Genetic variation in structure-function relationships for the inbred mouse lumbar vertebral body

Skeletal tissue regulation by catalase overexpression in mitochondria

Accumulation of oxidative damage from excess reactive oxygen species (ROS) may contribute to skeletal aging and mediate adverse responses to physiological challenges. Wild-type (WT) mice and transgenic mice (male, 16 wk of age) with human catalase targeted to the mitochondria (mCAT) were analyzed for skeletal responses to the remodeling stimuli of combined hind-limb unloading and exposure to ionizing radiation (¹³⁷Cs, 2 Gy). Treatment for 2 wk caused lipid peroxidation in the bones WT but not mCAT mice, showing that transgene expression mitigated oxidative stress. Ex vivo osteoblast colony growth rate was 95% greater in mCAT than WT mice and correlated with catalase activity levels (P < 0.005, r = 0.67), although terminal osteoblast and osteoclast differentiation were unaffected. mCAT mice had lower cancellous bone volume and cortical size than WT mice. Ambulatory control mCAT animals also displayed reduced cancellous and cortical structural properties compared with control WT mice. In mCAT but not WT mice, treatment caused an unexpectedly rapid radial expansion (+8% cortical area, +22% moment of inertia), reminiscent of compensatory bone growth during advancing age. In contrast, treatment caused similar structural deficits in cancellous tissue of mCAT and WT mice. In sum, mitochondrial ROS signaling via H₂O₂ was important for the acquisition of adult bone structure and catalase overexpression failed to protect cancellous tissue from treatment. In contrast, catabolic stimuli caused radial expansion in mCAT not WT mice, suggesting that mitochondrial ROS in skeletal cells act to suppress tissue turnover in response to remodeling challenges.

Age- and Strain-Related Differences in Bone Microstructure and Body Composition During Development in Inbred Male Mouse Strains

We explored age- and strain-related differences in bone microstructure and body composition in male C57BL/6J, DBA/2JRj and C3H/J mice. Bone microstructure of the femur, tibia and L4 was assessed by μCT at the age of 8, 16 and 24 weeks. The weight of several muscles and fat depots were measured at the same time points. At all timepoints, C3H/J mice had the thickest cortices followed by DBA/2JRj and C57BL/6J mice. Nevertheless, C57BL/6J mice had higher Tb.BV/TV and Tb.N, and lower Tb.Sp than DBA/2JRj and C3H/J mice at least at 24 weeks of age. Skeletal development patterns differed among strains. C57BL/6J and DBA/2JRj mice, but not C3H/J mice, experienced significant increases in the sum of the masses of 6 individual muscles by 24 weeks of age. In C57BL/6J and DBA/2JRj mice, the mass of selected fat depots reached highest values at 24 weeks, whist, in C3H/J mice, the highest values of fat depots masses were achieved at 16 weeks. Early strain differences in muscle and fat masses were largely diminished by 24 weeks of age. C3H/J and C57BL/6J mice displayed the most favorable cortical and trabecular bone parameters, respectively. Strain differences in body composition were less overt than strain specificity in bone microstructure, however, they possibly influenced aspects of skeletal development.

Dietary calcium intake and genetics have site-specific effects on peak trabecular bone mass and microarchitecture in male mice

Trabecular bone (Tb) is used for rapid exchange of calcium (Ca) in times of physiologic need and the site-specific characteristics of Tb may explain why certain sites are more vulnerable to osteoporosis. We hypothesized that peak trabecular bone mass (PTBM) and Tb microarchitecture are differentially regulated by dietary Ca intake, genetics, or Gene-by-Diet (GxD) interactions at the distal femur and the fifth lumbar (L5) vertebra. Male mice from 62 genetically distinct lines were fed basal (0.5%) or low (0.25%) Ca diets from 4 to 12 wks of age. Afterwards, the right femur and L5 vertebra were removed and trabecular bone was analyzed by μCT. In mice fed the basal diet, bone volume fraction (BV/TV), trabecular number (Tb.N), and connectivity density (Conn.D) were significantly higher in the L5 vertebra than femur. Femur Tb had a weaker, more rod-like structure than the L5 vertebrae while mice fed the low Ca diet developed rod-like structures at both sites. Dietary Ca restriction also caused a greater relative reduction of Tb.N and Conn.D in the femur than L5 vertebra, i.e. it was more harmful to the integrity of Tb microarchitecture in femur. Genetics was a major determinant of Tb at both sites, e.g. heritability of BV/TV on the basal diet = 0.65 (femur) and 0.68 (L5 vertebra). However, while GxD interactions altered the impact of dietary Ca restriction on Tb parameters at both sites, the effect was not uniform, e.g. some lines had site-specific responses to Ca restriction. The significance of our work is that there are site-specific effects of dietary Ca restriction and genetics that work independently and interactively to influence the attainment of PTBM and Tb microarchitecture.

Positive impact of low-dose, high-energy radiation on bone in partial- and/or full-weightbearing mice

Astronauts traveling beyond low Earth orbit will be exposed to galactic cosmic radiation (GCR); understanding how high energy ionizing radiation modifies the bone response to mechanical unloading is important to assuring crew health. To investigate this, we exposed 4-mo-old female Balb/cBYJ mice to an acute space-relevant dose of 0.5 Gy ⁵⁶Fe or sham (n = ~8/group); 4 days later, half of the mice were also subjected to a ground-based analog for 1/6 g (partial weightbearing) (G/6) for 21 days. Microcomputed tomography (µ-CT) of the distal femur reveals that ⁵⁶Fe exposure resulted in 65–78% greater volume and improved microarchitecture of cancellous bone after 21 d compared to sham controls. Radiation also leads to significant increases in three measures of energy absorption at the mid-shaft femur and an increase in stiffness of the L4 vertebra. No significant effects of radiation on bone formation indices are detected; however, G/6 leads to reduced % mineralizing surface on the inner mid-tibial bone surface. In separate groups allowed 21 days of weightbearing recovery from G/6 and/or ⁵⁶Fe exposure, radiation-exposed mice still exhibit greater bone mass and improved microarchitecture vs. sham control. However, femoral bone energy absorption values are no longer higher in the ⁵⁶Fe-exposed WB mice vs. sham controls. We provide evidence for persistent positive impacts of high-LET radiation exposure preceding a period of full or partial weightbearing on bone mass and microarchitecture in the distal femur and, for full weightbearing mice only and more transiently, cortical bone energy absorption values.

Crocin attenuates metabolic syndrome-induced osteoporosis in rats

In the current study, anti-osteoporotic activities of crocin were evaluated in a rat model of metabolic syndrome-induced osteoporosis. Metabolic syndrome was confirmed by increased body weight gain, increased fasting blood glucose, hyperinsulinemia, elevated mean arterial blood pressure, and increased serum triglycerides level. Crocin (5 or 10 mg/kg) protected against histological and architectural alteration in bone tissues. Further, it ameliorated the decline in the bone formation markers serum alkaline phosphatase activity and osteocalcin level and inhibited the rise in the bone resorption markers serum tartrate-resistant acid phosphatase and collagen cross-linking carboxyterminal telopeptide, type I. Crocin anti-inflammatory properties were confirmed by a significant decline in serum interleukin-6 and tumor necrosis factor-α. Crocin mitigated oxidative stress in femur distal epiphysis tissues. Mechanically, crocin enhanced both the longitudinal and perpendicular forces of femurs. The current data highlight a protective activity that can be attributed, at least partly, to its anti-inflammatory and antioxidant activities. PRACTICAL APPLICATIONS: Metabolic syndrome is a serious health problem. Its prevalence is present in approximately 25% of all adults. Complications of metabolic syndrome include osteoporosis. This poses high risk of fractures and represents a heavy health, social, and economic burden. The current study highlights the antiosteoporotic activities of crocin in an experimental model of osteoporosis. Thus, crocin and/or other structurally related carotenoids can be lead compounds for synthesizing more potent and bioavailable molecules. These are expected to be devoid of the hazardous adverse effects of the currently available medicaments.

Age Dependence of Systemic Bone Loss and Recovery Following Femur Fracture in Mice

The most reliable predictor of future fracture risk is a previous fracture of any kind. The etiology of this increased fracture risk is not fully known, but it is possible that fracture initiates systemic bone loss, leading to greater fracture risk at all skeletal sites. In this study, we investigated systemic bone loss and recovery after femoral fracture in young (3-month-old) and middle-aged (12-month-old) mice. Transverse femur fractures were created using a controlled impact, and whole-body bone mineral density (BMD), trabecular and cortical microstructure, bone mechanical properties, bone formation and resorption rates, mouse voluntary movement, and systemic inflammation were quantified at multiple time points post-fracture. We found that fracture led to decreased whole-body BMD in both young and middle-aged mice 2 weeks post-fracture; this bone loss was recovered by 6 weeks in young but not middle-aged mice. Similarly, trabecular bone volume fraction (BV/TV) of the L5 vertebral body was significantly reduced in fractured mice relative to control mice 2 weeks post-fracture (-11% for young mice, -18% for middle-aged mice); no significant differences were observed 6 weeks post-fracture. At 3 days post-fracture, we observed significant increases in serum levels of interleukin-6 and significant decreases in voluntary movement in fractured mice compared with control mice, with considerably greater changes in middle-aged mice than in young mice. At this time point, we also observed increased osteoclast number on L5 vertebral body trabecular bone of fractured mice compared with control mice. These data show that systemic bone loss occurs after fracture in both young and middle-aged mice, and recovery from this bone loss may vary with age. This systemic response could contribute to increased future fracture risk after fracture; these data may inform clinical treatment of fractures with respect to improving long-term skeletal health. © 2018 American Society for Bone and Mineral Research.

