, using sheep bone marrow stromal cells (BMSCs) as the cell source for both cartilaginous zones.

DESIGN

BMSCs were predifferentiated to chondrocytes, harvested and then grown on a porous calcium polyphosphate substrate in the presence of triiodothyronine (T3). T3 was withdrawn, and additional predifferentiated chondrocytes were placed on top of the construct and grown for 21 days.

RESULTS

This protocol yielded two distinct zones: hyaline cartilage that accumulated proteoglycans and collagen type II, and calcified cartilage adjacent to the substrate that additionally accumulated mineral and collagen type X. Constructs with the calcified interface had comparable compressive strength to native sheep osteochondral tissue and higher interfacial shear strength compared to control without a calcified zone.

CONCLUSION

This protocol improves on the existing scaffold-free approaches to cartilage tissue engineering by incorporating a calcified zone. Since this protocol employs no xenogeneic material, it will be appropriate for use in preclinical large-animal studies.

Structural differences in epiphyseal and physeal hypertrophic chondrocytes

We have observed that epiphyseal and physeal hypertrophic chondrocytes in BALB/c mice show considerable differences of light microscopic and ultrastructural appearance, even when the cells are at the same stage of differentiation. In addition, cell structure maintenance improved with tissue preparation controlled for osmolarity and for membrane stabilization using 0.5% ruthenium hexammine trichloride (RHT) for both light microscopy (LM) and electron microscopy (EM) or 0.5% lanthanum nitrate for LM. Physeal hypertrophic chondrocytes showed a gradual increase in size closer to the metaphysis and a change in shape as cells elongated along the long axis. The nucleus remained central, with uniformly dispersed chromatin, and the rough endoplasmic reticulum (RER) was randomly dispersed throughout cytoplasm with little to no presence against the cell membrane. Even the lowermost cells showed thin elongated or dilated cisternae of RER and intact cell membranes. Epiphyseal chondrocytes remained circular to oval with no elongation. Nucleus and RER were positioned as a complete transcellular central nucleocytoplasmic column or as an incomplete bud with RER of the column/bud always continuous with RER peripherally against the intact cell membrane. RER was densely packed with parallel cisternae with adjacent cytoplasm empty of organelles but often filled with circular deposits of moderately electron-dense material consistent with fat. Optimal technique for LM involved fixation using glutaraldehyde (GA) 1.3%, paraformaldehyde (PFA) 1% and RHT 0.5% (mOsm 606) embedded in JB-4 plastic and stained with 0.5% toluidine blue. Optimal technique for EM used fixation with GA 1.3%, PFA 1%, RHT 0.5% and cacodylate buffer 0.03 M (mOsm 511) and post-fixation including 1% osmium tetroxide. These observations lead to the possibility that the same basic cell, the hypertrophic chondrocyte, has differing functional mechanisms at different regions of the developing bone.

Novel EP4 receptor agonist-bisphosphonate conjugate drug (C1) promotes bone formation and improves vertebral mechanical properties in the ovariectomized rat model of postmenopausal bone loss

Current treatments for postmenopausal osteoporosis aim to either promote bone formation or inhibit bone resorption. The C1 conjugate drug represents a new treatment approach by chemically linking the antiresorptive compound alendronate (ALN) with the anabolic agent prostanoid EP4 receptor agonist (EP4a) through a linker molecule (LK) to form a conjugate compound. This enables the bone-targeting ability of ALN to deliver EP4a to bone sites and mitigate the systemic side effects of EP4a, while also facilitating dual antiresorptive and anabolic effects. In vivo hydrolysis is required to release the EP4a and ALN components for pharmacological activity. Our study investigated the in vivo efficacy of this drug in treating established bone loss using an ovariectomized (OVX) rat model of postmenopausal osteopenia. In a curative experiment, 3-month-old female Sprague-Dawley rats were OVX, allowed to lose bone for 7 weeks, then treated for 6 weeks. Treatment groups consisted of C1 conjugate at low and high doses, vehicle-treated OVX and sham, prostaglandin E2 (PGE2 ), and mixture of unconjugated ALN-LK and EP4a to assess the effect of conjugation. Results showed that weekly administration of C1 conjugate dose-dependently increased bone volume in trabecular bone, which partially or completely reversed OVX-induced bone loss in the lumbar vertebra and improved vertebral mechanical strength. The conjugate also dose-dependently stimulated endocortical woven bone formation and intracortical resorption in cortical bone, with high-dose treatment increasing the mechanical strength but compromising the material properties. Conjugation between the EP4a and ALN-LK components was crucial to the drug's anabolic efficacy. To our knowledge, the C1 conjugate represents the first time that a combined therapy using an anabolic agent and the antiresorptive compound ALN has shown significant anabolic effects which reversed established osteopenia.

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

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

Molecular mechanisms underlying skeletal growth arrest by cutaneous scarring

In pediatric surgeries, cutaneous scarring is frequently accompanied by an arrest in skeletal growth. The molecular mechanisms responsible for this effect are not understood. Here, we investigated the relationship between scar contracture and osteogenesis. An excisional cutaneous wound was made on the tail of neonatal mice. Finite element (FE) modeling of the wound site was used to predict the distribution and magnitude of contractile forces within soft and hard tissues. Morphogenesis of the bony vertebrae was monitored by micro-CT analyses, and vertebral growth plates were interrogated throughout the healing period using assays for cell proliferation, death, differentiation, as well as matrix deposition and remodeling. Wound contracture was grossly evident on post-injury day 7 and accompanying it was a significant shortening in the tail. FE modeling indicated high compressive strains localized to the dorsal portions of the vertebral growth plates and intervertebral disks. These predicted strain distributions corresponded to sites of increased cell death, a cessation in cell proliferation, and a loss in mineralization within the growth plates and IVD. Although cutaneous contracture resolved and skeletal growth rates returned to normal, vertebrae under the cutaneous wound remained significantly shorter than controls. Thus, localized contractile forces generated by scarring led to spatial alterations in cell proliferation, death, and differentiation that inhibited bone growth in a location-dependent manner. Resolution of cutaneous scarring was not accompanied by compensatory bone growth, which left the bony elements permanently truncated. Therefore, targeting early scar reduction is critical to preserving pediatric bone growth after surgery.

