Differential growth by growth plates as a function of multiple parameters of chondrocytic kinetics

Tissue and cellular morphological changes in growth plate explants under compression

The mechanisms by which mechanical loading may alter bone development within growth plates are still poorly understood. However, several growth plate cell or tissue morphological parameters are associated with both normal and mechanically modulated bone growth rates. The aim of this study was to quantify in situ the three-dimensional morphology of growth plate explants under compression at both cell and tissue levels. Growth plates were dissected from ulnae of immature swine and tested under 15% compressive strain. Confocal microscopy was used to image fluorescently labeled chondrocytes in the three growth plate zones before and after compression. Quantitative morphological analyses at both cell (volume, surface area, sphericity, minor/major radii) and tissue (cell/matrix volume ratio) levels were performed. Greater chondrocyte bulk strains (volume decrease normalized to the initial cell volume) were found in the proliferative (35.4%) and hypertrophic (41.7%) zones, with lower chondrocyte bulk strains (24.7%) in the reserve zone. Following compression, the cell/matrix volume ratio decreased in the reserve and hypertrophic zones by 24.3% and 22.6%, respectively, whereas it increased by 35.9% in the proliferative zone. The 15% strain applied on growth plate explants revealed zone-dependent deformational states at both tissue and cell levels. Variations in the mechanical response of the chondrocytes from different zones could be related to significant inhomogeneities in growth plate zonal mechanical properties. The ability to obtain in situ cell morphometry and monitor the changes under compression will contribute to a better understanding of mechanisms through which abnormal growth can be triggered.

Intraspecies disparity in growth rate is associated with differences in expression of local growth plate regulators

Disparities in longitudinal growth within a species can be partly explained by endocrinological differences. We hypothesized that regulatory networks acting locally in the growth plate may also be important. We tested this hypothesis by evaluating the IGF/IGFBP expression, the vitamin D pathway, and the PTHrP-Indian hedgehog (IHH) feedback loop in rib growth plates from 10- and 21-wk-old small- (Miniature Poodles, MP) and large-breed dogs (Great Danes, GD) using immunohistochemistry and quantitative (q)PCR. The rib growth plates of GD were 1.7 times thicker compared with those of MP, with larger proliferative (in absolute terms) and larger hypertrophic (in absolute and relative terms) zones. IGF/IGFBP gene expression profiling of the growth plates revealed decreased gene expression of igfbp2, -4, and -6 and an unaltered expression of igf-I and igf-II and their respective receptors in GD vs. MP. Immunohistochemistry and qPCR findings showed that the vitamin D pathway was more active in GD than in MP. Staining for 1α- and 24-hydroxylase was more abundant and intense in GD and the gene expressions of 1α-hydroxylase and the vitamin D receptor-driven 24-hydroxylase were six- and eightfold higher in GD vs. MP, respectively. Consistent with the immunohistochemistry findings, the expression of mRNA for components of the parathyroid hormone-related peptide (PTHrP)-IHH loop was different in GD compared with MP, with there being a relative threefold downregulation of Pthrp and a tenfold upregulation of Ihh in GD vs MP. These differences suggest that the effects of IHH in the regulation of chondrocyte proliferation and hypertrophy, both independently of PTHrP, can become more dominant during rapid growth rates. In conclusion, our data suggest that, in addition to modest endocrine differences, more pronounced changes in the expression of locally acting regulatory networks, such as the IGF system, vitamin D pathway, and PTHrP-IHH feedback loop are important contributors to within-species disparities in growth rates.

Exercise mitigates the stunting effect of cold temperature on limb elongation in mice by increasing solute delivery to the growth plate

Ambient temperature and physical activity modulate bone elongation in mammals, but mechanisms underlying this plasticity are a century-old enigma. Longitudinal bone growth occurs in cartilaginous plates, which receive nutritional support via delivery of solutes from the vasculature. We tested the hypothesis that chronic exercise and warm temperature promote bone lengthening by increasing solute delivery to the growth plate, measured in real time using in vivo multiphoton microscopy. We housed 68 weanling female mice at cold (16°C) or warm (25°C) temperatures and allowed some groups voluntary access to a running wheel. We show that exercise mitigates the stunting effect of cold temperature on limb elongation after 11 days of wheel running. All runners had significantly lengthened limbs, regardless of temperature, while nonrunning mice had shorter limbs that correlated with housing temperature. Tail length was impacted only by temperature, indicating that the exercise effect was localized to limb bones and was not a systemic endocrine reaction. In vivo multiphoton imaging of fluoresceinated tracers revealed enhanced solute delivery to tibial growth plates in wheel-running mice, measured under anesthesia at rest. There was a minimal effect of rearing temperature on solute delivery when measured at an intermediate room temperature (20°C), suggesting that a lasting increase in solute delivery is an important factor in exercise-mediated limb lengthening but may not play a role in temperature-mediated limb lengthening. These results are relevant to the study of skeletal evolution in mammals from varying environments and have the potential to fundamentally advance our understanding of bone elongation processes.

A key role for membrane transporter NKCC1 in mediating chondrocyte volume increase in the mammalian growth plate

The mechanisms that underlie growth plate chondrocyte volume increase and hence bone lengthening are poorly understood. Many cell types activate the Na-K-Cl cotransporter (NKCC) to bring about volume increase. We hypothesised that NKCC may be responsible for the volume expansion of hypertrophic chondrocytes. Metatarsals/metacarpals from 16 rat pups (P(7)) were incubated in the presence/absence of the specific NKCC inhibitor bumetanide and measurement of whole-bone lengths and histologic analysis of the growth plate were done after 24 hours. Fluorescent NKCC immunohistochemistry was visualised using a confocal laser scanning microscopy on seven rat tibial growth plates (P(7)). Microarray analysis was performed on mRNA isolated from proliferative and hypertrophic zone cells of tibial growth plates from five rats of each of three ages (P(49/53/58)). Exposure to bumetanide resulted in approximately 35% reduction (paired Student's t test, p < .05) of bone growth in a dose-dependent manner; histologic analysis showed that a reduction in hypertrophic zone height was responsible. Quantification of fluorescence immunohistochemistry revealed a significant (paired Student's t test, p < .05) change in NKCC from the intracellular space of proliferative cells to the cytosolic membrane of hypertrophic zone cells. Further, microarray analysis illustrated an increase in NKCC1 mRNA between proliferative and hypertrophic cells. The increase in NKCC1 mRNA in hypertrophic zone cells, its cellular localization, and reduced bone growth in the presence of the NKCC inhibitor bumetanide implicate NKCC in growth plate hypertrophic chondrocyte volume increase. Further investigation is warranted to determine the regulatory control of NKCC in the mammalian growth plate and the possible detrimental effect on bone growth with chronic exposure to loop diuretics.

Genome-wide pleiotropy of osteoporosis-related phenotypes: the Framingham Study

Genome-wide association studies offer an unbiased approach to identify new candidate genes for osteoporosis. We examined the Affymetrix 500K + 50K SNP GeneChip marker sets for associations with multiple osteoporosis-related traits at various skeletal sites, including bone mineral density (BMD, hip and spine), heel ultrasound, and hip geometric indices in the Framingham Osteoporosis Study. We evaluated 433,510 single-nucleotide polymorphisms (SNPs) in 2073 women (mean age 65 years), members of two-generational families. Variance components analysis was performed to estimate phenotypic, genetic, and environmental correlations (rho(P), rho(G), and rho(E)) among bone traits. Linear mixed-effects models were used to test associations between SNPs and multivariable-adjusted trait values. We evaluated the proportion of SNPs associated with pairs of the traits at a nominal significance threshold alpha = 0.01. We found substantial correlation between the proportion of associated SNPs and the rho(P) and rho(G) (r = 0.91 and 0.84, respectively) but much lower with rho(E) (r = 0.38). Thus, for example, hip and spine BMD had 6.8% associated SNPs in common, corresponding to rho(P) = 0.55 and rho(G) = 0.66 between them. Fewer SNPs were associated with both BMD and any of the hip geometric traits (eg, femoral neck and shaft width, section moduli, neck shaft angle, and neck length); rho(G) between BMD and geometric traits ranged from -0.24 to +0.40. In conclusion, we examined relationships between osteoporosis-related traits based on genome-wide associations. Most of the similarity between the quantitative bone phenotypes may be attributed to pleiotropic effects of genes. This knowledge may prove helpful in defining the best phenotypes to be used in genetic studies of osteoporosis.

