Bone adaptation and osteoporosis prevention in hibernating mammals.

Hibernating bears and rodents have evolved mechanisms to prevent disuse osteoporosis during the prolonged physical inactivity that occurs during hibernation. Serum markers and histological indices of bone remodeling in bears indicate reduced bone turnover during hibernation, which is consistent with organismal energy conservation. Calcium homeostasis is maintained by balanced bone resorption and formation since hibernating bears do not eat, drink, urinate, or defecate. Reduced and balanced bone remodeling protect bear bone structure and strength during hibernation, unlike the disuse osteoporosis that occurs in humans and other animals during prolonged physical inactivity. Conversely, some hibernating rodents show varying degrees of bone loss such as osteocytic osteolysis, trabecular loss, and cortical thinning. However, no negative effects of hibernation on bone strength in rodents have been found. More than 5000 genes in bear bone tissue are differentially expressed during hibernation, highlighting the complexity of hibernation induced changes in bone. A complete picture of the mechanisms that regulate bone metabolism in hibernators still alludes us, but existing data suggest a role for endocrine and paracrine factors such as cocaine- and amphetamine-regulated transcript (CART) and endocannabinoid ligands like 2-arachidonoyl glycerol (2-AG) in decreasing bone remodeling during hibernation. Hibernating bears and rodents evolved the capacity to preserve bone strength during long periods of physical inactivity, which contributes to their survival and propagation by allowing physically activity (foraging, escaping predators, and mating) without risk of bone fracture following hibernation. Understanding the biological mechanisms regulating bone metabolism in hibernators may inform novel treatment strategies for osteoporosis in humans.

Validation of Running Gait Event Detection Algorithms in a Semi-Uncontrolled Environment.

The development of lightweight portable sensors and algorithms for the identification of gait events at steady-state running speeds can be translated into the real-world environment. However, the output of these algorithms needs to be validated. The purpose of this study was to validate the identification of running gait events using data from Inertial Measurement Units (IMUs) in a semi-uncontrolled environment. Fifteen healthy runners were recruited for this study, with varied running experience and age. Force-sensing insoles measured normal foot-shoe forces and provided a standard for identification of gait events. Three IMUs were mounted to the participant, two bilaterally on the dorsal aspect of the foot and one clipped to the back of each participant's waistband, approximating their sacrum. The identification of gait events from the foot-mounted IMU was more accurate than from the sacral-mounted IMU. At running speeds <3.57 m s−1, the sacral-mounted IMU identified contact duration as well as the foot-mounted IMU. However, at speeds >3.57 m s−1, the sacral-mounted IMU overestimated foot contact duration. This study demonstrates that at controlled paces over level ground, we can identify gait events and measure contact time across a range of running skill levels.

The effects of neurectomy and hibernation on bone properties and the endocannabinoid system in marmots (Marmota flaviventris).

Hibernators have adapted a physiological mechanism allowing them to undergo long periods of inactivity without experiencing bone loss. However, the biological mechanisms that prevent bone loss are unknown. Previous studies found meaningful changes, between active and hibernating marmots, in the endocannabinoid system of many tissues, including bone. Cannabinoid receptors (CB1 and CB2) have divergent localization in bone. CB1 is predominately found on sympathetic nerve terminals, while CB2 is more abundant on bone cells and their progenitors. This study aimed to determine the contribution of innervation on endocannabinoid regulation of bone properties in hibernating (during torpor) and non-hibernating yellow-bellied marmots. Neurectomy, a model for disuse osteoporosis, was performed unilaterally in both hibernating and active marmots. Endocannabinoid concentrations were measured in bone marrow, cortical, and trabecular regions from fourth metatarsals of both hindlimbs using microflow chromatography-tandem quadrupole mass spectrometry. Trabecular bone architectural properties of fifth metatarsals were evaluated using micro-computed tomography. There were ligand-specific increases with neurectomy in active, but not hibernating, marmots. Trabecular bone architectural properties were not affected by neurectomy during hibernation, but did show some minor negative changes in active marmots. These findings suggest protection from bone loss in hibernating rodents is peripherally rather than centrally regulated. Furthermore, findings suggest even active marmots with normal metabolism are partially protected from disuse induced bone loss compared to laboratory rodents. Understanding the mechanism hibernators use to maintain bone density may guide development for novel bone loss prevention therapies.

