Steven D. Bain

Mechanical strain, induced noninvasively in the high-frequency domain, is anabolic to cancellous bone, but not cortical bone

Departing from the premise that it is the large-amplitude signals inherent to intense functional activity that define bone morphology, we propose that it is the far lower magnitude, high-frequency mechanical signals that continually barrage the skeleton during longer term activities such as standing, which regulate skeletal architecture. To examine this hypothesis, we proposed that brief exposure to slight elevations in these endogenous mechanical signals would suffice to increase bone mass in those bones subject to the stimulus. This was tested by exposing the hind limbs of adult female sheep (n = 9) to 20 min/day of low-level (0.3g), high-frequency (30 Hz) mechanical signals, sufficient to induce a peak of approximately 5 microstrain (micro epsilon) in the tibia. Following euthanasia, peripheral quantitative computed tomography (pQCT) was used to segregate the cortical shell from the trabecular envelope of the proximal femur, revealing a 34.2% increase in bone density in the experimental animals as compared with controls (p = 0.01). Histomorphometric examination of the femur supported these density measurements, with bone volume per total volume increasing by 32% (p = 0.04). This density increase was achieved by two separate strategies: trabecular spacing decreased by 36.1% (p = 0.02), whereas trabecular number increased by 45.6% (p = 0.01), indicating the formation of cancellous bone de novo. There were no significant differences in the radii of animals subject to the stimulus, indicating that the adaptive response was local rather than systemic. The anabolic potential of the signal was evident only in trabecular bone, and there were no differences, as measured by any assay, in the cortical bone. These data suggest that subtle mechanical signals generated during predominant activities such as posture may be potent determinants of skeletal morphology. Given that these strain levels are three orders of magnitude below strains that can damage bone tissue, we believe that a noninvasive stimulus based on this sensitivity has potential for treating skeletal complications such as osteoporosis.

Increased marrow-derived osteoprogenitor cells and endosteal bone formation in mice lacking thrombospondin 2.

The phenotype of thrombospondin 2 (TSP2)–null mice includes abnormalities in collagen fibrils and increases in ligamentous laxity, vascular density, and bleeding time. In this study, analyses by computerized tomography (CT) revealed that cortical density was increased in long bones of TSP2‐null mice. Histomorphometric analysis showed that the mid‐diaphyseal endosteal bone formation rate (BFR) of TSP2‐null mice was increased in comparison with that of wild‐type (WT) animals. Although microgeometric analysis showed that periosteal and endosteal radii were reduced, the mechanical properties of femurs from TSP2‐null mice were not significantly different from those of controls, presumably because of the concomitant increase in endosteal bone mass. Bone loss in ovariectomized mice was equivalent for WT and mutant mice, a finding that indicates that TSP2‐null animals are capable of normal bone resorption. To further explore the cellular basis for the increased endosteal BFR in TSP2‐null mice, marrow stromal cells (MSCs) were isolated and examined in vitro. These cells were found to be present in increased numbers in a colony forming unit (CFU) assay and showed an increased rate of proliferation in vitro. We conclude that TSP2 regulates the proliferation of osteoblast progenitors, directly or indirectly, and that in its absence endosteal bone formation is increased. (J Bone Miner Res 2000;15:851–862)

Local Modulation of Skeletal Growth and Bone Modeling in Poultry

Skeletal growth and bone modeling in poultry are regulated by complex interactions between the animals genetic potential and a host of systemic and localized factors (growth factors and cytokines) influencing bone biology. The objective of these interactions is to orchestrate the achievement of bone architecture that balances functionally appropriate morphology with the skeletons involvement in mineral homeostasis. Within this context, bone modeling in the growing animal represents an adaptive process that is distinct from bone remodeling, which is the term used to describe the resorption and formation of mineralized tissue that maintains skeletal mass and morphology in the adult. As many of the skeletal lesions that afflict poultry are the consequence of abnormalities in bone modeling, not bone remodeling, an appreciation of the differences between these two contrasting processes is a prerequisite for understanding the pathogenesis of skeletal lesions in poultry.

The anabolic effect of estrogen on endosteal bone formation in the mouse is attenuated by ovariohysterectomy: A role for the uterus in the skeletal response to estrogen?

