Loss of bone strength is dependent on skeletal site in disuse osteoporosis in rats

Progression of microstructural deterioration in load-bearing immobilization osteopenia

Purpose

Immobilization osteopenia is a major healthcare problem in clinical and social medicine. However, the mechanisms underlying this bone pathology caused by immobilization under load-bearing conditions are not yet fully understood. This study aimed to evaluate sequential changes to the three-dimensional microstructure of bone in load-bearing immobilization osteopenia using a fixed-limb rat model.

Materials and method

Eight-week-old specific-pathogen-free male Wistar rats were divided into an immobilized group and a control group (n = 60 each). Hind limbs in the immobilized group were fixed using orthopedic casts with fixation periods of 1, 2, 4, 8, and 12 weeks. Feeding and weight-bearing were freely permitted. Length of the right femur was measured after each fixation period and bone microstructure was analyzed by micro-computed tomography. The architectural parameters of cortical and cancellous bone were analyzed statistically.

Results

Femoral length was significantly shorter in the immobilized group than in the control group after 2 weeks. Total area and marrow area were significantly lower in the immobilized group than in the control group from 1 to 12 weeks. Cortical bone area, cortical thickness, and polar moment of inertia decreased significantly after 2 weeks. Some cancellous bone parameters showed osteoporotic changes at 2 weeks after immobilization and the gap with the control group widened as the fixation period extended (P < 0.05).

Conclusion

The present results indicate that load-bearing immobilization triggers early deterioration of microstructure in both cortical and cancellous bone after 2 weeks.

Spatiotemporal responses of trabecular and cortical bone to complete spinal cord injury in skeletally mature rats

Objective

Methods

Results

Conclusions

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Skeletally mature spinal cord transected rats display biphasic bone loss

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The osteoporosis manifests over slower time scales than in skeletally immature rats.

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Relevancy for testing efficacy of interventions against SCI-induced osteoporosis.

Hypobaric hypoxia deteriorates bone mass and strength in mice

Mountaineers at high altitude are at increased risk of acute mountain sickness as well as high altitude pulmonary and cerebral edema. A densitometric study in mountaineers has suggested that expeditions at high altitude decrease bone mineral density. Surprisingly, the in vivo skeletal effects of hypobaric hypoxia are largely unknown, and have not been studied using advanced contemporary methods to assess bone microstructure. Eighty-four 22-week-old female mice were divided into seven groups with 12 mice in each group: 1. Baseline; 2. Normobaric, 4 weeks; 3. Hypobaric hypoxia, 4 weeks; 4. Normobaric, 8 weeks; 5. Hypobaric hypoxia, 8 weeks; 6. Normobaric, 12 weeks; and 7. Hypobaric hypoxia, 12 weeks. Hypobaric hypoxia mice were housed in hypobaric chambers at an ambient pressure of 500 mbar (5500 m altitude), while normobaric mice were housed at sea level atmospheric pressure for 4, 8, or 12 weeks, respectively. Hypobaric hypoxia had a profound impact on femoral cortical bone and L4 trabecular bone, while the effect on femoral trabecular bone was less pronounced. Hypobaric hypoxia reduced the bone strength of the femoral mid-diaphysis and L4 at all time-points. At femoral cortical bone, hypobaric hypoxia reduced bone formation through fewer mineralizing surfaces and lower bone formation rate after 2 weeks. In addition, bone strength decreased, and C-terminal telopeptide of type I collagen (CTX-I) increased independently of the duration of exposure to simulated high altitude. At L4, hypobaric hypoxia resulted in a substantial reduction in bone volume fraction, trabecular thickness, and trabecular number after 4 weeks of exposure. Hypobaric hypoxia reduced bone strength and femoral bone mass, while femoral trabecular bone was much less affected, indicating the skeletal response to hypobaric hypoxia differ between cortical and trabecular bone. These findings provide initial preclinical support for future clinical studies in mountaineers to assess bone status and bone strength after exposure to prolonged high altitude exposure.

Differential Effects of Neurectomy and Botox-induced Muscle Paralysis on Bone Phenotype and Titanium Implant Osseointegration

Metabolic bone is highly innervated by both sensory and sympathetic nerves. In addition to skeletal development, neural regulation participates in local bone remodeling, which is important for successful osseointegration of titanium implants. Neurectomy is a model used to investigate the lack of neural function on bone homeostasis, but the relative impacts of direct denervation to bone or denervation-induced muscle paralysis are less well defined. To investigate this difference, we used two nerve intervention models, sciatic and femoral neurectomy (SFN) v. botox-induced muscle paralysis (BTX) and assessed the resulting femoral bone phenotype and Ti implant osseointegration. Male Sprague Dawley rats (19) were randomly divided into three groups: implant control (n = 5), SFN (n = 7), and BTX (n = 7). Ti implants (microrough/hydrophilic [modSLA], Institut Straumann AG) were placed in the distal metaphysis of each femur on day 24 post-SFN or BTX. Bone and muscle were examined on day 28 after implant insertion. Both nerve intervention models impaired osseointegration. MicroCT and histology indicated that both models had reduced trabecular bone formation. Only BTX reduced cortical bone formation and increased cortical bone porosity. BTX resulted in more bone loss characterized by the least trabecular and cortical bone, as well as osseointegration. Osteoblasts isolated from the tibia exhibited a model-specific phenotype when they were grown on Ti substrates in vitro. Neurectomy caused more severe muscle atrophy than botox injection. These results indicate that neural regulation directly modulates bone formation and osseointegration. Muscle paralysis modulated the effects of loss of neural inputs into bone, supporting the hypothesis that mechanical loading of bone is a factor in achieving successful osseointegration. The different effects of botox and neurectomy on bone phenotype indicated that the sensory and sympathetic nerves had a role in the osseointegration process.

A Systematic Review of Animal Models of Disuse-Induced Bone Loss

OBJECTIVE

Several different animal models are used to study disuse-induced bone loss. This systematic review aims to give a comprehensive overview of the animal models of disuse-induced bone loss and provide a detailed narrative synthesis of each unique animal model.

METHODS

PubMed and Embase were systematically searched for animal models of disuse from inception to November 30, 2019. In addition, Google Scholar and personal file archives were searched for relevant publications not indexed in PubMed or Embase. Two reviewers independently reviewed titles and abstracts for full-text inclusion. Data were extracted using a predefined extraction scheme to ensure standardization.

RESULTS

1964 titles and abstracts were screened of which 653 full-text articles were included. The most common animal species used to model disuse were rats (59%) and mice (30%). Males (53%) where used in the majority of the studies and genetically modified animals accounted for 7%. Twelve different methods to induce disuse were identified. The most frequently used methods were hindlimb unloading (44%), neurectomy (15%), bandages and orthoses (15%), and botulinum toxin (9%). The median time of disuse was 21 days (quartiles: 14 days, 36 days) and the median number of animals per group subjected to disuse was 10 (quartiles: 7, 14). Random group allocation was reported in 43% of the studies. Fewer than 5% of the studies justified the number of animals per group by a sample size calculation to ensure adequate statistical power.