Expression of Runx2 and type X collagen in vertebral growth plate of patients with adolescent idiopathic scoliosis

The different expression of type X collagen and Runx2 between the convex and concave side of vertebral growth plate in scoliosis may help to improve our understanding of the role that growth plate tissue play in the development or progression of idiopathic scoliosis. In this investigation, there were significant differences of the total expression of type X collagen, Runx2 protein, and Runx2 mRNA between convex side and concave side growth plates of the apex vertebrae (p < 0.05). The total expression of type X collagen in the concave side growth plates of the lower end vertebrae was higher than that in the same side growth plates of apex (p < 0.05). The total expression of Runx2 in the concave side growth plates in the upper and lower end vertebrae were higher than that in the concave side growth plates of apex (p < 0.05). The expression of type X collagen, Runx2, and Runx2 mRNA, the cell density of type X collagen and Runx2 positive chondrocytes, and histological changes between convex side and concave side of the vertebral growth plate indicated that the vertebral growth plate was affected by mechanical forces, which was a secondary change and could contribute to progression of adolescent idiopathic scoliosis.

Developmental engineering: a new paradigm for the design and manufacturing of cell-based products. Part II: from genes to networks: tissue engineering from the viewpoint of systems biology and network science

The field of tissue engineering is moving toward a new concept of "in vitro biomimetics of in vivo tissue development." In Part I of this series, we proposed a theoretical framework integrating the concepts of developmental biology with those of process design to provide the rules for the design of biomimetic processes. We named this methodology "developmental engineering" to emphasize that it is not the tissue but the process of in vitro tissue development that has to be engineered. To formulate the process design rules in a rigorous way that will allow a computational design, we should refer to mathematical methods to model the biological process taking place in vitro. Tissue functions cannot be attributed to individual molecules but rather to complex interactions between the numerous components of a cell and interactions between cells in a tissue that form a network. For tissue engineering to advance to the level of a technologically driven discipline amenable to well-established principles of process engineering, a scientifically rigorous formulation is needed of the general design rules so that the behavior of networks of genes, proteins, or cells that govern the unfolding of developmental processes could be related to the design parameters. Now that sufficient experimental data exist to construct plausible mathematical models of many biological control circuits, explicit hypotheses can be evaluated using computational approaches to facilitate process design. Recent progress in systems biology has shown that the empirical concepts of developmental biology that we used in Part I to extract the rules of biomimetic process design can be expressed in rigorous mathematical terms. This allows the accurate characterization of manufacturing processes in tissue engineering as well as the properties of the artificial tissues themselves. In addition, network science has recently shown that the behavior of biological networks strongly depends on their topology and has developed the necessary concepts and methods to describe it, allowing therefore a deeper understanding of the behavior of networks during biomimetic processes. These advances thus open the door to a transition for tissue engineering from a substantially empirical endeavor to a technology-based discipline comparable to other branches of engineering.

Evidence for pleiotropic factors in genetics of the musculoskeletal system

There are both theoretical and empirical underpinnings that provide evidence that the musculoskeletal system develops, functions, and ages as a whole. Thus, the risk of osteoporotic fracture can be viewed as a function of loading conditions and the ability of the bone to withstand the load. Both bone loss (osteoporosis) and muscle wasting (sarcopenia) are the two sides of the same coin, an involution of the musculoskeletal system. Skeletal loads are dominated by muscle action; both bone and muscle share environmental, endocrine and paracrine influences. Muscle also has an endocrine function by producing bioactive molecules, which can contribute to homeostatic regulation of both bone and muscle. It also becomes clear that bone and muscle share genetic determinants; therefore the consideration of pleiotropy is an important aspect in the study of the genetics of osteoporosis and sarcopenia. The aim of this review is to provide an additional evidence for existence of the tight genetic co-regulation of muscles and bones, starting early in development and still evident in aging. Recently, important papers were published, including those dealing with the cellular mechanisms and anatomic substrate of bone mechanosensitivity. Further evidence has emerged suggesting that the relationship between skeletal muscle and bone parameters extends beyond the general paradigm of bone responses to mechanical loading. We provide insights into several pathways and single genes, which apparently have a biologically plausible pleiotropic effect on both bones and muscles; the list is continuing to grow. Understanding the crosstalk between muscles and bones will translate into a conceptual framework aimed at studying the pleiotropic genetic relationships in the etiology of complex musculoskeletal disease. We believe that further progress in understanding the common genetic etiology of osteoporosis and sarcopenia will provide valuable insight into important biological underpinnings for both musculoskeletal conditions. This may translate into new approaches to reduce the burden of both conditions, which are prevalent in the elderly population.

The effect of rapamycin on bone growth in rabbits

mTOR is a nutrient-sensing protein kinase that regulates numerous cellular processes. Our prior studies using the mTOR inhibitor, rapamycin, indicate an important role for mTOR in chondrogenesis. We extended our observations to a physiological, in vivo model of bone growth, direct infusion of rapamycin into the proximal tibial growth plates of rabbits. Rapamycin or DMSO vehicle was infused directly into growth plates by an osmotic minipump for 8 weeks. Tibial growth was followed radiographically. At the end of the experiment, growth plates were recovered for histological analysis. Six animals were studied. No untoward effects of rapamycin infusion were found. Bone growth of limbs exposed to rapamycin was slower than control limbs, particularly during the period of most rapid growth. Histological analysis revealed that growth plate height in the rapamycin-infused limbs was reduced. Both the hypertrophic and proliferative zones were significantly smaller in the rapamycin-infused limbs. Direct infusion of rapamycin into proximal tibial growth plates decreased the size of the growth plate and inhibited overall long bone growth. Rapamycin appears to affect both the proliferative and hypertrophic zones of the tibial growth plate. Our results indicate that nutrients may exert a direct effect on long bone growth via mTOR-mediated modulation of chondrogenesis at the growth plate. and suggest that the possible inhibitory effects of rapamycin on skeletal growth warrant further attention before its use in children.

Ontogeny of the tessellated skeleton: insight from the skeletal growth of the round stingray Urobatis halleri

The majority of the skeleton of elasmobranch fishes (sharks, rays and relatives) is tessellated: uncalcified cartilage is overlain by a superficial rind of abutting, mineralized, hexagonal blocks called tesserae. We employed a diversity of imaging techniques on an ontogenetic series of jaw samples to investigate the development of the tessellated skeleton in a stingray (Urobatis halleri). We compared these data with the cellular changes that characterize cartilage calcification in bony skeletons. Skeletal growth is characterized by the appearance of tesserae as well as changes in chondrocyte shape, arrangement and density. Yolk sac embryos (35-56 mm disc width, DW) have untessellated lower jaw tissue wrapped in perichondrium and densely packed with chondrocytes. Chondrocyte density decreases dramatically after yolk sac absorption (histotroph stage: 57-80 mm DW) until the formation of tesserae, which are first visible using our techniques as thin (approximately 60 microm), sub-perichondral plaques. During the histotroph stage, flattened chondrocytes align parallel to the perichondrium at the tissue periphery, where we believe they are incorporated into developing tesserae to form the cell-rich laminae observed within tesserae; in older animals peripheral cells in the uncalcified phase are rounder and less uniformly oriented. By parturition (approximately 75 mm DW), cell density and the number of adjoining chondrocyte pairs (an indicator of cell division) have dropped to less than a third of their initial values; these remain low and tesserae continue to grow in size. The tessellated skeleton is a simple solution to the conundrum of growth in an endoskeleton with external mineralization and no remodeling. Although we see parallels with endochondral ossification (e.g. chondrocytes decreasing in density with age), the lack of chondrocyte hypertrophy and the fact that mineralization is sub-perichondral (not the case in mammalian cartilage) suggest that the similarities end there.