Dietary Advanced Glycation End Products Have Sex- and Age-Dependent Effects on Vertebral Bone Microstructure and Mechanical Function in Mice

Back pain is a leading cause of global disability that can arise from vertebral fracture and osteoporosis. Although poor general health and obesity are among the strongest risk factors for back pain, there is remarkably little known about how diet influences spinal diseases. Advanced glycation end-products (AGEs) are implicated in increased fracture risk, yet no studies investigated how dietary AGEs affect spinal health. We tested the hypothesis that high dietary AGE ingestion will diminish vertebral structure and function in a sex- and age-dependent manner. Female and male mice were fed low-AGE (L-AGE) or high-AGE (H-AGE) isocaloric diets for 6 and 18 months and multiple measurements of bone structure and function were taken. AGE levels in serum and cortical vertebrae were increased only for 6-month-old H-AGE female mice while blood glucose and body weight remained normal for all animals. When fed an H-AGE diet, 6-month-old female mice had inferior vertebral trabecular structure with decreased bone mineral density (BMD) and bone volume fraction. Biomechanical testing and analytical modeling further showed functional deterioration in 6-month-old H-AGE females with reduced shear and compression moduli, and maximum load to failure. At 18 months, H-AGE mice of both sexes had significant but small decreases in cortical BMD and thickness, yet functional biomechanical behaviors were not distinguishable from other aging changes. We conclude that an H-AGE diet, without diabetic or overweight conditions, diminished vertebral microstructure, mechanical behaviors, and fracture resistance in young female mice in a manner suggesting accelerated bone aging. © 2017 American Society for Bone and Mineral Research.

Dose-dependent effect of parathyroid hormone on fracture healing and bone formation in mice

BACKGROUND

Parathyroid hormone (PTH) is the only clinically approved osteoanabolic drug for osteoporosis treatment. However, PTH is not established for the treatment of fracture healing, and doses of PTH diverge significantly between different studies. We hypothesized that the effect of PTH on promoting fracture healing and bone formation is dose dependent.

MATERIALS AND METHODS

In vivo, mice were treated with PTH (10, 40, and 200 μg/kg) in a closed femoral fracture model. Fracture healing was analyzed after 4 weeks. The fourth lumbar vertebra was analyzed to assess systemic effects. In addition, osteoblasts from calvaria of mice were treated in vitro with PTH doses of 10^-5-50 nM, and their differentiation was analyzed after 26 days.

RESULTS

In vivo, PTH dose-dependently stimulated bone formation in the fracture callus and the vertebral body. However, PTH treatment did not increase biomechanical stiffness of the fractured femora in a dose-dependent manner. The increased bone formation in the 200 μg/kg group was associated with a depletion of osteoclasts, indicating diminished bone remodeling. Of interest, in vitro, we observed diminished mineralization with the highest doses of PTH in osteoblast cultures.

CONCLUSIONS

PTH dose-dependently stimulates bone formation in vivo. However, during fracture healing, this did not result in a dose-dependent increase of the mechanical stiffness of the fracture callus. Taken together, our in vivo and in vitro data indicate that the dose-dependent effects of PTH during fracture healing are based on the actions on multiple cell types, thereby influencing not only bone formation but also osteoclastic callus remodeling.

Femoral Neck External Size but not aBMD Predicts Structural and Mass Changes for Women Transitioning Through Menopause

The impact of adult bone traits on changes in bone structure and mass during aging is not well understood. Having shown that intracortical remodeling correlates with external size of adult long bones led us to hypothesize that age-related changes in bone traits also depend on external bone size. We analyzed hip dual-energy X-ray absorptiometry images acquired longitudinally over 14 years for 198 midlife women transitioning through menopause. The 14-year change in bone mineral content (BMC, R²  = 0.03, p = 0.015) and bone area (R²  = 0.13, p = 0.001), but not areal bone mineral density (aBMD, R²  = 0.00, p = 0.931) correlated negatively with baseline femoral neck external size, adjusted for body size using the residuals from a linear regression between baseline bone area and height. The dependence of the 14-year changes in BMC and bone area on baseline bone area remained significant after adjusting for race/ethnicity, postmenopausal hormone use, the 14-year change in weight, and baseline aBMD, weight, height, and age. Women were sorted into tertiles using the baseline bone area-height residuals. The 14-year change in BMC (p = 0.009) and bone area (p = 0.001) but not aBMD (p = 0.788) differed across the tertiles. This suggested that women showed similar changes in aBMD for different structural and biological reasons: women with narrow femoral necks showed smaller changes in BMC but greater increases in bone area compared to women with wide femoral necks who showed greater losses in BMC but without large compensatory increases in bone area. This finding is opposite to expectations that periosteal expansion acts to mechanically offset bone loss. Thus, changes in femoral neck structure and mass during menopause vary widely among women and are predicted by baseline external bone size but not aBMD. How these different structural and mass changes affect individual strength-decline trajectories remains to be determined. © 2017 American Society for Bone and Mineral Research.

The development of inter-strain variation in cortical and trabecular traits during growth of the mouse lumbar vertebral body

How cortical and trabecular bone co-develop to establish a mechanically functional structure is not well understood. Comparing early postnatal differences in morphology of lumbar vertebral bodies for three inbred mouse strains identified coordinated changes within and between cortical and trabecular traits. These early coordinate changes defined the phenotypic differences among the inbred mouse strains.

INTRODUCTION

Age-related changes in cortical and trabecular traits have been well studied; however, very little is known about how these bone tissues co-develop from day 1 of postnatal growth to establish functional structures by adulthood. In this study, we aimed to establish how cortical and trabecular tissues within the lumbar vertebral body change during growth for three inbred mouse strains that express wide variation in adult bone structure and function.

METHODS

Bone traits were quantified for lumbar vertebral bodies of female A/J, C57BL/6J (B6), and C3H/HeJ (C3H) inbred mouse strains from 1 to 105 days of age (n = 6-10 mice/age/strain).

RESULTS

Inter-strain differences in external bone size were observed as early as 1 day of age. Reciprocal and rapid changes in the trabecular bone volume fraction and alignment in the direction of axial compression were observed by 7 days of age. Importantly, the inter-strain difference in adult trabecular bone volume fraction was established by 7 days of age. Early variation in external bone size and trabecular architecture was followed by progressive increases in cortical area between 28 and 105 days of age, with the greatest increases in cortical area seen in the mouse strain with the lowest trabecular mass.

CONCLUSION

Establishing the temporal changes in bone morphology for three inbred mouse strains revealed that genetic variation in adult trabecular traits were established early in postnatal development. Early variation in trabecular architecture preceded strain-specific increases in cortical area and changes in cortical thickness. This study established the sequence of how cortical and trabecular traits co-develop during growth, which is important for identifying critical early ages to further focus on intervention studies that optimize adult bone strength.

Single dose of bisphosphonate preserves gains in bone mass following cessation of sclerostin antibody in Brtl/+ osteogenesis imperfecta model

Sclerostin antibody has demonstrated a bone-forming effect in pre-clinical models of osteogenesis imperfecta, where mutations in collagen or collagen-associated proteins often result in high bone fragility in pediatric patients. Cessation studies in osteoporotic patients have demonstrated that sclerostin antibody, like intermittent PTH treatment, requires sequential anti-resorptive therapy to preserve the anabolic effects in adult populations. However, the persistence of anabolic gains from either drug has not been explored clinically in OI, or in any animal model. To determine whether cessation of sclerostin antibody therapy in a growing OI skeleton requires sequential anti-resorptive treatment to preserve anabolic gains in bone mass, we treated 3week old Brtl/+ and wild type mice for 5weeks with SclAb, and then withdrew treatment for an additional 6weeks. Trabecular bone loss was evident following cessation, but was preserved in a dose-dependent manner with single administration of pamidronate at the time of cessation. In vivo longitudinal near-infrared optical imaging of cathepsin K activation in the proximal tibia suggests an anti-resorptive effect of both SclAb and pamidronate which is reversed after three weeks of cessation. Cortical bone was considerably less susceptible to cessation effects, and showed no structural or functional deficits in the absence of pamidronate during this cessation period. In conclusion, while SclAb induces a considerable anabolic gain in the rapidly growing Brtl/+ murine model of OI, a single sequential dose of antiresorptive drug is required to maintain bone mass at trabecular sites for 6weeks following cessation.

Congenic Strains Confirm the Pleiotropic Effect of Chromosome 4 QTL on Mouse Femoral Geometry and Biomechanical Performance

A pleiotropic quantitative trait locus (QTL) for bone geometry and mechanical performance in mice was mapped to distal chromosome 4 via an intercross of recombinant congenic mice HcB-8 and HcB-23. To study the QTL in isolation, we have generated C3H.B10-(rs6355453-rs13478087) (C.B.4.3) and C3H.B10-(rs6369860-D4Mit170) (C.B.4.2) congenic strains that harbor ~20 Mb and ~3 Mb, respectively, of chromosome 4 overlapping segments from C57BL/10ScSnA (B10) within the locus on a C3H/DiSnA (C3H) background. Using 3-point bend testing and standard beam equations, we phenotyped these mice for femoral mid-diaphyseal geometry and biomechanical performance. We analyzed the results via 2-way ANOVA, using sex and genotype as factors. In the C.B.4.3 strain, we found that homozygous B10/B10 male mice had smaller cross sectional area (CSA) and reduced total displacement than homozygous C3H/C3H mice. Sex by genotype interaction was also observed for maximum load and stiffness for C3H/C3H and B10/B10 mice, respectively. In C.B.4.2 strain, we found that homozygous B10/B10 mice had lower total displacement, post-yield displacement (PYD), stiffness, yield load and maximum load than mice harboring C3H allele. Sex by genotype interaction was observed in B10/B10 mice for perimeter, outer minor axis (OMA) and CSA. There were no significant differences in tissue level mechanical performance, which suggest that the QTL acts primarily on circumferential bone size. These data confirm the prior QTL mapping data and support other work demonstrating the importance of chromosome 4 QTL on bone modeling and bone responses to mechanical loading.