CONCLUSION

Multiple animal models of disuse-induced bone loss exist, and several species of animals have successfully been studied. The complexity of disuse-induced bone loss warrants rigid research study designs. This systematic review emphasized the need for standardization of animal disuse research and reporting.

The Effect of Normobaric Intermittent Hypoxia Therapy on Bone in Normal and Disuse Osteopenic Mice

Bromer, Frederik Duch, Mikkel Bo Brent, Michael Pedersen, Jesper Skovhus Thomsen, Annemarie Brüel, and Casper Bindzus Foldager. The effect of normobaric intermittent hypoxia therapy on bone in normal and disuse osteopenic mice. High Alt Med Biol. 22: 225-234, 2021. Background: Systemic intermittent hypoxia therapy (IHT) has been shown to elicit beneficial effects on multiple physiological systems. However, only few studies have investigated the effect of long-term normobaric IHT on bone mass and mechanical and microstructural properties. The aim of the present study was to examine the effect of IHT on bone in both healthy and osteopenic mice. Materials and Methods: Thirty mice were stratified into four groups: Ctrl, Ctrl+IHT, Botox, and Botox+IHT. Osteopenia was induced by injecting Botox into the right hindlimb of the mice causing paralysis and disuse. IHT animals were placed in a normobaric hypoxia-chamber (10% oxygen) for 1 hour twice daily 5 days/week. Animals were sacrificed after 21 days, and DEXA, micro-computed tomography, and mechanical testing were performed on the femora. Results: As expected, Botox resulted in a significant reduction of bone mineral content (-23.4%), area bone mineral density (-19.1%), femoral neck strength (F_max: -54.7%), bone volume fraction (bone volume/tissue volume: -41.8%), and trabecular thickness (-32.4%). IHT had no measurable effect on the bone properties in either healthy or osteopenic mice. Conclusion: The study confirmed that Botox led to loss of bone mass, deterioration of trabecular microstructure, and loss of bone strength. These changes were not influenced by IHT. Notably, IHT had no detrimental effect on bone in either healthy or osteopenic mice. This indicates that IHT of ailments outside of the skeletal system may be administered without causing harm to the bone.

Botulinum Toxin A and Osteosarcopenia in Experimental Animals: A Scoping Review

We conducted a scoping review to investigate the effects of intramuscular injection of Botulinum Toxin A (BoNT-A) on bone morphology. We investigated if the muscle atrophy associated with Injection of BoNT-A had effects on the neighboring bone. We used the search terms: osteopenia, bone atrophy, Botulinum Toxin A, Micro-CT, mice or rat. The following databases were searched: Medline, Embase, PubMed and the Cochrane Library, between 1990 and 2020. After removal of duplicates, 228 abstracts were identified of which 49 studies satisfied our inclusion and exclusion criteria. The majority of studies (41/49) reported a quantitative reduction in at least one measure of bone architecture based on Micro-CT. The reduction in the ratio of bone volume to tissue volume varied from 11% to 81% (mean 43%) according to the experimental set up and study time points. While longer term studies showed muscle recovery, no study showed complete recovery of all bone properties at the termination of the study. In experimental animals, intramuscular injection of BoNT-A resulted in acute muscle atrophy and acute degradation of the neighboring bone segment. These findings may have implications for clinical protocols in the use of Botulinum Toxin in children with cerebral palsy, with restraint recommended in injection protocols and consideration for monitoring bone density. Clinical studies in children with cerebral palsy receiving injections of Botulinum are indicated.

Activin type IIA decoy receptor and intermittent parathyroid hormone in combination overturns the bone loss in disuse-osteopenic mice

Damage of the lower motor neuron cell bodies or their axons results in reduced or abolished voluntary movement accompanied by a substantial loss of bone and muscle mass. Intermittent parathyroid hormone 1-34 (PTH) (teriparatide) is one of the most potent bone-anabolic treatment regimens. ActRIIA-mFc is an activin type IIA decoy receptor that increases bone mass mediated by inhibition of the activin receptor signaling pathway. We investigated whether PTH or ActRIIA-mFc alone or in combination could prevent loss of bone and muscle mass induced by injecting botulinum toxin A (BTX) into the right hind limb in mice. Seventy-two 16-week-old female C57BL/6 mice were allocated to the following groups: Baseline, Control, BTX, BTX + ActRIIA-mFc (10 mg/kg), BTX + PTH (100 μg/kg), and BTX + ActRIIA-mFc + PTH. The mice were sacrificed after three weeks of disuse and treatment. In contrast to monotherapy with PTH, ActRIIA-mFc alone or in combination with PTH was able partly or completely to prevent disuse-induced loss of whole femoral bone mass, trabecular thickness, and bone strength. Moreover, an additive effect of ActRIIA-mFc and PTH on areal bone mineral density and trabecular bone volume was found. In summary, ActRIIA-mFc and PTH in combination were more effective in preventing disuse-induced bone loss and deterioration of trabecular micro-architecture than either treatment alone.

Rodent model of disuse-induced bone loss by hind limb injection with botulinum toxin A

Bone loss materializes rapidly after immobilization or mechanical unloading. Hind limb injection with botulinum toxin A (BTX) is a highly reproducible animal model for disuse-induced bone loss. Here we describe an easy-to-use and enhanced version of the method employing multiple hind limb injections with BTX to induce a pervasive muscle paralysis and thereby disuse of the hind limb. Thirty-six 12-14-week-old female Wistar rats were stratified into three groups: Baseline (Base), Control (Ctrl), and BTX. Disuse was achieved by injecting BTX directly into the right quadriceps femoris muscle, the hamstring muscles, and the posterior calf muscles. The rats were sacrificed after six weeks, and the right rectus femoris muscle and femur were isolated and analyzed. Hind limb disuse resulted in a significant and substantial loss of both muscle mass and bone mass. The loss of bone mass was accompanied by a reduction of trabecular bone mass and a deterioration of the trabecular micro-architecture with a reduction of trabecular thickness and trabecular number compared to Ctrl. In addition, the trabeculae changed from a more plate-like towards a more rod-like shape as indicated by an increase in the structure model index.•Multiple injections with BTX targeting muscles on both the anterior and posterior thigh and the calf ensure a uniform and pervasive muscle paralysis and hind limb disuse.•Hind limb injections with BTX results in a substantial loss of muscle and bone mass and deterioration of the trabecular micro-architecture.•The induction of hind limb disuse with BTX is highly reproducible.

