Bone loss dynamics result in trabecular alignment in aging and ovariectomized rats

Altered extracellular matrix and mechanotransduction gene expression in rat bone tissue following long-term estrogen deficiency

Osteoporosis is primarily associated with bone loss, but changes in bone tissue matrix composition and osteocyte mechanotransduction have also been identified. However, the molecular mechanisms underlying these changes and their relation to bone loss are not fully understood. The objectives of this study were to (1) conduct comprehensive temporal gene expression analyses on cortical bone tissue from ovariectomized rats, with a specific focus on genes known to govern matrix degradation, matrix production, and mechanotransduction, and (2) correlate these findings with bone mass, trabecular and cortical microarchitecture, and mineral and matrix composition. Microarray data revealed 35 differentially expressed genes in the cortical bone tissue of the ovariectomized cohort. We report that catabolic gene expression abates after the initial accelerated bone loss period, which occurs within the first 4 wk of estrogen deficiency. However, in long-term estrogen deficiency, we report increased expression of genes associated with extracellular matrix deposition (Spp1, COL1A1, COL1A2, OCN) and mechanotransduction (Cx43) compared with age-matched controls and short-term estrogen deficiency. These changes coincided with increased heterogeneity of mineral-to-matrix ratio and collagen maturity, to which extracellular matrix markers COL1A1 and COL1A2 were positively correlated. Interestingly, mineral heterogeneity and collagen maturity, exhibited a negative correlation with PHEX and IFT88, associated with mechanosensory cilia formation and Hedgehog (Hh) signaling. This study provides the first insight into the underlying mechanisms governing secondary mineralization and heterogeneity of matrix composition of bone tissue in long-term estrogen deficiency. We propose that altered mechanobiological responses in long-term estrogen deficiency may play a role in these changes.

Biomechanical properties and clinical significance of cancellous bone in proximal femur: A review

Trabecular bone plays an important role in the load-bearing capacity of the femur. Understanding the structural characteristics, biomechanics, and mechanical conduction of the trabecular bone is of great value in studying the mechanism of fractures and formulating surgical plans. The past decade has witnessed unprecedented progress in imaging, biomechanics and finite element analysis techniques, translating into a better understanding of trabecular bone. This article reviews the research progress achieved over the years regarding femoral trabecular bone, especially on factors influencing the strength of the proximal femoral cancellous bone and cancellous bone microfractures and provides a comprehensive overview of the latest findings on proximal femoral trabecular bone and their clinical significance.

Temporal changes in cortical microporosity during estrogen deficiency associated with perilacunar resorption and osteocyte apoptosis: A pilot study

Osteocytes can actively regulate bone microporosity, through either perilacunar resorption or micropetrosis following apoptosis. Osteocyte apoptosis is more prevalent in estrogen deficiency and changes in the lacunar-canalicular network of osteocytes have been reported. Temporal changes in bone mineralisation and osteocytes cellular strains occur, which might be associated with osteocyte-driven microporosity changes, although time dependant changes in bone microporosity are not yet fully understood. In this pilot study we conducted micro-CT analysis, backscatter electron imaging and histological analysis of femoral cortical bone form an ovariectomized rat model of osteoporosis to investigate whether estrogen deficiency causes temporal changes in lacunar and vascular porosity. We also assessed MMP14 expression, lacunar occupancy and mineral infilling, as indicators of perilacunar resorption and micropetrosis. We report temporal changes in cortical microporosity in estrogen deficiency. Specifically, canalicular and vascular porosity initially increased (4 weeks post-OVX), coinciding with the period of rapid bone loss, whereas in the longer term (14 weeks post-OVX) lacunar and canalicular diameter decreased. Interestingly, these changes coincided with an increased prevalence of empty lacunae and osteocyte lacunae were observed to be more circular with a mineralised border around the lacunar space. In addition we report an increase in MMP14+ osteocytes, which also suggests active matrix degradation by these cells. Together these results provide an insight into the temporal changes in cortical microporosity during estrogen deficiency and suggest the likelihood of occurrence of both perilacunar resorption and osteocyte apoptosis leading to micropetrosis. We propose that microporosity changes arise due to processes driven by distinct populations of osteocytes, which are either actively resorbing their matrix or have undergone apoptosis and are infilling lacunae by micropetrosis.

Microstructure Determines Apparent-Level Mechanics Despite Tissue-Level Anisotropy and Heterogeneity of Individual Plates and Rods in Normal Human Trabecular Bone

Trabecular plates and rods determine apparent elastic modulus and yield strength of trabecular bone, serving as important indicators of bone's mechanical integrity in health and disease. Although trabecular bone's apparent-level mechanical properties have been widely reported, tissue mechanical properties of individual trabeculae have not been fully characterized. We systematically measured tissue mineral density (TMD)-dependent elastic modulus of individual trabeculae using microindentation and characterized its anisotropy as a function of trabecular type (plate or rod), trabecular orientation in the global coordinate (longitudinal, oblique, or transverse along the anatomic loading axis), and indentation direction along the local trabecular coordinate (axial or lateral). Human trabecular bone samples were scanned by micro-computed tomography for TMD and microstructural measurements. Individual trabecula segmentation was used to decompose trabecular network into individual trabeculae, where trabecular type and orientation were determined. We performed precise, selective indentation of trabeculae in each category using a custom-built, microscope-coupled microindentation device. Co-localization of TMD at each indentation site was performed to obtain TMD-to-modulus correlations. We found significantly higher TMD and tissue modulus in trabecular plates than rods. Regardless of trabecular type and orientation, axial tissue modulus was consistently higher than lateral tissue modulus, with ratios ranging from 1.13 to 1.41. Correlations between TMD and tissue modulus measured from axial and lateral indentations were strong but distinct: axial correlation predicted higher tissue modulus than lateral correlation at the same TMD level. To assess the contribution of experimentally measured anisotropic tissue properties of individual trabeculae to apparent-level mechanics, we constructed non-linear micro-finite element models using a new set of trabecular bone samples and compared model predictions to mechanical testing measurements. Heterogeneous anisotropic models accurately predicted apparent elastic modulus but were no better than a simple homogeneous isotropic model. Variances in tissue-level properties may therefore contribute nominally to apparent-level mechanics in normal human trabecular bone. © 2021 American Society for Bone and Mineral Research (ASBMR).

A role for estrogen in skin ageing and dermal biomechanics

The skin is the body's primary defence against the external environment, preventing infection and desiccation. Therefore, alterations to skin homeostasis, for example with skin ageing, increase susceptibility to skin disease and injury. Skin biological ageing is uniquely influenced by a combination of intrinsic and extrinsic (primarily photoageing) factors, with differential effects on skin structure and function. Interestingly, skin architecture rapidly changes following the menopause, as a direct result of reduced circulating 17β-estradiol. The traditional clinical benefit of estrogens are supported by recent experimental data, where 17β-estradiol supplementation prevents age-related decline in the skin's structural and mechanical properties. However, the off-target effects of 17β-estradiol continue to challenge therapeutic application. Here we discuss how ageing alters the physiological and structural properties of the dermal extracellular matrix, and explore how estrogen receptor-targeted therapies may restore the mechanical defects associated with skin ageing.

Peak trabecular bone microstructure predicts rate of estrogen-deficiency-induced bone loss in rats

Postmenopausal osteoporosis affects a large number of women worldwide. Reduced estrogen levels during menopause lead to accelerated bone remodeling, resulting in low bone mass and increased fracture risk. Both peak bone mass and the rate of bone loss are important predictors of postmenopausal osteoporosis risk. However, whether peak bone mass and/or bone microstructure directly influence the rate of bone loss following menopause remains unclear. Our study aimed to establish the relationship between peak bone mass/microstructure and the rate of bone loss in response to estrogen deficiency following ovariectomy (OVX) surgery in rats of homogeneous background by tracking the skeletal changes using in vivo micro-computed tomography (μCT) and three-dimensional (3D) image registrations. Linear regression analyses demonstrated that the peak bone microstructure, but not peak bone mass, was highly predictive of the rate of OVX-induced bone loss. In particular, the baseline trabecular thickness was found to have the highest correlation with the degree of OVX-induced bone loss and trabecular stiffness reduction. Given the same bone mass, the rats with thicker baseline trabeculae had a lower rate of trabecular microstructure and stiffness deterioration after OVX. Moreover, further evaluation to track the changes within each individual trabecula via our novel individual trabecular dynamics (ITD) analysis suggested that a trabecular network with thicker trabeculae is less likely to disconnect or perforate in response to estrogen deficiency, resulting a lower degree of bone loss. Taken together, these findings indicate that the rate of estrogen-deficiency-induced bone loss could be predicted by peak bone microstructure, most notably the trabecular thickness. Given the same bone mass, a trabecular bone phenotype with thin trabeculae may be a risk factor toward accelerated postmenopausal bone loss.

Secondary alterations in bone mineralisation and trabecular thickening occur after long-term estrogen deficiency in ovariectomised rat tibiae, which do not coincide with initial rapid bone loss

This study delineates the time sequence of changes in bone tissue mineralisation in ovariectomised rats. We report that changes in bone mineral distribution arise secondary to the initial rapid bone loss but coincide with trabecular thickening. We propose that these changes compensate for elevated stresses in remaining trabeculae after bone resorption.

