Genetic variation in the patterns of skeletal progenitor cell differentiation and progression during endochondral bone formation affects the rate of fracture healing

OMIBONE: Omics-driven computer model of bone regeneration for personalized treatment

Treatment of bone fractures are standardized according to the AO classification, which mainly refers to the mechanical stabilization required in a given situation but neglect individual differences due to patient's healing potential or accompanying diseases. Specially in elderly or immune-compromised patients, the complexity of individual constrains on a biological as well as mechanical level are hard to account for. Here, we introduce a novel framework that allows to predict bone regeneration outcome using combined proteomic and mechanical analyses in a computer model. The framework uses Ingenuity Pathway Analysis (IPA) software to link protein changes to alterations in biological processes and integrates these in an Agent-Based Model (ABM) of bone regeneration. This combined framework allows to predict bone formation and the potential of an individual to heal a given fracture setting. The performance of the framework was evaluated by replicating the experimental setup of a mouse femur fracture stabilized with an intramedullary pin. The model was informed by serum derived proteomics data. The tissue formation patterns were compared against experimental data based on x-ray and histology images. The results indicate the framework potential in predicting an individual's bone formation potential and hold promise as a concept to enable personalized bone healing predictions for a chosen fracture fixation.

Stimulation of fracture mineralization by salt-inducible kinase inhibitors

Introduction

Over 6.8 million fractures occur annually in the US, with 10% experiencing delayed- or non-union. Anabolic therapeutics like PTH analogs stimulate fracture repair, and small molecule salt inducible kinase (SIK) inhibitors mimic PTH action. This study tests whether the SIK inhibitor YKL-05-099 accelerates fracture callus osteogenesis.

Methods

126 female mice underwent femoral shaft pinning and midshaft fracture, receiving daily injections of PBS, YKL-05-099, or PTH. Callus tissues were analyzed via RT-qPCR, histology, single-cell RNA-seq, and μCT imaging. Biomechanical testing evaluated tissue rigidity. A hydrogel-based delivery system for PTH and siRNAs targeting SIK2/SIK3 was developed and tested.

Results

YKL-05-099 and PTH-treated mice showed higher mineralized callus volume fraction and improved structural rigidity. RNA-seq indicated YKL-05-099 increased osteoblast subsets and reduced chondrocyte precursors. Hydrogel-released siRNAs maintained target knockdown, accelerating callus mineralization.

Discussion

YKL-05-099 enhances fracture repair, supporting selective SIK inhibitors' development for clinical use. Hydrogel-based siRNA delivery offers targeted localized treatment at fracture sites.

Intramedullary implant stability affects patterns of fracture healing in mice with morphologically different bone phenotypes

Almost all prior mouse fracture healing models have used needles or K-wires for fixation, unwittingly providing inadequate mechanical stability during the healing process. Our contention is that the reported outcomes have predominantly reflected this instability, rather than the impact of diverse biological conditions, pharmacologic interventions, exogenous growth factors, or genetic considerations. This important issue becomes obvious upon a critical review of the literature. Therefore, the primary aim of this study was to demonstrate the significance of mouse-specific implants designed to provide both axial and torsional stability (Screw and IM Nail) compared to conventional pins (Needle and K-wires), even when used in mice with differently sized marrow canals and diverse genetic backgrounds. B6 (large medullary canal), DBA, and C3H (smaller medullary canals) mice were employed, all of which have different bone morphologies. Closed femoral fractures were created and stabilized with intramedullary implants that provide different mechanical conditions during the healing process. The most important finding of this study was that appropriately designed mouse-specific implants, providing both axial and torsional stability, had the greatest influence on bone healing outcomes regardless of the different bone morphologies encountered. For instance, unstable implants in the B6 strain (largest medullary canal) resulted in significantly greater callus, with a fracture region mainly comprising trabecular bone along with the presence of cartilage 28 days after surgery. The DBA and C3H strains (with smaller medullary canals) instead formed significantly less callus, and only had a small amount of intracortical trabeculation remaining. Moreover, with more stable fracture fixation a higher BV/TV was observed and cortices were largely restored to their original dimensions and structure, indicating an accelerated healing and remodeling process. These observations reveal that the diaphyseal cortical thickness, influenced by the genetic background of each strain, played a pivotal role in determining the amount of bone formation in response to the fracture. These findings are highly important, indicating the rate and type of tissue formed is a direct result of mechanical instability, and this most likely would mask the true contribution of the tested genes, genetic backgrounds, or various therapeutic agents administered during the bone healing process.

Overview of Skeletal Repair (Fracture Healing and Its Assessment)

The study of postnatal skeletal repair is of immense clinical interest. Optimal repair of skeletal tissue is necessary in all varieties of elective and reparative orthopedic surgical treatments. However, the repair of fractures is unique in this context in that fractures are one of the most common traumas that humans experience and are the end-point manifestation of osteoporosis, the most common chronic disease of aging. In the first part of this introduction the basic biology of fracture healing is presented. The second part discusses the primary methodological approaches that are used to examine repair of skeletal hard tissue and specific considerations for choosing among and implementing these approaches.

Generation of Closed Transverse Fractures in Small Animals

The most common procedure that has been developed for use in rats and mice to model fracture healing is described. The nature of the regenerative processes that may be assessed and the types of research questions that may be addressed with this model are briefly outlined. The detailed surgical protocol to generate closed simple transverse fractures is presented and general considerations when setting up an experiment using this model are described.

The biology and treatment of acute long-bones diaphyseal fractures: Overview of the current options for bone healing enhancement

Diaphyseal fractures represent a complex biological entity that could often end into impaired bone-healing, with delayed union and non-union occurring up to 10% of cases. The role of the modern orthopaedic surgeon is to optimize the fracture healing environment, recognize and eliminate possible interfering factors, and choose the best suited surgical fixation technique. The impaired reparative process after surgical intervention can be modulated with different surgical techniques, such as dynamization or exchange nailing after failed intramedullary nailing. Moreover, the mechanical stability of a nail can be improved through augmentation plating, bone grafting or external fixation techniques with satisfactory results. According to the "diamond concept", local therapies, such as osteoconductive scaffolds, bone growth factors, and osteogenic cells can be successfully applied in "polytherapy" for the enhancement of delayed union and non-union of long bones diaphyseal fractures. Moreover, systemic anti-osteoporosis anabolic drugs, such as teriparatide, have been proposed as off-label treatment for bone healing enhancement both in fresh complex shaft fractures and impaired unions, especially for fragility fractures. The article aims to review the biological and mechanical principles of failed reparative osteogenesis of diaphyseal fractures after surgical treatment. Moreover, the evidence about the modern non-surgical and pharmacological options for bone healing enhancement will discussed.

