Mechanism of longitudinal bone growth and its regulation by growth plate chondrocytes

Age-related changes in densitometry and histomorphometry of long bone during the pubertal growth spurt in male rats

Because experimental studies to determine the developmental toxicity of exposure to various substances in children are impossible, many studies use immature male rats. This study aimed to provide normative data for longitudinal bone growth with age during the puberty in male rats. In order to evaluate long bone growth and mineralization we examined bone size and bone density by dual-energy X-ray absorptiometry, analyzed histomorphometry of the growth plate, and serum hormone levels relevant to bone growth from postnatal day (PD)20 to PD60. The length and weight of long bones increased strongly by PD40, and no further increase was observed after PD50. On the other hand, tibial growth plate height decreased sharply after PD50 along with a reduction in the number of cells and columns, which was probably responsible for the absence of further lengthening of long bones. Parameters related to bone formation such as bone area ratio, and the thickness and number of trabeculae, also increased significantly between PD40 and PD50. Furthermore, serum levels of IGF-1 peaked at PD30 and testosterone increased rapidly on and after PD40, when IGF-1 levels were going down. These changes may participate in the parallel increase in mineral acquisition, as well as lengthening of long bones. Our findings provide comprehensive data for changes in bone density, histomorphometry of long bones, and hormone levels relevant to bone growth during the growth spurt. This will be useful for planning animal toxicological studies, particularly for deciding on the appropriate age of animals to use in given experiments.

A Modified Approach to Measuring Femoro-Epiphyseal Acetabular Roof Index Has Better Intraobserver and Interobserver Reliability Compared With the Original Femoro-Epiphyseal Acetabular Roof Index

PURPOSE

To propose a modified approach to measuring the femoro-epiphyseal acetabular roof (FEAR) index while still abiding by its definition and biomechanical basis, and to compare the intra- and interobserver reliabilities of the original and the modified FEAR index. To propose a classification for medial sourcil edges.

METHODS

We retrospectively reviewed a consecutive series of patients treated with periacetabular osteotomy and/or hip arthroscopy at a single institute. Patients with unilateral or bilateral symptomatic borderline hip(s) were included. Hips with remarkable osteoarthritis, deformities, history of previous surgery, or without symptoms were excluded. A modified FEAR index was defined using a best-fit circle to determine the sourcil line and 2 ancillary lines connecting femoral head and sourcil edges to determine epiphyseal line. Lateral center-edge angle, Sharp angle, Tönnis angle on all hips, as well as FEAR index with original and modified approaches, were measured. Intra- and interobserver reliability were calculated as intraclass correlation coefficients (ICCs) for the FEAR index with both approaches and other alignments. A classification was proposed to categorize medial sourcil edges. ICCs for the 2 approaches across different sourcil groups also were calculated.

RESULTS

After we reviewed 411 patients, 49 were finally included. Thirty-two patients (40 hips) were identified as having borderline dysplasia defined by a lateral center-edge angle of 18 to 25°. Intraobserver ICCs for the modified method were good to excellent for borderline hips; poor to excellent for developmental dysplasia of the hip; and moderate to excellent for normal hips. As for interobserver reliability, the modified approach outperformed original approach with moderate-to-good interobserver reliability (developmental dysplasia of the hip group, ICC = 0.650; borderline dysplasia group, ICC = 0.813; normal hip group, ICC = 0.709). The medial sourcil edge was classified to 3 groups upon its morphology. Type II (39.0%) and III (43.9%) sourcil were the dominant patterns. The sourcil classification had substantial intraobserver agreement (observer 4, kappa = 0.68; observer 1, kappa = 0.799) and moderate interobserver agreement (kappa = 0.465). The modified approach to FEAR index possessed greater interobserver reliability in all medial sourcil edge patterns.

CONCLUSIONS

The modified FEAR index has better intra- and interobserver reliability compared with the original approach in all hip groups and sourcil groups. Type II and III sourcil types account for the majority, to which the modified approach is better.

LEVEL OF EVIDENCE

Level II, development of diagnostic criteria (consecutive patients with consistently applied reference standard and blinding).

Ex vivo organotypic bone slice culture reveals preferential chondrogenesis after sustained growth plate injury

Postnatal bone growth primarily relies on chondrocyte proliferation and osteogenic differentiation within the growth plate (GP) via endochondral ossification. Despite its importance, the GP is vulnerable to injuries, affecting 15-30 % of bone fractures. These injuries may lead to growth discrepancies, influence bone length and shape, and negatively affecting the patient's quality of life. This study aimed to investigate the molecular and cellular physiological and pathophysiological regeneration following sustained growth plate injury (GPI) in an ex vivo rat femur organotypic culture (OTC) model. Specifically, focusing on postnatal endochondral ossification process. 300 μm thick ex vivo bone cultures with a 2 mm long horizontal GPI was utilized. After 15 days of cultivation, gene expression analysis, histological and immunohistochemistry staining's were conducted to analyze key markers of endochondral ossification. In our OTCs we observed a significant increase in Sox9 expression due to GPI at day 15. The Ihh-PTHrP feedback loop was affected, favoring chondrocyte proliferation and maturation. Ihh levels increased significantly on day 7 and day 15, while PTHrP was downregulated on day 7. GPI had no impact on osteoclast number and activity, but gene expression analysis indicated OTCs' efforts to inhibit osteoclast differentiation and activation, thereby reducing bone resorption. In conclusion, our study provides novel insights into the molecular and cellular mechanisms underlying postnatal bone growth and regeneration following growth plate injury (GPI). We demonstrate that chondrocyte proliferation and differentiation play pivotal roles in the regeneration process, with the Ihh-PTHrP feedback loop modulating these processes. Importantly, our ex vivo rat femur organotypic culture model allows for the detailed investigation of these processes, providing a valuable tool for future research in the field of skeletal biology and regenerative medicine.

Gli1 labels progenitors during chondrogenesis in postnatal mice

Skeletal growth promoted by endochondral ossification is tightly coordinated by self-renewal and differentiation of chondrogenic progenitors. Emerging evidence has shown that multiple skeletal stem cells (SSCs) participate in cartilage formation. However, as yet, no study has reported the existence of common long-lasting chondrogenic progenitors in various types of cartilage. Here, we identify Gli1+ chondrogenic progenitors (Gli1+ CPs), which are distinct from PTHrP+ or FoxA2+ SSCs, are responsible for the lifelong generation of chondrocytes in the growth plate, vertebrae, ribs, and other cartilage. The absence of Gli1+ CPs leads to cartilage defects and dwarfishness phenotype in mice. Furthermore, we show that the BMP signal plays an important role in self-renewal and maintenance of Gli1+ CPs. Deletion of Bmpr1α triggers Gli1+ CPs quiescence exit and causes the exhaustion of Gli1+ CPs, consequently disrupting columnar cartilage. Collectively, our data demonstrate that Gli1+ CPs are common long-term chondrogenic progenitors in multiple types of cartilage and are essential to maintain cartilage homeostasis.