Sex-Linked Skeletal Phenotype of Lysyl Oxidase Like-1 Mutant Mice

Lysyl oxidases are required for collagen and elastin cross-linking and extracellular matrix maturation including in bone. The lysyl oxidase family consists of lysyl oxidase (LOX) and 4 isoforms (LOXL1-4). Here we investigate whether deletion of LOXL1, which has been linked primarily to elastin maturation, leads to skeletal abnormalities. Left femurs (n = 8), L5 vertebrae (n = 8), and tibiae (n = 8) were analyzed by micro-computed tomography in 13-week-old wild-type (WT) and LOXL1-/- male and female mice. Right femurs (n = 8) were subjected to immunohistochemistry for LOXL1, and histochemical/histology analyses of osteoclasts and growth plates. Sera from all mice were analyzed for bone turnover markers. Results indicate strong expression of LOXL1 in wild-type growth plates in femurs. Significant deterioration of trabecular bone structure in long bones and vertebrae from female was observed but not from male, mutant mice compared with WT. Decreases in BV/TV, Conn.D, trabecular thickness, and number in the femoral distal metaphysis were observed in female, but not in male, mutant mice. Trabecular spacing was increased significantly in femurs of female mutant mice. Findings were similar in trabeculae of L5 vertebrae from female mutant mice. The number of TRAP positive osteoclasts at the trabecular bone surface was increased in female mutant mice compared with WT females, consistent with increased serum RANKL and decreased OPG levels. Analysis of bone turnover markers confirmed increased bone resorption as indicated by significantly elevated CTX-1 in the serum of female LOXL1-/- mice compared to their wild-type counterparts, as well as decreased bone formation as measured by decreased serum levels of PINP. Picrosirius red staining revealed a loss of heterogeneity in collagen organization in female LOXL1-/- mice only, with little to no yellow and orange birefringence. Organization was also impaired in chondrocyte columns in both female and male LOXL1-/- mice, but to a greater extent in females. Data indicate that LOXL1-/- mutant mice develop appendicular and axial skeletal phenotypes characterized by decreased bone volume fraction and compromised trabecular microstructure, predominantly in females.

Trabecular bone loss at a distant skeletal site following noninvasive knee injury in mice

Traumatic injuries can have systemic consequences, as the early inflammatory response after trauma can lead to tissue destruction at sites not affected by the initial injury. This systemic catabolism may occur in the skeleton following traumatic injuries such as anterior cruciate ligament (ACL) rupture. However, bone loss following injury at distant,unrelated skeletal sites has not yet been established. In the current study, we utilized a mouse knee injury model to determine whether acute knee injury causes a mechanically significant trabecular bone loss at a distant, unrelated skeletal site (L5 vertebral body).Knee injury was noninvasively induced using either high-speed (HS; 500 mm/s) or lowspeed(LS; 1 mm/s) tibial compression overload. HS injury creates an ACL rupture by midsubstance tear, while LS injury creates an ACL rupture with an associated avulsion bone fracture. At 10 days post-injury, vertebral trabecular bone structure was quantified using high-resolution microcomputed tomography (lCT), and differences in mechanical properties were determined using finite element modeling (FEM) and compressive mechanical testing. We hypothesized that knee injury would initiate a loss of trabecular bone structure and strength at the L5 vertebral body. Consistent with our hypothesis, we found significant decreases in trabecular bone volume fraction (BV/TV) and trabecular number at the L5 vertebral body in LS injured mice compared to sham (8.8% and 5.0%, respectively), while HS injured mice exhibited a similar, but lower magnitude response (5.1% and 2.5%, respectively). Contrary to our hypothesis, this decrease intrabecular bone structure did not translate to a significant deficit in compressive stiffness or ultimate load of the full trabecular body assessed by mechanical testing or FEM. However,we were able to detect significant decreases in compressive stiffness in both HS and LS injured specimens when FE models were loaded directly through the trabecular bone region (9.9% and 8.1%, and 3, respectively). This finding may be particularly important for osteoporotic fracture risk, as damage within vertebral bodies has been shown to initiate within the trabecular bone compartment. Altogether, these data point to a systemic trabecular bone loss as a consequence of fracture or traumatic musculoskeletal injury, which may be an underlying mechanism contributing to increased risk of refracture following an initial injury. This finding may have consequences for treatment of acute musculoskeletal injuries and the prevention of future bone fragility.

Thrombospondin-2 deficiency in growing mice alters bone collagen ultrastructure and leads to a brittle bone phenotype

Thrombospondin-2 (TSP2) is a matricellular protein component of the bone extracellular matrix. Long bones of adult TSP2-deficient mice have increased endosteal bone thickness due to expansion of the osteoblast progenitor cell pool, and these cells display deficits in osteoblastic potential. Here, we investigated the effects of TSP2 deficiency on whole bone geometric and mechanical properties in growing 6-wk-old male and female wild-type and TSP2-knockout (KO) mice. Microcomputed tomography and mechanical testing were conducted on femora and L2 vertebrae to assess morphology and whole bone mechanical properties. In a second series of experiments, femoral diaphyses were harvested from wild-type and TSP2-KO mice. Detergent-soluble type I collagen content was determined by Western blot of right femora. Total collagen content was determined by hydroxyproline analysis of left femora. In a third series of experiments, cortical bone was dissected from the anterior and posterior aspects of the femoral middiaphysis and imaged by transmission electron microscopy to visualize collagen fibrils. Microcomputed tomography revealed minimal structural effects of TSP2 deficiency. TSP2 deficiency imparted a brittle phenotype on cortical bone. Femoral tissue mineral density was not affected by TSP2 deficiency. Instead, transmission electron microscopy revealed less intensely stained collagen fibrils with altered morphology in the extracellular matrix assembled by osteoblasts on the anterior surface of TSP2-KO femora. Femoral diaphyseal bone displayed comparable amounts of total collagen, but the TSP2-KO bones had higher levels of detergent-extractable type I collagen. Together, our data suggest that TSP2 is required for optimal collagen fibrillogenesis in bone and thereby contributes to normal skeletal tissue quality.

Micro/Nanostructures and Mechanical Properties of Trabecular Bone in Ovariectomized Rats

Bone mechanical properties encompass both geometric and material factors, while the effects of estrogen deficiency on the material and structural characteristics of bone at micro- to nanoscales are still obscure. We performed a series of combined methodological experiments, including nanoindentation assessment of intrinsic material properties, atomic force microscopy (AFM) characterization of trabecular (Tb) nanostructure, and Tb microarchitecture and 2D BMD. At 15 weeks after surgery, we found significantly less Tb bone mineral density (BMD) at organ (-27%) and at tissue level (-12%), Tb bone volume fraction (-29%), Tb thickness (-14%), and Tb number (-17%) in ovariectomy (OVX) rats than in sham operated (SHAM) rats, while the structure model index (+91%) and Tb separation (+19%) became significantly greater. AFM images showed lower roughness Tb surfaces with loosely packed large nodular structures and less compacted interfibrillar space in OVX than in SHAM. However, no statistically significant changes were in the Tb intrinsic material properties-nanoindentation hardness, elastic modulus, and plastic deformation-nanoindentation depths, and residual areas. Therefore, estrogen deprivation results in a dramatic deterioration in Tb micro/nanoarchitectures, 3D volumetric BMD at both organ and tissue levels, and 2D BMD, but not in the nanomechanical properties of the trabeculae per se.

Characterization of complex, co-adapted skeletal biomechanics phenotypes: a needed paradigm shift in the genetics of bone structure and function

The genetic architecture of skeletal biomechanical performance has tremendous potential to advance our knowledge of the biological mechanisms that drive variation in skeletal fragility and osteoporosis risk. Research using traditional approaches that focus on specific gene pathways is increasing our understanding of how and to what degree those pathways may affect population-level variation in fracture susceptibility, and shows that known pathways may affect bone fragility through unsuspected mechanisms. Non-traditional approaches that incorporate a new appreciation for the degree to which bone traits co-adapt to functional loading environments, using a wide variety of redundant compensatory mechanisms to meet both physiological and mechanical demands, represent a radical departure from the dominant reductionist paradigm and have the potential to rapidly advance our understanding of bone fragility and identification of new targets for therapeutic intervention.

Intracortical bone remodeling variation shows strong genetic effects

Intracortical microstructure influences crack propagation and arrest within bone cortex. Genetic variation in intracortical remodeling may contribute to mechanical integrity and, therefore, fracture risk. Our aim was to determine the degree to which normal population-level variation in intracortical microstructure is due to genetic variation. We examined right femurs from 101 baboons (74 females, 27 males; aged 7-33 years) from a single, extended pedigree to determine osteon number, osteon area (On.Ar), haversian canal area, osteon population density, percent osteonal bone (%On.B), wall thickness (W.Th), and cortical porosity (Ct.Po). Through evaluation of the covariance in intracortical properties between pairs of relatives, we quantified the contribution of additive genetic effects (heritability [h (2)]) to variation in these traits using a variance decomposition approach. Significant age and sex effects account for 9 % (Ct.Po) to 21 % (W.Th) of intracortical microstructural variation. After accounting for age and sex, significant genetic effects are evident for On.Ar (h (2) = 0.79, p = 0.002), %On.B (h (2) = 0.82, p = 0.003), and W.Th (h (2) = 0.61, p = 0.013), indicating that 61-82 % of the residual variation (after accounting for age and sex effects) is due to additive genetic effects. This corresponds to 48-75 % of the total phenotypic variance. Our results demonstrate that normal, population-level variation in cortical microstructure is significantly influenced by genes. As a critical mediator of crack behavior in bone cortex, intracortical microstructural variation provides another mechanism through which genetic variation may affect fracture risk.