Radioprotectant combinations spare radiation-induced damage to the physis more than fractionation alone

PURPOSE

The aim of this study was to determine if fractionation and individual or combinations of radioprotectants could minimize damage to physeal longitudinal growth in an animal model to any greater extent than fractionation alone.

MATERIALS AND METHODS

Sixty-three weanling male Sprague-Dawley rats were randomized into seven equal groups. Five groups received a total 25 Gy radiation exposure in three equal fractions to the right knee with the left as non-irradiated control. For each group, pentoxifylline, misoprostol, and amifostine were given individually and amifostine was also given in combination with each of the other drugs prior to the radiation fractions. One group each received 25 Gy in one or three fractions without radioprotection. At six weeks, limb lengths and histomorphometry were assessed.

RESULTS

The single fraction of 25 Gy caused a mean tibial length discrepancy of 24.4%. Fractionation decreased this to 18.8% (p < 0.001). Beyond fractionation alone, the mean femoral length discrepancies were significantly decreased by each of the added individual and combination radioprotectant drugs (p < 0.0004). The smallest absolute femoral length discrepancy (11%) was achieved with fractionation and the combination of amifostine and misoprostol.

CONCLUSIONS

Radioprotectants may be beneficial in growth plate radioprotection, alone or in combination.

Leucine restriction inhibits chondrocyte proliferation and differentiation through mechanisms both dependent and independent of mTOR signaling

Linear growth in children is sensitive to nutritional status. Amino acids, in particular leucine, have been shown to regulate cell growth, proliferation, and differentiation through the mammalian target of rapamycin (mTOR), a nutrient-sensing protein kinase. Having recently demonstrated a role for mTOR in chondrogenesis, we hypothesized that leucine restriction, acting through mTOR, would inhibit growth plate chondrocyte proliferation and differentiation. The effect of leucine restriction was compared with that of the specific mTOR inhibitor, rapamycin. Leucine restriction produced a dose-dependent inhibition of fetal rat metatarsal explant growth. This was accounted by reduced cell proliferation and hypertrophy but not apoptosis. mTOR activity, as reflected by ribosomal protein S6 phosphorylation, was only partially inhibited by leucine restriction, whereas rapamycin abolished S6 phosphorylation. In chondrogenic ATDC5 cells, leucine restriction inhibited cell number, proteoglycan accumulation, and collagen X expression despite minimal inhibition of mTOR. Microarray analysis demonstrated that the effect of leucine restriction on ATDC5 cell gene expression differed from that of rapamycin. Out of 1,571 genes affected by leucine restriction and 535 genes affected by rapamycin, only 176 genes were affected by both. These findings indicate that the decreased chondrocyte growth and differentiation associated with leucine restriction is only partly attributable to inhibition of mTOR signaling. Thus nutrient restriction appears to directly modulate bone growth through unidentified mTOR-independent mechanisms in addition to the well-characterized mTOR nutrient-sensing pathway.

Spinal hemiepiphysiodesis decreases the size of vertebral growth plate hypertrophic zone and cells

BACKGROUND

Hemiepiphysiodesis is a potential method to treat idiopathic juvenile scoliosis early. The purpose of the present study was to investigate a mechanism of curve creation in the pig thoracic model of spinal hemiepiphysiodesis by determining whether the structure of the vertebral growth plate varied with distance from the stapled, concave side of the spine. The hypotheses were that the heights of the hypertrophic zone, hypertrophic cells, and disc would be decreased on the treated side of the treated level as compared with both an unstapled control level and the side opposite the staple.

METHODS

Custom spine staples were implanted into six midthoracic vertebrae in each of five skeletally immature pigs. After eight weeks, the spines were harvested and histological sections were prepared. Hypertrophic zone height, hypertrophic cell height and width, and disc height were measured at discrete coronal plane locations at stapled and unstapled thoracic levels. Differences between stapled and unstapled levels and locations were compared with use of mixed linear modeling for repeated measures, followed by regression models to determine growth plate intercept and slope across the plane by thoracic level.

RESULTS

Zone height, cell height, and cell width were lowest on the stapled side of the stapled level, with significant differences in the overall statistical model (p < 0.02). Disc heights were significantly reduced (p < 0.0001) at the stapled levels across the coronal plane.

CONCLUSIONS

Unilateral control of intervertebral joint motion decreased growth plate height, cell size, and disc height.

The effect of fluoroquinolone antibiotics on growing cartilage in the lamb model

BACKGROUND

The fluoroquinolones are a relatively new class of antimicrobials with an appealing spectrum of activity. Their use in pediatric medicine is limited because of the concern over possible growth inhibition, as published reports have documented articular cartilage damage in animal models after their administration. These data, extrapolated to include the epiphyseal cartilage, suggest that these agents may reduce growth rates, but limited human data are at the least equivocal, if not strictly contradictory to such claims. Specific investigations into the effects of fluoroquinolones on epiphyseal plate cartilage and growth velocity have not been performed.

METHODS

Gatifloxacin and ciprofloxacin were used as representative agents of the fluoroquinolone class. Each drug was administered to experimental lambs over a 14-day interval at a dose designed to reflect those used in pediatric medicine. Recumbent versus standing intervals were used to monitor for arthropathy. Upon completion of fluoroquinolone administration, lambs underwent double fluorochrome labeling for determination of growth velocity. Gross and microscopic analysis of articular cartilage was performed to assess for pathologic changes. Age- and sex-matched lambs served as controls.

RESULTS

Neither gatifloxacin nor ciprofloxacin negatively affected growth velocity of the proximal tibial growth plate as measured by double fluorochrome labeling. In addition, no difference between experimental and control lambs in regard to recumbent versus standing intervals was noted. Examination of the articular cartilage failed to suggest chondrotoxicity.

CONCLUSION

Fluoroquinolone antimicrobials do not affect growth velocity in the ovine model when administered along a dosing regimen that closely models that seen in pediatric medicine.

CLINICAL RELEVANCE

Fluoroquinolones may be acceptable for use in the pediatric population, as concerns over chondrotoxicity and growth inhibition may not be valid. These data suggest that expanded studies in lambs and other species, including humans, with differences in dosing and duration are justified to ultimately demonstrate clinical safety.

Rac1 signaling regulates CTGF/CCN2 gene expression via TGFbeta/Smad signaling in chondrocytes

OBJECTIVE

Connective tissue growth factor (CTGF) has been implicated in regulation of chondrocyte differentiation at multiple steps and has been implicated in the progression of diseases such as scleroderma and osteoarthritis. However, the pathways mediating the expression of CTGF/CCN2 and related factors in cartilage are not fully understood. We have previously shown that the Rho family of proteins and the actin cytoskeleton regulate both early and late chondrocyte differentiation.

RESULTS

Here we demonstrate that several CTGF/Cyr61/Nov (CCN) family members are differentially affected by either inhibition of actin polymerization (cytochalasin D treatment), promotion of actin polymerization (jasplakinolide treatment), inhibition of RhoA/rho kinase (ROCK) signaling (Y27632 treatment) and Rac1 signaling. We also show that the Smad site in the CTGF/CCN2 promoter is responsive to both Rac1 inhibition and cytochalasin D treatment, suggesting a role of TGFbeta/Smad signaling in mediating the effects of actin dynamics and Rac1.

CONCLUSION

Collectively, these data show that Rac1 and actin pathways control CTGF/CCN2 expression in chondrocytes which might be relevant to both skeletal development and associated diseases such as osteoarthritis.

