Weight loading young chicks inhibits bone elongation and promotes growth plate ossification and vascularization

Growth of long bones in European and Japanese quail from the 13th day of incubation to day 35 post-hatch

This study described the growth, morphometric, biomechanical, and chemical properties of the femur, tibiotarsus, and tarsometatarsus of European and Japanese quail. Analyses were performed at 13 and 15 days of incubation, at hatch, and at 4, 7, 10, 14, 21, 28, and 35 days post-hatch (n=6/subspecies/period). Bone specimens were analyzed by cone-beam computed tomography, biomechanical assays, chemical analyses, and histomorphometry. Variables were fitted by the Gompertz function and its derivative or assessed using the analysis of variance. Analysis of the derivative of Gompertz curves showed that the growth behavior of the tarsometatarsal bone was similar between quail subspecies, and the femur and tibiotarsus of European quail increased first in width and then in length, whereas the opposite occurred in Japanese quail. There was an interaction between quail subspecies and days of growth on femoral, tarsometatarsal, and tibiotarsal bone densities. Femoral and tibiotarsal cross-sectional areas were influenced by the interaction of quail subspecies and day of growth. Interaction effects were significant for breaking strength and phosphorus percentage. European and Japanese quail have different femoral and tibiotarsal growth patterns, especially in the first few days after hatching, whereas tarsometatarsal growth is similar between subspecies.

Ciliary IFT88 Protects Coordinated Adolescent Growth Plate Ossification From Disruptive Physiological Mechanical Forces

Compared with our understanding of endochondral ossification, much less is known about the coordinated arrest of growth defined by the narrowing and fusion of the cartilaginous growth plate. Throughout the musculoskeletal system, appropriate cell and tissue responses to mechanical force delineate morphogenesis and ensure lifelong health. It remains unclear how mechanical cues are integrated into many biological programs, including those coordinating the ossification of the adolescent growth plate at the cessation of growth. Primary cilia are microtubule-based organelles tuning a range of cell activities, including signaling cascades activated or modulated by extracellular biophysical cues. Cilia have been proposed to directly facilitate cell mechanotransduction. To explore the influence of primary cilia in the mouse adolescent limb, we conditionally targeted the ciliary gene Intraflagellar transport protein 88 (Ift88^fl/fl ) in the juvenile and adolescent skeleton using a cartilage-specific, inducible Cre (AggrecanCreER^T2 Ift88^fl/fl ). Deletion of IFT88 in cartilage, which reduced ciliation in the growth plate, disrupted chondrocyte differentiation, cartilage resorption, and mineralization. These effects were largely restricted to peripheral tibial regions beneath the load-bearing compartments of the knee. These regions were typified by an enlarged population of hypertrophic chondrocytes. Although normal patterns of hedgehog signaling were maintained, targeting IFT88 inhibited hypertrophic chondrocyte VEGF expression and downstream vascular recruitment, osteoclastic activity, and the replacement of cartilage with bone. In control mice, increases to physiological loading also impair ossification in the peripheral growth plate, mimicking the effects of IFT88 deletion. Limb immobilization inhibited changes to VEGF expression and epiphyseal morphology in Ift88cKO mice, indicating the effects of depletion of IFT88 in the adolescent growth plate are mechano-dependent. We propose that during this pivotal phase in adolescent skeletal maturation, ciliary IFT88 protects uniform, coordinated ossification of the growth plate from an otherwise disruptive heterogeneity of physiological mechanical forces. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Inconsistencies and Imprecision in the Nomenclature Used to Describe Primary Periphyseal Stress Injuries: Towards a Better Understanding

Stress injuries involving the epiphyseal-physeal-metaphyseal complex affecting the extremities of child and adolescent athletes were first described in the early 1950s. Initially observed in Little League baseball players, these injuries are now known to affect skeletally immature athletes in a variety of sports that involve high-impact repetitive overuse activities. Collectively known as primary periphyseal stress injuries, they may affect the long bones around the shoulder, elbow, wrist, hand, hip, knee, ankle, and foot of young athletes. These injuries respond well to timely treatment and relative rest, while non-compliance with non-operative treatment can produce skeletal growth disruption and resultant limb deformity. A major concern raised from the existing literature on primary periphyseal stress injuries is the long history of inconsistent and imprecise terminology used to describe these injuries. A variety of terms have been used to describe primary periphyseal stress injuries, including those which potentially misinform regarding who may be affected by these injuries and the true nature and pathophysiologic mechanisms involved. These imprecisions and inconsistencies arise, at least in part, from a misunderstanding or incomplete understanding of the nature and mechanism of primary periphyseal stress injuries. In this article, we examine the inconsistent and imprecise nomenclature historically used to describe primary periphyseal stress injuries. We also offer a novel framework for understanding the pathophysiologic mechanisms behind these injuries, and provide suggestions for more standard use of terminology and further research moving forward.