INTRODUCTION

Recent studies have shown that osteoporosis is not simply a disease of bone loss and microarchitectural degradation but that important changes in tissue composition also occur. Such changes may be a secondary response to early bone loss, but the time sequence of changes in bone mineral distribution is not fully understood. The objective of this study was to quantify the temporal effects of estrogen deficiency on trabecular mineral distribution in the tibia of ovariectomised (OVX) rats.

METHODS

Weekly in vivo micro-CT scans and morphometric and bone mineral density distribution analyses of the proximal tibia were conducted for the first 4 weeks of estrogen deficiency and then at 8, 14 and 34 weeks.

RESULTS

Here we report that although trabecular bone volume and architecture are significantly deteriorated within the first 4 weeks of estrogen deficiency, there is no change in the distribution of bone mineral within trabeculae during this initial period. The rate of bone loss in OVX animals dramatically reduced between week 4 and week 14, which coincided with the initiation of increases in trabecular thickness and mineralisation in the OVX group.

CONCLUSIONS

Together this study reveals for the first time that alterations in bone mineralisation and trabecular thickening arise secondary to the initial rapid bone loss. We propose that these secondary mineralisation changes act to reinforce the trabecular network in an attempt to compensate for the increased loading that ensues after severe bone loss. This study provides an insight into temporal changes in bone mineral distribution in estrogen deficiency.

Effect of ovariectomy on proximal tibia metaphysis and lumbar vertebral body in common marmoset monkeys

This study aimed to investigate the effect of estrogen withdrawal on bone tissue in adult female marmoset monkeys. In a 1-year follow-up study we used quantitative computer tomography to measure total bone mineral density (BMD) of the proximal tibia and the second-last lumbar vertebral body (L5/L6) before and 1, 3, 6, and 12 months after ovariectomy. Body mass did not significantly change during the 1-year observation period. However, a significant decline of total BMD after ovariectomy was observed in the proximal tibia but not in L5/L6. In addition, regression analysis showed a significant positive relationship between BMD and body mass in both tibia and L5/L6. The results of our study support the idea that ovariectomized marmoset monkeys may serve as a model to investigate bone loss related to decline of estrogen production.

Bone microarchitectural parameters can detect oxytocin induced changes prior to bone density on mitigating bone deterioration in rabbit osteoporosis model using micro-CT

Background

This study is aimed to determine the efficacy of X-Ray Microtomography (micro-CT) in predicting oxytocin (OT) treatment response in rabbit osteoporosis(OP) model.

Methods

Sixty-five rabbits were randomly divided into three groups: control group, ovariectomy (OVX) -vehicle and OVX-oxytocin group. The controls underwent sham surgery. OVX-vehicle and OVX-oxytocin groups were subjected to bilateral OVX. The rabbits in OVX-oxytocin group were injected with oxytocin. In the 0th, 4th, 8th, 10th and 12th weeks post OVX operation, bone mineral density (BMD) and bone micro-architectural parameters were measured in three groups.

Results

Bone mineral density (BMD), bone volume fraction (BV/TV), Trabecular Number (Tb.N), and Trabecular Thickness (Tb.Th) decreased, while Trabecular Spacing (Tb.Sp) and Structure Model Index (SMI) increased overtime in all the three groups. In OVX-oxytocin group, the bone deterioration tendency is slowing down compared with that of the OVX-vehicle group. The BMD of the OVX-oxytocin group was significantly lower than those in the OVX-vehicle group at 12th week (P = 0.017). BV/TV and Tb.Sp in OVX-oxytocin group changed significantly from 8th week (P = 0.043) and 12th week (P = 0.014), which is earlier than that of BMD and other bone micro-architectural parameters.

Conclusion

BV/TV and Tb.Sp changed prior to BMD and other bone micro-architectural parameters with oxytocin intervention, which indicate that they are more sensitive markers for predicting early osteoporosis and treatment monitoring when using micro-CT to evaluate osteoporosis rabbit model.

Structural Adaptations in the Rat Tibia Bone Induced by Pregnancy and Lactation Confer Protective Effects Against Future Estrogen Deficiency

The female skeleton undergoes substantial structural changes during the course of reproduction. Although bone mineral density recovers postweaning, reproduction may induce permanent alterations in maternal bone microarchitecture. However, epidemiological studies suggest that a history of pregnancy and/or lactation does not increase the risk of postmenopausal osteoporosis or fracture and may even have a protective effect. Our study aimed to explain this paradox by using a rat model, combined with in vivo micro-computed tomography (μCT) imaging and bone histomorphometry, to track the changes in bone structure and cellular activities in response to estrogen deficiency following ovariectomy (OVX) in rats with and without a reproductive history. Our results demonstrated that a history of reproduction results in an altered skeletal response to estrogen-deficiency-induced bone loss later in life. Prior to OVX, rats with a reproductive history had lower trabecular bone mass, altered trabecular microarchitecture, and more robust cortical structure at the proximal tibia when compared to virgins. After OVX, these rats underwent a lower rate of trabecular bone loss than virgins, with minimal structural deterioration. As a result, by 12 weeks post-OVX, rats with a reproductive history had similar trabecular bone mass, elevated trabecular thickness, and increased robustness of cortical bone when compared to virgins, resulting in greater bone stiffness. Further evaluation suggested that reproductive-history-induced differences in post-OVX trabecular bone loss were likely due to differences in baseline trabecular microarchitecture, particularly trabecular thickness. Rats with a reproductive history had a larger population of thick trabeculae, which may be protective against post-OVX trabecular connectivity deterioration and bone loss. Taken together, these findings indicate that reproduction-associated changes in bone microarchitecture appear to reduce the rate of bone loss induced by estrogen deficiency later in life, and thereby exert a long-term protective effect on bone strength. © 2018 American Society for Bone and Mineral Research.

The anti-epileptic drugs valproate, carbamazepine and levetiracetam cause bone loss and modulate Wnt inhibitors in normal and ovariectomised rats

Secondary osteoporosis is the major concern associated with long term intake of antiepileptic drugs (AEDs). Women are the vulnerable targets owing to post-menopausal bone loss. In the present work, we evaluated the effect of 10 weeks of treatment with AED therapy (carbamazepine, CBZ, 75 mg/kg; sodium valproate, SVP, 300 mg/kg; levetiracetam, LTM, 150 mg/kg) on bone mineral density and microarchitecture at femoral epiphysis, lumbar vertebrae and proximal tibia of normal and ovariectomised Wistar rats. In addition, we measured serum levels of vitamin D, receptor activator of nuclear factor kappa β-ligand (RANKL), procollagen type 1 amino-terminal propeptide (P1NP) and wnt inhibitors (sclerostin and DKK-1) following AED therapy. Micro-computed tomography analysis of bones revealed significant reduction in BMD at femur epiphysis and lumbar vertebrae with all the three AEDs evaluated. At proximal tibia, only CBZ showed a significant decline. The reduction in BMD was more pronounced in ovariectomised rats. AEDs also resulted in alteration of micro-CT parameters. These changes were accompanied by an increased serum RANKL with all AEDs while vitamin D levels were reduced only with CBZ treatment and P1NP levels were reduced with SVP and CBZ. Serum sclerostin levels were elevated following all AEDs in normal and ovariectomised rats except with CBZ in normal rats. However, increase in DKK-1 levels was observed with only LTM. Ovariectomy itself resulted in increased RANKL, sclerostin and DKK-1 and reduced vitamin D and P1NP levels. Significant differences were discernible between normal and ovariectomised rats treated with AEDs in all the parameters. However, while sclerostin increased further upon AEDs treatment, P1NP decreased with SVP and CBZ and serum DKK-1 levels showed a declining trend with all the three AEDs studied. We confirm adverse effects on bone following AEDs in female rats. Further, our results demonstrate for the first time that these effects are more pronounced in ovariectomised rats as compared to normal rats and that this could be related to estrogen deficiency which in turn enhances bone resorption via increased RANKL and reduces bone formation via increased sclerostin and reduced P1NP. Finally, our study demonstrated for the first time that AED treatment displayed changes in the serum levels of wnt inhibitors and hence modulation of wnt inhibitors might be partly involved in their adverse effects on bone.

Staphylococcus aureus protein A causes osteoblasts to hyper-mineralise in a 3D extra-cellular matrix environment

Osteomyelitis is an inflammatory bone infection that is caused most commonly by the opportunistic pathogen Staphylococcus aureus. Research into staphylococcal induced bone infection is typically conducted using traditional 2D in vitro culture settings, which is not fully representative of the dynamic in vivo environment. In this study we utilised a collagen glycosaminoglycan scaffold, previously developed for bone tissue engineering, as a representative 3D model of infection. The scaffold resisted degradation and retained its pore structure, which is important for cellular function and survival, when seeded with both cells and bacteria. Using this model, we showed that in the presence of S. aureus, osteoblast proliferation was reduced over 21 days. Interestingly however these cells were more metabolically active compared to the uninfected cells and demonstrated increased mineralisation. Protein A (SpA) is a virulence factor found on the surface of S. aureus and has been shown to interact with osteoblasts. When SpA was removed from the surface of S. aureus, the osteoblasts show comparable activity with the uninfected cells-demonstrating the importance of SpA in the interaction between bone cells and S. aureus. Our results suggest that infected osteoblasts are capable of over-compensating for bone loss and bone destruction by increasing mineralisation in a 3D environment, key elements required for ensuring bone strength. It also reinforces our previously established result that S. aureus SpA is a critical mediator in osteomyelitis and might be a potential novel drug target to treat osteomyelitis by preventing the interaction between S. aureus and osteoblasts.