C/EBPbeta Promotes transition from proliferation to hypertrophic differentiation of chondrocytes through transactivation of p57

BACKGROUND

Although transition from proliferation to hypertrophic differentiation of chondrocytes is a crucial step for endochondral ossification in physiological skeletal growth and pathological disorders like osteoarthritis, the underlying mechanism remains an enigma. This study investigated the role of the transcription factor CCAAT/enhancer-binding protein beta (C/EBPbeta) in chondrocytes during endochondral ossification.

METHODOLOGY/PRINCIPAL FINDINGS

Mouse embryos with homozygous deficiency in C/EBPbeta (C/EBPbeta-/-) exhibited dwarfism with elongated proliferative zone and delayed chondrocyte hypertrophy in the growth plate cartilage. In the cultures of primary C/EBPbeta-/- chondrocytes, cell proliferation was enhanced while hypertrophic differentiation was suppressed. Contrarily, retroviral overexpression of C/EBPbeta in chondrocytes suppressed the proliferation and enhanced the hypertrophy, suggesting the cell cycle arrest by C/EBPbeta. In fact, a DNA cell cycle histogram revealed that the C/EBPbeta overexpression caused accumulation of cells in the G0/G1 fraction. Among cell cycle factors, microarray and real-time RT-PCR analyses have identified the cyclin-dependent kinase inhibitor p57(Kip2) as the transcriptional target of C/EBPbeta. p57(Kip2) was co-localized with C/EBPbeta in late proliferative and pre-hypertrophic chondrocytes of the mouse growth plate, which was decreased by the C/EBPbeta deficiency. Luciferase-reporter and electrophoretic mobility shift assays identified the core responsive element of C/EBPbeta in the p57(Kip2) promoter between -150 and -130 bp region containing a putative C/EBP motif. The knockdown of p57(Kip2) by the siRNA inhibited the C/EBPbeta-induced chondrocyte hypertrophy. Finally, when we created the experimental osteoarthritis model by inducing instability in the knee joints of adult mice of wild-type and C/EBPbeta+/- littermates, the C/EBPbeta insufficiency caused resistance to joint cartilage destruction.

CONCLUSIONS/SIGNIFICANCE

C/EBPbeta transactivates p57(Kip2) to promote transition from proliferation to hypertrophic differentiation of chondrocytes during endochondral ossification, suggesting that the C/EBPbeta-p57(Kip2) signal would be a therapeutic target of skeletal disorders like growth retardation and osteoarthritis.

Effects of in vivo static compressive loading on aggrecan and type II and X collagens in the rat growth plate extracellular matrix

Mechanical loads are essential to normal bone growth, but excessive loads can lead to progressive deformities. In addition, growth plate extracellular matrix remodelling is essential to regulate the normal longitudinal bone growth process and to ensure physiological bone mineralization. In order to investigate the effects of static compression on growth plate extracellular matrix using an in vivo animal model, a loading device was used to precisely apply a compressive stress of 0.2 MPa for two weeks on the seventh caudal vertebra (Cd7) of rats during the pubertal growth spurt. Control, sham and loaded groups were studied. Growth modulation was quantified based on calcein labelling, and three matrix components (type II and X collagens, and aggrecan) were assessed using immunohistochemistry/safranin-O staining. As well, extracellular matrix components and enzymes (MMP-3 and -13, ADAMTS-4 and -5) were studied by qRT-PCR. Loading reduced Cd7 growth by 29% (p<0.05) and 15% (p=0.07) when compared to controls and shams respectively. No significant change could be observed in the mRNA expression of collagens and the proteolytic enzyme MMP-13. However, MMP-3 was significantly increased in the loaded group as compared to the control group (p<0.05). No change was observed in aggrecan and ADAMTS-4 and -5 expression. Low immunostaining for type II and X collagens was observed in 83% of the loaded rats as compared to the control rats. This in vivo study shows that, during pubertal growth spurt, two-week static compression reduced caudal vertebrae growth rates; this mechanical growth modulation occurred with decreased type II and X collagen proteins in the growth plate.

Bone growth during rapamycin therapy in young rats

BACKGROUND

Rapamycin is an effective immunosuppressant widely used to maintain the renal allograft in pediatric patients. Linear growth may be adversely affected in young children since rapamycin has potent anti-proliferative and anti-angiogenic properties.

METHODS

Weanling three week old rats were given rapamycin at 2.5 mg/kg daily by gavage for 2 or 4 weeks and compared to a Control group given equivalent amount of saline. Morphometric measurements and biochemical determinations for serum calcium, phosphate, iPTH, urea nitrogen, creatinine and insulin-growth factor I (IGF-I) were obtained. Histomorphometric analysis of the growth plate cartilage, in-situ hybridization experiments and immunohistochemical studies for various proteins were performed to evaluate for chondrocyte proliferation, chondrocyte differentiation and chondro/osteoclastic resorption.

RESULTS

At the end of the 2 weeks, body and tibia length measurements were shorter after rapamycin therapy associated with an enlargement of the hypertrophic zone in the growth plate cartilage. There was a decrease in chondrocyte proliferation assessed by histone-4 and mammalian target of rapamycin (mTOR) expression. A reduction in parathyroid hormone/parathyroid hormone related peptide (PTH/PTHrP) and an increase in Indian hedgehog (Ihh) expression may explain in part, the increase number of hypertrophic chondrocytes. The number of TRAP positive multinucleated chondro/osteoclasts declined in the chondro-osseous junction with a decrease in the receptor activator of nuclear factor kappa beta ligand (RANKL) and vascular endothelial growth factor (VEGF) expression. Although body and tibial length remained short after 4 weeks of rapamycin, changes in the expression of chondrocyte proliferation, chondrocyte differentiation and chondro/osteoclastic resorption which were significant after 2 weeks of rapamycin improved at the end of 4 weeks.

CONCLUSION

When given to young rats, 2 weeks of rapamycin significantly decreased endochondral bone growth. No catch-up growth was demonstrated at the end of 4 weeks, although markers of chondrocyte proliferation and differentiation improved. Clinical studies need to be done to evaluate these changes in growing children.

The effect of periosteal resection on tibial growth velocity measured by microtransducer technology in lambs

BACKGROUND

Disruption of the periosteum, whether traumatic or elective, has long been known to accelerate growth in the developing skeleton. However, the extent, timing, and mechanism of the resultant increase in growth velocity (if any) remain undefined. The primary research questions were: Does periosteal resection result in a change (increase) in growth velocity of a long bone at the growth plate? When does the effect start after the resection and for how long? Finally, which of several cellular mechanisms is most likely responsible for the change in growth velocity?

METHODS

Five lambs underwent proximal tibial growth plate periosteal resection with subsequent measurement of growth velocity by implantable microtransducers or fluorochrome labeling. This former technique provided real-time growth velocity data with a resolution of about 10 microm (width of a proliferative zone chondrocyte). These measurements were accurate at up to 4 weeks postoperative, as verified by fluorochrome labeling, and radiographic measurement. Two lambs were continued on the study for an additional 3 weeks. Histomorphometric and stereological assessments of chondrocytic kinetic parameters were performed on control and experimental tibiae after euthanasia.

RESULTS

Periosteal resection increased growth velocity in every lamb, at every time point, and in a consistent and sustained manner. Histomorphometric correlation to this phenomenon indicated that the cellular basis of this acceleration was most likely the result of hypertrophic chondrocyte axial elongation rather than changes in chondrocyte proliferation, magnitude of hypertrophic chondrocytic swelling, or increased matrix production.

CONCLUSIONS

Periosteal resection creates immediate and sustained acceleration of growth resulting from axial elongation of the hypertrophic chondrocyte. Although the increase in growth velocity was consistent, the absolute magnitude of the acceleration suggests that periosteal resection be considered as an adjunct to other primary procedures. Periosteal resection may serve as a useful clinical adjunct to provide a modest growth stimulus in cases of hemihypertrophy or angular limb deformity or to counteract the growth inhibition seen when performing distraction osteogenesis.