Reversibility of marrow adipose accumulation and reduction of trabecular bone in the epiphysis of the proximal tibia

Mechanical stimuli play an important role in the homeostasis of trabecular bone and marrow adipose tissue, particularly for the weight-bearing skeleton. Prolonged immobilization and disuse have been shown to reduce trabecular bone content and increase marrow adipose tissue in the bones of lower limb joints such as the knee. However, details on the temporal response of this relationship to prolonged immobilization and its reversibility is limited. Forty rats had one knee immobilized at 45° of flexion for 2, 4, 8, or 16 weeks and subsequently remobilized for 0 or 8 weeks. The contralateral knees were used as controls. Histomorphometric measures of trabecular bone and marrow adipose tissue (MAT) areas were conducted in the epiphysis of the proximal tibia. Knee immobilization for 4, 8, and 16 weeks significantly reduced trabecular bone area by -0.125, -0.139, and -0.161 mm²/mm², respectively, with corresponding 95 % CIs of [-0.012, -0.239], [-0.006, -0.273], and [-0.101, -0.221]. MAT area significantly increased at 2 and 16 weeks by +0.008 and +0.027 mm²/mm², respectively, with 95 % CIs of [0.014, 0.002] and [0.039, 0.016]. Remobilization for 8 weeks restored trabecular bone area compared to the contralateral knee and the magnitude of change was significantly greater for 8 and 16 weeks of immobilization with effect sizes of 1.69 and 1.86, respectively. The difference in MAT area between immobilized and contralateral knees were eliminated with remobilization. These results characterize the temporal response of trabecular bone and MAT in the epiphysis of the proximal tibia to joint immobilization and remobilization.

Decreased mandibular cortical bone quality after botulinum toxin injections in masticatory muscles in female adults

This study aimed to clarify how masticatory muscle atrophy induced by botulinum toxin (BTX) injection affects cortical bone quality of the mandible using 3D modeling technology. A total of 39 young (26.9 ± 6.0 years) and 38 post-menopausal (55.3 ± 6.3 years) females were included. Computed tomography (CT) images were obtained before and after 12 months of treatment. Predictor variables were application of a stabilization splint, and/or two times of BTX injection in the bilateral temporalis and masseter muscles within a six-month interval. Outcome variables were changes in average Hounsfield units (HU) and cortical thickness of region of interest (ROI). 3D mandibular models were reconstructed using CT images, and models were used to calculate average HU and cortical thickness of ROIs, including inferior half of the lateral surface of ascending ramus, coronoid process, and temporomandibular joint condyle. Cortical bone quality at muscle insertion site was influenced by decreased muscle thickness but seemed not to be affected by decreased functional loading. Reduced functional loading seemed to influence cortical bone quality of the condyles. These effects were more remarkable in post-menopausal females. Hence, decreased masticatory muscle thickness may lead to alterations of the mandibular cortical structures, especially in post-menopausal females.

Therapeutic Effects of Low-Intensity Pulsed Ultrasound on Osteoporosis in Ovariectomized Rats: Intensity-Dependent Study

This study investigated the effects of low-intensity pulsed ultrasound (LIPUS) of different spatial-average-temporal-average intensity (I_SATA) ranging from 15-150 mW/cm² on the treatment of osteoporosis in ovariectomized rats. Healthy 3-mo-old female Sprague-Dawley rats were randomly divided into nine groups (n = 12 per group): sham-ovariectomy (OVX) control group, OVX control group and OVX groups treated with LIPUS at seven different intensities (I_SATA: 15, 30, 50, 75, 100, 125 and 150 mW/cm², respectively). LIPUS was applied to bilateral femurs 12 wk post-OVX for 20 min/d for 6 wk. Micro-computed tomography, biomechanical tests, serum biochemical analysis and grip strength tests were performed to evaluate the therapeutic effects of LIPUS at different intensities. Results revealed that LIPUS intensity yielded strong correlations with bone mineral density and bone microstructure (R² = 0.57-0.83) and bone mechanical strength (R² = 0.80-0.97), and that the intensity of 150 mW/cm², instead of the 30 mW/cm² widely used in bone fracture healing, was most effective in maintaining bone mass among all the LIPUS signals between 15 and 150 mW/cm². This suggests that higher ultrasound intensity (i.e., 150 mW/cm²) may be more effective than lower intensity in mitigation of osteopenia and osteoporosis.

The Efficacy of PTH and Abaloparatide to Counteract Immobilization-Induced Osteopenia Is in General Similar

Immobilization results in a substantial bone loss and increased fracture risk. Powerful bone anabolic therapies are necessary to counteract the bone loss and reduce fracture risk during periods with immobilization. Intermittent parathyroid hormone 1-34 (PTH) (teriparatide) and PTH related peptide analog abaloparatide (ABL) are potent bone anabolic therapies acting through the same receptor, but induce different durations of signaling response. We investigated the efficacy of PTH or ABL in preventing immobilization-induced bone loss in rats in a direct mole-to-mole comparison. Immobilization was achieved by injecting botulinum toxin type A (BTX) into the right hindlimb musculature. Sixty 14-week-old female Wistar rats were allocated to the following groups: Baseline, Control, BTX, BTX + PTH (80 μg/kg/day), and BTX + ABL (77 μg/kg/day). Immobilization resulted in a substantial and significant reduction in bone mineral density (aBMD), metaphyseal and epiphyseal trabecular bone volume fraction (BV/TV) and trabecular thickness (Tb.Th), metaphyseal trabecular number (Tb.N), and femoral neck bone strength. Both PTH and ABL prevented the immobilization-induced decrease in aBMD, metaphyseal and epiphyseal Tb.Th, and metaphyseal Tb.N. In addition, PTH rescued the reduction in metaphyseal BV/TV and femoral neck strength, while ABL did not. However, the effect of PTH and ABL did not differ significantly for serum calcium, aBMD, metaphyseal, and epiphyseal BV/TV, Tb.Th, or Tb.N. In conclusion, in a mole-to-mole comparison the efficacy of PTH and ABL is similar in counteracting immobilization-induced reduction in bone mineral density, deterioration in trabecular microarchitecture, and decrease in bone strength.