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

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

Differential development of the distal and proximal femoral epiphysis and physis in mice

Many pathologic conditions of hip development result from aberrant vascularity with subsequent effects on physeal resorption and epiphyseal fusion. To elucidate the mechanisms of these developmental disorders, researchers have mainly focused on larger mammals as they have been well characterized and are known to provide a model similar to humans in which a secondary ossification center is formed through an independent blood supply followed by physeal resorption. Murine models of hip development, however, have never been characterized as it was previously assumed that all physes in rodents never resorb and therefore not suitable as a model of the human condition. The purpose of this study was to determine if murine hip development was comparable to humans as laboratory mice provide an ideal model in which genetic knockouts are readily available with a short developmental time span. Here we show for the first time the unique developmental patterns of the murine hip in which a secondary ossification center never develops. Instead, the epiphysis undergoes a prolonged phase of mineralization through chondrocyte, not osteoblast, dependent mechanisms. After skeletal growth, transphyseal vessels develop from the metaphysis resulting in resorption of the physis, ossification of the mineralized cartilage of the epiphysis, and epiphyseal fusion. Although the development of the murine hip is markedly different from that in humans, we believe that these findings have direct implications for studying the vascularity of developing bone, particularly during physeal resorption, chondrocyte-mediated mineralization and more importantly, developmental diseases of the human physis and epiphysis.

Articular cartilage development: a molecular perspective

In this article, development of articular cartilage and endochondral ossification is reviewed, from the perspective of both morphologic aspects of histogenesis and molecular biology, particularly with respect to key signaling molecules and extracellular matrix components most active in cartilage development. The current understanding of the roles of transforming growth factor β and associated signaling molecules, bone morphogenic proteins, and molecules of the Wnt-β catenin system in chondrogenesis are described. Articular cartilage development is a highly conserved complex biological process that is dynamic and robust in nature, which proceeds well without incident or failure in all joints of most young growing individuals.

Bony island within the articular cartilage of the knee in a child: a rare condition for early osteoarthritis

Articular cartilage is a specific type of connective tissue composed of hydrated proteoglycans within a matrix of collagen fibrils. In the elderly population, it shows degenerative changes that may results in osteoarthritis. The more severe form of osteoarthritis occasionally demonstrates bone formation within the cartilage, which is designated as a bony protuberance, however, such lesions are rare in children. This report presents the case of a 10-year-old boy with a bony protuberance within the articular cartilage of the knee. The patient initially complained of knee pain and he subsequently developed flexion contracture. Radiological and arthroscopic examinations revealed a bony protuberance in the articular cartilage and degenerative changes of the cartilage above it. He was successfully treated by the removal of the bony protuberance and osteochondral grafting. The bony protuberance may have caused cartilage degradation since the thickness of the cartilage above it was thinner than that around the lesion. The bony protuberance within the articular cartilage formed in the younger population may be a possible cause of osteoarthritis. This case is a noteworthy with regard to the pathogenesis of osteoarthritis.

Bony island within the articular cartilage of the knee in a child: a rare condition for early osteoarthritis

Articular cartilage is a specific type of connective tissue composed of hydrated proteoglycans within a matrix of collagen fibrils. In the elderly population, it shows degenerative changes that may results in osteoarthritis. The more severe form of osteoarthritis occasionally demonstrates bone formation within the cartilage, which is designated as a bony protuberance, however, such lesions are rare in children. This report presents the case of a 10-year-old boy with a bony protuberance within the articular cartilage of the knee. The patient initially complained of knee pain and he subsequently developed flexion contracture. Radiological and arthroscopic examinations revealed a bony protuberance in the articular cartilage and degenerative changes of the cartilage above it. He was successfully treated by the removal of the bony protuberance and osteochondral grafting. The bony protuberance may have caused cartilage degradation since the thickness of the cartilage above it was thinner than that around the lesion. The bony protuberance within the articular cartilage formed in the younger population may be a possible cause of osteoarthritis. This case is a noteworthy with regard to the pathogenesis of osteoarthritis.

Decreased length of telomeric DNA sequences and increased numerical chromosome aberrations in human osteoarthritic chondrocytes

Length of telomeric DNA sequences and numerical chromosome aberrations from uncultured human osteoarthritic (OA) articular chondrocytes were compared with those from peripheral blood leukocytes (PBL) from the same individual and from chondrocytes and PBL from control subjects. Cells were both obtained from 39 OA patients (age range: 43-80 years) and from 20 control subjects (age range: 39-94 years). Mean length of telomeric DNA sequences was determined using a quantitative real-time polymerase chain reaction (qPCR) assay and numerical chromosome aberrations were identified in interphase nuclei by Fluorescence In Situ Hybridization (FISH) using cocktails of specific DNA probes for chromosomes 7, 8 and for 18, X and Y. Chondrocytes revealed higher telomere size than PBL, both in control subjects and in OA patients, being 2 and 1.6 times higher respectively, thus revealing cell type specific differences. However, chondrocytes from OA patients showed significantly shorter telomere size than chondrocytes from control subjects (T/S ratio 1.64±0.41 vs. 1.99±0.54; mean±sd; p=0.008). Regarding the percentage of numerical chromosome aberrations, OA chondrocytes showed 1.7 times higher than chondrocytes from control subjects (19.80±3.31 vs.11.48±4.11; p<0.01) and 1.5 times average higher than that from PBL from the own OA patient (13.06±1.45; p<0.001). Moreover, PBL from OA patients also showed 1.4 times more anomalies than PBL from controls (13.06±1.45 vs. 9.54±1.61; p<0.001). No significant differences were found between chondrocytes and PBL in control subjects. Chromosome loss was the more frequent aneuploidy, mainly monosomy 18. The decreased telomere size and increased chromosome instability in chondrocytes from OA affected joints may imply a local advanced senescence that could contribute to the pathogenesis or progression of the degenerative articular disease. Moreover, the increased chromosomal abnormalities in PBL from OA patients suggest a more general accelerated senescence phenotype that could promote the age-related degenerative joint pathology.