Krogh's principle for musculoskeletal physiology and pathology.

August Krogh was a comparative physiologist who used frogs, guinea pigs, cats, dogs, and horses in his research that led to his Nobel Prize on muscle physiology. His idea to choose the most relevant organism to study problems in physiology has become known as Krogh's principle. Indeed, many important discoveries in physiology have been made using naturally occurring animal models. However, the majority of research today utilizes laboratory mouse and rat models to study problems in physiology. This paper discusses how Krogh's principle can be invoked in musculoskeletal research as a complementary approach to using standard laboratory rodent models for solving problems in musculoskeletal physiology. This approach may increase our ability to treat musculoskeletal diseases clinically. For example, it has been noted that progress in osteogenesis imperfecta research has been limited by the absence of a naturally occurring animal model. Several examples of naturally occurring animal models are discussed including osteoarthritis and osteosarcoma in dogs, resistance to disuse induced bone and skeletal muscle loss in mammalian hibernators, and bone phenotypic plasticity in fish lacking osteocytes. Many musculoskeletal diseases (e.g., osteoarthritis) occur naturally in companion animals, which may provide clues on etiology and progression of musculoskeletal diseases and accelerate the development of pharmaceutical therapies for humans.

Evolutionary physiology of bone: bone metabolism in changing environments.

Bone evolved to serve many mechanical and physiological functions. Osteocytes and bone remodeling first appeared in the dermal skeleton of fish, and subsequently adapted to various challenges in terrestrial animals occupying diverse environments. This review discusses the physiology of bone and its role in mechanical and calcium homeostases from an evolutionary perspective. We review how bone physiology responds to changing environments and the adaptations to unique and extreme physiological conditions.

Suppressed bone remodeling in black bears conserves energy and bone mass during hibernation.

Decreased physical activity in mammals increases bone turnover and uncouples bone formation from bone resorption, leading to hypercalcemia, hypercalcuria, bone loss and increased fracture risk. Black bears, however, are physically inactive for up to 6 months annually during hibernation without losing cortical or trabecular bone mass. Bears have been shown to preserve trabecular bone volume and architectural parameters and cortical bone strength, porosity and geometrical properties during hibernation. The mechanisms that prevent disuse osteoporosis in bears are unclear as previous studies using histological and serum markers of bone remodeling show conflicting results. However, previous studies used serum markers of bone remodeling that are known to accumulate with decreased renal function, which bears have during hibernation. Therefore, we measured serum bone remodeling markers (BSALP and TRACP) that do not accumulate with decreased renal function, in addition to the concentrations of serum calcium and hormones involved in regulating bone remodeling in hibernating and active bears. Bone resorption and formation markers were decreased during hibernation compared with when bears were physically active, and these findings were supported by histomorphometric analyses of bone biopsies. The serum concentration of cocaine and amphetamine regulated transcript (CART), a hormone known to reduce bone resorption, was 15-fold higher during hibernation. Serum calcium concentration was unchanged between hibernation and non-hibernation seasons. Suppressed and balanced bone resorption and formation in hibernating bears contributes to energy conservation, eucalcemia and the preservation of bone mass and strength, allowing bears to survive prolonged periods of extreme environmental conditions, nutritional deprivation and anuria.

The morphology of the interfacial tissue between bighorn sheep horn and bony horncore increases contact surface to enhance strength and facilitate load transfer from the horn to the horncore.