Botox-induced muscle paralysis alters intracortical porosity and osteocyte lacunar density in skeletally mature rats

Reduced mechanical loading can lead to disuse osteoporosis, resulting in bone fragility. Disuse models report macroscopic bone loss due to muscle inactivity and immobilization, yet only recently has there been quantification of the effects of disuse on the vascular pores and osteocyte network, which are believed to play an important role in mechanotransduction via interstitial fluid flow. The goal of this study was to perform a high-resolution analysis of the effects of muscle inactivity on intracortical porosity and osteocyte lacunar density in skeletally mature rats. Muscle paralysis was induced in 20-week-old female Sprague Dawley rats by injection of botulinum neurotoxin. Rats were injected in the right hindlimb muscles with either Botox (BTX, n = 8) or saline solution (CTRL, n = 8), with a third group used as baseline controls (n = 8). Four weeks after injection, Botox caused a ∼60% reduction in hindlimb muscle mass. High-resolution micro-CT analysis showed that Botox-induced muscle paralysis increased vascular canal porosity and reduced osteocyte lacunar density within the tibial metaphysis cortex. Cortical thickness and other areal properties were diminished in the proximal tibial metaphysis, whereas no differences were found in the mid-diaphysis. Within the BTX group, the injected limbs showed a lower cancellous bone volume fraction relative to the contralateral limb. These results indicate that diminished muscle activity alters the vascular canal porosity and osteocyte lacunar density in cortical bone, which could alter interstitial fluid flow, affecting molecular transport and the transmission of mechanical signals to osteocytes. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

A follistatin-based molecule increases muscle and bone mass without affecting the red blood cell count in mice

Inhibitors of the activin receptor signaling pathway (IASPs) have become candidate therapeutics for sarcopenia and bone remodeling disorders because of their ability to increase muscle and bone mass. However, IASPs utilizing activin type IIA and IIB receptors are also potent stimulators of erythropoiesis, a feature that may restrict their usage to anemic patients because of increased risk of venous thromboembolism. Based on the endogenous TGF-β superfamily antagonist follistatin (FST), a molecule in the IASP class, FST_ΔHBS-mFc, was generated and tested in both ovariectomized and naive BALB/c and C57BL/6 mice. In ovariectomized mice, FST_ΔHBS-mFc therapy dose-dependently increased cancellous bone mass up to 42% and improved bone microstructural indices. For the highest dosage of FST_ΔHBS-mFc (30 mg/kg, 2 times/wk), the increase in cancellous bone mass was similar to that observed with parathyroid hormone therapy (1-34, 80 µg/kg, 5 times/wk). Musculus quadriceps femoris mass dose-dependently increased up to 21% in ovariectomized mice. In both ovariectomized and naive mice, FST_ΔHBS-mFc therapy did not influence red blood cell count or hematocrit or hemoglobin levels. If the results are reproduced, a human FST_ΔHBS-mFc version could be applicable in patients with musculoskeletal conditions irrespective of hematocrit status.-Lodberg, A., van der Eerden, B. C. J., Boers-Sijmons, B., Thomsen, J. S., Brüel, A., van Leeuwen, J. P. T. M., Eijken, M. A follistatin-based molecule increases muscle and bone mass without affecting the red blood cell count in mice.

Paravertebral injection of botulinum toxin-A reduces lumbar vertebral bone quality

Aging has been associated with decreases in muscle strength and bone quality. In older patients, paravertebral muscle atrophy tends to coincide with vertebral osteoporosis. The purpose of this study was to investigate the effects of a paravertebral injection of botulinum toxin-A (BTX) on paravertebral muscle atrophy and lumbar vertebral bone quality. Forty 16-week-old female SD rats were randomly divided into four groups: (1) a control group (CNT); (2) a resection of erector spinae muscles group (RESM); (3) a botulinum toxin-A group (BTX), treated with 5U BTX by local injection into the paravertebral muscles bilaterally; and (4) a positive control group (OVX), treated by bilateral ovariectomy. Rats were sacrificed at 12 weeks post-surgery, and the lumbar vertebrae (L3-L6) were collected. Micro-CT scans showed that rats in the three experimental groups-particularly the OVX rats-had fewer trabeculae and trabecular connections than rats in the CNT group. BMD was significantly lower in rats in the OVX, RESM, and BTX groups than in the CNT group (p < 0.01). Vertebral compression testing revealed significantly lower maximum load, energy absorption, maximum stress, and elastic modulus values in the three experimental groups compared with the CNT group (p < 0.01); these parameters were lowest in the OVX group (p < 0.05). Our results demonstrate that local BTX injection causes sufficient muscle atrophy and dysfunction to result in local lumbar vertebral bone loss and quality deterioration in a model of paravertebral muscle atrophy. Clinical Significance: The muscular tissues surrounding the lumbar vertebrae should be preserved during clinical surgery to avoid loss of bone quality and mass in the adjacent bone. Maintaining paravertebral muscle strength is an important consideration for patients with early osteoporosis. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2664-2670, 2018.

Long-Term Quantitative Evaluation of Muscle and Bone Wasting Induced by Botulinum Toxin in Mice Using Microcomputed Tomography

Muscle and bone masses are highly correlated and muscles impose large loads on bone. Muscle wasting that accompanies bone loss has been poorly investigated. 21 female mice were spread into seven groups. At day 0, 18 mice received Botulinum toxin (BTX) injection in the quadriceps muscle to induce paralysis of the right hind limb; the left contralateral side was used as control. Mice were sacrificed at 7, 14, 21, 28, 56 and 90 days post-injection. A remaining group was sacrificed at day 0. Trabecular bone volume was determined by microcomputed tomography (microCT) at the distal femur and tibia proximal metaphyses on both sides. Limbs were immersed in an HgCl₂ solution allowing muscle visualization by microCT. On 2D sections, the cross-sectional areas and form-factors were measured for the quadriceps at mid-thigh and gastrocnemius at mid-leg and these muscles were dissected and weighed. Bone volume decreased in the paralysed side. Bone loss was maximal at 56 days followed by recuperation at 90 days. The cross-sectional areas of gastrocnemius and quadriceps were significantly lower in the paralysed limb from 7 days; the decrease was maximum at 21 days for the gastrocnemius and 28 days for the quadriceps. No difference in form-factors was found between the two limbs. Similar results were obtained with the anatomical method and significant correlations were obtained between the two methods. Quantitative analysis of muscle loss and recovery was possible by microCT after using a metallic contrast agent. Loss of bone secondary to muscle wastage induced by BTX and recovery showed a parallel evolution for bone and muscles.

Evaluation of the rivastigmine role against botulinum toxin-A-induced osteoporosis in albino rats: A biochemical, histological, and immunohistochemical study

The present study aimed to evaluate the role of rivastigmine against the effect of a single unilateral botulinum toxin-A (BTX-A) injection on the bone and bone marrow of adult albino rats 4 weeks after injection. Twenty-four Wistar albino rats were divided into four equal groups: group I, which received distilled water; group II, which received rivastigmine (0.3 mg/kg daily, intraperitoneally for 4 weeks); group III, which received BTX-A (4 IU in 0.2 mL physiological saline) single dose, intramuscularly; and group IV, which received BTX-A + rivastigmine. The results revealed that BTX-A induced a significant decrease in the calcium level with a significant increase in the phosphorus, alkaline phosphatase, C-reactive protein, and tumor necrosis factor α levels in serum. Furthermore, a significant increase in malondialdehyde with a significant decrease in reduced glutathione activities in both bone and bone marrow. Histologically, a distortion and thinning out of the compact bone and trabeculae of cancellous bone of the rat femur in the BTX-A group with an increase in adipocytes in adjacent bone marrow were detected. Immunohistochemically, Cluster of Differentiation 68 (CD68) showed a significant increase in both osteoclasts and bone marrow macrophage. Rivastigmine treatment could relieve the toxic effects induced by BTX-A. In conclusion, rivastigmine has a protective effect against the hazardous effects of BTX-A on bone and bone marrow.

