Stress fracture healing: fatigue loading of the rat ulna induces upregulation in expression of osteogenic and angiogenic genes that mimic the intramembranous portion of fracture repair

Zoledronate reduces loading-induced microdamage in cortical ulna of ovariectomized rats

As a daily physiological mechanism in bone, microdamage accumulation dissipates energy and helps to prevent fractures. However, excessive damage accumulation might bring adverse effects to bone mechanical properties, which is especially problematic among the osteoporotic and osteopenic patients treated by bisphosphonates. Some pre-clinical studies in the literature applied forelimb loading models to produce well-controlled microdamage in cortical bone. Ovariectomized animals were also extensively studied to assimilate human conditions of estrogen-related bone loss. In the present study, we combined both experimental models to investigate microdamage accumulation in the context of osteopenia and zoledronate treatment. Three-month-old normal and ovariectomized rats treated by saline or zoledronate underwent controlled compressive loading on their right forelimb to create in vivo microdamage, which was then quantified by barium sulfate contrast-enhanced micro-CT imaging. Weekly in vivo micro-CT scans were taken to evaluate bone (re)modeling and to capture microstructural changes over time. After sacrifice, three-point-bending tests were performed to assess bone mechanical properties. Results show that the zoledronate treatment can reduce cortical microdamage accumulation in ovariectomized rats, which might be explained by the enhancement of several bone structural properties such as ultimate force, yield force, cortical bone area and volume. The rats showed increased bone formation volume and surface after the generation of microdamage, especially for the normal and the ovariectomized groups. Woven bone formation was also observed in loaded ulnae, which was most significant in ovariectomized rats. Although all the rats showed strong correlations between periosteal bone formation and microdamage accumulation, the correlation levels were lower for the zoledronate-treated groups, potentially because of their lower levels of microdamage. The present study provides insights to further investigations of pharmaceutical treatments for osteoporosis and osteopenia. The same experimental concept can be applied in future studies on microdamage and drug testing.

Cells transiently expressing periostin are required for intramedullary intramembranous bone regeneration

Intramembranous bone regeneration plays an important role in fixation of intramedullary implants used in joint replacement and dental implants used in tooth replacement. Despite widespread recognition of the importance of intramembranous bone regeneration in these clinical procedures, the underlying mechanisms have not been well explored. A previous study that examined transcriptomic profiles of regenerating bone from the marrow space showed that increased periostin gene expression preceded increases in several osteogenic genes. We therefore sought to determine the role of cells transiently expressing periostin in intramedullary intramembranous bone regeneration. We used a genetic mouse model that allows tamoxifen-inducible fluorescent labeling of periostin expressing cells. These mice underwent ablation of the bone marrow cavity through surgical disruption, a well-established intramembranous bone regeneration model. We found that in intact bones, fluorescently labeled cells were largely restricted to the periosteal surface of cortical bone and were absent in bone marrow. However, following surgical disruption of the bone marrow cavity, cells transiently expressing periostin were found within the regenerating tissue of the bone marrow compartment even though the cortical bone remained intact. The source of these cells is likely heterogenous, including cells occupying the periosteal surface as well as pericytes and endothelial cells within the marrow cavity. We also found that diphtheria toxin-mediated depletion of cells transiently expressing periostin at the time of surgery impaired intramembranous bone regeneration in mice. These data suggest a critical role of periostin expressing cells in intramedullary intramembranous bone regeneration and may lead to novel therapeutic interventions to accelerate or enhance implant fixation.

VEGFA from osteoblasts is not required for lamellar bone formation following tibial loading

The relationship between osteogenesis and angiogenesis is complex. Normal bone development requires angiogenesis, mediated by vascular endothelial growth factor A (VEGFA). Studies have demonstrated through systemic inhibition or genetic modification that VEGFA is indispensable for several types of bone repair, presumably via its role in supporting angiogenesis. But a direct role for VEGFA within osteoblasts, in the absence of angiogenesis, has also been suggested. To address the question of whether VEGFA from osteoblasts supports bone formation directly, we applied anabolic loading to induce lamellar bone formation in mice, a process shown to be independent of angiogenesis. We hypothesized that VEGFA from osteoblasts is required for lamellar bone formation. To test this hypothesis, we applied axial tibial compression to inducible Cre/LoxP mice from three lines. Vegfafl/fl mice were crossed with Ubiquitin C (UBC), Osterix (Osx) and Dentin-Matrix Protein 1 (DMP1) Cre-ERT2 mice to target all cells, (pre)osteoblast-lineage cells, and mature osteoblasts and osteocytes, respectively. Genotype effects were determined by comparing control (Vegfafl/fl) and Cre+ (VegfaΔ) mice for each line. At 5 months of age tamoxifen was injected for 5 days followed by a 3-week clearance prior to loading. Female and male mice (N = 100) were loaded for 5 days to peak forces to engender -3100 με peak compressive strain and processed for dynamic histomorphometry (day 12). Percent MS/BS increased 20-70 % as a result of loading, with no effect of genotype in Osx or Dmp1 lines. In contrast, the UBC groups had a significant decrease in relative periosteal BFR/BS in VegfaΔ vs. Vegfafl/fl mice. The UBC line did not have any cortical bone phenotype in non-loaded femurs. In summary, dynamic histomorphometry data confirmed that tibial loading induces lamellar bone formation. Contrary to our hypothesis, there was no decrease in loading-induced bone formation in the Osx or Dmp1 lines in the absence of VEGFA. There was a decrease in bone formation in the UBC line where all cells were targeted. This result indicates that VEGFA from a non-osteoblast cell source supports loading-induced lamellar bone formation, although osteoblast/osteocyte VEGFA is dispensable. These findings support a paracrine model whereby non-osteoblast VEGFA supports lamellar bone formation, independent of angiogenesis.

Animal models of impaired long bone healing and tissue engineering- and cell-based in vivo interventions

Bone healing after injury typically follows a systematic process and occurs spontaneously under appropriate physiological conditions. However, impaired long bone healing is still quite common and may require surgical intervention. Various complications can result in different forms of impaired bone healing including nonunion, critical-size defects, or stress fractures. While a nonunion may occur due to impaired biological signaling and/or mechanical instability, a critical-size defect exhibits extensive bone loss that will not spontaneously heal. Comparatively, a stress fracture occurs from repetitive forces and results in a non-healing crack or break in the bone. Clinical standards of treatment vary between these bone defects due to their pathological differences. The use of appropriate animal models for modeling healing defects is critical to improve current treatment methods and develop novel rescue therapies. This review provides an overview of these clinical bone healing impairments and current animal models available to study the defects in vivo. The techniques used to create these models are compared, along with the outcomes, to clarify limitations and future objectives. Finally, rescue techniques focused on tissue engineering and cell-based therapies currently applied in animal models are specifically discussed to analyze their ability to initiate healing at the defect site, providing information regarding potential future therapies. In summary, this review focuses on the current animal models of nonunion, critical-size defects, and stress fractures, as well as interventions that have been tested in vivo to provide an overview of the clinical potential and future directions for improving bone healing.

Extracellular vesicles-encapsulated microRNA-29b-3p from bone marrow-derived mesenchymal stem cells promotes fracture healing via modulation of the PTEN/PI3K/AKT axis

Bone marrow-derived mesenchymal stem cells (BM-MSCs) are well-established as vital regulators of fracture healing, whereas angiogenesis is one of the critical processes during the course of bone healing. Accordingly, the current study sought to determine the functions of microRNA (miR)-29b-3p from BM-MSCs-derived extracellular vesicles (EVs) on the angiogenesis of fracture healing via the PTEN/PI3K/AKT axis. Firstly, BM-MSCs-EVs were extracted and identified. The lentiviral protocol was adopted to construct miR-29b-3p^KD-BMSCs or miR-negative control-BMSCs, which were then co-cultured with human umbilical vein endothelial cells (HUVECs) in vitro to determine the roles of EVs-encapsulated miR-29b-3p on the proliferation, migration, and angiogenesis of HUVECs in vitro with the help of a CCK-8 assay, scratch test, and tube formation assay. Subsequent database prediction, luciferase activity assay, RT-qPCR, and Western blot assay findings identified the downstream target gene of miR-29b-3p, PTEN, and a signaling pathway, PI3K/AKT. Furthermore, the application of si-PTEN attenuated the effects induced by miR-29b-3p^KD-EVs. Finally, a mouse model of femoral fracture was established with a locally instilled injection of equal volumes of BM-MSCs-EVs and miR-29b-3p^KD-BM-MSCs-EVs. Notably, the mice treated with BMSC-EVs presented with enhanced neovascularization at the fracture site, in addition to increased bone volume (BV), BV/tissue volume, and mean bone mineral density; whereas miR-29b-3p^KD-BMSCs-EVs-treated mice exhibited decreased vessel density with poor fracture healing capacity. Collectively, our findings elicited that BM-MSCs-EVs carrying miR-29b-3p were endocytosed by HUVECs, which consequently suppressed the PTEN expression and activated the PI3K/AKT pathway, thereby promoting HUVEC proliferation, migration, and angiogenesis, and ultimately facilitating fracture healing.