Of mice, rats and men: trabecular bone architecture in mammals scales to body mass with negative allometry

Body mass (BM) in mammal species spans over six orders of magnitude. Although trabecular bone contributes to the mechanical properties of bones, we know much less about how trabecular bone scales with BM than about how cortical bone scales with BM. We therefore conducted a meta-analysis of the existing literature to test in rodents, humans and other mammals, predicted scaling properties between BM and several trabecular parameters: bone volume fraction (BV/TV), trabecular number (Tb.N), trabecular thickness (Tb.Th), trabecular separation (Tb.Sp), connectivity density (ConnD) and degree of anisotropy (DA). Our results show that BV/TV and DA are independent of BM and that Tb.N, Tb.Th and Tb.Sp scale with negative allometry relative to BM. Rodents appear to have relatively thicker and fewer trabeculae than humans, and we propose it that is due to a minimum thickness threshold "imposed" on mechanically functional trabeculae. Consequently, rodents (mice and rats) and humans demonstrate two distinct mechanisms to achieve variations in BV/TV. Although Tb.Th variation is the main contributing factor for differences in BV/TV in humans, Tb.N variation is the main contributing factor for differences in BV/TV in rodents. Our results also demonstrate no correlation between Tb.N and Tb.Th within each taxon (mice, rats and humans). Since rodents are a common animal model for research on bone biomechanics, the evidence that trabecular bone parameters scale and correlate differently in rodents than in humans suggests that care should be applied when extrapolating bone biomechanical results from small animals to large-bodied humans.

Structural, compositional, and biomechanical alterations of the lumbar spine in rats with mucopolysaccharidosis type VI (Maroteaux-Lamy syndrome)

Mucopolysaccharidosis (MPS) VI is an inherited lysosomal storage disorder resulting from deficiency of N-acetylgalactosamine-4-sulfatase activity and subsequent accumulation of incompletely degraded dermatan sulfate (DS) containing glycosaminoglycans (GAGs). Painful spinal deformities are commonly found in MPS VI patients. We characterized lumbar spine structure, composition, and biomechanics in a naturally occurring rat MPS VI model and evaluated the role of MMP-13, ADAMTS-5 and TNF-α in modulating the observed changes. MPS VI rats had discs with large vacuolated cells and sizable nuclear defects. MPS spine segments also had structural and functional changes suggestive of spinal instability, including decreased nuclear pressurization, increased joint laxity and increased disc height index. These functional changes were at least partly associated with elevated ADAMTS-5, MMP-13, and TNF-α. Vertebral and endplate biomechanics were also affected by MPS VI with decreased failure load and stiffness. The discal and vertebral dysfunctions observed in MPS VI rats are likely to be associated with pathological spinal conditions, similar to those that afflict MPS patients. Our findings also suggest more broadly that abnormal accumulation of GAGs and the associated chronic pro-inflammatory and catabolic cascade may also be a source of spinal dysfunction.

Inhibitory effects of luteolin on titanium particle-induced osteolysis in a mouse model

Wear particles liberated from the surfaces of an implanted prosthesis are associated with peri-implant osteolysis and subsequent aseptic loosening. In the latter wear particle-induced inflammation and osteoclastogenesis have been identified as critical factors, and their inhibition as important steps in the treatment of affected patients, such as those undergoing total hip replacement. In this study the ability of luteolin to inhibit both titanium (Ti) particle-induced osteoclastogenesis in vitro and osteolysis in a murine calvaria Ti particle-induced model of osteolysis was examined. The results showed that luteolin, a highly potent and efficient inhibitor of tumor necrosis factor α (TNF-α) and interleukin-6 expression, inhibited Ti particle-induced inflammatory cytokine release, osteoclastogenesis, and bone resorption in bone marrow macrophages. Microcomputed tomography and histological analyses showed that the Ti particles caused significant bone resorption and increased TRAP(+) multinuclear osteoclasts in the murine calvarial model of osteolysis, whereas this was not the case in the luteolin treatment group, in which osteolytic suppression was accompanied by a decrease in both TNF-α production and serum levels of the osteoclast marker the C-terminal telopeptide fragment of type I collagen. These results support the use of luteolin as a natural compound in the prevention and treatment of aseptic loosening after total replacement arthroplasty.

The Change of Bone Metabolism in Ovariectomized Rats : Analyses of MicroCT Scan and Biochemical Markers of Bone Turnover

Objective

The purpose of this study was to verify the appropriateness of ovariectomized rats as the osteoporosis animal model.

Methods

Twelve female Sprague-Dawley rats underwent a sham operation (the sham group) or bilateral ovariectomy [the ovariectomy (OVX) group]. Eight weeks after operations, serum biochemical markers of bone turnover were analyzed; osteocalcin and alkaline phosphatase, which are sensitive biochemical markers of bone formation, and C-terminal telopeptide fragment of type I collagen C-terminus (CTX), which is a sensitive biochemical marker of bone resorption. Bone histomorphometric parameters and microarchitectural properties of 4th lumbar vertebrae were determined by micro-computed tomographic (CT) scan.

Results

The OVX group showed on average 75.4% higher osteocalcin and 72.5% higher CTX levels than the sham group, indicating increased bone turnover. Micro-CT analysis showed significantly lower bone mineral density (BMD) (p=0.005) and cortical BMD (p=0.021) in the OVX group. Furthermore, the OVX group was found to have a significantly lower trabecular bone volume fraction (p=0.002).

Conclusion

Our results showed that bone turnover was significantly increased and bone mass was significantly decreased 8 weeks after ovariectomy in rats. Thus, we propose that the ovariectomized rat model be considered a reproducible and reliable model of osteoporosis.

Assessment of lamellar level properties in mouse bone utilizing a novel spherical nanoindentation data analysis method

In this work, we demonstrate the viability of using our recently developed data analysis procedures for spherical nanoindentation in conjunction with Raman spectroscopy for studying lamellar-level correlations between the local composition and local mechanical properties in mouse bone. Our methodologies allow us to convert the raw load-displacement datasets to much more meaningful indentation stress-strain curves that accurately capture the loading and unloading elastic moduli, the indentation yield points, as well as the post-yield characteristics in the tested samples. Using samples of two different inbred mouse strains, A/J and C57BL/6J (B6), we successfully demonstrate the correlations between the mechanical information obtained from spherical nanoindentation measurements to the local composition measured using Raman spectroscopy. In particular, we observe that a higher mineral-to-matrix ratio correlated well with a higher local modulus and yield strength in all samples. Thus, new bone regions exhibited lower moduli and yield strengths compared to more mature bone. The B6 mice were also found to exhibit lower modulus and yield strength values compared to the more mineralized A/J strain.

Curcumin improves bone microarchitecture and enhances mineral density in APP/PS1 transgenic mice

Alzheimer's disease and osteoporosis are often observed to co-occur in clinical practice. The present study aimed to evaluate the bone microarchitecture and bone mineral density (BMD) of the proximal tibia in APP/PS1 transgenic mice by micro-computed tomography (micro-CT), and to search for evidence that curcumin can be used to reduce bone mineral losses and treat osteoporosis after senile dementia in these transgenic mice. Three-month-old female mice were divided into the following groups (n=9 per group): wild-type mice (WT group); APP/PS1 transgenic mice (APP group); and APP/PS1 transgenic mice with curcumin treatment (APP+Cur group). Between 9 and 12 months of age, the APP+Cur group were administered curcumin orally (600ppm). CT scans of the proximal tibia were taken at 6, 9 and 12 months. At 6 months, there were little differences in the structural parameters. At 9 months, the APP groups displayed loss of bone volume ratio (BV/TV), trabecular thickness (Tb.Th), trabecular number (Tb.N) and connectivity density (Conn.D) and increases in trabecular separation (Tb.Sp) and geometric degree of anisotropy (DA) (P<0.05 or P<0.01), with significant changes in the BMD parameters. At 12 months, curcumin treatment led to constant increases in the trabecular bone mass of the metaphysis and clearly improved the BMD. By the same time, we measured the TNF-α and IL-6 in the serum among the different groups at 6, 9 and 12 months by enzyme-linked immunoassay(ELISA). These results suggest that APP/PS1 transgenic mice are susceptible to osteoporosis, and that curcumin can prevent further deterioration of the bone structure and produce beneficial changes in bone turnover. The change of inflammation cytokine, including TNF-α and IL-6, may play an important role in the mechanisms of action of curcumin, but the detail mechanism remains unknown.