Ontogeny of skeletal maturation in the juvenile rat

Systemic regulation of the cellular processes that produce endochondral elongation and endochondral mineralization during postnatal skeletal maturation are not completely understood. In particular, a mechanism coupling the decline of cellular activity in the bone microenvironment to the onset of sexual maturity remains elusive. The purpose of this study was to empirically integrate the dynamic progression of bone mineral accrual and endochondral elongation as a function of animal age in growing male and female Sprague-Dawley rats. We used serial dual-energy X-ray absorptiometry (DXA) and radiography to study the temporal progression of bone growth and mineral accrual from weaning to adulthood. We observed that skeletal maturation proceeds in a pattern adequately described by the Gompertz function. During this period of growth, we found that serum markers of osteoblastic bone formation declined with age, while osteoclastic bone resorption activity remained unchanged. We also report a slight lag in the age at inflection in the rate of bone mineral accrual relative to the rate of tibial elongation and that both endochondral processes eventually come to asymptotic equilibrium by approximately 20 weeks of age. In addition, we studied tibial growth plate histomorphometry at select time points through 1 year of age. We report that, despite the histologic persistence of physeal cartilage, very little proliferative or elongative activity was measured in this tissue beyond 20 weeks of age. Taken together, these data provide insight to the temporal coordination of postnatal endochondral growth processes.

Forelimb versus hindlimb skeletal development in the big brown bat, Eptesicus fuscus: functional divergence is reflected in chondrocytic performance in Autopodial growth plates

The morphology of the chiropteran forelimb demonstrates musculoskeletal specializations for powered flight essentially unique among mammals, including extreme elongation of the distal skeletal elements. Recent studies have focused primarily on the relative timing and levels of gene expression during early stages of endochondral ossification in the chiropteran embryo for clues to the molecular basis of the evolutionary origins of flight in these species. The goal of the current study was to examine how elongation of skeletal elements of the forelimb autopod is achieved through a combination of cellular proliferation, cellular enlargement and matrix synthesis during a period of rapid postnatal growth in Eptesicus fuscus. Quantitative analyses were done of multiple performance parameters of growth plate chondrocytes during all phases of the differentiation cascade. Fourteen autopodial growth plates from the forelimb and hindlimb of one individual, as well as the proximal tibial growth plate, were collected and analyzed. Significant differences were seen in all performance parameters examined. Particularly striking were the differences between growth plates of the manus and pes in the size of the pool of chondrocytes in all cellular zones and rates of turnover of terminal cells. The magnitude of hypertrophy of chondrocytes in growth plates of the manus in E. fuscus far exceeded what has been reported previously in any species, even in rapidly elongating rodent long bones. Volume changes approaching x70 and height changes of 50-60 mum/cell (paralleling the direction of growth) occurred after proliferation in the most rapidly growing growth plates. The data demonstrate that final differences in lengths of homologous skeletal elements in the autopod of the forelimb and hindlimb of this species result not just from an initiating factor early in development, but from continued quantitative differences in chondrocytic performance during postnatal bone elongation as measured by multiple kinetic-based parameters.

Endochondral bone formation is involved in media calcification in rats and in men

Arterial media calcification is often considered a cell-regulated process resembling intramembranous bone formation, implying a conversion of vascular tissue into a bone-like structure without a cartilage intermediate. In this study, we examined the association of chondrocyte-specific marker expression with media calcification in arterial samples derived from rats with chronic renal failure (CRF) and from human transplant donors. CRF was induced in rats with a diet supplemented with adenine. Vascular calcification was evaluated histomorphometrically on Von Kossa-stained sections and the expression of the chondrocyte markers sox9 and collagen II with the osteogenic marker core-binding factor alpha1 (cbfa1) was determined immunohistochemically. Media calcification was detected in more than half of the rats with CRF. In over half of the rats with severe media calcification, a typical cartilage matrix was found by morphology. All of the animals with severe calcification showed the presence of chondrocyte-like cells expressing the markers sox9, collagen II, and cbfa1. Human aorta specimens showing mild to moderate media calcification also showed sox9, collagen II, and cbfa1 expression. The presence of chondrocytes in association with calcification of the media in aortas of rats with CRF mimics endochondral bone formation. The relevance of this association is further demonstrated by the chondrogenic conversion of medial smooth muscle cells in the human aorta.

Solute transport in growth plate cartilage: in vitro and in vivo

Bone elongation originates from cartilaginous discs (growth plates) at both ends of a growing bone. Here chondrocytes proliferate and subsequently enlarge (hypertrophy), laying down a matrix that serves as the scaffolding for subsequent bone matrix deposition. Because cartilage is generally avascular, all nutrients, oxygen, signaling molecules, and waste must be transported relatively long distances through the tissue for it to survive and function. Here we examine the transport properties of growth plate cartilage. Ex vivo, fluorescence photobleaching recovery methods are used in tissue explants. In vivo, multiphoton microscopy is used to image through an intact perichondrium and into the cartilage of anesthetized mice. Systemically introduced fluorescent tracers are monitored directly as they move from the vasculature into the cartilage. We demonstrate the existence of a relatively permissive region at the midplane of the growth plate, where chondrocytes transition from late proliferative to early hypertrophic stages and where paracrine communication is known to occur between chondrocytes and cells in the surrounding perichondrium. Transport in the living mouse is also significantly affected by fluid flow from the two chondro-osseus junctions, presumably resulting from a pressure difference between the bone vasculature and the cartilage.

Physiological death of hypertrophic chondrocytes

OBJECTIVE

Post-proliferative chondrocytes in growth cartilage are present in two forms, light and dark cells. These cells undergo hypertrophy and die by a mechanism that is morphologically distinct from apoptosis, but has not been characterized. The aims of the current study were to document the ultrastructural appearance of dying hypertrophic chondrocytes, and to establish a culture system in which the mechanism of their death can be examined.

DESIGN

Growth cartilage from fetal and growing postnatal horses was examined by electron microscopy. Chondrocytes were isolated from epiphyseal cartilage from fetal horses and grown in pellet culture, then examined by light and electron microscopy, and quantitative polymerase chain reaction.

RESULTS

In tissue specimens, it was observed that dying dark chondrocytes underwent progressive extrusion of cytoplasm into the extracellular space, whereas light chondrocytes appeared to disintegrate within the cellular membrane. Pellets cultured in 0.1% fetal calf serum (FCS) contained dying light and dark chondrocytes similar to those seen in vivo. Transforming growth factor-beta1 or 10% FCS increased the proportion of dark cells and induced cell death. Triiodothyronine increased the differentiation of dark and light cells and induced their death. Dark cells were associated with higher levels of matrix metalloproteinase-13 expression than light cells, and light cells were associated with higher levels of type II collagen expression.

CONCLUSIONS

Light and dark hypertrophic chondrocytes each undergo a distinctive series of non-apoptotic morphological changes as they die. Pellet culture can be used as a model of the two forms of physiological death of hypertrophic chondrocytes.