Postnatal Osterix but not DMP1 lineage cells significantly contribute to intramembranous ossification in three preclinical models of bone injury

Murine models of long-bone fracture, stress fracture, and cortical defect are used to discern the cellular and molecular mediators of intramembranous and endochondral bone healing. Previous work has shown that Osterix (Osx⁺) and Dentin Matrix Protein-1 (DMP1⁺) lineage cells and their progeny contribute to injury-induced woven bone formation during femoral fracture, ulnar stress fracture, and tibial cortical defect repair. However, the contribution of pre-existing versus newly-derived Osx⁺ and DMP1⁺ lineage cells in these murine models of bone injury is unclear. We addressed this knowledge gap by using male and female 12-week-old, tamoxifen-inducible Osx Cre_ERT2 and DMP1 Cre_ERT2 mice harboring the Ai9 TdTomato reporter allele. To trace pre-existing Osx⁺ and DMP1⁺ lineage cells, tamoxifen (TMX: 100 mg/kg gavage) was given in a pulse manner (three doses, 4 weeks before injury), while to label pre-existing and newly-derived lineage Osx⁺ and DMP1⁺ cells, TMX was first given 2 weeks before injury and continuously (twice weekly) throughout healing. TdTomato positive (TdT⁺) cell area and cell fraction were quantified from frozen histological sections of injured and uninjured contralateral samples at times corresponding with active woven bone formation in each model. We found that in uninjured cortical bone tissue, Osx Cre_ERT2 was more efficient than DMP1 Cre_ERT2 at labeling the periosteal and endosteal surfaces, as well as intracortical osteocytes. Pulse-labeling revealed that pre-existing Osx⁺ lineage and their progeny, but not pre-existing DMP1⁺ lineage cells and their progeny, significantly contributed to woven bone formation in all three injury models. In particular, these pre-existing Osx⁺ lineage cells mainly lined new woven bone surfaces and became embedded as osteocytes. In contrast, with continuous dosing, both Osx⁺ and DMP1⁺ lineage cells and their progeny contributed to intramembranous woven bone formation, with higher TdT⁺ tissue area and cell fraction in Osx⁺ lineage versus DMP1⁺ lineage calluses (femoral fracture and ulnar stress fracture). Similarly, Osx⁺ and DMP1⁺ lineage cells and their progeny significantly contributed to endochondral callus regions with continuous dosing only, with higher TdT⁺ chondrocyte fraction in Osx⁺ versus DMP1⁺ cell lineages. In summary, pre-existing Osx⁺ but not DMP1⁺ lineage cells and their progeny make up a significant amount of woven bone cells (particularly osteocytes) across three preclinical models of bone injury. Therefore, Osx⁺ cell lineage modulation may prove to be an effective therapy to enhance bone regeneration.

Double-edged effects caused by magnesium ions and alkaline environment regulate bioactivities of magnesium-incorporated silicocarnotite in vitro

Magnesium (Mg) is an important element for its enhanced osteogenic and angiogenic properties in vitro and in vivo, however, the inherent alkalinity is the adverse factor that needs further attention. In order to study the role of alkalinity in regulating osteogenesis and angiogenesis in vitro, magnesium-silicocarnotite [Mg-Ca₅(PO₄)₂SiO₄, Mg-CPS] was designed and fabricated. In this study, Mg-CPS showed better osteogenic and angiogenic properties than CPS within 10 wt.% magnesium oxide (MgO), since the adversity of alkaline condition was covered by the benefits of improved Mg ion concentrations through activating Smad2/3-Runx2 signaling pathway in MC3T3-E1 cells and PI3K-AKT signaling pathway in human umbilical vein endothelial cells in vitro. Besides, provided that MgO was incorporated with 15 wt.% in CPS, the bioactivities had declined due to the environment consisting of higher-concentrated Mg ions, stronger alkalinity and lower Ca/P/Si ions caused. According to the results, it indicated that bioactivities of Mg-CPS in vitro were regulated by the double-edged effects, which were the consequence of Mg ions and alkaline environment combined. Therefore, if MgO is properly incorporated in CPS, the improved bioactivities could cover alkaline adversity, making Mg-CPS bioceramics promising in orthopedic clinical application for its enhancement of osteogenesis and angiogenesis in vitro.

How faithfully does intramembranous bone regeneration recapitulate embryonic skeletal development?

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

Proliferating osteoblasts are necessary for maximal bone anabolic response to loading in mice

Following mechanical loading, osteoblasts may arise via activation, differentiation, or proliferation to form bone. Our objective was to ablate proliferating osteoblast lineage cells in order to investigate the importance of these cells as a source for loading-induced bone formation. We utilized 3.6Col1a1-tk mice in which replicating osteoblast lineage cells can be ablated in an inducible manner using ganciclovir (GCV). Male and female mice were aged to 5- and 12-months and subjected to 5 days of tibial compression. "Experimental" mice were tk-positive, treated with GCV; "control" mice were either tk-negative treated with GCV, or tk-positive treated with PBS. We confirmed that experimental mice had a decrease in tk-positive cells that arose from proliferation. Next, we assessed bone formation after loading to low (7N) and high (11N) forces and observed that periosteal bone formation rate in experimental mice was reduced by approximately 70% for both forces. Remarkably, woven bone formation induced by high-force loading was blocked in experimental mice. Loading-induced lamellar bone formation was diminished but not prevented in experimental mice. We conclude that osteoblast proliferation induced by mechanical loading is a critical source of bone forming osteoblasts for maximal lamellar formation and is essential for woven bone formation.

HLA-B27-mediated activation of TNAP phosphatase promotes pathogenic syndesmophyte formation in ankylosing spondylitis

Ankylosing spondylitis (AS) is a type of axial inflammation. Over time, some patients develop spinal ankylosis and permanent disability; however, current treatment strategies cannot arrest syndesmophyte formation completely. Here, we used mesenchymal stem cells (MSCs) from AS patients (AS MSCs) within the enthesis involved in spinal ankylosis to delineate that the HLA-B27-mediated spliced X-box-binding protein 1 (sXBP1)/retinoic acid receptor-β (RARB)/tissue-nonspecific alkaline phosphatase (TNAP) axis accelerated the mineralization of AS MSCs, which was independent of Runt-related transcription factor 2 (Runx2). An animal model mimicking AS pathological bony appositions was established by implantation of AS MSCs into the lumbar spine of NOD-SCID mice. We found that TNAP inhibitors, including levamisole and pamidronate, inhibited AS MSC mineralization in vitro and blocked bony appositions in vivo. Furthermore, we demonstrated that the serum bone-specific TNAP (BAP) level was a potential prognostic biomarker to predict AS patients with a high risk for radiographic progression. Our study highlights the importance of the HLA-B27-mediated activation of the sXBP1/RARB/TNAP axis in AS syndesmophyte pathogenesis and provides a new strategy for the diagnosis and prevention of radiographic progression of AS.

Transcriptional profiling of intramembranous and endochondral ossification after fracture in mice

Bone fracture repair represents an important clinical challenge with nearly 1 million non-union fractures occurring annually in the U.S. Gene expression differs between non-union and healthy repair, suggesting there is a pattern of gene expression that is indicative of optimal repair. Despite this, the gene expression profile of fracture repair remains incompletely understood. In this work, we used RNA-seq of two well-established murine fracture models to describe gene expression of intramembranous and endochondral bone formation. We used top differentially expressed genes, enriched gene ontology terms and pathways, callus cellular phenotyping, and histology to describe and contrast these bone formation processes across time. Intramembranous repair, as modeled by ulnar stress fracture, and endochondral repair, as modeled by femur full fracture, exhibited vastly different transcriptional profiles throughout repair. Stress fracture healing had enriched differentially expressed genes associated with bone repair and osteoblasts, highlighting the strong osteogenic repair process of this model. Interestingly, the PI3K-Akt signaling pathway was one of only a few pathways uniquely enriched in stress fracture repair. Full fracture repair involved a higher level of inflammatory and immune cell related genes than did stress fracture repair. Full fracture repair also differed from stress fracture in a robust downregulation of ion channel genes following injury, the role of which in fracture repair is unclear. This study offers a broad description of gene expression in intramembranous and endochondral ossification across several time points throughout repair and suggests several potentially intriguing genes, pathways, and cells whose role in fracture repair requires further study.

VEGFA From Early Osteoblast Lineage Cells (Osterix+) Is Required in Mice for Fracture Healing

Bone formation via intramembranous and endochondral ossification is necessary for successful healing after a wide range of bone injuries. The pleiotropic cytokine, vascular endothelial growth factor A (VEGFA) has been shown, via nonspecific pharmacologic inhibition, to be indispensable for angiogenesis and ossification following bone fracture and cortical defect repair. However, the importance of VEGFA expression by different cell types during bone healing is not well understood. We sought to determine the role of VEGFA from different osteoblast cell subsets following clinically relevant models of bone fracture and cortical defect. Ubiquitin C (UBC), Osterix (Osx), or Dentin matrix protein 1 (Dmp1) Cre-ERT2 mice (male and female) containing floxed VEGFA alleles (VEGFA^fl/fl ) were either given a femur full fracture, ulna stress fracture, or tibia cortical defect at 12 weeks of age. All mice received tamoxifen continuously starting 2 weeks before bone injury and throughout healing. UBC Cre-ERT2 VEGFA^fl/fl (UBC cKO) mice, which were used to mimic nonspecific inhibition, had minimal bone formation and impaired angiogenesis across all bone injury models. UBC cKO mice also exhibited impaired periosteal cell proliferation during full fracture, but not stress fracture repair. Osx Cre-ERT2 VEGFA^fl/fl (Osx cKO) mice, but not Dmp1 Cre-ERT2 VEGFA^fl/fl (Dmp1 cKO) mice, showed impaired periosteal bone formation and angiogenesis in models of full fracture and stress fracture. Neither Osx cKO nor Dmp1 cKO mice demonstrated significant impairments in intramedullary bone formation and angiogenesis following cortical defect. These data suggest that VEGFA from early osteolineage cells (Osx+), but not mature osteoblasts/osteocytes (Dmp1+), is critical at the time of bone injury for rapid periosteal angiogenesis and woven bone formation during fracture repair. Whereas VEGFA from another cell source, not from the osteoblast cell lineage, is necessary at the time of injury for maximum cortical defect intramedullary angiogenesis and osteogenesis. © 2019 American Society for Bone and Mineral Research.