The horns of bighorn sheep rams are permanent cranial appendages used for high energy head-to-head impacts during interspecific combat. The horns attach to the underlying bony horncore by a layer of interfacial tissue that facilitates load transfer between the impacted horn and underlying horncore, which has been shown to absorb substantial energy during head impact. However, the morphology and mechanical properties of the interfacial tissue were previously unknown. Histomorphometry was used to quantify the interfacial tissue composition and morphology and lap-shear testing was used to quantify its mechanical properties. Histological analyses revealed the interfacial tissue is a complex network of collagen and keratin fibers, with collagen being the most abundant protein. Sharpey's fibers provide strong attachment between the interfacial tissue and horncore bone. The inner horn surface displayed microscopic porosity and branching digitations which increased the contact surface with the interfacial tissue by approximately 3-fold. Horn-horncore samples tested by lap-shear loading failed primarily at the horn surface, and the interfacial tissue displayed non-linear strain hardening behavior similar to other soft tissues. The elastic properties of the interfacial tissue (i.e., low- and high-strain shear moduli) were comparable to previously measured values for the equine laminar junction. The interfacial tissue contact surface was positively correlated with the interfacial tissue shear strength (1.23 ± 0.21 MPa), high-strain shear modulus (4.5 ± 0.7 MPa), and strain energy density (0.38 ± 0.07 MJ/m3). STATEMENT OF SIGNIFICANCE: The bony horncore in bighorn sheep rams absorbs energy to reduce brain cavity accelerations and mitigate brain injury during head butting. The interfacial zone between the horn and horncore transfers energy from the impacted horn to the energy absorbing horncore but has been largely neglected in previous models of bighorn sheep ramming since interfacial tissue properties were previously unknown. This study quantified the morphology and mechanical properties of the horn-horncore interfacial tissue to better understand structure-property relationships that contribute to energy transfer during ramming. Results from this study will improve models of bighorn sheep ramming used to study mechanisms of brain injury mitigation and may inspire novel materials and structures for brain injury prevention in humans.

Scaling of Haversian canal surface area to secondary osteon bone volume in ribs and limb bones.

Studies of secondary osteons in ribs have provided a great deal of what is known about remodeling dynamics. Compared with limb bones, ribs are metabolically more active and sensitive to hormonal changes, and receive frequent low-strain loading. Optimization for calcium exchange in rib osteons might be achieved without incurring a significant reduction in safety factor by disproportionally increasing central canal size with increased osteon size (positive allometry). By contrast, greater mechanical loads on limb bones might favor reducing deleterious consequences of intracortical porosity by decreasing osteon canal size with increased osteon size (negative allometry). Evidence of this metabolic/mechanical dichotomy between ribs and limb bones was sought by examining relationships between Haversian canal surface area (BS, osteon Haversian canal perimeter, HC.Pm) and bone volume (BV, osteonal wall area, B.Ar) in a broad size range of mature (quiescent) osteons from adult human limb bones and ribs (modern and medieval) and various adult and subadult non-human limb bones and ribs. Reduced major axis (RMA) and least-squares (LS) regressions of HC.Pm/B.Ar data show that rib and limb osteons cannot be distinguished by dimensional allometry of these parameters. Although four of the five rib groups showed positive allometry in terms of the RMA slopes, nearly 50% of the adult limb bone groups also showed positive allometry when negative allometry was expected. Consequently, our results fail to provide clear evidence that BS/BV scaling reflects a rib versus limb bone dichotomy whereby calcium exchange might be preferentially enhanced in rib osteons.

Estimation of gait events and kinetic waveforms with wearable sensors and machine learning when running in an unconstrained environment.

Wearable sensors and machine learning algorithms are becoming a viable alternative for biomechanical analysis outside of the laboratory. The purpose of this work was to estimate gait events from inertial measurement units (IMUs) and utilize machine learning for the estimation of ground reaction force (GRF) waveforms. Sixteen healthy runners were recruited for this study, with varied running experience. Force sensing insoles were used to measure normal foot-shoe forces, providing a proxy for vertical GRF and a standard for the identification of gait events. Three IMUs were mounted on each participant, two bilaterally on the dorsal aspect of each foot and one clipped to the back of each participant's waistband, approximating their sacrum. Participants also wore a GPS watch to record elevation and velocity. A Bidirectional Long Short Term Memory Network (BD-LSTM) was used to estimate GRF waveforms from inertial waveforms. Gait event estimation from both IMU data and machine learning algorithms led to accurate estimations of contact time. The GRF magnitudes were generally underestimated by the machine learning algorithm when presented with data from a novel participant, especially at faster running speeds. This work demonstrated that estimation of GRF waveforms is feasible across a range of running velocities and at different grades in an uncontrolled environment.

Estimation of ground reaction force waveforms during fixed pace running outside the laboratory.

