Fundamental limits on longitudinal bone growth: growth plate senescence and epiphyseal fusion

Toward a Robust Definition of Sport Sex

Elite individual sports in which success depends on power, speed, or endurance are conventionally divided into male and female events using traditional binary definitions of sex. Male puberty creates durable physical advantages due to the 20- to 30-fold increase in circulating testosterone producing a sustained uplift in men's muscle, bone, hemoglobin, and cardiorespiratory function resulting from male puberty and sustained during men's lives. These male physical advantages provide strong justification for a separate protected category of female events allowing women to achieve the fame and fortune from success they would be denied if competing against men. Recent wider social acceptance of transgender individuals, together with the less recognized involvement of intersex individuals, challenge and threaten to defeat the sex classifications for elite individual female events. This can create unfair advantages if seeking inclusion into elite female events of unmodified male-bodied athletes with female gender identity who have gained the physical advantages of male puberty. Based on reproductive physiology, this paper proposes a working definition of sport sex based primarily on an individual's experience of male puberty and can be applied to transgender and various XY intersex conditions. Consistent with the multidimensionality of biological sex (chromosomal, genetic, hormonal, anatomical sex), this definition may be viewed as a multistrand cable whose overall strength survives when any single strand weakens or fails, rather than as a unidimensional chain whose strength is only as good as its weakest link.

Exercise for optimizing bone health after hormone-induced increases in bone stiffness

Hormones and mechanical loading co-regulate bone throughout the lifespan. In this review, we posit that times of increased hormonal influence on bone provide opportunities for exercise to optimize bone strength and prevent fragility. Examples include endogenous secretion of growth hormones and sex steroids that modulate adolescent growth and exogenous administration of osteoanabolic drugs like teriparatide, which increase bone stiffness, or its resistance to external forces. We review evidence that after bone stiffness is increased due to hormonal stimuli, mechanoadaptive processes follow. Specifically, exercise provides the mechanical stimulus necessary to offset adaptive bone resorption or promote adaptive bone formation. The collective effects of both decreased bone resorption and increased bone formation optimize bone strength during youth and preserve it later in life. These theoretical constructs provide physiologic foundations for promoting exercise throughout life.

Effect of high-fructose consumption in pregnancy on the bone growth of offspring rats

Growing evidence suggests that bone health is programmed in early life. Maternal diet may influence the skeletal development of offspring. We aimed to determine the possible effects of high-fructose intake during pregnancy on different aspects of long bone morphology in the offspring of rats and to initially explore the possible mechanisms. Pregnant Sprague-Dawley rats were randomly divided into four groups and intragastrically administered the same dose of distilled water (CON, n = 12), 20 g/kg/day glucose (GLU, n = 12), 10 g/kg/day fructose (LFRU, n = 12), or 20 g/kg/day fructose (HFRU, n = 12) for 21 days during gestation. Computed tomography was used to analyze the cortical and cancellous bones of the distal femur of the offspring rats, and circulating bone metabolic biomarkers were measured using enzyme immunoassay. The results showed that high-fructose intake during pregnancy could decrease body weight, impair glucose metabolism, and increase serum leptin and uric acid in offspring. The offspring in the HFRU group had higher levels of the N-terminal propeptide of type I procollagen (PINP) and the C-telopeptide of type I collagen (CTX). The bone mean density (BMD), the total cross-sectional area inside the periosteal envelope (Tt.Ar), cortical bone area (Ct.Ar), medullary (or marrow) area (Ma.Ar), and trabecular mean density of the offspring in the HFRU group were lower than those in the CON group. Tartrate-resistant acid phosphatase (Trap) staining showed that high-fructose intake during pregnancy could increase the number of osteoclasts and increase the absorption area. Our results suggested that excessive fructose intake during pregnancy could inhibit skeletal development in offspring. Thus, attention to fructose intake during pregnancy is important for bone development in offspring.

Delayed epiphyseal closure in an adult with panhypopituitarism detected by 99mTc-MDP bone SPECT/CT

We reported a 23-year-old male patient with panhypopituitarism who underwent two resections for craniopharyngioma and received postoperative hormone replacement therapy. The 99mTc-MDP bone scan revealed focal high uptake of radioactive nuclide in multiple large joints. The SPECT/CT demonstrated the focal high uptake in their metaphysis. Thus, delayed epiphyseal closure was considered.

Impact loading intensifies cortical bone (re)modeling and alters longitudinal bone growth of pubertal rats

Physical exercise is important for musculoskeletal development during puberty, which builds bone mass foundation for later in life. However, strenuous levels of training might bring adverse effects to bone health, reducing longitudinal bone growth. Animal models with various levels of physical exercise were largely used to provide knowledge to clinical settings. Experiments from our previous studies applied different levels of mechanical loading on rat tibia during puberty accompanied by weekly in vivo micro-CT scans. In the present article, we apply 3D image registration-based methods to retrospectively analyze part of the previously acquired micro-CT data. Longitudinal bone growth, growth plate thickness, and cortical bone (re)modeling were evaluated from rats' age of 28-77 days. Our results show that impact loading inhibited proximal bone growth throughout puberty. We hypothesize that impact loading might bring different growth alterations to the distal and proximal growth plates. High impact loading might lead to pathological consequence of osteochondrosis and catch-up growth due to growth inhibition. Impact loading also increased cortical bone (re)modeling before and after the peak proximal bone growth period of young rats, of which the latter case might be caused by the shift from modeling to remodeling as the dominant activity toward the end of rat puberty. We confirm that the tibial endosteum is more mechano-sensitive than the periosteum in response to mechanical loading. To our knowledge, this is the first study to follow up bone growth and bone (re)modeling of young rats throughout the entire puberty with a weekly time interval.

DTI assessment of the maturing growth plate of the knee in adolescents and young adults

PURPOSE

To assess the growth plates of the knee in a healthy population of young adults and adolescents using DTI, and to correlate the findings with chronological age and skeletal maturation.

METHODS

A prospective, cross-sectional study to assess the tibial and femoral growth plates with DTI in 155 healthy volunteers aged between 14.0 and 21 years old. Echo-planar DTI with 15 directions and b value of 0 and 600 s/mm² was performed on a 3 T whole-body scanner.

RESULTS

A relationship was observed between chronological age and most DTI metrics (fractional anisotropy, mean diffusivity, and radial diffusivity), tract length and volume. (No significant relationship could be seen for axonal diffusivity and tract length.) Subdivision according to skeletal maturation showed the greatest tract lengths and volumes seen in stage 4b and not 4a. The intra-observer agreement was significant (P = 0.01) for all the measured variables, but agreement varied (femur 0.53 - 0.98; tibia 0.58 - 0.98). Spearman's correlation showed a significant correlation for age (P = 0.05; P = 0.01) as well as for the fractional anisotropy value within all variables in both femur and tibia. Tract number and volume had a similar correlation with most variables, especially the DTI metrics, and would seem to be interchangeable.

CONCLUSION

The current study indicates that DTI metrics could be a tool to assess the skeletal maturation process of the growth plate and its activity. Tractography seems promising to assess the activity of the growth plate in a younger population but must be used with caution in the more mature growth plate.

Cellular senescence: when growth stimulation meets cell cycle arrest

At the very moment of cell-cycle arrest, the cell is not senescent yet. For several days in cell culture, the arrested cell is acquiring a senescent phenotype. What is happening during this geroconversion? Cellular enlargement (hypertrophy) and hyperfunctions (lysosomal and hyper-secretory) are hallmarks of geroconversion.