Crosstalk between chitosan and cell signaling pathways

The field of tissue engineering (TE) experiences its most exciting time in the current decade. Recent progresses in TE have made it able to translate into clinical applications. To regenerate damaged tissues, TE uses biomaterial scaffolds to prepare a suitable backbone for tissue regeneration. It is well proven that the cell-biomaterial crosstalk impacts tremendously on cell biological activities such as differentiation, proliferation, migration, and others. Clarification of exact biological effects and mechanisms of a certain material on various cell types promises to have a profound impact on clinical applications of TE. Chitosan (CS) is one of the most commonly used biomaterials with many promising characteristics such as biocompatibility, antibacterial activity, biodegradability, and others. In this review, we discuss crosstalk between CS and various cell types to provide a roadmap for more effective applications of this polymer for future uses in tissue engineering and regenerative medicine.

Single injection of PTH improves osteoclastic parameters of remodeling at a stress fracture site in rats

Stress fractures (SFx) result from repetitive cyclical loading of bone. They are frequent athletic injuries and underlie atypical femoral fractures following long-term bisphosphonate (BP) therapy. We investigated the effect of a single PTH injection on the healing of SFx in the rat ulna. SFx was induced in 120 female Wistar rats (300 ± 15 g) during a single loading session. A single PTH (8 µg.100g^-1 ) or vehicle (VEH) saline injection was administered 24 h after loading. Rats were divided into four groups (n = 15) and ulnae were examined 1, 2, 6, or 10 weeks following SFx. Two Toluidine Blue and TRAP-stained sections of the SFx were examined for histomorphometric analysis using Osteomeasure™ software. An increase in osteoclast number (N.Oc) and perimeter (Oc.Pm) was observed two weeks following PTH treatment (p < 0.01). At 6 weeks, bone formation was the main activity in BMUs. At 10 weeks, the proportion of healing along the SFx line remained 50% greater in PTH groups (p = 0.839), leading to a 43% reduction in the porosity area of BMU (p = 0.703). The main effect of time was a significant variable along the entire SFx remodeling cycle, with significant interactions between time and treatment type affecting (N.Oc) (p = 0.047) and (Oc.Pm) (p = 0.002). We conclude that a single PTH injection increases osteoclastogenesis by the second week of the remodeling cycle in a SFx in vivo. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Experimental Validation of the Radiographic Union Score for Tibial Fractures (RUST) Using Micro-Computed Tomography Scanning and Biomechanical Testing in an in-Vivo Rat Model

BACKGROUND

The Radiographic Union Score for Tibial fractures (RUST) and the modified version of the system, mRUST, are popular standards for assessing fracture-healing progress with use of radiographs. To our knowledge, this is the first study to experimentally validate the ability of RUST and mRUST to accurately assess bone-healing progression with use of both micro-computed tomography (micro-CT) scanning and biomechanical testing.

METHODS

Adult male rats (n = 29) underwent osteotomy with a midshaft fracture gap repaired with use of a polyetheretherketone plate. Anteroposterior and lateral radiographs were made of the repaired femora prior to rat death at end points of 5, 6, 7, 8, 9, and 17 weeks, and 2 fellowship-trained orthopaedic trauma surgeons independently assigned RUST and mRUST scores to repaired femora. The repaired and intact contralateral femora were then dissected. Bones underwent dissection, micro-CT scanning, and biomechanical torsion testing at the end points.

RESULTS

RUST scores ranged from 5 to 12 and mRUST scores ranged from 5 to 16. Intraclass correlation coefficients (ICCs) were 0.89 (95% confidence interval [CI]: 0.78 to 0.94) for RUST and 0.86 (95% CI: 0.74 to 0.93) for mRUST, which fall within the "almost perfect agreement" category for ICCs. Spearman rank correlation coefficients (RS) showed correlation of RUST (RS range, 0.456 to 0.818) and mRUST (RS range, 0.519 to 0.862) with micro-CT measurements of mineralized callus volume (BV), total callus volume (TV), and BV/TV ratio, but less so with bone mineral density (BMD). Additionally, RUST (RS range, 0.524 to 0.863) and mRUST (RS range, 0.434 to 0.850) were correlated with some biomechanical properties. A RUST score of 10 or an mRUST score of 15 may be considered the threshold above which a plated bone is "healed" because, at these scores, 120% or 140% of failure torque, respectively, was achieved by the repaired femora as compared with the intact contralateral femora.

CONCLUSIONS

RUST and mRUST both show strong statistical correlations with micro-CT and biomechanical parameters.

CLINICAL RELEVANCE

RUST and mRUST scoring systems provide clinicians with validated, reliable, and available tools to assess the progress of fracture-healing.

The Interaction of BMP2-Induced Defect Healing in Rat and Fixator Stiffness Modulates Matrix Alignment and Contraction

Successful fracture healing requires a tight interplay between mechanical and biological cues. In vitro studies illustrated that mechanical loading modulates bone morphogenetic protein (BMP) signaling. However, in the early phases of large bone defect regeneration in vivo, the underlying mechanisms leading to this mechanosensation remained unknown. We investigated the interaction of BMP2 stimulation and mechanical boundary conditions in a rat critical‐sized femoral defect model (5 mm) stabilized with three distinctly different external fixator stiffness. Defects were treated with 5 μg rhBMP2 loaded on an absorbable collagen sponge. Early matrix alignment was monitored by second‐harmonic generation imaging. Bony bridging of defects and successive healing was monitored by histology at day 7 and day 14 as well as in vivo microCT at days 10, 21, and 42 post‐operation. Femora harvested at day 42 were characterized mechanically assessing torsional load to failure ex vivo. At tissue level, differences between groups were visible at day 14 with manifest bone formation in the microCT. Histologically, we observed prolonged chondrogenesis upon flexible fixation, whereas osteogenesis started earlier after rigid and semirigid fixation. At later time points, there was a boost of bone tissue formation upon flexible fixation, whereas other groups already displayed signs of tissue maturation. Based on gene expression profiling, we analyzed the mechanobiological interplay. Already at day 3, these analyses revealed differences in expression pattern, specifically of genes involved in extracellular matrix formation. Gene regulation correlating with fixator stiffness was pronounced at day 7 comprising genes related to immunological processes and cellular contraction. The influence of loading on matrix contraction was further investigated and confirmed in a 3D bioreactor. Taken together, we demonstrate an early onset of mechanical conditions influencing BMP2‐induced defect healing and shed light on gene regulatory networks associated with extracellular matrix organization and contraction that seemed to directly impact healing outcomes. © 2018 The Authors. JBMR Plus is published by Wiley Periodicals, Inc. on behalf of the American Society for Bone and Mineral Research.

Spatiotemporal characterization of microdamage accumulation in rat ulnae in response to uniaxial compressive fatigue loading

Repetitive fatigue loading can induce microdamage accumulation in bone matrix, which results in impaired mechanical properties and increased fracture susceptibility. However, the spatial distribution and time-variant process of microdamage accumulation in fatigue-loaded skeleton, especially for linear microcracks which are known to initiate bone remodeling, remain not fully understood. In this study, the time-varying process of the morphology and distribution of microcracks in rat ulnae subjected to uniaxial compressive fatigue loading was investigated. Right forelimbs of thirty four-month-old male Sprague-Dawley rats were subjected to one bout of cyclic ramp loading with 0.67 Hz at a normalized peak force of 0.055 N/g body weight for 6000 cycles, and the contralateral left ulnae were not loaded as the control samples. Ten rats were randomly euthanized on Days 3, 5, and 7 post fatigue loading. Our findings via two-dimensional histomorphometric measurements based on basic fuchsin staining and three-dimensional quantifications using contrast-enhanced micro-computed tomography (MicroCT) with precipitated BaSO₄ staining demonstrated that the accumulation of linear microcracks (increase in the amount of linear microcracks) on Day 5 was significantly higher than that on Day 3 and Day 7 post fatigue loading. Our histological and histomorphometric results revealed that linear microcrack density (Cr.Dn) in the tensile cortex at Days 3, 5 and 7 post fatigue loading was significantly higher than that in the compressive side, whereas linear microcrack length (Cr.Le) in the tensile cortex at Day 3 was significantly lower than that in the compressive cortex. Our findings revealed that microcrack accumulation exhibited a non-linear time-varying process at 3, 5 and 7 days post axial compressive fatigue loading (with observable peak Cr.Dn at Day 5). Our findings also revealed distinct distribution of microcrack density and morphology in rat ulnae with tensile and compressive strains, as characterized by more microcracks accumulated in tensile cortices, and longer cracks shown in compressive cortices.

