Postmenopausal osteoporosis as a failure of bone's adaptation to functional loading: a hypothesis

Bone density in the adolescent athlete

Adolescence is a critical time for bone mass accrual, and increases in bone mass through puberty are dependent on rising levels of gonadal steroids, growth hormone and insulin like growth factor-1. Many high school girls are involved in athletic activities, and as many as 23.5% of adolescent athletes have been reported to develop amenorrhea. This review focuses on (1) factors that determine which athletes are likely to develop amenorrhea, such as a negative energy balance state, low levels of leptin and high levels of ghrelin, and (2) the impact of hypogonadism in athletes on bone metabolism. Beneficial effects of increased mechanical loading from athletic activity do not appear to protect against the deleterious effects of hypogonadism in adolescent athletes.

Bone metabolism in adolescent athletes with amenorrhea, athletes with eumenorrhea, and control subjects

OBJECTIVE

We hypothesized that, despite increased activity, bone density would be low in athletes with amenorrhea, compared with athletes with eumenorrhea and control subjects, because of associated hypogonadism and would be associated with a decrease in bone formation and increases in bone-resorption markers.

METHODS

In a cross-sectional study, we examined bone-density measures (spine, hip, and whole body) and body composition by using dual-energy radiograph absorptiometry and assessed fasting levels of insulin-like growth factor I and bone-turnover markers (N-terminal propeptied of type 1 procollagen and N-telopeptide) in 21 athletes with amenorrhea, 18 athletes with eumenorrhea, and 18 control subjects. Subjects were 12 to 18 years of age and of comparable chronologic and bone age.

RESULTS

Athletes with amenorrhea had lower bone-density z scores at the spine and whole body, compared with athletes with eumenorrhea and control subjects, and lower hip z scores, compared with athletes with eumenorrhea. Lean mass did not differ between groups. However, athletes with amenorrhea had lower BMI z scores than did athletes with eumenorrhea and lower insulin-like growth factor I levels than did control subjects. Levels of both markers of bone turnover were lower in athletes with amenorrhea than in control subjects. BMI z scores, lean mass, insulin-like growth factor I levels, and diagnostic category were important independent predictors of bone mineral density z scores.

CONCLUSIONS

Although they showed no significant differences in lean mass, compared with athletes with eumenorrhea and control subjects, athletes with amenorrhea had lower bone density at the spine and whole body. Insulin-like growth factor I levels, body-composition parameters, and menstrual status were important predictors of bone density. Follow-up studies are necessary to determine whether amenorrhea in athletes adversely affects the rate of bone mass accrual and therefore peak bone mass.

The bone-muscle ratio of fetal lambs is affected more by maternal nutrition during pregnancy than by maternal size

Bone formation and loss are related to the strain imposed on bone by muscle forces. Bone mineral content (BMC) and lean mass (LM) of fetal lambs was determined at day 140 of pregnancy in 8 groups of ewes, which were of either large or small body size, on either high (ad libitum) or maintenance pasture intake from day 21 of pregnancy, or carrying either singletons or twins. BMC and LM (using DXA scanning) of fetal hindquarters/spine were corrected to leg length. BMC and LM were less in twin than singleton groups (P < 0.001). Large ewes on high intake produced single fetuses with a (group mean) BMC/LM ratio that was higher (P < 0.002) than that in fetuses of large ewes with singletons on maintenance intake or twins on either high or maintenance intakes, the ratios of which were not different. In single fetuses from small ewes on high intake, the BMC/LM ratio was higher than those from small ewes with singletons on maintenance intake or twins on either high or maintenance intakes, the ratios of which were not different. The ratio was not different in singleton fetuses of ewes on high intake, whether they were large or small. Different fetal environments resulted in a given amount of muscle being associated with a higher or lower bone mass. Dietary intake during pregnancy was more important than maternal size in affecting the ratio. We conclude that intrauterine environmental factors may be important in determining bone mass postnatally, and possibly later in life.

Ovariectomy sensitizes rat cortical bone to whole-body vibration

This study was designed to determine the modulatory effect of estrogen on mechanical stimulation in bone. Trabecular and cortical bone compartments of ovariectomized rats exposed to whole-body vibration of different amplitudes were evaluated by peripheral quantitative computed tomographic (pQCT) analysis and histomorphometry and compared to controls not exposed to vibration. Rats underwent whole-body vibration (20 minutes/day, 5 days/week) on a vibration platform for 2 months. The control rats were placed on the platform without vibration for the same time. We divided rats into six groups: a sham control (SHAM); a sham vibrated (SHAM-V) at 30 Hz, 0.6 g; a SHAM-V at 30 Hz, 3g; an ovariectomized control (OVX); an ovariectomized vibrated (OVX-V) at 30 Hz, 0.6 g; and an OVX-V at 30 Hz, 3g. In vivo, pQCT analyses of the tibiae were performed at the start of the experiment and after 4 and 8 weeks. After 8 weeks the tibiae were excised for histomorphometric and for in vitro pQCT analyses. In the SHAM-V group, vibration had no effect upon the different bone parameters. In the OVX-V group, vibration induced a significant increase compared to the OVX group of the cortical and medullary areas (P < 0.01) and of the periosteal (P < 0.01) and endosteal (P < 0.05) perimeters at the 3 g vibration. The strain strength index increased in the OVX-V group significantly (P < 0.01) at the higher vibration. The results showed that low-amplitude, high-frequency whole-body vibration is anabolic to bone in OVX animals. The osteogenic potential is limited to the modeling of the bone cortex and depends on the amplitude of the vibration.

Constitutively active parathyroid hormone receptor signaling in cells in osteoblastic lineage suppresses mechanical unloading-induced bone resorption

Multiple signaling pathways participate in the regulation of bone remodeling, and pathological negative balance in the regulation results in osteoporosis. However, interactions of signaling pathways that act comprehensively in concert to maintain bone mass are not fully understood. We investigated roles of parathyroid hormone receptor (PTH/PTHrP receptor) signaling in osteoblasts in unloading-induced bone loss using transgenic mice. Hind limb unloading by tail suspension reduced bone mass in wild-type mice. In contrast, signaling by constitutively active PTH/PTHrP receptor (caPPR), whose expression was regulated by the osteoblast-specific Col1a1 promoter (Col1a1-caPPR), suppressed unloading-induced reduction in bone mass in these transgenic mice. In Col1a1-caPPR transgenic (Tg) mice, hind limb unloading suppressed bone formation parameters in vivo and mineralized nodule formation in vitro similarly to those observed in wild-type mice. In addition, serum osteocalcin levels and mRNA expression levels of type I collagen, Runx2 and Osterix in bone were suppressed by unloading in both wild-type mice and Tg mice. However, in contrast to unloading-induced enhancement of bone resorption parameters in wild-type mice, Col1a1-caPPR signaling suppressed, rather than enhanced, osteoclast number and osteoclast surface as well as urinary deoxypyridinoline excretion upon unloading. Col1a1-caPPR signaling also suppressed mRNA expression levels of RANK and c-fms in bone upon unloading. Although the M-CSF and monocyte chemoattractant protein 1 (MCP-1) mRNA levels were enhanced in control Tg mice, these levels were suppressed in unloaded Tg mice. These results indicated that constitutive activation of PTH/PTHrP receptor signaling in osteoblastic cells suppresses unloading-induced bone loss specifically through the regulation of osteoclastic activity.