Magnetic resonance and diffusion tensor imaging of the adolescent rabbit growth plate of the knee

PURPOSE

To assess the ability of MRI-DTI to evaluate growth plate morphology and activity compared with that of histomorphometry and micro-CT in rabbits.

METHODS

The hind limbs of female rabbits aged 16, 20, and 24 wk (n = 4 per age group) were studied using a 9.4T MRI scanner with a multi-gradient echo 3D sequence and DTI in 14 directions (b-value = 984 s/mm² ). After MRI, the right and left hind limb were processed for histological analysis and micro-CT, respectively. The Wilcoxon signed-rank test was used to evaluate the height and volume of the growth plate. Intraclass correlation and Pearson correlation coefficient were used to evaluate the association between DTI metrics and age.

RESULTS

The growth plate height and volume were similar for all modalities at each time point and age. Age was correlated with all tractography and DTI metrics in both the femur and tibia. A correlation was also observed between all the metrics at both sites. Tract number and volume declined with age; however, tract length did not show any changes. The fractional anisotropy color map showed lateral diffusion centrally in the growth plate and perpendicular diffusion in the hypertrophic zone, as verified by histology and micro-CT.

CONCLUSION

MRI-DTI may be useful for evaluating the growth plates.

Exploring adolescence as a key life history stage in bioarchaeology

Adolescence is a unique period in the life history of an individual. It is characterized by a myriad of changes that bioarchaeologists are only just coming to appreciate, related to sexual maturation, linear growth, immunological transformation, and emotional and cognitive development. New methods allow us to measure this age of transition through the stages of the adolescent growth, as a proxy for the physical development associated with sexual maturation (puberty). This review outlines ways bioarchaeologists may draw on research developments from the fields of human biology, evolutionary theory and neurobiology to advance a more holistic approach to the study of adolescence in the past. It considers current theoretical and analytical approaches to highlight the research potential of this critical stage of life history. This synthesis integrates the most recent research in the medical sciences concerned with body and brain development, and outlines the biological processes involved with sexual and physical maturation of the adolescent. The goal of this review is to help inform potentially rewarding areas of research that bioarchaeologists can contribute to and draw from, as well as the challenges and limitations, theoretical and methodological questions, and ways in which we can develop the study of adolescence in the discipline going forward.

How do bones grow? A mathematical description of the mechanobiological behavior of the epiphyseal plate

Growth modulation is an emerging method for the treatment of skeletal deformities originating in the long bones or the vertebral bodies. It requires the controlled application of mechanical loads to the affected bone, causing an alteration of the growth and ossification process occurring in a cartilaginous region called epiphyseal growth plate or physis. In order to avoid the possibility of under- or over-correction, quantification of the applied forces is necessary. Pursuing this goal, here we propose a phenomenological model of mechanobiological effects on the epiphyseal growth plate, based on the observed similarity between the mechanobiologically induced growth and viscoelastic material behavior. The model incorporates mechanical loading effects on growth direction, growth rate and ossification speed; it also allows to evaluate the occurrence of transient effects. Model consistency was tested against a rather large set of experiments existing in the literature. A generic simplified geometrical model of bones was established for this. Analytical solutions for growth and ossification evolution were obtained for different loading conditions, allowing to test the ability of the model to describe bone growth under various kinds of mechanical loading conditions. Model-predicted changes regarding epiphyseal growth plate thickness as well as longitudinal growth speed are consistent with experiments in which static tension or compression were applied to long bones. Results suggest that when the mechanical load is sinusoidally variable, conflicting data existing in the literature could be explained by a previously unconsidered effect of the the applied load initial phase. The model can accurately fit data regarding torsional loads effects on growth. Mechanobiological data for humans is very scarce. For this reason, when possible, the model parameters values were estimated, for the proposed generic geometry, after growth measurements in animal models available in the literature. Although it is not possible to assert their validity for humans, the proposed model along with the obtained parameters values give a rational foundation to be used in more advanced computational studies.

Roles of Sodium Hydrogen Exchanger (NHE1) and Anion Exchanger (AE2) across Chondrocytes Plasma Membrane during Longitudinal Bone Growth

Mammalian long bone growth occurs through endochondral ossification, majorly regulated by the controlled enlargement of chondrocytes at the growth plate (GP). This study aimed to investigate the roles of Na+/H+ (sodium hydrogen exchanger (NHE1)) and HCO3− (anion exchanger [AE2]) during longitudinal bone growth in mammals. Bones from P10 SpragueDawley rat pups were cultured exvivo in the presence or absence of NHE1 and AE2 inhibitors to determine their effect on long bone growth. Gross morphometry, histomorphometry, and immunohistochemistry were used to assess the bone growth. The results revealed that the culture of the bones in the presence of NHE1 and AE2 inhibitors reduces bone growth significantly (p < 0.05) by approximately 11%. The inhibitor significantly (p < 0.05) reduces bone growth velocity and the length of the hypertrophic chondrocyte zone without any effect on the total GP length. The total GP chondrocyte density was significantly (p < 0.05) reduced, but hypertrophic chondrocyte densities remained constant. NHE1 fluorescence signaling across the GP length was higher than AE2, and their localization was significantly (p < 0.05) inhibited at the hypertrophic chondrocytes zone. The GP lengthening was majorly driven by an increase in the overall GP chondrocyte and hypertrophic chondrocyte densities apart from the regulatory volume phenomenon. This may suggest that NHE1 and AE2 could have a regulatory role in long bone growth.

Dietary Complex and Slow Digestive Carbohydrates Promote Bone Mass and Improve Bone Microarchitecture during Catch-Up Growth in Rats

Catch-up growth is a process that promotes weight and height gains to recover normal growth patterns after a transient period of growth inhibition. Accelerated infant growth is associated with reduced bone mass and quality characterized by poor bone mineral density (BMD), content (BMC), and impaired microarchitecture. The present study evaluated the effects of a diet containing slow (SDC) or rapid (RDC) digestible carbohydrates on bone quality parameters during the catch-up growth period in a model of diet-induced stunted rats. The food restriction period negatively impacted BMD, BMC, and microarchitecture of appendicular and axial bones. The SDC diet was shown to improve BMD and BMC of appendicular and axial bones after a four-week refeeding period in comparison with the RDC diet. In the same line, the micro-CT analysis revealed that the trabecular microarchitecture of tibiae and vertebrae was positively impacted by the dietary intervention with SDC compared to RDC. Furthermore, features of the cortical microstructure of vertebra bones were also improved in the SDC group animals. Similarly, animals allocated to the SDC diet displayed modest improvements in growth plate thickness, surface, and volume compared to the RDC group. Diets containing the described SDC blend might contribute to an adequate bone formation during catch-up growth thus increasing peak bone mass, which could be linked to reduced fracture risk later in life in individuals undergoing transient undernutrition during early life.

gp130/STAT3 signaling is required for homeostatic proliferation and anabolism in postnatal growth plate and articular chondrocytes

Growth of long bones and vertebrae is maintained postnatally by a long-lasting pool of progenitor cells. Little is known about the molecular mechanisms that regulate the output and maintenance of the cells that give rise to mature cartilage. Here we demonstrate that postnatal chondrocyte-specific deletion of a transcription factor Stat3 results in severely reduced proliferation coupled with increased hypertrophy, growth plate fusion, stunting and signs of progressive dysfunction of the articular cartilage. This effect is dimorphic, with females more strongly affected than males. Chondrocyte-specific deletion of the IL-6 family cytokine receptor gp130, which activates Stat3, phenocopied Stat3-deletion; deletion of Lifr, one of many co-receptors that signals through gp130, resulted in a milder phenotype. These data define a molecular circuit that regulates chondrogenic cell maintenance and output and reveals a pivotal positive function of IL-6 family cytokines in the skeletal system with direct implications for skeletal development and regeneration.