Effect of low-intensity whole-body vibration on bone defect repair and associated vascularization in mice

Low-intensity whole-body vibration (LIWBV) may stimulate bone healing, but the involvement of vascular ingrowth, which is essential for bone regeneration, has not been well examined. We thus investigated the LIWBV effect on vascularization during early-stage bone healing. Mice aged 13 weeks were subjected to cortical drilling on tibial bone. Two days after surgery (day 0), mice were exposed daily to sine-wave LIWBV at 30 Hz and 0.1 g peak-to-peak acceleration for 20 min/day (Vib) or were sham-treated (sham). Following vascular casting with a zirconium-based contrast agent on days 6, 9, or 12 and sacrifice, vascular and bone images were obtained by K-edge subtraction micro-CT using synchrotron lights. Bone regeneration advanced more in the Vib group from days 9 to 12. The vascular volume fraction decreased from days 6 to 9 in both groups; however, from days 9 to 12, it was increased in shams, while it stabilized in the Vib group. The vascular volume fraction tended to be or was smaller in the Vib group on days 6 and 12. The vessel number density was higher on day 9 but lower on day 12 in the Vib group. These results suggest that the LIWBV-promoted bone repair is associated with the modulation of vascularization, but additional studies are needed to determine the causality of this association.

Distinct Osteomimetic Response of Androgen-Dependent and Independent Human Prostate Cancer Cells to Mechanical Action of Fluid Flow: Prometastatic Implications

BACKGROUND AND METHODS

Prostate cancer frequently expresses an osteomimetic phenotype, but it is unclear how it is regulated and what biological and clinical implications it confers. Because mechanical forces physiologically regulate bone-remodeling activity in osteocytes, we hypothesized that mechanical action of fluid flow (MAFF) at the cancer microenvironment may similarly foster prostate cancer cell osteomimicry.

RESULTS

We showed that in vitro MAFF on androgen-dependent (LNCap) and androgen-independent (PC3) prostate cancer cells remarkably increased OPG, VEGF, RunX2, PTH1R, and PTHrP gene expression in both cell lines irrespective of their androgen dependency. MAFF also altered the cytokine secretion pattern of prostate cancer cells, including Ang2, SCF, and TNFα increase with TRAIL decrease in the supernatant of both cell lines; preferential increase of Leptin and PDGF-BB in LnCap and of VEGF, IL-8, and G-CSF in PC3; and exclusive increase of FGFβ, MIF, and PECAM-1 with HGF decrease in LnCap, and of TGBβ1, HGF, M-CSF, CXCL1, and CCL7 with NGF decrease in PC3. Murine MLO-Y4 osteocyte-conditioned medium (CM) abrogated M-CSF, G-CSG, IL-8, TNFα, and FGFβ secretion-stimulating activity of mechanical stimulation on PC3 cells, and did the opposite effect on LnCap cells. However, MAFF fostered osteomimetic gene expression response of PC3 cells, but not of LnCap cells, to mechanically stimulated osteocyte-CM. Moreover, it abrogated TNFα and IL-8 secretion inhibitory effect of osteocyte-CM on mechanically stimulated PC3 cells and G-CSF, TNFα, and FGFβ-stimulating effect on mechanically stimulated LnCap cells.

CONCLUSIONS

MAFF activated osteoblast-like phenotype of prostate cancer cells and altered their responses to osteocyte soluble factors. It also induced osteocyte production of osteomimetic gene expression- and cytokine secretion-stimulating factors for prostate cancer cells, particularly, when they were mechanically stimulated. Importantly, MAFF induced a prometastatic response in androgen-independent prostate cancer cells, suggesting the interest of mechanical stimulation-dependent transcription and secretion patterns as diagnostic biomarkers, and as therapeutic targets for the screening of bone-metastasizing phenotype inhibitors upregulated during prostate cancer cell response to MAFF at the cancer microenvironment. Prostate 77:321-333, 2017. © 2016 Wiley Periodicals, Inc.

Hedgehog signaling mediates woven bone formation and vascularization during stress fracture healing

Hedgehog (Hh) signaling is critical in developmental osteogenesis, and recent studies suggest it may also play a role in regulating osteogenic gene expression in the post-natal setting. However, there is a void of studies directly assessing the effect of Hh inhibition on post-natal osteogenesis. This study utilized a cyclic loading-induced ulnar stress fracture model to evaluate the hypothesis that Hh signaling contributes to osteogenesis and angiogenesis during stress fracture healing. Immediately prior to loading, adult rats were given GDC-0449 (Vismodegib - a selective Hh pathway inhibitor; 50mg/kg orally twice daily), or vehicle. Hh signaling was upregulated in response to stress fracture at 3 days (Ptch1, Gli1 expression), and was markedly inhibited by GDC-0449 at 1 day and 3 days in the loaded and non-loaded ulnae. GDC-0449 did not affect Hh ligand expression (Shh, Ihh, Dhh) at 1 day, but decreased Shh expression by 37% at 3 days. GDC-0449 decreased woven bone volume (-37%) and mineral density (-17%) at 7 days. Dynamic histomorphometry revealed that the 7 day callus was composed predominantly of woven bone in both groups. The observed reduction in woven bone occurred concomitantly with decreased expression of Alpl and Ibsp, but was not associated with differences in early cellular proliferation (as determined by callus PCNA staining at 3 days), osteoblastic differentiation (Osx expression at 1 day and 3 days), chondrogenic gene expression (Acan, Sox9, and Col2α1 expression at 1 day and 3 days), or bone resorption metrics (callus TRAP staining at 3 days, Rankl and Opg expression at 1 day and 3 days). To evaluate angiogenesis, vWF immunohistochemistry showed that GDC-0449 reduced fracture callus blood vessel density by 55% at 3 days, which was associated with increased Hif1α gene expression (+30%). Dynamic histomorphometric analysis demonstrated that GDC-0449 also inhibited lamellar bone formation. Lamellar bone analysis of the loaded limb (directly adjacent to the woven bone callus) showed that GDC-0449 significantly decreased mineral apposition rate (MAR) and bone formation rate (BFR/BS) (-17% and -20%, respectively). Lamellar BFR/BS in the non-loaded ulna was also significantly decreased (-37%), indicating that Hh signaling was required for normal bone modeling. In conclusion, Hh signaling plays an important role in post-natal osteogenesis in the setting of stress fracture healing, mediating its effects directly through regulation of bone formation and angiogenesis.

Bmp2 conditional knockout in osteoblasts and endothelial cells does not impair bone formation after injury or mechanical loading in adult mice

Post-natal osteogenesis after mechanical trauma or stimulus occurs through either endochondral healing, intramembranous healing or lamellar bone formation. Bone morphogenetic protein 2 (BMP2) is up-regulated in each of these osteogenic processes and is expressed by a variety of cells including osteoblasts and vascular cells. It is known that genetic knockout of Bmp2 in all cells or in osteo-chondroprogenitor cells completely abrogates endochondral healing after full fracture. However, the importance of BMP2 from differentiated osteoblasts and endothelial cells is not known. Moreover, the importance of BMP2 in non-endochondral bone formation such as intramembranous healing or lamellar bone formation is not known. Using inducible and tissue-specific Cre-lox mediated targeting of Bmp2 in adult (10-24 week old) mice, we assessed the role of BMP2 expression globally, by osteoblasts, and by vascular endothelial cells in endochondral healing, intramembranous healing and lamellar bone formation. These three osteogenic processes were modeled using full femur fracture, ulnar stress fracture, and ulnar non-damaging cyclic loading, respectively. Our results confirmed the requirement of BMP2 for endochondral fracture healing, as mice in which Bmp2 was knocked out in all cells prior to fracture failed to form a callus. Targeted deletion of Bmp2 in osteoblasts (osterix-expressing) or vascular endothelial cells (vascular endothelial cadherin-expressing) did not impact fracture healing in any way. Regarding non-endochondral bone formation, we found that BMP2 is largely dispensable for intramembranous bone formation after stress fracture and also not required for lamellar bone formation induced by mechanical loading. Taken together our results indicate that osteoblasts and endothelial cells are not a critical source of BMP2 in endochondral fracture healing, and that non-endochondral bone formation in the adult mouse is not as critically dependent on BMP2.

Chitosan nanofiber scaffold improves bone healing via stimulating trabecular bone production due to upregulation of the Runx2/osteocalcin/alkaline phosphatase signaling pathway

Osteoblasts play critical roles in bone formation. Our previous study showed that chitosan nanofibers can stimulate osteoblast proliferation and maturation. This translational study used an animal model of bone defects to evaluate the effects of chitosan nanofiber scaffolds on bone healing and the possible mechanisms. In this study, we produced uniform chitosan nanofibers with fiber diameters of approximately 200 nm. A bone defect was surgically created in the proximal femurs of male C57LB/6 mice, and then the left femur was implanted with chitosan nanofiber scaffolds for 21 days and compared with the right femur, which served as a control. Histological analyses revealed that implantation of chitosan nanofiber scaffolds did not lead to hepatotoxicity or nephrotoxicity. Instead, imaging analyses by X-ray transmission and microcomputed tomography showed that implantation of chitosan nanofiber scaffolds improved bone healing compared with the control group. In parallel, microcomputed tomography and bone histomorphometric assays further demonstrated augmentation of the production of new trabecular bone in the chitosan nanofiber-treated group. Furthermore, implantation of chitosan nanofiber scaffolds led to a significant increase in the trabecular bone thickness but a reduction in the trabecular parameter factor. As to the mechanisms, analysis by confocal microscopy showed that implantation of chitosan nanofiber scaffolds increased levels of Runt-related transcription factor 2 (Runx2), a key transcription factor that regulates osteogenesis, in the bone defect sites. Successively, amounts of alkaline phosphatase and osteocalcin, two typical biomarkers that can simulate bone maturation, were augmented following implantation of chitosan nanofiber scaffolds. Taken together, this translational study showed a beneficial effect of chitosan nanofiber scaffolds on bone healing through stimulating trabecular bone production due to upregulation of Runx2-mediated alkaline phosphatase and osteocalcin gene expressions. Our results suggest the potential of chitosan nanofiber scaffolds for therapy of bone diseases, including bone defects and bone fractures.