Genetic variation in mice affects closed femoral fracture pattern outcomes

The purpose of this study was to determine whether differences in structural and material properties of bone between different mouse strains influence the fracture patterns produced under experimental fracture conditions. Femurs of C57BL/6 (B6), C3H/HeJ (C3H), and DBA/2 (DBA) strains were evaluated using micro-computed tomography (μCT), measurements derived from radiographic images and mechanical testing to determine differences in the geometry and mechanical properties. A fracture device was used to create femoral fractures on freshly sacrificed animals using a range of kinetic energies (∼20-80mJ) which were classified as transverse, oblique, or comminuted. B6 femurs had the lowest bone volume/total volume (BV/TV) and bone mineral density (BMD), thinnest cortex, and had the most variable fracture patterns, with 77.5% transverse, 15% oblique, and 7.5% comminuted fractures. In contrast, C3H had the highest BV/TV, BMD, and thickest cortices, resulting in 97.5% transverse, 2.5% oblique, and 0% comminuted fractures. DBA had an intermediate BV/TV and thickness of cortices, with BMD similar to C3H, resulting in 92.9% transverse, 7.1% oblique, and 0% comminuted fractures. A binomial logistic regression confirmed that bone morphometry was the single strongest predictor of the resulting fracture pattern. This study demonstrated that the reproducibility of closed transverse femoral fractures was most influenced by the structural and material properties of the bone characteristics in each strain, rather than the kinetic energy or body weight of the mice. This was evidenced through geometric analysis of X-ray and μCT data, and further supported by the bone mineral density measurements from each strain, derived from μCT. Furthermore, this study also demonstrated that the use of lower kinetic energies was more than sufficient to reproducibly create transverse fractures, and to avoid severe tissue trauma. The creation of reproducible fracture patterns is important as this often dictates the outcomes of fracture healing, and those studies that do not control this potential variability could lead to a false interpretation of the results.

Clustering of temporal gene expression data with mixtures of mixed effects models with a penalized likelihood

MOTIVATION

Clustering algorithms like K-Means and standard Gaussian mixture models (GMM) fail to account for the structure of variability of replicated data or repeated measures over time. Additionally, a priori cluster number assumptions add an additional complexity to the process. Current methods to optimize cluster labels and number can be inaccurate or computationally intensive for temporal gene expression data with this additional variability.

RESULTS

An extension to a model-based clustering algorithm is proposed using mixtures of mixed effects polynomial regression models and the EM algorithm with an entropy penalized log-likelihood function (EPEM). The EPEM is used to cluster temporal gene expression data with this additional variability. The addition of random effects in our model decreased the misclassification error when compared to mixtures of fixed effects models or other methods such as K-Means and GMM. Applying our method to microarray data from a fracture healing study revealed distinct temporal patterns of gene expression.

AVAILABILITY AND IMPLEMENTATION

https://github.com/darlenelu72/EPEM-GMM.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Hypophosphatemia Regulates Molecular Mechanisms of Circadian Rhythm

Transcriptomic analysis showed that the central circadian pathway genes had significantly altered expression in fracture calluses from mice fed a low phosphate diet. This led us to hypothesize that phosphate deficiency altered the circadian cycle in peripheral tissues. Analysis of the expression of the central clock genes over a 24-36 hour period in multiple peripheral tissues including fracture callus, proximal tibia growth plate and cardiac tissues after 12 days on a low phosphate diet showed higher levels of gene expression in the hypophosphatemia groups (p < 0.001) and a 3 to 6 hour elongation of the circadian cycle. A comparative analysis of the callus tissue transcriptome genes that were differentially regulated by hypophosphatemia with published data for the genes in bone that are diurnally regulated identified 1879 genes with overlapping differential regulation, which were shown by ontology assessment to be associated with oxidative metabolism and apoptosis. Network analysis of the central circadian pathway genes linked their expression to the up regulated expression of the histone methyltransferase gene EZH2, a gene that when mutated in both humans and mice controls overall skeletal growth. These data suggest that phosphate is an essential metabolite that controls circadian function in both skeletal and non skeletal peripheral tissues and associates its levels with the overall oxidative metabolism and skeletal growth of animals.

Transcriptional Mechanisms of Secondary Fracture Healing

PURPOSE OF REVIEW

Growing evidence supports the critical role of transcriptional mechanisms in promoting the spatial and temporal progression of bone healing. In this review, we evaluate and discuss new transcriptional and post-transcriptional regulatory mechanisms of secondary bone repair, along with emerging evidence for epigenetic regulation of fracture healing.

RECENT FINDINGS

Using the candidate gene approach has identified new roles for several transcription factors in mediating the reactive, reparative, and remodeling phases of fracture repair. Further characterization of the different epigenetic controls of fracture healing and fracture-driven transcriptome changes between young and aged fracture has identified key biological pathways that may yield therapeutic targets. Furthermore, exogenously delivered microRNA to post-transcriptionally control gene expression is quickly becoming an area with great therapeutic potential. Activation of specific transcriptional networks can promote the proper progression of secondary bone healing. Targeting these key factors using small molecules or through microRNA may yield effective therapies to enhance and possibly accelerate fracture healing.

BMPR1A antagonist differentially affects cartilage and bone formation during fracture healing

A soluble form of BMP receptor type 1A (mBMPR1A-mFC) acts as an antagonist to endogenous BMPR1A and has been shown to increase bone mass in mice. The goal of this study was to examine the effects of mBMPR1A-mFC on secondary fracture healing. Treatment consisted of 10 mg/kg intraperitoneal injections of mBMPR1A-mFC twice weekly in male C57BL/6 mice. Treatment beginning at 1, 14, and 21 days post-fracture assessed receptor function during endochondral bone formation, at the onset of secondary bone formation, and during coupled remodeling, respectively. Control animals received saline injections. mBMPR1A-mFC treatment initiated on day 1 delayed cartilage maturation in the callus and resulted in large regions of fibrous tissue. Treatment initiated on day 1 also increased the amount of mineralized tissue and up-regulated many bone-associated genes (p = 0.002) but retarded periosteal bony bridging and impaired strength and toughness at day 35 (p < 0.035). Delaying the onset of treatment to day 14 or 21 partially mitigated these effects and produced evidence of accelerated coupled remodeling. These results indicate that inhibition of the BMPR1A-mediated signaling has negative effects on secondary fracture healing that are differentially manifested at different stages of healing and within different cell populations. These effects are most pronounced during the endochondral period and appear to be mediated by selective inhibition of BMPRIA signaling within the periosteum. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:2096-2105, 2016.

Intrinsic Sex-Linked Variations in Osteogenic and Adipogenic Differentiation Potential of Bone Marrow Multipotent Stromal Cells

Bone formation and aging are sexually dimorphic. Yet, definition of the intrinsic molecular differences between male and female multipotent mesenchymal stromal cells (MSCs) in bone is lacking. This study assessed sex-linked differences in MSC differentiation in 3-, 6-, and 9-month-old C57BL/6J mice. Analysis of tibiae showed that female mice had lower bone volume fraction and higher adipocyte content in the bone marrow compared to age-matched males. While both males and females lost bone mass in early aging, the rate of loss was higher in males. Similar expression of bone- and adipocyte-related genes was seen in males and females at 3 and 9 months, while at 6 months, females exhibited a twofold greater expression of these genes. Under osteogenic culture conditions, bone marrow MSCs from female 3- and 6-month-old mice expressed similar levels of bone-related genes, but significantly greater levels of adipocyte-related genes, than male MSCs. Female MSCs also responded to rosiglitazone-induced suppression of osteogenesis at a 5-fold lower (10 nM) concentration than male MSCs. Female MSCs grown in estrogen-stripped medium showed similar responses to rosiglitazone as MSCs grown in serum containing estrogen. MSCs from female mice that had undergone ovariectomy before sexual maturity also were sensitive to rosiglitazone-induced effects on osteogenesis. These results suggest that female MSCs are more sensitive to modulation of differentiation by PPARγ and that these differences are intrinsic to the sex of the animal from which the MSCs came. These results also may explain the sensitivity of women to the deleterious effects of rosiglitazone on bone.