Effect of short-term treatment with alendronate on ulnar bone adaptation to cyclic fatigue loading in rats

Targeted remodeling of fatigue-injured bone involves activation of osteoclastic resorption followed by local bone formation by osteoblasts. We studied the effect of parenteral alendronate (ALN) on bone adaptation to cyclic fatigue. The ulnae of 140 rats were cyclically loaded unilaterally until 40% loss of stiffness developed. We used eight treatment groups: (1) baseline control; (2) vehicle (sterile saline) and (3) alendronate before fatigue, no adaptation (Pre-VEH, Pre-ALN, respectively); (4) vehicle and (5) alendronate during adaptation to fatigue (Post-VEH, Post-ALN, respectively); (6) vehicle before fatigue and during adaptation (Pre-VEH/Post-VEH); (7) alendronate before fatigue and vehicle during adaptation (Pre-ALN/Post-VEH); (8) alendronate before fatigue and during adaptation (Pre-ALN/Post-ALN). Bones from half the rats/group were tested mechanically; remaining bones were examined histologically. The following variables were quantified: volumetric bone mineral density (vBMD); ultimate force (F(u)); stiffness (S); work-to-failure (U); cortical area (Ct.Ar); new woven bone tissue area (Ne.Wo.B.T.Ar); resorption space density (Rs.N/T.Ar). Microcracking was only seen in fatigue-loaded ulnae. A significant effect of alendronate on vBMD was not found. Preemptive treatment with alendronate did not protect the ulna from structural degradation during fatigue. After fatigue, recovery of mechanical properties by adaptation occurred; here a significant alendronate effect was not found. An alendronate-specific effect on adaptive Ne.Wo.B.T.Ar was not found. In the fatigue-loaded ulna, Rs.N/T.Ar was increased in vehicle-treated adapted groups, but not alendronate-treated adapted groups, when compared with baseline control. These data suggest that short-term alendronate treatment does not protect bone from fatigue in this model. Inhibition of remodeling may reduce microcrack repair over time.

Who's afraid of the big bad Wolff?: "Wolff's law" and bone functional adaptation

"Wolff's law" is a concept that has sometimes been misrepresented, and frequently misunderstood, in the anthropological literature. Although it was originally formulated in a strict mathematical sense that has since been discredited, the more general concept of "bone functional adaptation" to mechanical loading (a designation that should probably replace "Wolff's law") is supported by much experimental and observational data. Objections raised to earlier studies of bone functional adaptation have largely been addressed by more recent and better-controlled studies. While the bone morphological response to mechanical strains is reduced in adults relative to juveniles, claims that adult morphology reflects only juvenile loadings are greatly exaggerated. Similarly, while there are important genetic influences on bone development and on the nature of bone's response to mechanical loading, variations in loadings themselves are equally if not more important in determining variations in morphology, especially in comparisons between closely related individuals or species. The correspondence between bone strain patterns and bone structure is variable, depending on skeletal location and the general mechanical environment (e.g., distal vs. proximal limb elements, cursorial vs. noncursorial animals), so that mechanical/behavioral inferences based on structure alone should be limited to corresponding skeletal regions and animals with similar basic mechanical designs. Within such comparisons, traditional geometric parameters (such as second moments of area and section moduli) still give the best available estimates of in vivo mechanical competence. Thus, when employed with appropriate caution, these features may be used to reconstruct mechanical loadings and behavioral differences within and between past populations.

Wnt/beta-catenin signaling is a component of osteoblastic bone cell early responses to load-bearing and requires estrogen receptor alpha

The Wnt/beta-catenin pathway has been implicated in bone cell response to their mechanical environment. This response is the origin of the mechanism by which bone cells adjust bone architecture to maintain bone strength. Osteoporosis is the most widespread failure of this mechanism. The degree of osteoporotic bone loss in men and women is related to bio-available estrogen. Here we report that in osteoblastic ROS 17/2.8 cells and primary osteoblast cultures, a single short period of dynamic mechanical strain, as well as the glycogen synthase kinase-3beta (GSK-3beta) inhibitor LiCl, increased nuclear accumulation of activated beta-catenin and stimulated TCF/LEF reporter activity. This effect was blocked by the estrogen receptor (ER) modulators ICI 182,780 and tamoxifen and was absent in primary osteoblast cultures from mice lacking ERalpha. Microarray expression data for 25,000 genes from total RNA extracted from tibiae of wild-type mice within 24 h of being loaded in vivo showed differential gene regulation between loaded and contralateral non-loaded bones of 10 genes established to be involved in the Wnt pathway. Only 2 genes were involved in loaded tibiae from mice lacking ERalpha (ERalpha(-/-)). Together these data suggest that Wnt/beta-catenin signaling contributes to bone cell early responses to mechanical strain and that its effectiveness requires ERalpha. Reduced effectiveness of bone cell responses to bone loading, associated with estrogen-related decline in ERalpha, may contribute to the failure to maintain structurally appropriate bone mass in osteoporosis in both men and women.

Effects of physical activity on evolution of proximal femur structure in a younger elderly population

INTRODUCTION

For a fixed weight, a wider bone of standardised length is stiffer. But moving the cortices away from the centre of mass risks creating structural (elastic) instability, and hip fractures have been postulated to occur as a consequence of buckling of the thinned supero-lateral femoral neck cortex during a fall. We hypothesised that stereotyped physical activity (e.g., walking) may help conserve bending resistance (section modulus, Z) through redistribution of bone tissue, but it might be at the expense of supero-lateral cortical stability.

METHODS

Hip structural analysis (HSA) software applied to DXA scans was used to derive measurements of section modulus and distances of a cross-section's centre of mass from the supero-lateral cortical margin (lateral distance, in cm). DXA scans were obtained on 1361 men and women in the EPIC-Norfolk population-based prospective cohort study. Up to 4 repeat DXA scans were done in 8 years of follow-up. Weight, height and activities of daily living were assessed on each occasion. A detailed physical activity and lifestyle questionnaire was administered at baseline. The lateral distance was measured on three narrow cross-sections with good precision: narrow neck (NN, coefficient of variation 2.6%), intertrochanter (IT) and shaft (S). A linear mixed model was used to assess associations with predictors.