N-cadherin and β1 integrin coordinately regulate growth plate cartilage architecture

Spatial and temporal regulation of chondrocyte maturation in the growth plate drives growth of many bones. One essential event to generate the ordered cell array characterizing growth plate cartilage is the formation of chondrocyte columns in the proliferative zone via 90-degree rotation of daughter cells to align with the long axis of the bone. Previous studies have suggested crucial roles for cadherins and integrin β1 in column formation. The purpose of this study was to determine the relative contributions of cadherin- and integrin-mediated cell adhesion in column formation. Here we present new mechanistic insights generated by application of live time-lapse confocal microscopy of cranial base explant cultures, robust genetic mouse models, and new quantitative methods to analyze cell behavior. We show that conditional deletion of either the cell-cell adhesion molecule Cdh2 or the cell-matrix adhesion molecule Itgb1 disrupts column formation. Compound mutants were used to determine a potential reciprocal regulatory interaction between the two adhesion surfaces and identified that defective chondrocyte rotation in a N-cadherin mutant was restored by a heterozygous loss of integrin β1. Our results support a model for which integrin β1, and not N-cadherin, drives chondrocyte rotation and for which N-cadherin is a potential negative regulator of integrin β1 function.

Hypertrophic chondrocytes at the junction of musculoskeletal structures

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

α-parvin controls chondrocyte column formation and regulates long bone development

Endochondral ossification requires proper control of chondrocyte proliferation, differentiation, survival, and organization. Here we show that knockout of α-parvin, an integrin-associated focal adhesion protein, from murine limbs causes defects in endochondral ossification and dwarfism. The mutant long bones were shorter but wider, and the growth plates became disorganized, especially in the proliferative zone. With two-photon time-lapse imaging of bone explant culture, we provide direct evidence showing that α-parvin regulates chondrocyte rotation, a process essential for chondrocytes to form columnar structure. Furthermore, loss of α-parvin increased binucleation, elevated cell death, and caused dilation of the resting zones of mature growth plates. Single-cell RNA-seq analyses revealed alterations of transcriptome in all three zones (i.e., resting, proliferative, and hypertrophic zones) of the growth plates. Our results demonstrate a crucial role of α-parvin in long bone development and shed light on the cellular mechanism through which α-parvin regulates the longitudinal growth of long bones.

Effects of acute femoral head ischemia on the growth plate and metaphysis in a piglet model of Legg-Calvé-Perthes disease

OBJECTIVE

To determine the effects of acute (≤7 days) femoral head ischemia on the proximal femoral growth plate and metaphysis in a piglet model of Legg-Calvé-Perthes disease (LCPD). We hypothesized that qualitative and quantitative histological assessment would identify effects of ischemia on endochondral ossification.

DESIGN

Unilateral femoral head ischemia was surgically induced in piglets, and femurs were collected for histological assessment at 2 (n = 7) or 7 (n = 5) days post-ischemia. Samples were assessed qualitatively, and histomorphometry of the growth plate zones and primary spongiosa was performed. In a subset of samples at 7 days, hypertrophic chondrocytes were quantitatively assessed and immunohistochemistry for TGFβ1 and Indian hedgehog was performed.

RESULTS

By 2 days post-ischemia, there was significant thinning of the proliferative and hypertrophic zones, by 63 μm (95% CI -103, -22) and -19 μm (95% CI -33, -5), respectively. This thinning persisted at 7 days post-ischemia. Likewise, at 7 days post-ischemia, the primary spongiosa was thinned to absent by an average of 311 μm (95% CI -542, -82) in all ischemic samples. TGFβ1 expression was increased in the hypertrophic zone at 7 days post-ischemia.

CONCLUSIONS

Alterations to the growth plate zones and metaphysis occurred by 2 days post-ischemia and persisted at 7 days post-ischemia. Our findings suggest that endochondral ossification may be disrupted at an earlier time point than previously reported and that growth disruption may occur in the piglet model as occurs in some children with LCPD.

The Radiographic Femoroepiphyseal Acetabular Roof Index Is a Reliable and Reproducible Diagnostic Tool in Patients Undergoing Hip-Preservation Surgery: A Systematic Review

PURPOSE

To assess the utility of the femoroepiphyseal acetabular roof (FEAR) index as a diagnostic tool in hip-preservation surgery.

METHODS

MEDLINE, EMBASE, and PubMed were searched from database inception until May 2022 for literature addressing the utility of the FEAR index in patients undergoing hip-preservation surgery, and the results are presented descriptively.

RESULTS

Overall, there were a total of 11 studies comprising 1,458 patients included in this review. The intraobserver agreement for the FEAR index was reported by 3 of 11 studies (intraclass correlation coefficient range = 0.86-0.99), whereas the interobserver agreement was reported by 8 of 11 studies (intraclass correlation coefficient range = 0.776-1). Among the 5 studies that differentiated between hip instability and hip impingement, the mean FEAR index in 319 patients in the instability group ranged from 3.01 to 13.3°, whereas the mean FEAR index in 239 patients in the impingement group ranged from -10 to -0.77° and the mean FEAR index in 105 patients in the control group ranged from -13 to -7.7°. Three studies defined a specific cutoff value for the FEAR index, with 1 study defining a cutoff value of 5°, which correctly predicted treatment decision between periacetabular osteotomy versus osteochondroplasty 79% of the time with an AUC of 0.89, whereas another defined a cutoff of 2°, which correctly predicted treatment 90% of the time and the last study set a threshold of 3°, which provided an AUC of 0.86 for correctly predicting treatment decision.

CONCLUSIONS

This review demonstrates that the FEAR index has a high agreement and consistent application, making it a useful diagnostic tool in hip-preservation surgery particularly in patients with borderline dysplastic hips. However, given the variability in FEAR index cutoff values across studies, there is no absolute consensus value that dictates treatment decision.

LEVEL OF EVIDENCE

Level IV; Systematic Review of Level II-IV studies.

Effect of light intensity on growth performance and bone development of tibia in broilers

Light management affects the health outcomes and growth performance of broiler chickens. However, the effects of different light intensities on growth performance and its association with tibia development of broilers remain unclear. In the present study, 462 Ross male broilers were divided into seven treatment groups with 6 replicates (11 birds per replicate), and then were subjected to different light intensity levels (0.5, 2, 5, 7, 9, 13 or 19 Lx) for 42 days. The results demonstrated that broilers under lower light intensity (2, 5Lx) obtained higher body weight (p < 0.05) and feed conversion ratio (p < 0.05). Lower light intensity exposure had no effects on the length, width, weight, breaking strength and the mineral density of the tibia (p > 0.05), but led to increased ash content and phosphorus during the starter phase (p < 0.05). Also, plasma levels of calcium (Ca), phosphorus (P) and alkaline phosphatase were increased in response to lower light intensity conditions (p < 0.05), but decreased under higher light intensity (p < 0.05), indicating dynamic mineral metabolic and depositional activity to light intensity. In addition, broilers exposed to lower intensity (0.5 Lx, 2 Lx and 5 Lx) during the starter phase had decreased hypertrophic chondrocytes (p < 0.05), but did not affect resting zone chondrocytes and proliferative chondrocytes of the growth plate (p > 0.05). In contrast, the light intensity did not affect the growth performance and the development of the tibia of broilers during the finishing phase. In summary, we demonstrated that lower light intensity promoted the growth performance and the bone development of broilers. Application of lower light intensity at the starter phase might be a management strategy for broiler industries.

The effect of ureteropelvic junction obstruction and pyeloplasty on somatic growth during infancy

Background

Evidence regarding the impact of perinatal ureteropelvic junction obstruction (UPJO) and surgical correction during infancy, on somatic growth are scarce. Understanding these impacts could help advise parents and aid in treatment decision making.

Objectives

To assess the impact of unilateral UPJO and surgical correction on somatic growth in infants diagnosed antenatally and treated during infancy.

Design

A retrospective bi-institutional analysis of somatic growth in patients under 2 years who underwent dismembered pyeloplasty for the treatment of UPJO was conducted.