Continuous treatment with a low-dose β-agonist reduces bone mass by increasing bone resorption without suppressing bone formation

The sympathetic nervous system regulates bone remodeling through the β-adrenergic receptor (β-AR). However, the systemic roles of adrenergic actions on bone remodeling through the β-AR are largely unknown. In this study, we examined the dose effect of continuous treatment with isoprenaline, a nonspecific β-AR agonist, on bone remodeling. Male C57BL/6J mice were intrasubcutaneously administrated with four different doses (5, 25, 50, or 100 μg/g daily) of isoprenaline or vehicle using an osmotic pump for 2 weeks. The region of high-turnover cancellous bone was analyzed by microcomputed tomography (μCT). Continuous isoprenaline treatment caused a ~35.7% decline in the femoral cancellous bone volume fraction (BV/TV) at all doses (5-100 μg/g daily). Furthermore, continuous isoprenaline treatment weakened the bone mechanical properties in the trunk of lumbar vertebra 4 (L4). These parameters did not show significant differences between doses. Histomorphometric analysis revealed that isoprenaline doses of 50 μg/g daily or less did not significantly inhibit bone formation parameters, such as bone formation rate (BFR) and mineral surface/bone surface (MS/BS). Only the highest dose (100 μg/g daily) of isoprenaline significantly inhibited BFR and MS/BS. On the other hand, osteoclast number/bone surface (Oc.N/BS) was enhanced approximately 2.4-fold and osteoclast surface/bone surface (Oc.S/BS) was increased 2.0-fold by all doses of continuous isoprenaline treatment. The osteoclast parameters plateaued at the lowest dose (5 μg/g daily) of continuous isoprenaline treatment. These results indicate that chronic stimulation of β-AR with low-dose agonist treatment induces bone loss mainly via enhanced bone resorption.

Effects of long-term supplementation with omega-3 fatty acids on longitudinal changes in bone mass and microstructure in mice

A diet rich in omega-3s has previously been suggested to prevent bone loss. However, evidence for this has been limited by short exposure to omega-3 fatty acids (FAs). We investigated whether a diet enriched in eicosapentaenoic acid (EPA) or docosahexaenoic acid (DHA) for the entire adult life of mice could improve bone microstructure and strength. Thirty female mice received a diet enriched in DHA or EPA or an isocaloric control diet from 3 to 17 months of age. Changes in bone microstructure were analyzed longitudinally and biomechanical properties were analysed by a three-point bending test. Bone remodelling was evaluated by markers of bone turnover and histomorphometry. Trabecular bone volume in caudal vertebrae was improved by EPA or DHA at 8 months (+26.6% and +17.2%, respectively, compared to +3.8% in controls, P=.01), but not thereafter. Trabecular bone loss in the tibia was not prevented by omega-3 FAs (BV/TV -94%, -93% and -97% in EPA, DHA and controls, respectively). EPA improved femur cortical bone volume (+8.1%, P<.05) and thickness (+4.4%, P<.05) compared to controls. EPA, but not DHA, reduced age-related decline of osteocalcin (-70% vs. -83% in controls, P<.05). EPA and DHA increased leptin levels (7.3±0.7 and 8.5±0.5 ng ml⁻¹, respectively, compared to 4.5±0.9 ng ml⁻¹ in controls, P=.001); however, only EPA further increased IGF-1 levels (739±108 ng ml⁻¹, compared to 417±58 ng ml⁻¹ in controls, P=.04). These data suggest that long-term intake of omega-3 FA, particularly EPA, may modestly improve the structural and mechanical properties of cortical bone by an increase in leptin and IGF-1 levels, without affecting trabecular bone loss.

Measuring the dynamic mechanical response of hydrated mouse bone by nanoindentation

This study demonstrates a novel approach to characterizing hydrated bone's viscoelastic behavior at lamellar length scales using dynamic indentation techniques. We studied the submicron-level viscoelastic response of bone tissue from two different inbred mouse strains, A/J and B6, with known differences in whole bone and tissue-level mechanical properties. Our results show that bone having a higher collagen content or a lower mineral-to-matrix ratio demonstrates a trend towards a larger viscoelastic response. When normalized for anatomical location relative to biological growth patterns in the antero-medial (AM) cortex, bone tissue from B6 femora, known to have a lower mineral-to-matrix ratio, is shown to exhibit a significantly higher viscoelastic response compared to A/J tissue. Newer bone regions with a higher collagen content (closer to the endosteal edge of the AM cortex) showed a trend towards a larger viscoelastic response. Our study demonstrates the feasibility of this technique for analyzing local composition-property relationships in bone. Further, this technique of viscoelastic nanoindentation mapping of the bone surface at these submicron length scales is shown to be highly advantageous in studying subsurface features, such as porosity, of wet hydrated biological specimens, which are difficult to identify using other methods.

Heavy ion irradiation and unloading effects on mouse lumbar vertebral microarchitecture, mechanical properties and tissue stresses

Astronauts are exposed to both musculoskeletal disuse and heavy ion radiation in space. Disuse alters the magnitude and direction of forces placed upon the skeleton causing bone remodeling, while energy deposited by ionizing radiation causes free radical formation and can lead to DNA strand breaks and oxidative damage to tissues. Radiation and disuse each result in a net loss of mineralized tissue in the adult, although the combined effects, subsequent consequences for mechanical properties and potential for recovery may differ. First, we examined how a high dose (2 Gy) of heavy ion radiation ((56)Fe) causes loss of mineralized tissue in the lumbar vertebrae of skeletally mature (4 months old), male, C57BL/6 mice using microcomputed tomography and determined the influence of structural changes on mechanical properties using whole bone compression tests and finite element analyses. Next, we tested if a low dose (0.5 Gy) of heavy particle radiation prevents skeletal recovery from a 14-day period of hindlimb unloading. Irradiation with a high dose of (56)Fe (2 Gy) caused bone loss (-14%) in the cancellous-rich centrum of the fourth lumbar vertebra (L4) 1 month later, increased trabecular stresses (+27%), increased the propensity for trabecular buckling and shifted stresses to the cortex. As expected, hindlimb unloading (14 days) alone adversely affected microarchitectural and mechanical stiffness of lumbar vertebrae, although the reduction in yield force was not statistically significant (-17%). Irradiation with a low dose of (56)Fe (0.5 Gy) did not affect vertebrae in normally loaded mice, but significantly reduced compressive yield force in vertebrae of unloaded mice relative to sham-irradiated controls (-24%). Irradiation did not impair the recovery of trabecular bone volume fraction that occurs after hindlimb unloaded mice are released to ambulate normally, although microarchitectural differences persisted 28 days later (96% increase in ratio of rod- to plate-like trabeculae). In summary, (56)Fe irradiation (0.5 Gy) of unloaded mice contributed to a reduction in compressive strength and partially prevented recovery of cancellous microarchitecture from adaptive responses of lumbar vertebrae to skeletal unloading. Thus, irradiation with heavy ions may accelerate or worsen the loss of skeletal integrity triggered by musculoskeletal disuse.

Guidelines for assessment of bone microstructure in rodents using micro-computed tomography

Use of high-resolution micro-computed tomography (microCT) imaging to assess trabecular and cortical bone morphology has grown immensely. There are several commercially available microCT systems, each with different approaches to image acquisition, evaluation, and reporting of outcomes. This lack of consistency makes it difficult to interpret reported results and to compare findings across different studies. This article addresses this critical need for standardized terminology and consistent reporting of parameters related to image acquisition and analysis, and key outcome assessments, particularly with respect to ex vivo analysis of rodent specimens. Thus the guidelines herein provide recommendations regarding (1) standardized terminology and units, (2) information to be included in describing the methods for a given experiment, and (3) a minimal set of outcome variables that should be reported. Whereas the specific research objective will determine the experimental design, these guidelines are intended to ensure accurate and consistent reporting of microCT-derived bone morphometry and density measurements. In particular, the methods section for papers that present microCT-based outcomes must include details of the following scan aspects: (1) image acquisition, including the scanning medium, X-ray tube potential, and voxel size, as well as clear descriptions of the size and location of the volume of interest and the method used to delineate trabecular and cortical bone regions, and (2) image processing, including the algorithms used for image filtration and the approach used for image segmentation. Morphometric analyses should be based on 3D algorithms that do not rely on assumptions about the underlying structure whenever possible. When reporting microCT results, the minimal set of variables that should be used to describe trabecular bone morphometry includes bone volume fraction and trabecular number, thickness, and separation. The minimal set of variables that should be used to describe cortical bone morphometry includes total cross-sectional area, cortical bone area, cortical bone area fraction, and cortical thickness. Other variables also may be appropriate depending on the research question and technical quality of the scan. Standard nomenclature, outlined in this article, should be followed for reporting of results.

Age-related changes in bone structure and strength in female and male BALB/c mice

Mice may be useful for studies of skeletal aging, but there are limited data on changes in bone structure and strength over their life span. We obtained bones from female and male BALB/c mice at ages 2, 4, 7, 12, and 20 months and evaluated their structural, densitometric, and mechanical properties. MicroCT of the mid-diaphysis of the femur and radius indicated that during skeletal growth (2-7 months) bone cross-sectional size (area, moment of inertia) increased rapidly; during aging (7-20 months) cortical area was maintained, while moment of inertia continued to increase. Bones from females were smaller than those from males at young ages but not at later ages. Changes in whole-bone stiffness and strength reflected the changes in bone size, with a rapid increase from 2 to 7 months, followed by little or no change. In contrast, energy-to-fracture declined with aging. Cortical tissue mineral density increased during growth and was maintained with aging. MicroCT of trabecular bone revealed age-related changes that were site-dependent. The proximal tibia showed a clear pattern of age-related decline in trabecular BV/TV, with progressive decreases after 4 months in both sexes; lumbar vertebra L5 had more modest age-related declines; in contrast, caudal vertebra Ca7 had increasing BV/TV with aging. Overall, we found no evidence that females had more pronounced age-related deterioration than males. We conclude that bones from aging female and male BALB/c mice exhibit many of the changes seen in humans and are therefore a clinically relevant model for studies of skeletal aging.