New insights into function of the growth plate

The mammalian growth plate is a complex structure which is essential for the elongation of long bones. However, an understanding of how the growth plate functions at the cellular level is lacking. This review, summarises the factors involved in growth-plate regulation, its failure and the consequence of injury. We also describe some of the cellular mechanisms which underpin the increase in volume of the growth-plate chondrocyte which is the major determinant of the rate and extent of bone lengthening. We show how living in situ chondrocytes can be imaged using 2-photon laser scanning microscopy to provide a quantitative analysis of their volume. This approach should give better understanding of the cellular control of bone growth in both healthy and failed growth plates.

Age and pattern of the onset of differential growth among growth plates in rats

Differential growth is the phenomenon whereby growth plates in the same individual at the same time all have uniquely different axial growth velocities. Differential growth is clearly present in the adolescent skeleton. In this study we ask two questions. When and by what pattern does the phenomenon of differential growth begin? Second, to what extent are the development of differential growth velocities correlated with changes in hypertrophic chondrocyte volume and/or with changes in chondrocytic production/turnover? Four growth plates (proximal and distal radial; proximal and distal tibial) were studied at 24 different time points in Long-Evans rats between the 17th gestational day (when differential growth does not exist) and postnatal day 27 (when differential growth is well established). Growth velocities were measured using fluorochrome labeling. Using stereological methodology, multiple chondrocytic kinetic parameters were measured for all growth plates. Elongation of the proximal radial growth plate decreases relative to elongation in the other three growth plates in the late fetal phase. Differential growth is fully expressed at postnatal day 13 when the other three growth plates start to decrease daily elongation at different rates. Differential growth is primarily associated with differences in hypertrophic cell volume manifested when growth deceleration occurs. This study also illustrates that differential growth is superimposed on systemic regulators that affect all growth plates simultaneously. The most dramatic illustration of this is the sharp decline in growth velocity in all four growth plates that occurs perinatally.

Histomorphometric evidence of growth plate recovery potential after fractionated radiotherapy: an in vivo model

This study evaluated the hypothesis that early growth plate radiorecovery is evident by growth rate, histomorphometric and immunohistochemical end points after exposure to clinically relevant fractionated radiation in vivo. Twenty-four weanling 5-week-old male Sprague-Dawley rats were randomized into eight groups. In each animal, the right distal femur and proximal tibia were exposed to five daily fractions of 3.5 Gy (17.5 Gy) with the left leg serving as a control. Rats were killed humanely at 7, 8, 9, 10, 11, 14, 15 and 16 days after the first day of radiation exposure. Quantitative end points calculated included individual zonal and overall growth plate heights, area matrix fraction, OTC-labeled growth rate, chondrocyte clone volume and numeric density, and BrdU immunohistochemical labeling for proliferative index. Transient postirradiation reductions occurred early and improved during observation for growth rate, proliferative indices, transitional/hypertrophic zone matrix area fraction, proliferative height, and clonal volume. Reserve and hypertrophic zone height remained increased during the period of observation. The current model, using a more clinically relevant fractionation scheme than used previously, shows early evidence of growth plate recovery and provides a model that can be used to correlate temporal changes in RNA and protein expression during the early period of growth plate recovery.

Phosphorylation of GSK-3beta by cGMP-dependent protein kinase II promotes hypertrophic differentiation of murine chondrocytes

cGMP-dependent protein kinase II (cGKII; encoded by PRKG2) is a serine/threonine kinase that is critical for skeletal growth in mammals; in mice, cGKII deficiency results in dwarfism. Using radiographic analysis, we determined that this growth defect was a consequence of an elongated growth plate and impaired chondrocyte hypertrophy. To investigate the mechanism of cGKII-mediated chondrocyte hypertrophy, we performed a kinase substrate array and identified glycogen synthase kinase-3beta (GSK-3beta; encoded by Gsk3b) as a principal phosphorylation target of cGKII. In cultured mouse chondrocytes, phosphorylation-mediated inhibition of GSK-3beta was associated with enhanced hypertrophic differentiation. Furthermore, cGKII induction of chondrocyte hypertrophy was suppressed by cotransfection with a phosphorylation-deficient mutant of GSK-3beta. Analyses of mice with compound deficiencies in both protein kinases (Prkg2(-/-)Gsk3b(+/-)) demonstrated that the growth retardation and elongated growth plate associated with cGKII deficiency were partially rescued by haploinsufficiency of Gsk3b. We found that beta-catenin levels decreased in Prkg2(-/-) mice, while overexpression of cGKII increased the accumulation and transactivation function of beta-catenin in mouse chondroprogenitor ATDC5 cells. This effect was blocked by coexpression of phosphorylation-deficient GSK-3beta. These data indicate that hypertrophic differentiation of growth plate chondrocytes during skeletal growth is promoted by phosphorylation and inactivation of GSK-3beta by cGKII.

Nutrition-induced catch-up growth at the growth plate

The effect of 40% food restriction (FR) and replenishment on the growth hormone (GH) and insulin-like growth factor-I (IGF-I) axis in the epiphyseal growth plate (EGP) was examined in a mouse model. Changes in RNA and protein levels were evaluated with real time PCR and immunohistochemistry, respectively, and serum levels of IGF-I and leptin were measured with radioimmunoassay. Dramatic changes in weight, tibial length and EGP height were observed following 10 days of 40% FR. The protein levels of IGF-I receptor (IGF-IR) and GH receptor (GHR), which were reduced during FR, increased during catch-up growth without an apparent change in the level of their RNA. The levels of type II and X collagens were unchanged. Serum IGF-I and leptin levels were reduced during FR and increased during catch-up growth. Following 40% FR, there was a significant decrease in the level of GHR and IGF-IR in the EGP which may explain the reduced effect of GH treatment in malnourished animals and children.

The heterozygous disproportionate micromelia (dmm) mouse: morphological changes in fetal cartilage precede postnatal dwarfism and compared with lethal homozygotes can explain the mild phenotype

The disproportionate micromelia (Dmm) mouse has a mutation in the C-propeptide coding region of the Col2a1 gene that causes lethal dwarfism when homozygous (Dmm/Dmm) but causes only mild dwarfism observable approximately 1-week postpartum when heterozygous (Dmm/+). The purpose of this study was 2-fold: first, to analyze and quantify morphological changes that precede the expression of mild dwarfism in Dmm/+ animals, and second, to compare morphological alterations between Dmm/+ and Dmm/Dmm fetal cartilage that may correlate with the marked skeletal differences between mild and lethal dwarfism. Light and electron transmission microscopy were used to visualize structure of chondrocytes and extracellular matrix (ECM) of fetal rib cartilage. Both Dmm/+ and Dmm/Dmm fetal rib cartilage had significantly larger chondrocytes, greater cell density, and less ECM per unit area than +/+ littermates. Quantitative RT-PCR showed a decrease in aggrecan mRNA in Dmm/+ vs +/+ cartilage. Furthermore, the cytoplasm of chondrocytes in Dmm/+ and Dmm/Dmm cartilage was occupied by significantly more distended rough endoplasmic reticulum (RER) compared with wild-type chondrocytes. Fibril diameters and packing densities of +/+ and Dmm/+ cartilage were similar, but Dmm/Dmm cartilage showed thinner, sparsely distributed fibrils. These findings support the prevailing hypothesis that a C-propeptide mutation could interrupt the normal assembly and secretion of Type II procollagen trimers, resulting in a buildup of proalpha1(II) chains in the RER and a reduced rate of matrix synthesis. Thus, intracellular entrapment of proalpha1(II) seems to be primarily responsible for the dominant-negative effect of the Dmm mutation in the expression of dwarfism.

Histomorphometric analysis of an adolescent distal tibial physis prior to growth plate closure

PURPOSE

Our current understanding of the rate and pattern of physeal closure is based on roentgenographic, magnetic resonance imaging, and qualitative histological studies. The purpose of this report is to provide a detailed histomorphometric/stereological analysis of a distal tibial human growth plate in the process of physiological epiphysiodesis.