Trabecular architecture in the sciuromorph femoral head: allometry and functional adaptation

BACKGROUND

Sciuromorpha (squirrels and close relatives) are diverse in terms of body size and locomotor behavior. Individual species are specialized to perform climbing, gliding or digging behavior, the latter being the result of multiple independent evolutionary acquisitions. Each lifestyle involves characteristic loading patterns acting on the bones of sciuromorphs. Trabecular bone, as part of the bone inner structure, adapts to such loading patterns. This network of thin bony struts is subject to bone modeling, and therefore reflects habitual loading throughout lifetime. The present study investigates the effect of body size and lifestyle on trabecular structure in Sciuromorpha.

METHODS

Based upon high-resolution computed tomography scans, the femoral head 3D inner microstructure of 69 sciuromorph species was analyzed. Species were assigned to one of the following lifestyle categories: arboreal, aerial, fossorial and semifossorial. A cubic volume of interest was selected in the center of each femoral head and analyzed by extraction of various parameters that characterize trabecular architecture (degree of anisotropy, bone volume fraction, connectivity density, trabecular thickness, trabecular separation, bone surface density and main trabecular orientation). Our analysis included evaluation of the allometric signals and lifestyle-related adaptation in the trabecular parameters.

RESULTS

We show that bone surface density, bone volume fraction, and connectivity density are subject to positive allometry, and degree of anisotropy, trabecular thickness, and trabecular separation to negative allometry. The parameters connectivity density, bone surface density, trabecular thickness, and trabecular separation show functional signals which are related to locomotor behavior. Aerial species are distinguished from fossorial ones by a higher trabecular thickness, lower connectivity density and lower bone surface density. Arboreal species are distinguished from semifossorial ones by a higher trabecular separation.

CONCLUSION

This study on sciuromorph trabeculae supplements the few non-primate studies on lifestyle-related functional adaptation of trabecular bone. We show that the architecture of the femoral head trabeculae in Sciuromorpha correlates with body mass and locomotor habits. Our findings provide a new basis for experimental research focused on functional significance of bone inner microstructure.

Proper Positioning and Restraint of a Rat Hind Limb for Focused High Resolution Imaging of Bone Micro-architecture Using In Vivo Micro-computed Tomography

The use of in vivo micro-computed tomography (µCT) is a powerful tool which involves the non-destructive imaging of internal structures at high resolutions in live animal models. This allows for repeated imaging of the same rodent over time. This feature not only reduces the total number of rodents required in an experimental design and thereby reduces the inter-subject variation that can arise, but also allows researchers to assess longitudinal or life-long responses to an intervention. To acquire high quality images that can be processed and analyzed to more accurately quantify outcomes of bone micro-architecture, users of in vivo µCT scanners must properly anesthetize the rat, and position and restrain the hind limb. To do this, it is imperative that the rat be anesthetized to a level of complete relaxation, and that pedal reflexes are lost. These guidelines may be modified for each individual rat, as the rate of isoflurane metabolism can vary depending on strain and body size. Proper technique for in vivo µCT image acquisition enables accurate and consistent measurement of bone micro-architecture within and across studies.

Preclinical Models for Skeletal Research: How Commonly Used Species Mimic (or Don’t) Aspects of Human Bone

Preclinical studies play an indispensable role in exploring the biological regulation of the musculoskeletal system. They are required in all drug development pipelines where both small and large animal models are needed to understand efficacy and side effects. This brief review highlights 4 aspects of human bone, longitudinal bone growth, intracortical remodeling, collagen/mineral interface, and age-related changes, and discusses how various animal models recapitulate (or don’t) these aspects of human skeletal physiology.

Evaluation of the effects of photobiomodulation on vertebras in two rat models of experimental osteoporosis

The aim of this study was to evaluate the effects of photobiomodulation (PBM) on cancellous bone in rat models of ovariectomized induced osteoporosis (OVX-D) and glucocorticoid-induced osteoporosis (GIOP). The experiment comprised of nine groups. A group of healthy rats was used for baseline evaluations. The OVX-D rats were further divided into groups as follows: control rats with osteoporosis, OVX-D rats that received alendronate (1 mg/kg 60 days), OVX-D rats treated with pulsed wave laser (890 nm, 80 Hz, 900 s, 0.0061 W/cm², 5.5 J/cm², three times a week, 60 days), and OVX-D rats treated with alendronate + pulsed laser. Dexamethasone was administered to the remaining rats that were split into four groups: control, alendronate-treated rats, laser-treated rats, and GIOP rats treated with alendronate + laser. T12, L1, L2, and L3 vertebrae were subjected to laser. Results of the current study demonstrated that OVX-D and GIOP significantly decreased some stereological parameters, and type 1 collagen gene expression compared to the healthy group. There was a significant increase in osteoclast number in both OVX-D and glucocorticoid administration compared to the healthy group. However, the detrimental effect of the OVX-D procedure on bone was more serious than glucocorticoid administration. Results showed that laser alone had a detrimental effect on trabecular bone volume, and cortical bone volume in groups GIOP and OVX-D compared to those in the healthy group. Alendronate significantly improved total vertebral bone volume, trabecular bone volume, and cortical bone volume, in GIOP and OVX-D groups compared to the laser-treated groups. Furthermore, the alendronate + laser in OVX-D rats and GIOP rats produced significantly increased osteoblast number and type 1 collagen gene expression and caused a significant decrease in osteoclast number compared to the controls.

Can the alendronate dosage be altered when combined with high-frequency loading in osteoporosis treatment?

Alendronate therapy has been associated with serious side effects. Altering the alendronate concentration and combining with high-frequency loading as mechanical intervention was explored in this animal study as a treatment for osteoporosis. The bone anabolic potency of high-frequency loading was overruled by the different alendronate dosages applied in the present study. Further exploration of reduced hormonal therapy associated with mechanical interventions in osteoporosis treatment should be sought.

INTRODUCTION

The aim of the present study was to investigate the effect of alendronate (ALN) administration at two different dosages, associated or not with high-frequency (HF) loading, on the bone microstructural response.

METHODS

Sixty-four female Wistar rats were used, of which 48 were ovariectomized (OVX) and 16 were sham-operated (shOVX). The OVX animals were divided into three groups: two groups were treated with alendronate, at a dosage of 2 mg/kg (ALN(₂)) or at a reduced dosage of 1 mg/kg (ALN(₁)) three times per week. A third OVX group did not receive pharmaceutical treatment. All four groups were mechanically stimulated via whole body vibration (WBV) at HF (up to 150 Hz) or left untreated (shWBV). ALN and HF were administered for 6 weeks, starting at 10-week post-(sh)OVX. Tibia bone structural parameters were analyzed using ex vivo microcomputed tomography.

RESULTS

Trabecular bone loss and structural deterioration resulting from ovariectomy were partially restored by ALN administration, demonstrated by the improvement of trabecular patter factor (Tb.Pf), trabecular separation (Tb.Sp), and structure model index (SMI) of the ALN groups compared to that of the OVX group, regardless of the applied dosage [ALN(₂) or ALN(₁)] or mechanical loading regime (shWBV or WBV). However, a significant positive effect of the ALN(₁) administration on trabecular (decrease of Tb.Sp and SMI) and cortical bone (increase of cortical thickness) microarchitecture compared to that of the OVX status group was observed for both loading regimes was not seen for ALN(₂). Furthermore, HF loading resulted in cortical bone changes, with an increased trabeculary area and endocortical perimeter. Finally, the benefits of a combined therapy of ALN with HF loading could not be discerned in the present experimental conditions.

CONCLUSIONS

The bone anabolic potency of HF loading was overruled by the ALN dosages applied in the present study. Further altering the ALN dosage combined with robust mechanical stimuli needs to be considered in osteoporosis research and eventually therapy.

Repeated irradiation from micro-computed tomography scanning at 2, 4 and 6 months of age does not induce damage to tibial bone microstructure in male and female CD-1 mice

Long-term effects of repeated in vivo micro-computed tomography (μCT) scanning at key stages of growth and bone development (ages 2, 4 and 6 months) on trabecular and cortical bone structure, as well as developmental patterns, have not been studied. We determined the effect of repetitive μCT scanning at age 2, 4 and 6 months on tibia bone structure of male and female CD-1 mice and characterized developmental changes. At 2, 4 and 6 months of age, right tibias were scanned using in vivo μCT (Skyscan 1176) at one of three doses of radiation per scan: 222, 261 or 460 mGy. Left tibias of the same mice were scanned only at 6 months to serve as non-irradiated controls to determine whether recurrent radiation exposure alters trabecular and cortical bone structure at the proximal tibia. In males, eccentricity was lower (P<0.05) in irradiated compared with non-irradiated tibias (222 mGy group). Within each sex, all other structural outcomes were similar between irradiated and non-irradiated tibias regardless of dose. Trabecular bone loss occurred in all mice due to age while cortical development continued to age 6 months. In conclusion, repetitive μCT scans at various radiation doses did not damage trabecular or cortical bone structure of proximal tibia in male and female CD-1 mice. Moreover, scanning at 2, 4 and 6 months of age highlight the different developmental time course between trabecular and cortical bone. These scanning protocols can be used to investigate longitudinal responses of bone structures to an intervention.

