Suppression of cancer-associated bone loss through dynamic mechanical loading.

Patients afflicted with or being treated for cancer constitute a distinct and alarming subpopulation who exhibit elevated fracture risk and heightened susceptibility to developing secondary osteoporosis. Cancer cells uncouple the regulatory processes central for the adequate regulation of musculoskeletal tissue. Systemically taxing treatments to target tumors or disrupt the molecular elements driving tumor growth place considerable strain on recovery efforts. Skeletal tissue is inherently sensitive to mechanical forces, therefore attention to exercise and mechanical loading as non-pharmacological means to preserve bone during treatment and in post-treatment rehabilitative efforts have been topics of recent focus. This review discusses the dysregulation that cancers and the ensuing metabolic dysfunction that confer adverse effects on musculoskeletal tissues. Additionally, we describe foundational mechanotransduction pathways and the mechanisms by which they influence both musculoskeletal and cancerous cells. Functional and biological implications of mechanical loading at the tissue and cellular levels will be discussed, highlighting the current understanding in the field. Herein, in vitro, translational, and clinical data are summarized to consider the positive impact of exercise and low magnitude mechanical loading on tumor-bearing skeletal tissue.

The ability of low-magnitude mechanical signals to normalize bone turnover in adolescents hospitalized for anorexia nervosa.

We sought to determine whether low-magnitude mechanical stimulation (LMMS) normalizes bone turnover among adolescents hospitalized for anorexia nervosa (AN). Brief, daily LMMS prevents the decline in bone turnover typically seen during bed rest in AN. LMMS may have application for patients with AN in the inpatient setting to protect bone health. INTRODUCTION: Malnourished adolescents with AN requiring medical hospitalization are at high risk for rapid reduction in skeletal quality. Even short-term bed rest can suppress normal patterns of bone turnover. We sought to determine whether LMMS normalizes bone turnover among adolescents hospitalized for complications of AN. METHODS: In this randomized, double-blind trial, we prospectively enrolled adolescent females (n = 41) with AN, age 16.3 ± 1.9 years (mean ± SD) and BMI 15.6 ± 1.7 kg/m2. Participants were randomized to stand on a platform delivering LMMS (0.3 g at 32-37 Hz) or placebo platform for 10 min/day for 5 days. Serum markers of bone formation [bone-specific alkaline phosphatase (BSAP)], turnover [osteocalcin (OC)], and bone resorption [serum C-telopeptides (CTx)] were measured. From a random coefficients model, we constructed estimates and confidence intervals for all outcomes. RESULTS: BSAP decreased by 2.8% per day in the placebo arm (p = 0.03) but remained stable in the LMMS group (p = 0.51, pdiff = 0.04). CTx did not change with placebo (p = 0.56) but increased in the LMMS arm (+6.2% per day, p = 0.04; pdiff = 0.01). Serum OC did not change in either group (p > 0.70). CONCLUSIONS: Bed rest during hospitalization for patients with AN is associated with a suppression of bone turnover, which may contribute to diminished bone quality. Brief, daily LMMS prevents a decline in bone turnover during bed rest in AN. Protocols prescribing strict bed rest may not be appropriate for protecting bone health for these patients. LMMS may have application for these patients in the inpatient setting.

Marrow adipogenesis and bone loss that parallels estrogen deficiency is slowed by low-intensity mechanical signals.