Effects of amino acid levels during rearing on Cobb 500 slow-feathering broiler breeders: 1. Growth and development

Adequate pullet nutrition is essential to reach BW and suitable body composition for reproduction. An experiment was conducted to determine the effects of 4 dietary amino acid (AA) levels on BW, flock uniformity, body conformation, organ, leg, and feathering development of broiler breeder pullets during the rearing phase from 5 to 24 wk. A total of 1,360 Cobb-500 slow-feathering (SF) pullets were randomly placed in 16-floor pens with 85 females per pen. Diets with corn, soybean meal, and wheat-midds were formulated without protein restriction maintaining minimum ratios between essential AA and Lys on a digestible (dig) ideal basis. Treatments consisted of 4 dietary AA levels with 80% (low-AA), 90% (moderate-AA), 100% (standard-AA), and 110% (high-AA) of the Cobb-Vantress recommendations guided by dig Lys using balanced protein. Up to 4 wk, all pullets were fed a common starter crumble diet. Grower and developer mash diets were fed to pullets from 5 to 15 wk and from 16 to 24 wk, respectively. Pullets fed standard-AA and high-AA diets were heavier (P < 0.001) than those fed low-AA diets at 10, 15, and 20 wk of age. High-AA diets resulted in better (P = 0.040) flock uniformity at 10 wk. Pullets fed a high-AA diet had the highest (P = 0.041) relative breast weight at 20 wk of age and the lowest (P = 0.044) deposits of abdominal fat at 15 wk of age. Fleshing increased (P < 0.05) as AA content rise in the diet, while the relative shank length (P < 0.001) and the number of wing juvenile feathers (P = 0.004) decreased. Pullets fed the lowest dietary AA level had the longest (P = 0.007) small intestine relative to BW at 10 wk of age, but a smaller (P = 0.001) liver than those fed moderate and standard-AA diets at 20 wk of age. Dietary AA levels have important effects on pullet BW, fleshing, and organ development during rearing with potential reproductive performance impacts.

Part 2. Review and meta-analysis of studies on modulation of longitudinal bone growth and growth plate activity: A micro-scale perspective

Macro-scale changes in longitudinal bone growth resulting from mechanical loading were shown in Part 1 of this review to depend on load magnitude, anatomical location, and species. While no significant effect on longitudinal growth was observed by varying frequency and amplitude of cyclic loading, such variations, in addition to loading duration and species, were shown to affect the morphology, viability, and gene and protein expression within the growth plate. Intermittent compression regimens were shown to preserve or increase growth plate height while stimulating increased chondrocyte presence in the hypertrophic zone relative to persistent and static loading regimens. Gene and protein expressions related to matrix synthesis and degradation, as well as regulation of chondrocyte apoptosis were shown to exhibit magnitude-, frequency-, and duration-dependent responses to loading regimen. Chondrocyte viability was shown to be largely preserved within physiological bounds of magnitude, frequency, amplitude, and duration. Persistent static loading was shown to be associated with overall growth plate height in tension only, reducing it in compression, while affecting growth plate zone heights differently across species and encouraging mineralization relative to intermittent cyclic loading. Lateral loading of the growth plate, as well as microfluidic approaches are relatively understudied, and age, anatomical location, and species effects within these approaches are undefined. Understanding the micro-scale effects of varied loading regimes can assist in the development of growth modulation methods and device designs optimized for growth plate viability preservation or mineralization stimulation based on patient age and anatomical location.

Secondary ossification center induces and protects growth plate structure

Growth plate and articular cartilage constitute a single anatomical entity early in development but later separate into two distinct structures by the secondary ossification center (SOC). The reason for such separation remains unknown. We found that evolutionarily SOC appears in animals conquering the land - amniotes. Analysis of the ossification pattern in mammals with specialized extremities (whales, bats, jerboa) revealed that SOC development correlates with the extent of mechanical loads. Mathematical modeling revealed that SOC reduces mechanical stress within the growth plate. Functional experiments revealed the high vulnerability of hypertrophic chondrocytes to mechanical stress and showed that SOC protects these cells from apoptosis caused by extensive loading. Atomic force microscopy showed that hypertrophic chondrocytes are the least mechanically stiff cells within the growth plate. Altogether, these findings suggest that SOC has evolved to protect the hypertrophic chondrocytes from the high mechanical stress encountered in the terrestrial environment.

Mechanical stimulation of growth plate chondrocytes: Previous approaches and future directions

Growth plate cartilage resides near the ends of long bones and is the primary driver of skeletal growth. During growth, both intrinsically and extrinsically generated mechanical stresses act on chondrocytes in the growth plate. Although the role of mechanical stresses in promoting tissue growth and homeostasis has been strongly demonstrated in articular cartilage of the major skeletal joints, effects of stresses on growth plate cartilage and bone growth are not as well established. Here, we review the literature on mechanobiology in growth plate cartilage at macroscopic and microscopic scales, with particular emphasis on comparison of results obtained using different methodological approaches, as well as from whole animal and in vitro experiments. To answer these questions, macroscopic mechanical stimulators have been developed and applied to study mechanobiology of growth plate cartilage and chondrocytes. However, the previous approaches have tested a limited number of stress conditions, and the mechanobiology of a single chondrocyte has not been well studied due to limitations of the macroscopic mechanical stimulators. We explore how microfluidics devices can overcome these limitations and improve current understanding of growth plate chondrocyte mechanobiology. In particular, microfluidic devices can generate multiple stress conditions in a single platform and enable real-time monitoring of metabolism and cellular behavior using optical microscopy. Systematic characterization of the chondrocytes using microfluidics will enhance our understanding of how to use mechanical stresses to control the bone growth and the properties of tissue-engineered growth plate cartilage.

A Microfluidic Platform for Stimulating Chondrocytes with Dynamic Compression

Mechanical stimuli are known to modulate biological functions of cells and tissues. Recent studies have suggested that compressive stress alters growth plate cartilage architecture and results in growth modulation of long bones of children. To determine the role of compressive stress in bone growth, we created a microfluidic device actuated by pneumatic pressure, to dynamically (or statically) compress growth plate chondrocytes embedded in alginate hydrogel cylinders. In this article, we describe detailed methods for fabricating and characterizing this device. The advantages of our protocol are: 1) Five different magnitudes of compressive stress can be generated on five technical replicates in a single platform, 2) It is easy to visualize cell morphology via a conventional light microscope, 3) Cells can be rapidly isolated from the device after compression to facilitate downstream assays, and 4) The platform can be applied to study mechanobiology of any cell type that can grow in hydrogels.