Intramembranous bone regeneration differs among common inbred mouse strains following marrow ablation

Various intact and post-injury bone phenotypes are heritable traits. In this study, we sought to determine if intramembranous bone regeneration following marrow ablation differed among common inbred mouse strains and to identify how early the differences appear. We found a ∼four-fold difference in the regenerated bone volume 21 days after marrow ablation in females from four inbred mouse strains: FVB/N (15.7 ± 8.1%, mean and standard deviation), C3H/He (15.5 ± 4.2%), C57BL/6 (12.2 ± 5.2%), and BALB/c (4.0 ± 4.4%); with BALB/c different from FVB/N (p = 0.007) and C3H/He (p = 0.002). A second experiment showed that FVB/N compared to BALB/c mice had more regenerated bone 7 and 14 days after ablation (p < 0.001), while at 21 days FVB/N mice had a greater fraction of mineralizing surface (p = 0.008) without a difference in mineral apposition rate. Thus, differences among strains are evident early during intramembranous bone regeneration following marrow ablation and appear to be associated with differences in osteogenic cell recruitment, but not osteoblast activity. The amount of regenerating bone was not correlated with other heritable traits such as the intact bone phenotype or soft tissue wound healing, suggesting that there may be independent genetic pathways for these traits.

Animal Models for Evaluation of Bone Implants and Devices

Bone implants and devices are a rapidly growing field within biomedical research, and implants have the potential to significantly improve human and animal health. Animal models play a key role in initial product development and are important components of nonclinical data included in applications for regulatory approval. Pathologists are increasingly being asked to evaluate these models at the initial developmental and nonclinical biocompatibility testing stages, and it is important to understand the relative merits and deficiencies of various species when evaluating a new material or device. This article summarizes characteristics of the most commonly used species in studies of bone implant materials, including detailed information about the relevance of a particular model to human bone physiology and pathology. Species reviewed include mice, rats, rabbits, guinea pigs, dogs, sheep, goats, and nonhuman primates. Ultimately, a comprehensive understanding of the benefits and limitations of different model species will aid in rigorously evaluating a novel bone implant material or device.

A simple critical-sized femoral defect model in mice

While bone has a remarkable capacity for regeneration, serious bone trauma often results in damage that does not properly heal. In fact, one tenth of all limb bone fractures fail to heal completely due to the extent of the trauma, disease, or age of the patient. Our ability to improve bone regenerative strategies is critically dependent on the ability to mimic serious bone trauma in test animals, but the generation and stabilization of large bone lesions is technically challenging. In most cases, serious long bone trauma is mimicked experimentally by establishing a defect that will not naturally heal. This is achieved by complete removal of a bone segment that is larger than 1.5 times the diameter of the bone cross-section. The bone is then stabilized with a metal implant to maintain proper orientation of the fracture edges and allow for mobility. Due to their small size and the fragility of their long bones, establishment of such lesions in mice are beyond the capabilities of most research groups. As such, long bone defect models are confined to rats and larger animals. Nevertheless, mice afford significant research advantages in that they can be genetically modified and bred as immune-compromised strains that do not reject human cells and tissue. Herein, we demonstrate a technique that facilitates the generation of a segmental defect in mouse femora using standard laboratory and veterinary equipment. With practice, fabrication of the fixation device and surgical implantation is feasible for the majority of trained veterinarians and animal research personnel. Using example data, we also provide methodologies for the quantitative analysis of bone healing for the model.

Mouse models of bone healing: fracture, marrow ablation, and distraction osteogenesis

Three commonly used murine surgical models of bone healing [closed fracture with intramedullary fixation, distraction osteogenesis (DO), and marrow ablation by reaming] are presented. Detailed surgical protocols for each model are outlined. The nature of the regenerative processes and the types of research questions that may be addressed with these models are briefly outlined. The relative strengths and weaknesses of these models are compared to a number of other surgical models that are used to address similar research questions.

Bone morphology in 46 BXD recombinant inbred strains and femur-tibia correlation

We examined the bone properties of BXD recombinant inbred (RI) mice by analyzing femur and tibia and compared their phenotypes of different compartments. 46 BXD RI mouse strains were analyzed including progenitor C57BL/6J (n = 16) and DBA/2J (n = 15) and two first filial generations (D2B6F1 and B6D2F1). Strain differences were observed in bone quality and structural properties (P < 0.05) in each bone profile (whole bone, cortical bone, or trabecular bone). It is well known that skeletal phenotypes are largely affected by genetic determinants and genders, such as bone mineral density (BMD). While genetics and gender appear expectedly as the major determinants of bone mass and structure, significant correlations were also observed between femur and tibia. More importantly, positive and negative femur-tibia associations indicated that genetic makeup had an influence on skeletal integrity. We conclude that (a) femur-tibia association in bone morphological properties significantly varies from strain to strain, which may be caused by genetic differences among strains, and (b) strainwise variations were seen in bone mass, bone morphology, and bone microarchitecture along with bone structural property.

Constructing the toolbox: Patient-specific genetic factors of altered fracture healing

The multifaceted sequence of events that follow fracture repair can be further complicated when considering risk factors for impaired union, present in a large and growing percentage of the population. Risk factors such as diabetes, substance abuse, and poor nutrition affect both the young and old alike, and have been shown to dramatically impair the body's natural healing processes. To this end, biotherapeudic interventions such as ultrasound, electrical simulation, growth factor treatment (BMP-2, BMP-7, PDGF-BB, FGF-2) have been evaluated in preclinical models and in some cases are used widely for patients with established non-union or risk/indication or impaired healing (ie. ultrasound, BMP-2, etc.). Despite the promise of these interventions, they have been shown to be reliant on patient compliance and can produce adverse side-effects such as heterotopic ossification. Gene and cell therapy approaches have attempted to apply controlled regimens of these factors and have produced promising results. However, there are safety and efficacy concerns that may limit the translation of these approaches. In addition, none of the above mentioned approaches consider genetic variation between individual patients. Several clinical and preclinical studies have demonstrated a genetic component to fracture repair and that SNPs and genetic background variation play major roles in the determination of healing outcomes. Despite this, there is a need for preclinical data to dissect the mechanism underlying the influence of specific gene loci on the processes of fracture healing, which will be paramount in the future of patient-centered interventions for fracture repair.

Overview of skeletal repair (fracture healing and its assessment)

The study of postnatal skeletal repair is of immense clinical interest. Optimal repair of skeletal tissue is necessary in all varieties of elective and reparative orthopedic surgical treatments. However, the repair of fractures is unique in this context in that fractures are one of the most common traumas that humans experience and are the end-point manifestation of osteoporosis, the most common chronic disease of aging. In the first part of this introduction the basic biology of fracture healing is presented. The second part discusses the primary methodological approaches that are used to examine repair of skeletal hard tissue and specific considerations for choosing among and implementing these approaches.