Necrostatin-1 treatment inhibits osteocyte necroptosis and trabecular deterioration in ovariectomized rats

Estrogen (E2) deficiency has been associated with accelerated osteocyte apoptosis. Our previous study showed necroptosis accelerated the loss of osteocytes in E2 deficiency-induced osteoporosis in rats in addition to apoptosis, but the mechanism involved remains. Necroptosis is a caspase-independent form of programmed cell death. In the necroptosis pathway, receptor interaction proteins 1 and 3 (RIP1/3) play vital roles. Necrostatin-1 (Nec-1) has been confirmed to be a specific inhibitor of necroptosis. However, the effect of Nec-1 on postmenopausal osteoporosis remains ambiguous. The aim of this study was to investigate the effect of Nec-1 on osteocytes in ovariectomized (OVX) rats. We found that an increased number of necroptotic osteocytes was related to the production of tumor necrosis factor-alpha (TNF-α) in OVX rats. Treatment with Nec-1 significantly decreased RIP1 and RIP3 expression in OVX rats and inhibited osteocyte necroptosis induced by TNF-α in vitro. Both E2 and Nec-1 treatment markedly ameliorated trabecular bone deterioration. Nec-1 also significantly elevated the levels of bone formation markers and decreased bone resorption markers. These data suggest that the role of Nec-1 on alleviating bone loss might be associated with Nec-1 restraining TNF-α-induced osteocyte necroptosis in rats with E2 deficiency-induced osteoporosis. This process may represent a novel therapeutic strategy for the treatment of postmenopausal osteoporosis.

Longitudinal Use of Micro-computed Tomography Does Not Alter Microarchitecture of the Proximal Tibia in Sham or Ovariectomized Sprague-Dawley Rats

In vivo micro-computed tomography (μCT) provides the ability to measure longitudinal changes to tibia microarchitecture, but the effect of this radiation is not well understood. The right proximal tibia of Sprague-Dawley rats (n = 12/group) randomized to Sham-control (Sham) or ovariectomy (OVX) surgery at 12 weeks of age was scanned using μCT at 13, 17, 21, and 25 weeks of age, at a resolution of 18 μm and a radiation dose of 603 mGy. The left proximal tibia was scanned only at 25 weeks of age to serve as an internal non-irradiated control. Repeated irradiation did not affect tibia microarchitecture in Sham or OVX groups, although there was an increase in cortical eccentricity (P < 0.05). All trabecular outcomes and cortical BMD were different (P < 0.05) between groups after only 1 week post-surgery and differences persisted to study endpoint. Characteristic changes to trabecular bone were observed in OVX rats over time. Interactions of time and hormone status were found for cortical BMD (P < 0.001), Ps. Pm., and Ec. Pm. (P < 0.05). Repeated irradiation of the tibia at 13, 17, 21, and 25 weeks does not cause adverse effects to microarchitecture, regardless of hormone status. This radiation dose can be applied over a typical 3-month study period to comprehensively understand how an intervention alters tibia microarchitecture without confounding effects of radiation.

The RIP1-RIP3 Complex Mediates Osteocyte Necroptosis after Ovariectomy in Rats

Osteocyte apoptosis has been reported to play a central role in bone remodeling. In addition to apoptosis, other mechanisms may be involved in osteocyte loss. This study aimed to investigate the effect of necroptosis on osteocytes in ovariectomized (OVX) rats. Ninety-six female Sprague-Dawley rats were randomly divided into an OVX group and a sham group. At 0, 4, 8 and 12 weeks after surgery, specimens from each group (n = 12 each) were harvested. Bone mineral density (BMD) and body weight were measured. Transmission electron microscopy (TEM) and micro-CT were used to observe the changes in cellular morphology and bone microarchitecture induced by estrogen deficiency. Osteocyte apoptosis and necroptosis were evaluated via TUNEL and immunofluorescence staining for active caspase-3. At 8 weeks after ovariectomy, a greater number of osteocytes with typical necrotic morphological features were TUNEL positive but negative for active caspase-3. Western blotting, quantitative real-time PCR and immunofluorescence assessments demonstrated that the levels of receptor-interacting serine/threonine protein kinase 1 (RIP1) and RIP3 in osteocytes were significantly increased at 8 weeks after ovariectomy. These data are the first to suggest that necroptosis accelerates osteocyte loss under conditions of estrogen deficiency-induced osteoporosis in OVX rats. These findings provide evidence of a potential mechanism through which osteocyte necroptosis is associated with postmenopausal osteoporosis.

Minimizing Interpolation Bias and Precision Error in In Vivo µCT-Based Measurements of Bone Structure and Dynamics

In vivo µCT imaging allows for high-resolution, longitudinal evaluation of bone properties. Based on this technology, several recent studies have developed in vivo dynamic bone histomorphometry techniques that utilize registered µCT images to identify regions of bone formation and resorption, allowing for longitudinal assessment of bone remodeling. However, this analysis requires a direct voxel-by-voxel subtraction between image pairs, necessitating rotation of the images into the same coordinate system, which introduces interpolation errors. We developed a novel image transformation scheme, matched-angle transformation (MAT), whereby the interpolation errors are minimized by equally rotating both the follow-up and baseline images instead of the standard of rotating one image while the other remains fixed. This new method greatly reduced interpolation biases caused by the standard transformation. Additionally, our study evaluated the reproducibility and precision of bone remodeling measurements made via in vivo dynamic bone histomorphometry. Although bone remodeling measurements showed moderate baseline noise, precision was adequate to measure physiologically relevant changes in bone remodeling, and measurements had relatively good reproducibility, with intra-class correlation coefficients of 0.75-0.95. This indicates that, when used in conjunction with MAT, in vivo dynamic histomorphometry provides a reliable assessment of bone remodeling.

Altered mechanical environment of bone cells in an animal model of short- and long-term osteoporosis

Alterations in bone tissue composition during osteoporosis likely disrupt the mechanical environment of bone cells and may thereby initiate a mechanobiological response. It has proved challenging to characterize the mechanical environment of bone cells in vivo, and the mechanical environment of osteoporotic bone cells is not known. The objective of this research is to characterize the local mechanical environment of osteocytes and osteoblasts from healthy and osteoporotic bone in a rat model of osteoporosis. Using a custom-designed micromechanical loading device, we apply strains representative of a range of physical activity (up to 3000 με) to fluorescently stained femur samples from normal and ovariectomized rats. Confocal imaging was simultaneously performed, and digital image correlation techniques were applied to characterize cellular strains. In healthy bone tissue, osteocytes experience higher maximum strains (31,028 ± 4213 με) than osteoblasts (24,921 ± 3,832 με), whereas a larger proportion of the osteoblast experiences strains >10,000 με. Most interestingly, we show that osteoporotic bone cells experience similar or higher maximum strains than healthy bone cells after short durations of estrogen deficiency (5 weeks), and exceeded the osteogenic strain threshold (10,000 με) in a similar or significantly larger proportion of the cell (osteoblast, 12.68% vs. 13.68%; osteocyte, 15.74% vs. 5.37%). However, in long-term estrogen deficiency (34 weeks), there was no significant difference between bone cells in healthy and osteoporotic bone. These results suggest that the mechanical environment of bone cells is altered during early-stage osteoporosis, and that mechanobiological responses act to restore the mechanical environment of the bone tissue after it has been perturbed by ovariectomy.

Molecular mechanisms of osteoporotic hip fractures in elderly women

A common manifestation of age-related bone loss and resultant osteoporosis are fractures of the hip. Age-related osteoporosis is thought to be determined by a number of intrinsic factors including genetics, hormonal changes, changes in levels of oxidative stress, or an inflammatory status associated with the aging process. The aim of this study was to investigate gene expression and bone architecture in bone samples derived from elderly osteoporotic women with hip fractures (OP) in comparison to bone samples from age matched women with osteoarthritis of the hip (OA). Femoral heads and adjacent neck tissue were collected from 10 women with low-trauma hip fractures (mean age 83±6) and consecutive surgical hip replacement. Ten bone samples from patients undergoing hip replacement due to osteoarthritis (mean age 80±5) served as controls. One half of each bone sample was subjected to gene expression analysis. The second half of each bone sample was analyzed by microcomputed tomography. From each half, samples from four different regions, the central and subcortical region of the femoral head and neck, were analyzed. We could show a significantly decreased expression of the osteoblast related genes RUNX2, Osterix, Sclerostin, WNT10B, and Osteocalcin, a significantly increased ratio of RANKL to Osteoprotegerin, and a significantly increased expression of the enzymes superoxide dismutase 2 (SOD2) and glutathione peroxidase GPX3, and of the inflammatory cytokine IL6 in bone samples from hip fracture patients compared to controls. Major microstructural changes in OP bone were seen in the neck and were characterized by a significant decrease of bone volume, trabecular number, and connectivity density and a significant increase of trabecular separation. In conclusion, our data give evidence for a decreased expression of osteoblast related genes and increased expression of osteoclast related genes. Furthermore, increased expression of SOD2 and GPX3 suggest increased antioxidative activity in bone samples from elderly osteoporotic women with hip fractures.