UNLABELLED: Ovariectomized mice were used to assess the ability of low-intensity vibrations to protect bone microarchitecture and marrow composition. Results indicate that low-intensity vibrations (LIV), introduced 2 weeks postsurgery, slows marrow adipogenesis in OVX mice but does not restore the bone within the period studied. However, immediate application of LIV partially protects quality. INTRODUCTION: The aim of this study was to evaluate consequences of estrogen depletion on bone marrow (BM) phenotype and bone microarchitecture, and effects of mechanical signals delivered as LIV on modulating these changes. METHODS: LIV (0.3 g, 90 Hz) was applied to C57BL/6 mice immediately following ovariectomy or 2 weeks postestrogen withdrawal for 2 (ST-LIV) or 6 weeks (LT-LIV), respectively. Sham-operated age-matched controls (ST-AC, LT-AC) and ovariectomized controls (ST-OVX, LT-OVX) received sham LIV treatment. Bone microstructure was evaluated through µCT and BM adipogenesis through histomorphometry, serum markers, and genes expression analysis. RESULTS: LT-OVX increased BM adipogenesis relative to LT-AC (+136 %, p ≤ 0.05), while LT-LIV introduced for 6w suppressed this adipose encroachment (-55 %, p ≤ 0.05). In parallel with the fatty marrow, LT-OVX showed a marked loss of trabecular bone, -40 % (p ≤ 0.05) in the first 2 weeks following ovariectomy compared to LT-AC. Application of LT-LIV for 6w following this initial 2w bone loss failed to restore the lost trabeculae but did initiate an anabolic response as indicated by increased serum alkaline phosphatase (+26 %, p ≤ 0.05). In contrast, application of LIV immediately following ovariectomy was more efficacious in the protection of trabecular bone, with a +29 % (p > 0.05) greater BV/TV compared to ST-OVX at the 2w time period. CONCLUSIONS: LIV can mitigate adipocyte accumulation in OVX marrow and protect it by favoring osteoblastogenesis over adipogenesis. These data also emphasize the rapidity of bone loss with OVX and provide perspective in the timing of treatments for postmenopausal osteoporosis where sooner is better than later.

Is bone formation induced by high-frequency mechanical signals modulated by muscle activity?

Bone formation and resorption are sensitive to both external loads arising from gravitational loading as well to internal loads generated by muscular activity. The question as to which of the two sources provides the dominant stimulus for bone homeostasis and new bone accretion is arguably tied to the specific type of activity and anatomical site but it is often assumed that, because of their purportedly greater magnitude, muscle loads modulate changes in bone morphology. High-frequency mechanical signals may provide benefits at low- (<1g) and high- (>1g) acceleration magnitudes. While the mechanisms by which cells perceive high-frequency signals are largely unknown, higher magnitude vibrations can cause large muscle loads and may therefore be sensed by pathways similar to those associated with exercise. Here, we review experimental data to examine whether vibrations applied at very low magnitudes may be sensed directly by transmittance of the signal through the skeleton or whether muscle activity modulates, and perhaps amplifies, the externally applied mechanical stimulus. Current data indicate that the anabolic and anti-catabolic effects of whole body vibrations on the skeleton are unlikely to require muscular activity to become effective. Even high-frequency signals that induce bone matrix deformations of far less than five microstrain can promote bone formation in the absence of muscular activity. This independence of cells on large strains suggests that mechanical interventions can be designed that are both safe and effective.

Quantification of adiposity in small rodents using micro-CT.

Non-invasive three-dimensional imaging of live rodents is a powerful research tool that has become critical for advances in many biomedical fields. For investigations into adipose development, obesity, or diabetes, accurate and precise techniques that quantify adiposity in vivo are critical. Because total body fat mass does not accurately predict health risks associated with the metabolic syndrome, imaging modalities should be able to stratify total adiposity into subcutaneous and visceral adiposity. Micro-computed tomography (micro-CT) acquires high-resolution images based on the physical density of the material and can readily discriminate between subcutaneous and visceral fat. Here, a micro-CT based method to image the adiposity of live rodents is described. An automated and validated algorithm to quantify the volume of discrete fat deposits from the computed tomography is available. Data indicate that scanning the abdomen provides sufficient information to estimate total body fat. Very high correlations between micro-CT determined adipose volumes and the weight of explanted fat pads demonstrate that micro-CT can accurately monitor site-specific changes in adiposity. Taken together, in vivo micro-CT is a non-invasive, highly quantitative imaging modality with greater resolution and selectivity, but potentially lower throughput, than many other methods to precisely determine total and regional adipose volumes and fat infiltration in live rodents.

Development of diet-induced fatty liver disease in the aging mouse is suppressed by brief daily exposure to low-magnitude mechanical signals.