Runx2 is required for early stages of endochondral bone formation but delays final stages of bone repair in Axin2-deficient mice

Runx2 and Axin2 regulate skeletal development. We recently determined that Axin2 and Runx2 molecularly interact in differentiating osteoblasts to regulate intramembranous bone formation, but the relationship between these factors in endochondral bone formation was unresolved. To address this, we examined the effects of Axin2 deficiency on the cleidocranial dysplasia (CCD) phenotype of Runx2(+/-) mice, focusing on skeletal defects attributed to improper endochondral bone formation. Axin2 deficiency unexpectedly exacerbated calvarial components of the CCD phenotype in the Runx2(+/-) mice; the endocranial layer of the frontal suture, which develops by endochondral bone formation, failed to mineralize in Axin2(-/-):Runx2(+/-) mice, resulting in a cartilaginous, fibrotic and larger fontanel than observed in Runx2(+/-) mice. Transcripts associated with cartilage development (e.g., Acan, miR140) were expressed at higher levels, whereas blood vessel morphogenesis transcripts (e.g., Slit2) were suppressed in Axin2(-/-):Runx2(+/-) calvaria. Cartilage maturation was impaired, as primary chondrocytes from double mutant mice demonstrated delayed differentiation and produced less calcified matrix in vitro. The genetic dominance of Runx2 was also reflected during endochondral fracture repair, as both Runx2(+/-) and double mutant Axin2(-/-):Runx2(+/-) mice had enlarged fracture calluses at early stages of healing. However, by the end stages of fracture healing, double mutant animals diverged from the Runx2(+/-) mice, showing smaller calluses and increased torsional strength indicative of more rapid end stage bone formation as seen in the Axin2(-/-) mice. Taken together, our data demonstrate a dominant role for Runx2 in chondrocyte maturation, but implicate Axin2 as an important modulator of the terminal stages of endochondral bone formation.

Role of Fas and Treg cells in fracture healing as characterized in the fas-deficient (lpr) mouse model of lupus

Previous studies showed that loss of tumor necrosis factor α (TNFα) signaling delayed fracture healing by delaying chondrocyte apoptosis and cartilage resorption. Mechanistic studies showed that TNFα induced Fas expression within chondrocytes; however, the degree to which chondrocyte apoptosis is mediated by TNFα alone or dependent on the induction of Fas is unclear. This question was addressed by assessing fracture healing in Fas-deficient B6.MRL/Fas(lpr) /J mice. Loss of Fas delayed cartilage resorption but also lowered bone fraction in the calluses. The reduced bone fraction was related to elevated rates of coupled bone turnover in the B6.MRL/Fas(lpr) /J calluses, as evidenced by higher osteoclast numbers and increased osteogenesis. Analysis of the apoptotic marker caspase 3 showed fewer positive chondrocytes and osteoclasts in calluses of B6.MRL/Fas(lpr) /J mice. To determine if an active autoimmune state contributed to increased bone turnover, the levels of activated T cells and Treg cells were assessed. B6.MRL/Fas(lpr) /J mice had elevated Treg cells in both spleens and bones of B6.MRL/Fas(lpr) /J but decreased percentage of activated T cells in bone tissues. Fracture led to ∼30% to 60% systemic increase in Treg cells in both wild-type and B6.MRL/Fas(lpr) /J bone tissues during the period of cartilage formation and resorption but either decreased (wild type) or left unchanged (B6.MRL/Fas(lpr) /J) the numbers of activated T cells in bone. These results show that an active autoimmune state is inhibited during the period of cartilage resorption and suggest that iTreg cells play a functional role in this process. These data show that loss of Fas activity specifically in chondrocytes prolonged the life span of chondrocytes and that Fas synergized with TNFα signaling to mediate chondrocyte apoptosis. Conversely, loss of Fas systemically led to increased osteoclast numbers during later periods of fracture healing and increased osteogenesis. These findings suggest that retention of viable chondrocytes locally inhibits osteoclast activity or matrix proteolysis during cartilage resorption.

Osteotomy and fracture fixation in children and teenagers

Significant changes have occurred recently in fixation methods following fracture or osteotomy in children and teenagers. Children have benefited the most from these advances. A child's growth is anatomically and physiologically ensured by the growth plate and periosteum. The need to keep the periosteum intact during trauma cases has led to the introduction of flexible intramedullary nailing. We will review the basic principles of this safe, universally adopted technique, and also describe available material, length and diameter options. The problems and the limitations of this method will be discussed extensively. In orthopedics, the desire to preserve the periosteum has led to the use of locking compression plates. Because of their low profile and high stability, they allow the micromovements essential for bone union. These new methods reduce the immobilization period and allow autonomy to be regained more quickly, which is especially important in children with neurological impairment. The need to preserve the growth plate, which is well known in pediatric surgery, is reviewed with the goal of summarizing current experimental data on standard fracture and osteotomy fixation methods. Adjustable block stop wires provide better control over compression. These provide an alternate means of fixation between K-wires and screws (now cannulated) and have contributed to the development of minimally invasive surgical techniques. The aim of this lecture is to provide a rationale for the distinct technical features of pediatric surgery, while emphasizing the close relationship between the physiology of growth, bone healing and technical advances.

Generation of closed transverse fractures in small animals

The most common procedure that has been developed for use in rats and mice to model fracture healing is described. The nature of the regenerative processes that may be assessed and the types of research questions that may be addressed with this model are briefly outlined. The detailed surgical protocol to generate closed simple transverse fractures is presented, and general considerations when setting up an experiment using this model are described.

Skeletal trauma generates systemic BMP2 activation that is temporally related to the mobilization of CD73+ cells

The relationship between BMP2 expression and the recruitment of skeletogenic stem cells was assessed following bone marrow reaming. BMP2 expression was examined using transgenic mice in which β-galactosidase had been inserted into the coding region of BMP2. Stem cell mobilization was analyzed by FACS analysis using CD73, a marker associated with bone marrow stromal stem cells. BMP2 expression was induced in endosteal lining cells, cortical osteocytes and periosteal cells in both the reamed and in contralateral bones. BMP2 mRNA expression in the reamed bone showed an early peak within the first 24 h of reaming followed by a later peak at 7 days, while contralateral bones only showed the 7 days peak of expression. FACS analysis sorting on CD73 positive cells showed a 50% increase of these cells at 3 and 14 days in the marrow of the injured bone and a single peak at 14 days of the marrow cell population of the contralateral bone. A ∼20% increase of CD73 positive cells was seen in the peripheral blood 2 days after reaming. These data showed that traumatic bone injury caused a systemic induction of BMP2 expression and that this increase is correlated with the mobilization of CD73 positive cells.