RESULTS

Ageing was associated with medial shifting of the centre of mass, so that lateral distance increased. Both greater weight and height were associated with greater lateral distance (P<0.0001). Among physical activity-related variables, walking/cycling for >1 h/day (P=0.025), weekly time spent on moderate impact activity (P=0.003), forced expiratory volume in 1 s (NN and IT, P<0.026) and lifetime physical activity (IT, P<0.0001) were associated with higher lateral distance. However, after adjusting for these variables, activities of daily living scores (NN, P<0.0001) and weekly time spent on low impact hip flexing activities were associated with shorter lateral distance (P=0.001). Greater baseline lateral distance was significantly associated with increased risk of subsequent hip fracture (n=26) in females (P<0.05, all regions) independently of age, height and bone mineral content.

CONCLUSION

The age-related shift medially of the centre of mass of the femoral neck and trochanter may have adverse effects on fracture resistance in the event of a fall, so compromising the beneficial effects of walking on fitness, strength and risk of falling. The role of more diverse physical activity patterns in old age that impose loading on the supero-lateral cortex of the femur, involving for example hip flexion and stretching, needs investigation for their ability to correct this medial shifting of the centre of mass.

Sexual dimorphism affects tibia size and shape but not tissue-level mechanical properties

Understanding how growth influences adult bone morphology and tissue quality should provide important insight into why females show a greater incidence of stress fractures early in life and fragility fractures later in life compared to males. The objective of this study was to test whether females acquire similar tissue-level mechanical properties as males by the time peak bone properties are established. Standardized beams of bone were machined from the tibial diaphyses of 14 young, adult females ranging in age from 22 to 46 years. Data for males (n=17, age=17-46 years) were taken from a prior study. Measures of tissue-level mechanical properties, including stiffness, strength, ductility, toughness, and damageability, were compared between sexes using t-tests. The relationship between cross-sectional morphology and tissue-level mechanical properties was also examined. Males and females showed nearly identical tissue-level mechanical properties. Both sexes also showed similar age-related degradation of mechanical properties and a similar relationship between cross-sectional morphology and tissue quality. However, for all body sizes, female tibiae were smaller relative to body size (i.e., less robust) compared to males. The results indicated that sex-specific growth patterns affected transverse bone size, but did not affect tissue-level mechanical properties. This, combined with the observation that young, adult female long bones are undersized relative to body size, suggests that adult females would be expected to accumulate more damage under intense loading compared to males. This may be a contributing factor to the greater incidence of stress fractures observed for female military recruits.

Does reduced skeletal loading account for age-related bone loss?

A leading theory suggests that decreasing activity and muscle mass is the main cause of age-related bone loss. However, in a population-based study of 375 women and 325 men (age, 21-97 years), we failed to find a close correspondence between these variables and changes in bone strength with aging.

INTRODUCTION

It has been suggested that bone strength is homeostatically adapted to habitual skeletal loading conditions and that bone loss could, therefore, result simply from age-related reductions in physical activity and muscle mass, but this notion has not been explored in detail.

MATERIALS AND METHODS

In a stratified random sample of Rochester, MN, women and men 21-97 years of age, indices of bone strength, flexural rigidity (EI), and axial rigidity (EA) were estimated from central QCT measurements at the femoral neck and lumbar spine and pQCT measurements at the ultradistal radius, whereas habitual skeletal loading was assessed using lean body mass, total skeletal muscle mass (TSM), body weight, and physical activity. Using regression analysis, we tested the hypothesis (H(o)) that bone strength per unit load did not vary with age.

RESULTS AND CONCLUSIONS

In these cross-sectional data, the null hypothesis of no age-related change was rejected in 72% of the strength-to-load ratios tested. For example, the ratio of femoral neck EI to TSM increased by 0.19%/year in women (p = 0.008) and by 0.49%/year in men (p < 0.001). There was no close correspondence between changes in habitual load and changes in bone strength, nor any consistent pattern. Moreover, interindividual variation in the strength-to-load ratios was substantial. These data suggest that the notion of reduced skeletal loading as the primary basis for age-related bone loss is oversimplified.

Current perspectives on NMDA-type glutamate signalling in bone

Bone is a complex, evolving tissue, architecturally defined by the activities of osteoclasts and osteoblasts that continually resorb and replace the mineralised matrix. Numerous regulatory mechanisms exist to control bone remodelling and the maintenance of bone mass. The consequences of inappropriate or uncoupled bone resorption and formation are significant and invariably lead to different disease states, the most prevalent being osteoporosis. In recent years, much attention has focused on unravelling the systemic and local signalling interactions that influence the differentiation and function of bone cells with a view to developing our understanding of bone biology and identifying potential new targets for therapeutic intervention. Several lines of evidence indicate that neurotransmitters and neuromodulators have influential roles to play in the regulation of bone remodelling and much of this research has involved analysis of the excitatory amino acid glutamate. This review will summarise current understanding of glutamate signalling in bone cells, addressing specifically the function of N-methyl-D-aspartate (NMDA)-type glutamate receptor signalling mechanisms, and will address the functional significance and future prospects for this area of research.

Independent and combined contributions of cancellous and cortical bone deficits to vertebral fracture risk in postmenopausal women

Using iliac bone histomorphometry on 78 patients with vertebral fracture and 66 healthy postmenopausal women, cortical thickness discriminated at least as well as any cancellous bone structural index between the two groups. Subjects with a deficit in both cortical and cancellous bone had much greater likelihood of fracture.

INTRODUCTION

Vertebral fracture is often attributed to disproportional loss of cancellous bone, but fracture patients may have deficits in cortical and cancellous bone. Accordingly, we examined the contribution of cortical and cancellous bone deficits, separately and together, to the likelihood of vertebral fracture.

MATERIALS AND METHODS

Iliac bone histomorphometry was performed in 78 white woman with clinically apparent vertebral fracture, 66 healthy postmenopausal women, and 38 healthy premenopausal women. We measured cancellous bone volume (Cn.BV/TV), trabecular number (Tb.N), trabecular thickness (Tb.Th), cortical bone volume (Ct.BV/TV), and cortical thickness (Ct.Th). For each variable, a value of >1 SD below the mean in premenopausal women was treated as a putative risk factor, and its association with the presence or absence of fracture was determined by OR calculated by logistic regression and by receiver operating characteristic (ROC) curve analysis. Subsets of fracture and control subjects were separately matched for Cn.BV/TV and Ct.Th.

RESULTS

All structural indices differed between fracture patients and controls except Ct.BV/TV. There was a weak but highly significant correlation between Cn.BV/TV and Ct.Th in the entire group (r = 0.389, r(2) = 0.151 p < 0.001). Many control subjects had a high value for one of these variables and a low value for the other. Ct.Th., Cn.BV/TV, and Tb.N were all significantly associated with vertebral fracture (ORs, 4.4-5.8; ROC area under the curve [AUC], 0.74-0.85). In subjects matched for Cn.BV/TV, Ct.Th was reduced by 29% (OR, 5.0), and in subjects matched for Ct.Th, Cn.BV/TV was reduced by 27% (OR, 5.0). In patients with deficits in both cortical and cancellous bone, the ORs ( 28-35 ) were much higher.