The age-induced decline in the body's ability to fight disease is exacerbated by obesity and metabolic disease. Using a mouse model of diet-induced obesity, the combined challenge of a high-fat diet and age on liver morphology and biochemistry was characterized, while evaluating the potential of 15 min per day of high frequency (90 Hz), extremely low-magnitude (0.2 G) mechanical signals (LMMS) to suppress lipid accumulation in the liver. Following a 36-week protocol (animals 43 weeks of age), suppression of hepatomegaly and steatosis was reflected by a 29% lower liver mass in LMMS animals as compared with controls. Average triglyceride content was 101.7+/-19.4 microg mg(-1) tissue in the livers of high-fat diet control (HFD) animals, whereas HFD+LMMS animals realized a 27% reduction to 73.8+/-22.8 microg mg(-1) tissue. In HFD+LMMS animals, liver free fatty acids were also reduced to 0.026+/-0.009 microEq mg(-1) tissue from 0.035+/-0.005 microEq mg(-1) tissue in HFD. Moderate to severe micro- and macrovesicular steatosis in HFD was contrasted to a 49% reduction in area covered by the vacuoles of at least 15 microm(2) in size in HFD+LMMS animals. These data provide preliminary evidence of the ability of LMMS to attenuate the progression of fatty liver disease, most likely achieved indirectly by suppressing adipogenesis and thus the total adipose burden through life, thereby reducing a downstream challenge to liver morphology and function.

Devastation of bone tissue in the appendicular skeleton parallels the progression of neuromuscular disease.

A mouse model of spinal muscular atrophy with respiratory distress (SMARD1) was used to study the consequences of neuromuscular degenerative disease on bone quantity and morphology. Histomorphometry and micro-computed tomography were used to assess the cortical and cancellous bone in the tibia, femur and humerus of adult neuromuscular degeneration (nmd) mice (up to 21w) and age-matched wild-type controls (WT). At 21w, the average lengths of the humerus, tibia and femur were 15%, 10%, and 10% shorter in the nmd mice, respectively. The midshaft of the humerus, tibia and femur of nmd mice had 41%, 47% and 34% less cortical bone than the WT. In the humeral, tibial, and femoral metaphyses of the nmd mice, there was 50%, 78%, and 85% less trabecular bone volume, and 58%, 92%, and 94% less trabecular connectivity than the WT. NMD cortical bone had less than half of the 42% active surface measured in the WT, yet the mineral apposition rate of those surfaces were similar between strains (nmd: 1.80 microm x day(-1); WT: 2.05 microm x day(-1)). Osteoclast number and activity levels did not differ across strains. These data emphasize that neuromuscular degeneration as a result of immunoglobulin S-mu binding protein-2 (Ighmbp2) mutation will compromise several critical parameters of bone quantity and architecture, the most severe occurring in the trabecular compartment.

In vivo quantification of subcutaneous and visceral adiposity by micro-computed tomography in a small animal model.

Accurate and precise techniques that identify the quantity and distribution of adipose tissue in vivo are critical for investigations of adipose development, obesity, or diabetes. Here, we tested whether in vivo micro-computed tomography (microCT) can be used to provide information on the distribution of total, subcutaneous and visceral fat volume in the mouse. Ninety C57BL/6J mice (weight range: 15.7-46.5 g) were microCT scanned in vivo at 5 months of age and subsequently sacrificed. Whole body fat volume (base of skull to distal tibia) derived from in vivo microCT was significantly (p<0.001) correlated with the ex vivo tissue weight of discrete perigonadal (R(2)=0.94), and subcutaneous (R(2)=0.91) fat pads. Restricting the analysis of tissue composition to the abdominal mid-section between L1 and L5 lumbar vertebrae did not alter the correlations between total adiposity and explanted fat pad weight. Segmentation allowed for the precise discrimination between visceral and subcutaneous fat as well as the quantification of adipose tissue within specific anatomical regions. Both the correlations between visceral fat pad weight and microCT determined visceral fat volume (R(2)=0.95, p<0.001) as well as subcutaneous fat pad weight and microCT determined subcutaneous fat volume (R(2)=0.91, p<0.001) were excellent. Data from these studies establish in vivo microCT as a non-invasive, quantitative tool that can provide an in vivo surrogate measure of total, visceral, and subcutaneous adiposity during longitudinal studies. Compared to current imaging techniques with similar capabilities, such as microMRI or the combination of DEXA with NMR, it may also be more cost-effective and offer higher spatial resolutions.