Analysis of Association between Morphometric Parameters of Growth Plate and Bone Growth of Tibia in Mice and Humans

OBJECTIVE

The purposes of this study are to evaluate which growth plate parameters are associated with bone growth in mice and to compare the mouse results with those in humans.

DESIGN

The sagittal sections of the proximal growth plate of the mouse tibia from neonate to young adult stages were subjected to histomorphometric and functional analyses. The radiographic images of tibias of human patients until puberty were analyzed to obtain the tibia length and the proximal growth plate height. It was found that a linear correlation best modeled the relationship between the growth plate variables with the tibia growth rate and length.

RESULTS

In mice, total height, resting zone height, combined height of the proliferation and prehypertrophic zones, proliferation activity, and the total width of tibia growth plate showed high linear correlation with tibia bone length and bone growth rate, but the hypertrophic zone height and the growth plate area did not. In both mice and humans, the total growth plate width of tibia was found to have the strongest correlation with tibia length and growth rate.

CONCLUSIONS

The results validated that growth plate total height, the height of the resting zone and cell proliferation activity are appropriate parameters to evaluate the balance between growth plate activity and bone growth in mice, consistent with previous reports. The study also provided a new growth plate parameter candidate, growth plate width for growth plate activity evaluation in both mouse and human tibia bone.

Predilection for developing a hematogenous orthopaedic implant-associated infection in older versus younger mice

Background

The pathogenesis of hematogenous orthopaedic implant-associated infections (HOIAI) remains largely unknown, with little understanding of the influence of the physis on bacterial seeding. Since the growth velocity in the physis of long bones decreases during aging, we sought to evaluate the role of the physis on influencing the development of Staphylococcus aureus HOIAI in a mouse model comparing younger versus older mice.

Methods

In a mouse model of HOIAI, a sterile Kirschner wire was inserted retrograde into the distal femur of younger (5–8-week-old) and older (14–21-week-old) mice. After a 3-week convalescent period, a bioluminescent Staphylococcus aureus strain was inoculated intravenously. Bacterial dissemination to operative and non-operative legs was monitored longitudinally in vivo for 4 weeks, followed by ex vivo bacterial enumeration and X-ray analysis.

Results

In vivo bioluminescence imaging and ex vivo CFU enumeration of the bone/joint tissue demonstrated that older mice had a strong predilection for developing a hematogenous infection in the operative legs but not the non-operative legs. In contrast, this predilection was less apparent in younger mice as the infection occurred at a similar rate in both the operative and non-operative legs. X-ray imaging revealed that the operative legs of younger mice had decreased femoral length, likely due to the surgical and/or infectious insult to the more active physis, which was not observed in older mice. Both age groups demonstrated substantial reactive bone changes in the operative leg due to infection.

Conclusions

The presence of an implant was an important determinant for developing a hematogenous orthopaedic infection in older but not younger mice, whereas younger mice had a similar predilection for developing periarticular infection whether or not an implant was present. On a clinical scale, diagnosing HOIAI may be difficult particularly in at-risk patients with limited examination or other data points. Understanding the influence of age on developing HOIAI may guide clinical surveillance and decision-making in at-risk patients.

Comparison of reliability of magnetic resonance imaging using cartilage and T1-weighted sequences in the assessment of the closure of the growth plates at the knee

Background

Growth development is traditionally evaluated with plain radiographs of the hand and wrist to visualize bone structures using ionizing radiation. Meanwhile, MRI visualizes bone and cartilaginous tissue without radiation exposure.

Purpose

To determine the state of growth plate closure of the knee in healthy adolescents and young adults and compare the reliability of staging using cartilage sequences and T1-weighted (T1W) sequence between pediatric and general radiologists.

Material and Methods

A prospective, cross-sectional study of MRI of the knee with both cartilage and T1W sequences was performed in 395 male and female healthy subjects aged between 14.0 and 21.5 years old. The growth plate of the femur and the tibia were graded using a modified staging scale by two pediatric and two general radiologists. Femur and tibia were graded separately with both sequences.

Results

The intraclass correlation was overall excellent. The inter- and intra-observer agreement for pediatric radiologists on T1W was 82% (κ = 0.73) and 77% (κ = 0.65) for the femur and 90% (κ = 0.82) and 87% (κ = 0.75) for the tibia. The inter-observer agreement for general radiologists on T1W was 69% (κ = 0.56) for the femur and 56% (κ = 0.34) for the tibia. Cohen’s kappa coefficient showed a higher inter- and intra-observer agreement for cartilage sequences than for T1W: 93% (κ = 0.86) and 89% (κ = 0.79) for the femur and 95% (κ = 0.90) and 91% (κ = 0.81) for the tibia.

Conclusion

Cartilage sequences are more reliable than T1W sequence in the assessment of the growth plate in adolescents and young adults. Pediatric radiology experience is preferable.

Network-regulated organ allometry: The developmental regulation of morphological scaling

Morphological scaling relationships, or allometries, describe how traits grow coordinately and covary among individuals in a population. The developmental regulation of scaling is essential to generate correctly proportioned adults across a range of body sizes, while the mis-regulation of scaling may result in congenital birth defects. Research over several decades has identified the developmental mechanisms that regulate the size of individual traits. Nevertheless, we still have poor understanding of how these mechanisms work together to generate correlated size variation among traits in response to environmental and genetic variation. Conceptually, morphological scaling can be generated by size-regulatory factors that act directly on multiple growing traits (trait-autonomous scaling), or indirectly via hormones produced by central endocrine organs (systemically regulated scaling), and there are a number of well-established examples of such mechanisms. There is much less evidence, however, that genetic and environmental variation actually acts on these mechanisms to generate morphological scaling in natural populations. More recent studies indicate that growing organs can themselves regulate the growth of other organs in the body. This suggests that covariation in trait size can be generated by network-regulated scaling mechanisms that respond to changes in the growth of individual traits. Testing this hypothesis, and one of the main challenges of understanding morphological scaling, requires connecting mechanisms elucidated in the laboratory with patterns of scaling observed in the natural world. This article is categorized under: Establishment of Spatial and Temporal Patterns > Regulation of Size, Proportion, and Timing Comparative Development and Evolution > Organ System Comparisons Between Species.

Growing up different in Neolithic China: A contextualised case study and differential diagnosis of a young adult with skeletal dysplasia

This paper presents a case study of a young adult from the late Neolithic Yangshao cultural period site (∼3300-2900 years BC) of Guanjia () located in Henan Province on the Central Plains of China, who has evidence for skeletal dysplasia characterised by proportional stunting of the long bones and a small axial skeleton, generalised osteopenia, and non-fusion of epiphyses. We provide a detailed differential diagnosis of skeletal dysplasia with paediatric onset and conclude that this is likely a form of hypopituitarism or hypothyroidism, an extremely rare finding within the archaeological context. This paper highlights the issues of distinguishing the forms of proportional dwarfism in palaeopathology because of the considerable variation in manifestation of these conditions. Finally, we assess whether there were any health and social implications for this person and community through the consideration of a bioarchaeology of care approach across the lifecourse, burial context, and information on social perceptions of 'difference' in the community. ：： （3300～2900）。，，，，。，，。。，。，、、""，。.