A soluble activin type IIA receptor mitigates the loss of femoral neck bone strength and cancellous bone mass in a mouse model of disuse osteopenia

Disuse causes a rapid and substantial bone loss distinct in its pathophysiology from the bone loss associated with cancers, age, and menopause. While inhibitors of the activin-receptor signaling pathway (IASPs) have been shown to prevent ovariectomy- and cancer-induced bone loss, their application in a model of disuse osteopenia remains to be tested. Here, we show that a soluble activin type IIA receptor (ActRIIA-mFc) increases diaphyseal bone strength and cancellous bone mass, and mitigates the loss of femoral neck bone strength in the Botulinum Toxin A (BTX)-model of disuse osteopenia in female C57BL/6J mice. We show that ActRIIA-mFc treatment preferentially stimulates a dual-effect (anabolic-antiresorptive) on the periosteal envelope of diaphyseal bone, demonstrating in detail the effects of ActRIIA-mFc on cortical bone. These observations constitute a previously undescribed feature of IASPs that mediates at least part of their ability to mitigate detrimental effects of unloading on bone tissue. The study findings support the application of IASPs as a strategy to combat bone loss during disuse.

PTH (1-34) and growth hormone in prevention of disuse osteopenia and sarcopenia in rats

Osteopenia and sarcopenia develops rapidly during disuse. The study investigated whether intermittent parathyroid hormone (1-34) (PTH) and growth hormone (GH) administered alone or in combination could prevent or mitigate disuse osteopenia and sarcopenia in rats. Disuse was achieved by injecting 4IU botulinum toxin A (BTX) into the right hindlimb musculature of 12-14-week-old female Wistar rats. Seventy-two rats were divided into six groups: 1. Baseline; 2. Ctrl; 3. BTX; 4. BTX+GH; 5. BTX+PTH; 6. BTX+PTH+GH. PTH (1-34) (60μg/kg/day) and GH (5mg/kg/day). The animals were sacrificed after 6weeks of treatment. Sarcopenia was established by histomorphometry, while the skeletal properties were determined using DXA, μCT, mechanical testing, and dynamic bone histomorphometry. Disuse resulted in lower muscle mass (-63%, p<0.05), trabecular BV/TV (-28%, p<0.05), Tb.Th (-11%, p<0.05), lower diaphyseal cortical thickness (-10%, p<0.001), and lower bone strength at the distal femoral metaphysis (-27%, p<0.001) compared to Ctrl animals. PTH fully counteracted the immobilization-induced lower BV/TV, Tb.Th, and distal femoral metaphyseal strength. GH increased muscle mass (+17%, p<0.05) compared to BTX, but did not prevent the immobilization-induced loss of bone strength, BV/TV, and cortical trabecular thickness. Combination of PTH and GH increased distal femoral metaphyseal bone strength (+45%, p<0.001), BV/TV (+50%, p<0.05), Tb.Th (+40%, p<0.05), and whole femoral aBMD (+15%, p<0.001) compared to BTX and muscle mass (+21%, p<0.05) compared to BTX+PTH. In conclusion, PTH and GH in combination is more efficient at preventing the disuse-related deterioration of bone strength, density, and micro-architecture than either PTH or GH given as monotherapy. Furthermore, GH, either alone or in combination with PTH, attenuated disuse-induced loss of muscle mass. The combination of PTH and GH resulted in a more effective treatment than PTH and GH as monotherapy.

Long-Term High-Dose Resveratrol Supplementation Reduces Bone Mass and Fracture Strength in Rats

Resveratrol (RSV) is a natural polyphenolic compound. A recent study suggests a positive effect on BMD in men; however, the underlying changes in microstructure and strength remain unknown. We aimed to investigate the effects of RSV on the skeleton in hindlimb-immobilized and non-immobilized rats. Seventy-two female Wistar rats were divided into six groups. Two baseline (BSL) groups underwent short-term diet intervention for 4 weeks before sacrifice [phytoestrogen-deficient diet (PD) (BSL + PD) or RSV diet (600 mg/kg body weight/day) (BSL + RSV)]. Four groups were injected in the right hindlimb with botulinum toxin (BTX) (immobilized) or saline (non-immobilized), and fed either PD diet or RSV diet 4 weeks pre-injection and 6 weeks post-injection before sacrifice (BTX + PD, BTX + RSV, PD, and RSV, respectively). DXA, µCT, dynamic histomorphometry, and mechanical tests were performed. Short-term RSV treatment did not affect bone parameters, whereas long-term RSV exposure had a consistent negative impact on non-immobilized rats (RSV vs. PD); whole femoral aBMD (p = 0.01) and distal femoral metaphyseal Tb.N (p = 0.01), Tb.Sp (p = 0.02), and BV/TV (p = 0.07). At the femoral mid-diaphysis, RSV increased periosteal resorption (p = 0.01) and increased endosteal formation (p = 0.02), while mineralization was unaffected. In addition, RSV reduced femoral mid-diaphyseal three-point bending strength (p = 0.03) and stiffness (p = 0.04). BTX-induced immobilization resulted in significant bone loss and reduced bone strength; however, RSV supplementation was unable to prevent this. In conclusion, long-term high-dose RSV reduced bone mass and fracture strength and did not prevent immobilization-induced bone loss in rats.