Adipogenesis is inhibited by brief, daily exposure to high-frequency, extremely low-magnitude mechanical signals.

Obesity, a global pandemic that debilitates millions of people and burdens society with tens of billions of dollars in health care costs, is deterred by exercise. Although it is presumed that the more strenuous a physical challenge the more effective it will be in the suppression of adiposity, here it is shown that 15 weeks of brief, daily exposure to high-frequency mechanical signals, induced at a magnitude well below that which would arise during walking, inhibited adipogenesis by 27% in C57BL/6J mice. The mechanical signal also reduced key risk factors in the onset of type II diabetes, nonesterified free fatty acid and triglyceride content in the liver, by 43% and 39%, respectively. Over 9 weeks, these same signals suppressed fat production by 22% in the C3H.B6-6T congenic mouse strain that exhibits accelerated age-related changes in body composition. In an effort to understand the means by which fat production was inhibited, irradiated mice receiving bone marrow transplants from heterozygous GFP+ mice revealed that 6 weeks of these low-magnitude mechanical signals reduced the commitment of mesenchymal stem cell differentiation into adipocytes by 19%, indicating that formation of adipose tissue in these models was deterred by a marked reduction in stem cell adipogenesis. Translated to the human, this may represent the basis for the nonpharmacologic prevention of obesity and its sequelae, achieved through developmental, rather than metabolic, pathways.

Rapid establishment of chemical and mechanical properties during lamellar bone formation.

The development of prophylaxes and treatments of bone diseases that can effectively increase the strength of bone as a structure necessitates a better understanding of the time course by which chemical properties define the stiffness of the material during primary and secondary mineralization. It was hypothesized that these processes would be relatively slow in the actively growing skeleton. Seven-week-old Sprague-Dawley female rats (n = 8) were injected with multiple fluorochrome labels over a time span of 3 weeks and killed. Chemical and mechanical properties of the tibial mid-diaphysis were spatially characterized between the endocortical and periosteal surface by in situ infrared microspectroscopy and nanoindentation. The phosphate-to-protein ratio of bone 2-6 days old was 20% smaller at the periosteal surface and 22% smaller at the endocortical surface (P < 0.05 each) compared to older intracortical regions. The ratios of carbonate to protein, crystallinity, type A/type B carbonate, collagen cross-linking, and bone elastic modulus did not differ significantly between bone 2-6, 10-14, and 8-22 days old and intracortical regions. Intracortical properties of 10-week-old rats, except for the carbonate-to-protein ratio which was 23% smaller (P < 0.01), were not significantly different from intracortical matrix properties of young adult rats (5 months, n = 4). Spatially, the phosphate-to-protein ratio (R(2) = 0.33) and the phosphate-to-carbonate ratio (R(2) = 0.55) were significantly correlated with bone material stiffness, while the combination of all chemical parameters raised the R(2) value to 0.83. These data indicate that lamellar bone has the ability to quickly establish its mechanical and chemical tissue properties during primary and secondary mineralization even when the skeleton experiences rapid growth.

Patterns of strain in the macaque tibia during functional activity.

The strain environment of the tibial midshaft of two female macaques was evaluated through in vivo bone strain experiments using three rosette gauges around the circumference of the bones. Strains were collected for a total of 123 walking and galloping steps as well as several climbing cycles. Principal strains and the angle of the maximum (tensile) principal strain with the long axis of the bone were calculated for each gauge site. In addition, the normal strain distribution throughout the cross section was determined from the longitudinal normal strains (strains in the direction of the long axis of the bone) at each of the three gauge sites, and at the corresponding cross-sectional geometry of the bone. This strain distribution was compared with the cross-sectional properties (area moments) of the midshaft. For both animals, the predominant loading regime was found to be bending about an oblique axis running from anterolateral to posteromedial. The anterior and part of the medial cortex are in tension; the posterior and part of the lateral cortex are in compression. The axis of bending does not coincide with the maximum principal axis of the cross section, which runs mediolaterally. The bones are not especially buttressed in the plane of bending, but offer the greatest strength anteroposteriorly. The cross-sectional geometry therefore does not minimize strain or bone tissue. Peak tibial strains are slightly higher than the peak ulnar strains reported earlier for the same animals (Demes et al. [1998] Am J Phys Anthropol 106:87-100). Peak strains for both the tibia and the ulna are moderate in comparison to strains recorded during walking and galloping activities in nonprimate mammals.