Differential β3 and β1 Integrin Expression in Bone Marrow and Cortical Bone of Estrogen Deficient Rats

Integrin-based (β3 ) attachments to the extracellular matrix (ECM) on osteocyte cell processes have recently been proposed to play an important role in facilitating osteocyte mechanosensation. However, it is not yet known whether integrin expression is altered in the mechanoregulatory osteocytes during osteoporosis. The objective of this study was to test the hypothesis that the expression of integrin-based mechanosensory complexes (β1 and β3 integrins) is altered as a direct response to estrogen deficiency, in an estrogen deficient animal model of osteoporosis. Four weeks post-operatively, immunohistochemistry was used to detect for β1 and β3 integrin subunits in bone tissue and marrow of ovariectomized (OVX; N = 4) and SHAM (N = 4) operated animals. A tartrate resistant acid phosphatase (TRAP) control stain was performed to quantify the presence of osteoclasts in the bone marrow and bone surfaces. Image analysis was performed to quantify expression patterns in different biological compartments, that is, bone marrow, endosteum, and cortical bone. Our results showed that β1 integrins were ubiquitously expressed throughout the bone and marrow, for both OVX and SHAM groups. β3 integrin subunit expression was lower in bone cells from osteoporotic animals compared to controls, whereas β3 expression in marrow cells did not differ significantly between groups. At the endosteum no difference was observed in β3 integrin subunit expression. As expected, the number of osteoclasts was higher in the OVX group validating an imbalance in bone remodeling. We propose that a reduction in β3 integrin expression in osteocytes might impair mechanosensation by bone cells during estrogen deficiency.

Multiscale Modeling of Trabecular Bone Marrow: Understanding the Micromechanical Environment of Mesenchymal Stem Cells During Osteoporosis

Mechanical loading directs the differentiation of mesenchymal stem cells (MSCs) in vitro and it has been hypothesized that the mechanical environment plays a role in directing the cellular fate of MSCs in vivo. However, the complex multicellular composition of trabecular bone marrow means that the precise nature of mechanical stimulation that MSCs experience in their native environment is not fully understood. In this study, we developed a multiscale model that discretely represents the cellular constituents of trabecular bone marrow and applied this model to characterize mechanical stimulation of MCSs in vivo. We predicted that cell-level strains in certain locations of the trabecular marrow microenvironment were greater in magnitude (maximum ε12 = ∼24,000 με) than levels that have been found to result in osteogenic differentiation of MSCs in vitro (>8000 με), which may indicate that the native mechanical environment of MSCs could direct cellular fate in vivo. The results also showed that cell–cell adhesions could play an important role in mediating mechanical stimulation within the MSC population in vivo. The model was applied to investigate how changes that occur during osteoporosis affected mechanical stimulation in the cellular microenvironment of trabecular bone marrow. Specifically, a reduced bone volume (BV) resulted in an overall increase in bone deformation, leading to greater cell-level mechanical stimulation in trabecular bone marrow (maximum ε12 = ∼48,000 με). An increased marrow adipocyte content resulted in slightly lower levels of stimulation within the adjacent cell population due to a shielding effect caused by the more compliant behavior of adipocytes (maximum ε12 = ∼41,000 με). Despite this reduction, stimulation levels in trabecular bone marrow during osteoporosis remained much higher than those predicted to occur under healthy conditions. It was found that compensatory mechanobiological responses that occur during osteoporosis, such as increased trabecular stiffness and axial alignment of trabeculae, would be effective in returning MSC stimulation in trabecular marrow to normal levels. These results have provided novel insight into the mechanical stimulation of the trabecular marrow MSC population in both healthy and osteoporotic bone, and could inform the design three-dimensional (3D) in vitro bioreactor strategies techniques, which seek to emulate physiological conditions.

Trabecular bone recovers from mechanical unloading primarily by restoring its mechanical function rather than its morphology

Upon returning to normal ambulatory activities, the recovery of trabecular bone lost during unloading is limited. Here, using a mouse population that displayed a large range of skeletal susceptibility to unloading and reambulation, we tested the impact of changes in trabecular bone morphology during unloading and reambulation on its simulated mechanical properties. Female adult mice from a double cross of BALB/cByJ and C3H/HeJ strains (n=352) underwent 3wk of hindlimb unloading followed by 3wk of reambulation. Normally ambulating mice served as controls (n=30). As quantified longitudinally by in vivo μCT, unloading led to an average loss of 43% of trabecular bone volume fraction (BV/TV) in the distal femur. Finite element models of the μCT tomographies showed that deterioration of the trabecular structure raised trabecular peak Von-Mises (PVM) stresses on average by 27%, indicating a significant increase in the risk of mechanical failure compared to baseline. Further, skewness of the Von-Mises stress distributions (SVM) increased by 104% with unloading, indicating that the trabecular structure became inefficient in resisting the applied load. During reambulation, bone of experimental mice recovered on average only 10% of its lost BV/TV. Even though the addition of trabecular tissue was small during reambulation, PVM and SVM as indicators of risk of mechanical failure decreased by 56% and 57%, respectively. Large individual differences in the response of trabecular bone, together with a large sample size, facilitated stratification of experimental mice based on the level of recovery. As a fraction of all mice, 66% of the population showed some degree of recovery in BV/TV while in 89% and 87% of all mice, PVM and SVM decreased during reambulation, respectively. At the end of the reambulation phase, only 8% of the population recovered half of the unloading induced losses in BV/TV while 50% and 49% of the population recovered half of the unloading induced deterioration in PVM and SVM, respectively. The association between morphological and mechanical variables was strong at baseline but progressively decreased during the unloading and reambulation cycles. The preferential recovery of trabecular micromechanical properties over bone volume fraction emphasizes that mechanical demand during reambulation does not, at least initially, seek to restore bone's morphology but its mechanical integrity.

Inhibited osteoclastic bone resorption through alendronate treatment in rats reduces severe osteoarthritis progression

Osteoarthritis (OA) is a non-rheumatoid joint disease characterized by progressive degeneration of extra-cellular cartilage matrix (ECM), enhanced subchondral bone remodeling, osteophyte formation and synovial thickening. Alendronate (ALN) is a potent inhibitor of osteoclastic bone resorption and results in reduced bone remodeling. This study investigated the effects of pre-emptive use of ALN on OA related osteoclastic subchondral bone resorption in an in vivo rat model for severe OA. Using multi-modality imaging we measured effects of ALN treatment within cartilage and synovium. Severe osteoarthritis was induced in left rat knees using papain injections in combination with a moderate running protocol. Twenty rats were treated with subcutaneous ALN injections and compared to twenty untreated controls. Animals were longitudinally monitored for 12weeks with in vivo μCT to measure subchondral bone changes and SPECT/CT to determine synovial macrophage activation using a folate-based radiotracer. Articular cartilage was analyzed at 6 and 12weeks with ex vivo contrast enhanced μCT and histology to measure sulfated-glycosaminoglycan (sGAG) content and cartilage thickness. ALN treatment successfully inhibited subchondral bone remodeling. As a result we found less subchondral plate porosity and reduced osteophytosis. ALN treatment did not reduce subchondral sclerosis. However, after the OA induction phase, ALN treatment protected cartilage ECM from degradation and reduced synovial macrophage activation. Surprisingly, ALN treatment also improved sGAG content of tibia cartilage in healthy joints. Our data was consistent with the hypothesis that osteoclastic bone resorption might play an important role in OA and may be a driving force for progression of the disease. However, our study suggest that this effect might not solely be effects on osteoclastic activity, since ALN treatment also influenced macrophage functioning. Additionally, ALN treatment and physical activity exercised a positive effect in healthy control joints, which increased cartilage sGAG content. More research on this topic might lead to novel insights as to improve cartilage quality.

The use of micro-CT to study bone architecture dynamics noninvasively

High-resolution micro-CT has become a standard tool in the evaluation of bone architecture. It has recently progressed from an invasive tool for bone specimens into an in vivo tool for small animals. The combination of novel sophisticated evaluation methods, such as registration (matching) of sequential scans and computer simulation models will further evolve in vivo micro-CT into an optimal tool for small animal phenotyping and contemporary approaches for drug discovery relating to the skeleton.:

3D image registration is critical to ensure accurate detection of longitudinal changes in trabecular bone density, microstructure, and stiffness measurements in rat tibiae by in vivo microcomputed tomography (μCT)

In the recent decade, in vivo μCT scanners have become available to monitor temporal changes in rodent bone in response to diseases and treatments. We investigated short-term and long-term precision of in vivo μCT measurements of trabecular bone density, microstructure and stiffness of rat tibiae and tested whether they can be improved by 3D image registration. Rats in the short-term precision group underwent baseline and follow-up scans within the same day (n = 15) and those in the long-term precision group were scanned at day 0 and day 14 (n = 16) at 10.5 μm voxel size. A 3D image-registration scheme was applied to register the trabecular bone compartments of baseline and follow-up scans. Prior to image registration, short-term precision ranged between 0.85% and 2.65% in bone volume fraction (BV/TV), trabecular number, thickness, and spacing (Tb.N*, Tb.Th*, Tb.Sp*), trabecular bone mineral density and tissue mineral density (Tb.BMD, and Tb.TMD), and was particularly high in structure model index (SMI), connectivity density (Conn.D), and stiffness (4.29%-8.83%). Image registration tended to improve the short-term precision, but the only statistically significant improvement was in Tb.N*, Tb.TMD, and stiffness. On the other hand, unregistered comparisons between day-0 and day-14 scans suggested significant increases in BV/TV, Tb.N*, Tb.Th*, Conn.D, and Tb.BMD and decrease in Tb.Sp* and SMI. However, the percent change in each parameter from registered comparisons was significantly different from unregistered comparisons. Registered results suggested a significant increase in BV/TV, Tb.BMD, and stiffness over 14 days, primarily caused by increased Tb.Th* and Tb.TMD. Due to the continuous growth of rodents, the direct comparisons between the unregistered baseline and follow-up scans were driven by changes due to global bone modeling instead of local remodeling. Our results suggested that 3D image registration is critical for detecting changes due to bone remodeling activities in rodent trabecular bone by in vivo μCT imaging.