CONCLUSIONS

Deficits in cortical bone (reduced value for Ct.Th) and in cancellous bone (reduced values for Cn.BV/TV or Tb.N) were equally effective in discriminating between subjects with and without vertebral fracture. With a deficit in both cortical and cancellous bone, the association with vertebral fracture was much stronger. Vertebral fracture is not the result of disproportionate loss of cancellous bone in the patients as a whole, although individual patients may have relatively greater deficits in either cancellous or cortical bone.

Influence of forces on peri-implant bone

Occlusal forces affect an oral implant and the surrounding bone. According to bone physiology theories, bones carrying mechanical loads adapt their strength to the load applied on it by bone modeling/remodeling. This also applies to bone surrounding an oral implant. The response to an increased mechanical stress below a certain threshold will be a strengthening of the bone by increasing the bone density or apposition of bone. On the other hand, fatigue micro-damage resulting in bone resorption may be the result of mechanical stress beyond this threshold. In the present paper literature dealing with the relationship between forces on oral implants and the surrounding bone is reviewed. Randomized controlled as well as prospective cohorts studies were not found. Although the results are conflicting, animal experimental studies have shown that occlusal load might result in marginal bone loss around oral implants or complete loss of osseointegration. In clinical studies an association between the loading conditions and marginal bone loss around oral implants or complete loss of osseointegration has been stated, but a causative relationship has not been shown.

Fluid flow induces Rankl expression in primary murine calvarial osteoblasts

Mechanical loading of bone generates fluid flow within the mineralized matrix that exerts fluid shear stress (FSS) on cells. We examined effects of FSS on receptor activator of nuclear factor kappa B ligand (RANKL), a critical factor for osteoclast formation. Primary murine osteoblasts were subjected to pulsatile FSS (5 Hz, 10 dynes/cm(2)) for 1 h and then returned to static culture for varying times (post-FSS). Protein levels were measured by Western analysis and mRNA by Northern analysis, RT-PCR and quantitative PCR. There were 20- to 40-fold increases in RANKL mRNA at 2-4 h post-FSS. RANKL protein was induced by 2 h post-FSS and remained elevated for at least 8 h. Effects were independent of cyclooxygenase-2 activity. Small increases (up to three-fold) in mRNA of the decoy receptor for RANKL, osteoprotegerin, were seen. Five min of FSS, followed by static culture, was as effective in stimulating RANKL mRNA as 4 h of continuous FSS. FSS induced cAMP activity, and H-89, a protein kinase A (PKA) inhibitor, blocked the FSS induction of RANKL. H-89 also inhibited the PKC pathway, but specific PKC inhibitors, GF109203X and Go6983, did not inhibit FSS-induced RANKL. FSS induced phosphorylation of ERK1/2, and PD98059, an inhibitor of the ERK pathway, inhibited the FSS induction of RANKL mRNA 60%-90%. Thus, brief exposure to FSS resulted in sustained induction of RANKL expression after stopping FSS, and this induction was dependent on PKA and ERK signaling pathways. Increased RANKL after mechanical loading may play a role in initiating bone remodeling.

Osteocytes use estrogen receptor alpha to respond to strain but their ERalpha content is regulated by estrogen

The role of mechanical strain and estrogen status in regulating ERalpha levels in bone cells was studied in female rats. OVX is associated with decreased ERalpha protein expression/osteocyte, whereas habitual strain and artificial loading has only a small but positive effect, except on the ulna's medial surface, where artificial loading stimulates reversal of resorption to formation.

INTRODUCTION

Osteoporosis is the most widespread failure of bones' ability to match their architectural strength to their habitual load bearing. In men and women, the severity of bone loss is associated with bioavailability of estrogen. This association could result from the estrogen receptor (ER) involvement in bone cells' adaptive response to loading.

MATERIALS AND METHODS

In vivo semiquantitative analysis of the amount of ERalpha protein per osteocyte was performed in immuno-cytochemically stained sections from control and loaded rat ulna, as well as tibias of ovariectomy (OVX) and sham-operated female rats. In vitro, the effect of exogenous estrogen (10(-8) M) and mechanical strain (3400 microepsilon, 1 Hz, 600 cycles) on the expression of ERalpha mRNA levels was assessed in ROS 17/2.8 cells in monolayers using real-time PCR and ER promoter activity. ERalpha translocation in response to exogenous estrogen and mechanical strain was assessed in both ROS 17/2.8 and MLO-Y4 cells.

RESULTS

More than 90 percent of tibial osteocytes express ERalpha, the level/osteocyte being higher in cortical than cancellous bone. OVX is associated with decreased ERalpha protein expression/osteocyte, whereas in the ulna habitual strain and that caused by artificial loading had only a small but positive effect, except on the medial surface, where loading stimulates reversal of resorption to formation. In unstimulated osteocytes and osteoblasts in situ, and osteocyte-like and osteoblast-like cells in vitro, ERalpha is predominantly cytoplasmic. In vitro, both strain and estrogen stimulate transient ERalpha translocation to the nucleus and transient changes in ERalpha mRNA. Strain but not estrogen also induces discrete membrane localization of ERalpha.

CONCLUSIONS

Bone cells' responses to both strain and estrogen involve ERalpha, but only estrogen regulates its cellular concentration. This is consistent with the hypothesis that bone loss associated with estrogen deficiency is a consequence of reduction in ERalpha number/activity associated with lower estrogen concentration reducing the effectiveness of bone cells' anabolic response to strain.

Two-week longitudinal survey of bone architecture alteration in the hindlimb-unloaded rat model of bone loss: sex differences

The goal of this study was to determine, through a longitudinal follow-up, whether sex influences bone adaptation during simulated weightlessness. Twelve-week-old male and female Wistar rats were hindlimb unweighted for 2 wk, and the time course of bone alteration was monitored in vivo by means of densitometry and unbiased three-dimensional quantitative microcomputed tomography at 7 and 14 days. Compared with male rats, female rats had twice more cancellous bone volume at the proximal tibia at baseline, and this bone volume continued to increase, whereas in males it stabilized. Conversely, cortical area was greater in males than in females, and in both sexes cortical bone was still expanding. Hindlimb unloading resulted in larger reductions in males than in females in both cortical and cancellous compartments. In females, trabecular thickness and number decreased mildly, whereas in males trabecular number was dramatically reduced. In both sexes, the trabecular network became less connected and more rod-like shaped. Bone cellular activities evaluated by histomorphometry showed decreased bone formation rate in both sexes and increased resorption activity only in males. In conclusion, in female rats unloaded-related cancellous alterations reversed the growing process, whereas in males, which show lower growth process, it induced an accentuation of age-related cancellous bone changes for most of the parameters.