Differential expression of neuroleukin in osseous tissues and its involvement in mineralization during osteoblast differentiation.

Osteoblast differentiation is a multistep process that involves critical spatial and temporal regulation of cellular processes marked by the presence of a large number of differentially expressed molecules. To identify key functional molecules, we used differential messenger RNA (mRNA) display and compared RNA populations isolated from the defined transition phases (proliferation, matrix formation, and mineralization) of the MC3T3-E1 osteoblast-like cell line. Using this approach, a complementary DNA (cDNA) fragment was isolated and identified as neuroleukin (NLK), a multifunctional cytokine also known as autocrine motility factor (AMF), phosphoglucose isomerase (PGI; phosphohexose isomerase [PHI]), and maturation factor (MF). Northern analysis showed NLK temporal expression during MC3T3-E1 cell differentiation with a 3.5-fold increase during matrix formation and mineralization. Immunocytochemical studies revealed the presence of NLK in MC3T3-E1 cells as well as in the surrounding matrix, consistent with a secreted molecule. In contrast, the NLK receptor protein was detected primarily on the cell membrane. In subsequent studies, a high level of NLK expression was identified in osteoblasts and superficial articular chondrocytes in bone of 1-, 4-, and 8-month-old normal mice, as well as in fibroblasts, proliferating chondrocytes, and osteoblasts within a fracture callus. However, NLK was not evident in hypertrophic chondrocytes or osteocytes. In addition, treatment of MC3T3 cells with 6-phosphogluconic acid (6PGA; a NLK inhibitor) resulted in diminishing alkaline phosphatase (ALP) activity and mineralization in MC3T3-E1 cells, especially during the matrix formation stage of differentiating cells. Taken together, these data show specific expression of NLK in discrete populations of bone and cartilage cells and suggest a possible role for this secreted protein in bone development and regeneration.

Biology of bone and how it orchestrates the form and function of the skeleton.

The principal role of the skeleton is to provide structural support for the body. While the skeleton also serves as the body's mineral reservoir, the mineralized structure is the very basis of posture, opposes muscular contraction resulting in motion, withstands functional load bearing, and protects internal organs. Although the mass and morphology of the skeleton is defined, to some extent, by genetic determinants, it is the tissue's ability to remodel--the local resorption and formation of bone--which is responsible for achieving this intricate balance between competing responsibilities. The aim of this review is to address bone's form-function relationship, beginning with extensive research in the musculoskeletal disciplines, and focusing on several recent cellular and molecular discoveries which help understand the complex interdependence of bone cells, growth factors, physical stimuli, metabolic demands, and structural responsibilities. With a clinical and spine-oriented audience in mind, the principles of bone cell and molecular biology and physiology are presented, and an attempt has been made to incorporate epidemiologic data and therapeutic implications. Bone research remains interdisciplinary by nature, and a deeper understanding of bone biology will ultimately lead to advances in the treatment of diseases and injuries to bone itself.

Inhibition of osteopenia by low magnitude, high-frequency mechanical stimuli.

The identification of anabolic agents for the treatment of metabolic bone disease is a highly prized, and elusive, goal. In searching for the osteogenic (bone-producing) constituents within mechanical stimuli, it was determined that high frequency (10-100 Hz) and low magnitude (<10 microstrain) stimuli were capable of augmenting bone mass and morphology, thereby benefiting both bone quantity and quality. Using animal models, it is shown that these mechanical signals can double bone-formation rates, inhibit disuse osteoporosis and increase the strength of trabecular bone by 25%. Considering that the magnitude of these mechanical signals are several orders of magnitude below those which cause damage to the bone tissue, it is proposed that this modality could be useful in the treatment of metabolic bone diseases.