HIF-1α regulates bone formation after osteogenic mechanical loading

HIF-1 is a transcription factor typically associated with angiogenic gene transcription under hypoxic conditions. In this study, mice with HIF-1α deleted in the osteoblast lineage (ΔHIF-1α) were subjected to damaging or non-damaging mechanical loading known to produce woven or lamellar bone, respectively, at the ulnar diaphysis. By microCT, ΔHIF-1α mice produced significantly less woven bone than wild type (WT) mice 7days after damaging loading. This decrease in woven bone volume and extent was accompanied by a significant decrease in vascularity measured by immunohistochemistry against vWF. Additionally, osteocytes, rather than osteoblasts, appear to be the main bone cell expressing HIF-1α following damaging loading. In contrast, 10days after non-damaging mechanical loading, dynamic histomorphometry measurements demonstrated no impairment in loading-induced lamellar bone formation in ΔHIF-1α mice. In fact, both non-loaded and loaded ulnae from ΔHIF-1α mice had increased bone formation compared with WT ulnae. When comparing the relative increase in periosteal bone formation in loaded vs. non-loaded ulnae, it was not different between ΔHIF-1α mice and controls. There were no significant differences observed between WT and ΔHIF-1α mice in endosteal bone formation parameters. The increases in periosteal lamellar bone formation in ΔHIF-1α mice are attributed to non-angiogenic effects of the knockout. In conclusion, these results demonstrate that HIF-1α is a pro-osteogenic factor for woven bone formation after damaging loading, but an anti-osteogenic factor for lamellar bone formation under basal conditions and after non-damaging loading.

Initiation and early control of tissue regeneration - bone healing as a model system for tissue regeneration

INTRODUCTION

Tissue regeneration in itself is a fascinating process that promises repeated renewal of tissue and organs.

AREAS COVERED

This article aims to illustrate the different strategies available to control tissue regeneration at a very early stage, using bone as an exemplary tissue. The aspects of a controlled inflammatory cascade to achieve a balanced immune response, cell therapeutic approaches for improved tissue formation and angiogenesis, guiding the organization of newly formed extracellular matrix by biomaterials, the relevance of mechanical signals for tissue regeneration processes, and the chances and limitations of growth factor treatments are discussed.

EXPERT OPINION

The currently available knowledge is reviewed and perspectives for potential new targets are given. This is done under the assumption that early identification of risk patients as well as the application of early intervention strategies is possible.

Osteocyte expression of caspase-3, COX-2, IL-6 and sclerostin are spatially and temporally associated following stress fracture initiation

Stress fractures (SFxs) are debilitating injuries and exact mechanisms that initiate their repair incompletely understood. We hypothesised that osteocyte apoptosis and expression of cytokines and proteins such as sclerostin, VEGF, TGF-β, COX-2 and IL-6 were early signalling events to facilitate the formation of periosteal woven bone and recruitment of osteoclast precursors to the site of remodelling. A SFx was created in the right ulna of mature female wistar rats using cyclic end loading. Rats were killed 1, 4 and 7 days after loading (n=5 per group). Standard histological staining was used to examine SFx morphology and immunohistochemistry to detect the localisation of these proteins and in situ hybridisation to detect mRNA along the SFx line or gene expression to quantify the target genes. Unloaded ulnae served as controls. The labelling index of caspase-3, COX-2 and IL-6 was significantly elevated in the region of SFxs at all time points compared with controls (P<0.001). In addition, the labelling index of sclerostin protein was significantly reduced in osteocytes adjacent to the SFx region when compared with controls at all three time points (P<0.001). Both VEGF and TGF-β expressions were only localised in the woven bone. These data reinforce the involvement of osteocyte apoptosis in the healing of fatigue damage in bone, and demonstrate that local regulation of sclerostin, COX-2 and IL-6 are important signalling events associated with new bone formation and SFx remodelling.

Antagonizing the αv β3 integrin inhibits angiogenesis and impairs woven but not lamellar bone formation induced by mechanical loading

Angiogenesis and osteogenesis are critically linked, although the role of angiogenesis is not well understood in osteogenic mechanical loading. In this study, either damaging or non-damaging cyclic axial compression was used to generate woven bone formation (WBF) or lamellar bone formation (LBF), respectively, at the mid-diaphysis of the adult rat forelimb. αv β3 integrin-targeted nanoparticles or vehicle was injected intravenously after mechanical loading. β3 integrin subunit expression on vasculature was maximal 7 days after damaging mechanical loading, but was still robustly expressed 14 days after loading. Accordingly, targeted nanoparticle delivery in WBF-loaded limbs was increased compared with non-loaded limbs. Vascularity was dramatically increased after WBF loading (+700% on day 14) and modestly increased after LBF loading (+50% on day 14). This increase in vascularity was inhibited by nanoparticle treatment in both WBF- and LBF-loaded limbs at days 7 and 14 after loading. Decreased vascularity led to diminished woven, but not lamellar, bone formation. Decreased woven bone formation resulted in impaired structural properties of the skeletal repair, particularly in post-yield behavior. These results demonstrate that αv β3 integrin-mediated angiogenesis is critical for recovering fracture resistance after bone injury but is not required for bone modeling after modest mechanical strain. © 2014 American Society for Bone and Mineral Research.

Mechanistic, mathematical model to predict the dynamics of tissue genesis in bone defects via mechanical feedback and mediation of biochemical factors

The link between mechanics and biology in the generation and the adaptation of bone has been well studied in context of skeletal development and fracture healing. Yet, the prediction of tissue genesis within - and the spatiotemporal healing of - postnatal defects, necessitates a quantitative evaluation of mechano-biological interactions using experimental and clinical parameters. To address this current gap in knowledge, this study aims to develop a mechanistic mathematical model of tissue genesis using bone morphogenetic protein (BMP) to represent of a class of factors that may coordinate bone healing. Specifically, we developed a mechanistic, mathematical model to predict the dynamics of tissue genesis by periosteal progenitor cells within a long bone defect surrounded by periosteum and stabilized via an intramedullary nail. The emergent material properties and mechanical environment associated with nascent tissue genesis influence the strain stimulus sensed by progenitor cells within the periosteum. Using a mechanical finite element model, periosteal surface strains are predicted as a function of emergent, nascent tissue properties. Strains are then input to a mechanistic mathematical model, where mechanical regulation of BMP-2 production mediates rates of cellular proliferation, differentiation and tissue production, to predict healing outcomes. A parametric approach enables the spatial and temporal prediction of endochondral tissue regeneration, assessed as areas of cartilage and mineralized bone, as functions of radial distance from the periosteum and time. Comparing model results to histological outcomes from two previous studies of periosteum-mediated bone regeneration in a common ovine model, it was shown that mechanistic models incorporating mechanical feedback successfully predict patterns (spatial) and trends (temporal) of bone tissue regeneration. The novel model framework presented here integrates a mechanistic feedback system based on the mechanosensitivity of periosteal progenitor cells, which allows for modeling and prediction of tissue regeneration on multiple length and time scales. Through combination of computational, physical and engineering science approaches, the model platform provides a means to test new hypotheses in silico and to elucidate conditions conducive to endogenous tissue genesis. Next generation models will serve to unravel intrinsic differences in bone genesis by endochondral and intramembranous mechanisms.

Arising as a consequence of trauma, tumor resection, removal of necrotic or infected tissue, and congenital abnormalities, critical-sized defects are too large to heal spontaneously and therefore require surgical intervention. New surgical approaches harness the regenerative power of the periosteum, a tissue membrane covering most bones, which provides a niche for stem cells and plays a key role in healing after injury. The interplay of mechanical, cellular and biochemical mechanisms involved in periosteum-mediated tissue genesis and healing remains elusive, providing the impetus for the current study. Here, we develop a mechanistic, mathematical model to predict the dynamics of tissue genesis by periosteum-derived stem cells within a bone defect surrounded by periosteum or a periosteum substitute. A mechanical finite element model is coupled with a model of cellular dynamics to simulate a tested clinical scenario in which the patient's own periosteum is left around the defect after injury. Model predictions incorporating mechanical feedback match spatiotemporal patterns of bone tissue regeneration observed in a series of in vivo ovine experiments. Through combination of computational, physical and engineering science approaches, the model platform provides a means to test new hypotheses in silico. This will provide criteria conducive to endogenous tissue genesis that can be tested in follow on experiments.

Long bone structure and strength depend on BMP2 from osteoblasts and osteocytes, but not vascular endothelial cells

The importance of bone morphogenetic protein 2 (BMP2) in the skeleton is well known. BMP2 is expressed in a variety of tissues during development, growth and healing. In this study we sought to better identify the role of tissue-specific BMP2 during post-natal growth and to determine if BMP2 knockout affects the ability of terminally differentiated cells to create high quality bone material. We targeted BMP2 knockout to two differentiated cell types known to express BMP2 during growth and healing, early-stage osteoblasts and their progeny (osterix promoted Cre) and vascular endothelial cells (vascular-endothelial-cadherin promoted Cre). Our objectives were to assess post-natal bone growth, structure and strength. We hypothesized that removal of BMP2 from osteogenic and vascular cells (separately) would result in smaller skeletons with inferior bone material properties. At 12 and 24 weeks of age the osteoblast knockout of BMP2 reduced body weight by 20%, but the vascular knockout had no effect. Analysis of bone in the tibia revealed reductions in cortical and cancellous bone size and volume in the osteoblast knockout, but not in the vascular endothelial knockout. Furthermore, forelimb strength testing revealed a 30% reduction in ultimate force at both 12 and 24 weeks in the osteoblast knockout of BMP2, but no change in the vascular endothelial knockout. Moreover, mechanical strength testing of femurs from osteoblast knockout mice demonstrated an increased Young's modulus (greater than 35%) but decreased post-yield displacement (greater than 50%) at both 12 and 24 weeks of age. In summary, the osteoblast knockout of BMP2 reduced bone size and altered mechanical properties at the whole-bone and material levels. Osteoblast-derived BMP2 has an important role in post-natal skeletal growth, structure and strength, while vascular endothelial-derived BMP2 does not.