SummaryIn the mouse, the anabolic effect of estrogen on the uterus and its stimulatory effect on endosteal bone formation are well documented. When these observations are coupled with the recent description of uterine-derived bone cell mitogens, it raises the possibility that uterine hypertrophy in response to estrogen might lead to the production and release of factors that participate in the skeletons anabolic response to estrogen. To determine if the stimulatory effects of estrogen on endosteal bone formation and uterine tissue in the mouse are related, we have studied this specific skeletal response to ovariectomy (OVX) and ovariohysterectomy (OHTX), and to two levels of 17β-estradiol (17β-E2). To assess treatment effects, 48 Swiss-webster mice were assigned to six groups: OHTX/oil vehicle, OVX/oil vehicle, OHTX/150 μg 17β-E2, OHTX/300 μg 17β-E2, OVX/150 μg 17β-E2, and OVX/300 μg 17β-E2. Animals were treated once per week with vehicle or the respective 17β-E2 dose. To quantitate bone formation, fluorochrome labels were administered at the beginning and end of the experimental period. At the conclusion of the 5-week study, tibiae were processed undecalcified for embedding in methyl methacrylate plastic. Cross-sectional areal properties and bone formation rates were quantitated from 30 μm mid-diaphyseal sections using a Bioquant Bone Morphometry system. Compared with the vehicle-treated OVX and OHTX mice, 150 μg of 17β-E2 administered once per week significantly increased cortical bone areas (P<0.05) but cortical bone widths and the ratio of cortical bone area to total bone area was increased only in estrogen-treated OVX mice (P<0.01). The attenuation of bone formation in the OHTX mice was even more apparent in animals treated with 300 μg 17β-E2. Endosteal mineral apposition and bone formation, cortical bone widths, and cortical bone ratios were all significantly reduced in OHTX mice compared with OVX animals treated with the same 17β-E2 dose. Indeed, the 17β-E2-induced cortical bone increases in the OVX animals were reduced 50% by OHTX. These results suggest that the anabolic effects of high-dose 17β-E2 on endosteal bone formation in the mouse are modulated by estrogens uterotrophic activity, and are therefore consistent with the hypothesis that the uterus may produce and release factors with the capacity to stimulate bone formation.

Structural Aspects of Bone Resorption

Bone remodeling in the adult skeleton is the process whereby bone matrix is first removed by osteoclasts and then replaced by osteoblasts. The cellular basis of the remodeling cycle was first described by Frost, who named this physiologic relationship the basic multicellular unit or the BMU [26, 28]. Known also as a bone remodeling unit or BRU [70], the BMU describes the sequential activities of osteoclasts and osteoblasts that are spatially and temporally coupled to ensure that the removal of mineralized matrix is replaced by an equivalent quantity of new bone [27, 48, 69]. Thus, over innumerable remodeling cycles the collective activities of individual BMUs ensure that an organism’s bone mass remains in constant balance and its skeletal architecture relatively unchanged [29, 43, 71]. It is now evident from investigations of the bone remodeling cycle that the skeleton’s mass and its corresponding architecture are dependent on the coordinated activity of the BMU, which is commonly separated into four distinct phases: activation, resorption, reversal, and formation [71, 85, 93, 94]. In both cortical and cancellous bone, the BMU life cycle begins with the activation of a quiescent (i.e., resting) bone surface in response to an enabling stimulus that triggers differentiation and recruitment of osteoclast precursors, which fuse to form mature, multinucleated, bone-resorbing osteoIntroduction

Static Preload Inhibits Loading-Induced Bone Formation: STATIC PRELOAD INHIBITS BONE ADAPTATION

Nearly all exogenous loading models of bone adaptation apply dynamic loading superimposed upon a time invariant static preload (SPL) in order to ensure stable, reproducible loading of bone. Given that SPL may alter aspects of bone mechanotransduction (eg, interstitial fluid flow), we hypothesized that SPL inhibits bone formation induced by dynamic loading. As a first test of this hypothesis, we utilized a newly developed device that enables stable dynamic loading of the murine tibia with SPLs ≥ −0.01 N. We subjected the right tibias of BALB/c mice (4‐month‐old females) to dynamic loading (−3.8 N, 1 Hz, 50 cycles/day, 10 s rest) superimposed upon one of three SPLs: −1.5 N, −0.5 N, or −0.03 N. Mice underwent exogenous loading 3 days/week for 3 weeks. Metaphyseal trabecular bone adaptation (μCT) and midshaft cortical bone formation (dynamic histomorphometry) were assessed following euthanasia (day 22). Ipsilateral tibias of mice loaded with a −1.5‐N SPL demonstrated significantly less trabecular bone volume/total volume (BV/TV) than contralateral tibias (−12.9%). In contrast, the same dynamic loading superimposed on a −0.03‐N SPL significantly elevated BV/TV versus contralateral tibias (12.3%) and versus the ipsilateral tibias of the other SPL groups (−0.5 N: 46.3%, −1.5 N: 37.2%). At the midshaft, the periosteal bone formation rate (p.BFR) induced when dynamic loading was superimposed on −1.5‐N and −0.5‐N SPLs was significantly amplified in the −0.03‐N SPL group (>200%). These data demonstrate that bone anabolism induced by dynamic loading is markedly inhibited by SPL magnitudes commonly implemented in the literature (ie, −0.5 N, −1.5 N). The inhibitory impact of SPL has not been recognized in bone adaptation models and, as such, SPLs have been neither universally reported nor standardized. Our study therefore identifies a previously unrecognized, potent inhibitor of mechanoresponsiveness that has potentially confounded studies of bone adaptation and translation of insights from our field.