Methods

We evaluated patients who were diagnosed with unilateral hydronephrosis during pre-natal ultrasound screening for detection of fetal anomalies between May 2015 and October 2020. The height and weight of patients who were diagnosed with UPJO were recorded at the age of 1 month, time of surgery, and 6 months after surgery. Standard deviation scores (SDSs) for height and weight were calculated and compared.

Results

Forty-eight patients under the age of 2 years were included in the analysis. Median age and weight at pyeloplasty were 6.9 months and 7.5 kg. At 1 month, the median SDS for weight in the entire cohort was -0.30 [interquartile range (IQR): -1.0 to 0.63] and the median SDS for height was -0.26 (IQR: -1.08 to 0.52). In 22.9% of patients (11/48), weight and height were below -1 age-appropriate standard deviations, and 6.3% (3/48) were below -2 standard deviations, suggesting growth restriction. When comparing SDS for the entire cohort, there was no significant difference corelated to measurement time or effect of surgery. In the growth restricted cohort, we found a significant improvement in linear growth for height, which was demonstrated between birth and surgery as well as after surgery.

Conclusion

Infants with unilateral UPJO diagnosed antenatally as a single anomaly may be at an increased risk of somatic growth restriction in comparison with the general population. In children with growth restriction at time of birth, height seems to improve regardless of surgical treatment. Pyeloplasty during infancy does not seem to negatively affect somatic growth. These findings can be used to counsel parents regarding the potential effects of UPJO and pyeloplasty.

Cellular basis of differential endochondral growth in Lake Malawi cichlids

BACKGROUND

The shape and size of skeletal elements is determined by embryonic patterning mechanisms as well as localized growth and remodeling during post-embryonic development. Differential growth between endochondral growth plates underlies many aspects of morphological diversity in tetrapods but has not been investigated in ray-finned fishes. We examined endochondral growth rates in the craniofacial skeletons of two cichlid species from Lake Malawi that acquire species-specific morphological differences during postembryonic development and quantified cellular mechanisms underlying differential growth both within and between species.

RESULTS

Cichlid endochondral growth rates vary greatly (50%-60%) between different growth zones within a species, between different stages for the same growth zone, and between homologous growth zones in different species. Differences in cell proliferation and/or cell enlargement underlie much of this differential growth, albeit in different proportions. Strikingly, differences in extracellular matrix production do not correlate with growth rate differences.

CONCLUSIONS

Differential endochondral growth drives many aspects of craniofacial morphological diversity in cichlids. Cellular proliferation and enlargement, but not extracellular matrix deposition, underlie this differential growth and this appears conserved in Osteichthyes. Cell enlargement is observed in some but not all cichlid growth zones and the degree to which it occurs resembles slower growing mammalian growth plates.

Primary Periphyseal Stress Injuries of the Fingers in Adolescent Climbers: A Critical Review

ABSTRACT

The worldwide rise in popularity of climbing and development of climbing as a competitive sport is reflected by its debut at the 2021 Summer Olympic Games in Tokyo. Digital primary periphyseal stress injuries in adolescent climbers may pose a significant risk to long-term skeletal health. The aim of this article is to critically review research on the diagnosis and management of primary periphyseal stress injuries of the fingers in adolescent climbers. We adopted a systematic approach to searching for relevant literature. Articles were identified after searches of the following electronic databases: Discover, Academic Search Complete, PubMed, Embase, SPORTDiscus, and ScienceDirect. Conclusive evidence suggests digital primary periphyseal stress injuries are a consequence of repetitive microtrauma. Pain reported by adolescent climbers on the dorsal aspect of the proximal interphalangeal joint should be investigated promptly to avoid serious negative consequences. Clinicians should be aware of the efficacy of imaging techniques to inform a clinical diagnosis. A conservative management approach is preferred but in rare cases surgical intervention may be necessary. A diagnostic and therapeutic algorithm for digital primary periphyseal stress injuries is presented.

Growing Pains: The Need for Engineered Platforms to Study Growth Plate Biology

Growth plates, or physis, are highly specialized cartilage tissues responsible for longitudinal bone growth in children and adolescents. Chondrocytes that reside in growth plates are organized into three distinct zones essential for proper function. Modeling key features of growth plates may provide an avenue to develop advanced tissue engineering strategies and perspectives for cartilage and bone regenerative medicine applications and a platform to study processes linked to disease progression. In this review, a brief introduction of the growth plates and their role in skeletal development is first provided. Injuries and diseases of the growth plates as well as physiological and pathological mechanisms associated with remodeling and disease progression are discussed. Growth plate biology, namely, its architecture and extracellular matrix organization, resident cell types, and growth factor signaling are then focused. Next, opportunities and challenges for developing 3D biomaterial models to study aspects of growth plate biology and disease in vitro are discussed. Finally, opportunities for increasingly sophisticated in vitro biomaterial models of the growth plate to study spatiotemporal aspects of growth plate remodeling, to investigate multicellular signaling underlying growth plate biology, and to develop platforms that address key roadblocks to in vivo musculoskeletal tissue engineering applications are described.

Vitamin D deficiency and anatomical region alters porcine growth plate properties

Ossification of growth plate cartilage mediates longitudinal extension of long bones. Biomechanical and biochemical disruptions of growth plate function may lead to abnormal bone growth. In humans and animals, severe dietary vitamin D deficiency can lead to rickets which features growth plate widening, resulting in abnormalities in growth. However, effects of marginal vitamin D deficiencies on growth plates are not well understood. The purpose of this study was to examine the effects of a vitamin D deficient diet in the 26-day nursery phase on mechanical properties (ultimate normal stress, ultimate shear stress, ultimate strain, and tangent modulus) of porcine growth plate. Standard uniaxial tensile tests were applied on bone-growth plate-bone sections and the total stress was decomposed into normal stress and shear stress. Ultimate shear stress and ultimate strain traits were lower in the vitamin D deficient group than in the control. Regional differences were observed in all four variables. Ultimate normal stress was higher in the anterior region, which was consistent with a previous study. Sex differences were detected in ultimate normal stress, which was higher in females than in males. Interestingly, the classical finding of growth plate widening seen in severe vitamin D deficiency was not observed in the pigs with marginal vitamin D deficiency utilized in this study.

The synovial microenvironment suppresses chondrocyte hypertrophy and promotes articular chondrocyte differentiation

During the development of the appendicular skeleton, the cartilaginous templates undergo hypertrophic differentiation and remodels into bone, except for the cartilage most adjacent to joint cavities where hypertrophic differentiation and endochondral bone formation are prevented, and chondrocytes instead form articular cartilage. The mechanisms that prevent hypertrophic differentiation and endochondral bone formation of the articular cartilage have not been elucidated. To explore the role of the synovial microenvironment in chondrocyte differentiation, osteochondral allografts consisting of articular cartilage, epiphyseal bone, and growth plate cartilage from distal femoral epiphyses of inbred Lewis rats expressing enhanced green fluorescent protein from a ubiquitous promoter were transplanted either in inverted or original (control) orientation to matching sites in wildtype littermates, thereby allowing for tracing of transplanted cells and their progenies. We found that no hypertrophic differentiation occurred in the growth plate cartilage ectopically placed at the joint surface. Instead, the transplanted growth plate cartilage, with time, remodeled into articular cartilage. This finding suggests that the microenvironment at the articular surface inhibits hypertrophic differentiation and supports articular cartilage formation. To explore this hypothesis, rat chondrocyte pellets were cultured with and without synoviocyte-conditioned media. Consistent with the hypothesis, hypertrophic differentiation was inhibited and expression of the articular surface marker lubricin (Prg4) was dramatically induced when chondrocyte pellets were exposed to synovium- or synoviocyte-conditioned media, but not to chondrocyte- or osteoblast-conditioned media. Taken together, we present evidence for a novel mechanism by which synoviocytes, through the secretion of a factor or factors, act directly on chondrocytes to inhibit hypertrophic differentiation and endochondral bone formation and promote articular cartilage formation. This mechanism may have important implications for articular cartilage development, maintenance, and regeneration.