Severe Spinal Cord Injury Causes Immediate Multi-cellular Dysfunction at the Chondro-Osseous Junction

Spinal cord injury is associated with rapid bone loss and arrested long bone growth due to mechanisms that are poorly understood. In this study, we sought to determine the effects of severe T10 contusion spinal cord injury on the sublesional bone microenvironment in adolescent rats. A severe lower thoracic (vertebral T10) spinal cord injury was generated by weight drop (10 g×50 mm). Severely injured and body weight-matched uninjured male Sprague-Dawley rats were studied. At 3 and 5 days post-injury, we performed histological analysis of the distal femoral metaphysis, TUNEL assay, immunohistochemistry, real-time PCR, and western blot analysis compared to uninjured controls. We observed severe hindlimb functional deficits typical of this model. We detected uncoupled remodeling with increased osteoclast activity in the absence of osteoblast activity. We detected osteoblast, osteocyte, and chondrocyte apoptosis with suppressed osteoblast and chondrocyte proliferation and growth plate arrest due to spinal cord injury. We also detected altered gene expression in both whole bone extracts and bone marrow monocytes following spinal cord injury. We conclude that spinal cord injury results in altered gene expression of key regulators of osteoblast and chondrocyte activity. This leads to premature cellular apoptosis, suppressed cellular proliferation, growth plate arrest, and uncoupled bone remodeling in sublesional bone with unopposed osteoclastic resorption.

Functional role of Runx3 in the regulation of aggrecan expression during cartilage development

Runx2 and Runx3 are known to be expressed in the growth plate during endochondral bone formation. Here we addressed the functional role of Runx3 as distinct from Runx2 by using two models of postnatal bone repair: fracture healing that proceeds by an endochondral process and marrow ablation that proceeds by only an intramembranous process. Both Runx2 and Runx3 mRNAs were differentially up regulated during fracture healing. In contrast, only Runx2 showed increased expression after marrow ablation. During fracture healing, Runx3 was expressed earlier than Runx2, was concurrent with the period of chondrogenesis, and coincident with maximal aggrecan expression a protein associated with proliferating and permanent cartilage. Immunohistological analysis showed Runx3 protein was also expressed by chondrocytes in vivo. In contrast, Runx2 was expressed later during chondrocyte hypertrophy, and primary bone formation. The functional activities of Runx3 during chondrocyte differentiation were assessed by examining its regulatory actions on aggrecan gene expression. Aggrecan mRNA levels and aggrecan promoter activity were enhanced in response to the over-expression of either Runx2 and Runx3 in ATDC5 chondrogenic cell line, while sh-RNA knocked down of each Runx protein showed that only Runx3 knock down specifically suppressed aggrecan mRNA expression and promoter activity. ChIP assay demonstrated that Runx3 interactions were selective to sites within the aggrecan promoter and were only observed during early periods of chondrogenesis before hypertrophy. Our studies suggest that Runx3 positively regulates aggrecan expression and suggest that its function is more limited to cartilage development than to bone. In aggregate these data further suggest that the various members of the Runx transcription factors are involved in the coordination of chondrocyte development, maturation, and hypertrophy during endochondral bone formation.

Assessment of contrast-enhanced computed tomography for imaging of cartilage during fracture healing

Assessment of the early stages of fracture healing via X-rays and computed tomography is limited by the low radio-opacity of cartilage. We validated a method of contrast-enhanced computed tomography (CECT) for non-destructive identification of cartilage within a healing fracture callus. Closed, stabilized fractures in femora of C57BL/6 mice were harvested on post-operative day 9.5 and imaged ex vivo with micro-computed tomography (µCT) before and after incubation in a cationic contrast agent that preferentially accumulates in cartilage due to the high concentration of sulfated glycosaminoglycans in the tissue. Co-registration of the pre- and post-incubation images, followed by image subtraction, enabled two- and three-dimensional delineation of mineralized tissue, soft callus, and cartilage. The areas of cartilage and callus identified with CECT were compared to those identified with the gold-standard method of histomorphometry. No difference was found between the areas of cartilage measured by the two methods (p = 0.999). Callus area measured by CECT was smaller than, but strongly predictive of (R(2) = 0.80, p < 0.001), the corresponding histomorphometric measurements. CECT also enabled qualitative identification of mineralized cartilage. These findings indicate that the CECT method provides accurate, quantitative, and non-destructive visualization of the shape and composition of the fracture callus, even during the early stages of repair when little mineralized tissue is present. The non-destructive nature of this method would allow subsequent analyses, such as mechanical testing, to be performed on the callus, thus enabling higher-throughput, comprehensive investigations of bone healing.

The emergence of mechanoregulated endochondral ossification in evolution

The differentiation of skeletal tissue phenotypes is partly regulated by mechanical forces. This mechanoregulatory aspect of tissue differentiation has been the subject of many experimental and computational investigations. However, little is known about what factors promoted the emergence of mechanoregulated tissue differentiation in evolution, even though mechanoregulated tissue differentiation, for example during development or healing of adult bone, is crucial for vertebrate phylogeny. In this paper, we use a computational framework to test the hypothesis that the emergence of mechanosensitive genes that trigger endochondral ossification in evolution will stabilise in the population and create a variable mechanoregulated response, if the endochondral ossification process enhances fitness for survival. The model combines an evolutionary algorithm that considers genetic change with a mechanoregulated fracture healing model in which the fitness of animals in a population is determined by their ability to heal their bones. The simulations show that, with the emergence of mechanosensitive genes through evolution enabling skeletal cells to modulate their synthetic activities, novel differentiation pathways such as endochondral ossification could have emerged, which when favoured by natural selection is maintained in a population. Furthermore, the model predicts that evolutionary forces do not lead to a single optimal mechanoregulated response but that the capacity of endochondral ossification exists with variability in a population. The simulations correspond with many existing findings about the mechanosensitivity of skeletal tissues in current animal populations, therefore indicating that this kind of multi-level models could be used in future population based simulations of tissue differentiation.

Heterotopic mineralization (ossification or calcification) in tendinopathy or following surgical tendon trauma

Heterotopic tendon mineralization (ossification or calcification), which may be a feature of tendinopathy or which may develop following surgical trauma (repair or graft harvest), has not received much attention. The purpose of this article is to review the prevalence, mechanisms and consequences of heterotopic tendon mineralization and to identify the gaps in our current understanding. We focus on endochondral heterotopic ossification and draw on knowledge of the mechanisms of this process in other tissues and conditions. Finally, we introduce a novel murine Achilles tendon needle injury model, which will enable us to further study the mechanisms and biomechanical consequences of tendon mineralization.

Fracture healing in protease-activated receptor-2 deficient mice

Protease-activated receptor-2 (PAR-2) provides an important link between extracellular proteases and the cellular initiation of inflammatory responses. The effect of PAR-2 on fracture healing is unknown. This study investigates the in vivo effect of PAR-2 deletion on fracture healing by assessing differences between wild-type (PAR-2(+/+)) and knock-out (PAR-2(-/-)) mice. Unilateral mid-shaft femur fractures were created in 34 PAR-2(+/+) and 28 PAR-2(-/-) mice after intramedullary fixation. Histologic assessments were made at 1, 2, and 4 weeks post-fracture (wpf), and radiographic (plain radiographs, micro-computed tomography (µCT)) and biomechanical (torsion testing) assessments were made at 7 and 10 wpf. Both the fractured and un-fractured contralateral femur specimens were evaluated. Polar moment of inertia (pMOI), tissue mineral density (TMD), bone volume fraction (BV/TV) were determined from µCT images, and callus diameter was determined from plain radiographs. Statistically significant differences in callus morphology as assessed by µCT were found between PAR-2(-/-) and PAR-2(+/+) mice at both 7 and 10 wpf. However, no significant histologic, plain radiographic, or biomechanical differences were found between the genotypes. The loss of PAR-2 was found to alter callus morphology as assessed by µCT but was not found to otherwise effect fracture healing in young mice.