Effect of Long-Term Diving on the Morphology and Growth of the Distal Radial Epiphyseal Plate of Young Divers: A Magnetic Resonance Imaging Study

OBJECTIVE

To investigate the effects of long-term diving on the morphology and growth of the distal radial epiphyseal plate in young divers.

STUDY DESIGN

Cohort study.

SETTING

Guangzhou Sport University.

PARTICIPANTS

Thirty-eight professional divers, aged 10 to 17 years, and 25 age-matched volunteers.

INTERVENTIONS

Each subject received a physical examination at the beginning of the study and underwent bilateral magnetic resonance imaging of the wrist. The divers were divided into 2 groups depending on the status of the epiphyseal plate: group A (positive distal radial epiphyseal plate injury) and group B (no positive distal radial epiphyseal plate injury). A third group, group C, consisted of the 25 volunteers.

MAIN OUTCOME MEASURES

The frequency of distal radial epiphyseal plate injury and the thickness of the distal radial epiphyseal plate were analyzed across the 3 groups.

RESULTS

Twenty-nine cases (29/76, 38.15%) of distal radial epiphyseal plate injury were observed in 20 divers (20/38, 52.63%). The incidence of injury to the right hand was higher than that for the left (P = 0.009). There were statistically significant differences (P = 0.000) among the 3 groups in terms of epiphyseal plate thickness; group A > group B > group C.

CONCLUSIONS

Distal radial epiphyseal plate injury is common in divers, and more injuries are seen in the right hand. Moreover, growth of the radius was impaired in divers relative to controls. We consider that loading during diving may influence growth of the epiphyseal plate in either a transient or permanent manner.

The primary cilium as a signaling nexus for growth plate function and subsequent skeletal development

The primary cilium is a solitary, antenna-like sensory organelle with many important roles in cartilage and bone development, maintenance, and function. The primary cilium's potential role as a signaling nexus in the growth plate makes it an attractive therapeutic target for diseases and disorders associated with bone development and maintenance. Many signaling pathways that are mediated by the cilium-such as Hh, Wnt, Ihh/PTHrP, TGFβ, BMP, FGF, and Notch-are also known to influence endochondral ossification, primarily by directing growth plate formation and chondrocyte behavior. Although a few studies have demonstrated that these signaling pathways can be directly tied to the primary cilium, many pathways have yet to be evaluated in context of the cilium. This review serves to bridge this knowledge gap in the literature, as well as discuss the cilium's importance in the growth plate's ability to sense and respond to chemical and mechanical stimuli. Furthermore, we explore the importance of using the appropriate mechanism to study the cilium in vivo and suggest IFT88 deletion is the best available technique. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:533-545, 2018.

In situ deformation of growth plate chondrocytes in stress-controlled static vs dynamic compression

Longitudinal bone growth in children/adolescents occurs through endochondral ossification at growth plates and is influenced by mechanical loading, where increased compression decreases growth (i.e., Hueter-Volkmann Law). Past in vivo studies on static vs dynamic compression of growth plates indicate that factors modulating growth rate might lie at the cellular level. Here, in situ viscoelastic deformation of hypertrophic chondrocytes in growth plate explants undergoing stress-controlled static vs dynamic loading conditions was investigated. Growth plate explants from the proximal tibia of pre-pubertal rats were subjected to static vs dynamic stress-controlled mechanical tests. Stained hypertrophic chondrocytes were tracked before and after mechanical testing with a confocal microscope to derive volumetric, axial and lateral cellular strains. Axial strain in hypertrophic chondrocytes was similar for all groups, supporting the mean applied compressive stress's correlation with bone growth rate and hypertrophic chondrocyte height in past studies. However, static conditions resulted in significantly higher lateral (p<0.001) and volumetric cellular strains (p≤0.015) than dynamic conditions, presumably due to the growth plate's viscoelastic nature. Sustained compression in stress-controlled static loading results in continued time-dependent cellular deformation; conversely, dynamic groups have less volumetric strain because the cyclically varying stress limits time-dependent deformation. Furthermore, high frequency dynamic tests showed significantly lower volumetric strain (p=0.002) than low frequency conditions. Mechanical loading protocols could be translated into treatments to correct or halt progression of bone deformities in children/adolescents. Mimicking physiological stress-controlled dynamic conditions may have beneficial effects at the cellular level as dynamic tests are associated with limited lateral and volumetric cellular deformation.

Bone growth resumption following in vivo static and dynamic compression removals on rats

Mechanical loadings influence bone growth and are used in pediatric treatments of musculoskeletal deformities. This in vivo study aimed at evaluating the effects of static and dynamic compression application and subsequent removal on bone growth, mineralization and neuropathic pain markers in growing rats. Forty-eight immature rats (28 days old) were assigned in two groups (2- and 4 weeks experiment duration) and four subgroups: control, sham, static, and dynamic. Controls had no surgery. A micro-loading device was implanted on the 6th and 8th caudal vertebrae of shams without loading, static loading at 0.2 MPa or dynamic loading at 0.2 MPa ± 30% and 0.1 Hz. In 2-week subgroups, compression was maintained for 15 days prior to euthanasia, while in 4- week subgroups, compression was removed for 10 additional days. Growth rates, histomorphometric parameters and mineralization intensity were quantified and compared. At 2 weeks, growth rates and growth plate heights of loaded groups (static/dynamic)were significantly lower than shams (p b 0.01).However, at 4 weeks, both growth rates and growth plate heights of loaded groups were similar to shams. At 4 weeks, alizarin red intensity was significantly higher in dynamics compared to shams (p b 0.05) and controls (p b 0.01). Both static and dynamic compressions enable growth resumption after loading removal, while preserving growth plate histomorphometric integrity. However, mineralization was enhanced after dynamic loading removal only. Dynamic loading showed promising results for fusionless treatment approaches for musculoskeletal deformities.