METHODS

A human distal tibial growth plate was sampled in three regions (anterior, central, and posterior), with each region further separated medially, in the middle, and laterally. The regions were assessed for the location and extent of bony bar formation as well as for physeal height. Companion sections from optimally fixed tissue in the distal 100 microm of the hypertrophic zone were analyzed for hypertrophic chondrocytic volumes.

RESULTS

Physis closure started in the middle of the central region of the growth plate, with 46% of the volume in this area occupied by trans-physeal bridging bone. The growth plate was also narrowed with the lowest physeal heights evident in the middle of the central and anterior regions of the physis. Disruption of the regular columns of the physis was evident with the cells arranged in clusters with intervening areas of acellularity. The average hypertrophic cell volume was 5,900 microm(3) and did not significantly differ between different areas of the physis.

CONCLUSIONS

This is the first characterization of closure in a human distal tibial growth plate via optimum fixation and stereological techniques. The studied physis was during the earliest phases of closure and provides stereological support that the distal tibial physis closes in a central to medial direction.

Regulation of immature cartilage growth by IGF-I, TGF-beta1, BMP-7, and PDGF-AB: role of metabolic balance between fixed charge and collagen network

Cartilage growth may involve alterations in the balance between the swelling tendency of proteoglycans and the restraining function of the collagen network. Growth factors, including IGF-I, TGF-beta1, BMP-7, and PDGF-AB, regulate chondrocyte metabolism and, consequently, may regulate cartilage growth. Immature bovine articular cartilage explants from the superficial and middle zones were incubated for 13 days in basal medium or medium supplemented with serum, IGF-I, TGF-beta1, BMP-7, or PDGF-AB. Variations in tissue size, accumulation of proteoglycan and collagen, and tensile properties were assessed. The inclusion of serum, IGF-I, or BMP-7 resulted in expansive tissue growth, stimulation of proteoglycan deposition but not of collagen, and a diminution of tensile integrity. The regulation of cartilage metabolism by TGF-beta1 resulted in tissue homeostasis, with maintenance of size, composition, and function. Incubation in basal medium or with PDGF-AB resulted in small volumetric and compositional changes, but a marked decrease in tensile integrity. These results demonstrate that the phenotype of cartilage growth, and the associated balance between proteoglycan content and integrity of the collagen network, is regulated differentially by certain growth factors.

Accelerated hypertrophic chondrocyte kinetics in GDF-7 deficient murine tibial growth plates

The Growth/Differentiation Factors (GDFs) are a subgroup of the Bone Morphogenetic Proteins (BMPs) well known for their role in joint formation and chondrogenesis. Mice deficient in one of these signaling molecules, GDF-5, have recently been shown to exhibit a decreased rate of endochondral bone growth in the proximal tibia due to a significantly longer hypertrophic phase duration. GDF-7 is a related family member, which exhibits a high degree of sequence identity with GDF-5. The purpose of the present study was to determine whether GDF-7 deficiency also alters the endochondral bone growth rate in mice and, if so, how this is achieved. Stereologic and cell kinetic parameters in proximal tibial growth plates from 5-week-old female GDF-7 -/- mice and wild type control littermates were examined. GDF-7 deficiency resulted in a statistically significant increase in growth rate (+26%; p = 0.0084) and rate of cell loss at the chondrosseous junction (+25%; p = 0.0217). Cells from GDF-7 deficient mice also exhibited a significantly shorter hypertrophic phase duration compared to wild type controls (-27%; p = 0.0326). These data demonstrate that, in the absence of GDF-7, the rate of endochondral bone growth is affected through the modulation of hypertrophic phase duration in growth plate chondrocytes. These findings further support a growing body of evidence implicating the GDFs in the formation, maturation, and maintenance of healthy cartilage.

mTOR signaling contributes to chondrocyte differentiation

The mammalian Target Of Rapamycin (mTOR) is a nutrient-sensing protein kinase that regulates numerous cellular processes. Fetal rat metatarsal explants were used as a physiological model to study the effect of mTOR inhibition on chondrogenesis. Insulin significantly enhanced their growth. Rapamycin significantly diminished this response to insulin through a selective effect on the hypertrophic zone. Cell proliferation (bromodeoxyuridine incorporation) was unaffected by rapamycin. Similar observations were made when rapamycin was injected to embryonic day (E) 19 fetal rats in situ. In the ATDC5 chondrogenic cell line, rapamycin inhibited proteoglycan accumulation and collagen X expression. Rapamycin decreased content of Indian Hedgehog (Ihh), a regulator of chondrocyte differentiation. Addition of Ihh to culture medium reversed the effect of rapamycin. We conclude that modulation of mTOR signaling contributes to chondrocyte differentiation, perhaps through its ability to regulate Ihh. Our findings support the hypothesis that nutrients, acting through mTOR, directly influence chondrocyte differentiation and long bone growth.

Genetics of the musculoskeletal system: a pleiotropic approach

The risk of osteoporotic fracture can be viewed as a function of loading conditions and the ability of the bone to withstand the load. Skeletal loads are dominated by muscle action. Recently, it has become clear that bone and muscle share genetic determinants. Involution of the musculoskeletal system manifests as bone loss (osteoporosis) and muscle wasting (sarcopenia). Therefore, the consideration of pleiotropy is an important aspect in the study of the genetics of osteoporosis and sarcopenia. This Perspective will provide the evidence for a shared genetic influence on bone and muscle. We will start with an overview of accumulating evidence that physical exercise produces effects on the adult skeleton, seeking to unravel some of the contradictory findings published thus far. We will provide indications that there are pleiotropic relationships between bone structure/mass and muscle mass/function. Finally, we will offer some insights and practical recommendations as to the value of studying shared genetic factors and will explore possible directions for future research. We consider several related questions that together comprise the general paradigm of bone responses to mechanical loading and the relationship between muscle strength and bone parameters, including the genetic factors that modulate these responses. We believe that further progress in understanding the common genetic etiology of osteoporosis and sarcopenia will provide valuable insight into important biological underpinnings for both conditions and may translate into new approaches to reduce the burdens of both conditions through improved diagnosis, prevention, and early targeted treatment.

Bioengineering cartilage growth, maturation, and form

Cartilage of articular joints grows and matures to achieve characteristic sizes, forms, and functional properties. Through these processes, the tissue not only serves as a template for bone growth but also yields mature articular cartilage providing joints with a low-friction, wear-resistant bearing material. The study of cartilage growth and maturation is a focus of both cartilage biologists and bioengineers with one goal of trying to create biologic tissue substitutes for the repair of damaged joints. Experimental approaches both in vivo and in vitro are being used to better understand the mechanisms and regulation of growth and maturation processes. This knowledge may facilitate the controlled manipulation of cartilage size, shape, and maturity to meet the criteria needed for successful clinical applications. Mathematical models are also useful tools for quantitatively describing the dynamically changing composition, structure and function of cartilage during growth and maturation and may aid the development of tissue engineering solutions. Recent advances in methods of cartilage formation and culture which control the size, shape, and maturity of these tissues are numerous and provide contrast to the physiologic development of cartilage.

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.

Developmental basis of limb length in rodents: evidence for multiple divisions of labor in mechanisms of endochondral bone growth

Mammals are remarkably diverse in limb lengths and proportions, but the number and kind of developmental mechanisms that contribute to length differences between limb bones remain largely unknown. Intra- and interspecific differences in bone length could result from variations in the cellular processes of endochondral bone growth, creating differences in rates of chondrocyte proliferation or hypertrophy, variation in the shape and size of chondrocytes, differences in the number of chondrocytes in precursor populations and throughout growth, or a combination of these mechanisms. To address these questions, this study compared cellular mechanisms of endochondral bone growth in cross-sectional ontogenetic series of the appendicular skeleton of two rodent species: the mouse (Mus musculus) and Mongolian gerbil (Meriones unguiculatus). Results indicate that multiple cellular processes of endochondral bone growth contribute to phenotypic differences in limb bone length. The data also suggest that separate developmental processes contribute to intraspecific length differences in proximal versus distal limb bones, and that these proximo-distal mechanisms are distinct from mechanisms that contribute to interspecific differences in limb bone length related to body size. These developmental "divisions of labor" are hypothesized to be important features of vertebrate limb development that allow (1) morphology in the autopods to evolve independently of the proximal limb skeleton, and (2) adaptive changes in limb proportions related to locomotion to evolve independently of evolutionary changes in body size.