In laboratory experiments, biomechanical data collections with wearable technologies and machine learning have been promising. Despite the development of lightweight portable sensors and algorithms for the identification of gait events and estimation of kinetic waveforms, machine learning models have yet to be used to full potential. We propose the use of a Long Short Term Memory network to map inertial data to ground reaction force data gathered in a semi-uncontrolled environment. Fifteen healthy runners were recruited for this study, with varied running experience: novice to highly trained runners (<15 min 5 km race), and ages ranging from 18 to 64 years old. Force sensing insoles were used to measure normal foot-shoe forces, providing the standard for identification of gait events and measurement of kinetic waveforms. Three inertial measurement units (IMUs) were mounted to each participant, two bilaterally on the dorsal aspect of the foot and one clipped to the back of each participant's waistband, approximating their sacrum. Data input into the Long Short Term Memory network were from the three IMUs and output were estimated kinetic waveforms, compared against the standard of the force sensing insoles. The range of RMSE for each stance phase was from 0.189-0.288 BW, which is similar to multiple previous studies. Estimation of foot contact had an r 2 = 0.795. Estimation of kinetic variables varied, with peak force presenting the best output with an r 2 = 0.614. In conclusion, we have shown that at controlled paces over level ground a Long Short Term Memory network can estimate 4 s temporal windows of ground reaction force data across a range of running speeds.

How the geometry and mechanics of bighorn sheep horns mitigate the effects of impact and reduce the head injury criterion.

Male bighorn sheep (Ovis canadensis) participate in seasonal ramming bouts that can last for hours, yet they do not appear to suffer significant brain injury. Previous work has shown that the keratin-rich horn and boney horncore may play an important role in mitigating brain injury by reducing brain cavity accelerations through energy dissipating elastic mechanisms. However, the extent to which specific horn shapes (such as the tapered spiral of bighorn sheep) may reduce accelerations post-impact remains unclear. Thus, the goals of this work were to (a) quantify bighorn sheep horn shape, particularly the cross-sectional areal properties related to bending that largely dictate post-impact deformations, and (b) investigate the effects of different tapered horn shapes on reducing post-impact accelerations in an impact model with finite element analysis. Cross-sectional areal properties indicate bighorn sheep horns have a medial-lateral bending preference at the horn tip (p= 0.006), which is likely to dissipate energy through medial-lateral horn tip oscillations after impact. Finite element modeling showed bighorn sheep native horn geometry reduced the head injury criterion (HIC15) by 48% compared to horns with cross-sections rotated by 90° to have a cranial-caudal bending preference, and by 125% compared to a circular tapered spiral model. These results suggest that the tapered spiral horn shape of bighorn sheep is advantageous for dissipating energy through elastic mechanisms following an impact. These findings can be used to broadly inform the design of improved safety equipment and impact systems.

Structure-property relationships of velar bone tissue from the energy absorbing horncore of bighorn sheep rams.

Velar bone is the material that fills the horncore of bighorn sheep rams. The architectural dimensions of velar bone are orders of magnitude larger than trabecular bone, and velae are more sail-like compared to strut-like trabeculae. Velar bone is important for energy absorption and reduction of brain cavity accelerations during high energy head impacts, but velar bone material properties were previously unknown. It was hypothesized that velar bone tissue would have properties that are beneficial for increased energy absorption at the material level. Solid velar bone beams were tested using dynamic mechanical analysis and three-point bending to quantify mechanical properties. Additionally, the porosity, osteon population density, and mineral content of the solid velar sails were quantified. The velar bone damping factor (∼0.03 - 0.06) and modulus of toughness (3.9 ± 0.4 MJ/m3) were lower than other mammalian cortical bone tissues. The solid bony sails have a bending modulus (8.6 ± 0.5 GPa) that lies within the range of bending moduli values previously reported for individual trabecular struts and cortical bone tissue. The solid velar bone sails had porosity (6.7 ± 0.9 %) and bone mineral content (66 ± 1 %) in the range of cortical bone values. Interestingly, velar sails contained osteons, which are rarely found in trabecular struts. The velar bone osteon population density (5.8 ± 0.9 osteons/mm2) is in the low end of the range of values reported for cortical bone in other mammals. STATEMENT OF SIGNIFICANCE: Bighorn sheep rams sustain high energy head impacts during intraspecific combat without overt signs of brain injury. Previous studies have shown that the bony horncore plays a critical role in energy absorption and reduction of brain cavity accelerations post impact, which has implications for concussion prevention in humans. However, the material properties of the horncore velar bone were previously unknown. This study quantified the material properties and structure-property relationships of the horncore velar bone at the tissue level. Results from this study will improve our understanding of how bighorn sheep mitigate brain injury during head-to-head impacts and may inspire the design of novel materials for energy absorption applications (i.e., helmets materials that reduce concussion occurrence in humans).