Skeletal Blood Flow in Bone Repair and Maintenance

Bone is a highly vascularized tissue, although this aspect of bone is often overlooked. In this article, the importance of blood flow in bone repair and regeneration will be reviewed. First, the skeletal vascular anatomy, with an emphasis on long bones, the distinct mechanisms for vascularizing bone tissue, and methods for remodeling existing vasculature are discussed. Next, techniques for quantifying bone blood flow are briefly summarized. Finally, the body of experimental work that demonstrates the role of bone blood flow in fracture healing, distraction osteogenesis, osteoporosis, disuse osteopenia, and bone grafting is examined. These results illustrate that adequate bone blood flow is an important clinical consideration, particularly during bone regeneration and in at-risk patient groups.

Nitric oxide-mediated vasodilation increases blood flow during the early stages of stress fracture healing

Despite the strong connection between angiogenesis and osteogenesis in skeletal repair conditions such as fracture and distraction osteogenesis, little is known about the vascular requirements for bone formation after repetitive mechanical loading. Here, established protocols of damaging (stress fracture) and nondamaging (physiological) forelimb loading in the adult rat were used to stimulate either woven or lamellar bone formation, respectively. Positron emission tomography was used to evaluate blood flow and fluoride kinetics at the site of bone formation. In the group that received damaging mechanical loading leading to woven bone formation (WBF), (15)O water (blood) flow rate was significantly increased on day 0 and remained elevated 14 days after loading, whereas (18)F fluoride uptake peaked 7 days after loading. In the group that received nondamaging mechanical loading leading to lamellar bone formation (LBF), (15)O water and (18)F fluoride flow rates in loaded limbs were not significantly different from nonloaded limbs at any time point. The early increase in blood flow rate after WBF loading was associated with local vasodilation. In addition, Nos2 expression in mast cells was increased in WBF-, but not LBF-, loaded limbs. The nitric oxide (NO) synthase inhibitor N(ω)-nitro-l-arginine methyl ester was used to suppress NO generation, resulting in significant decreases in early blood flow rate and bone formation after WBF loading. These results demonstrate that NO-mediated vasodilation is a key feature of the normal response to stress fracture and precedes woven bone formation. Therefore, patients with impaired vascular function may heal stress fractures more slowly than expected.

Periostin deficiency increases bone damage and impairs injury response to fatigue loading in adult mice

Bone damage removal and callus formation in response to fatigue loading are essential to prevent fractures. Periostin (Postn) is a matricellular protein that mediates adaptive response of cortical bone to loading. Whether and how periostin influences damage and the injury response to fatigue remains unknown. We investigated the skeletal response of Postn(-/-) and Postn(+/+) mice after fatigue stimulus by axial compression of their tibia. In Postn(+/+) mice, cracks number and surface (CsNb, CsS) increased 1h after fatigue, with a decrease in strength compared to non-fatigued tibia. At 15 days, CsNb had started to decline, while CtTV and CtBV increased in fatigued vs non-fatigued tibia, reflecting a woven bone response that was present in 75% of the fatigued bones. Cortical porosity and remodelling also prominently increased in the fatigued tibia of Postn(+/+) mice. At 30 days, paralleling a continuous removal of cortical damage, strength of the fatigued tibia was similar to the non-fatigue tibia. In Postn(-/-) mice, cracks were detectable even in the absence of fatigue, while the amount of collagen crosslinks and tissue hardness was decreased compared to Postn(+/+). Fatigue significantly increased CsNb and CsS in Postn(-/-), but was not associated with changes in CtTV and CtBV, as only 16% of the fatigued bones formed some woven bone. Cortical porosity and remodelling did not increase either after fatigue in Postn(-/-), and the level of damage remained high even after 30 days. As a result, strength remained compromised in Postn(-/-) mice. Contrary to Postn(+/+), which osteocytic lacunae showed a change in the degree of anisotropy (DA) after fatigue, Postn(-/-) showed no DA change. Hence periostin appears to influence bone materials properties, damage accumulation and repair, including local modeling/remodeling processes in response to fatigue. These observations suggest that the level of periostin expression could influence the propensity to fatigue fractures.

Construction of adipose scaffold for bone repair with gene engineering bone cells

The bone defect repairing is still a challenge in orthopedics. As the gene engineering bones have been used in the bone repairing clinic, the scaffold construction is a critical fact to be considered. This study aims to construct optimal scaffolds using adipose tissue in the bone repair together with the gene engineering osteocytes. Rat adipose stem cells (ASC) were prepared; the cells were transduced with the OCT-4 gene carrying lentiviral vectors (OCT-4-Lv). Artificial bone defects were created in the rat femoral bone. The bone defects were filled up with adipose scaffolds and shaped by using surrounding muscles and supported with orthopedic splints. ASCs with or without transducing the OCT-4-Lv were injected into the adipose scaffolds. The rats were sacrificed 12 weeks after the surgery. After receiving the OCT-4-Lv, the expressions of OCT-4, RUNX2 and osteocalcin were detected in the ASCs. X-ray examination showed that rats received the OCT-4-Lv transduced ASCs together with the adipose pad had new bone formation in the defect area; none of the control rats showed any new bone formation in situ. The results were supported by histological assessment. Using adipose scaffold and OCT-4-modified ASC transplantation can repair bone defects.

Subtraction micro-computed tomography of angiogenesis and osteogenesis during bone repair using synchrotron radiation with a novel contrast agent

Quantitative three-dimensional (3D) imaging of angiogenesis during bone repair remains an experimental challenge. We developed a novel contrast agent containing 0.07- to 0.1-μm particles of zirconium dioxide (ZrCA) and established subtraction μCT using synchrotron radiation (sSRCT) for quantitative imaging of angiogenesis and bone repair. This method was applied to a rat model of tibial bone repair 3 days (DAY3; n = 2), 5 days (DAY5; n = 8), or 10 days (DAY10; n = 8) after drill-hole injury. Using the same drill-hole defect model, its potential use was illustrated by comparison of bone repair between hindlimbs subjected to mechanical unloading (n = 6) and normal weight bearing (n = 6) for 10 days. Following vascular casting with ZrCA, the defect site was scanned with 17.9- and 18.1-keV X-rays. In the latter, image contrast between ZrCA-filled vasculature and bone was enhanced owing to the sharp absorption jump of zirconium dioxide at 18.0 keV (k-edge). The two scan data sets were reconstructed with 2.74-μm voxel resolution, registered by mutual information, and digitally subtracted to extract the contrast-enhanced vascular image. K2HPO4 phantom solutions were scanned at 17.9 keV for quantitative evaluation of bone mineral. Angiogenesis had already started, but new bone formation was not found on DAY3. New bone emerged near the defect boundary on DAY5 and took the form of trabecular-like structure invaded by microvessels on DAY10. Vascular and bone volume fractions, blood vessel and bone thicknesses, and mineralization were higher on DAY10 than on DAY5. All these parameters were found to be decreased after 10 days of hindlimb unloading, indicating the possible involvement of angiogenesis in bone repair impairment caused by reduced mechanical stimuli. In conclusion, the combined technique of sSRCT and ZrCA vascular casting is suitable for quantitative 3D imaging of angiogenesis and its surrounding bone regeneration. This method will be useful for better understanding the linkage between angiogenesis and bone repair.

Angiogenesis and intramembranous osteogenesis

BACKGROUND

Angiogenesis is likely critical for the process of intramembranous osteogenesis; however, the developmental relationship between blood vessels and bone mineralization is not well studied within intramembranous bones. Given its importance, changes in angiogenesis regulation are likely to contribute to evolutionarily and medically relevant craniofacial variation.

RESULTS

We summarize what is known about the association between angiogenesis and intramembranous osteogenesis, supplementing with information from the better-studied processes of endochondral ossification and distraction osteogenesis. Based on this review, we introduce a model of angiogenesis during early intramembranous osteogenesis as well as a series of null hypotheses to be tested.

CONCLUSIONS

This model can serve as a basis of future research on the spatio-temporal association and regulatory interactions of mesenchymal, vascular, and bone cells, which will be required to illuminate the potential effects of angiogenesis dysregulation on craniofacial skeletal phenotypes.

Bmp2 in osteoblasts of periosteum and trabecular bone links bone formation to vascularization and mesenchymal stem cells

We generated a new Bmp2 conditional-knockout allele without a neo cassette that removes the Bmp2 gene from osteoblasts (Bmp2-cKO(ob)) using the 3.6Col1a1-Cre transgenic model. Bones of Bmp2-cKO(ob) mice are thinner, with increased brittleness. Osteoblast activity is reduced as reflected in a reduced bone formation rate and failure to differentiate to a mature mineralizing stage. Bmp2 in osteoblasts also indirectly controls angiogenesis in the periosteum and bone marrow. VegfA production is reduced in Bmp2-cKO(ob) osteoblasts. Deletion of Bmp2 in osteoblasts also leads to defective mesenchymal stem cells (MSCs), which correlates with the reduced microvascular bed in the periosteum and trabecular bones. Expression of several MSC marker genes (α-SMA, CD146 and Angiopoietin-1) in vivo, in vitro CFU assays and deletion of Bmp2 in vitro in α-SMA(+) MSCs support our conclusions. Critical roles of Bmp2 in osteoblasts and MSCs are a vital link between bone formation, vascularization and mesenchymal stem cells.