An in-situ fluorescence-based optical extensometry system for imaging mechanically loaded bone

The application and quantification of well-controlled tissue strains is required for investigations into mechanisms of tissue adaptation within the musculoskeletal system. Although many commercial and custom extensometry systems exist for large biological samples, integrated loading/strain measurement for small samples is not as readily available. Advanced imaging modules such as laser scanning microscopy provide in situ, minimally invasive tools to probe cellular and molecular processes with high spatiotemporal resolution. Currently, a need exists to devise loading/strain measurement systems that can be integrated with such advanced imaging modules. We describe the development and validation of a fluorescence-based, optical extensometry system directly integrated within a confocal microscopy platform. This system allows in situ measurement of surface strain and is compatible with the direct imaging of cellular processes within small bone samples. This optical extensometry system can accurately and reproducibly measure physiologically relevant surface strains (200 to 3000 microstrain) in beams machined from various well-characterized materials, including bovine femoral cortex, and in intact murine tibia. This simple system provides a powerful tool to further our investigation of the relationships between mechanical loading, fluid and solute transport, and mechanosensation within the musculoskeletal system.

Collagen fibril morphology and mechanical properties of the Achilles tendon in two inbred mouse strains

The relationship between collagen fibril morphology and the functional behavior of tendon tissue has been investigated in numerous experimental studies. Several of these studies suggest that larger fibril radius is a primary determinant of higher tendon stiffness and strength; others have shown that factors apart from fibril radius (such as fibril-fibril interactions) may be critical to improved tendon strength. In the present study, we investigate these factors in two inbred mouse strains that are widely used in skeletal structure-function research: C57BL/6J (B6) and C3H/HeJ (C3H). The aim was to establish a quantitative baseline that will allow one to assess how regulation of tendon extracellular matrix architecture affects tensile mechanical properties. We specifically focused on collagen fibril structure and glycosaminoglycan (GAG) content--the two primary constituents of tendon by dry weight--and their potential functional interactions. For this purpose, Achilles tendons from both groups were tested to failure in tension. Tendon collagen morphology was analyzed from transmission electron microscopy images of tendon sections perpendicular to the longitudinal axis. Our results showed that the two inbred strains are macroscopically similar, but C3H mice have a higher elastic modulus (P < 0.05). Structurally, C3H mice showed a larger collagen fibril radius compared to B6 mice (96 +/- 7 nm and 80 +/- 10 nm respectively). Tendons from C3H mice also showed smaller specific fibril surface (0.015 +/- 0.001 nm nm(-2) vs. 0.017 +/- 0.003 nm nm(-2) in the B6 tendons, P < 0.05), and accordingly a lower concentration of GAGs (0.60 +/- 0.07 microg mg(-1) vs. 0.83 +/- 0.11 microg mg(-1), P < 0.05). As in other studies of tendon structure and function, larger collagen fibril radius appears to be associated with stiffer tendon, but this functional difference could also be attributed to reduced potential surface area exchange between fibrils and the surrounding proteoglycan-rich matrix, in which the hydrophilic GAG side chains may promote inter-fibril sliding. This study provides an architectural and functional baseline for a comparative murine model that can be used to investigate the genetic regulation of tendon biomechanics.

The effects of luteolin on osteoclast differentiation, function in vitro and ovariectomy-induced bone loss

Flavonoids, a group of polyphenolic compounds abundant in plants, are known to prevent bone loss in ovariectomized (OVX) animal models. Inhibition of osteoclast differentiation and bone resorption is considered as an effective therapeutic approach in the treatment of postmenopausal bone loss. Luteolin, a plant flavonoid, has potent anti-inflammatory properties both in vivo and vitro. In this study, we found that luteolin markedly decreased the differentiation of both bone marrow mononuclear cells and Raw264.7 cells into osteoclasts. Luteolin also inhibited the bone resorptive activity of differentiated osteoclasts. We further investigated the effects of luteolin on ovariectomy-induced bone loss using micro-computed tomography, biomechanical tests and serum markers assay for bone remodeling. Oral administration of luteolin (5 and 20 mg/kg per day) to OVX mice caused significant increase in bone mineral density and bone mineral content of trabecular and cortical bones in the femur as compared to those of OVX controls, and prevented decreases of bone strength indexes induced by OVX surgery. Serum biochemical markers assays revealed that luteolin prevents OVX-induced increases in bone turnover. These data strongly suggest that luteolin has the potential for prevention of bone loss in postmenopausal osteoporosis by reducing both osteoclast differentiation and function.

OPG knockout mouse teeth display reduced alveolar bone mass and hypermineralization in enamel and dentin

Recent studies showed that local injection or upregulation of OPG gene would result in early temporal retardation of tooth development. It was assumed that this retardation might cause defective tooth mineralization and pulp formation as the long-term effects. However, since those OPG treatments were transient, any possible long-term effects of OPG addition could not be assessed previously. In the present study, a high-resolution microCT was used to evaluate the long-term effect of OPG gene deprivation on the mineralization and morphology of mouse tooth. Our results showed that the mineralization of alveolar bone in OPG(-/-) mouse tooth was decreased while those of enamel and dentin were increased, compared with the wild-type (WT) group. The labial and lingual dentin thicknesses of OPG(-/-) group were significantly higher and with larger area in enamel and dentin than those of WT group. The size of pulp chamber was also substantially decreased in OPG(-/-) mouse incisor. Different responses in mineralization and morphogenesis to OPG gene deprivation were found between bone and tooth. These effects may be independent of the early odontogenesis, and further studies are warranted to investigate the molecular mechanism of the effect of OPG gene expression on bone formation and later tooth development.

Adverse effects of valproate on bone: defining a model to investigate the pathophysiology

PURPOSE

Bone disease and fractures are common with chronic antiepileptic drug (AED) therapy, but the underlying mechanisms are poorly understood. This study aimed to characterize adverse bone effects of valproate and to identify mouse strains either resistant or sensitive to these effects.

METHODS

Seven mouse strains (n = 40/strain; 10/diet) were screened for the effect of chronic (8 weeks) valproate treatment (0, 2, 4, and 6 g/kg food) on total bone mineral content (BMC, by dual energy x-ray absorptiometry). In a confirmatory study the effect of valproate (0 or 4 g/kg food) over 16 weeks was assessed in five of the mouse strains (n = 60/strain; 30/diet) identified in the screening phase as either sensitive or resistant. Ex vivo volumetric bone measures and structural changes were assessed using peripheral quantitative computed tomography (pQCT) and histomorphometry.

RESULTS

Chronic valproate treatment reproducibly affected bone in C3H/HeJ mice, with a 9.1% (p < 0.01) reduction in total BMC and a 10.7% (p < 0.01) reduction in trabecular volumetric density, indicating a sensitive strain to AED-induced bone loss. Histomorphometry was consistent, revealing reductions in trabecular volume (19.6%, p < 0.05) and number (14.3%, p < 0.04), and a 19.9% (p < 0.05) increase in trabecular separation. In contrast the A/J mice were reproducibly resistant to the bone effects.

CONCLUSION

Mouse strains sensitive and resistant to the adverse bone effects of chronic valproate treatment were identified. The strain-specific effects suggest a role of genetic factors in the pathogenesis of AED-induced bone disease. This novel model provides a new, powerful tool to investigate the pathophysiology and therapy of AED-associated bone disease.

Functional dependence of cancellous bone shear properties on trabecular microstructure evaluated using time-lapsed micro-computed tomographic imaging and torsion testing

When compressed axially, cancellous bone often fails at an oblique angle along well-defined bands, highlighting the importance of cancellous bone shear properties. Torsion testing to determine shear properties of cancellous bone has often been conducted under conditions appropriate only for axisymmetric specimens comprised of homogeneous and isotropic materials. However, most cancellous bone specimens do not meet these stringent test conditions. Therefore, we studied the application of the stepwise torsion testing system in biologic specimens with viscoelastic behavior. We explore the functional dependence of cancellous bone shear properties on trabecular microstructure and its spatial distribution, specifically the contribution of the subregion with the minimum polar moment of inertia to the overall failure properties. Torsional properties of whale trabecular specimens obtained by the incremental application of stepwise torque were not different from those obtained via continuous testing. Average polar moment of inertia accounted for 82 and 67% of the variation in shear modulus and shear stress, respectively. However, torsional properties were better predicted by the subregion with minimum polar moment of inertia, describing 87 and 74% of the variation in shear modulus and shear stress. The use of a novel torsion testing system for nonhomogeneous, orthotropic cancellous bone using stepwise application of torsion and simultaneous micro-computed tomographic imaging was further studied. Most importantly, a heterogeneous cancellous bone microstructural environment, the subregion with the minimum polar moment of inertia, hence the weakest spatial distribution of bone, predicted the shear properties for the entire bone volume.

Cross-sectional geometry of the femoral midshaft in baboons is heritable

A great deal of research into the determinants of bone strength has unequivocally demonstrated that variation in bone strength is highly subject to genetic factors. Increasing attention in skeletal genetic studies is being paid to indicators of bone quality that complement studies of BMD, including studies of the genetic control of bone geometry. The aim of this study is to investigate the degree to which normal population-level variation in femoral midshaft geometry in a population of pedigreed baboons (Papio hamadryas spp.) can be attributed to the additive effect of genes. Using 110 baboons (80 females, 30 males), we 1) characterize normal variation in midshaft geometry of the femur with regard to age and sex, and 2) determine the degree to which the residual variation is attributable to additive genetic effects. Cross-sectional area (CSA), minimum (I(MIN)) and maximum (I(MAX)) principal moments of inertia, and polar moment of inertia (J) were calculated from digitized images of transverse midshaft sections. Maximum likelihood-based variance decomposition methods were used to estimate the mean effects of age, sex, and genes. Together age and sex effects account for approximately 56% of the variance in each property. In each case the effect of female sex is negative and that of age is positive, although of a lower magnitude than the effect of female sex. Increased age is associated with decreased mean cross-sectional geometry measures in the oldest females. Residual h(2) values range from 0.36 to 0.50, reflecting genetic effects accounting for 15% to 23% of the total phenotypic variance in individual properties. This study establishes the potential of the baboon model for the identification of genes that regulate bone geometric properties in primates. This model is particularly valuable because it allows for experimental designs, environmental consistency, availability of tissues, and comprehensive assessments of multiple integrated bone phenotypes that are not possible in human populations. The baboon is of particular importance in genetic studies, because it provides results that are likely highly relevant to the human condition due to the phylogenetic proximity of baboons to humans.