Endurance exercise and growth hormone improve bone formation in young and growth-retarded chronic kidney disease rats

BACKGROUND

Childhood chronic kidney disease (CKD) is associated with both short stature and abnormal bone mineralization. Normal longitudinal growth depends on proper maturation of epiphyseal growth plate (EGP) chondrocytes, leading to the formation of trabecular bone in the primary ossification centre. We have recently shown that linear growth impairment in CKD is associated with impaired EGP growth hormone (GH) receptor signalling and that exercise improved insulin-like growth factor I (IGF-I) signalling in CKD-related muscle atrophy.

METHODS

In this study, 20-day-old rats underwent 5/6 nephrectomy (CKD) or sham surgery (C) and were exercised with treadmill, with or without GH supplementation.

RESULTS

CKD-related growth retardation was associated with a widened EGP hypertrophic zone. This was not fully corrected by exercise (except for tibial length). Exercise in CKD improved the expression of EGP key factors of endochondral ossification such as IGF-I, vascular endothelial growth factor (VEGF), receptor activator of nuclear factor kappa-B ligand (RANKL) and osteocalcin. Combining GH treatment with treadmill exercise for 2 weeks improved the decreased trabecular bone volume in CKD, as well as the expression of growth plate runt-related transcription factor 2, RANKL, metalloproteinase 13 and VEGF, while GH treatment alone could not do that.

CONCLUSIONS

Treadmill exercise improves tibial bone linear growth, as well as growth plate local IGF-I. When combined with GH treatment, running exercise shows beneficial effects on trabecular bone formation, suggesting the potential benefit of this combination for CKD-related short stature and bone disease.

The growth plate's response to load is partially mediated by mechano-sensing via the chondrocytic primary cilium

Mechanical load plays a significant role in bone and growth-plate development. Chondrocytes sense and respond to mechanical stimulation; however, the mechanisms by which those signals exert their effects are not fully understood. The primary cilium has been identified as a mechano-sensor in several cell types, including renal epithelial cells and endothelium, and accumulating evidence connects it to mechano-transduction in chondrocytes. In the growth plate, the primary cilium is involved in several regulatory pathways, such as the non-canonical Wnt and Indian Hedgehog. Moreover, it mediates cell shape, orientation, growth, and differentiation in the growth plate. In this work, we show that mechanical load enhances ciliogenesis in the growth plate. This leads to alterations in the expression and localization of key members of the Ihh-PTHrP loop resulting in decreased proliferation and an abnormal switch from proliferation to differentiation, together with abnormal chondrocyte morphology and organization. Moreover, we use the chondrogenic cell line ATDC5, a model for growth-plate chondrocytes, to understand the mechanisms mediating the participation of the primary cilium, and in particular KIF3A, in the cell's response to mechanical stimulation. We show that this key component of the cilium mediates gene expression in response to mechanical stimulation.

Effect of rearing environment on bone growth of pullets

Alternative housing systems for laying hens provide mechanical loading and help reduce bone loss. Moreover, achieving greater peak bone mass during pullet phase can be crucial to prevent fractures in the production period. The aim of this study was to determine the housing system effects on bone quality of pullets. Tibiae and humeri of White Leghorn pullets reared in conventional cages (CCs) and a cage-free aviary (AV) system were studied. At 16 wk, 120 birds at random from each housing system were euthanized. Right and left tibiae and humeri were collected and further analyzed. Cortical bone density and thickness were measured using computed tomography. Periosteal and endosteal dimensions were measured at the fracture site during mechanical testing. At 4, 8, 12, and 16 wk, serum concentrations of osteocalcin and hydroxylysyl pyridinoline were analyzed as markers of bone formation and resorption. Cortical bone density was higher (P<0.05) in humeri of AV pullets, and tibiae were denser (P<0.05) for AV pullets in the distal section of the bone compared to CC pullets. Ash content was higher (P<0.05) in AV humeri with no difference in tibiae ash content. Tibiae and humeri of AV pullets had a thicker cortex than the CC pullets (P<0.05). Additionally, the tibiae and humeri of AV pullets had greater (P<0.05) second moment of areas than the CC pullets. While some bone material properties between groups were different (P<0.05), the differences were so small (<7%) that they likely have no clinical significance. Serum osteocalcin concentrations were not different between the treatments, but hydroxylsyl pyridinoline concentrations were higher in CC pullets at 12 wk compared to the AV pullets and the effect reversed at 16 wk (P<0.05). These findings indicate that tibiae and humeri respond differently to load bearing activities during growth. The improved load bearing capability and stiffness in bones of AV pullets were related to increased cross-sectional geometry.