Estimation of an early meaningful time point of bone parameter changes in application to an osteoporotic rat model with in vivo microcomputed tomography measurements

The commonly used preclinical animal model of postmenopausal osteoporosis is the mature ovariectomized rat, whereby cessation of ovarian oestrogen production consequently results in bone volume reduction. The study aim was to precisely define the time course of structural changes resulting from ovariectomy and thereby reduce the time animals have to be treated to judge the effects of osteoporosis treatment. For this purpose, we assessed architectural changes by microcomputed tomography ( μCT) during 10 weeks following ovariectomy or sham surgery at two-week intervals. Moreover, the trabecular microarchitecture of the lumbar vertebrae was assessed after necropsy. Besides this, serum biomarkers of bone turnover were determined. These data were in a new approach additionally correlated to femur mRNA expression profiles. We selected the osteoblast marker genes osteocalcin and type I collagen as well as the two osteoclast marker genes cathepsin k and tartrate-resistant acid phosphatase 5. The gene expression analysis suggested an activation of osteoblasts as well as octeoclasts. The significantly induced serum levels of osteocalcin and collagen degradation fragments also revealed this higher rate of bone turnover. Our results indicate that as soon as four weeks after ovariectomy the bone volume fraction exhibited a decline of 30% and 50% of the connectivity density. In addition, significant decreases of trabecular number and thickness as well as of the bone volume fraction were only observed in vertebrae of ovariectomized animals. Interestingly, changes of trabecular morphology were also found in the sham animals as a consequence of senescence.

Difference in intraosseous blood vessel volume and number in osteoporotic model mice induced by spinal cord injury and sciatic nerve resection

In the present study, we examined intraosseous blood vessel parameters of the tibial metaphysis in mice using microcomputed tomography (µCT) to investigate the relationship between post-nerve-injury osteoporosis and local intraosseous blood vessel volume and number. Mice were randomly divided into groups receiving spinal cord injury (SCI), sciatic nerve resection group (NX), or intact controls (30 mice/group). Four weeks after surgery, mice were perfused with silicone and the distribution of intraosseous blood vessels analyzed by μCT. The bone density, μCT microstructure, biomechanical properties, and the immunohistochemical and biochemical indicators of angiogenesis were also measured. The SCI group showed significantly reduced tibial metaphysis bone density, μCT bone microstructure, tibial biomechanical properties, indicators of angiogenesis, and intraosseous blood vessel parameters compared to the NX group. Furthermore, the spinal cord-injured mice exhibited significantly decreased intraosseous blood vessel volume and number during the development of osteoporosis. In conclusion, these data suggest that decreased intraosseous blood vessel volume and number may play an important role in the development of post-nerve-injury osteoporosis.

Differential age-related bone architecture changes between female and male STR/Ort mice

The incidence of spontaneous osteoarthritis (OA) in female STR/Ort mice is much lower than that observed in male STR/Ort mice; however, the reason for the differential incidence of OA between sexes has not been elucidated. Here, we investigated and compared age- and sex-related bone mineral density and architectural changes in male and female STR/Ort mice. Bone architecture and bone mineral density (BMD) of femurs were examined in 5-, 10-, 15-, 20-, and 35-week-old male and female STR/Ort mice by microscopic computed tomography (µCT). Angular degrees of internal tibial torsion (ADITT) were also measured in mice at 5, 15, and 35 weeks of age. Earlier decreases of cancellous volume and BMD were found in male STR/Ort mice. Using µCT, an age-related decline of bone marrow space in femoral diaphysis was observed in both males and females but was more dramatic in females. In addition, an earlier increase of ADITT was observed in male STR/Ort mice, suggesting that internal rotation of the tibia may contribute to OA. Age- and sex-related bone architectural changes clearly differ between male and female STR/Ort mice. These differences in bone structure, particularly ADITT, may explain the differential incidence of OA in STR/Ort mice.

Bone loss following spinal cord injury in a rat model

The current study was undertaken to follow the time course of bone loss in the proximal tibia of rats over several weeks following thoracic contusion spinal cord injury (SCI) of varying severity. It was hypothesized that bone loss would be more pronounced in the more severely injured animals, and that hindlimb weight bearing would help prevent bone loss. Twenty-six female Sprague-Dawley rats (200-225 g, 6-7 weeks old) received standard thoracic (T9) injuries at energies of 6.25, 12.5, 25, or 50 g-cm. The rats were scored weekly for hindlimb function during locomotion. At 0, 2 or 3, and 8 weeks, high-resolution micro-CT images of each right tibia were obtained. Mechanical indentation testing was done to measure the compressive strength of the cancellous bone structure. The 6.25 g-cm group showed near normal locomotion, the 12.5 and 25 g-cm groups showed the ability to frequently or occasionally generate weight-supported plantar steps, respectively, and the 50 g-cm group showed only movement without weight-supported plantar stepping. The 6.25, 12.5 and 25 g-cm groups remained at the same level of bone volume fraction (cancBV/TV=0.24±0.07), while the 50 g-cm group experienced severe bone loss (67%), resulting in significantly lower (p<0.05) bone volume fraction (cancBV/TV=0.11±0.05) at 8 weeks. Proximal tibia cancellous bone strength was reduced by approximately 50% in these severely injured rats. Instead of a linear proportionality between injury severity and bone loss, there appears to be a distinct functional threshold, marked by occasional weight-supported stepping, above which bone loss does not occur.

Longitudinal assessment of in vivo bone dynamics in a mouse tail model of postmenopausal osteoporosis

Recently, it has been shown that transient bone biology can be observed in vivo using time-lapse micro-computed tomography (μCT) in the mouse tail bone. Nevertheless, in order for the mouse tail bone to be a model for human disease, the hallmarks of any disease must be mimicked. The aim of this study was to investigate whether postmenopausal osteoporosis could be modeled in caudal vertebrae of C57Bl/6 mice, considering static and dynamic bone morphometry as well as mechanical properties, and to describe temporal changes in bone remodeling rates. Twenty C57Bl/6 mice were ovariectomized (OVX, n = 11) or sham-operated (SHM, n = 9) and monitored with in vivo μCT on the day of surgery and every 2 weeks after, up to 12 weeks. There was a significant decrease in bone volume fraction for OVX (-35%) compared to SHM (+16%) in trabecular bone (P < 0.001). For OVX, high-turnover bone loss was observed, with the bone resorption rate exceeding the bone formation rate (P < 0.001). Furthermore there was a significant decrease in whole-bone stiffness for OVX (-16%) compared to SHM (+11%, P < 0.001). From these results we conclude that the mouse tail vertebra mimics postmenopausal bone loss with respect to these parameters and therefore might be a suitable model for postmenopausal osteoporosis. When evaluating temporal changes in remodeling rates, we found that OVX caused an immediate increase in bone resorption rate (P < 0.001) and a delayed increase in bone formation rate (P < 0.001). Monitoring transient bone biology is a promising method for future research.

Cortical bone resorption following muscle paralysis is spatially heterogeneous

Mechanical loading of the skeleton, as induced by muscle function during activity, plays a critical role in maintaining bone homeostasis. It is not understood, however, whether diminished loading (and thus diminished mechanical stimuli) directly mediates the bone resorption that is associated with disuse. Our group has recently developed a murine model in which we have observed rapid and profound bone loss in the tibia following transient paralysis of the calf muscles. As cortical bone loss is achieved via rapid endocortical expansion without alterations in periosteal morphology, we believe this model holds unique potential to explore the spatial relation between altered mechanical stimuli and subsequent bone resorption. Given the available literature, we hypothesized that endocortical resorption following transient muscle paralysis would be spatially homogeneous. To test this hypothesis, we first validated an image registration algorithm that quantified site-specific cortical bone alterations with high precision and accuracy. We then quantified endocortical expansion in the tibial diaphysis within 21 days following transient muscle paralysis and found that, within the analyzed mid-diaphyseal region (3.15 mm), site-specific bone loss was focused on the anterior surface in the proximal region but shifted to the posterior surface at the distal end of the analyzed volume. This site-specific, and highly repeatable biologic response suggests active osteoclast chemotaxis or focal activation of osteoclastic resorption underlies the spatially consistent endocortical resorption induced by transient muscle paralysis. Clarifying this relation holds potential to yield unique insight into how the removal of factors critical for bone homeostasis acutely precipitates local modulation of cellular responses within bone.