Short-term delay of puberty causes a transient reduction in bone strength in growing female rats

Multiple factors affect the structural development of the skeleton; in particular, estrogen levels during growth are an important factor in the pathogenesis of bone fragility. The delay of menarche and infrequent menstrual cycles decrease estrogen levels during adolescence and decrease peak bone mass. The aim of this study was to determine if delayed puberty through administration of a GnRH antagonist initiated prior to the onset of the first estrus cycle would delay the increase in estrogen levels and impede bone strength development in female rats. Twenty-three-day-old female Sprague-Dawley rats were randomly assigned to one of four groups; 1) short-term control group (C-ST) (n = 12), 2) long-term control (C-LT) (n = 12), 3) short-term GnRH antagonist group (G-ST) (n = 12) and 4) long-term GnRH antagonist group (G-LT) (n = 12). Injections (0.2 ml) of either saline or GnRH antagonist (100 microg/day) (Cetrotide, Serono, Inc) were given intraperitoneally for a duration of 18 days. Pubertal and gonadal development was retarded as indicated by a delay in vaginal opening (an indicator of pubertal onset), lower ovarian and uterine weights and lower estradiol levels in the short-term experimental animals (G-ST). However, at maturity (G-LT), there were no significant differences found in these measures. A delay in the timing of puberty significantly attenuated the development of femoral bone strength at 6 weeks of age. Peak moment, yield moment and stiffness in the G-ST group were all significantly less than the C-ST group. Cortical width was significantly attenuated due to the increased percentage of marrow area per total bone area in the G-ST group. However, femoral bone strength was recovered at maturity (G-LT). In summary, a transient delay in pubertal timing has short-term effects on bone strength development. In the current animal model of delaying puberty through GnRH antagonist injections, there appears to be no long-term effects on bone strength.

Relation of body composition, fat mass, and serum lipids to osteoporotic fractures and bone mineral density in Chinese men and women

BACKGROUND

Higher fat mass may be an independent risk factor for osteoporosis and osteoporotic fractures.

OBJECTIVE

We aimed to determine the independent contribution of fat mass to osteoporosis and to estimate the risk of osteoporotic fractures in relation to body weight, lean mass, and other confounders.

DESIGN

This was a community-based, cross-sectional study of 7137 men, 4585 premenopausal women, and 2248 postmenopausal women aged 25-64 y. Total-body and hip bone mineral content (BMC) and bone mineral density (BMD) and body composition were measured by dual-energy X-ray absorptiometry. Serum lipids were measured. Sex- and menopause-specific multiple generalized linear models were applied.

RESULTS

Across 5-kg strata of body weight, fat mass was significantly inversely associated with BMC in the whole body and total hip. When we compared the highest quartile with the lowest quartile of percentage fat mass in men, premenopausal women, and postmenopausal women, the adjusted odds ratios (95% CIs) of osteoporosis defined by hip BMD were 5.2 (2.1, 13.2), 5.0 (1.7, 15.1), and 6.9 (4.3, 11.2), respectively. Significant linear trends existed for higher risks of osteoporosis, osteopenia, and nonspine fractures with higher percentage fat mass. Significant negative relations were found between whole-body BMC and cholesterol, triacylglycerol, LDL, and the ratio of HDL to LDL in all groups.

CONCLUSIONS

Risks of osteoporosis, osteopenia, and nonspine fractures were significantly higher for subjects with higher percentage body fat independent of body weight, physical activity, and age. Thus, fat mass has a negative effect on bone mass in contrast with the positive effect of weight-bearing itself.

Unloading induces osteoblastic cell suppression and osteoclastic cell activation to lead to bone loss via sympathetic nervous system

Osteoporosis is one of the major health problems in our modern world. Especially, disuse (unloading) osteoporosis occurs commonly in bedridden patients, a population that is rapidly increasing due to aging-associated diseases. However, the mechanisms underlying such unloading-induced pathological bone loss have not yet been fully understood. Since sympathetic nervous system could control bone mass, we examined whether unloading-induced bone loss is controlled by sympathetic nervous tone. Treatment with beta-blocker, propranolol, suppressed the unloading-induced reduction in bone mass. Conversely, beta-agonist, isoproterenol, reduced bone mass in loaded mice, and under such conditions, unloading no longer further reduced bone mass. Analyses on the cellular bases indicated that unloading-induced reduction in the levels of osteoblastic cell activities, including mineral apposition rate, mineralizing surface, and bone formation rate, was suppressed by propranolol treatment and that isoproterenol-induced reduction in these levels of bone formation parameters was no longer suppressed by unloading. Unloading-induced reduction in the levels of mineralized nodule formation in bone marrow cell cultures was suppressed by propranolol treatment in vivo. In addition, loss of a half-dosage in the dopamine beta-hydroxylase gene suppressed the unloading-induced bone loss and reduction in mineralized nodule formation. Unloading-induced increase in the levels of osteoclastic activities such as osteoclast number and surface as well as urinary deoxypyridinoline was all suppressed by the treatment with propranolol. These observations indicated that sympathetic nervous tone mediates unloading-induced bone loss through suppression of bone formation by osteoblasts and enhancement of resorption by osteoclasts.

Identifying genes that regulate bone remodeling as potential therapeutic targets

Bone remodeling, a coupled process involving bone resorption and formation, is initiated by mechanical signals and is controlled by local and systemic factors that regulate osteoblast and osteoclast differentiation and function. An excess of resorption over formation leads to the bone loss and increased propensity to fracture that is characteristic of osteoporosis. A newly described inhibitor of osteoblast differentiation, Ciz, interferes with bone morphogenic protein signaling. As a consequence, Ciz-deficient mice develop increased bone mass.

Effect of oral contraceptive treatment on bone mass acquisition in skeletally immature young female rats

The objective of the present study was to investigate the effect of oral contraceptive (OC) treatment on bone mass accrual in skeletally immature young female rats. Animals in the baseline group were killed at the beginning of the experiment and were subjected to bone density assessment by peripheral quantitative computerized tomography (pQCT). The control group was fed a base diet free of phytoestrogens, while animals in the contraceptive group received the same base diet mixed with 2.67 microg desogestrel/100 g body weight and 0.0533 microg ethinyl estradiol/100 g body weight. The duration of the treatment period was 16 weeks. Densitometric measurements by dual energy x-ray absorptiometry and serum bone markers assessment were carried out at baseline, at 8 weeks and at 16 weeks, while pQCT densitometry took place after sacrifice. All bone mineral density and bone mineral content indices measured by dual energy x-ray absorptiometry increased significantly throughout the study period in both the OC and control group. Concerning pQCT measurements, animals in both the OC and the control group had significantly higher cortical density compared with baseline (midtibia: p=.0003 and .0003, respectively). Total area and periosteal circumference were significantly higher in OC group, both in proximal (p=.003 and .003, respectively) and midtibia (p=.048 and .042, respectively) compared with baseline. Osteoprotegerin serum levels increased in both groups, and at the end of the experiment, circulating osteoprotegerin was significantly higher in the OC group compared with controls (p=.032). At the end of the experiment, carboxyl-terminal telopeptides of collagen type I levels were significantly lower in the OC-treated animals compared with controls (p=.046). Our results suggest that OC administration to skeletally immature female rats allows normal bone accrual and may even improve bone geometry. This effect may be mediated through enhanced inhibition of bone resorption.