Hypodynamia Alters Bone Quality and Trabecular Microarchitecture

Disuse induces a rapid bone loss in humans and animals; hypodynamia/sedentarity is now recognized as a risk factor for osteoporosis. Hypodynamia also decreases bone mass but its effects are largely unknown and only few animal models have been described. Hypodynamic chicken is recognized as a suitable model of bone loss but the effects on the quality have not been fully explored. We have used ten chickens bred in a large enclosure (FREE group); ten others were confined in small cages with little space to move around (HYPO group). They were sacrificed at 53 days and femurs were evaluated by microcomputed tomography (microCT) and nanoindentation. Sections (4 µm thick) were analyzed by Fourier Transform InfraRed Microspectroscopy (FTIR) to see the effects on mineralization and collagen and quantitative backscattered electron imaging (qBEI) to image the mineral of the bone matrix. Trabecular bone volume and microarchitecture were significantly altered in the HYPO group. FTIR showed a significant reduction of the mineral-to-matrix ratio in the HYPO group associated with an increase in the carbonate content and an increase in crystallinity (calculated as the area ratio of subbands located at 1020 and 1030 cm^-1) indicating a poor quality of the mineral. Collagen maturity (calculated as the area ratio of subbands located at 1660 and 1690 cm^-1) was significantly reduced in the HYPO group. Reduced biomechanical properties were observed at the tissue level. Confined chicken represents a new model for the study of hypodynamia because bone changes are not created by a surgical lesion or a traumatic method. Animals have a reduced bone mass and present with an altered bone matrix quality which is less mineralized and whose collagen contains less crosslinks than in control chicken.

Role of endoplasmic reticulum stress in disuse osteoporosis

Osteoporosis is a major skeletal disease with low bone mineral density, which leads to an increased risk of bone fracture. Salubrinal is a synthetic chemical that inhibits dephosphorylation of eukaryotic translation initiation factor 2 alpha (eIF2α) in response to endoplasmic reticulum (ER) stress. To understand possible linkage of osteoporosis to ER stress, we employed an unloading mouse model and examined the effects of salubrinal in the pathogenesis of disuse osteoporosis. The results presented several lines of evidence that osteoclastogenesis in the development of osteoporosis was associated with ER stress, and salubrinal suppressed unloading-induced bone loss. Compared to the age-matched control, unloaded mice reduced the trabecular bone area/total area (B.Ar/T.Ar) as well as the number of osteoblasts, and they increased the osteoclasts number on the trabecular bone surface in a time-dependent way. Unloading-induced disuse osteoporosis significantly increased the expression of Bip, p-eIF2α and ATF4 in short-term within 6h of tail suspension, but time-dependent decreased in HU2d to HU14d. Furthermore, a significant correlation of ER stress with the differentiation of osteoblasts and osteoclasts was observed. Administration of salubrinal suppressed the unloading-induced decrease in bone mineral density, B.Ar/T.Ar and mature osteoclast formation. Salubrinal also increased the colony-forming unit-fibroblasts and colony-forming unit-osteoblasts. It reduced the formation of mature osteoclasts, suppressed their migration and adhesion, and increased the expression of Bip, p-eIF2α and ATF4. Electron microscopy showed that rough endoplasmic reticulum expansion and a decreased number of ribosomes on ER membrane were observed in osteoblast of unloading mice, and the abnormal ER expansion was significantly improved by salubrinal treatment. A TUNEL assay together with CCAAT/enhancer binding protein homologous protein (CHOP) expression indicated that ER stress-induced osteoblast apoptosis was rescued by salubrinal. Collectively, the results support the notion that ER stress plays a key role in the pathogenesis of disuse osteoporosis, and salubrinal attenuates unloading-induced bone loss by altering proliferation and differentiation of osteoblasts and osteoclasts via eIF2α signaling.

Immobilization and long-term recovery results in large changes in bone structure and strength but no corresponding alterations of osteocyte lacunar properties

The ability of osteocytes to demineralize the perilacunar matrix, osteocytic osteolysis, and thereby participate directly in bone metabolism, is an aspect of osteocyte biology that has received increasing attention during the last couple of years. The aim of the present work was to investigate whether osteocyte lacunar properties change during immobilization and subsequent recovery. A rat cortical bone model with negligible Haversian remodeling effects was used, with temporary immobilization of one hindlimb induced by botulinum toxin. Several complementary techniques covering multiple length scales enabled correlation of osteocyte lacunar properties to changes observed on the organ and tissue level of femoral bone. Bone structural parameters measured by μCT and mechanical properties were compared to sub-micrometer resolution SR μCT data mapping an unprecedented number (1.85 million) of osteocyte lacunae. Immobilization induced a significant reduction in aBMD, bone volume, tissue volume, and load to fracture, as well as the muscle mass of rectus femoris. During the subsequent recovery period, the bone structural and mechanical properties were only partly regained in spite of a long-term (28weeks) study period. No significant changes in osteocyte lacunar volume, density, oblateness, stretch, or orientation were detected upon immobilization or subsequent recovery. In conclusion, the bone architecture and not osteocyte lacunar properties or bone material characteristics dominate the immobilization response as well as the subsequent recovery.

Organ and tissue level properties are more sensitive to age than osteocyte lacunar characteristics in rat cortical bone

Modeling and remodeling induce significant changes of bone structure and mechanical properties with age. Therefore, it is important to gain knowledge of the processes taking place in bone over time. The rat is a widely used animal model, where much data has been accumulated on age-related changes of bone on the organ and tissue level, whereas features on the nano- and micrometer scale are much less explored. We investigated the age-related development of organ and tissue level bone properties such as bone volume, bone mineral density, and load to fracture and correlated these with osteocyte lacunar properties in rat cortical bone. Femora of 14 to 42-week-old female Wistar rats were investigated using multiple complementary techniques including X-ray micro-computed tomography and biomechanical testing. The body weight, femoral length, aBMD, load to fracture, tissue volume, bone volume, and tissue density were found to increase rapidly with age at 14–30 weeks. At the age of 30–42 weeks, the growth rate appeared to decrease. However, no accompanying changes were found in osteocyte lacunar properties such as lacunar volume, ellipsoidal radii, lacunar stretch, lacunar oblateness, or lacunar orientation with animal age. Hence, the evolution of organ and tissue level properties with age in rat cortical bone is not accompanied by related changes in osteocyte lacunar properties. This suggests that bone microstructure and bone matrix material properties and not the geometric properties of the osteocyte lacunar network are main determinants of the properties of the bone on larger length scales.

Vertical Trabeculae are Thinned More Than Horizontal Trabeculae in Skeletal-Unloaded Rats

Skeletal unloading results in a rapid thinning of the trabecular bone network, but it is unknown whether vertical and horizontal trabeculae are equally affected. Therefore, the purpose of the present study was to investigate whether horizontal and vertical trabeculae were thinned similarly during skeletal unloading in rats. Fifty-seven 16-week-old female Wistar rats were randomized into six groups: baseline; control 4 weeks; botulinum toxin A (BTX) 4 weeks; control 8 weeks; BTX 8 weeks; and two BTX injections 8 weeks (BTX + BTX8). The BTX animals were injected in the right hind limb with 4 IU BTX at the start of the study, while the BTX + BTX8 were also injected with 2 IU BTX after 4 weeks. The animals were killed after 0, 4, or 8 weeks. The distal femoral metaphyses were μCT scanned, and the strengths of the femoral necks, mid-diaphyses, and distal femoral metaphyses were ascertained. Disuse resulted in a significant loss of BV/TV, thinning of the trabeculae, and decrease in the degree of anisotropy, and in a significant reduced bone strength after both 4 and 8 weeks. The ratio of horizontal to vertical trabecular thickness (Tb.Th.horz/Tb.Th.vert) and the ratio of horizontal to vertical bone volume (BV.horz/BV.vert) were significantly higher in BTX animals than in control animals. In addition, the horizontal and vertical trabecular thickness probability density functions were more similar in BTX animals than in control animals. In conclusion, skeletal unloading decreased BV/TV, Tb.Th, the degree of anisotropy, and mechanical strength, while BV.horz/BV.vert and Tb.Th.horz/Tb.Th.vert were increased. This indicates that the more loaded vertical trabeculae are pronouncedly more thinned than the less loaded supporting horizontal trabeculae during unloading.