Self-repair of rat cortical bone microdamage after fatigue loading in vivo

Bone microdamage can be repaired through bone remodeling induced by loading. In this study, a loading device was developed for improved efficiency and the self-repair process of bone microdamage was studied in ovariectomized rats. First, four-point bending fixtures capable of holding two live rats simultaneously were designed. Rats were loaded and subjected to a sinusoidal wave for 10,000 cycles. They were then divided into four groups to evaluate time points from 1 to 4 weeks in the microdamage repair process. The loaded right ulna was used for microdamage parameter analysis, and the loaded right radius was tested for mechanical properties. In all groups, microdamage consisted primarily of microcracks, which were observed in bone surrounding the force-bearing point. The values of the microdamage parameters were significantly lower at 3 weeks than at 2 weeks. However, none of the differences in mechanical properties between any four groups were statistically significant. This study shows that the improved application of loading in the form of bending for double-rat simultaneous administration was practical and efficient. These results suggest that microdamage was repaired between 2 weeks to 3 weeks after fatigue damage and microdamage is a more sensitive index of bone quality than mechanical properties.

Angiogenesis is required for stress fracture healing in rats

Although angiogenesis and osteogenesis are critically linked, the importance of angiogenesis for stress fracture healing is unknown. In this study, mechanical loading was used to create a non-displaced stress fracture in the adult rat forelimb. Fumagillin, an anti-angiogenic agent, was used as the water soluble analogue TNP-470 (25mg/kg) as well as incorporated into lipid-encapsulated α(v)β(3) integrin targeted nanoparticles (0.25mg/kg). In the first experiment, TNP-470 was administered daily for 5 days following mechanical loading, and changes in gene expression, vascularity, and woven bone formation were quantified. Although no changes in vascularity were detected 3 days after loading, treatment-related downregulation of angiogenic (Pecam1) and osteogenic (Bsp, Osx) genes was observed at this early time point. On day 7, microCT imaging of loaded limbs revealed diminished woven bone formation in treated limbs compared to vehicle treated limbs. In the second experiment, α(v)β(3) integrin targeted fumagillin nanoparticles were administered as before, albeit with a 100-fold lower dose, and changes in vascularity and woven bone formation were determined. There were no treatment-related changes in vessel count or volume 3 days after loading, although fewer angiogenic (CD105 positive) blood vessels were present in treated limbs compared to vehicle treated limbs. This result manifested on day 7 as a reduction in total vascularity, as measured by histology (vessel count) and microCT (vessel volume). Similar to the first experiment, treated limbs had diminished woven bone formation on day 7 compared to vehicle treated limbs. These results indicate that angiogenesis is required for stress fracture healing, and may have implications for inducing rapid repair of stress fractures.

VEGF and bone cell signalling: an essential vessel for communication?

Vascular endothelial growth factor (VEGF) is an endothelial cell survival factor and is required for effective coupling of angiogenesis and osteogenesis. Although central to bone homeostasis, repair and the pathobiology that affect these processes, the precise mechanisms coupling endothelial cell function within bone formation and remodelling remain unclarified. This review will (i) focus on the potential directionality of VEGF signalling in adult bone by identifying the predominant source of VEGF within the bone microenvironment, (ii) will summarize current VEGF receptor expression studies by bone cells and (iii) will provide evidence for a role for VEGF signalling during postnatal repair and osteoporosis. A means of understanding the directionality of VEGF signalling in adult bone would allow us to most effectively target angiogenic pathways in diseases characterized by changes in bone remodelling rates and enhance bone repair when compromised.

Quantification of skeletal blood flow and fluoride metabolism in rats using PET in a pre-clinical stress fracture model

PURPOSE

Blood flow is an important factor in bone production and repair, but its role in osteogenesis induced by mechanical loading is unknown. Here, we present techniques for evaluating blood flow and fluoride metabolism in a pre-clinical stress fracture model of osteogenesis in rats.

PROCEDURES

Bone formation was induced by forelimb compression in adult rats. (15)O water and (18)F fluoride PET imaging were used to evaluate blood flow and fluoride kinetics 7 days after loading. (15)O water was modeled using a one-compartment, two-parameter model, while a two-compartment, three-parameter model was used to model (18)F fluoride. Input functions were created from the heart, and a stochastic search algorithm was implemented to provide initial parameter values in conjunction with a Levenberg-Marquardt optimization algorithm.

RESULTS

Loaded limbs are shown to have a 26% increase in blood flow rate, 113% increase in fluoride flow rate, 133% increase in fluoride flux, and 13% increase in fluoride incorporation into bone as compared to non-loaded limbs (p < 0.05 for all results).

CONCLUSIONS

The results shown here are consistent with previous studies, confirming this technique is suitable for evaluating the vascular response and mineral kinetics of osteogenic mechanical loading.

Adaptive and Injury Response of Bone to Mechanical Loading

Bone responds to supraphysiological mechanical loads by increasing bone formation. Depending on the applied strain magnitude (and other loading parameters) the response can be either adaptive (mostly lamellar bone) or injury (mostly woven bone). Seminal studies of Hert, Lanyon, and Rubin originally established the basic "rules" of bone mechanosensitivity. These were reinforced by subsequent studies using non-invasive rodent loading models, most notably by Turner et al. More recent work with these models have been able to explore the structural, transcriptional, and molecular mechanisms which distinguish the two responses (lamellar vs. woven). Wnt/Lrp signaling has emerged as a key mechanoresponsive pathway for lamellar bone. However, there is still much to study with regard to effects of ageing, osteocytes, other signaling pathways, and the molecular regulation that modulates lamellar vs. woven bone formation. This review summarizes not only the historical findings but also the current data for these topics.

A SHORT-TERM FREE-FALL LANDING ENHANCES BONE FORMATION AND BONE MATERIAL PROPERTIES

To investigate the effects of a short-term free-fall landing course on local bone metabolism and biomaterial properties, 32 female Wistar rats (7 week old) were randomly assigned to three groups: L30 (n = 11), L10 (n = 11) and CON (n = 10). Animals in the L30 and L10 groups were subjected to 30 and 10 free-fall landings per day, respectively, from a height of 40 cm for five consecutive days. Animals' ulnae were studied using methods of dynamic histomorphometry, tissue geometry, biomaterial measurements and collagen fiber orientation (CFO) analysis. In dynamic histomorphometry analysis, periosteal as well as endosteal mineral apposition rates (MAR, μm/day) were significantly higher in L30 group than in the CON group (p < 0.05). In addition, the periosteal bone formation rate (BFR/BS, μm2/μm3/year) was significantly higher in the L10 and L30 groups (p < 0.05). The ulnae of the animals in the two landing groups were higher in post-yield energy without significant changes in CFO, tissue size or tissue weight measurements. In conclusion, a short-term free-fall landing training produced a slight, but significant, higher bone formation in the ulnae of young female rats. Enhanced tissue biomaterial properties did not accompany size-related changes, suggesting that bone adapting to mechanical loading begins with changes in tissue-level properties.

Differential gene expression from microarray analysis distinguishes woven and lamellar bone formation in the rat ulna following mechanical loading

Formation of woven and lamellar bone in the adult skeleton can be induced through mechanical loading. Although much is known about the morphological appearance and structural properties of the newly formed bone, the molecular responses to loading are still not well understood. The objective of our study was to use a microarray to distinguish the molecular responses between woven and lamellar bone formation induced through mechanical loading. Rat forelimb loading was completed in a single bout to induce the formation of woven bone (WBF loading) or lamellar bone (LBF loading). A set of normal (non-loaded) rats were used as controls. Microarrays were performed at three timepoints after loading: 1 hr, 1 day and 3 days. Confirmation of microarray results was done for a select group of genes using quantitative real-time PCR (qRT-PCR). The micorarray identified numerous genes and pathways that were differentially regulated for woven, but not lamellar bone formation. Few changes in gene expression were evident comparing lamellar bone formation to normal controls. A total of 395 genes were differentially expressed between formation of woven and lamellar bone 1 hr after loading, while 5883 and 5974 genes were differentially expressed on days 1 and 3, respectively. Results suggest that not only are the levels of expression different for each type of bone formation, but that distinct pathways are activated only for woven bone formation. A strong early inflammatory response preceded an increase in angiogenic and osteogenic gene expression for woven bone formation. Furthermore, at later timepoints there was evidence of bone resorption after WBF loading. In summary, the vast coverage of the microarray offers a comprehensive characterization of the early differences in expression between woven and lamellar bone formation.

Impaired bone healing pattern in mice with ovariectomy-induced osteoporosis: A drill-hole defect model

OBJECTIVE

To establish a drill-hole defect model in osteoporotic mouse femur by comparing temporal cortical bone healing pattern between OVX-induced osteoporotic bone and sham-operated bone.