Development of the terminally differentiated state sensitizes epiphyseal chondrocytes to apoptosis through caspase-3 activation

The maturation of epiphyseal chondrocytes is accompanied by dramatic changes in energy metabolism and shifts in proteins concerned with the induction of apoptosis. We evaluated the role of mitochondria in this process by evaluating the membrane potential (Delta psi m) of chondrocytes of embryonic tibia and the epiphyseal growth plate. We observed that there was a maturation-dependent change in fluorescence, indicating a fall in the Delta psi m. The level of mitochondrial Bcl-2 was decreased during maturation, while in the same time period there was an obvious increase in Bax levels in the mitochondrial fraction of the terminally differentiated chondrocytes. Bcl(xL), another anti-apoptotic protein, was also robustly expressed in the mitochondrial fraction, but its expression was not dependent on the maturation status of the chondrocytes. We found that caspase-3 was present throughout the growth plate and in hypertrophic cells in culture. We blocked caspase-3 activity and found that alkaline phosphatase staining and mineral formation was decreased, and the cells had lost their characteristic shape. Moreover, we noted that the undifferentiated cells were insensitive to elevated concentrations of inorganic phosphate (Pi). It is concluded that during hypertrophy, the change in membrane potential, the increased binding of a pro-apoptotic protein to mitochondria, and the activation of caspase-3 serve to prime cells for apoptosis. Only when the terminally differentiated chondrocytes are challenged with low levels of apoptogens there is activation of apoptosis.

Restoration of growth plate function following radiotherapy is driven by increased proliferative and synthetic activity of expansions of chondrocytic clones

Radiation therapy encompassing an active epiphysis can negatively impact the potential for bone growth by disrupting cell-cycle progression and accelerating apoptosis and terminal differentiation in physeal chondrocytes. Despite functional derangement following radiation exposure, the irradiated growth plate retains a capacity for regeneration and recovery of growth. The purpose of this study was to characterize the initial sequence of events leading to functional growth recovery in irradiated weanling rat growth plates. We hypothesized that growth in an irradiated epiphysis would be partially restored due to the expansion of chondrocytic clones. Stereological histomorphometry was used to compare chondrocytic cell and matrix turnover between the first and second week following irradiation, and to determine the relative contribution of each of the cellular and extracellular matrix (ECM) compartments to growth. We found that restoration of growth in the irradiated limb was strongly associated with the proliferative activity and production of ECM by these chondrocytic clones, as they expand in average volume, but not in numerical density. We conclude that chondrocytes forming expansive clones and exhibiting increased mitotic and matrix synthesis activity initiate the early restoration of function in the irradiated growth plate, and would be a logical target for strategies to restore full growth potential.

Fate of the hypertrophic chondrocyte: microenvironmental perspectives on apoptosis and survival in the epiphyseal growth plate

The goal of this review is to examine the fate of the hypertrophic chondrocyte in the epiphyseal growth plate and consider the impact of the cartilage microenvironment on cell survival and apoptosis. Early investigations pointed to a direct role of the hypertrophic chondrocyte in osteogenesis. The terminally differentiated cells were considered to undergo a dramatic change in shape, size, and phenotype, and assume the characteristics of an osteoblast. While some studies have supported the notion of transdifferentiation, much of the evidence in favor of reprogramming epiphyseal chondrocytes is circumstantial and based on microscopic evaluation of cells that are present at the chondro-osseous junction. Although these investigations provided a novel perspective on endochondral bone formation, they were flawed by the failure to consider the importance of stem cells in osseous tissue formation. Subsequent studies indicated that many, if not all, of the cells of the cartilage plate die through the induction of apoptosis. With respect to agents that mediate apoptosis, at the chondro-osseous junction, solubilization of mineral and hydrolysis of organic matrix constituents by septoclasts generates high local concentrations of ions, peptides, and glycans, and secreted matrix metalloproteins. Individually, and in combination, a number of these agents serve as potent chondrocyte apoptogens. We present a new concept: hypertrophic cells die through the induction of autophagy. In the cartilage microenvironment, combinations of local factors cause chondrocytes to express an initial survival phenotype and oxidize their own structural macromolecules to generate ATP. While delaying death, autophagy leads to a state in which cells are further sensitized to changes in the local microenvironment. One such change is similar to ischemia reperfusion injury, a condition that leads to tissue damage and cell death. In the growth cartilage, an immediate effect of this type of injury is sensitization to local apoptogens. These two concepts (type II programmed cell death and ischemia reperfusion injury) emphasize the importance of the local microenvironment, in particular pO(2), in directing chondrocyte survival and apoptosis.

Endocrine regulation of the growth plate

Longitudinal bone growth occurs at the growth plate by endochondral ossification. Within the growth plate, chondrocyte proliferation, hypertrophy, and cartilage matrix secretion result in chondrogenesis. The newly formed cartilage is invaded by blood vessels and bone cells that remodel the newly formed cartilage into bone tissue. This process of longitudinal bone growth is governed by a complex network of endocrine signals, including growth hormone, insulin-like growth factor I, glucocorticoid, thyroid hormone, estrogen, androgen, vitamin D, and leptin. Many of these signals regulate growth plate function, both by acting locally on growth plate chondrocytes and also indirectly by modulating other endocrine signals in the network. Some of the local effects of hormones are mediated by changes in paracrine factors that control chondrocyte proliferation and differentiation. Many human skeletal growth disorders are caused by abnormalities in the endocrine regulation of the growth plate. This review provides an overview of the endocrine signals that regulate longitudinal bone growth, their interactions, and the mechanisms by which they affect growth plate chondrogenesis.

Transient disturbance in physeal morphology is associated with long-term effects of nitrogen-containing bisphosphonates in growing rabbits

Bisphosphonates have clinical benefit in children with severe osteogenesis imperfecta or osteoporosis and potential benefit in children with Perthes disease or undergoing distraction osteogenesis. However, there is concern about the effects of bisphosphonates on the physis and bone length. In 44 growing rabbits, zoledronic acid caused a transient disruption of physeal morphology, retention of cartilaginous matrix in trabeculae and cortical bone of the metaphysis, and a minor decrement in tibial bone length at maturity.