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.

A FoxA2+ long-term stem cell population is necessary for growth plate cartilage regeneration after injury

Longitudinal bone growth, achieved through endochondral ossification, is accomplished by a cartilaginous structure, the physis or growth plate, comprised of morphologically distinct zones related to chondrocyte function: resting, proliferating and hypertrophic zones. The resting zone is a stem cell-rich region that gives rise to the growth plate, and exhibits regenerative capabilities in response to injury. We discovered a FoxA2+group of long-term skeletal stem cells, situated at the top of resting zone, adjacent the secondary ossification center, distinct from the previously characterized PTHrP+ stem cells. Compared to PTHrP+ cells, FoxA2+ cells exhibit higher clonogenicity and longevity. FoxA2+ cells exhibit dual osteo-chondro-progenitor activity during early postnatal development (P0-P28) and chondrogenic potential beyond P28. When the growth plate is injured, FoxA2+ cells expand in response to trauma, and produce physeal cartilage for growth plate tissue regeneration.

Radial shockwave treatment promotes chondrogenesis in human growth plate and longitudinal bone growth in rabbits

OBJECTIVE

The process of longitudinal bone growth occurs at the growth plate where the chondrocytes undergo apparent structural and molecular changes to promote growth. Recent reports suggest that radial shockwave treatment (rSWT) stimulates bone length in cultured fetal rat metatarsals. Therefore, we investigated if rSWT has similar growth promoting effects on cultured human growth plate fragments and addressed the same in a preclinical in vivo rabbit model by subjecting their growth plates to rSWT.

METHODS

Short-term effects of high-energy rSWT were evaluated in a unique model of cultured human growth plate cartilage (n = 5) wherein samples exposed to rSWT were assessed for chondrogenic markers at 24 h in comparison to unexposed samples obtained from the same limb. Local in vivo effects were studied in six-week-old rabbits who had their distal femurs exposed to four weekly sessions of rSWT at low- and high-energy levels (n = 4 each). At sacrifice, histomorphometric and immunohistochemistry analyses were performed. For effect on longitudinal growth, proximal tibiae of 22-week-old rabbits (n = 12) were asymmetrically exposed to rSWT; the contralateral side served as untreated controls. At sacrifice, the final bone length was measured.

RESULTS

In the ex vivo model of cultured human growth plate cartilage, rSWT exposure upregulated SOX9 and COL2A1 compared to control. In the immature rabbit model, an increased number of proliferative chondrocytes and column density was seen for both the energy levels. In the adolescent rabbits, an increase in tibial length was observed after the fourth session of high-energy rSWT and until six-weeks after rSWT compared to the untreated limb.

CONCLUSIONS

Our preliminary experimental results suggest that rSWT may serve as a non-invasive treatment and possibly a safe strategy to stimulate longitudinal bone growth. However, further studies are needed to assess the in vivo effects of rSWT in models of disturbed bone growth.

Structural Changes in Trabecular Bone, Cortical Bone and Hyaline Cartilage as Well as Disturbances in Bone Metabolism and Mineralization in an Animal Model of Secondary Osteoporosis in Clostridium perfringens Infection

There is no information regarding whether changes in the microbiological balance of the gastrointestinal tract as a result of an infection with Clostridium perfringens influence the development of metabolic bone disorders. The experiment was carried out on male broiler chickens divided into two groups: control (n = 10) and experimental (n = 10). The experimental animals were infected with Clostridium perfringens between 17 and 20 days of age. The animals were euthanized at 42 days of age. The structural parameters of the trabecular bone, cortical bone, and hyaline cartilage as well as the mineralization of the bone were determined. The metabolism of the skeletal system was assessed by determining the levels of bone turnover markers, hormones, and minerals in the blood serum. The results confirm that the disturbed composition of the gastrointestinal microflora has an impact on the mineralization and metabolism of bone tissue, leading to the structural changes in cortical bone, trabecular bone, and hyaline cartilage. On the basis of the obtained results, it can be concluded that changes in the microenvironment of the gastrointestinal tract by infection with C. perfringens may have an impact on the earlier development of osteoporosis.

Properties of Cartilage-Subchondral Bone Junctions: A Narrative Review with Specific Focus on the Growth Plate

OBJECTIVE

The purpose of this narrative review is to summarize what is currently known about the structural, chemical, and mechanical properties of cartilage-bone interfaces, which provide tissue integrity across a bimaterial interface of 2 very different structural materials. Maintaining these mechanical interfaces is a key factor for normal bone growth and articular cartilage function and maintenance.

MATERIALS AND METHODS

A comprehensive search was conducted using Google Scholar and PubMed/Medline with a specific focus on the growth plate cartilage-subchondral bone interface. All original articles, reviews in journals, and book chapters were considered. Following a review of the overall structural and functional characteristics of the physis, the literature on histological studies of both articular and growth plate chondro-osseous junctions is briefly reviewed. Next the literature on biochemical properties of these interfaces is reviewed, specifically the literature on elemental analyses across the cartilage-subchondral bone junctions. The literature on biomechanical studies of these junctions at the articular and physeal interfaces is also reviewed and compared.

RESULTS

Unlike the interface between articular cartilage and bone, growth plate cartilage has 2 chondro-osseous junctions. The reserve zone of the mature growth plate is intimately connected to a plate of subchondral bone on the epiphyseal side. This interface resembles that between the subchondral bone and articular cartilage, although much less is known about its makeup and formation.

CONCLUSION

There is a notably paucity of information available on the structural and mechanical properties of reserve zone-subchondral epiphyseal bone interface. This review reveals that further studies are needed on the microstructural and mechanical properties of chondro-osseous junction with the reserve zone.

Characterization of the growth plate-bone interphase region using cryo-FIB SEM 3D volume imaging

The interphase region at the base of the growth plate includes blood vessels, cells and mineralized tissues. In this region, cartilage is mineralized and replaced with bone. Blood vessel extremities permeate this space providing nutrients, oxygen and signaling factors. All these different components form a complex intertwined 3D structure. Here we use cryo-FIB SEM to elaborate this 3D structure without removing the water. As it is challenging to image mineralized and unmineralized tissues in a hydrated state, we provide technical details of the parameters used. We obtained two FIB SEM image stacks that show that the blood vessels are in intimate contact not only with cells, but in some locations also with mineralized tissues. There are abundant red blood cells at the extremities of the vessels. We also documented large multinucleated cells in contact with mineralized cartilage and possibly also with bone. We observed membrane bound mineralized particles in these cells, as well as in blood serum, but not in the hypertrophic chondrocytes. We confirm that there is an open pathway from the blood vessel extremities to the mineralizing cartilage. Based on the sparsity of the mineralized particles, we conclude that mainly ions in solution are used for mineralizing cartilage and bone, but these are augmented by the supply of mineralized particles.