Dynamic acoustic radiation force retains bone structural and mechanical integrity in a functional disuse osteopenia model

Disuse osteopenia and bone loss have been extensively reported in long duration space mission and long term bed rest. The pathology of the bone loss is similar to osteoporosis but highly confined to weight bearing bones. The current anabolic and/or anti-resorptive drugs have systemic effects and are costly over extended time, with concerns of long term fracture risk. This study use Low Intensity Pulsed Ultrasound (LIPUS) as a non-invasive acoustic force and anabolic stimulus to countermeasure disuse induced bone loss. Four-month old C57BL/6 mice were randomized into five groups, 1) age-matched (AM), 2) non-suspended sham (NS), 3) non-suspended-LIPUS (NU), 4) suspended sham (SS), and 5) suspended-LIPUS (SU) groups. After four weeks of suspension, μCT analyses showed significant decreases in trabecular bone volume fraction (BV/TV) (-36%, p<0.005), bone tissue mineral density (TMD) (-3%, p<0.05), trabecular thickness (Tb.Th) (-12.5%, p<0.005), and increase in bone surface/bone volume (+BS/BV) (+16%, p<0.005), relative to age-matched (AM). The application of LIPUS for 20 min/day for 5 days/week, significantly increased TMD (+3%, p<0.05), Tb.Th (+6%, p<0.05), and decreased BS/BV (-10%, p<0.005), relative to suspension alone (SS) mice. Histomorphometry analyses showed a breakdown of bone microstructure under disuse conditions consist with μCT results. In comparison to SS mice, LIPUS treated bone showed increased structural integrity with increased bone formation rates at metaphysical endosteal and trabecular surfaces (+0.104±0.07 vs 0.031±0.30 μm(3)/μm(2)/day) relative to SS. Four-point bending mechanical tests of disused SS femurs showed reduced elastic modulus (-53%, p<0.05), yield (-33%, p<0.05) and ultimate strength (-45%, p<0.05) at the femoral diaphysis relative to AM bone. LIPUS stimulation mitigated the adverse effects of disuse on bone elastic modulus (+42%, p<0.05), yield strength (+29%, p<0.05), and ultimate strength (+39%, p<0.05) relative to SS femurs. LIPUS provides the essential mechanical stimulus to retain bone morphological and mechanical integrity in disuse conditions. This study demonstrates LIPUS potential as regional therapeutic agent to countermeasure disuse induced bone loss while maintaining bone's integrity.

Immobilization induced osteopenia is strain specific in mice

Immobilization causes rapid and massive bone loss. By comparing Botulinum Toxin A (BTX)-induced bone loss in mouse strains with different genetic backgrounds we investigated whether the genetic background had an influence on the severity of the osteopenia. Secondly, we investigated whether BTX had systemic effects on bone. Female mice from four inbred mouse strains (BALB/cJ, C57BL/6 J, DBA/2 J, and C3H/HeN) were injected unilaterally with BTX (n = 10/group) or unilaterally with saline (n = 10/group). Mice were euthanized after 21 days, and the bone properties evaluated using μCT, DXA, bone histomorphometry, and mechanical testing. BTX resulted in substantially lower trabecular bone volume fraction (BV/TV) and trabecular thickness in all mouse strains. The deterioration of BV/TV was significantly greater in C57BL/6 J (− 57%) and DBA/2 J (− 60%) than in BALB/cJ (− 45%) and C3H/HeN (− 34%) mice. The loss of femoral neck fracture strength was significantly greater in C57BL/6 J (− 47%) and DBA/2 J (− 45%) than in C3H (− 25%) mice and likewise the loss of mid-femoral fracture strength was greater in C57BL/6 J (− 17%), DBA/2 J (− 12%), and BALB/cJ (− 9%) than in C3H/HeN (− 1%) mice, which were unaffected. Using high resolution μCT we found no evidence of a systemic effect on any of the microstructural parameters of the contralateral limb. Likewise, there was no evidence of a systemic effect on the bone strength in any mouse strain. We did, however, find a small systemic effect on aBMD in DBA/2 J and C3H/HeN mice. The present study shows that BTX-induced immobilization causes the greatest loss of cortical and trabecular bone in C57BL/6 J and DBA/2 J mice. A smaller loss of bone microstructure and fracture strength was seen in BALB/cJ mice, while the bone microstructure and fracture strength of C3H/HeN mice were markedly less affected. This indicates that BTX-induced loss of bone is mouse strain dependent. We found only minimal systemic effects of BTX.

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Botulinum Toxin A (Botox) causes only minimal systemic effects in mice.

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The degree of immobilization induced osteopenia is highly strain specific in mice.

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The greatest degree of bone loss was observed with C57BL/6 J and DBA/2 J mice followed by BALB/cJ mice after Botox-injection.

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C3H/HeN mice had the smallest bone loss following Botox-injection.

Bone mass and bone quality are altered by hypoactivity in the chicken

Disuse induces a rapid bone loss in adults; sedentarity is now recognized as a risk factor for osteoporosis. Hypoactivity or confinement also decrease bone mass in adults but their effects are largely unknown and only few animal models have been described. We have used 10 chickens of the rapidly growing strain 857K bred in a large enclosure (FREE group); 10 others were confined in small cages with little space to move around (HYPO group). They were sacrificed at 53 days and femurs and tibias were evaluated by texture analysis, dual energy X-ray densitometry, microcomputed tomography (microCT) and histomorphometry. Hypoactivity had no effect on the length and diameter of the bones. Bone mineral density (BMD), microCT (trabecular bone volume and trabecular microarchitecture) and texture analysis were always found significantly reduced in the animals of the HYPO group. BMD was reduced at both femur and tibia diaphysises; BMD of the metaphysis was significantly reduced in the femur but not in the tibia. An increase in osteoid volume and surfaces was noted in the HYPO group. However, there was no alteration of the mineral phase as the osteoid thickness did not differ from control animals. Bone loss was much more pronounced at the lower femur metaphysis than at the upper metaphysis of the tibia. At the tibia, only microarchitectural changes of trabecular bone could be evidenced. The confined chicken represents a new method for the study of hypodynamia since these animals do not have surgical lesions.