METHODS

3-month-old female C57BL/6 mice were randomly divided into an ovariectomy group (OVX) and a sham-operated group (Sham). At 6 weeks post-surgery, 7 mice from each group were sacrificed to examine the distal femur and femoral shaft by both micro-CT and mechanical testing for confirming established osteoporosis induced by OVX. In the remaining mice, a cortical bone defect 0.8mm in diameter was created on the mid-diaphysis of the right femur. The local repair process at days 0, 3, 7, 10, 14 and 21 after creation of the drill-hole was in vivo monitored by high-resolution micro-CT scanning. At each time point, each animal was scanned four times and was removed from the scanner between scans to determine reproducibility. Mice were sacrificed at each time point (n=12 at days 0, 3, 7, 10 and 14; n=20 at day 21). Before sacrifice, sera were collected to examine expression of bone formation marker P1NP (procollagen type I N-terminal propeptide) and bone resorption marker CTX (C-terminal telopeptide of type I collagen). After sacrifice, callus samples were collected and subjected to the following analyses: micro-CT-based angiography; histological examination; immunohistochemical staining to determine estrogen receptor expression; quantitative real-time PCR analysis of collagen type I, collagen type II, collagen type X, osteocalcin, tartrate-resistant acid phosphatase, estrogen receptor alpha (ER alpha) and estrogen receptor beta (ER beta) gene expression; and three-point mechanical testing.

RESULTS

At 6 weeks post-surgery, OVX mice had significantly lower bone mass, impaired bone micro architecture and compromised mechanical properties compared to the Sham mice. In vivo micro-CT analysis revealed that the bone volume fraction in the defect region was significantly lower in the OVX group from day 10 to day 21 post-injury as compared to the Sham group, and was significantly lower in the intra-medulla region in the OVX group from day 7 to day 14 as compared to the Sham group, consistent with the histological data. Analysis of bone biochemical markers indicated that circulating P1NP levels normalized by baseline in the OVX mice were significantly lower than in the Sham mice from day 7 to day 10, and that temporal expression of circulating CTX levels normalized by baseline was also lower in the OVX mice as compared to the Sham mice. These results were consistent with quantitative real-time PCR analysis. ER alpha mRNA expression was significantly lower in the OVX mice, whereas ER beta mRNA expression was significantly higher in the OVX mice as compared to the Sham mice at all time points examined, consistent with immunohistochemical staining. The restoration of femoral mechanical property, determined based on ultimate load and energy-to-failure, was significantly lower in the OVX mice than in the Sham mice. In addition, in vivo micro-CT scanning for quantifying new bone formation in the defect site was highly reproducible in this model.

CONCLUSION

The bone healing of the drill-hole defect was impaired in mice with OVX-induced osteoporosis. The present study provides a model to investigate the functional role of specific gene in osteoporotic bone healing and may facilitate development of novel therapeutic strategies for promoting osteoporotic bone healing.

Small-animal PET/CT assessment of bone microdamage in ovariectomized rats

Microdamage in bone contributes to bone fragility in postmenopausal women. Therefore, it is important to find a noninvasive method to detect microdamage in living bone. PET with (18)F-fluoride has been used for skeletal imaging in clinical studies. However, few studies are undertaken to investigate bone microdamage associated with osteoporosis in vivo using noninvasive means. The aim of our study was to analyze the impact of osteoporosis due to estrogen deficiency on the occurrence of microdamage by observing the change in the uptake of (18)F-fluoride in the tibiae of ovariectomized rats after fatigue loading with small-animal PET/CT. We also explored the feasibility of noninvasive detection of bone microdamage in vivo using a small-animal PET/CT scanner specially designed for rodent study.

METHODS

Rats were randomized into 2 groups: ovariectomy and sham surgery. These rats were imaged using a dedicated small-animal PET scanner with (18)F-fluoride after the left tibiae were loaded cyclically under the axial compression. The fluoride uptake values were quantified in the tibial mid shafts, and the tibia was obtained for histomorphometric measurements of bone microdamage and osteocyte density. Bone mineral density at the fourth lumbar vertebra and right femur were measured using dual-energy x-ray absorptiometry.

RESULTS

PET image intensity was significantly increased (P < 0.05) in the loaded tibia of the ovariectomy group, compared with that of the sham group. Histomorphometry showed that both crack density and crack length in the loaded tibia were significantly higher (P < 0.05) in ovariectomized rats than in sham rats. The PET image intensity in the loaded tibia was significantly positively correlated with crack length and crack density (which show in histomorphometric measurement) (P < 0.05).

CONCLUSION

Both small-animal PET/CT and histomorphometric measurement provided evidence that bone microdamage is significantly increased after estrogen depletion. The strong correlation between these 2 measurements suggests that small-animal PET/CT is a useful noninvasive means to detect bone microdamage in vivo.

Hierarchies of damage induced loss of mechanical properties in calcified bone after in vivo fatigue loading of rat ulnae

During fatigue loading of whole bone, damage to bone tissue accumulates, coalesces and leads to fractures. Whether damage affects tissue material properties similarly at the nanoscale (less than 1 μm), microscale (less than 1 mm), and whole bone scale has not been fully evaluated. Therefore, in this study, we examine scale-dependent loss of calcified tissue material properties in rat ulnae, after fatigue loading of rat forearms using the forearm compression model. In vivo fatigue loading was conducted on the right forearms until a displacement end-point was reached. The non-fatigued left forearms served as contralateral controls. Subsequently, three-point bending tests to failure on excised ulnae demonstrated a 41% and 49% reduction in the stiffness and ultimate strength as compared to contralateral control ulnae, respectively. Depth-sensing microindentation demonstrated an average decrease in material properties, such as elastic modulus and hardness, of 28% and 29% respectively. Nanoindentation measured elastic modulus and hardness were reduced by 26% and 29% in damaged bone relative to contralateral controls, respectively. The increased loss of whole bone material properties compared to tissue material properties measured using indentation is mainly attributed to the presence of a macrocrack located in the medial compressive region at the site of peak strains. The similar magnitude of changes in material properties by microindentation and nanoindentation is attributed to damage that may originate at an even smaller scale, as inferred from 10% differences in connectivity of osteocyte canaliculi in damaged bone.

Short-term low-strain vibration enhances chemo-transport yet does not stimulate osteogenic gene expression or cortical bone formation in adult mice

Development of low-magnitude mechanical stimulation (LMMS) based treatment strategies for a variety of orthopaedic issues requires better understanding of mechano-transduction and bone adaptation. Our overall goal was to study the tissue and molecular level changes in cortical bone in response to low-strain vibration (LSV: 70 Hz, 0.5 g, 300 με) and compare these to changes in response to a known anabolic stimulus: high-strain compression (HSC: rest inserted loading, 1000 με). Adult (6-7 months) C57BL/6 mice were used for the study and non-invasive axial compression of the tibia was used as a loading model. We first studied bone adaptation at the tibial mid-diaphysis, using dynamic histomorphometry, in response to daily loading of 15 min LSV or 60 cycles HSC for 5 consecutive days. We found that bone formation rate and mineral apposition rate were significantly increased in response to HSC but not LSV. The second aim was to compare chemo-transport in response to 5 min of LSV versus 5 min (30 cycles) of HSC. Chemo-transport increased significantly in response to both loading stimuli, particularly in the medial and the lateral quadrants of the cross section. Finally, we evaluated the expression of genes related to mechano-responsiveness, osteoblast differentiation, and matrix mineralization in tibias subjected to 15 min LSV or 60 cycles HSC for 1 day (4-h time point) or 4 consecutive days (4-day time point). The expression level of most of the genes remained unchanged in response to LSV at both time points. In contrast, the expression level of all the genes changed significantly in response to HSC at the 4-h time point. We conclude that short-term, low-strain vibration results in increased chemo-transport, yet does not stimulate an increase in mechano-responsive or osteogenic gene expression, and cortical bone formation in tibias of adult mice.

Comparing histological, vascular and molecular responses associated with woven and lamellar bone formation induced by mechanical loading in the rat ulna

Osteogenesis occurs by formation of woven or lamellar bone. Little is known about the molecular regulation of these two distinct processes. We stimulated periosteal bone formation at the ulnar mid-diaphysis of adult rats using a single bout of forelimb compression. We hypothesized that loading that stimulates woven bone formation induces higher over-expression of genes associated with cell proliferation, angiogenesis and osteogenesis compared to loading that stimulates lamellar bone formation. We first confirmed that a single bout of 100 cycles of loading using either a rest-inserted (0.1 Hz) or haversine (2 Hz) waveform (15 N peak force) was non-damaging and increased lamellar bone formation (LBF loading). Woven bone formation (WBF loading) was stimulated using a previously described, damaging fatigue loading protocol (2 Hz, 1.3 mm disp., 18 N peak force). There were dramatic differences in gene expression levels (based on qRT-PCR) between loading protocols that produced woven and lamellar bone. In contrast, gene expression levels were not different between LBF loading protocols using a rest-inserted or haversine waveform. Cell proliferation markers Hist4 and Ccnd1 were strongly upregulated (5- to 17-fold) 1 and 3 days after WBF loading, prior to woven bone formation, but not after LBF loading. The angiogenic genes Vegf and Hif1a were upregulated within 1 h after WBF loading and were strongly up on days 1-3 (3- to 15-fold). In sharp contrast, we observed only a modest increase (<2-fold) in Vegfa and Hif1a expression on day 3 following LBF loading. Consistent with these relative differences in gene expression, vascular perfusion 3 days after loading revealed significant increases in vessel number and volume following WBF loading, but not after LBF loading. Lastly, bone formation markers (Runx2, Osx, Bsp) were more strongly upregulated for woven (4- to 89-fold) than for lamellar bone (2-fold), consistent with the differences in new bone volume observed 10 days after loading. In summary, robust early increases both molecularly and histologically for cell proliferation and angiogenesis precede woven bone formation, whereas lamellar bone formation is associated with only a modest upregulation of molecular signals at later timepoints.