First inventory of resorption lacunae on rods and plates of trabecular bone as observed by scanning electron microscopy

In the present study a novel systematic distribution scheme of resorption lacunae (RL) was applied using scanning electron microscopy. RL, classified as either reticulate patch resorption lacunae (RPR) or as longitudinally extended resorption lacunae (LER) [11, were analyzed and quantified according to their localizations on rods (middle, nodes or both) and plates (central or peripheral) in standardized segments from the femoral head of 24 Caucasian subjects without bone disease. Age and gender variations were explored. No clear gender-related distribution pattern could be detected on plates. On rods of males, however, the distribution of RL tended to be higher at the nodes, but seemed to be more prevalent in the middle or extended from the middle to the nodes of rods in females. Certain other non-conclusive tendencies in relation to age, gender, type of RL and localization were observed.

Estrogen receptor beta: the antimechanostat?

We have known for sometime that sex hormones influence the growth, preservation, and loss of bone tissue in the skeleton. However, we are only beginning to recognize how estrogen influences the responsiveness of the skeleton to exercise. Frost's mechanostat theory proposes that estrogen reduces the mechanical strain required to initiate an osteogenic response, but this may only occur at the endocortical and trabecular bone surfaces. The discovery of estrogen receptors alpha and beta may help us to understand the bone surface-specific effects of exercise. Findings from estrogen receptor knockout mice suggest that the activity of ERalpha may explain the positive interaction between estrogen and exercise on bone formation near marrow, that is, endocortical and trabecular bone surfaces. Estrogen inhibits the anabolic exercise response at the periosteal surface, and this we propose is due to the activation of ERbeta. Signaling through this receptor retards periosteal bone formation and suppresses gains in bone size and bone strength, and for these reasons it behaves as an antimechanostat.

Modulation of bone ingrowth of rabbit femur titanium implants by in vivo axial micromechanical loading

Titanium implants commonly used in orthopedics and dentistry integrate into host bone by a complex and coordinated process. Despite increasingly well illustrated molecular healing processes, mechanical modulation of implant bone ingrowth is poorly understood. The objective of the present study was to determine whether micromechanical forces applied axially to titanium implants modulate bone ingrowth surrounding intraosseous titanium implants. We hypothesized that small doses of micromechanical forces delivered daily to the bone-implant interface enhance implant bone ingrowth. Small titanium implants were placed transcortically in the lateral aspect of the proximal femur in 15 New Zealand White rabbits under general anesthesia and allowed to integrate with the surrounding bone for 6 wk. Micromechanical forces at 200 mN and 1 Hz were delivered axially to the right femur implants for 10 min/day over 12 consecutive days, whereas the left femur implants served as controls. The average bone volume 1 mm from mechanically loaded implants (n = 15) was 73 +/- 12%, which was significantly greater than the average bone volume (52 +/- 21%) of the contralateral controls (n = 15) (P < 0.01). The average number of osteoblast-like cells per endocortical bone surface was 55 +/- 8 cells/mm(2) for mechanically loaded implants, which was significantly greater than the contralateral controls (35 +/- 6 cells/mm(2)) (P < 0.01). Dynamic histomorphometry showed a significant increase in mineral apposition rate and bone-formation rate of mechanically stressed implants (3.8 +/- 1.2 microm/day and 2.4 +/- 1.0 microm(3).microm(-2).day(-1), respectively) than contralateral controls (2.2 +/- 0.92 microm/day and 1.2 +/- 0.60 microm(3).microm(-2).day(-1), respectively; P < 0.01). Collectively, these data suggest that micromechanical forces delivered axially on intraosseous titanium implants may have anabolic effects on implant bone ingrowth.

Osteocyte lacuna size and shape in women with and without osteoporotic fracture

Osteocytes have been hypothesized to control the amount and location of bone tissue which is resorbed or formed, based on the strain magnitude they perceive, and therefore may play a role in the bone loss of osteoporosis. The shape of osteocyte lacunae influences the mechanical strain applied to the osteocyte; thus, it is important to quantify their shape to further understand the mechanical environment of this cell. Previous studies of the size and shape of lacunae have been contradictory and limited to two-dimensional measurements on iliac crest biopsies. This investigation measured the size and shape of osteocyte lacunae in trabecular bone near a typical fracture site from three-dimensional image sets obtained by confocal microscopy. Bone tissue specimens were obtained from individuals undergoing hip replacement subsequent to fracture, and matched cadaveric specimens without fracture. After extensive image processing to differentiate the lacunae from the matrix, the volume and anisotropy of the lacuna were determined. No significant difference was found in the size (volume) or shape (anisotropy) of the lacunae between women with and without osteoporotic fracture, although there was a large range of sizes and shapes in both groups. These results suggest that the size or shape of the lacunae, which influences the strain in osteocytes, does not play a role in osteoporotic fracture. In addition, this study provides geometric measures of lacunae that are important in computational modeling of the mechanical environment of osteocytes.

Femoral bone structural geometry adapts to mechanical loading and is influenced by sex steroids: the Penn State Young Women's Health Study

We used 10 years of longitudinal data from Penn State Young Women's Health Study to explore predictors of adult bone structural geometry and strength. One hundred twelve participants were enrolled in the study at age 12. We report findings on the 76 participants who remained in the study for 10 years. Measurements were recorded biannually for the first 4 years and annually thereafter. Proximal femur DXA scans (Hologic QDR 2000) were taken from 17-22 years and analyzed using a hip structure analysis program to assess areal bone mineral density (BMD, g/cm2), subperiosteal width, cortical thickness, bone cross-sectional area (CSA), and section modulus (Z) at the narrow neck and femoral shaft. Total body lean mass (g) was measured with DXA total body scans. Nutrition, anthropometry, and sex steroids [testosterone (T) and estradiol (E2)] were measured from ages 12-22 years. Multiple regression models were used to assess predictors of change in bone variables (17-22 years) and absolute bone values (average of age 21 and 22 years, n = 79). Neck Z (+3.1%) and width (+1.3%), but not BMD (-0.8%), increased significantly from age 17 to 22 years. At the shaft, all variables increased (+1.0-4.0%, P < 0.01). After controlling for baseline (age 17) height, weight and bone measurement, weight change (neck) or lean mass (shaft), and age of menarche were the primary predictors of change in bone strength. After controlling for height and weight, only lean mass predicted absolute young adult Z at both the neck (r2 = 0.48, P < 0.01) and the shaft (r2 = 0.67, P < 0.01). When lean mass was removed from the model, sports exercise score replaced lean mass as a predictor of Z at both neck (r2 = 0.40, P < 0.01) and shaft (r2 = 0.60, P < 0.01) sites. For neck and shaft cortical thickness and BMD, both estradiol and sports score/lean mass were positive predictors (r2 = 0.15-0.40, P < 0.01). For neck bone width, testosterone levels (negative) and lean mass (positive) were significant (r2 = 0.48). Results were similar for each geometric variable at the shaft site. These data suggest that bone adapts its bending strength primarily to mechanical loading (represented by lean mass and sports exercise score) and that sex steroids are associated with bone geometric structure.