Early initiation of endochondral ossification of mouse femur cultured in hydrogel with different mechanical stiffness

Mineralization is one of the most important processes in normal bone tissue development and in disease condition. Developing a novel and standardized in vitro model system that can readily monitor both cellular dynamics and mineralization is crucial for better understanding the bone tissue development and growth. Recent studies indicated that the mechanical environment is a critical condition in mineralization. We hypothesized that hydrogel with different mechanical stiffness can provide a biomimetic mechanical environment that can modulate bone tissue growth and mineralization. A femur of mouse embryo (embryonic day 16) was embedded in agarose hydrogel (2-60 kPa) and cultured in an osteogenic medium for a week. Microcomputed tomography (μCT) results revealed enhanced mineralization was detected in the femur head cultured in the gel condition, whereas no mineralization in the femur head cultured in the control (floating culture) condition. The mineralized region was corresponding to the region of secondary ossification center. Both histological and quantitative analyses indicated that the mineralized region of femur head cultured in 10 kPa gel condition was the highest and the mineralized area was significantly larger than that cultured in 2, 40, and 60 kPa gel condition. Immunofluorescence results indicated the enhanced mineralization caused by the higher chondrogenic differentiation at that region. This enhancement mainly relating to the mechanical forces and not to the oxygen tension was also confirmed. Since this system enhances and shortens the mineralization procedure compared with the conventional two-dimensional or three-dimensional cell culture system, this hydrogel system would be one of the unique models for better understanding the mineralized tissue development.

Environmental temperature impact on bone and cartilage growth

Environmental temperature can have a surprising impact on extremity growth in homeotherms, but the underlying mechanisms have remained elusive for over a century. Limbs of animals raised at warm ambient temperature are significantly and permanently longer than those of littermates housed at cooler temperature. These remarkably consistent lab results closely resemble the ecogeographical tenet described by Allen's "extremity size rule," that appendage length correlates with temperature and latitude. This phenotypic growth plasticity could have adaptive significance for thermal physiology. Shortened extremities help retain body heat in cold environments by decreasing surface area for potential heat loss. Homeotherms have evolved complex mechanisms to maintain tightly regulated internal temperatures in challenging environments, including "facultative extremity heterothermy" in which limb temperatures can parallel ambient. Environmental modulation of tissue temperature can have direct and immediate consequences on cell proliferation, metabolism, matrix production, and mineralization in cartilage. Temperature can also indirectly influence cartilage growth by modulating circulating levels and delivery routes of essential hormones and paracrine regulators. Using an integrated approach, this article synthesizes classic studies with new data that shed light on the basis and significance of this enigmatic growth phenomenon and its relevance for treating human bone elongation disorders. Discussion centers on the vasculature as a gateway to understanding the complex interconnection between direct (local) and indirect (systemic) mechanisms of temperature-enhanced bone lengthening. Recent advances in imaging modalities that enable the dynamic study of cartilage growth plates in vivo will be key to elucidating fundamental physiological mechanisms of long bone growth regulation.

Regulation of PTHrP expression by cyclic mechanical strain in postnatal growth plate chondrocytes

Mechanical loading has been widely considered to be a crucial regulatory factor for growth plate development, but the exact mechanisms of this regulation are still not completely understood. In the growth plate, parathyroid hormone-related protein (PTHrP) regulates chondrocyte differentiation and longitudinal growth. Cyclic mechanical strain has been demonstrated to influence growth plate chondrocyte differentiation and metabolism, whereas the relationship between cyclic mechanical strain and PTHrP expression is not clear. The objective of this study was to investigate whether short-term cyclic tensile strain regulates PTHrP expression in postnatal growth plate chondrocytes in vitro and to explore whether the organization of cytoskeletal F-actin microfilaments is involved in this process. To this end, we obtained growth plate chondrocytes from 2-week-old Sprague-Dawley rats and sorted prehypertrophic and hypertrophic chondrocytes using immunomagnetic beads coated with anti-CD200 antibody. The sorted chondrocytes were subjected to cyclic tensile strain of varying magnitude and duration at a frequency of 0.5 Hz. We found that cyclic strain regulates PTHrP expression in a magnitude- and time-dependent manner. Incubation of chondrocytes with cytochalasin D, an actin microfilament-disrupting reagent, blocked the induction of PTHrP expression in response to strain. The results suggest that short-term cyclic tensile strain induces PTHrP expression in postnatal growth plate prehypertrophic and hypertrophic chondrocytes and that PTHrP expression by these chondrocytes may subsequently affect growth plate development. The results also support the idea that the organization of cytoskeletal F-actin microfilaments plays an important role in mechanotransduction.

Expression of matrix metalloproteinases in human growth plate chondrocytes is enhanced at high levels of mechanical loading

Matrix metalloproteinases (MMPs), responsible for extracellular matrix remodelling and angiogenesis, might play a major role in the response of the growth plate to detrimental loads that lead to overuse injuries in young athletes. In order to test this hypothesis, human growth plate chondrocytes were subjected to mechanical forces equal to either physiological loads, near detrimental or detrimental loads for two hours. In addition, these cells were exposed to physiological loads for up to 24 hours. Changes in the expression of MMPs -2, -3 and -13 were investigated.

We found that expression of MMPs in cultured human growth plate chondrocytes increases in a linear manner with increased duration and intensity of loading. We also showed for the first time that physiological loads have the same effect on growth plate chondrocytes over a long period of time as detrimental loads applied for a short period.

These findings confirm the involvement of MMPs in overuse injuries in children. We suggest that training programmes for immature athletes should be reconsidered in order to avoid detrimental stresses and over-expression of MMPs in the growth plate, and especially to avoid physiological loads becoming detrimental.

Cite this article: Bone Joint J 2013;95-B:568–73.