The osteogenic and angiogenic potential of microRNA-26a delivered via a non-viral delivery peptide for bone repair.

Bone-related injuries and diseases are among the most common causes of morbidity worldwide. Current bone-regenerative strategies such as auto- and allografts are invasive by nature, with adverse effects such as pain, infection and donor site morbidity. MicroRNA (miRNA) gene therapy has emerged as a promising area of research, with miRNAs capable of regulating multiple gene pathways simultaneously through the repression of post-transcriptional mRNAs. miR-26a is a key regulator of osteogenesis and has been found to be upregulated following bone injury, where it induces osteodifferentiation of mesenchymal stem cells (MSCs) and facilitates bone formation. This study demonstrates, for the first time, that the amphipathic, cell-penetrating peptide RALA can efficiently deliver miR-26a to MSCs in vitro to regulate osteogenic signalling. Transfection with miR-26a significantly increased expression of osteogenic and angiogenic markers at both gene and protein level. Using a rat calvarial defect model with a critical size defect, RALA/miR-26a NPs were delivered via an injectable, thermo-responsive Cs-g-PNIPAAm hydrogel to assess the impact on both rate and quality of bone healing. Critical defects treated with the RALA/miR-26a nanoparticles (NPs) had significantly increased bone volume and bone mineral density at 8 weeks, with increased blood vessel formation and mechanical properties. This study highlights the utility of RALA to deliver miR-26a for the purpose of bone healing within an injectable biomaterial, warranting further investigation of dose-related efficacy of the therapeutic across a range of in vivo models.

Development of miR-26a-activated scaffold to promote healing of critical-sized bone defects through angiogenic and osteogenic mechanisms.

Very large bone defects significantly diminish the vascular, blood, and nutrient supply to the injured site, reducing the bone's ability to self-regenerate and complicating treatment. Delivering nanomedicines from biomaterial scaffolds that induce host cells to produce bone-healing proteins is emerging as an appealing solution for treating these challenging defects. In this context, microRNA-26a mimics (miR-26a) are particularly interesting as they target the two most relevant processes in bone regeneration-angiogenesis and osteogenesis. However, the main limitation of microRNAs is their poor stability and issues with cytosolic delivery. Thus, utilising a collagen-nanohydroxyapatite (coll-nHA) scaffold in combination with cell-penetrating peptide (RALA) nanoparticles, we aimed to develop an effective system to deliver miR-26a nanoparticles to regenerate bone defects in vivo. The microRNA-26a complexed RALA nanoparticles, which showed the highest transfection efficiency, were incorporated into collagen-nanohydroxyapatite scaffolds and in vitro assessment demonstrated the miR-26a-activated scaffolds effectively transfected human mesenchymal stem cells (hMSCs) resulting in enhanced production of vascular endothelial growth factor, increased alkaline phosphatase activity, and greater mineralisation. After implantation in critical-sized rat calvarial defects, micro CT and histomorphological analysis revealed that the miR-26a-activated scaffolds improved bone repair in vivo, producing new bone of superior quality, which was highly mineralised and vascularised compared to a miR-free scaffold. This innovative combination of osteogenic collagen-nanohydroxyapatite scaffolds with multifunctional microRNA-26a complexed nanoparticles provides an effective carrier delivering nanoparticles locally with high efficacy and minimal off-target effects and demonstrates the potential of targeting osteogenic-angiogenic coupling using scaffold-based nanomedicine delivery as a new "off-the-shelf" product capable of healing complex bone injuries.

Preservation of bone mass and structure in hibernating black bears (Ursus americanus) through elevated expression of anabolic genes.

Physical inactivity reduces mechanical load on the skeleton, which leads to losses of bone mass and strength in non-hibernating mammalian species. Although bears are largely inactive during hibernation, they show no loss in bone mass and strength. To obtain insight into molecular mechanisms preventing disuse bone loss, we conducted a large-scale screen of transcriptional changes in trabecular bone comparing winter hibernating and summer non-hibernating black bears using a custom 12,800 probe cDNA microarray. A total of 241 genes were differentially expressed (P < 0.01 and fold change >1.4) in the ilium bone of bears between winter and summer. The Gene Ontology and Gene Set Enrichment Analysis showed an elevated proportion in hibernating bears of overexpressed genes in six functional sets of genes involved in anabolic processes of tissue morphogenesis and development including skeletal development, cartilage development, and bone biosynthesis. Apoptosis genes demonstrated a tendency for downregulation during hibernation. No coordinated directional changes were detected for genes involved in bone resorption, although some genes responsible for osteoclast formation and differentiation (Ostf1, Rab9a, and c-Fos) were significantly underexpressed in bone of hibernating bears. Elevated expression of multiple anabolic genes without induction of bone resorption genes, and the down regulation of apoptosis-related genes, likely contribute to the adaptive mechanism that preserves bone mass and structure through prolonged periods of immobility during hibernation.