INTRODUCTION

Data from growing animal models suggest that bisphosphonates cause retention of longitudinal cartilaginous septa at the chondro-osseous junction, extension of trabeculae to the metaphyseal-diaphyseal junction, and varying dose-dependent effects on longitudinal growth. However, there is a lack of data regarding effects of intermittent use of nitrogen-containing bisphosphonates on the physis and on tibial length in models reaching maturity.

MATERIALS AND METHODS

Contralateral tibias of juvenile rabbits were examined after right tibial distraction osteogenesis from two previous studies. Animals were randomized to receive 0.1 mg/kg zoledronic acid (ZA) IV at 8 weeks of age (ZA*1) or 8 and 10 weeks of age (ZA*2) or saline. Body mass was analyzed from 5 to 44 weeks of age; tibial length and proximal physeal-metaphyseal histology and histomorphometry were analyzed at 8-52 weeks of age.

RESULTS

Tibial length was 3% less at 14 weeks of age in the ZA*2-treated versus saline group (p<0.05) in both studies, and this difference persisted at maturity in the long-term study group (26 weeks of age, p<0.05). Total body mass gain from 5 to 26 weeks of age was 14% less in ZA*2-treated than saline animals (p<0.05). Rate of weight gain from 8 to 10 weeks of age was 76% less in ZA*2 compared with saline animals (p<0.05). Radiographs showed radiodense lines in the metaphyses of ZA-treated bones, corresponding to the number of doses. Histologically, lines resulting from the first dose of ZA contained longitudinal cartilaginous matrix cores surrounded by bone, whereas those from the second dose contained spherical cores of matrix caused by transient disruption of physeal morphology after the first dose of ZA. Resorption of these lines at later times was radiographically and histologically evident, but remnants of cartilaginous matrix remained in the cortical bone of ZA-treated animals.

CONCLUSIONS

ZA treatment within the final 13.5% of the rabbit tibial growth period caused a transient disruption in physeal morphology and resorption associated with retention of cartilaginous matrix and coinciding with a persistent 3% decrement in tibial length. Disruption of physeal morphology and potential loss of bone length should be considered when administering nitrogen-containing bisphosphonates to children before closure of the major physes.

The role of sex hormones in the kinetics of chondrocytes in the growth plate

Sex hormones play important roles in the regulation of the proliferation, maturation and death of chondrocytes in the epiphyseal growth plate. We have investigated the effects of male castration on the cell kinetics of chondrocytes as defined by the numbers of proliferating and dying cells. The growth plates of normal rabbits and animals castrated at eight weeks of age were obtained at 10, 15, 20 and 25 weeks of age.

Our study suggested that castration led to an increase in apoptosis and a decrease in the proliferation of chondrocytes in the growth plate. In addition, the number of chondrocytes in the castrated rabbits was less than that of normal animals of the same age.

The pathophysiology of osteoarthritis

Osteoarthritis (OA) is a complex disease whose pathogenesis includes the contribution of biomechanical and metabolic factors which, altering the tissue homeostasis of articular cartilage and subchondral bone, determine the predominance of destructive over productive processes. A key role in the pathophysiology of articular cartilage is played by cell/extra-cellular matrix (ECM) interactions, which are mediated by cell surface integrins. In a physiologic setting, integrins modulate cell/ECM signaling, essential for regulating growth and differentiation and maintaining cartilage homeostasis. During OA, abnormal integrin expression alters cell/ECM signaling and modifies chondrocyte synthesis, with the following imbalance of destructive cytokines over regulatory factors. IL-1, TNF-alpha and other pro-catabolic cytokines activate the enzymatic degradation of cartilage matrix and are not counterbalanced by adequate synthesis of inhibitors. The main enzymes involved in ECM breakdown are metalloproteinases (MMPs), which are sequentially activated by an amplifying cascade. MMP activity is partially inhibited by the tissue inhibitors of MMPs (TIMPs), whose synthesis is low compared with MMP production in OA cartilage. Intriguing is the role of growth factors such as TGF-beta, IFG, BMP, NGF, and others, which do not simply repair the tissue damage induced by catabolic factors, but play an important role in OA pathogenesis.

Matrix vesicles and calcification

Matrix vesicles (MVs) are extracellular, 100 nM in diameter, membrane-invested particles selectively located at sites of initial calcification in cartilage, bone, and predentin. The first crystals of apatitic bone mineral are formed within MVs close to the inner surfaces of their investing membranes. Matrix vesicle biogenesis occurs by polarized budding and pinching-off of vesicles from specific regions of the outer plasma membranes of differentiating growth plate chondrocytes, osteoblasts, and odontoblasts. Polarized release of MVs into selected areas of developing matrix determines the nonrandom distribution of calcification. Initiation of the first mineral crystals, within MVs (phase 1), is augmented by the activity of MV phosphatases (eg, alkaline phosphatase, adenosine triphosphatase and pyrophosphatase) plus calcium-binding molecules (eg, annexin I and phosphatidyl serine), all of which are concentrated in or near the MV membrane. Phase 2 of biologic mineralization begins with crystal release through the MV membrane, exposing preformed hydroxyapatite crystals to the extracellular fluid. The extracellular fluid normally contains sufficient Ca2+ and PO4(3-) to support continuous crystal proliferation, with preformed crystals serving as nuclei (templates) for the formation of new crystals by a process of homologous nucleation. In diseases such as osteoarthritis, crystal deposition arthritis, and atherosclerosis, MVs initiate pathologic calcification, which, in turn, augments disease progression.

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

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

Alendronate affects long bone length and growth plate morphology in the oim mouse model for Osteogenesis Imperfecta

Alendronate, a bisphosphonate drug, has shown promise in reducing remodeling and bone loss in postmenopausal osteoporosis. Alendronate acts directly on the osteoclast, inhibiting its resorption capability. This inhibition of osteoclast activity has led to the use of bisphosphonates in the treatment of the osteogenesis imperfecta condition. Treatment of osteogenesis imperfecta with bisphosphonates enhances bone strength, but the consequences on linear bone growth are not well defined. Using the oim mouse model for type III osteogenesis imperfecta, two doses of alendronate, low (0.125 mg/kg/wk) and high (2.5 mg/kg/wk) were administered weekly via intraperitoneal injection starting at 4 weeks of age and ending at 12 weeks of age to assess the effects of alendronate on humerus and ulna length. The higher dose of alendronate reduced humerus and ulna length in the oim/wt and wt/wt genotypes for both sexes (P < 0.05). The oim/oim humerus and ulna were not significantly affected by the higher dose of alendronate in females, but reduced bone length in males (P < 0.0085). Proximal humerus growth plate area was affected by both genotype and alendronate dose and growth plate diameter was increased at the chondro-osseous junction by both alendronate doses (P < 0.011). Genotype and alendronate dose affected growth plate height. The oim/oim genotype displayed taller growth plates. The high dosage of alendronate increased overall growth plate height, particularly within the hypertrophic zone, which suggests a failure of vascular invasion-induced apoptosis in the hypertrophic cells. In conclusion, these results indicate that high doses of alendronate (>2.5 mg/kg/wk) inhibit long bone length in mice through alteration of the growth plate and possibly reduced resorption at the chondro-osseous junction.