The osmotic sensitivity of rat growth plate chondrocytes in situ; clarifying the mechanisms of hypertrophy

Bone elongation is predominantly driven by the volume expansion of growth plate chondrocytes. This mechanism was initially believed to be "hypertrophy", describing a proportional increase of cell water and organelles. However, morphometrical analysis subsequently assumed the increase to be "swelling", resulting in a disproportionate increase of cell water (osmotically active fraction). Histological approaches were performed on fixed tissue, and for the "swelling" assumption to be valid, the osmotic sensitivity of living cells before and during volume increase should differ. To test this, analysis of images acquired by 2-photon laser scanning microscopy (2PLSM) were used to determine the osmotic sensitivity, and osmotically active/inactive proportions of in situ chondrocytes from 15 living rat growth plates exposed to varying media osmolarities ( approximately 0-580 mOsm). The dimensions of cell volume swelling in hypotonic media were different to the preferential lengthening seen in vivo, confirming the complexity of directional cell volume increase. Boyle-van't Hoff analysis of cell volume over the range of media osmolarity indicated no significant difference (Student's t-test) in the osmotically inactive fraction, 39.5 +/- 2.9% and 47.0 +/- 4.3% (n = 13) for proliferative and hypertrophic zones, respectively, or the sensitivity of volume to changes in media osmolarity (proliferative 15.5 +/- 0.8 and hypertrophic zone 15.5 +/- 1.2%volume . Osm). The osmotic fractions did not change as chondrocytes progress from proliferative to hypertrophic regions of the growth plate. Our data suggest cell volume increase by hypertrophy may play a greater role in cell enlargement than swelling, and should be re-evaluated as a mechanism responsible for growth plate chondrocyte volume increase and hence bone elongation.

Snail1 is a transcriptional effector of FGFR3 signaling during chondrogenesis and achondroplasias

Achondroplasias are the most common genetic forms of dwarfism in humans. They are associated with activating mutations in FGFR3, which signal through the Stat and MAPK pathways in a ligand-independent manner to impair chondrocyte proliferation and differentiation. Snail1 has been implicated in chondrocyte differentiation as it represses Collagen II and aggrecan transcription in vitro. Here we demonstrate that Snail1 overexpression in the developing bone leads to achondroplasia in mice. Snail1 acts downstream of FGFR3 signaling in chondrocytes, regulating both Stat and MAPK pathways. Moreover, FGFR3 requires Snail1 during bone development and disease as the inhibition of Snail1 abolishes its signaling even through achondroplastic- and thanatophoric-activating FGFR3 forms. Significantly, Snail1 is aberrantly upregulated in thanatophoric versus normal cartilages from stillborns. Thus, Snail activity may likely be considered a target for achondroplasia therapies.

Postnatal bone elongation of the manus versus pes: analysis of the chondrocytic differentiation cascade in Mus musculus and Eptesicus fuscus

Bones elongate postnatally by endochondral ossification as cells of the cartilaginous growth plate undergo a differentiation cascade of proliferation, cellular hypertrophy and matrix synthesis. Interspecific comparisons of homologous bones elongating at different rates has been a useful approach for studying the dynamics of this process. The purpose of this study was to measure quantitative stereological parameters of growth plates of the third digit of the manus and pes of the laboratory mouse, and make comparisons to chondrocytic performance parameters in the homologous bones of the big brown bat, Eptesicus fuscus, where extremely rapid postnatal elongation of bones of the manus is associated with skeletal modifications for powered flight. Measurements were made across all zones of forelimb and hindlimb autopod growth plates by dividing each growth plate into strata of equal height (from thirteen 200-mum-high strata in the metacarpus to five 40-mum-high strata in phalangeal bones of the pes). Results indicate that all chondrocytic performance parameters known to quantitatively contribute to the elongation potential of a growth plate change together. A significant finding was that in growth plates of the chiropteran manus, final hypertrophic cell size and shape were achieved early in the zone of hypertrophy, indicating that interstitial expansion of the growth plate resulting from the incremental chondrocytic height increase in the direction of elongation was completed soon after the transition from the cessation of proliferation to the initiation of hypertrophy. This is unlike what has been reported in most mammalian growth plates previously analyzed, but is the situation in the proximal tibial growth plate of rapidly growing frogs and precocial birds. This suggests that a similar adaptation for stabilization of a rapidly elongating bone has evolved independently in three widely separated groups that have in common rapid growth in limbs to be used for early active, powered locomotion.

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.

Histomorphological study of the spinal growth plates from the convex side and the concave side in adolescent idiopathic scoliosis

Asymmetrical growth of the vertebrae has been implicated as one possible etiologic factor in the pathogenesis of adolescent idiopathic scoliosis. The longitudinal vertebral growth derives from the endochondral ossification of the vertebral growth plate. In the present study, the growth plates from the convex and concave side of the vertebrae were characterized by the method of histology and immunohistochemistry to evaluate the growth activity, cell proliferation, and apoptosis. Normal zoned architectures were observed in the convex side of the growth plate and disorganized architectures in the concave side. The histological grades were significantly different between the convex and the concave side of the growth plate in the apex vertebrae (P < 0.05). The histological difference was also found significant statistically between end vertebrae and apex vertebrae in the concave side of vertebral growth plates (P < 0.05). The proliferative potential indexes and apoptosis indexes of chondrocytes in the proliferative and hypertrophic zone in the convex side were significantly higher than that in the concave side in the apex vertebral growth plate (P < 0.05). There was a significant difference of the proliferative potential index (proliferating cell nuclear antigen, PCNA index) between convex side and concave side at the upper end vertebra (P < 0.05). The difference of the proliferative potential index and apoptosis index were found significant statistically in the concave side of the vertebral growth plate between end vertebrae and apex vertebrae (P < 0.05). The same result was also found for the apoptosis index (terminal deoxynucleotidyl transferase mediated deoxyuridine triphosphate biotin nick end labeling assay, TUNEL index) in the convex side of vertebral growth plate between end vertebrae and apex vertebrae (P < 0.05). Some correlation were found between radiographic measurements and proliferation and apoptosis indexes. The difference in histological grades and cellular activity between the convex and concave side indicated that the bilateral growth plate of the vertebrae in AIS patients have different growth kinetics which may affect the curve progression.

Leptin stimulates parathyroid hormone related peptide expression in the endochondral growth plate

We have previously shown that growth plate chondrocytes expressed the long form of leptin receptor, and that within the growth plate, leptin stimulated cell proliferation and differentiation and epiphyseal growth in a balanced manner. These three cell processes are known to be regulated by the interactions of parathyroid hormone-related peptide (PTHrP) and Indian hedgehog (Ihh) protein. The aim of the present study was to examine the effect of leptin on the PTHrP/Ihh feedback loop. The effect of leptin was investigated in vivo in pair-fed experiments in ICR mice, and ex vivo in mandibular condyle explants incubated with leptin. Immunohistochemistry and in situ hybridization showed that in the pair-fed in vivo system as well as in the organ culture, leptin increased the level of PTHrP and reduced that of Ihh. Leptin may affect chondrocyte proliferation and differentiation by activating the PTHrP/Ihh growth-restraining feedback loop in the postnatal growth plate.