Identifying the homology of the short human pisiform and its lost ossification center

Background

The pisiform and calcaneus are paralogous bones of the wrist and ankle and are the only carpal and tarsal, respectively, to develop from two ossification centers with an associated growth plate in mammals. Human pisiforms and calcanei have undergone drastic evolutionary changes since our last common ancestor with chimpanzees and bonobos. The human pisiform is truncated and has lost an ossification center with the associated growth plate, while the human calcaneus has expanded and retained two ossification centers and a growth plate. Mammalian pisiforms represent a wide range of morphologies but extremely short pisiforms are rare and ossification center loss is even rarer. This raises the question of whether the sole human pisiform ossification center is homologous to the primary center or the secondary center of other species. We performed an ontogenetic study of pisiform and calcaneus ossification patterns and timing in macaques, apes, and humans (n = 907) from museum skeletal collections to address this question.

Results

Human pisiforms ossify irregularly and lack characteristic features of other primates while they develop. Pisiform primary and secondary center ossification timing typically matches that of the calcaneus of non-human primates, while the human pisiform corresponds with calcaneal secondary center ossification. Finally, human pisiforms ossify at the same dental stages as pisiform and calcaneal secondary centers in other hominoids.

Conclusions

These data indicate that the human pisiform is homologous to the pisiform epiphysis of other species, and that humans have lost a primary ossification center and associated growth plate while retaining ossification timing of the secondary center. This represents an exceptional evolutionary event and demonstrates a profound developmental change in the human wrist that is unusual not only among primates, but among mammals.

Indirubin-3'-oxime stimulates chondrocyte maturation and longitudinal bone growth via activation of the Wnt/β-catenin pathway

Researchers have shown increased interest in determining what stimulates height. Currently, many children undergo precocious puberty, resulting in short stature due to premature closure of the growth plate. However, the current approach for height enhancement is limited to growth hormone treatment, which often results in side effects and clinical failure and is costly. Although recent studies have indicated the importance of paracrine signals in the growth plate for longitudinal bone growth, height-stimulating agents targeting the signaling pathways involved in growth plate maturation remain unavailable in the clinic. The Wnt/β-catenin pathway plays a major role in the maturation of growth plate chondrocytes. In this study, by using an ex vivo tibial culture system, we identified indirubin-3′-oxime (I3O) as a compound capable of enhancing longitudinal bone growth. I3O promoted chondrocyte proliferation and differentiation via activation of the Wnt/β-catenin pathway in vitro. Intraperitoneal injection of I3O in adolescent mice increased growth plate height along with incremental chondrocyte maturation. I3O promoted tibial growth without significant adverse effects on bone thickness and articular cartilage. Therefore, I3O could be a potential therapeutic agent for increasing height in children with growth retardation.

A compound that stimulates longitudinal bone growth could lead to safer treatments for children with short stature. Growth hormone treatments can normalize development in some children with growth hormone deficiency, but the side effects can be severe. Researchers led by Kang-Yell Choi at Yonsei University, Seoul, South Korea, have determined that chemical stimulation of a critical cell signaling pathway involving bone growth may offer a better approach for growing taller. Longitudinal growth of bones is driven by the proliferation and differentiation of cartilage cells in the growth plate. Choi and colleagues screened a chemical library and identified a compound derived from traditional Chinese herbs, which efficiently promotes this bone growth process in cultured cartilage cells and in the tibias of three-week-old mice. This compound appears safe, suggesting a potentially better avenue for promoting height growth.

A tale of the good and bad: Cell senescence in bone homeostasis and disease

Historically, cellular senescence has been viewed as an irreversible cell-cycle arrest process with distinctive phenotypic alterations that were implicated primarily in aging and tumor suppression. Recent discoveries suggest that cellular senescence represents a series of diverse, dynamic, and heterogeneous cellular states with the senescence-associated secretory phenotype (SASP). Although senescent cells typically contribute to aging and age-related diseases, accumulating evidence has shown that they also have important physiological functions during embryonic development, late pubertal bone growth cessation, and adulthood tissue remodeling. Here, we review the recent research on cellular senescence and SASP, highlighting the key pathways that mediate senescence cell-cycle arrest and initiate SASP. We also summarize recent literature on the role of cellular senescence in maintaining bone homeostasis and mediating age-associated osteoporosis, discussing both the beneficial and adverse roles of cellular senescence in bone during different physiological stages, including bone development, childhood bone growth, adulthood bone remodeling, and bone aging.

CXXC5 mediates growth plate senescence and is a target for enhancement of longitudinal bone growth

Longitudinal bone growth ceases with growth plate senescence during puberty. However, the molecular mechanisms of this phenomenon are largely unexplored. Here, we examined Wnt-responsive genes before and after growth plate senescence and found that CXXC finger protein 5 (CXXC5), a negative regulator of the Wnt/β-catenin pathway, was gradually elevated with reduction of Wnt/β-catenin signaling during senescent changes of rodent growth plate. Cxxc5 ^-/- mice demonstrated delayed growth plate senescence and tibial elongation. As CXXC5 functions by interacting with dishevelled (DVL), we sought to identify small molecules capable of disrupting this interaction. In vitro screening assay monitoring CXXC5-DVL interaction revealed that several indirubin analogs were effective antagonists of this interaction. A functionally improved indirubin derivative, KY19382, elongated tibial length through delayed senescence and further activation of the growth plate in adolescent mice. Collectively, our findings reveal an important role for CXXC5 as a suppressor of longitudinal bone growth involving growth plate activity.

Postnatal ex vivo rat model for longitudinal bone growth investigations

BACKGROUND

Chondrocytes in the growth plate (GP) undergo increases in volume during different cascades of cell differentiation during longitudinal bone growth. The volume increase is reported to be the most significant variable in understanding the mechanism of long bone growth.

METHODS

Forty-five postnatal Sprague-Dawley rat pups, 7-15 days old were divided into nine age groups (P7-P15). Five pups were allocated to each group. The rats were sacrificed and tibia and metatarsal bones were harvested. Bone lengths were measured after 0, 24, 48, and 72 hours of ex vivo incubation. Histology of bones was carried out, and GP lengths and chondrocyte densities were determined.

RESULTS

There were significant differences in bone length among the age groups after 0 and 72 hours of incubation. Histological sectioning was possible in metatarsal bone from all age groups, and in tibia from 7- to 13-day-old rats. No significant differences in tibia and metatarsal GP lengths were seen among different age groups at 0 and 72 hours of incubation. Significant differences in chondrocyte densities along the epiphyseal GP of the bones between 0 and 72 hours of incubation were observed in most of the age groups.

CONCLUSION

Ex vivo growth of tibia and metatarsal bones of rats aged 7-15 days old is possible, with percentage growth rates of 23.87 ± 0.80% and 40.38 ± 0.95% measured in tibia and metatarsal bone, respectively. Histological sectioning of bones was carried out without the need for decalcification in P7-P13 tibia and P7-P15 metatarsal bone. Increases in chondrocyte density along the GP influence overall bone elongation.