Functional interactions among morphologic and tissue quality traits define bone quality

BACKGROUND

Advances in diagnostic and treatment regimens that aim to reduce fracture incidence will benefit from a better understanding of how bone morphology and tissue quality define whole-bone mechanical properties.

QUESTIONS/PURPOSES

The goal of this article was to review what is known about the interactions among morphologic and tissue quality traits and how these interactions contribute to bone quality (ie, whole-bone mechanical function). Several questions were addressed. First, how do interactions among morphology and tissue quality traits relate to functional adaptation? Second, what are the emergent patterns of functionally adapted trait sets in long bones? Third, how effective is phenotypic integration at establishing function across a population? Fourth, what are the emergent patterns of functionally adapted trait sets in corticocancellous structures? Fifth, how do functional interactions change with aging?

METHODS

A literature review was conducted with papers identified primarily through citations listed in reference sections as well as general searches using Google Scholar and PubMed.

RESULTS

The interactions among adult traits or phenotypic integration are an emergent property of the compensatory mechanisms complex systems used to establish function or homeostasis. Traits are not regulated independently but vary simultaneously (ie, covary) in specific ways to establish function. This covariation results in individuals acquiring unique sets of traits to establish bone quality.

CONCLUSIONS AND CLINICAL RELEVANCE

Biologic constraints imposed on the skeletal system result in a population showing a pattern of trait sets that is predictable based on external bone size and that can be used to identify individuals with reduced bone quality relative to their bone size and body size.

Simulated evolution of the vertebral body based on basic multicellular unit activities

A numerical model based on the theory of bone remodeling is proposed to predict the evolution of trabecular bone architecture within the vertebral body and to investigate the process of degeneration in vertebral bone. In this study, particular attention is paid on the description of microstructure changes during the aging process. To take into account the effect of basic multicellular units (BMUs), a set of computational algorithms has been developed. It is assumed that BMU activation probability depends on the state of damaged bone tissue (damage accumulation, ω), which is evaluated according to previous research concerning bone fatigue damage. Combining these algorithms with the finite-element method (FEM), the microstructure of vertebral bone has been predicted for up to 8 simulated years. Moreover, biomechanical material properties have been monitored to investigate the changes of vertebral bone with age. This study shows that the simulation based on BMU activities has the potential to define and predict the morphological evolution of the vertebral body. It can be concluded that the novel algorithms incorporating the coupled effects of both adaptive remodeling and microdamage remodeling could be utilized to gain greater insight into the mechanism of bone loss in the elderly population.

In vivo CT quantification of trabecular bone dynamics in mice after sciatic neurectomy using monochromatic synchrotron radiation

We demonstrated the capability of in vivo synchrotron radiation CT (SRCT) in analyzing short-term changes in trabecular bone architecture (TBA) and the degree of bone mineralization (DBM) in small animals. Mice underwent unilateral sciatic neurectomy (SN) and sham operation on the contralateral side (SO) at 13 weeks of age. In vivo SRCT scans (11.7-μm cubic voxel) were made of both knees 7 and 17 days (group 1, n = 7) or only 17 days (group 2, n = 6) after surgery. In three mice in group 2, one knee was scanned twice on the same day in different orientations for reproducibility testing. Two scan data sets of the tibial proximal metaphysis acquired at different time points (group 1) or at the same time point (group 2) were registered for detecting differences in volume fraction (BV/TV), trabecular thickness (Tb.Th), trabecular number (Tb.N), connectivity density (Conn.D), and mean DBM (mDBM). The reproducibility test showed small errors of <2.5% in the TBA indexes and <3.0% in mDBM, while mismatched bone regions amounted to >25%. In group 1, Tb.Th increased but Tb.N and Conn.D decreased in both SN and SO; BV/TV and mDBM increased only in SO; accordingly, BV/TV, Tb.Th, and mDBM became lower in SN than in SO. No significant interaction between SN and irradiation was found; the SN effects on TBA and DBM were similar between groups 1 and 2, although synchrotron irradiation led to higher Tb.Th and lower Tb.N in group 1. In conclusion, in vivo SRCT has potential use for detecting short-term bone dynamics of small animals.

Reduced local blood supply to the tibial metaphysis is associated with ovariectomy-induced osteoporosis in mice

OBJECTIVE

To investigate angiogenesis of the tibial metaphysis in ovariectomized mice with microcomputed tomography, as well as to detect the expression of vascular endothelial growth factor (VEGF) in the metaphysis, and to explore the relationship between osteoporosis and local blood supply to bones.

METHODS

Sixty mice were randomly divided into an ovariectomy group (n = 30) and a control group (n = 30). Four weeks after ovariectomy, the mice were killed and the distribution of vessels in the tibial metaphysis was determined after silicone rubber perfusion. In addition, the expression of VEGF of the tibial metaphysis was immunohistochemically determined and bone mineral density, microarchitecture, and biomechanics were tested.

RESULTS

The bone mineral density, biomechanical parameters, number of microvessels, and expression of VEGF were significantly reduced in the tibial metaphysis of ovariectomized mice, whose bone microarchitecture was also disrupted.

CONCLUSION

In this study, it was found that reduced local blood supply to the tibial metaphysis may be associated with ovariectomy-induced osteoporosis.

Relationship between age, skeletal site, and time post-ovariectomy on bone mineral and trabecular microarchitecture in rats

The ovariectomized (OVX) rat is widely used in osteoporosis research, but no standard model exists. The individual effects of rat age, skeletal site, and time post-ovariectomy (post-OVX) on bone have been examined. However, the relationship between them is not yet fully explored. This study examined how various combinations of rat age, skeletal site, and time post-OVX affect bone mineral and microarchitecture. The rats used were 12 (n = 28), 24 (n = 28), and 44 (n = 31) weeks old. In each age group, approximately half underwent OVX and other half underwent Sham surgeries. Bone mineral (content and density) and trabecular morphology was assessed at 2, 5, 10, 15, 20, 25, and 30 weeks post-surgery. Sites examined included the proximal tibia, spine, distal femur, and proximal femur. Overall, the proximal tibia showed the earliest and greatest differences between OVX and Sham groups. The 24-week-old group showed the best osteoporotic response. The 12-week-old group showed growth effects, whilst the 44-week-old group showed aging effects. The response of certain sites to OVX was also found to depend on the rat age used. These findings may aid in explaining discrepancies reported in the literature as well as synergistic combinations that may signify advanced conditions. © 2010 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 29:189-196, 2011.

Oestrogen is important for maintenance of cartilage and subchondral bone in a murine model of knee osteoarthritis

INTRODUCTION

Oestrogen depletion may influence onset and/or progression of osteoarthritis. We investigated in an ovariectomized mouse model the impact of oestrogen loss and oestrogen supplementation on articular cartilage and subchondral bone in tibia and patella, and assessed bone changes in osteoarthritis development.

METHODS

C3H/HeJ mice were divided into four groups: sham-operated, oestrogen depletion by ovariectomy (OVX), OVX with estradiol supplementation (OVX+E) and OVX with bisphosphonate (OVX+BP). Each mouse had one knee injected with low-dose iodoacetate (IA), and the contralateral knee was injected with saline. Cartilage was analysed histologically 12 weeks postsurgery; bone changes were monitored over time using in vivo micro-computed tomography.

RESULTS

In tibiae, OVX alone failed to induce cartilage damage, but OVX and IA combination significantly induced cartilage damage. In patellae, OVX alone induced significant cartilage damage, which was enhanced by IA. In both tibiae and patellae, OVX in combination with IA significantly decreased subchondral cortical thickness in an additive manner. OVX+E and OVX+BP inhibited tibial and patellar subchondral cortical thinning, inhibited patellar and tended to diminish tibial cartilage damage. In patellae, IA interacted with BP, leading to increased subchondral cortical and trabecular bone.

CONCLUSIONS

This study demonstrates the significance of oestrogen for articular cartilage and subchondral bone and maintenance of healthy joints, supporting an etiological role for altered oestrogen signaling in osteoarthritis either by directly affecting cartilage or increasing susceptibility for an osteoarthritis trigger. The data strongly support the concept of involvement of subchondral bone plate in osteoarthritis.

Anabolic and antiresorptive drugs improve trabecular microarchitecture and reduce fracture risk following radiation therapy

Many patients with symptomatic bone metastases receive radiation therapy, even though radiation is known to have potential adverse effects on bone. We hypothesized that the concurrent use of a bisphosphonate drug (zoledronic acid, ZA) or a combination of ZA plus an anabolic agent (parathyroid hormone, PTH) would lead to improvements in the microarchitecture and mechanical properties of irradiated bone. Human breast cancer cells were injected into the distal femur of 56 female nude mice, which were then divided into four groups: no treatment (0 Gy), radiation administered 4 weeks postinjection (20 Gy), radiation plus ZA (12.5 microg/kg weekly from weeks 4 to 12) (20 Gy + ZA), and radiation followed by ZA (25 microg/kg weekly from weeks 4 to 8) and PTH(1-34) (100 microg microg/kg daily from weeks 8 to 12) (20 Gy + ZA + PTH). Left limbs served as normal control bones. Bone loss over the 12-week study was tracked with serial radiography and bone densitometry. At the end of the study, micro-computed tomography and mechanical testing were used to quantify bone microarchitecture and bone strength. Radiation alone failed to prevent tumor-induced decreases in bone mineral density (BMD), trabecular bone volume, and bone strength. Treatment with 20 Gy + ZA or 20 Gy + ZA + PTH as adjuncts to radiation was effective at preserving trabecular bone architecture and bone strength at normal levels. ZA reduced the risk of mechanical fragility following irradiation of a lytic bone lesion. Supplemental use of PTH did not result in further increases in bone strength but was associated with significant increases in BMD and bone mass, suggesting that it may be beneficial in enhancing bone architecture following radiation therapy.