Regulation of chondrocyte differentiation by the actin cytoskeleton and adhesive interactions

Chondrocyte differentiation is a multi-step process characterized by successive changes in cell morphology and gene expression. In addition to tight regulation by numerous soluble factors, these processes are controlled by adhesive events. During the early phase of the chondrocyte life cycle, cell-cell adhesion through molecules such as N-cadherin and neural cell adhesion molecule (N-CAM) is required for differentiation of mesenchymal precursor cells to chondrocytes. At later stages, for example in growth plate chondrocytes, adhesion signaling from extracellular matrix (ECM) proteins through integrins and other ECM receptors such as the discoidin domain receptor (DDR) 2 (a collagen receptor) and Annexin V is necessary for normal chondrocyte proliferation and hypertrophy. Cell-matrix interactions are also important for chondrogenesis, for example through the activity of CD44, a receptor for Hyaluronan and collagens. The roles of several signaling molecules involved in adhesive signaling, such as integrin-linked kinase (ILK) and Rho GTPases, during chondrocyte differentiation are beginning to be understood, and the actin cytoskeleton has been identified as a common target of these adhesive pathways. Complete elucidation of the pathways connecting adhesion receptors to downstream effectors and the mechanisms integrating adhesion signaling with growth factor- and hormone-induced pathways is required for a better understanding of physiological and pathological skeletal development.

Distinct developmental changes in the distribution of calcium, phosphorus and sulphur during fetal growth-plate development

Gradients in the concentrations of free phosphate (Pi) and calcium (Ca) exist in fully developed growth zones of long bones and ribs, with the highest concentrations closest to the site of mineralization. As high concentrations of Pi and Ca induce chondrocyte maturation and apoptosis, it has been hypothesized that Ca and Pi drive chondrocyte differentiation in growth plates. This study aimed to determine whether gradients in the important spectral elements phosphorus (P), Ca and sulphur (S) are already present in early stages of development, or whether they gradually develop with maturation of the growth zone. We quantified the concentration profiles of Ca, P, S, chloride and potassium at four different stages of early development of the distal growth plates of the porcine femurs, using particle-induced X-ray emission and forward- and backward-scattering spectrometry with a nuclear microprobe. A Ca concentration gradient towards the mineralized area and a stepwise increase in S was found to develop slowly with tissue maturation. The increase in S co-localizes with the onset of proliferation. A P gradient was not detected in the earliest developmental stages. High Ca levels, which may induce chondrocyte maturation, are present near the mineralization front. As total P concentrations do not correspond with former free Pi measurements, we hypothesize that the increase of free Pi towards the bone-forming site results from enzymatic cleavage of bound phosphate.

Mechanical behavior of the lamb growth plate in response to asymmetrical loading: a model for Blount disease

Blount disease is a deformity of the knee as a result of abnormal mechanical forces known to influence the growth of the physis. Despite existing studies on mechanical forces on chondrocyte cultures or limited growth plate specimens, very little information characterizes the whole growth plate to asymmetrical loading. In this study, we evaluate the response of 5 ovine proximal tibial growth plates to asymmetrical mechanical loading. Fresh proximal tibia specimens were mounted, and compressive forces were applied via a servohydraulic test frame (MTS Systems Corporation, Minneapolis, Minn) machine at standardized locations while transducers recorded the displacement at different locations. With this method, we demonstrate that loading (cyclical or static) on 1 edge of the tibial surface results in compression through the physis under the site of pressure. In addition, we record statistically significant tensile displacement opposite the compressed side (P < 0.001); this effect diminished as loading cell moved central on the tibial surface. We further show that growth plate topography influences the amount of tension and compression observed. From this study, we conclude that asymmetrical loading (such as that observed in Blount disease) may lead to compression (which retards growth) but also develops tension on the convex side (which may be a mechanism to increase deformity via Depelch phenomenon). The relationship of physeal architecture (more undulations-less physeal strain) may explain why greater deformity is observed on the tibial side of the knee in adolescent Blount disease than on the femoral side.

Alterations in the growth plate associated with growth modulation by sustained compression or distraction

Sustained mechanical load is known to modulate endochondral growth in the immature skeleton, but it is not known what causes this mechanical sensitivity. This study aimed to quantify alterations in parameters of growth plate performance associated with mechanically altered growth rate. Vertebral and proximal tibial growth plates of immature rats and cattle, and rabbit (proximal tibia only) were subjected to different magnitudes of sustained loading, which altered growth rates by up to 53%. The numbers of proliferative chondrocytes, their rate of proliferation, and the amount of chondrocytic enlargement occurring in the hypertrophic zone were quantified. It was found that reduced growth rate with compression and increased growth rate with distraction were associated with corresponding changes in the number of proliferative chondrocytes per unit width of growth plate, and in the final (maximum) chondrocytic height in the hypertrophic zone (overall correlation coefficients 0.38 and 0.56 respectively). According to multiple linear regression coefficients for these two variables (0.72 and 1.39 respectively), chondrocytic enlargement made a greater contribution to altered growth rates.

Histologic, risser sign, and digital skeletal age evaluation for residual spine growth potential in Chinese female idiopathic scoliosis

STUDY DESIGN

A prospective study.

OBJECTIVE

To ascertain the correlation between histologic grades (HGs) of vertebral growth plates and Risser grades as well as DSA stages in the Chinese female idiopathic scoliosis (IS) patients; to identify whether digital skeletal age (DSA) is a reliable indicator for accurate evaluation of the spinal residual growth potential.

SUMMARY OF BACKGROUND DATA

DSA is considered one of the more important indicators for representing the peak height velocity (PHV) typically and predicting spinal growth potential. The correlation between HGs of growth plates and DSA stages in IS patients is unclear.

METHODS

Thirty-nine Chinese female patients were available for this study. Superior and inferior growth plates were obtained at each level when anterior approach surgeries were performed. Histologic examinations were conducted after the specimens were processed. Of these patients, 28 cases were evaluated by DSA stages in this study. Correlations between histologic grades, Risser grades, menarchal status, and chronologic age were analyzed in 39 patients. Correlations between histologic grades, DSA, menarchal status, and chronologic age were analyzed in 28 patients.

RESULTS

There was a negative correlation between the following: HGs and Risser grades in 39 patients (r = -0.645, P = 0.000-0.05), HGs and menarchal status in patients in Risser 4 (r = -0.710, P = 0.002-0.05), HGs and DSA stages in 28 cases (r = -0.541, P = 0.003-0.05), and HGs and menarchal status in patients in DSA Stage III (r = -0.591, P = 0.006-0.05). Statistical significance of growth activity of growth plates was found between patients in Risser Grades 0 to 1 and those in Risser Grades 2 to 5 (P = 0.020-0.05) and patients in DSA Stage II and those in DSA Stage III (P = 0.014-0.05).

CONCLUSION

DSA may be a reliable indicator for predicting the spinal residual growth potential in IS patients, but it should be correlated with menarchal status and chronologic ages.

Age- and site-specific decline in insulin-like growth factor-I receptor expression is correlated with differential growth plate activity in the mouse hindlimb

The proximal and distal growth plates of the principal long bones do not contribute equally to longitudinal growth. Most forelimb elongation occurs at the shoulder and wrist, while most hindlimb growth occurs at the knee. This study examined whether insulin-like growth factor-I (IGF-I), a potent growth regulator, could underlie this variation via differential receptor expression. The spatiotemporal distribution of the IGF-I receptor (IGF-IR) was mapped in hindlimb growth plates (overall and within regional zones) from immature mice using immunohistochemistry. Growth activity was assessed by size/morphology of the growth plate and proliferating cell nuclear antigen (PCNA) expression. Both IGF-IR and PCNA staining declined considerably with age in the proximal femur and distal tibia (hip and ankle), but expression remained high in the more active distal femur and proximal tibia (knee) throughout growth. Growth plate size decreased with age in all sites, but the absolute and relative decline in IGF-IR in the hips and ankles of older mice indicated a site-specific loss of IGF-I sensitivity in these less active regions. These results suggest that regulation of the IGF-IR may at least partially mediate differential long bone growth, thereby providing a local mechanism for altering skeletal proportions absent modification of systemic hormone levels.