Integrating gross morphology and bone histology to assess skeletal maturity in early dinosauromorphs: new insights from Dromomeron (Archosauria: Dinosauromorpha)

Understanding growth patterns is central to properly interpreting paleobiological signals in tetrapods, but assessing skeletal maturity in some extinct clades may be difficult when growth patterns are poorly constrained by a lack of ontogenetic series. To overcome this difficulty in assessing the maturity of extinct archosaurian reptiles—crocodylians, birds and their extinct relatives—many studies employ bone histology to observe indicators of the developmental stage reached by a given individual. However, the relationship between gross morphological and histological indicators of maturity has not been examined in most archosaurian groups. In this study, we examined the gross morphology of a hypothesized growth series of Dromomeron romeri femora (96.6–144.4 mm long), the first series of a non-dinosauriform dinosauromorph available for such a study. We also histologically sampled several individuals in this growth series. Previous studies reported that D. romeri lacks well-developed rugose muscle scars that appear during ontogeny in closely related dinosauromorph taxa, so integrating gross morphology and histological signal is needed to determine reliable maturity indicators for early bird-line archosaurs. We found that, although there are small, linear scars indicating muscle attachment sites across the femur, the only rugose muscle scar that appears during ontogeny is the attachment of the M. caudofemoralis longus, and only in the largest-sampled individual. This individual is also the only femur with histological indicators that asymptotic size had been reached, although smaller individuals possess some signal of decreasing growth rates (e.g., decreasing vascular density). The overall femoral bone histology of D. romeri is similar to that of other early bird-line archosaurs (e.g., woven-bone tissue, moderately to well-vascularized, longitudinal vascular canals). All these data indicate that the lack of well-developed femoral scars is autapomorphic for this species, not simply an indication of skeletal immaturity. We found no evidence of the high intraspecific variation present in early dinosaurs and other dinosauriforms, but a limited sample size of other early bird-line archosaur growth series make this tentative. The evolutionary history and phylogenetic signal of gross morphological features must be considered when assessing maturity in extinct archosaurs and their close relatives, and in some groups corroboration with bone histology or with better-known morphological characters is necessary.

Heat-Induced Limb Length Asymmetry Has Functional Impact on Weight Bearing in Mouse Hindlimbs

Limb length inequality results from many types of musculoskeletal disorders. Asymmetric weight bearing from a limb length discrepancy of less than 2% can have debilitating consequences such as back problems and early-onset osteoarthritis. Existing treatments include invasive surgeries and/or drug regimens that are often only partially effective. As a noninvasive alternative, we previously developed a once daily limb-heating model using targeted heat on one side of the body for 2 weeks to unilaterally increase bone length by up to 1.5% in growing mice. In this study, we applied heat for 1 week to determine whether these small differences in limb length are functionally significant, assessed by changes in hindlimb weight bearing. We tested the hypothesis that heat-induced limb length asymmetry has a functional impact on weight bearing in mouse hindlimbs. Female 3-week-old C57BL/6 mice (N = 12 total) were treated with targeted intermittent heat for 7 days (40 C for 40 min/day). High-resolution x-ray (N = 6) and hindlimb weight bearing data (N = 8) were acquired at the start and end of the experiments. There were no significant left-right differences in starting tibial length or hindlimb weight bearing. After 1-week heat exposure, tibiae (t = 7.7, p < 0.001) and femora (t = 11.5, p < 0.001) were ~1 and 1.4% longer, respectively, on the heat-treated sides (40 C) compared to the non-treated contralateral sides (30 C). Tibial elongation rate was over 6% greater (t = 5.19, p < 0.001). Hindlimb weight bearing was nearly 20% greater (t = 11.9, p < 0.001) and significantly correlated with the increase in tibial elongation rate on the heat-treated side (R² = 0.82, p < 0.01). These results support the hypothesis that even a small limb length discrepancy can cause imbalanced weight distribution in healthy mice. The increase in bone elongation rate generated by localized heat could be a way to equalize limb length and weight bearing asymmetry caused by disease or trauma, leading to new approaches with better outcomes by using heat to lengthen limbs and reduce costly side effects of more invasive interventions.

The enigmatic relationship between epiphyseal fusion and bone development in primates

Epiphyseal fusion in primates is a process that occurs in a regular sequence spanning a period of years and thus provides biological anthropologists with a useful marker of maturity that can be used to assess age and stage of development. Despite the many studies that have catalogued fusion timing and sequence pattern, comparatively little research has been devoted to understanding why these sequences exist in the first place. Answering this question is not necessarily intuitive; indeed, given that neither taxonomic affinities nor recent adaptations have been clearly defined, it is a challenge to explain this process in evolutionary terms. In all mammals, there is a tendency for the fusion of epiphyses at joints to occur close in sequence, and this has been proposed to relate to locomotor adaptations. Further consideration of the evidence suggests that linking locomotor behavior to sequence data alone is difficult to prove and may require a different type of evidence. Epiphyseal fusion should be considered in the context of other parameters that affect the developing skeleton, including how joint morphology relates to growth in length, as well as other possible morphological constraints. In recent years, developmental biology has been providing a better understanding of the molecular regulators of epiphyseal fusion. At some point in the near future, we may be able to link our understanding of the genetics of fusion timing to the possible selective mechanisms that are responsible for these sequences.

Programmed cell senescence in skeleton during late puberty

Mesenchymal stem/progenitor cells (MSPCs) undergo rapid self-renewal and differentiation, contributing to fast skeletal growth during childhood and puberty. It remains unclear whether these cells change their properties during late puberty to young adulthood, when bone growth and accrual decelerate. Here we show that MSPCs in primary spongiosa of long bone in mice at late puberty undergo normal programmed senescence, characterized by loss of nestin expression. MSPC senescence is epigenetically controlled by the polycomb histone methyltransferase enhancer of zeste homolog 2 (Ezh2) and its trimethylation of histone H3 on Lysine 27 (H3K27me3) mark. Ezh2 maintains the repression of key cell senescence inducer genes through H3K27me3, and deletion of Ezh2 in early pubertal mice results in premature cellular senescence, depleted MSPCs pool, and impaired osteogenesis as well as osteoporosis in later life. Our data reveals a programmed cell fate change in postnatal skeleton and unravels a regulatory mechanism underlying this phenomenon.

Mesenchymal stem cells are essential for bone development, but it is unclear if their activity is maintained after late puberty, when bone growth decelerates. The authors show that during late puberty in mice, these cells undergo senescence under the epigenetic control of Ezh2.

Possible effects of an early diagnosis and treatment in patients with growth hormone deficiency: the state of art

Growth hormone deficiency (GHD) is a relatively uncommon and heterogeneous endocrine disorder presenting in childhood with short stature. However, during the neonatal period, the metabolic effects of GHD may to require prompt replacement therapy to avoid possible life-threatening complications. An increasing amount of data suggests the importance of an early diagnosis and treatment of GHD because of its auxological, metabolic, and neurodevelopmental features with respect to the patients diagnosed and treated later in life.

The available results show favourable auxological outcomes for patients with GHD diagnosed and treated with r-hGH early in life compared with those from patients with GHD who do not receive this early diagnosis and treatment. Because delayed referral for GHD diagnosis and treatment is still frequent, these results highlight the need for more attention in the diagnosis and treatment of GHD.