Induction of apoptosis in chondrocytes by tumor necrosis factor-alpha

Tumor necrosis factor alpha (TNF-alpha) induces apoptosis in a number of cell types and plays an essential role in bone remodeling, both stimulating the proliferation of osteoblasts and activating osteoclasts. During endochondral ossification, apoptosis of chondrocytes occurs concurrently with new bone formation and the resorption and replacement of mineralized cartilage with woven bone. In the present study, the role of TNF-alpha in promoting chondrocyte apoptosis was examined. Chondrocyte cell populations, enriched in either hypertrophic or non-hypertrophic cells, were isolated from the cephalic and caudal portions of 17-day chick embryo sterna, respectively, and treated in vitro with 0.1-10 nM recombinant human TNF-alpha. As a positive control, apoptosis was also induced by Fas receptor antibody binding. Dye exclusion assays of the live/dead ratios of cells showed that TNF-alpha caused a dose-dependent 1.5- and 2.0-fold increase in the number of dead cells in both hypertrophic and non-hypertrophic chondrocytes. Induction of apoptosis was independently assayed by measurement of interleukin-1beta-converting enzyme (ICE) activity, and analyzed by a semi-quantitative determination of DNA fragmentation. When compared to untreated cells, these analyses also showed dose-dependent increases in TNF-alpha induced apoptosis in both chondrocyte populations, with increases in the levels of ICE activity for all doses of TNF-alpha (from approximately 5 to approximately 20 fold). Osteoblasts, however, were not affected by treatment with TNF-alpha or by Fas antibody/protein G induction. Immunostaining of chondrocytes for Fas receptor and caspase-2 protein expression showed that most of the chondrocytes expressed these two markers of apoptosis after treatment with TNF-alpha. Although cell killing and ICE induction were higher in the more hypertrophic cells, TNF-alpha induced apoptosis in both hypertrophic and non-hypertrophic chondrocyte populations. These results demonstrate that apoptosis may be induced in both hypertrophic and non-hypertrophic chondrocytes through both Fas and TNF-alpha receptor mediated signaling, and suggest that chondrocytes are more sensitive to apoptotic effects of TNF-alpha within the skeletal lineage than are osteoblasts.

Phosphate-induced chondrocyte apoptosis is linked to nitric oxide generation

An elevation in inorganic phosphate (P(i)) concentration activates epiphyseal chondrocyte apoptosis. To determine the mechanism of apoptosis, tibial chondrocytes were treated with P(i), and nitrate/nitrite (NO/NO) levels were determined. P(i) induced a threefold increase in the NO/NO concentration; inhibitors of nitric oxide (NO) synthase activity and P(i) transport significantly reduced NO/NO levels and prevented cell death. Furthermore, a dose-dependent increase in cell death was observed after exposure of chondrocytes to S-nitrosoglutathione. P(i) increased caspase 3 activity 2.7-fold. Both caspase 1 and caspase 3 inhibitors protected chondrocytes from P(i)-induced apoptosis. P(i) caused a significant decrease in the mitochondrial membrane potential, while NO synthase inhibitors maintained mitochondrial function. While P(i) caused thiol depletion, inhibition of P(i) uptake or NO generation served to maintain glutathione levels. The results suggest that NO serves to mediate key metabolic events linked to P(i)-dependent chondrocyte apoptosis.

Uncoupling of chondrocyte death and vascular invasion in mouse galectin 3 null mutant bones

Galectin 3 is a beta-galactoside binding protein which localizes to the cytoplasm of proliferative, mature, and hypertrophic chondrocytes in the growth plate cartilage of developing long bones. To elucidate the function of galectin 3 during bone development, we examined the epiphyseal femurs and tibias of fetal mice carrying a null mutation for the galectin 3 gene. Detailed histological and ultrastructural studies identified abnormalities in the cells of the proliferative, mature, and hypertrophic zones and in the extracellular matrix of the hypertrophic zone, as well as a reduction in the total number of hypertrophic chondrocytes. The expression patterns of several chondrocyte and bone cell markers were analyzed and revealed a subtle modification of Ihh expression in the galectin 3 mutant growth plate. A striking difference was observed at the chondrovascular junction where many empty lacunae are present. In addition, large numbers of condensed chondrocytes exhibiting characteristic signs of cell death were found in the late hypertrophic zone, indicating that the rate of chondrocyte death is increased in the mutants. These results suggest a role for galectin 3 as a regulator of chondrocyte survival. In addition, this unique phenotype shows that the elimination of chondrocytes and vascular invasion can be uncoupled and indicates that galectin 3 may play a role in the coordination between chondrocyte death and metaphyseal vascularization.

Bone morphogenetic protein (BMP) localization in developing human and rat growth plate, metaphysis, epiphysis, and articular cartilage

We assessed the distribution and relative staining intensity of bone morphogenetic protein (BMP)-1-7 by immunohistochemistry in tibial growth plates, epiphyses, metaphyses, and articular cartilage in one 21-week and one 22-week human fetus and in five 10-week-old Sprague-Dawley rats. In the rats, articular cartilage was also examined. BMP proteins were mostly cytoplasmic, with negligible matrix staining. Highest BMP levels were seen in (a) hypertrophic and calcifying zone chondrocytes of growth plate (BMP-1-7), (b) osteoblasts and/or osteoprogenitor fibroblasts and vascular cells of the metaphyseal cortex and medulla (BMP-1-6), (c) osteoclasts of the metaphysis and epiphysis (BMP-1,-4,-5, and -6), and (d) mid to deep zone articular chondrocytes of weanling rats (BMP-1-7). BMP staining in osteoclasts, an unexpected finding, was consistently strong with BMP-4, -5, and -6 but was variable and dependent on osteoclast location with BMP-2,-3, and -7. BMP-1-7 were moderately to intensely stained in vascular canals of human fetal epiphyseal cartilage by endothelial cells and pericytes. BMP-1,-3,-5,-6, and -7 were localized in hypertrophic chondrocytes adjacent to cartilage canals. We conclude that BMP expression is associated with maturing chondrocytes of growth plate and articular cartilage, and may play a role in chondrocyte differentiation and/or apoptosis. BMP appears to be expressed by osteoclasts and might be involved in the intercellular "cross-talk" between osteoclasts and neighboring osteoprogenitor cells at sites of bone remodeling.