Feature Identification With a Heuristic Algorithm and an Unsupervised Machine Learning Algorithm for Prior Knowledge of Gait Events.

The purpose of this study was to compare a heuristic feature identification algorithm with output from the Beta Process Auto Regressive Hidden Markov Model (BP-AR-HMM) utilizing minimally sampled (≤ 100 Hz) human locomotion data for identification of gait events prior to their occurrence. Data were collected from 16 participants (21-64 years) using a single gyroscopic sensor in an inertial measurement unit on the dorsum of the foot, across multiple locomotion modes, including level ground walking and running (across speeds 0.8 m s-1 - 3.0 m s-1), ramps and stairs. Identification of gait events, initial contact (IC) and toe off (TO) with the heuristic algorithm, was 94% across locomotion modes. The features identified prior to initial contact had a lead time of 186.32 ± 86.70 ms, while TO had a lead time of 63.96 ± 46.30 ms. The BP-AR-HMM identified features that indicated an impending IC and TO with 99% accuracy, with a lead time of 59.41 ± 54.41 ms for IC and 90.79 ± 35.51 ms for TO. These approaches are consistent in their identification of gait events and have the potential to be utilized for classification and prediction of locomotion mode.

Granular PEG hydrogels mediate osteoporotic MSC clustering via N-cadherin influencing the pro-resorptive bias of their secretory profile.

Postmenopausal osteoporosis results from a pro-resorptive bone environment, which decreases bone mineral density causing increased fracture risk. Bone marrow derived mesenchymal stem/stromal cells (MSCs) secrete factors involved in bone homeostasis, but osteoporosis mediated changes to their secretions remain understudied. Herein, we examined the secretome of MSCs isolated from ovariectomized rats (OVX rMSCs), a model of post-menopausal osteoporosis, as a function of cell-cell interactions. Specifically, we controlled clustering of OVX and SHAM rMSCs by assembling them in granular hydrogels synthesized from poly(ethylene glycol) microgels with average diameters of ∼10, 100, and 200 µm. We directed both the sizes of rMSC clusters (single cells to ∼30 cells/cluster) and the percentages of cells within clusters (∼20-90%) by controlling the scaffold pore dimensions. Large clusters of OVX rMSCs had a pro-resorptive secretory profile, with increased concentrations of Activin A, CXCL1, CX3CL1, MCP-1, TIMP-1, and TNF-ɑ, compared to SHAM rMSCs. As this pro-resorptive bias was only observed in large cell clusters, we characterized the expression of several cadherins, mediators of cell-cell contacts. N-cadherin expression was elevated (∼4-fold) in OVX relative to SHAM rMSCs, in both cell clusters and single cells. Finally, TIMP-1 and MCP-1 secretion was only decreased in large cell clusters of OVX rMSCs when N-cadherin interactions were blocked, highlighting the dependence of OVX rMSC secretion of pro-resorptive cytokines on N-cadherin mediated cell-cell contacts. Further elucidation of the N-cadherin mediated osteoporotic MSC secretome may have implications for developing therapies for postmenopausal osteoporosis. STATEMENT OF SIGNIFICANCE: Postmenopausal osteoporosis is a prevalent bone disorder that affects tens of millions of women worldwide. This disease is characterized by severe bone loss resulting from a pro-resorptive bone marrow environment, where the rates of bone resorption outpace the rates of bone deposition. The paracrine factors secreted by bone marrow MSCs can influence cell types responsible for bone homeostasis, but the osteoporosis-mediated changes to MSC secretory properties remains understudied. In this study, we used PEG-based porous granular scaffolds to study the influence of cell clustering on the secretory properties of osteoporotic MSCs. We observed increased secretion of several pro-resorptive factors by osteoporotic MSCs in large clusters. Further, we explored the dependence of this altered secretion profile on N-cadherin mediated cell-cell contacts.