Despite these very encouraging data regarding metabolic and neurodevelopmental features, further studies are needed to better characterize these findings. Overall, the importance of early diagnosis and treatment of GHD needs to be addressed.

Altered paracrine signaling from the injured knee joint impairs postnatal long bone growth

Regulation of organ growth is a poorly understood process. In the long bones, the growth plates (GPs) drive elongation by generating a scaffold progressively replaced by bone. Although studies have focused on intrinsic GP regulation, classic and recent experiments suggest that local signals also modulate GP function. We devised a genetic mouse model to study extrinsic long bone growth modulation, in which injury is specifically induced in the left hindlimb, such that the right hindlimb serves as an internal control. Remarkably, when only mesenchyme cells surrounding postnatal GPs were killed, left bone growth was nevertheless reduced. GP signaling was impaired by altered paracrine signals from the knee joint, including activation of the injury response and, in neonates, dampened IGF1 production. Importantly, only the combined prevention of both responses rescued neonatal growth. Thus, we identified signals from the knee joint that modulate bone growth and could underlie establishment of body proportions.

DOI:http://dx.doi.org/10.7554/eLife.27210.001

As bones grow, their size is carefully controlled and coordinated with the growth of the other organs in the body. The mechanisms that control organ size also help the body to recover from injury, and play a key role in controlling body size and proportions. Over the course of evolution, these mechanisms have likely changed to produce the distinct body sizes and proportions seen in humans and other animals.

Despite their importance, it is not well understood how signals from both inside and outside an organ work together to regulate its size. In growth disorders this signaling goes wrong, which can lead to a person having unusual proportions such as a very short stature or having one leg shorter than the other. Currently, most growth disorders that affect leg proportions are treated with painful surgical procedures. Researchers would like to know how bone growth is affected by signals from the surrounding tissues because this could help them to develop new non-invasive treatments for these conditions.

Long bones, for example those in the leg, grow from structures near their ends called growth plates. Roselló-Díez et al. have now engineered mice in which an injury shortly after birth caused cells in the knee in the rear left leg to die off. At the same time, the rear right leg of the mice developed as normal, allowing the growth of the two legs to be compared. Roselló-Díez et al. found that the left leg of these mice grew more slowly than the right leg, even though none of the cells in the growth plate of the left leg bone had been damaged. Further investigation revealed that this was because the injury caused an imbalance between the growth-promoting and growth-restricting signals that are produced by the fat pad and articular cartilage in the knee joint. Restoring the lost balance allowed the left leg bone to grow to a more normal length.

In the future, boosting bone growth signals might provide a way to treat conditions like dwarfism or leg-length discrepancies. Understanding how different tissues influence body proportions could also help researchers to investigate how different animals evolved different body proportions.

DOI:http://dx.doi.org/10.7554/eLife.27210.002

Accelerated Skeletal Maturation in Disorders of Retinoic Acid Metabolism: A Case Report and Focused Review of the Literature

Nutritional excess of vitamin A, a precursor for retinoic acid (RA), causes premature epiphyseal fusion, craniosynostosis, and light-dependent retinopathy. Similarly, homozygous loss-of-function mutations in CYP26B1, one of the major RA-metabolizing enzymes, cause advanced bone age, premature epiphyseal fusion, and craniosynostosis. In this paper, a patient with markedly accelerated skeletal and dental development, retinal scarring, and autism-spectrum disease is presented and the role of retinoic acid in longitudinal bone growth and skeletal maturation is reviewed. Genetic studies were carried out using SNP array and exome sequencing. RA isomers were measured in the patient, family members, and in 18 age-matched healthy children using high-performance liquid chromatography coupled to tandem mass spectrometry. A genomic SNP array identified a novel 8.3 megabase microdeletion on chromosome 10q23.2-23.33. The 79 deleted genes included CYP26A1 and C1, both major RA-metabolizing enzymes. Exome sequencing did not detect any variants that were predicted to be deleterious in the remaining alleles of these genes or other known retinoic acid-metabolizing enzymes. The patient exhibited elevated plasma total RA (16.5 vs. 12.6±1.5 nM, mean±SD, subject vs. controls) and 13-cisRA (10.7 nM vs. 6.1±1.1). The findings support the hypothesis that elevated RA concentrations accelerate bone and dental maturation in humans. CYP26A1 and C1 haploinsufficiency may contribute to the elevated retinoic acid concentrations and clinical findings of the patient, although this phenotype has not been reported in other patients with similar deletions, suggesting that other unknown genetic or environmental factors may also contribute.

Multiple Effects of Growth Hormone in the Body: Is it Really the Hormone for Growth?

In this review, we analyze the effects of growth hormone on a number of tissues and organs and its putative role in the longitudinal growth of an organism. We conclude that the hormone plays a very important role in maintaining the homogeneity of tissues and organs during the normal development of the human body or after an injury. Its effects on growth do not seem to take place during the fetal period or during the early infancy and are mediated by insulin-like growth factor I (IGF-I) during childhood and puberty. In turn, IGF-I transcription is dependent on an adequate GH secretion, and in many tissues, it occurs independent of GH. We propose that GH may be a prohormone, rather than a hormone, since in many tissues and organs, it is proteolytically cleaved in a tissue-specific manner giving origin to shorter GH forms whose activity is still unknown.

Energy expenditure and intake during puberty in healthy nonobese adolescents: a systematic review

BACKGROUND

Puberty is a time of rapid growth and changing energy requirements and is a risk period for obesity. There is little high-quality evidence on the pubertal alterations of energy expenditure and intake, and this has limited our understanding of energy balance during this important life stage.

OBJECTIVE

The purpose of this study was to summarize existing evidence on pubertal energy expenditure and intake in healthy nonobese adolescents.

DESIGN

Studies were identified through CINAHL, the Cochrane Library, Embase, MEDLINE, and Web of Science databases up to August 2015. Articles presenting objectively measured data for basal or resting metabolic rate (BMR/RMR), total daily energy expenditure (TDEE), and/or energy intake (EI) for ≥2 categories of puberty were included. Relevant data adjusted for fat-free mass (FFM) also were extracted. Data were dichotomized into prepubertal and pubertal groups and compared through the use of standardized mean differences (SMDs). Heterogeneous study methodologies precluded meta-analysis.

RESULTS

The search netted 6770 articles, with 12 included for review. From these, 6 of 9 studies supported significantly higher absolute BMR/RMR during puberty (SMD: 1.10-5.93), and all of the studies favored significantly higher absolute TDEE during puberty (SMD: 0.46-9.55). These corresponded to a 12% difference and an 18% difference in absolute BMR/RMR and TDEE, respectively. Results adjusted for FFM were equivocal, with 3 studies favoring higher (1 significantly) and 3 favoring significantly lower adjusted BMR/RMR during puberty. Only 1 study reported EI, showing 41% and 25% greater absolute intakes in pubertal males and females, respectively. These differences were not significant after adjustment for FFM.

CONCLUSIONS

Reasonably consistent evidence exists to support higher absolute BMR/RMR and TDEE in pubertal than in prepubertal adolescents. Differences are largely accounted for by FFM, among other potential factors such as growth- and puberty-related hormones. This review argues for further research into hormonal influences on pubertal energy balance and subsequent effects on obesity risk.