Cartilage turnover in embryonic chick tibial explant cultures

Growth plate cartilage regulates the rate of growth and ultimate length of several bones in the skeleton. Chondrocytes within the growth plate proliferate, differentiate, enlarge, and die. The extracellular matrix undergoes synthesis, reorganization, and eventually degradation. The majority of research in growth plate physiology has focused on the proliferation and differentiation of chondrocytes as well as proteins they produce for the extracellular matrix. However, little is known about the transition from hypertrophic to apoptotic chondrocytes or the regulation of terminal degradation of cartilage prior to bone formation. An explant culture has been developed to study cartilage differentiation using 12-d-old embryonic chick tibiae. We have modified the explant culture and are using it to further elucidate mechanisms involved in the regulation of growth plate cartilage turnover. In our cultures, chondrocytes mature and then die, completely degrading the cartilage in approximately 16 d. The matrix undergoes a predictable pattern of degradation in which proteoglycans followed by collagen are removed. Increases in matrix metalloproteinase activity and nitric oxide production are detected in cartilage concurrently with release of proteoglycans into media. Inhibitors of nitric oxide inhibit nitric oxide production and proteoglycan degradation, suggesting that nitric oxide, at least in part, regulates growth plate cartilage turnover in the explant culture. Information gained from using this explant culture will aid in understanding the regulation of growth plate cartilage turnover in vivo and potentially help determine the cause of bone growth diseases such as tibial dyschondroplasia.

Differential expression of egr-1 in osteoarthritic compared to normal adult human articular cartilage

OBJECTIVE

The purpose of this study was to identify genes that are differentially expressed in normal versus osteoarthritic human articular cartilage as either potential novel therapeutic targets or diagnostic markers of this disease.

DESIGN

mRNA was isolated from histologically normal and osteoarthritic adult human articular cartilage. The Differential Display technique was employed which identified differentially expressed genes in the normal and diseased tissue. Northern and reverse Northern hybridization were used to confirm the gene expression pattern. Immunohistochemistry and in-situ hybridization were used to localize expression of Egr-1 protein and mRNA respectively in cartilage.

RESULTS

A transcription factor, early growth response protein-1 (Egr-1) was found to be down-regulated more than six-fold in multiple human OA cartilage samples when compared to normal tissue. Immunohistochemistry indicated that Egr-1 was expressed throughout normal adult cartilage, in deep-, mid- and superficial-zones. In contrast, in OA cartilage there was expression of Egr-1 mRNA and protein only in the chondrocytes undergoing cloning.

CONCLUSIONS

Egr-1 is differentially expressed in OA versus normal cartilage and because of its role in transcriptional activation and repression and regulation of proliferation, differentiation and apoptosis, Egr-1 may play an important role in the pathogenesis of OA. Up-regulation of Egr-1 may therefore provide a novel therapeutic approach for either the prevention or treatment of OA.

Computer model of endochondral growth and ossification in long bones: biological and mechanobiological influences

Endochondral growth and ossification, the processes by which cartilage increases in size and is replaced by bone, are affected by biological factors such as intrinsic genetic makeup and systemic chemical agents. In addition, these processes are affected by epigenetic mechanical factors: they may be accelerated in regions of intermittent high shear stress and decelerated in regions of intermittent high hydrostatic pressure. Previous models of bone development have not incorporated both biological and mechanobiological influences on endochondral growth and ossification. We have implemented a finite element analysis to model a developing bone rudiment from 8 weeks of gestational development to approximately 2 years after birth. As a function of time, we calculated a maturity index that reflects the progression of a region of cartilage through the endochondral ossification sequence of proliferation, hypertrophy, mineralization, and replacement by bone. We calculated a specific growth rate for each region of cartilage and estimated overall longitudinal growth of the rudiment. Regions of cartilage replaced by bone were remodeled. The results from the maturity index can be compared with distributions of proliferative, hypertrophic, and mineralized cartilage seen on histology at various stages in development. The results of the simulation predicted prenatal and postnatal developmental events, including formation of a secondary ossific nucleus, a layer of articular cartilage, and a growth plate. Our results demonstrate the necessity to include biological and mechanobiological influences when endochondral growth and ossification are considered.

Extracellular phosphate ions cause apoptosis of terminally differentiated epiphyseal chondrocytes

Epiphyseal chondrocytes end their life cycle through apoptosis. While this event provides a mechanism for the removal of terminally differentiated cells from cartilage, agents that promote this physiological process have not been defined. To address this issue, using a cell culture technique that models events that take place in the growth plate, we asked the following questions: Can agents that promote chondrocyte maturation and cartilage mineralization serve as specific triggers for cell death? Are chondrocytes susceptible to apoptogens at a singular developmental stage? Treatment of embryonic tibial chondrocytes with inorganic phosphate (Pi) induced death in a dose- and time-dependent manner. Within 48 hr, 3 mM Pi increased chondrocyte death by 30%; lower concentrations of Pi induced death after 48 hr. To ascertain if death was due to apoptosis, we evaluated Pi-induced death by a number of different methods and compared the results to those induced by the apoptogen, staurosporine. Analysis of the death process indicated that cartilage cells shared many of the common biological features of the apoptotic process. Thus, there was DNA fragmentation, terminal deoxynucleotidyl transferase (TUNEL) labeling, an increase in cells in the sub-G1 fraction of the cell cycle, and morphological evidence of apoptosis. To explore the specificity of the Pi effect, the experiment was repeated using embryonic sternal cephalic and caudal chondrocytes, cells that are at an earlier developmental stage than the terminally differentiated tibial cells. We noted that these cells remained vital despite a major increase in the medium Pi content. Results of this study suggest that Pi is a stage-specific inducer of apoptosis in maturing chondrocytes and that this role may be linked to chondrocyte maturation and mineralization of the extracellular matrix.