Transient overexpression of sonic hedgehog alters the architecture and mechanical properties of trabecular bone

Bone formation and remodeling involve coordinated interactions between osteoblasts and osteoclasts through signaling networks involving a variety of molecular pathways. We hypothesized that overexpression of Sonic hedgehog (Shh), a morphogen with a crucial role in skeletal development, would stimulate osteoblastogenesis and bone formation in adult animals in vivo. Systemic administration of adenovirus expressing the N-terminal form of Shh into adult mice resulted in a primary increase in osteoblasts and their precursors. Surprisingly, however, this was associated with altered trabecular morphology, decreased bone volume, and decreased compressive strength in the vertebrae. Whereas no change was detected in the number of osteoclast precursors, bone marrow stromal cells from Shh-treated mice showed enhanced osteoclastogenic potential in vitro. These effects were mediated by the PTH/PTH-related protein (PTHrP) pathway as evidenced by increased sensitivity to PTH stimulation and upregulation of the PTH/PTHrP receptor (PPR). Together, these data show that Shh has stimulatory effects on osteoprogenitors and osteoblasts in adult animals in vivo, which results in bone remodeling and reduced bone strength because of a secondary increase in osteoclastogenesis.

Total-body irradiation of postpubertal mice with (137)Cs acutely compromises the microarchitecture of cancellous bone and increases osteoclasts

Ionizing radiation can cause substantial tissue degeneration, which may threaten the long-term health of astronauts and radiotherapy patients. To determine whether a single dose of radiation acutely compromises structural integrity in the postpubertal skeleton, 18-week-old male mice were exposed to (137)Cs gamma radiation (1 or 2 Gy). The structure of high-turnover, cancellous bone was analyzed by microcomputed tomography (microCT) 3 or 10 days after irradiation and in basal controls (tissues harvested at the time of irradiation) and age-matched controls. Irradiation (2 Gy) caused a 20% decline in tibial cancellous bone volume fraction (BV/TV) within 3 days and a 43% decline within 10 days, while 1 Gy caused a 28% reduction 10 days later. The BV/TV decrement was due to increased spacing and decreased thickness of trabeculae. Radiation also increased ( approximately 150%) cancellous surfaces lined with tartrate-resistant, acid phosphatase-positive osteoclasts, an index of increased bone resorption. Radiation decreased lumbar vertebral BV/TV 1 month after irradiation, showing the persistence of cancellous bone loss, although mechanical properties in compression were unaffected. In sum, a single dose of gamma radiation rapidly increased osteoclast surface in cancellous tissue and compromised cancellous microarchitecture in the remodeling appendicular and axial skeleton of postpubertal mice.

Phenotypic integration among trabecular and cortical bone traits establishes mechanical functionality of inbred mouse vertebrae

Conventional approaches to identifying quantitative trait loci (QTLs) regulating bone mass and fragility are limited because they examine cortical and trabecular traits independently. Prior work examining long bones from young adult mice and humans indicated that skeletal traits are functionally related and that compensatory interactions among morphological and compositional traits are critical for establishing mechanical function. However, it is not known whether trait covariation (i.e., phenotypic integration) also is important for establishing mechanical function in more complex, corticocancellous structures. Covariation among trabecular, cortical, and compositional bone traits was examined in the context of mechanical functionality for L(4) vertebral bodies across a panel of 16-wk-old female AXB/BXA recombinant inbred (RI) mouse strains. The unique pattern of randomization of the A/J and C57BL/6J (B6) genome among the RI panel provides a powerful tool that can be used to measure the tendency for different traits to covary and to study the biology of complex traits. We tested the hypothesis that genetic variants affecting vertebral size and mass are buffered by changes in the relative amounts of cortical and trabecular bone and overall mineralization. Despite inheriting random sets of A/J and B6 genomes, the RI strains inherited nonrandom sets of cortical and trabecular bone traits. Path analysis, which is a multivariate analysis that shows how multiple traits covary simultaneously when confounding variables like body size are taken into consideration, showed that RI strains that tended to have smaller vertebrae relative to body size achieved mechanical functionality by increasing mineralization and the relative amounts of cortical and trabecular bone. The interdependence among corticocancellous traits in the vertebral body indicated that variation in trabecular bone traits among inbred mouse strains, which is often thought to arise from genetic factors, is also determined in part by the adaptive response to variation in traits describing the cortical shell. The covariation among corticocancellous traits has important implications for genetic analyses and for interpreting the response of bone to genetic and environmental perturbations.

Type 2 diabetic mice demonstrate slender long bones with increased fragility secondary to increased osteoclastogenesis

Type 2 diabetics often demonstrate normal or increased bone mineral density, yet are at increased risk for bone fracture. Furthermore, the anti-diabetic oral thiazolidinediones (PPARgamma agonists) have recently been shown to increase bone fractures. To investigate the etiology of possible structural and/or material quality defects, we have utilized a well-described mouse model of Type 2 diabetes (MKR). MKR mice exhibit muscle hypoplasia from birth with reduced mass by the pre-diabetic age of 3 weeks. A compensatory hyperplasia ensues during early (5 weeks) development; by 6-8 weeks muscle is normal in structure and function. Adult whole-bone mechanical properties were determined by 4-point bending to test susceptibility to fracture. Micro-computed tomography and cortical bone histomorphometry were utilized to assess static and dynamic indices of structure, bone formation and resorption. Osteoclastogenesis assays were performed from bone marrow-derived non-adherent cells. The 8-week and 16-week, but not 3-week, male MKR had slender (i.e., narrow relative to length) femurs that were 20% weaker (p<0.05) relative to WT control femurs. Tissue-level mineral density was not affected. Impaired periosteal expansion during early diabetes resulted from 250% more, and 40% less of the cortical bone surface undergoing resorption and formation, respectively (p<0.05). Greater resorption persisted in adult MKR on both periosteal and endosteal surfaces. Differences were not limited to cortical bone as the distal femur metaphysis of 16 week MKR contained less trabecular bone and trabecular separation was greater than in WT by 60% (p<0.05). At all ages, MKR marrow-derived cultures demonstrated the ability for enhanced osteoclast differentiation in response to M-CSF and RANK-L. Taken together, the MKR mouse model suggests that skeletal fragility in Type 2 diabetes may arise from reduced transverse bone accrual and increased osteoclastogenesis during growth that is accelerated by the diabetic/hyperinsulinemic milieu. Further, these results emphasize the importance of evaluating diabetic bone based on morphology in addition to bone mass.

Specimen size and porosity can introduce error into microCT-based tissue mineral density measurements

The accurate measurement of tissue mineral density, rho(m), in specimens of unequal size or quantities of bone mineral using polychromatic microCT systems is important, since studies often compare samples with a range of sizes and bone densities. We assessed the influence of object size on microCT measurements of rho(m) using (1) hydroxyapatite rods (HA), (2) precision-manufactured aluminum foams (AL) simulating trabecular bone structure, and (3) bovine cortical bone cubes (BCt). Two beam-hardening correction (BHC) algorithms, determined using a 200 and 1200 mg/cm(3) HA wedge phantom, were used to calculate rho(m) of the HA and BCt. The 200 mg/cm(3) and an aluminum BHC algorithm were used to calculate the linear attenuation coefficients of the AL foams. Equivalent rho(m) measurements of 500, 1000, and 1500 mg HA/cm(3) rods decreased (r(2)>0.96, p<0.05 for all) as HA rod diameter increased in the 200 mg/cm(3) BHC data. Errors averaged 8.2% across these samples and reached as high as 29.5%. Regression analyses suggested no size effects in the 1200 mg/cm(3) BHC data but differences between successive sizes still reached as high as 13%. The linear attenuation coefficients of the AL foams increased up to approximately 6% with increasing volume fractions (r(2)>0.81, p<0.05 for all) but the strength of the size-related error was also BHC dependent. Equivalent rho(m) values were inversely correlated with BCt cube size (r(2)>0.92, p<0.05). Use of the 1200 mg/cm(3) BHC ameliorated the size-related artifact compared to the 200 mg/cm(3) BHC but errors with this BHC were still significant and ranged between 5% and 12%. These results demonstrate that object size, structure, and BHC algorithm can influence microCT measurements of rho(m). Measurements of rho(m) of specimens of unequal size or quantities of bone mineral must be interpreted with caution unless appropriate steps are taken to minimize these potential artifacts.

Zoledronate reverses mandibular bone loss in osteoprotegerin-deficient mice

To characterize the changes in osteoprotegerin-deficient (OPG-/-) mice mandibles and the possible mandibular bone loss prevention by zoledronate. This preventive effect in the mandible differed from that in the proximal tibia and was independent of the OPG pathway.

INTRODUCTION

The study aimed to characterize both the changes in the mandible in osteoprotegerin-deficient (OPG-/-) mice and possible mandibular bone loss prevention by zoledronate.