Micro-computed tomography assessment of the progression of fracture healing in mice

The mouse fracture model is ideal for research into the pathways of healing because of the availability of genetic and transgenic mice and the ability to create cell-specific genetic mutations. While biomechanical tests and histology are available to assess callus integrity and tissue differentiation, respectively, micro-computed tomography (μCT) analysis has increasingly been utilized in fracture studies because it is non-destructive and provides descriptions of the structural and compositional properties of the callus. However, the dynamic changes of μCT properties that occur during healing are not well defined. Thus, the purpose of this study was to determine which μCT properties change with the progression of fracture repair and converge to values similar to unfractured bone in the mouse femur fracture model. A unilateral femur fracture was performed in C57BL/6 mice and intramedullary fixation performed. Fractured and un-fractured contralateral specimens were harvested from groups of mice between 2 and 12 weeks post-fracture. Parameters describing callus based on μCT were obtained, including polar moment of inertia (J), bending moment of inertia (I), total volume (TV), tissue mineral density (TMD), total bone volume fraction (BV/TV), and volumetric bone mineral density (vBMD). For comparison, plain radiographs were used to measure the callus diameter (D) and area (A); and biomechanical properties were evaluated using either three-point bending or torsion. The μCT parameters J, I, TV, and TMD converged toward their respective values of the un-fractured femurs over time, although significant differences existed between the two sides at every time point evaluated (p<0.05). Radiograph measurement D changed with repair progression in similar manner to TV. In contrast, BV/TV and BMD increased and decreased over time with statistical differences between callus and un-fractured bone occurring sporadically. Similarly, none of the biomechanical properties were found to distinguish consistently between the fractured and un-fractured femur. Micro-CT parameters assessing callus structure and size (J, I, and TV) were more sensitive to changes in callus over time post-fracture than those assessing callus substance (TMD, BV/TV, and BMD). Sample size estimates based on these results indicate that utilization of μCT requires fewer animals than biomechanics and thus is more practical for evaluating the healing femur in the mouse fracture model.

A2B adenosine receptor promotes mesenchymal stem cell differentiation to osteoblasts and bone formation in vivo

The differentiation of osteoblasts from their precursors, mesenchymal stem cells, is an important component of bone homeostasis as well as fracture healing. The A2B adenosine receptor (A2BAR) is a Gα(s)/α(q)-protein-coupled receptor that signals via cAMP. cAMP-mediated signaling has been demonstrated to regulate the differentiation of mesenchymal stem cells (MSCs) into various skeletal tissue lineages. Here, we studied the role of this receptor in the differentiation of MSCs to osteoblasts. In vitro differentiation of bone marrow-derived MSCs from A2BAR KO mice resulted in lower expression of osteoblast differentiation transcription factors and the development of fewer mineralized nodules, as compared with WT mice. The mechanism of effect involves, at least partially, cAMP as indicated by experiments involving activation of the A2BAR or addition of a cAMP analog during differentiation. Intriguingly, in vivo, microcomputed tomography analysis of adult femurs showed lower bone density in A2BAR KO mice as compared with WT. Furthermore, A2BAR KO mice display a delay in normal fracture physiology with lower expression of osteoblast differentiation genes. Thus, our study identified the A2BAR as a new regulator of osteoblast differentiation, bone formation, and fracture repair.

Urine matrix metalloproteinases (MMPs) as biomarkers for the progression of fracture healing

Whilst the majority of fractures heal normally, it is estimated that ∼10% of fractures exhibit some level of delayed or impaired healing. Although radiography is the primary diagnostic tool to assess the progression of fracture healing, radiographic features only qualitatively correlate with tissue level increases in mineral content and do not quantitatively measure underlying biological processes that are associated with the progression of healing. Specific metaloproteinases have been shown to be essential to processes of both angiogenesis and mineralised cartilage resorption and bone remodelling at different phases of fracture healing. The aim of this study was to determine the potential of using a simple urine based assay of the activity of two MMPs as a means of assessing the biological progression of fracture healing through the endochondral phase of healing. Using a standard mid-diaphyseal murine model of femoral fracture, MMP9 and MMP13 proteins and enzymatic activity levels were quantified in the urine of mice across the time-course of fracture healing and compared to the mRNA and protein expression profiles in the calluses. Both urinary MMP9 and MMP13 protein and enzymatic activity levels, assessed by Western blot, zymogram and specific MMP fluorometric substrate assays, corresponded to mRNA expression and immunohistologic assays of the proteins within callus tissues. These studies suggest that urinary levels of MMP9 and MMP13 may have potential as metabolic markers to monitor the progression of fracture healing.

The transcriptome of fracture healing defines mechanisms of coordination of skeletal and vascular development during endochondral bone formation

Fractures initiate one round of endochondral bone formation in which callus cells differentiate in a synchronous manner that temporally phenocopies the spatial variation of endochondral development of a growth plate. During fracture healing C57BL/6J (B6) mice initiate chondrogenesis earlier and develop more cartilage than bone, whereas C3H/HeJ (C3H) mice initiate osteogenesis earlier and develop more bone than cartilage. Comparison of the transcriptomes of fracture healing in these strains of mice identified the genes that showed differences in timing and quantitative expression and encode for the variations in endochondral bone development of the two mouse strains. The complement of strain-dependent differences in gene expression was specifically associated with ontologies related to both skeletal and vascular formation. Moreover, the differences in gene expression associated with vascular tissue formation during fracture healing were correlated with the underlying differences in development and function of the cardiovascular systems of these two strains of mice. Significant differences in gene expression associated with bone morphogenetic protein/transforming growth factor β (BMP/TGF-β) signal-transduction pathways were identified between the two strains, and a network of differentially expressed genes specific to the MAP kinase cascade was further defined as a subset of the genes of the BMP/TGF-β pathways. Other signal-transduction pathways that showed significant strain-specific differences in gene expression included the RXR/PPAR and G protein-related pathways. These data identify how bone and vascular regeneration are coordinated through expression of common sets of transcription and morphogenetic factors and suggest that there is heritable linkage between vascular and skeletal tissue development during postnatal regeneration.

MRT letter: Contrast-enhanced computed tomographic imaging of soft callus formation in fracture healing

Formation of a cartilaginous soft callus at the site of a bone fracture is a pivotal stage in the healing process. Noninvasive, or even nondestructive, imaging of soft callus formation can be an important tool in experimental and pre-clinical studies of fracture repair. However, the low X-ray attenuation of cartilage renders the soft callus nearly invisible in radiographs. This study utilized a recently developed, cationic, iodinated contrast agent in conjunction with micro-computed tomography to identify cartilage in fracture calluses in the femora of C57BL/6J and C3H/HeJ mice. Fracture calluses were scanned before and after incubation in the contrast agent. The set of pre-incubation images was registered against and then subtracted from the set of post-incubation images, resulting in a three-dimensional map of the locations of cartilage in the callus, as labeled by the contrast agent. This map was then compared to histology from a previous study. The results showed that the locations where the contrast agent collected in relatively high concentrations were similar to those of the cartilage. The contrast agent also identified a significant difference between the two strains of mice in the percentage of the callus occupied by cartilage, indicating that this method of contrast-enhanced computed tomography may be an effective technique for nondestructive, early evaluation of fracture healing.