Chondrocyte death is linked to development of a mitochondrial membrane permeability transition in the growth plate

In the companion article, we reported that the local phosphate (Pi) concentration triggers apoptosis in epiphyseal chondrocytes. The goal of the current investigation was to evaluate the apoptotic process in relationship to the energy status of cells in the growth plate. For these studies, we used sections of the adolescent growth plate, as well as cells isolated from the tissue. We found that there was a maturation-dependent loss of mitochondrial function in growth plate chondrocytes and these cells generated energy by glycolysis. Since treatment with the uncoupler 2,4-dinitrophenol as well as the site-specific inhibitors antimycin A and rotenone failed to elicit a further increase in the activity of the glycolytic pathway, we concluded that oxidative metabolism was minimum in these cells. Flow cytometric studies of growth plate cells and confocal microscopy of growth plate sections using the mitochondrial probes Rh123 and DiOC6(3) provided unequivocal evidence that there was loss of mitochondrial membrane potential in hypertrophic cells. Furthermore, the intrinsic fluorescence of the flavoprotein lipoamide dehydrogenase complex of the electron transport chain revealed that the mitochondria were in an oxidized state. Finally, we assessed Bcl-2 expression in these cells. Although immunohistochemical and Western blot analysis showed that the chick cells contained a low level of the anti-apoptotic protein Bcl-2, reverse transcription-polymerase chain reaction (RT-PCR) analysis indicated that transcripts were present in chondrocytes. Based on these observations, we suggest that terminally differentiated chondrocytes undergo a maturation-dependent loss of mitochondrial function. In concert with the low expression of Bcl-2, they become sensitive to signals for programmed cell death. We hypothesize that Pi triggers apoptosis in these energy-compromised cells by promoting a mitochondrial membrane transition, thereby inducing the death process.

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

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

Ultrastructural aspects of cartilage formation, mineralization, and degeneration during primary antler growth in fallow deer (Dama dama)

Due to their rapid growth, regular replacement and easy accessibility, deer antlers are considered a useful model for the study of cartilage and bone differentiation and mineralization in mammals. The present study describes, for the first time, the cellular and extracellular matrix changes associated with cartilage formation, mineralization and degeneration in primary antlers on the ultrastructural level. Growing primary antlers of 3 to 4 cm length were obtained from six fallow bucks, aged about 10 months. It was shown that the chondroblasts were derived from progenitor cells of the antler perichondrium and differentiated into mature chondrocytes that subsequently underwent hypertrophic changes. Concomitant with cell hypertrophy, formation of a lacunar and a perilacunar extracellular matrix was observed, the latter containing numerous collagenous fibers. Mineralization of the extracellular matrix occurred via matrix vesicles and the formation of apatite crystals at distinct sites of the collagenous fibers. The hypertrophic chondrocytes of the mineralized cartilage then degenerated, a process that was also occasionally observed in more distally located cells surrounded by still unmineralized matrix. No morphological indications of a transdifferentiation of hypertrophic chondrocytes into bone forming cells, i.e., co-occurrence of a degenerating chondrocyte and a viable osteogenic cell in intact lacunae, were found. The cellular and extracellular matrix changes seen in primary antlers resemble those described for secondary antlers. Our results further indicate that the hypertrophic chondrocytes of primary antlers eventually undergo apoptosis, thereby providing further evidence that metaplastic conversion of cartilage into bone does not play a role in antler growth.

Active role of chondrocyte apoptosis in endochondral ossification

Endochondral ossification involves an ordered progression from cell division through hypertrophic differentiation to cell death. The apoptotic nature of chondrocyte death was first suggested by characteristic changes in morphology; and more recently by the pattern of DNA fragmentation and other characteristic features of apoptosis. In situ detection, although controversial, suggests that DNA fragmentation probably does not occur until late in hypertrophic differentiation. From observations of key features of apoptosis, including activation of the caspase cascade, changes in mitochondrial function, and expression of apoptosis inhibitors, the commitment of chondrocytes to apoptosis, appears to occur very early in hypertrophic differentiation. It is proposed that these changes produce effects that go far beyond the process of cell death and exert a focal influence on the endochondral ossification processes occurring in the microenvironment of the growth cartilage vascular interface. Endochondral ossification processes mediated by chondrocyte apoptosis may include intracellular calcium accumulation and release; matrix calcification; matrix resorption; maintenance of growth plate homeostasis; attraction of blood vessels and osteoblast precursors; and stimulation of bone formation.

Phenotypic and biochemical consequences of collagen X mutations in mice and humans

Skeletal biology has entered an exciting period with the technological advances in murine transgenesis and human genetics. This review focuses on how these two approaches are being used to address the role of collagen X, the major extracellular matrix component of the focal zone of endochondral ossification, the hypertrophic cartilage zone. The hypothesized role of this unique collagen in skeletal morphogenesis and the phenotypic and biochemical consequences resulting from the disruption of its function are discussed. Specifically, data from three murine models, including transgenic mice with a dominant interference phenotype for collagen X, and two sets of mice with an inactivated collagen X gene through gene targeting and homologous recombination, as well as the human disorder of Schmid metaphyseal chondrodysplasia resulting from mutations in collagen X, are summarized and compared. Several inconsistencies and unresolved issues regarding the murine and human phenotypes and the function of collagen X are discussed.

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

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

MMP-9/gelatinase B is a key regulator of growth plate angiogenesis and apoptosis of hypertrophic chondrocytes

Homozygous mice with a null mutation in the MMP-9/gelatinase B gene exhibit an abnormal pattern of skeletal growth plate vascularization and ossification. Although hypertrophic chondrocytes develop normally, apoptosis, vascularization, and ossification are delayed, resulting in progressive lengthening of the growth plate to about eight times normal. After 3 weeks postnatal, aberrant apoptosis, vascularization, and ossification compensate to remodel the enlarged growth plate and ultimately produce an axial skeleton of normal appearance. Transplantation of wild-type bone marrow cells rescues vascularization and ossification in gelatinase B-null growth plates, indicating that these processes are mediated by gelatinase B-expressing cells of bone marrow origin, designated chondroclasts. Growth plates from gelatinase B-null mice in culture show a delayed release of an angiogenic activator, establishing a role for this proteinase in controlling angiogenesis.