A tool for solving bone growth related problems using finite elements adaptive meshes

Long bones geometry changes in response to longitudinal growth in the epiphyseal plates and hydroxyapatite apposition in the periosteum. Due to its relevance for growth modulation and orthotics performance, researchers have extensively modeled these phenomena, using the finite elements method for it almost since the introduction of modern computers. This is a rather complex task that, besides the inherent difficulty of solving the models equations, requires considering a moving boundary. Here, the development of a new computational tool for its resolution is described. A generalized formulation of these problems is established based on the most common approaches taken in the literature and a novel finite elements algorithm is proposed for its resolution. The later allows a significant reduction of the spatial discretization requirements, the computational cost and the numerical errors associated with more classical approaches. The potentiality of the method is demonstrated by its application to three cases of practical interest, namely, hemiepiphysiodesis treatment, growth in the distal femur and bone remodeling around hip prosthesis. Eight relevant cases of study and an open source implementation of the proposed algorithm are also provided as supplementary material.

Cilia kinases in skeletal development and homeostasis

Primary cilia are dynamic compartments that regulate multiple aspects of cellular signaling. The production, maintenance, and function of cilia involve more than 1000 genes in mammals, and their mutations disrupt the ciliary signaling which manifests in a plethora of pathological conditions-the ciliopathies. Skeletal ciliopathies are genetic disorders affecting the development and homeostasis of the skeleton, and encompass a broad spectrum of pathologies ranging from isolated polydactyly to lethal syndromic dysplasias. The recent advances in forward genetics allowed for the identification of novel regulators of skeletogenesis, and revealed a growing list of ciliary proteins that are critical for signaling pathways implicated in bone physiology. Among these, a group of protein kinases involved in cilia assembly, maintenance, signaling, and disassembly has emerged. In this review, we summarize the functions of cilia kinases in skeletal development and disease, and discuss the available and upcoming treatment options.

Role of extracellular vesicles from adipose tissue- and bone marrow-mesenchymal stromal cells in endothelial proliferation and chondrogenesis

The secretome of mesenchymal stromal cells (MSCs) derived from different tissue sources is considered an innovative therapeutic tool for regenerative medicine. Although adipose tissue‐and bone marrow‐derived MSCs (ADSCs and BMSCs, respectively) share many biological features, the different tissue origins can be mirrored by variations in their secretory profile, and in particular in the secreted extracellular vesicles (EVs). In this study, we carried out a detailed and comparative characterization of middle‐ and small‐sized EVs (mEVs and sEVs, respectively) released by either ADSCs or BMSCs. Their involvement in an endochondral ossification setting was investigated using ex vivo metatarsal culture models that allowed to explore both blood vessel sprouting and bone growth plate dynamics. Although EVs separated from both cell sources presented similar characteristics in terms of size, concentration, and marker expression, they exhibited different characteristics in terms of protein content and functional effects. ADSC‐EVs overexpressed pro‐angiogenic factors in comparison to the BMSC‐counterpart, and, consequently, they were able to induce a significant increase in endothelial cord outgrowth. On the other hand, BMSC‐EVs contained a higher amount of pro‐differentiation and chemotactic proteins, and they were able to prompt growth plate organization. The present study highlights the importance of selecting the appropriate cell source of EVs for targeted therapeutic applications.

Chondrocytes in the resting zone of the growth plate are maintained in a Wnt-inhibitory environment

Chondrocytes in the resting zone of the postnatal growth plate are characterized by slow cell cycle progression, and encompass a population of parathyroid hormone-related protein (PTHrP)-expressing skeletal stem cells that contribute to the formation of columnar chondrocytes. However, how these chondrocytes are maintained in the resting zone remains undefined. We undertook a genetic pulse-chase approach to isolate slow cycling, label-retaining chondrocytes (LRCs) using a chondrocyte-specific doxycycline-controllable Tet-Off system regulating expression of histone 2B-linked GFP. Comparative RNA-seq analysis identified significant enrichment of inhibitors and activators for Wnt signaling in LRCs and non-LRCs, respectively. Activation of Wnt/β-catenin signaling in PTHrP+ resting chondrocytes using Pthlh-creER and Apc-floxed allele impaired their ability to form columnar chondrocytes. Therefore, slow-cycling chondrocytes are maintained in a Wnt-inhibitory environment within the resting zone, unraveling a novel mechanism regulating maintenance and differentiation of PTHrP+ skeletal stem cells of the postnatal growth plate.

Impairment of Cyclo-oxygenase-2 Function Results in Abnormal Growth Plate Development and Bone Microarchitecture but Does Not Affect Longitudinal Growth of the Long Bones in Skeletally Immature Mice

OBJECTIVE

Despite the general awareness that cyclo-oxygenase-2 (COX-2) is crucial for endochondral ossification, the role of COX-2 in skeletal development is largely unknown. We hypothesized that inhibition or genetic loss of COX-2 leads to impaired growth plate development and consequently impaired postnatal development of the long bones.

DESIGN

Skeletally immature (5 weeks old) B6;129S-Ptgs2^tm1Jed/J wildtype mice were treated for 10 weeks with celecoxib (daily oral administration 10 mg/kg) or placebo and compared with B6;129S-Ptgs2^tm1Jed/J homozygous knockout mice (n = 12 per group).

RESULTS

Fifteen weeks postnatally, no significant difference in growth plate (zone) thickness was found between groups. However, significantly higher proteoglycan content and lower expression levels of collagen type II and X staining in the growth plates of celecoxib-treated mice, and to a lesser extent in COX-2 knockout mice. In addition, a significantly decreased cell number and cell size were observed in the hypertrophic zone of the growth plates of both experimental groups. Micro-computed tomography analysis of the subchondral bone region directly beneath the growth plate showed significantly higher bone density and trabecular thickness, following celecoxib treatment. Despite the detected differences in growth plate extracellular matrix composition and subchondral bone morphology, no difference was found in the length of the tibia in celecoxib-treated mice or COX-2 knockout mice.

CONCLUSIONS

Genetic loss of COX-2 or treatment with celecoxib did not result in detectable differences in gross murine formation of the tibia or femur. However, there were notable phenotypic features detected in the maturation of the growth plate (hypertrophic zone and subchondral bone) as a result of the celecoxib treatment.

Premature Growth Plate Closure Caused by a Hedgehog Cancer Drug Is Preventable by Co-Administration of a Retinoid Antagonist in Mice

The growth plates are key engines of skeletal development and growth and contain a top reserve zone followed by maturation zones of proliferating, prehypertrophic, and hypertrophic/mineralizing chondrocytes. Trauma or drug treatment of certain disorders can derange the growth plates and cause accelerated maturation and premature closure, one example being anti-hedgehog drugs such as LDE225 (Sonidegib) used against pediatric brain malignancies. Here we tested whether such acceleration and closure in LDE225-treated mice could be prevented by co-administration of a selective retinoid antagonist, based on previous studies showing that retinoid antagonists can slow down chondrocyte maturation rates. Treatment of juvenile mice with an experimental dose of LDE225 for 2 days (100 mg/kg by gavage) initially caused a significant shortening of long bone growth plates, with concomitant decreases in chondrocyte proliferation; expression of Indian hedgehog, Sox9, and other key genes; and surprisingly, the number of reserve progenitors. Growth plate involution followed with time, leading to impaired long bone lengthening. Mechanistically, LDE225 treatment markedly decreased the expression of retinoid catabolic enzyme Cyp26b1 within growth plate, whereas it increased and broadened the expression of retinoid synthesizing enzyme Raldh3, thus subverting normal homeostatic retinoid circuitries and in turn accelerating maturation and closure. All such severe skeletal and molecular changes were prevented when LDE-treated mice were co-administered the selective retinoid antagonist CD2665 (1.5 mg/kg/d), a drug targeting retinoid acid receptor γ, which is most abundantly expressed in growth plate. When given alone, CD2665 elicited the expected maturation delay and growth plate expansion. In vitro data showed that LDE225 acted directly to dampen chondrogenic phenotypic expression, a response fully reversed by CD2665 co-treatment. In sum, our proof-of-principle data indicate that drug-induced premature growth plate closures can be prevented or delayed by targeting a separate phenotypic regulatory mechanism in chondrocytes. The translation applicability of the findings remains to be studied. © 2021 American Society for Bone and Mineral Research (ASBMR).