The influence of an insulin-like growth factor I gene promoter polymorphism on hip bone geometry and the risk of nonvertebral fracture in the elderly: the Rotterdam Study

The absence of the wildtype allele of a promoter polymorphism of the IGF-I gene is associated with increased risk (1.5; 95% CI, 1.1-2.0) of fragility fracture in women (n = 4212) but not in men (n = 2799). An approximation of hip bone geometry (from DXA) suggested the polymorphism is associated with bone strength and stability in gender-specific ways.

INTRODUCTION

Previously, we found a CA-repeat promoter polymorphism in the insulin-like growth factor I (IGF-I) gene associated with IGF-I levels and BMD in postmenopausal women, but the relationship with fractures is unclear. In this large population-based study of elderly men and women, we examined the association between this IGF-I promoter polymorphism with parameters of bone geometry and the occurrence of fractures.

MATERIAL AND METHODS

Within the Rotterdam Study, a prospective population-based cohort, the IGF-I polymorphism was analyzed in relation to incident nonvertebral fractures in 2799 men and 4212 women followed on average for 8.6 years. Furthermore, we estimated structural parameters of hip bone geometry indirectly from DXA outputs of the femoral neck in 2372 men and 3114 women. We studied neck width, cortical thickness, and the cortical buckling ratio and the section modulus as indexes of bone stability and bending strength.

RESULTS

Women heterozygotes and noncarriers of the allele had, respectively, 1.2 (95% CI, 1.0-1.5) and 1.5 (95% CI, 1.1-2.0) increased risk of having a fragility fracture at older age compared with homozygotes for the 192-bp allele (p trend = 0.0007). In men, fracture risk was not influenced by the polymorphism. Compared with homozygotes for the 192-bp allele, noncarrier males had approximately 1% narrower femoral necks and 2.2% lower section moduli (p trend < 0.05). Noncarrier females had 1.7% thinner cortices and 1.6% higher buckling ratios (p trend < 0.05) but no significant differences in femoral neck widths and section moduli. In women with low body mass index, genotype differences in bone strength (section modulus) and fracture risk were accentuated (p interaction = 0.05). The genotype-dependent differences in hip bone geometry did not fully explain the genotype-dependent differences in fracture risk.

CONCLUSIONS

The CA-repeat promoter polymorphism in the IGF-I gene is associated with the risk for fragility fracture at old age in women and with bone structure in both genders.

Bone's mechanostat: a 2003 update

The still-evolving mechanostat hypothesis for bones inserts tissue-level realities into the former knowledge gap between bone's organ-level and cell-level realities. It concerns load-bearing bones in postnatal free-living bony vertebrates, physiologic bone loading, and how bones adapt their strength to the mechanical loads on them. Voluntary mechanical usage determines most of the postnatal strength of healthy bones in ways that minimize nontraumatic fractures and create a bone-strength safety factor. The mechanostat hypothesis predicts 32 things that occur, including the gross anatomical bone abnormalities in osteogenesis imperfecta; it distinguishes postnatal situations from baseline conditions at birth; it distinguishes bones that carry typical voluntary loads from bones that have other chief functions; and it distinguishes traumatic from nontraumatic fractures. It provides functional definitions of mechanical bone competence, bone quality, osteopenias, and osteoporoses. It includes permissive hormonal and other effects on bones, a marrow mediator mechanism, some limitations of clinical densitometry, a cause of bone "mass" plateaus during treatment, an "adaptational lag" in some children, and some vibration effects on bones. The mechanostat hypothesis may have analogs in nonosseous skeletal organs as well.

Mechanical loading influences bone mass through estrogen receptor alpha

Mechanical loading influences bone mass and architecture through a cascade of cellular events that involve estrogen receptor alpha (ERalpha). An implication of this is that bone architecture is more adaptive to mechanical loading when the estrogen receptor number is high, as during adolescence, and less sensitive when the estrogen receptor number is low, as occurs postmenopausally, during amenorrhea, or after ovariectomy.

Altered tissue properties induce changes in cancellous bone architecture in aging and diseases

The mechanical properties of cancellous bone depend on its architecture and the tissue properties of the mineralized matrix. The architecture is continuously adapted to external loads. In this paper, it was assumed that changes in tissue properties leading to changes in tissue deformation can induce adaptation of the architecture. We asked whether changes in cancellous bone architecture with aging and in e.g. early osteoarthrosis can be explained from changes in tissue properties. This was investigated using computer models in which the cancellous architecture was adapted to external loads. Bone tissue with deformations below a certain threshold was resorbed, deformations above another threshold induced formation. Deformations between these two boundaries, in the 'lazy zone', did not induce bone adaptation. The effects of changes in bone tissue stiffness on bone mass, global stiffness and architecture were investigated. The bone gain (40-60%) resulting from a 50% decrease in tissue stiffness (simulating diseased tissue) was much larger than the bone loss (2-30%) resulting from a 50% increase in tissue stiffness (simulating highly mineralized, old tissue). The adaptation induced by a decrease in tissue stiffness resulted in an almost constant stiffness in the main load bearing direction, but the transversal stiffness decreased. An increased tissue stiffness resulted in a higher stiffness in the main direction and overcompensation in the transversal directions: the global stiffness could become even smaller than the stiffness of the original model. Concluding, we showed that changes in trabecular bone in aging and diseases can be partly explained from changes in tissue properties.

Osteoblast-like cells from estrogen receptor alpha knockout mice have deficient responses to mechanical strain

INTRODUCTION

In vivo, bones' osteogenic response to mechanical loading involves proliferation of surface osteoblasts. This response is replicated in vitro and involves ERK-mediated activation of the estrogen receptor (ER) alpha and upregulation of estrogen response element activity. This proliferative response can be blocked by selective estrogen receptor modulators and increased by transfection of additional ERalpha.

MATERIALS AND METHODS

We have now investigated the mechanisms of ER involvement in osteoblast-like cells' early responses to strain by comparing the responses of primary cultures of these cells derived from homozygous ERalpha knockout (ERKO) mice (ERalpha-/-) with those from their wildtype (ERalpha+/+) and heterozygous (ERalpha+/-) littermates and from ER/beta knockout (BERKO) mice (ERbeta+/+, ERbeta+/-, and ERbeta-/-).