Differential phosphorylation of paxillin in response to surface-bound serum proteins during early osteoblast adhesion.

An early signaling event during the adhesion and spreading of cells is integrin-mediated tyrosine phosphorylation of the cytoskeletal adaptor protein paxillin and the non-receptor tyrosine kinase pp125(FAK) at focal contacts. To determine the influence of surface-charge and -adsorbed adhesion proteins on this signaling pathway, paxillin phosphorylation was examined during attachment of MC3T3-E1 osteoblast-like cell onto charged and uncharged polystyrene, and on adsorbed layers of serum proteins, fibronectin (Fn), vitronectin (Vn), a mixture of Fn and Vn, and albumin. Paxillin phosphorylation was induced 2.4-fold (P < 0.05) on charged vs uncharged polystyrene only in the presence of serum proteins. Activation of paxillin via Fn or Vn alone, or in combination, resulted in significantly lower phosphorylation signals compared to whole serum (41 +/- 6.9%, P < 0.05, 45 +/- 5.9%, P < 0.05, and 76 +/- 9.8%, P < 0.075, respectively). Confocal laser microscopy confirmed increased co-localization of phosphotyrosine and paxillin at protruding lamellopodia of spreading osteoblasts on charged vs uncharged serum-pretreated polystyrene. Taken together, these data suggest that subtle differences in surface characteristics mediate effects on adhering cells via adsorbed serum proteins involving the cytoskeletal adaptor protein paxillin.

Osteoblastic networks with deficient coupling: differential effects of magnetic and electric field exposure.

A gap junction-deficient cell line was utilized to test whether intercellular coupling plays a significant role in modulating the influence of biophysical stimuli such as extracellular electrical currents. ROS 17/2.8 cells, an osteosarcoma cell line, along with a control transfected cell line and a connexin 43-gap junction-deficient cell line, were exposed to a time-changing magnetic flux (30 Hz, 1.8 milliTesla) sufficient to induce an electric field in the cultures on the order of 2 mV/m. Field exposure inhibited cell growth independent of gap junctional coupling, while alkaline phosphatase activity was found to be dependent on gap junctional coupling. These findings can be interpreted to suggest that magnetic and electric field exposures have differential effects on cell cultures, with magnetic field exposure inhibiting cell growth through a mechanism independent of gap junctional coupling, while the alteration in enzyme activity appears to be stimulated by the induced electric field in a gap junction-dependent manner.

Temporal expression of the chondrogenic and angiogenic growth factor CYR61 during fracture repair.

The repair of a fractured bone is a complex biological event that essentially recapitulates embryonic development and requires the activity of a number of different cell types undergoing proliferation, migration, adhesion, and differentiation, while at the same time expressing a host of different genes. To identify such genes, we employed differential display and compared messenger RNA (mRNA) populations isolated from postfracture (PF) day 5 calluses to those of intact rat femurs. One such gene in which expression was up-regulated at PF day 5 is identified as CYR61, a member of the CCN family of secreted regulatory proteins. CYR61 is a growth factor that stimulates chondrogenesis and angiogenesis. We show that its mRNA expression during fracture repair is regulated temporally, with elevated levels seen as early as PF day 3 and day 5, rising dramatically at PF day 7 and day 10, and finally declining at PF day 14 and day 21. At the highest peak of expression (PF day 7 and day 10, which correlates with chondrogenesis), CYR61 mRNA levels are approximately 10-fold higher than those detected in intact femurs. Similarly, high protein levels are detected throughout the reparative phase of the callus, particularly in fibrous tissue and periosteum, and in proliferating chondrocytes, osteoblasts, and immature osteocytes. The secreted form of CYR61 also was detected within the newly made osteoid. No labeling was detected in hypertrophic chondrocytes or in mature cortical osteocytes. These results suggest that CYR61 plays a significant role in cartilage and bone formation and may serve as an important regulator of fracture healing.

Quantifying the strain history of bone: spatial uniformity and self-similarity of low-magnitude strains.