METHODS

Twenty-eight 6-week-old female mice (C57BL/6J), including OPG-/- (n = 21) and wild-type (WT) (n = 7) mice, were assigned to four groups after 2 weeks of acclimatization to local vivarium conditions: wild mice with vehicle (WT group); OPG-/- mice with vehicle (OPG-/- group); and OPG-/- mice that were subcutaneously injected with either 50 or 150 microg/kg zoledronate (Zol-50 and Zol-150 groups, respectively). Mice were sacrificed at 4 weeks after these treatments and after fasting for 12 h. Sera were harvested for biochemical analyses. The right mandible and tibia of each mouse were selected for microCT analysis. Student's t-test was performed for comparisons of bone parameters at different sites in the WT group. Analysis of variance (ANOVA) was used to compare the biomarkers and bone parameters in the different treatment groups.

RESULTS

Serum bone-specific alkaline phosphatase (B-ALP) and tartrate-resistant acid phosphatase 5b (TRACP-5b) were significantly decreased in WT mice as compared to the levels in the OPG-/- mice (P < 0.05). Zoledronate treatment decreased the high serum B-ALP activity observed in OPG-/- mice to the levels seen in WT mice, while serum TRACP-5b concentrations were decreased to levels even lower than those in WT mice. There were substantial variations in BMD and microstructure of the mandibular and proximal tibial trabeculae. Mandibular bone loss was less affected by OPG gene deprivation than the proximal tibia was. Both zoledronate groups showed greater BMD, trabecular BV/TV, Tb.Th, Tb.N, and Conn.D and a significant decrease in Tb.Sp and SMI as compared to the findings in OPG-/- mice (P < 0.05). However, higher apparent BMD and more compact plate-like trabeculae were observed in the mandible after treatment with zoledronate as compared to the findings in the proximal tibia. No significant differences were found in any parameter in both zoledronate groups.

CONCLUSIONS

The present study showed that zoledronate could reverse the significant bone loss in mice mandibles that was induced by OPG gene deficiency. This preventive effect, which was accompanied with considerable inhibition of bone turnover, differed in the mandible and in the proximal tibia and was independent of the OPG pathway.

Fourier transform infrared imaging microspectroscopy and tissue-level mechanical testing reveal intraspecies variation in mouse bone mineral and matrix composition

Fracture susceptibility is heritable and dependent upon bone morphology and quality. However, studies of bone quality are typically overshadowed by emphasis on bone geometry and bone mineral density. Given that differences in mineral and matrix composition exist in a variety of species, we hypothesized that genetic variation in bone quality and tissue-level mechanical properties would also exist within species. Sixteen-week-old female A/J, C57BL/6J (B6), and C3H/HeJ (C3H) inbred mouse femora were analyzed using Fourier transform infrared imaging and tissue-level mechanical testing for variation in mineral composition, mineral maturity, collagen cross-link ratio, and tissue-level mechanical properties. A/J femora had an increased mineral-to-matrix ratio compared to B6. The C3H mineral-to-matrix ratio was intermediate of A/J and B6. C3H femora had reduced acid phosphate and carbonate levels and an increased collagen cross-link ratio compared to A/J and B6. Modulus values paralleled mineral-to-matrix values, with A/J femora being the most stiff, B6 being the least stiff, and C3H having intermediate stiffness. In addition, work-to-failure varied among the strains, with the highly mineralized and brittle A/J femora performing the least amount of work-to-failure. Inbred mice are therefore able to differentially modulate the composition of their bone mineral and the maturity of their bone matrix in conjunction with tissue-level mechanical properties. These results suggest that specific combinations of bone quality and morphological traits are genetically regulated such that mechanically functional bones can be constructed in different ways.

Bone morphometry strongly predicts cortical bone stiffness and strength, but not toughness, in inbred mouse models of high and low bone mass

Inbred strains of mice make useful models to study bone properties. Our aim was to compare bone competence and cortical morphometric parameters of two inbred strains to better determine the role of bone structure and geometry in the process of bone failure. Morphometric analysis was performed on 20 murine femora with a low bone mass (C57BL/6J; B6) and 20 murine femora with a high bone mass (C3H/HeJ; C3H) using desktop microCT. The bones were tested under three-point bending to measure their mechanical properties. Results showed that the C3H strain is a more reproducible model regarding bone morphometric and mechanical phenotypes than the B6 strain. Bone strength, stiffness, yield force, yield displacement, and toughness, as well as morphometric traits, were all significantly different between the two strains, whereas postyield displacement was not. It was found that bone volume, cortical thickness, and cross-sectional area predicted almost 80% (p < 0.05) of bone stiffness, strength, and yield force. Nevertheless, cortical bone postyield properties such as bone toughness could not be explained by morphometry, but postyield whitening was observed in that phase. In conclusion, we found that morphometric parameters are strong predictors of preyield but not postyield properties. The lack of morphometric influence on bone competence in the postyield phase in combination with the observed postyield whitening confirmed the important contribution of ultrastructure and microdamage in the process of overall bone failure behavior, especially in the postyield phase.

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

These studies examined how genetic differences that regulate architectural and bone material properties would be expressed during fracture healing and determine whether any of these features would affect rates of healing as defined by regain of strength. Controlled fractures were generated in three inbred strains of mice: A/J, C57Bl/6J (B6), and C3H/HeJ (C3H). Both the A/J and B6 strains showed faster healing than the C3H strain based on regains in strength and stiffness. Strain-specific architectural features such as moment of inertia, cross-sectional area, and cortical thickness were all recapitulated during the development of the callus tissues. None of these traits were directly relatable to rates of fracture healing. However, rates of healing were related to variations in the temporal patterns of chondrogenic and osteogenic lineage development. The B6 strain expressed the highest percentage of cartilage gene products and had the longest period of chondrocyte maturation and hypertrophy. The slowest healing strain (C3H) had the shortest period of chondrogenic development and earliest initiation of osteogenic development. Although the A/J strain showed an almost identical pattern of chondrogenic development as the C3H strain, A/J initiated osteogenic development several days later than C3H during fracture healing. Long bone growth plates at 28 days after birth showed similar strain-specific variation in cartilage tissue development as seen in fracture healing. Thus, the B6 strain had the largest growth plate heights, cell numbers per column, and the largest cell size, whereas the C3H columns were the shortest, had the smallest number of cells per column, and showed the smallest cell sizes. These results show that (1) different strains of mice express variations of skeletal stem cell lineage differentiation and (2) that these variations affect the rate of fracture healing.

Differential effects of bone structural and material properties on bone competence in C57BL/6 and C3H/He inbred strains of mice

The femoral neck is a relevant and sensitive site for studying the degree of osteopenia. Engineering principles predict that bone structural parameters, like cross-sectional geometry, are important determinants of bone mechanical parameters. Mechanical parameters are also directly affected by the material properties of the bone tissue. However, the relative importance of structural and material properties is still unknown. The aim of this study was to compare bone competence and structural parameters between a murine strain showing a low bone mass phenotype, C57BL/6 (B6), and another one showing a high bone mass phenotype, C3H/He (C3H), in order to better determine the role of bone structure and geometry in bone failure behavior. Murine femora of 12- and 16-week-old B6 and 12- and 16-week-old C3H inbred strains were mechanically tested under axial loading of the femoral head. In order to assess the structural properties, we performed three-dimensional morphometric analyses in five different compartments of the mouse femur using micro-computed tomography. The mechanical tests revealed that B6 femora became stiffer, stronger, and tougher at 12-16 weeks, while bone brittleness stayed constant. C3H bone stiffness increased, but strength remained constant, work to failure decreased, and bone became more brittle. These age effects indicated that B6 did not reach peak bone properties at 16 weeks of age and C3H did reach maximal skeletal biomechanical properties before 16 weeks of age. Our investigations showed that 83% of the strength of the femoral neck in the B6 strain was explained by cortical thickness at this location; in contrast, in C3H none of the mechanical properties of the femoral neck was explained by bone structural parameters. The relative contributions of bone structural and material properties on bone strength are different in B6 and C3H. We hypothesize that these different contributions are related to differences at the ultrastructural level of bone that affect bone failure.

Percolation theory relates corticocancellous architecture to mechanical function in vertebrae of inbred mouse strains

Complex corticocancellous skeletal sites such as the vertebra or proximal femur are connected networks of bone capable of transferring mechanical loads. Characterizing these structures as networks may allow us to quantify the load transferring behavior of the emergent system as a function of the connected cortical and trabecular components. By defining the relationship between certain physical bone traits and mechanical load transfer pathways, a clearer picture of the genetic determinants of skeletal fragility can be developed. We tested the hypothesis that the measures provided by network percolation theory will reveal that different combinations of cortical, trabecular, and compositional traits lead to significantly different load transfer pathways within the vertebral bodies among inbred mouse strains. Gross morphologic, micro-architectural, and compositional traits of L5 vertebrae from 15 week old A/J (A), C57BL6/J (B6), and C3H/HeJ (C3H) inbred mice (n=10/strain) were determined using micro-computed tomography. Measures included total cross-sectional area, bone volume fraction, trabecular number, thickness, spacing, cortical area, and tissue mineral density. Two-dimensional coronal sections were converted to network graphs with the cortical shell considered as one highly connected node. Percolation parameters including correlation length (average number of connected nodes between superior and inferior surfaces), chemical length (minimum number of connected nodes between surfaces), and backbone mass (strut number) were measured. Analysis of the topology of the connected bone networks showed that A and B6 mice transfer load through trabecular pathways in the middle of the vertebral body in addition to the cortical shell. C3H mice transfer load primarily through the highly mineralized cortical shell. Thus, the measures provided by percolation theory provide a quantitative approach to study how different combinations of cortical and trabecular traits lead to mechanically functional structures. The data further emphasize the interdependent nature of these physical bone traits suggesting similar genetic variants may affect both trabecular and cortical bone. Therefore, developing a network approach to study corticocancellous architecture during growth should further our understanding of the biological basis of skeletal fragility and, thus, provide novel engineering approaches to studying the genetic basis of fracture risk.