Growth, growth potential, and influences on adult height in the transgender and gender-diverse population

The sexually dimorphic trait of height is one aspect of the experience of transgender and gender‐diverse (TGD) individuals that may influence their gender dysphoria and satisfaction with their transition. In this article, we have reviewed the current knowledge of the factors that contribute to one's final adult height and how it might be affected in TGD youth who have not experienced their gonadal puberty in the setting of receiving gonadotropin‐releasing hormone analog (GnRHa) and gender‐affirming hormonal treatment. Additional research is needed to characterize the influence of growth and final adult height on the lived experience of TGD youth and adults and how to best assess their growth, predict their final adult height, and how medical transition can be potentially modified to help them meet their goals.

WNT5B in Physiology and Disease

WNT5B, a member of the WNT family of proteins that is closely related to WNT5A, is required for cell migration, cell proliferation, or cell differentiation in many cell types. WNT5B signals through the non-canonical β-catenin-independent signaling pathway and often functions as an antagonist of canonical WNT signaling. Although WNT5B has a high amino acid identity with WNT5A and is often assumed to have similar activities, WNT5B often exhibits unique expression patterns and functions. Here, we describe the distinct effects and mechanisms of WNT5B on development, bone, adipose tissue, cardiac tissue, the nervous system, the mammary gland, the lung and hematopoietic cells, compared to WNT5A. We also highlight aberrances in non-canonical WNT5B signaling contributing to diseases such as osteoarthritis, osteoporosis, obesity, type 2 diabetes mellitus, neuropathology, and chronic diseases associated with aging, as well as various cancers.

High resolution 3D structures of mineralized tissues in health and disease

A thorough knowledge of the structures of healthy mineralized tissues, such as bone or cartilage, is key to understanding the pathological changes occurring during disease. Such knowledge enables the underlying mechanisms that are responsible for pathology to be pinpointed. One high-resolution 3D method in particular - focused ion beam-scanning electron microscopy (FIB-SEM) - has fundamentally changed our understanding of healthy vertebrate mineralized tissues. FIB-SEM can be used to study demineralized matrix, the hydrated components of tissue (including cells) using cryo-fixation and even untreated mineralized tissue. The latter requires minimal sample preparation, making it possible to study enough samples to carry out studies capable of detecting statistically significant differences - a pre-requisite for the study of pathological tissues. Here, we present an imaging and characterization strategy for tissue structures at different length scales, describe new insights obtained on healthy mineralized tissues using FIB-SEM, and suggest future research directions for both healthy and diseased mineralized tissues.

The effect of maternal HMB supplementation on bone mechanical and geometrical properties, as well as histomorphometry and immunolocalization of VEGF, TIMP2, MMP13, BMP2 in the bone and cartilage tissue of the humerus of their newborn piglets

The presented experiment focuses on assessing the impact of HMB (hydroxy-β-methobutyrate) supplementation of mothers during pregnancy on the development of the skeletal system of their offspring. For this purpose, an experiment was carried out on 12 clinically healthy sows of the Great White Poland breed, which were divided randomly into two groups the control and the HMB group. All animals were kept under standard conditions and received the same feed for pregnant females. In contrast, females from the HMB group between 70 and 90 days were supplemented with 3-hydroxy-3-methylbutyle in the amount of 0.2g/kg b.w/day. Immediately after birth, the piglets were also divided into groups based on: sex, and presence or lack HMB supplementation, and subsequently were euthanized and humerus bones from all piglets were collected. Mother's HMB supplementation during pregnancy affected the multiple index of their offspring. The higher humerus mass and length was observed with the greater effect in males. Maternal supplementation also influenced on the geometrical and mechanical properties of the humerus as in the case of mass, this effect was higher in males. Also, the collagen structure of the compacted and trabecular bone changed under the HMB addition. Maternal supplementation also affected the expression of selected proteins in growth cartilage and trabecular bone. The obtained results show that the administration to the mother during pregnancy by the HMB significantly affects the development of the humerus in many ways. The obtained results also confirm the utility of such experiments in understanding of the importance of the pregnancy diet as an develop and adaptable factor of offspring organisms and are the base for further research in that area as well as in the protein markers expression area.

Fibroblast Growth Factor 9 (FGF9) negatively regulates the early stage of chondrogenic differentiation

Fibroblast growth factor signaling is essential for mammalian bone morphogenesis and growth, involving membranous ossification and endochondral ossification. FGF9 has been shown to be an important regulator of endochondral ossification; however, its role in the early differentiation of chondrocytes remains unknown. Therefore, in this study, we aimed to determine the role of FGF9 in the early differentiation of chondrogenesis. We found an increase in FGF9 expression during proliferating chondrocyte hypertrophy in the mouse growth plate. Silencing of FGF9 promotes the growth of ATDC5 cells and promotes insulin-induced differentiation of ATDC5 chondrocytes, which is due to increased cartilage matrix formation and type II collagen (col2a1) and X (col10a1), Acan, Ihh, Mmp13 gene expression. Then, we evaluated the effects of AKT, GSK-3β, and mTOR. Inhibition of FGF9 significantly inhibits phosphorylation of AKT and GSK-3β, but does not affected the activation of mTOR. Furthermore, phosphorylation of inhibited AKT and GSK-3β was compensated using the AKT activator SC79, and differentiation of ATDC5 cells was inhibited. In conclusion, our results indicate that FGF9 acts as an important regulator of early chondrogenesis partly through the AKT/GSK-3β pathway.

Bone and growth: basic principles behind rare disorders

The heterogeneity of "rare bone disorders" can be explained by the number of molecules and regulatory pathways which are responsible for bone health and normal stature. In this article, the most important basic principles behind bone homeostasis from development to structure and regulation of the growing skeleton are summarized. The aim is to provide the reader with some theoretical background to understand the nature of the different main groups of disorders affecting bone stability, longitudinal growth and disturbances of calcium and phosphate homeostasis.

The Formation of the Epiphyseal Bone Plate Occurs via Combined Endochondral and Intramembranous-Like Ossification

The formation of the epiphyseal bone plate, the flat bony structure that provides strength and firmness to the growth plate cartilage, was studied in the present study by using light, confocal, and scanning electron microscopy. Results obtained evidenced that this bone tissue is generated by the replacement of the lower portion of the epiphyseal cartilage. However, this process differs considerably from the usual bone tissue formation through endochondral ossification. Osteoblasts deposit bone matrix on remnants of mineralized cartilage matrix that serve as a scaffold, but also on non-mineralized cartilage surfaces and as well as within the perivascular space. These processes occur simultaneously at sites located close to each other, so that, a core of the sheet of bone is established very quickly. Subsequently, thickening and reshaping occurs by appositional growth to generate a dense parallel-fibered bone structurally intermediate between woven and lamellar bone. All these processes occur in close relationship with a cartilage but most of the bone tissue is generated in a manner that may be considered as intramembranous-like. Overall, the findings here reported provide for the first time an accurate description of the tissues and events involved in the formation of the epiphyseal bone plate and gives insight into the complex cellular events underlying bone formation at different sites on the skeleton.