We hypothesize that when a broad spectrum of bone strain is considered, strain history is similar for different bones in different species. Using a data collection protocol with a fine resolution, mid-diaphyseal strains were measured in vivo for both weightbearing and non-weightbearing bones in three species: dog, sheep, and turkey, with strain information collected continuously while the animals performed their natural daily activities. The daily strain history was quantified by both counting cyclic strain events (to quantify the distribution of strains of different magnitudes) and by estimating the average spectral characteristics of the strain (to quantify the frequency content of the strain signals). Counting of the daily (12-24 h) strain events show that large strains (> 1000 microstrain) occur relatively few times a day, while very small strains (< 10 microstrain) occur thousands of times a day. The lower magnitude strains (< approximately 200 microstrain) are found to be more uniform around the bone cross-section than the higher magnitude, peak strains. Strain dynamics are found to be well described by a power-law relationship and exhibit self-similar characteristics. These data lead to the suggestion that the organization of bone tissue is driven by the continual barrage of activity spanning a wide but consistent range of frequency and amplitude, and until the mechanism of bone's mechanosensory system is fully understood, all portions of bone's strain history should be considered to possibly play a role in bone adaptation.

Osteoclastogenesis is repressed by mechanical strain in an in vitro model.

Functional loading provides a site-specific signal for the regulation of bone mass and morphology. To determine if strain can inhibit the resorptive component of bone remodeling, osteoclast formation was assessed in marrow cultures plated on flexible membranes subjected to 5% strain for 10 cycles/minute, 24 hours per day. Cultures strained during days 2 through 7 inhibited osteoclast formation to 61+/-7% of control cultures (p < 0.05), a degree of inhibition similar to that observed when the cultures were subjected to strains during only days 2 through 4 but also evaluated on day 7 (67+/-4% of control; p < 0.05). In contrast, straining of cultures during days 5 through 7 had little influence on inhibiting the formation of osteoclasts (94+/-5% of control; no significant difference). The nonuniformly strained substrate was subdivided into three concentric rings. and cultures were used to examine the site-specificity of the inhibition caused by strain. Osteoclast formation in the outermost boundary, which was distended from 3.6 to 5%, was 41+/-7% of that observed in outer regions of control wells. The inhibitory potential of mechanical strain was reduced within the middle ring (73+/-6% of control osteoclasts: p < 0.01), where the strain ranged from 0.2 to 3.6%. The central region, which experienced strains equivalent to those in the middle ring (0.2 to -4% strain), showed inhibition of osteoclast formation to a similar degree (75+/-6% of control). Media harvested from strained cultures failed to inhibit osteoclast formation in unstrained cultures; this implies that the inhibitory effect of strain depended on the direct interaction of the cell with the substrate rather than by a humoral factor. A second device, where a uniform strain was delivered at 1.8% throughout the entire plate, inhibited osteoclast recruitment to 48+/-3.6%, emphasizing that uniform strain in the absence of shear stress constrains osteoclast recruitment. These in vitro experiments can but model the complex environment generated by in vivo mechanical strains: however, they provide the first direct evidence that strain must be considered as inhibitory to osteoclast recruitment.

Changes in postural muscle dynamics as a function of age.

Histologic studies have demonstrated both a decrease in size and loss in number of type II muscle fibers with increasing age. Although these age-related histologic changes are believed to result in decreased strength and functional capacity, age-related changes in muscle force dynamics have not been clearly elucidated. Using vibromyographic (VMG) techniques, we recorded muscle activity of the soleus in 40 healthy adult volunteers spanning the age range of 20-82 years to test whether changes in postural muscle dynamics, in the frequency range of 0.1-50 Hz, were also associated with age. Although muscle dynamics below 15 Hz do not change with aging, the 30-50 Hz frequency components of the VMG were found to change significantly with advancing age (r = -.619, p = .0001). This was observed in both sexes independently. The observed age-related changes in muscle force dynamics demonstrate distinct physiologic alterations in muscle fiber activity. Further research will be required to fully elucidate the relationship between age-related changes in muscle fiber activity and other age-related conditions such as postural instability and osteoporosis.