TGF β-1 administration during ex vivo expansion of human articular chondrocytes in a serum-free medium redirects the cell phenotype toward hypertrophy

Cell-based cartilage resurfacing requires ex vivo expansion of autologous articular chondrocytes. Defined culture conditions minimize expansion-dependent phenotypic alterations but maintenance of the cells' differentiation potential must be carefully assessed. Transforming growth factor β-1 (TGF β-1) positively regulates the expression of several cartilage proteins, but its therapeutic application in damaged cartilage is controversial. Thus we evaluated the phenotypic outcomes of cultured human articular chondrocytes exposed to TGF β-1 during monolayer expansion in a serum-free medium. After five doublings cells were transferred to micromass cultures to assess their chondrogenic differentiation, or replated in osteogenic medium. Immunocytostainings of micromasses of TGF-expanded cells showed loss of aggrecan and type II collagen. Positivity was evidenced for RAGE, IHH, type X collagen and for apoptotic cells, paralleling a reduction of BCL-2 levels, suggesting hypertrophic differentiation. TGF β-1-exposed cells also evidenced increased mRNA levels for bone sialoprotein, osteopontin, matrix metalloproteinase-13, TIMP-3, VEGF and SMAD7, enhanced alkaline phosphatase activity and pyrophosphate availability. Conversely, SMAD3 mRNA and protein contents were reduced. After osteogenic induction, only TGF-expanded cells strongly mineralized and impaired p38 kinase activity, a contributor of chondrocytes' differentiation. To evaluate possible endochondral ossification progression, we seeded the chondrocytes on hydroxyapatite scaffolds, subsequently implanted in an in vivo ectopic setting, but cells failed to reach overt ossification; nonetheless, constructs seeded with TGF-exposed cells displayed blood vessels of the host vascular supply with enlarged diameters, suggestive of vascular remodeling, as in bone growth. Thus TGF-exposure during articular chondrocytes expansion induces a phenotype switch to hypertrophy, an undesirable effect for cells possibly intended for tissue-engineered cartilage repair.

A MATHEMATICAL MODEL OF THE GROWTH PLATE

The growth plate is a structure formed of cells called chondrocytes; these are arranged in columns and provide the elongation of bone due to their proliferation and hypertrophy. In each column, we can see chondrocytes in their proliferating state, which are constantly dividing, and in hypertrophic state, which grow in a nearly spherical shape. These cells express different proteins and molecules throughout their half-life and exhibit a special behavior depending on their local mechanical and biochemical environments. This article develops a mathematical model that describes the relationship of geometry, growth by proliferation and hypertrophy, and vascular invasion with biochemical and mechanical factors present during endochondral ossification.

The primary cilium as a dual sensor of mechanochemical signals in chondrocytes

The primary cilium is an immotile, solitary, and microtubule-based structure that projects from cell surfaces into the extracellular environment. The primary cilium functions as a dual sensor, as mechanosensors and chemosensors. The primary cilia coordinate several essential cell signaling pathways that are mainly involved in cell division and differentiation. A primary cilium malfunction can result in several human diseases. Mechanical loading is sense by mechanosensitive cells in nearly all tissues and organs. With this sensation, the mechanical signal is further transduced into biochemical signals involving pathways such as Akt, PKA, FAK, ERK, and MAPK. In this review, we focus on the fundamental functional and structural features of primary cilia in chondrocytes and chondrogenic cells.

Primary cilia modulate Ihh signal transduction in response to hydrostatic loading of growth plate chondrocytes

Indian hedgehog (Ihh) is a key component of the regulatory apparatus governing chondrocyte proliferation and differentiation in the growth plate. Recent studies have demonstrated that the primary cilium is the site of Ihh signaling within the cell, and that primary cilia are essential for bone and cartilage formation. Primary cilia are also postulated to act as mechanosensory organelles that transduce mechanical forces acting on the cell into biological signals. In this study, we used a hydrostatic compression system to examine Ihh signal transduction under the influence of mechanical load. Our results demonstrate that hydrostatic compression increased both Ihh gene expression and Ihh-responsive Gli-luciferase activity. These increases were aborted by disrupting the primary cilia structure with chloral hydrate. These results suggest that growth plate chondrocytes respond to hydrostatic loading by increasing Ihh signaling, and that the primary cilium is required for this mechano-biological signal transduction to occur.

The role of matrix gla protein in ossification and recovery of the avian growth plate

Extracellular matrix mineralization is an essential physiologic process in bone, teeth, and hypertrophic cartilage. Matrix Gla protein (MGP), an inhibitor of mineralization, is expressed by chondrocytes and vascular smooth muscle cells to inhibit calcification of those soft tissues. Tibial dyschondroplasia (TD), a skeletal abnormality apparent as a plug of non-vascularized, non-mineralized, white opaque cartilage in the tibial growth plate of avian species can serve as a good model for studying process and genes involved in matrix mineralization and calcification. In this work, we studied the involvement of MGP in the development of TD, as well as in the processes of spontaneous and induced recovery from this syndrome. First, we found that during normal bone development, MGP is expressed in specific time and locations, starting from wide-spread expression in the yet un-ossified diaphysis during embryonic development, to specific expression in hypertrophic chondrocytes adjacent to the chondro-osseous junction and the secondary ossification center just prior to calcification. In addition, we show that MGP is not expressed in the impaired TD lesion, however when the lesion begins to heal, it strongly express MGP prior to its calcification. Moreover, we show that when calcification is inhibited, a gap is formed between the expression zones of MGP and BMP2 and that this gap is closed during the healing process. To conclude, we suggest that MGP, directly or through interaction with BMP2, plays a role as ossification regulator that acts prior to ossification, rather then simple inhibitor.