Thirteen-lined ground squirrels (Ictidomys tridecemlineatus) show microstructural bone loss during hibernation but preserve bone macrostructural geometry and strength.

Lack of activity causes bone loss In most animals. Hibernating bears have physiological processes to prevent cortical and trabecular bone loss associated with reduced physical activity, but different mechanisms of torpor among hibernating species may lead to differences in skeletal responses to hibernation. There are conflicting reports regarding whether small mammals experience bone loss during hibernation. To investigate this phenomenon, we measured cortical and trabecular bone properties in physically active and hibernating juvenile and adult 13-lined ground squirrels (Ictidomys tridecemlineatus, previous genus name Spermophilus). Cortical bone geometry, strength and mineral content were similar in hibernating compared with active squirrels, suggesting that hibernation did not cause macrostructural cortical bone loss. Osteocyte lacunar size increased (linear regression, P=0.001) over the course of hibernation in juvenile squirrels, which may indicate an osteocytic role in mineral homeostasis during hibernation. Osteocyte lacunar density and porosity were greater (+44 and +59%, respectively; P<0.0001) in hibernating compared with active squirrels, which may reflect a decrease in osteoblastic activity (per cell) during hibernation. Trabecular bone volume fraction in the proximal tibia was decreased (-20%; P=0.028) in hibernating compared with physically active adult squirrels, but was not different between hibernating and active juvenile squirrels. Taken together, these data suggest that 13-lined ground squirrels may be unable to prevent microstructural losses of cortical and trabecular bone during hibernation, but importantly may possess a biological mechanism to preserve cortical bone macrostructure and strength during hibernation, thus preventing an increased risk of bone fracture during remobilization in the spring.

Material properties of bighorn sheep (Ovis canadensis) horncore bone with implications for energy absorption during impacts.

Bighorn sheep rams participate in high impact head-butting without overt signs of brain injury, thus providing a naturally occurring animal model for studying brain injury mitigation. Previously published finite element modeling showed that both the horn and bone materials play important roles in reducing brain cavity accelerations during ramming. However, in that study the elastic modulus of bone was assumed to be similar to that of human bone since the modulus of ram bone was unknown. Therefore, the goal of this study was to quantify the mechanical properties, mineral content, porosity, and microstructural organization of horncore cortical bone from juvenile and adult rams. Mineral content and elastic modulus increased with horn size, and porosity decreased. However, modulus of toughness did not change with horn size. This latter finding raises the possibility that the horncore cortical bone has not adapted exceptional toughness despite an extreme loading environment and may function primarily as an interface material between the horn and the porous bone within the horncore. Thus, geometric properties of the horn and horncore, including the porous bone architecture, may be more important for energy absorption during ramming than the horncore cortical bone. Results from this study can be used to improve accuracy of finite element models of bighorn sheep ramming to investigate these possibilities moving forward.

User Independent Estimations of Gait Events With Minimal Sensor Data.

GOAL: The purpose of this study was to provide an initial examination of the utility of the Beta Process - Auto Regressive - Hidden Markov Model (BP-AR-HMM) for the prior identification of gait events. A secondary objective was to determine whether the output of the model could be used for classification and prediction of locomotion states. METHODS: In this study we utilized the output of the BP-AR-HMM to develop user-independent identification of gait events and gait classification from an idealized three-dimensional acceleration signal. The input acceleration data were collected from two walking (1.4 and 1.6 ms-1) and two running (2.6 and 3.0 ms-1) steady state speeds, and during two dynamic walk to run and run to walk transitions (1.8-2.4 and 2.4-1.8 ms-1) on an instrumented force treadmill. RESULTS: The BP-AR-HMM identified 9 unique states. Of these, two states, 4 and 1, were utilized to estimate initial contact and toe off, respectively. The lead time from the first instance of state 4 to initial contact was 0.13 ± 0.02 s. Similarly, the first instance of state 1 occurred 0.14 ± 0.03 s before toe off. Two other states (3 and 7) were examined for possible utilization in a probabilistic model for the prediction of pending locomotion state transitions. CONCLUSION: The identification of gait events prior to their occurrence by the BP-AR-HMM appears to be an approach that can minimize the quantity of sensor data in an offline approach. Furthermore, there is evidence it could also be used as a basis to build a probabilistic model to estimate locomotion transitions.