Small animal bone healing models: standards, tips, and pitfalls results of a consensus meeting

Small animal fracture models have gained increasing interest in fracture healing studies. To achieve standardized and defined study conditions, various variables must be carefully controlled when designing fracture healing experiments in mice or rats. The strain, age and sex of the animals may influence the process of fracture healing. Furthermore, the choice of the fracture fixation technique depends on the questions addressed, whereby intra- and extramedullary implants as well as open and closed surgical approaches may be considered. During the last few years, a variety of different, highly sophisticated implants for fracture fixation in small animals have been developed. Rigid fixation with locking plates or external fixators results in predominantly intramembranous healing in both mice and rats. Locking plates, external fixators, intramedullary screws, the locking nail and the pin-clip device allow different degrees of stability resulting in various amounts of endochondral and intramembranous healing. The use of common pins that do not provide rotational and axial stability during fracture stabilization should be discouraged in the future. Analyses should include at least biomechanical and histological evaluations, even if the focus of the study is directed towards the elucidation of molecular mechanisms of fracture healing using the largely available spectrum of antibodies and gene-targeted animals to study molecular mechanisms of fracture healing. This review discusses distinct requirements for the experimental setups as well as the advantages and pitfalls of the different fixation techniques in rats and mice.

Variability observed in mechano-regulated in vivo tissue differentiation can be explained by variation in cell mechano-sensitivity

Computational simulations of tissue differentiation have been able to capture the main aspects of tissue formation/regeneration observed in animal experiments-except for the considerable degree of variability reported. Understanding and modelling the source of this variability is crucial if computational tools are to be developed for clinical applications. The objective of this study was to test the hypothesis that differences in cell mechano-sensitivity between individuals can explain the variability of tissue differentiation patterns observed experimentally. Simulations of an experiment of tissue differentiation in a mechanically loaded bone chamber were performed. Finite element analysis was used to determine the biophysical environment, and a lattice-modelling approach was used to simulate cell activity. Differences in cell mechano-sensitivity among individuals were modelled as differences in cell activity rates, with the activation of cell activities regulated by the mechanical environment. Predictions of the tissue distribution in the chambers produced the two different classes of results found experimentally: (i) chambers with a layer of bone across the chamber covered by a layer of cartilage on top and (ii) chambers with almost no bone, mainly fibrous tissue and small islands of cartilage. This indicates that the differing cellular response to the mechanical environment (i.e., subject-specific mechano-sensitivity) could be a reason for the different outcomes found when implants (or tissue engineered constructs) are used in a population.

Lysyl oxidase-like-2 (LOXL2) is a major isoform in chondrocytes and is critically required for differentiation

The lysyl oxidase family is made up of five members: lysyl oxidase (LOX) and lysyl oxidase-like 1-4 (LOXL1-LOXL4). All members share conserved C-terminal catalytic domains that provide for lysyl oxidase or lysyl oxidase-like enzyme activity; and more divergent propeptide regions. LOX family enzyme activities catalyze the final enzymatic conversion required for the formation of normal biosynthetic collagen and elastin cross-links. The importance of lysyl oxidase enzyme activity to normal bone development has long been appreciated, but regulation and roles for specific LOX isoforms in bone formation in vivo is largely unexplored. Fracture healing recapitulates aspects of endochondral bone development. The present study first investigated the expression of all LOX isoforms in fracture healing. A remarkable coincidence of LOXL2 expression with the chondrogenic phase of fracture healing was found, prompting more detailed analyses of LOXL2 expression in normal growth plates, and LOXL2 expression and function in developing ATDC5 chondrogenic cells. Data show that LOXL2 is expressed by pre-hypertrophic and hypertrophic chondrocytes in vivo, and that LOXL2 expression is regulated in vitro as a function of chondrocyte differentiation. Moreover, LOXL2 knockdown studies in vitro show that LOXL2 expression is required for ATDC5 chondrocyte cell line differentiation through regulation of SNAIL and SOX9, important transcription factors that control chondrocyte differentiation. Taken together, data provide evidence that LOXL2, like LOX, is a multifunctional protein. LOXL2 promotes chondrocyte differentiation by mechanisms that are likely to include roles as both a regulator and an effector of chondrocyte differentiation.

Healing of non-displaced fractures produced by fatigue loading of the mouse ulna

We developed a fatigue loading protocol in mice to produce a non-displaced ulnar fracture in vivo, and characterized the early healing response. Using adult (5 month) C57Bl/6 mice, we first determined that cyclic compression of the forelimb under load-control leads to increasing applied displacement and, eventually, complete fracture. We then subjected the right forelimbs of 80 mice to cyclic loading (2 Hz; peak force approximately 4N) and limited the displacement increase to 0.75 mm (60% of the average displacement increase at complete fracture). This fatigue protocol created a partial, non-displaced fracture through the medial cortex near the ulnar mid-shaft, and reduced ulnar strength and stiffness by >50%. Within 1 day, there was significant upregulation of genes related to hypoxia (Hif1a) and osteogenesis (Bmp2, Bsp) in loaded ulnae compared to non-loaded, contralateral controls. The gene expression response peaked in magnitude near day 7 (e.g., Osx upregulated 8-fold), and included upregulation of FGF-family genes (e.g., Fgfr3 up 6-fold). Histologically, a localized periosteal response was seen at the site of the fracture; by day 7 there was abundant periosteal woven bone surrounding a region of cartilage. From days 7 to 14, the woven bone became denser but did not increase in area. By day 14, the woven-bone response resulted in complete recovery of ulnar strength and stiffness, restoring mechanical properties to normal levels. In the future, the fatigue loading approach can be used create non-displaced bone fractures in transgenic and knockout mice to study the mechanisms by which the skeleton rapidly repairs damage.

Genetic variation in the structural pattern of osteoclast activity during post-natal growth of mouse femora

While the spatial activity of osteoblasts has been associated with modeling of bones during development, few studies have examined if variation in the spatial activity of osteoclasts also contributes to the morphogenesis of skeletal tissues. We examined this question by histomorphometric analysis and reconstructing the three-dimensional spatial distribution of osteoclasts in the femora of three inbred strains of male mice (A/J, C57BL/6J [B6], and C3H/HeJ [C3H]) that have differing skeletal, structural, and material properties. Our data show that total osteoclast surface area and osteoclast numbers are related to the overall bone density, but not related to the development of bone diameter or overall cortical area. The analysis of the spatial distribution of the osteoclasts showed that the asymmetrical mid-diaphyseal distribution of osteoclasts in A/J and B6 compared to the more uniform distribution of these cells around the circumference in the C3H mice was consistent with the more ellipsoid shape of A/J and B6 femora compared to the more circular mid-diaphyseal shape of the femora in the C3H mice. The statistically 2- to 3-fold fewer cells on the periosteal surface in the C3H compared to either the B6 or A/J mice is also consistent with the greater cortical thickness that is seen for the C3H mice compared to either B6 or A/J strains. In vitro studies of osteoclastogenesis and the expression of numerous phenotypic properties of osteoclasts prepared from the three strains of mice showed that A/J and B6 mice developed statistically greater numbers of tartrate resistant acid phosphatase (TRAP) positive cells and expressed statistically higher levels of multiple mRNAs that are unique to differentiated osteoclasts than those isolated from the C3H strain. In summary, the 3D reconstructions and histomorphometric analysis suggest that genetic differences lead to spatial variation in the distribution of osteoclasts. These variations in spatial distribution of osteoclasts in turn contribute in part to the development of the structural variations of the femora that are seen in the three strains of mice. In vitro studies suggest that intrinsic genetic variation in osteoclastogenesis and their phenotypic expression may contribute to the differences in their functional activities that give rise to the unique spatial distributions of these cells in bones.