Regulation of Long Bone Growth in Vertebrates; It Is Time to Catch Up

The regulation of organ size is essential to human health and has fascinated biologists for centuries. Key to the growth process is the ability of most organs to integrate organ-extrinsic cues (eg, nutritional status, inflammatory processes) with organ-intrinsic information (eg, genetic programs, local signals) into a growth response that adapts to changing environmental conditions and ensures that the size of an organ is coordinated with the rest of the body. Paired organs such as the vertebrate limbs and the long bones within them are excellent models for studying this type of regulation because it is possible to manipulate one member of the pair and leave the other as an internal control. During development, growth plates at the end of each long bone produce a transient cartilage model that is progressively replaced by bone. Here, we review how proliferation and differentiation of cells within each growth plate are tightly controlled mainly by growth plate-intrinsic mechanisms that are additionally modulated by extrinsic signals. We also discuss the involvement of several signaling hubs in the integration and modulation of growth-related signals and how they could confer remarkable plasticity to the growth plate. Indeed, long bones have a significant ability for "catch-up growth" to attain normal size after a transient growth delay. We propose that the characterization of catch-up growth, in light of recent advances in physiology and cell biology, will provide long sought clues into the molecular mechanisms that underlie organ growth regulation. Importantly, catch-up growth early in life is commonly associated with metabolic disorders in adulthood, and this association is not completely understood. Further elucidation of the molecules and cellular interactions that influence organ size coordination should allow development of novel therapies for human growth disorders that are noninvasive and have minimal side effects.

Hox11 genes regulate postnatal longitudinal bone growth and growth plate proliferation

Hox genes are critical regulators of skeletal development and Hox9-13 paralogs, specifically, are necessary for appendicular development along the proximal to distal axis. Loss of function of both Hoxa11 and Hoxd11 results in severe malformation of the forelimb zeugopod. In the radius and ulna of these mutants, chondrocyte development is perturbed, growth plates are not established, and skeletal growth and maturation fails. In compound mutants in which one of the four Hox11 alleles remains wild-type, establishment of a growth plate is preserved and embryos develop normally through newborn stages, however, skeletal phenotypes become evident postnatally. During postnatal development, the radial and ulnar growth rate slows compared to wild-type controls and terminal bone length is reduced. Growth plate height is decreased in mutants and premature growth plate senescence occurs along with abnormally high levels of chondrocyte proliferation in the reserve and proliferative zones. Compound mutants additionally develop an abnormal curvature of the radius, which causes significant distortion of the carpal elements. The progressive bowing of the radius appears to result from physical constraint caused by the disproportionately slower growth of the ulna than the radius. Collectively, these data are consistent with premature depletion of forelimb zeugopod progenitor cells in the growth plate of Hox11 compound mutants, and demonstrate a continued function for Hox genes in postnatal bone growth and patterning.

Evidence that estrogen hastens epiphyseal fusion and cessation of longitudinal bone growth by irreversibly depleting the number of resting zone progenitor cells in female rabbits

With age, growth plate cartilage undergoes programmed senescence, eventually causing cessation of bone elongation and epiphyseal fusion. Estrogen accelerates this developmental process. We hypothesized that senescence occurs because progenitor cells in the resting zone are depleted in number and that estrogen acts by accelerating this depletion. To test this hypothesis, juvenile ovariectomized rabbits received injections of estradiol cypionate or vehicle for 5 weeks, and then were left untreated for an additional 5 weeks. Exposure to estrogen accelerated the normal decline in growth plate height and in the number of proliferative and hypertrophic chondrocytes. Five weeks after discontinuation of estrogen treatment, these structural parameters remained advanced, indicating an irreversible advancement in structural senescence. Similarly, transient estrogen exposure hastened epiphyseal fusion. Estrogen also caused a more rapid decline in functional parameters of growth plate senescence, including growth rate, proliferation rate, and hypertrophic cell size. However, in contrast to the structural parameters, once the estrogen treatment was discontinued, the growth rate, chondrocyte proliferation rate, and hypertrophic cell size all normalized, suggesting that estrogen has a reversible, suppressive effect on growth plate function. In addition, estrogen accelerated the normal loss of resting zone chondrocytes with age. This decrease in resting zone cell number did not appear to be due to apoptosis. However, it was maintained after the estrogen treatment stopped, suggesting that it represents irreversible depletion. The findings are consistent with the hypothesis that estrogen causes irreversible depletion of progenitor cells in the resting zone, thus irreversibly accelerating structural senescence and hastening epiphyseal fusion. In addition, estrogen reversibly suppresses growth plate function.

Growth hormone therapy and craniofacial bones: a comprehensive review

Growth hormone (GH) has significant effects on linear bone growth, bone mass and bone metabolism. The primary role of GH supplementation in children with GH deficiency, those born small for gestational age or with other types of disorders in somatic development is to increase linear growth. However, GH therapy seems to elicit varying responses in the craniofacial region. Whereas the effects of GH administration on somatic development are well documented, comparatively little is known of its effects on the craniofacial region. The purpose of this review was to search the literature and compile results from both animal and human studies related to the impact of GH on craniofacial growth.

Serum thyroid hormone levels in healthy children from birth to adulthood and in short children born small for gestational age

CONTEXT

Age-appropriate reference ranges for thyroid hormones are required for detecting pediatric thyroid dysfunction. Data on thyroid hormones and peripheral thyroid metabolism in short children born small for gestational age (SGA) before and during GH treatment are lacking.

OBJECTIVES

Our objectives were to obtain pediatric thyroid hormone reference ranges; to investigate thyroid hormones in short SGA children before puberty, during puberty, and during postponement of puberty by GnRH analog; and to evaluate thyroid hormones during GH treatment.

PATIENTS AND DESIGN

In 512 healthy children (225 females; 0-18 yr), free T(4) (FT(4)), TSH, total T(4), T(3), rT(3), and T(4)-binding globulin were determined. Reference ranges were calculated using the linearity, median, and skewness method. In 125 short SGA children (62 females; mean age 11.3 yr), thyroid hormones were analyzed before and after 2 yr of GH treatment and additional GnRH analog.

RESULTS

Thyroid references showed wide ranges postnatally and age-specific patterns thereafter, similar in boys and girls. Untreated short SGA children had similar FT(4) and T(4) levels as the reference population but significantly higher T(3), rT(3), and T(4)-binding globulin levels. During puberty and during GH treatment, FT(4) and rT(3) significantly decreased, whereas T(3) significantly increased.

CONCLUSION

Age-specific thyroid reference ranges are presented. Puberty and GH treatment both induce changes in peripheral thyroid metabolism, resulting in more biologically active T(3) at the expense of less inactive rT(3), possibly mediated by IGF-I. GH treatment induces altered peripheral thyroid metabolism but does not result in thyroid dysfunction.

Synthesizing genome-wide association studies and expression microarray reveals novel genes that act in the human growth plate to modulate height

Previous meta-analysis of genome-wide association (GWA) studies has identified 180 loci that influence adult height. However, each GWA locus typically comprises a set of contiguous genes, only one of which presumably modulates height. We reasoned that many of the causative genes within these loci influence height because they are expressed in and function in the growth plate, a cartilaginous structure that causes bone elongation and thus determines stature. Therefore, we used expression microarray studies of mouse and rat growth plate, human disease databases and a mouse knockout phenotype database to identify genes within the GWAS loci that are likely required for normal growth plate function. Each of these approaches identified significantly more genes within the GWA height loci than at random genomic locations (P < 0.0001 each), supporting the validity of the approach. The combined analysis strongly implicates 78 genes in growth plate function, including multiple genes that participate in PTHrP-IHH, BMP and CNP signaling, and many genes that have not previously been implicated in the growth plate. Thus, this analysis reveals a large number of novel genes that regulate human growth plate chondrogenesis and thereby contribute to the normal variations in human adult height. The analytic approach developed for this study may be applied to GWA studies for other common polygenic traits and diseases, thus providing a new general strategy to identify causative genes within GWA loci and to translate genetic associations into mechanistic biological insights.