Additive effect of PTH (1-34) and zoledronate in the prevention of disuse osteopenia in rats

Immobilization is known to cause a rapid bone loss due to increased osteoclastic bone resorption and decreased osteoblastic bone formation. Zoledronate (Zln) is a potent anti-resorptive pharmaceutical, while intermittent PTH is a potent bone anabolic agent. The aim of the present study was to investigate whether PTH or Zln alone or in combination could prevent immobilization-induced osteopenia. Immobilization was achieved by injecting 4IU Botox (BTX) into the right hind limb musculature. Seventy-two 16-week-old female Wistar rats were randomized into 6 groups; baseline (Base), control (Ctrl), BTX, BTX+PTH, BTX+Zln, and BTX+PTH+Zln. PTH (1-34) (80μg/kg) was given 5days/week and Zln (100μg/kg) was given once at study start. The animals were killed after 4weeks of treatment. The bone properties were evaluated using DEXA, μCT, dynamic bone histomorphometry, and mechanical testing. BTX resulted in lower femoral trabecular bone volume fraction (BV/TV) (-25%, p<0.05), lower tibial trabecular bone formation rate (BFR/BS) (-29%, p<0.05), and lower bone strength (Fmax) at the distal femur (-19%, p<0.001) compared with Ctrl. BTX+PTH resulted in higher femoral BV/TV (+31%, p<0.05), higher tibial trabecular BFR/BS (+297%, p<0.05), and higher Fmax at the distal femur (+11%, p<0.05) compared with BTX. BTX+Zln resulted in higher femoral BV/TV (+36%, p<0.05), lower tibial trabecular BFR/BS (-93%, p<0.05), and higher Fmax at the distal femur (+10%, p<0.05) compared with BTX. BTX+PTH+Zln resulted in higher femoral BV/TV (+70%, p<0.001), higher tibial trabecular BFR/BS (+59%, p<0.05), and higher Fmax at the distal femur (+32%, p<0.001) compared with BTX. In conclusion, BTX-induced immobilization led to lower BV/TV, BFR/BS, and Fmax. In general, PTH or Zln alone prevented the BTX-induced osteopenia, whereas PTH and Zln given in combination not only prevented, but also increased BV/TV and BFR/BS, and maintained Fmax at the distal femoral metaphysis compared with Ctrl.

Reduced gravitational loading does not account for the skeletal effect of botulinum toxin-induced muscle inhibition suggesting a direct effect of muscle on bone

Intramuscular injection of botulinum toxin (botox) into rodent hindlimbs has developed as a useful model for exploring muscle-bone interactions. Botox-induced muscle inhibition rapidly induces muscle atrophy and subsequent bone loss, with the latter hypothesized to result from reduced muscular loading of the skeleton. However, botox-induced muscle inhibition also reduces gravitational loading (as evident by reduced ground reaction forces during gait) which may account for its negative skeletal effects. The aim of this study was to investigate the skeletal effect of botox-induced muscle inhibition in cage control and tail suspended mice, with tail suspension being used to control for the reduced gravitational loading associated with botox. Female C57BL/6J mice were injected unilaterally with botox and contralaterally with vehicle, and subsequently exposed to tail suspension or normal cage activities for 6 weeks. Botox-induced muscle inhibition combined with tail suspension had the largest detrimental effect on the skeleton, causing the least gains in midshaft tibial bone mass, cortical area and cortical thickness, greatest gains in midshaft tibial medullary area, and lowest proximal tibial trabecular bone volume fraction. These data indicate botox-induced muscle inhibition has skeletal effects over and above any effect it has in altering gravitational loading, suggesting that muscle has a direct effect on bone. This effect may be relevant in the development of strategies targeting musculoskeletal health.

Calcified cartilage islands in rat cortical bone

Rats display little to no haversian remodeling of cortical bone. This fact, combined with the endochondral formation of cortical bone, means that rat femoral cortical bone contains highly mineralized cartilage islands in a central band of mid-femoral cross sections. We demonstrate that these islands have a significantly higher degree of mineralization than the surrounding bone, using quantitative backscattered electron imaging. The cartilaginous nature of the islands was verified by immunostaining for collagen type II. Toluidine blue staining of longitudinal sections and three-dimensional synchrotron radiation X-ray tomographic microscopy confirmed that the islands are elongated along the femoral long axis. Nanoindentation revealed significantly higher values of both reduced modulus and hardness in the islands compared to the surrounding bone, reflecting a higher degree of mineralization. The calcified cartilage islands were distributed in a central zone of the bone, from the growth plates through the mid-femoral bone. The presence of these cartilage islands and their possible effect on mechanical properties could be an additional reason why haversian remodeling is observed in higher-order species.

PTH (1-34), but not strontium ranelate counteract loss of trabecular thickness and bone strength in disuse osteopenic rats

PTH and strontium ranelate (SrR) have both been shown to reduce bone loss induced by immobilization. PTH is a potent bone anabolic agent, whereas SrR has been suggested to be an antiresorptive as well as a bone anabolic agent. The aim of the study was to investigate whether PTH, SrR, and PTH and SrR in combination could counteract immobilization-induced bone loss in a rat model. Immobilization was induced by injecting 4IU Botox (BTX) into the muscles of the right hind limb. Seventy-two female Wistar rats, 3-months-old, were divided into the following groups: Baseline, Controls, BTX, BTX+PTH, BTX+SrR, and BTX+PTH+SrR (n=12 in each group). PTH was given as injections (SC) at a dosage of 60μg/kg/d, and SrR as 900mg/kg/d in the diet. The experiment lasted for 4weeks. BTX resulted in lower trabecular bone formation rate (-68%) and periosteal bone formation rate (-91%), and a higher fraction of osteoclast-covered surfaces (+53%) compared with controls. This was accompanied by significantly lower trabecular bone volume fraction (-24%), trabecular thickness (-16%), and bone strength (-14% to -32% depending on site). PTH alone counteracted immobilization-induced losses in trabecular (4-fold increase vs. BTX) and periosteal (5-fold increase vs. BTX) bone formation rate, trabecular thickness (+25% vs. BTX) and femoral neck strength (+24% vs. BTX). In contrast, SrR did not influence BTX-induced loss of bone formation rate, trabecular bone volume fraction, trabecular thickness, or bone strength. Finally, no additive effect was found when PTH and SrR treatments were combined. In conclusion, PTH counteracted loss in bone architecture and bone strength in immobilized rats, whereas as no effect of SrR was found. Moreover, no additional effect was found by combining PTH with SrR.