In vivo dynamic bone growth modulation is less detrimental but as effective as static growth modulation

Longitudinal bone growth, which occurs in growth plates, has important implications in pediatric orthopedics. Mechanical loads are essential to normal bone growth, but excessive loads can lead to progressive deformities. In order to compare the effects of in vivo static and dynamic loading on bone growth rate and growth plate histomorphometry, a finely controlled, normalized and equivalent compression was applied for a period of two weeks on the seventh caudal vertebra (Cd7) of rats during their pubertal growth spurt. The load was sustained (0.2MPa, 0.0Hz) in the static group and sinusoidally oscillating (0.2MPa±30%, 0.1Hz) in the dynamic group. Control and sham (operated but no load applied) groups were also studied. Cd7 growth rate was statistically reduced by 19% (p<0.001) for both static and dynamic groups when compared to the sham group. Loading effects on growth plate histomorphometry were greater in the static than dynamic groups with significant reductions (p<0.001) observed for growth plate thickness, proliferative chondrocyte number per column and hypertrophic chondrocyte height in the static group when compared to the sham group. Significant differences (p<0.01) were also found between static and dynamic groups for growth plate thickness and proliferative chondrocyte number per column while the difference nearly reached significance (p=0.014) for hypertrophic chondrocyte height. This in vivo study shows that static and dynamic loading are equally effective in modulating bone growth of rat caudal vertebrae. However, dynamic loading causes less detrimental effects on growth plate histomorphometry compared to static loading. This knowledge is greatly relevant for the improvement and/or development of new minimally invasive approaches, which are based on the local modulation of bone growth, to correct several progressive musculoskeletal deformities.

1,25(OH)2D3 alters growth plate maturation and bone architecture in young rats with normal renal function

Whereas detrimental effects of vitamin D deficiency are known over century, the effects of vitamin D receptor activation by 1,25(OH)₂D₃, the principal hormonal form of vitamin D, on the growing bone and its growth plate are less clear. Currently, 1,25(OH)₂D₃ is used in pediatric patients with chronic kidney disease and mineral and bone disorder (CKD-MBD) and is strongly associated with growth retardation. Here, we investigate the effect of 1,25(OH)₂D₃ treatment on bone development in normal young rats, unrelated to renal insufficiency. Young rats received daily i.p. injections of 1 µg/kg 1,25(OH)₂D₃ for one week, or intermittent 3 µg/kg 1,25(OH)₂D₃ for one month. Histological analysis revealed narrower tibial growth plates, predominantly in the hypertrophic zone of 1,25(OH)₂D₃-treated animals in both experimental protocols. This phenotype was supported by narrower distribution of aggrecan, collagens II and X mRNA, shown by in situ hybridization. Concomitant with altered chondrocyte maturation, 1,25(OH)₂D₃ increased chondrocyte proliferation and apoptosis in terminal hypertrophic cells. In vitro treatment of the chondrocytic cell line ATDC5 with 1,25(OH)₂D₃ lowered differentiation and increased proliferation dose and time-dependently. Micro-CT analysis of femurs from 1-week 1,25(OH)₂D₃-treated group revealed reduced cortical thickness, elevated cortical porosity, and higher trabecular number and thickness. 1-month administration resulted in a similar cortical phenotype but without effect on trabecular bone. Evaluation of fluorochrome binding with confocal microscopy revealed inhibiting effects of 1,25(OH)₂D₃ on intracortical bone formation. This study shows negative effects of 1,25(OH)₂D₃ on growth plate and bone which may contribute to the exacerbation of MBD in the CKD pediatric patients.

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

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

Involvement of matrix metalloproteinases in the growth plate response to physiological mechanical load

Enzymes from the matrix metalloproteinase (MMP) family play a crucial role in growth-plate vascularization and ossification via proteolytic cleavage and remodeling of the extracellular matrix. Their regulation in the growth plate is crucial for normal matrix assembly. Endochondral ossification, which takes place at the growth plates, is influenced by mechanical loading. Using an in vivo avian model for mechanical loading, we have found increased blood penetration into the growth plates of loaded chicks. The purpose of this work was to study the involvement of MMP-2, -3, -9, -13, and -16 in the growth plate's response to loading and in the catch-up growth resulting from load release. We found that mechanical loading, as well as release from load, upregulated MMP-2, -9, and -13 expressions. In contrast, MMP-3, associated with cartilage injuries, and its associated protein connective tissue growth factor (CTGF), were downregulated by the load. However, after release from load, MMP-3 was upregulated and CTGF levels were elevated and caught up with the control. MMP-3 and CTGF were also downregulated after 60 min of mechanical stretching in vitro. These results demonstrate the central role of MMPs in the growth plate's response to mechanical loading, as well as in the catch-up growth followed load release.

Growth plate mechanics and mechanobiology. A survey of present understanding

The longitudinal growth of long bones occurs in growth plates where chondrocytes synthesize cartilage that is subsequently ossified. Altered growth and subsequent deformity resulting from abnormal mechanical loading is often referred to as mechanical modulation of bone growth. This phenomenon has key implications in the progression of infant and juvenile musculoskeletal deformities, such as adolescent idiopathic scoliosis, hyperkyphosis, genu varus/valgus and tibia vara/valga, as well as neuromuscular diseases. Clinical management of these deformities is often directed at modifying the mechanical environment of affected bones. However, there is limited quantitative and physiological understanding of how bone growth is regulated in response to mechanical loading. This review of published work addresses the state of knowledge concerning key questions about mechanisms underlying biomechanical modulation of bone growth. The longitudinal growth of bones is apparently controlled by modifying the numbers of growth plate chondrocytes in the proliferative zone, their rate of proliferation, the amount of chondrocytic hypertrophy and the controlled synthesis and degradation of matrix throughout the growth plate. These variables may be modulated to produce a change in growth rate in the presence of sustained or cyclic mechanical load. Tissue and cellular deformations involved in the transduction of mechanical stimuli depend on the growth plate tissue material properties that are highly anisotropic, time-dependent, and that differ in different zones of the growth plate and with developmental stages. There is little information about the effects of time-varying changes in volume, water content, osmolarity of matrix, etc. on differentiation, maturation and metabolic activity of chondrocytes. Also, the effects of shear forces and torsion on the growth plate are incompletely characterized. Future work on growth plate mechanobiology should distinguish between changes in the regulation of bone growth resulting from different processes, such as direct stimulation of the cell nuclei, physico-chemical stimuli, mechanical degradation of matrix or cellular components and possible alterations of local blood supply.