Transforming growth factor alpha controls the transition from hypertrophic cartilage to bone during endochondral bone growth

We have recently identified transforming growth factor alpha (TGFα) as a novel growth factor involved in the joint disease osteoarthritis. The role of TGFα in normal cartilage and bone physiology however, has not been well defined.

PURPOSE

The objective of this study was to determine the role of TGFα in bone development through investigation of the Tgfa knockout mouse.

METHODS

The gross skeletons as well as the cartilage growth plates of Tgfa knockout mice and their control littermates were examined during several developmental stages ranging from newborn to ten weeks old.

RESULTS

Knockout mice experienced skeletal growth retardation and expansion of the hypertrophic zone of the growth plate. These phenotypes were transient and spontaneously resolved by ten weeks of age. Tgfa knockout growth plates also had fewer osteoclasts along the cartilage/bone interface. Furthermore, knockout mice expressed less RUNX2, RANKL, and MMP13 mRNA in their cartilage growth plates than controls did.

CONCLUSIONS

Tgfa knockout mice experience a delay in bone development, specifically the conversion of hypertrophic cartilage to true bone. The persistence of the hypertrophic zone of the growth plate appears to be mediated by a decrease in MMP13 and RANKL expression in hypertrophic chondrocytes and a resulting reduction in osteoclast recruitment. Overall, TGFα appears to be an important growth factor regulating the conversion of cartilage to bone during the process of endochondral ossification.

Organization of the Indian hedgehog--parathyroid hormone-related protein system in the postnatal growth plate

In embryonic growth cartilage, Indian hedgehog (Ihh) and parathyroid hormone-related protein (PTHrP) participate in a negative feedback loop that regulates chondrocyte differentiation. Postnatally, this region undergoes major structural and functional changes. To explore the organization of the Ihh–PTHrP system in postnatal growth plate, we microdissected growth plates of 7-day-old rats into their constituent zones and assessed expression of genes participating in the h–PTHrP feedback loop. Ihh, Patched 1, Smoothened, Gli1, Gli2, Gli3, and Pthr1 were expressed in regions analogous to the expression domains in embryonic growth cartilage. However, PTHrP was expressed in resting zone cartilage, a site that differs from the embryonic source, the periarticular cells. We then used mice in which lacZ has replaced coding sequences of Gli1 and thus serves as a marker for active hedgehog signaling. At 1, 4, 8, and 12 weeks of age, lacZ expression was detected in a pattern analogous to that of embryonic cartilage. The findings support the hypothesis that the embryonic Ihh–PTHrP feedback loop is maintained in the postnatal growth plate except that the source of PTHrP has shifted to a more proximal location in the resting zone.

Expansions of NK-like αβT cells with chronologic aging: novel lymphocyte effectors that compensate for functional deficits of conventional NK cells and T cells

As the repertoire of αβT cell receptors (TCR) contracts with advancing age, there is an associated age-dependent accumulation of oligoclonal T cells expressing of a variety of receptors (NKR), normally expressed on natural killer (NK) cells. Evidences for differential regulation of expression of particular NKRs between T cells and NK cells suggest that NKR expression on T cells is physiologically programmed rather than a random event of the aging process. Experimental studies show NKRs on aged αβT cells may function either as independent receptors, and/or as costimulatory receptors to the TCR. Considering the reported deficits of conventional αβTCR-driven activation and also functional deficits of classical NK cells, NKR(+) αβT cells likely represent novel immune effectors that are capable of combining innate and adaptive functions. Inasmuch as immunity is a determinant of individual fitness, the type and density of NKRs could be important contributing factors to the wide heterogeneity of health characteristics of older adults, ranging from institutionalized frail elders who are unable to mount immune responses to functionally independent community-dwelling elders who exhibit protective immunity. Understanding the biology of NKR(+) αβT cells could lead to new avenues for age-specific intervention to improve protective immunity.

Growth plate senescence and catch-up growth

Longitudinal bone growth is rapid in prenatal and early postnatal life, but then slows with age and eventually ceases. This growth deceleration is caused primarily by a decrease in chondrocyte proliferation, and is associated with other structural, functional, and molecular changes collectively termed growth plate senescence. Current evidence suggests that growth plate senescence occurs because the progenitor chondrocytes in the resting zone have a limited replicative capacity which is gradually exhausted with increasing cell division. In addition, recent experimental findings from laboratory and clinical studies suggest that growth plate senescence explains the phenomenon of catch-up growth. Growth-inhibiting conditions such as glucocorticoid excess and hypothyroidism delay the program of growth plate senescence. Consequently, growth plates are less senescent after these conditions resolve and therefore grow more rapidly than is normal for age, resulting in catch-up growth.

Growth-inhibiting conditions slow growth plate senescence

The mammalian growth plate undergoes programmed senescence during juvenile life, causing skeletal growth to slow with age. We previously found that hypothyroidism in rats slowed both growth plate chondrocyte proliferation and growth plate senescence, suggesting that senescence is not dependent on age per se but rather on chondrocyte proliferation. However, one alternative explanation is that the observed slowing of growth plate senescence is a specific consequence of hypothyroidism. We reasoned that, if delayed senescence is a general consequence of growth inhibition, rather than a specific result of hypothyroidism, then senescence would also be slowed by other growth-inhibiting conditions. In this study, we therefore used tryptophan deficiency to temporarily inhibit growth in newborn rats for 4 weeks. We then allowed the animals to recover and studied the effects on growth plate senescence. We found that structural, functional, and molecular markers of growth plate senescence were delayed by prior tryptophan deficiency, indicating that the developmental program of senescence had occurred more slowly during the period of growth inhibition. Taken together with previous studies in hypothyroid rats, our findings support the hypothesis that delayed senescence is a general consequence of growth inhibition and hence that growth plate senescence is not simply a function of time per se but rather depends on growth.

Effects of glucocorticoids on the growth plate

Glucocorticoids have a direct, inhibitory effect on the growth plate, as demonstrated by in vivo and organ culture studies. Glucocorticoids slow longitudinal bone growth by inhibiting chondrocyte proliferation, hypertrophy, and cartilage matrix synthesis. The molecular mediators of these effects are poorly understood. Glucocorticoids also delay growth plate senescence. The decreased rate of senescence appears to be a consequence of the growth inhibition and, in particular, may occur because glucocorticoids slow proliferation of stem-like cells in the resting zone and therefore conserve the limited proliferative capacity of these cells. This slowing of senescence appears to explain the phenomenon of catch-up growth following transient glucocorticoid exposure. After the exposure, the growth plate is less senescent, and therefore grows more rapidly than is normal for age. Glucocorticoids cause growth inhibition and subsequent catch-up growth not only in terms of longitudinal bone growth at the growth plate but also in terms of cross-sectional bone growth at the periosteum. Whether the underlying mechanisms are analogous to those at the growth plate is not known.