Apoptosis in the Extraosseous Calcification Process

Extraosseous calcification is a pathologic mineralization process occurring in soft connective tissues (e.g., skin, vessels, tendons, and cartilage). It can take place on a genetic basis or as a consequence of acquired chronic diseases. In this last case, the etiology is multifactorial, including both extra- and intracellular mechanisms, such as the formation of membrane vesicles (e.g., matrix vesicles and apoptotic bodies), mitochondrial alterations, and oxidative stress. This review is an overview of extraosseous calcification mechanisms focusing on the relationships between apoptosis and mineralization in cartilage and vascular tissues, as these are the two tissues mostly affected by a number of age-related diseases having a progressively increased impact in Western Countries.

Making and shaping endochondral and intramembranous bones

Skeletal elements have a diverse range of shapes and sizes specialized to their various roles including protecting internal organs, locomotion, feeding, hearing, and vocalization. The precise positioning, size, and shape of skeletal elements is therefore critical for their function. During embryonic development, bone forms by endochondral or intramembranous ossification and can arise from the paraxial and lateral plate mesoderm or neural crest. This review describes inductive mechanisms to position and pattern bones within the developing embryo, compares and contrasts the intrinsic vs extrinsic mechanisms of endochondral and intramembranous skeletal development, and details known cellular processes that precisely determine skeletal shape and size. Key cellular mechanisms are employed at distinct stages of ossification, many of which occur in response to mechanical cues (eg, joint formation) or preempting future load‐bearing requirements. Rapid shape changes occur during cellular condensation and template establishment. Specialized cellular behaviors, such as chondrocyte hypertrophy in endochondral bone and secondary cartilage on intramembranous bones, also dramatically change template shape. Once ossification is complete, bone shape undergoes functional adaptation through (re)modeling. We also highlight how alterations in these cellular processes contribute to evolutionary change and how differences in the embryonic origin of bones can influence postnatal bone repair.

Compares and contrasts Endochondral and intramembranous bone development

Reviews embryonic origins of different bones

Describes the cellular and molecular mechanisms of positioning skeletal elements.

Describes mechanisms of skeletal growth with a focus on the generation of skeletal shape

Determinate growth is predominant and likely ancestral in squamate reptiles

Body growth is typically thought to be indeterminate in ectothermic vertebrates. Indeed, until recently, this growth pattern was considered to be ubiquitous in ectotherms. Our recent observations of a complete growth plate cartilage (GPC) resorption, a reliable indicator of arrested skeletal growth, in many species of lizards clearly reject the ubiquity of indeterminate growth in reptiles and raise the question about the ancestral state of the growth pattern. Using X-ray micro-computed tomography (µCT), here we examined GPCs of long bones in three basally branching clades of squamate reptiles, namely in Gekkota, Scincoidea and Lacertoidea. A complete loss of GPC, indicating skeletal growth arrest, was the predominant finding. Using a dataset of 164 species representing all major clades of lizards and the tuataras, we traced the evolution of determinate growth on the phylogenetic tree of Lepidosauria. The reconstruction of character states suggests that determinate growth is ancestral for the squamate reptiles (Squamata) and remains common in the majority of lizard lineages, while extended (potentially indeterminate) adult growth evolved several times within squamates. Although traditionally associated with endotherms, determinate growth is coupled with ectothermy in this lineage. These findings combined with existing literature suggest that determinate growth predominates in both extant and extinct amniotes.

Role of the fibroblast growth factor 19 in the skeletal system

Fibroblast growth factor family (FGFs) is a kind of cytokine that plays an important role in growth, development, metabolism and disease. During bone development, multiple FGFs and fibroblast growth factor receptors (FGFRs) play important roles. Previous reports have elucidated the great importance of FGF1, 2, 4, 6, 7, 8, 9, 10, and 18 in bone development, and FGF21 and 23 in bone homeostasis and bone regulation. FGF19 was initially found in the human foetal brain, and its gene location is related to osteoporosis pseudoglioma syndrome. Presently, gene chip detection has repeatedly found that FGF19 shows spatiotemporal specificity of gene expression in bone development and bone-related diseases, as well as differences in the protein level, indicating that FGF19 affects the skeletal system. Considering the current insufficient understanding of FGF19 and its potential function in the skeletal system, this review aims to introduce the background of FGF19 in bone, summarise the research progress of FGF19 in the skeletal system, and discuss the role and therapeutic potential of FGF19 in bone development and bone-related diseases.

Femoral anteversion: significance and measurement

Femoral neck anteversion (FNA) is the angle between the femoral neck and femoral shaft, indicating the degree of torsion of the femur. Differences in FNA affect the biomechanics of the hip, through alterations in factors such as moment arm lengths and joint loading. Altered gait associated with differences in FNA may also contribute to the development of a wide range of skeletal disorders including osteoarthritis. FNA varies by up to 30° within apparently healthy adults. FNA increases substantially during gestation and thereafter decreases steadily until maturity. There is some evidence of a further decrease at a much lower rate during adulthood into old age, but the mechanisms behind it have never been studied. Development of FNA appears to be strongly influenced by mechanical forces experienced during everyday movements. This is evidenced by large differences in FNA in groups where movement is impaired, such as children born breech or individuals with neuromuscular conditions such as cerebral palsy. Several methods can be used to assess FNA, which may yield different values by up to 20° in the same participant. While MRI and CT are used clinically, limitations such as their cost, scanning time and exposure to ionising radiation limit their applicability in longitudinal and population studies, particularly in children. More broadly, applicable measures such as ultrasound and functional tests exist, but they are limited by poor reliability and validity. These issues highlight the need for a valid and reliable universally accepted method. Treatment for clinically problematic FNA is usually de-rotational osteotomy; passive, non-operative methods do not have any effect. Despite observational evidence for the effects of physical activity on FNA development, the efficacy of targeted physical activity remains unexplored. The aim of this review is to describe the biomechanical and clinical consequences of FNA, factors influencing FNA and the strengths and weaknesses of different methods used to assess FNA.

How faithfully does intramembranous bone regeneration recapitulate embryonic skeletal development?

Postnatal intramembranous bone regeneration plays an important role during a wide variety of musculoskeletal regeneration processes such as fracture healing, joint replacement and dental implant surgery, distraction osteogenesis, stress fracture healing, and repair of skeletal defects caused by trauma or resection of tumors. The molecular basis of intramembranous bone regeneration has been interrogated using rodent models of most of these conditions. These studies reveal that signaling pathways such as Wnt, TGFβ/BMP, FGF, VEGF, and Notch are invoked, reminiscent of embryonic development of membranous bone. Discoveries of several skeletal stem cell/progenitor populations using mouse genetic models also reveal the potential sources of postnatal intramembranous bone regeneration. The purpose of this review is to compare the underlying molecular signals and progenitor cells that characterize embryonic development of membranous bone and postnatal intramembranous bone regeneration.