Simulation of biological growth

Researchers concerned with the growth of biological tissue often use models that predict the growth as a function of a mechanical stimulus such as stress, strain or elastic energy. However, a general theory for bulk growth should consider that the mechanical stimulus may only be one of many factors contributing to growth. Another important factor could be time, as living tissues can be assumed to have a pre-programmed directional biological growth that is independent of mechanical stimuli. This paper has two objectives: the first is to introduce the concept of directional biological growth within a well developed growth theory, the second is to present the computational methods by which three-dimensional growth that encompasses time and stress effects can be simulated using commercially available finite element analysis software.

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

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

Gene expression profiles of dynamically compressed single chondrocytes and chondrons

A chondrocyte produces a hydrated pericellular matrix (PCM); together they form a chondron. Previous work has shown that the presence of the PCM influences the biological response of chondrocytes to loading. The objective of this study was to determine the gene expression profiles of enzymatically isolated single chondrocytes and chondrons in response to dynamic compression. Cartilage specific extracellular matrix components and transcription factors were examined. Following dynamic compression, chondrocytes and chondrons showed variations in gene expression profiles. Aggrecan, Type II collagen and osteopontin gene expression were significantly increased in chondrons. Lubricin gene expression decreased in both chondrons and chondrocytes. Dynamic compression had no effect on SOX9 gene expression. Our results demonstrate a clear role for the PCM in interfacing the mechanical signalling in chondrocytes in response to dynamic compression. Further investigation of single chondrocytes and chondrons from different zones within articular cartilage may further our understanding of cartilage mechanobiology.

The effect of weight loading and subsequent release from loading on the postnatal skeleton

INTRODUCTION

The relationship between load and the structure and mechanical properties of mature bones has been thoroughly described. In contrast, this relationship has been studied much less in immature bones, which consist of bony tissue and cartilaginous growth plate, during the postnatal period. This paper describes the effect of an externally applied load on the bones of young fast-growing chicks; in particular, we examine the effect on the growth plate, which regulates longitudinal bone growth, and the consequences in terms of bone structural and mechanical properties.

MATERIALS AND METHODS

The tibial growth plates from chicks subjected to external load and control chicks, immediately after loading and following 5 days of load release, were studied by histological staining and quantitative PCR. The contralateral tibiae were mechanically tested by three-point bending and their structural features determined by micro-CT.

RESULTS

At the end of the external loading period, the tibias of the experimental group were shorter and their growth plate narrower than in controls. However, at this time point, effects were not yet apparent in the bones' structural or mechanical parameters. After a further 5 days of no external load, bones and growth plates of the experimental group demonstrated the phenomenon of 'catch-up': the thickness of the growth plate exceeded that of the control; however the relative expression of genes controlling chondrocyte differentiation (collagen II and X) did not change, while the expression of factors related to growth-plate ossification (osteopontin, alkaline phosphatase) and cartilage and bone calcification (matrix and bone Gla proteins) was upregulated as a result of the catch-up process. At this time, however, the tibiae of the experimental group showed inferior mechanical and structural properties relative to the control group.

CONCLUSION

External loading during bone elongation negatively affects the mechanical and structural properties of the skeleton. The effect is first noticeable in the growth plate, which regulates bone growth, and is exhibited in the bone phenotype after a lag period.

Validation of a genu valgum model in a rabbit hind limb

Numerous techniques exist to correct pediatric angular deformity by asymmetrically inhibiting physeal growth. Despite decades of surgical experience, little is known about the determinants of success or failure of temporary hemiepiphysiodesis. We lack a basic understanding of tolerances and kinematics of the surgically restrained physis. Furthermore, little is known about the influence of implant design and placement on efficacy of deformity correction and rebound growth. We have undertaken a pilot research study with the goal of producing genu valgum in New Zealand white rabbits. This report comprises our initial experience and observations in performing hemiepiphysiodesis with staples and 2-hole plate techniques. The experimental hypotheses proposed by this article are as follows: (1) a staple or plate applied to the proximal lateral tibial physis of a rabbit hind limb will reliably create a valgus deformity of the knee; (2) the plate or staple will create this deformity without permanently damaging the proximal tibial physis; and (3) provided the implant remains in situ, there will be no difference between the plate and staple constructs with respect to the magnitude or rate of deformity produced. Further studies will aim to use this model to investigate technical issues related to physeal instrumentation.

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

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

Matrix metalloproteinase expression and localization in turkey (Meleagris gallopavo) during the endochondral ossification process1

Vertebrate long bones are formed by endochondral ossification, a process accompanied by changes in extracellular matrix synthesis and remodeling, performed mainly by the matrix metalloproteinases (MMP). The temporal/spatial expression patterns of 5 members of the MMP family known to be important for endochondral ossification were studied, for the first time, in the turkey growth plate during embryonic and juvenile stages. The expression of MMP-2 was detected in the proliferative zone, MMP-3, MMP-9, and MMP-13 in cells lining the blood vessels; MMP-13 was also detected in hypertrophic chondrocytes. The MMP-16 expression was detected in the reserve zone of the growth plate. These results present a detailed survey of turkey MMP, serving as a data source (atlas) for further studies in this subject.