Detecting early bone changes using in vivo micro-CT in ovariectomized, zoledronic acid-treated, and sham-operated rats

SUMMARY

This study monitored in vivo the effect on bone microarchitecture of initiating antiresorptive treatment with zoledronic acid in rats at 2 weeks following ovariectomy, an early phase at which major degenerative bone changes have been found to occur. The treatment still facilitated the full reversal of cancellous bone loss in rat tibia, highlighting the importance of the time point of initiation of antiresorptive treatment.

INTRODUCTION

Injection of zoledronic acid in rats at time of ovariectomy has been found to fully preserve tibial bone microarchitecture over time, whereas injection at 8 weeks after ovariectomy has shown partial bone recovery. This study investigated the effect on microarchitecture of initiating antiresorptive treatment in the early phase following ovariectomy, at 2 weeks, a time point at which major degenerative changes in the bone have been found to occur.

METHODS

Female Sprague-Dawley rats were divided into ovariectomized group, ovariectomized group treated with zoledronic acid, and sham-operated group. In vivo micro-CT scanning of rat tibiae and morphometric analysis were performed at 0, 2, 4, 8, and 12 weeks after ovariectomy, with zoledronic acid treatment beginning 2 weeks after ovariectomy. Data were first analyzed with repeated measures analysis of variance (longitudinal study design) and then without repeated measures (cross-sectional study design).

RESULTS

The ovariectomized group demonstrated dramatic bone loss, first detected at week 2. Conversely, at week 4, the zoledronic acid-treated group returned microstructural parameters to baseline values. Remarkable increases in bone parameters were found after 6 weeks of treatment and maintained similar to sham group until the end. The longitudinal study design provided earlier detection of bone changes compared to the cross-sectional study design.

CONCLUSIONS

Treatment with zoledronic acid as late as 2 weeks after ovariectomy still facilitates the full reversal of cancellous bone loss in the rat tibia.

Discordant effects of vitamin D deficiency in trabecular and cortical bone architecture and strength in growing rodents

We have previously shown that vitamin D deficiency in young male rats results in significant reduction in femoral trabecular bone volume (BV/TV). However, the effects of vitamin D deficiency and its impact on other relevant skeletal sites remain unclear. Ten week old male Sprague-Dawley rats were fed various levels of vitamin D3 (2, 4, 8, and 12 IU/day) with standard Ca (0.4%) until 30 weeks of age and achieved stable serum 25-hydroxyvitamin D3 (25D) levels between 16 and 117 nmol/L. At time of death, femora, L2 vertebrae and tibiae were processed for bone histomorphometric analyses and tibial cortical strength by 3-point mechanical testing. A significant association between serum 25D and trabecular bone occurred for both the distal femoral metaphysis (R2=0.34, P<0.05) and L2 vertebrae (R2=0.24, P<0.05). Tibia mid-shaft cortical bone was not, however, changed in terms of total volume, periosteal surface or endosteal surface as a function of vitamin D status. Furthermore, no changes to mechanical and intrinsic properties of the cortices were observed. We conclude that cortical bone is maintained under conditions of vitamin D deficiency in preference to cancellous bone in young growing rats.

Mechanobiological regulation of the remodelling cycle in trabecular bone and possible biomechanical pathways for osteoporosis

BACKGROUND

The rapid loss of trabeculae as observed during osteoporosis is attributed to pathological changes in the bone remodelling process. In this study, it is proposed that osteoporosis is due to altered signals resulting from either (i) a decrease in the mechanosensitivity of the sensor cells or (ii) an increase in the bone tissue elastic modulus.

METHODS

To test these hypotheses, a mechanobiological algorithm was developed and applied to simulate the remodelling cycle in a realistic trabecular strut. The model is based on the supposition that bone resorption is initiated either to remove damaged tissue or when strains fall below a lower threshold; bone formation is triggered when strains exceed an upper threshold.

FINDINGS

Applying this algorithm to a realistic trabecula, resorption and subsequent refilling of a cavity was simulated. Results showed that decreases in the mechanosensitivity (simulated by increasing the upper strain threshold) led to under-refilling of cavities. A critical sensitivity was found to exist, above which perforation of the strut due to osteoclastic resorption occurred. It was also found that increases in the bone tissue elastic modulus lead to an increased propensity for trabecular perforation.

INTERPRETATION

It may be concluded that if cells become less mechanosensitive, or if increases in the elastic modulus of trabecular bone tissue occurs, the possibility of trabecular perforation and therefore the rapid loss of bone mass increases. If this is true, the preservation of the bone mineral content or maintenance of bone cell mechanosensitivity are potential therapeutic strategies for the prevention of osteoporosis.

Perspective on post-menopausal osteoporosis: establishing an interdisciplinary understanding of the sequence of events from the molecular level to whole bone fractures

Current drug treatments for post-menopausal osteoporosis cannot eliminate bone fractures, possibly because the mechanisms responsible for bone loss are not fully understood. Although research within various disciplines has significantly advanced the state of knowledge, fundamental findings are not widely understood between different disciplines. For that reason, this paper presents noteworthy experimental findings from discrete disciplines focusing on post-menopausal osteoporosis. These studies have established that, in addition to bone loss, significant changes in bone micro-architecture, tissue composition and micro-damage occur. Cellular processes and molecular signalling pathways governing pathological bone resorption have been identified to a certain extent. Ongoing studies endeavour to determine how such changes are initiated at the onset of oestrogen deficiency. It emerges that, because of the discrete nature of previous research studies, the sequence of events that lead to bone fracture is not fully understood. In this paper, two sequences of multi-scale changes are proposed and the experimental challenges that need to be overcome to fully define this sequence are outlined. Future studies must comprehensively characterize the time sequence of molecular-, cellular- and tissue-level changes to attain a coherent understanding of the events that ultimately lead to bone fracture and inform the future development of treatments for post-menopausal osteoporosis.

Osteocyte apoptosis and control of bone resorption following ovariectomy in mice

INTRODUCTION

Osteocyte apoptosis has been linked to bone resorption resulting from estrogen depletion and other resorptive stimuli; however, precise spatial and temporal relationships between the two events have not been clearly established. The purpose of this study was to characterize the patterns of osteocyte apoptosis in relation to bone resorption following ovariectomy to test whether osteocyte apoptosis occurs preferentially in areas known to activate resorption. Moreover, we report that osteocyte apoptosis is necessary to initiate endocortical remodeling in response to estrogen withdrawal.

MATERIALS AND METHODS

Adult female C57BL/6J mice (17 weeks old) underwent either bilateral ovariectomy (OVX), or sham surgery (SHAM) and were euthanized on days 3, 7, 14, or 21 days after OVX. Diaphyseal cross-sections were stained by immunohistochemistry for activated caspase-3 as a marker of apoptosis. The percentages of caspase-positive stained osteocytes (Casp+Ot.) were measured along major and minor anatomical axes around the femoral diaphysis to evaluate the distribution of osteocyte apoptosis after estrogen loss; resorption surface was measured at the adjacent endocortical regions. In a second study to test whether osteocyte apoptosis plays a regulatory role in the initiation of bone resorption, a group of OVX mice received the pan-caspase inhibitor, QVDOPh, to inhibit osteocyte apoptosis. Remaining experimental and sham groups received either QVD or Vehicle.

RESULTS

OVX increased osteocyte apoptosis in a non-uniform distribution throughout the femoral diaphyses. Increases in Casp+osteocytes were predominantly located in the posterior diaphyseal cortex. Here, the number of apoptotic osteocytes 4- to 7-fold higher than sham controls (p<0.005) by day 3 post-OVX and remained elevated. Increases in resorption post-OVX also occurred along the posterior endocortical surface overlying the region of osteocyte apoptosis, but these increases occurred only at 14 and 21 days post-OVX (p<0.002) well after the increases in osteocyte apoptosis. Treatment with QVD in OVX animals suppressed osteocyte apoptosis, with levels in QVD-treated samples equivalent to baseline. Moreover, the increases in osteoclastic resorption normally observed after estrogen loss did not occur in OVX mice treated with QVD.

CONCLUSIONS

The results of this study demonstrate that osteocyte apoptosis following estrogen loss occur regionally, rather than uniformly throughout the cortex. We also showed that estrogen loss increased osteocyte apoptosis. Apoptotic osteocytes were overwhelmingly localized within the posterior cortical region, the location where endocortical resorption was subsequently activated in ovariectomized mice. Finally, the increases in osteoclastic resorption normally observed after estrogen withdrawal did not occur in the absence of osteocyte apoptosis indicating that this apoptosis is necessary to activate endocortical remodeling following estrogen loss.