Climatic influence on the growth pattern of Panthasaurus maleriensis from the Late Triassic of India deduced from paleohistology

Metoposaurids are representatives of the extinct amphibian clade Temnospondyli, found on almost every continent exclusively in the Late Triassic deposits. Osteohistologically, it is one of the best-known temnospondyl groups, analyzed with a wide spectrum of methods, such as morphology, morphometry, bone histology or computed modelling. The least known member of Metoposauridae is Panthasaurus maleriensis from the Pranhita-Godavari basin in Central India, being geographically the most southern record of this family. For the first time the bone histology of this taxon was studied with a focus on the intraspecific variability of the histological framework and the relationship between the observed growth pattern and climatic and/or environmental conditions. The studied material includes thin-sections of five long bones, a rib, an ilium and an intercentrum belonging most likely to eight individuals ranging from different ontogenetic stages. All bones have a large medullary region with progressively increasing remodeling, surrounded by a lamellar-zonal tissue type. The primary cortex consists of parallel-fibered matrix showing various degrees of organization, less organized collagen fibers in the zones and higher organized in the annuli. Growth marks occur in the form of alternating zones and annuli in every bone except the ilium and the intercentrum. The vascularity becomes less dense towards the outermost cortex in all sampled limb bones. Towards the outermost cortex the zone thickness is decreasing, in contrast to the avascular annuli, that become thicker or are of the same thickness. The growth pattern of P. maleriensis is uniform and represents changes in ontogenetic development. Multiple resting lines are prominent in the outer annuli of the limb bones and the rib and they presumably indicate climatic and environmental influence on the growth pattern. Therefore, a prolonged phase of slowed-down growth occurred during the unfavorable phase, but a complete cessation of growth indicated by Lines of Arrested Growth (LAGs) is not recorded in the studied samples. Based on the histological framework we conclude that the climate had an impact on the growth pattern. As we do not see any LAGs in the Indian metoposaurid, we assume that the local climate was relatively mild in India during the Late Triassic. A similar prolonged phase of slowed down growth without the occurrence of LAGs was observed in Metoposaurus krasiejowensis from the Late Triassic of Krasiejów (Poland). This is in contrast to Moroccan metoposaurid Dutuitosaurus ouazzoui from the Late Triassic of Argana Basin, where LAGs are regularly deposited throughout ontogeny indicating most likely harsher climatic conditions.

Mechanobiological based long bone growth model for the design of limb deformities correction devices

A mechanobiological model of bone growth aimed for the design of medical devices for the treatment of limb deformities during childhood and adolescence was developed. Dimensional analysis was introduced as a tool for the systematic evaluation of the influence attributed to different factors that might modify the bone growth process. Simplifications were proposed, allowing the reduction of bone growth relevant parameters to four non-dimensional numbers, representing the chondrocyte sensitivity to stress, the epiphyseal plate geometry, the bone rigidity and the time. Benchmark situations considered for model validation were bone growth under normal conditions and an epiphyseal stapling treatment. A finite elements approach was used to analyze bone growth in the distal portion of the femur. Results are shown to be consistent with corresponding clinical data published in the literature, which indicates the potential of the here proposed method for the design of specific devices and treatments.

Wnt signaling in chondroprogenitors during long bone development and growth

Wnt signaling together with other signaling pathways governs cartilage development and the growth plate function during long bone formation and growth. β-catenin-dependent Wnt signaling is a specific lineage determinant of skeletal mesenchymal cells toward chondrogenic or osteogenic direction. Once cartilage forms and the growth plate organize, Wnt signaling continues to regulate proliferation and differentiation of the growth plate chondrocytes. Although chondrocytes in the growth plate have a high capacity to proliferate, new cells must be supplied to the growth plate from chondroprogenitor population. Advances in in vivo cell tracking techniques have demonstrated the importance of Wnt signaling in driving tissue renewal. The Wnt-responsive cells, genetically marked by the Wnt-reporter system, are found as stem cells in various tissues. Similarly, Wnt-responsive cells are found in the periphery of the growth plate and expanded to constitute entire column structure, indicating that Wnt signaling participates in the regulation of chondroprogenitors in the growth plate. This review will discuss advancements in research of progenitors in the growth plate, specifically focusing on Wnt/β-catenin signaling.

Home for a rest: stem cell niche of the postnatal growth plate

The resting zone houses a group of slowly proliferating 'reserve' chondrocytes and has long been speculated to serve as the stem cell niche of the postnatal growth plate. But are these resting chondrocytes bona fide stem cells? Recent technological advances in lineage tracing and next-generation sequencing have finally allowed researchers to answer this question. Several recent studies have also shed light into the signaling pathways and molecular mechanisms involved in the maintenance of resting chondrocytes, thus providing us with important new insights into the role of the resting zone in the paracrine and endocrine regulation of childhood bone growth.

Innovative Three-Dimensional Microscopic Analysis of Uremic Growth Plate Discloses Alterations in the Process of Chondrocyte Hypertrophy: Effects of Growth Hormone Treatment

Chronic kidney disease (CKD) alters the morphology and function of the growth plate (GP) of long bones by disturbing chondrocyte maturation. GP chondrocytes were analyzed in growth-retarded young rats with CKD induced by adenine intake (AD), control rats fed ad libitum (C) or pair-fed with the AD group (PF), and CKD rats treated with growth hormone (ADGH). In order to study the alterations in the process of GP maturation, we applied a procedure recently described by our group to obtain high-quality three-dimensional images of whole chondrocytes that can be used to analyze quantitative parameters like cytoplasm density, cell volume, and shape. The final chondrocyte volume was found to be decreased in AD rats, but GH treatment was able to normalize it. The pattern of variation in the cell cytoplasm density suggests that uremia could be causing a delay to the beginning of the chondrocyte hypertrophy process. Growth hormone treatment appears to be able to compensate for this disturbance by triggering an early chondrocyte enlargement that may be mediated by Nkcc1 action, an important membrane cotransporter in the GP chondrocyte enlargement.

The Actions of IGF-1 in the Growth Plate and Its Role in Postnatal Bone Elongation

PURPOSE OF REVIEW

Bone elongation is a complex process driven by multiple intrinsic (hormones, growth factors) and extrinsic (nutrition, environment) variables. Bones grow in length by endochondral ossification in cartilaginous growth plates at ends of developing long bones. This review provides an updated overview of the important factors that influence this process.

RECENT FINDINGS

Insulin-like growth factor-1 (IGF-1) is the major hormone required for growth and a drug for treating pediatric skeletal disorders. Temperature is an underrecognized environmental variable that also impacts linear growth. This paper reviews the current state of knowledge regarding the interaction of IGF-1 and environmental factors on bone elongation. Understanding how internal and external variables regulate bone lengthening is essential for developing and improving treatments for an array of bone elongation disorders. Future studies may benefit from understanding how these unique relationships could offer realistic new approaches for increasing bone length in different growth-limiting conditions.

Developmental and Evolutionary Allometry of the Mammalian Limb Skeleton

The variety of limb skeletal proportions enables a remarkable diversity of behaviors that include powered flight in bats and flipper-propelled swimming in whales using extremes of a range of homologous limb architectures. Even within human limbs, bone lengths span more than an order of magnitude from the short finger and toe bones to the long arm and leg bones. Yet all of this diversity arises from embryonic skeletal elements that are each a very similar size at formation. In this review article, I survey what is and is not yet known of the development and evolution of skeletal proportion at multiple hierarchical levels of biological organization. These include the cellular parameters of skeletal elongation in the cartilage growth plate, genes associated with differential growth, and putative gene regulatory mechanisms that would allow both covariant and independent evolution of the forelimbs and hindlimbs and of individual limb segments. Although the genetic mechanisms that shape skeletal proportion are still largely unknown, and most of what is known is limited to mammals, it is becoming increasingly apparent that the diversity of bone lengths is an emergent property of a complex system that controls elongation of individual skeletal elements using a genetic toolkit shared by all.