BMP2 is essential for post natal osteogenesis but not for recruitment of osteogenic stem cells

The effects of BMP2 on bone marrow stromal cell differentiation and bone formation after bone marrow ablation were determined using C57 BL/6J (B6) mice. Inhibition of BMP2 expression with lentiviral BMP2 shRNA prevented both mineralized nodule formation in vitro and bone formation in vivo, and blocked the expression of Runx2 and osterix, transcriptional determinants of terminal osteogenic differentiation. No effect was observed on the expression of Sox9, a transcription factor, which is the one of the first transcriptional determinant to be expressed in committed chondroprogenitor and osteoprogenitor cells. In vitro studies showed that exogenously added BMP7 rescued the expression of osterix and enhanced the expression of Sox9, but had no effect on the expression of Runx2, while it only partially recovered the development of mineral deposition in the cultures. On the other hand, the exogenous addition of BMP2 rescued both Runx2 and osterix expression, did not enhance the expression of Sox9, but fully recovered the inhibition of mineral deposition in the cultures. Using antibodies against CD146 and Sox9, immunohistological examination of the cell populations found in the medullary space three days after bone marrow ablation, showed qualitatively equal numbers of cells expressing these skeletal progenitor and stem cell markers in control and BMP2 shRNA treated animals. Fluorescence Activated Cell Sorting (FACS) analysis of the cells found with the marrow cavities at three days after marrow ablation using CD146 antibody showed near equal numbers of immunopositive cells in both control and shRNA treated animals. In summary, the differences observed in vitro for BMP2 and BMP7 on osteogenic gene expression and mineralization suggest that they have differing effects on bone cell differentiation. These results further demonstrate that in vivo BMP2 is a central morphogenetic regulator of post natal osteoprogenitor differentiation, but does not affect recruitment of progenitors to the osteoblastic lineage.

Transcriptional analysis of fracture healing and the induction of embryonic stem cell-related genes

Fractures are among the most common human traumas. Fracture healing represents a unique temporarily definable post-natal process in which to study the complex interactions of multiple molecular events that regulate endochondral skeletal tissue formation. Because of the regenerative nature of fracture healing, it is hypothesized that large numbers of post-natal stem cells are recruited and contribute to formation of the multiple cell lineages that contribute to this process. Bayesian modeling was used to generate the temporal profiles of the transcriptome during fracture healing. The temporal relationships between ontologies that are associated with various biologic, metabolic, and regulatory pathways were identified and related to developmental processes associated with skeletogenesis, vasculogenesis, and neurogenesis. The complement of all the expressed BMPs, Wnts, FGFs, and their receptors were related to the subsets of transcription factors that were concurrently expressed during fracture healing. We further defined during fracture healing the temporal patterns of expression for 174 of the 193 genes known to be associated with human genetic skeletal disorders. In order to identify the common regulatory features that might be present in stem cells that are recruited during fracture healing to other types of stem cells, we queried the transcriptome of fracture healing against that seen in embryonic stem cells (ESCs) and mesenchymal stem cells (MSCs). Approximately 300 known genes that are preferentially expressed in ESCs and approximately 350 of the known genes that are preferentially expressed in MSCs showed induction during fracture healing. Nanog, one of the central epigenetic regulators associated with ESC stem cell maintenance, was shown to be associated in multiple forms or bone repair as well as MSC differentiation. In summary, these data present the first temporal analysis of the transcriptome of an endochondral bone formation process that takes place during fracture healing. They show that neurogenesis as well as vasculogenesis are predominant components of skeletal tissue formation and suggest common pathways are shared between post-natal stem cells and those seen in ESCs.

Quantitative measures of femoral fracture repair in rats derived by micro-computed tomography

Although fracture healing is frequently studied in pre-clinical models of long bone fractures using rodents, there is a dearth of objective quantitative techniques to assess successful healing. Biomechanical testing is possibly the most quantitative and relevant to a successful clinical outcome, but it is a destructive technique providing little insight into the cellular mechanisms associated with healing. The advent of X-ray computed tomography (CT) has provided the opportunity to quantitatively and non-destructively assess bone structure and density, but it is unknown how measurements derived using this technology relate to successful healing. To examine possible relationships, we used a pre-clinical model to test for statistically significant correlations between quantitative characteristics of the callus by micro-CT (microCT) and the bending strength, stiffness, and energy-to-failure of the callus as assessed by three-point bending of excised bones. A closed, transverse fracture was generated in the mid-shaft of rat femurs by impact loading. Shortly thereafter, the rats received a one-time, local injection of either the vehicle or one of four doses of lovastatin. Following sacrifice after 4 weeks of healing, fractured femurs were extracted for microCT analysis and then three-point bending. Setting the region of interest to be 3.2 mm above and below the fracture line, we acquired standard and new microCT-derived measurements. The mineralized callus volume and the mineral density of the callus correlated positively with callus strength (rxy = -0.315, p = 0.016 and rxy = 0.444, p<0.0005, respectively) and stiffness (rxy = -0.271, p = 0.040 and rxy = 0.325, p = 0.013, respectively), but the fraction of the callus that mineralized and the moment of inertia of the callus did not. This fraction did correlate with energy-to-failure (rxy = -0.343, p = 0.0085). Of the microCT-derived measurements, quantifying defects within the outer bridging cortices of the callus produced the strongest correlation with both callus strength (rxy = 0.557, p<0.0001) and stiffness (rxy = 0.468, p = 0.0002). By both reducing structural defects and increasing mineralization, lovastatin appears to increase the callus strength.

The diamond concept--open questions

In the Trauma and Orthopaedic discipline, tissue engineering attempts to convert scientific knowledge into new products and methods of treatment in order to advance the repair, replacement, or regeneration of tissues such as bone, cartilage, tendon and ligament. Currently, tissue engineering strategies are based mainly on cell and tissue-based approaches. We have previously reported that the standard tissue engineering approach to provide solutions for impaired fracture healing, bone restoration and regeneration must include the utilisation of growth factors, scaffolds, mesenchymal stem cells and an optimal mechanical environment (diamond concept). These strategies are already benefiting patients, but as our understanding of the physiological processes increases, a number of questions come up requiring clarification and answers. In this article, important issues which continue to remain obscured are discussed.