RESULTS

Whereas ERalpha+/+, ERalpha+/-, ERbeta+/+, and ERbeta-/- cells proliferate in response to a single 10-minute period of cyclic strain, ERalpha-/- cells do not. Transfection of fully functional, but not mutant, ERalpha rescues the proliferative response to strain in these cells. The strain-related response of ERalpha-/- cells is also deficient in that they show no increased activity of an AP-I driven reporter vector and no strain-related increases in NO production. Their strain-related increase in prostacyclin production is retained. They proliferate in response to fibroblast growth factor-2 but not insulin-like growth factor (IGF)-I or IGF-II, showing the importance of ERalpha in the IGF axis and the ability of ERalpha-/- cells to proliferate normally in response to a mitogenic stimulus that does not require functional ERalpha.

CONCLUSIONS

These data indicate ERalpha's obligatory involvement in a number of early responses to mechanical strain in osteoblast-like cells, including those that result in proliferation. They support the hypothesis that reduction in ERalpha expression or activity after estrogen withdrawal results in a less osteogenic response to loading. This could be important in the etiology of postmenopausal osteoporosis.

Emerging neuroskeletal signalling pathways: a review

Recent work has demonstrated that neurotransmitters, signalling molecules primarily associated with the nervous system, can have profound effects on the skeleton. Bone cells express a broad range of neurotransmitter receptors and transporters, and respond to receptor activation by initiating diverse intracellular signalling pathways, which modulate cellular function. Evidence of neuronal innervation in skeletal tissues, neurotransmitter release directly from bone cells and functional effects of pharmacological manipulation support the existence of a complex and functionally significant neurotransmitter-mediated signalling network in bone. This review aims to concisely summarise our current understanding of how neurotransmitters affect the skeletal system, focusing on their origin, cellular targets and functional effects in bone.

At the crossroads of skeletal responses to estrogen and exercise

The recent observation that mice deficient in estrogen receptor alpha (ERalpha) have an impaired response to mechanical strain suggests that ERalpha plays an important role in mediating the response of the skeleton to mechanical loading as well as to estrogen. In view of previous findings that estrogen deficiency leads to a fall in ERalpha numbers, postmenopausal bone loss might result from the impaired response of bone to mechanical strain caused by deficient ERalpha signalling.

Hip section modulus, a measure of bending resistance, is more strongly related to reported physical activity than BMD

We hypothesized that measures of physical activity would have a closer relationship with section modulus (SM), an indicator of bending resistance, than with bone mineral density (BMD) because physical activity might expand the bony envelope, which tends to reduce BMD for a constant bone mineral content. Four hundred twenty-three men and 436 women (mean age 72 years, SD =3) were recruited from a prospective population-based cohort study to a study of hip bone loss. Hip BMD was measured on two occasions 2-5 years apart (mean 2.7, DXA-Hologic 1,000 W). Hip structural analysis (HSA) software was used to calculate SM and BMD from the DXA scans on three narrow regions: the narrow neck (NN), intertrochanter (IT) and shaft (S). A physical activity and lifestyle questionnaire was administered at baseline. Multivariate repeated measures analysis of variance was used to model the associations between personal attributes (weight, height, age), physical activity and lifestyle variables with SM, cross-sectional area (CSA), sub-periosteal diameter (PD) and BMD. Men and women were analysed together after tests for interactions with gender, which were found not to be significant. In all regions female gender was associated with having lower values of all outcomes, and body weight was positively associated with all outcomes, i.e., SM, CSA, PD and BMD ( P<0.0001). Sub-periosteal diameter was positively associated with reported lifetime physical activity (IT and S, P<0.0001). There was a significant decline of BMD with age at the NN and S regions ( P<0.026), and the PD increased with age (NN and S, P<0.019). Previous fracture history was associated with having lower values of BMD, SM and CSA (except for S; P<0.022). Both section modulus and CSA were positively associated with heavy physical activity after age 50 years in all regions ( P<0.019), whereas NN BMD was the only BMD associate of heavy physical activity after 50 ( P=0.036). Time spent per week on recreational activities classified as no impact activity was positively associated with BMD, CSA and SM (multivariate P<0.016). In conclusion, proximal femur diameter is associated positively with reported life-long physical activity. If this is mediated through a loading related effect on sub-periosteal expansion, BMD would be an unsatisfactory outcome measure in physical activity studies since it is inversely related to projected bone area. SM in contrast was associated with several measures of recent physical activity and relates more directly to the bending experienced by the proximal femur in response to a given load. These data are consistent with an effect of mechanical loading to regulate bone strength through an anabolic effect maximal in the subperiosteal cortex, where the highest loading-related strains are experienced.

Estrogen and bone--a reproductive and locomotive perspective

The primary function of the skeleton is locomotion, and the primary function of estrogen is reproduction. When the skeleton is considered within this locomotive context, the onset of estrogen secretion at puberty leads to packing of mechanically excess mineral into female bones for reproductive needs. Accordingly, the unpacking of this reproductive safety deposit at menopause denotes the origin of type I osteoporosis.

INTRODUCTION

According to the prevailing unitary model of involutional osteoporosis, female postmenopausal bone loss can be described as having an initial accelerated, transient phase (type I), followed by a gradual continuous phase (type II). Estrogen withdrawal is generally accepted as the primary cause of the type I osteoporosis. Thus, the quest to uncover the origin of type I osteoporosis has focused on the estrogen withdrawal-related skeletal changes at and around the menopause. However, considering that the cyclical secretion of estrogen normally begins in early adolescence and continues over the entire fertile period, one could argue that focusing on perimenopause alone may be too narrow.

MATERIALS AND METHODS

This is not a systematic review of the literature on the skeletal function of estrogen(s), but rather, an introduction of a novel structure- and locomotion-oriented perspective to this particular issue through pertinent experimental and clinical studies.

RESULTS AND CONCLUSIONS

When considering locomotion as the primary function of the skeleton and integrating the classic findings of the pubertal effects of estrogen on female bones and the more recent hypothesis-driven experimental and clinical studies on estrogen and mechanical loading on bone within this context, a novel evolution-based explanation for the role of estrogen in controlling female bone mass can be outlined: the onset of estrogen secretion at puberty induces packing of mechanically excess bone into female skeleton for needs of reproduction (pregnancy and lactation). Accordingly, the unpacking of this reproductive safety deposit of calcium at menopause denotes the accelerated phase of bone loss and thus the origin of type I osteoporosis.

Reduced iliac cancellous osteocyte density in patients with osteoporotic vertebral fracture

Iliac cancellous osteocyte density declines with age, but its relationship to vertebral fracture pathogenesis is unknown. We performed iliac bone biopsy in 44 women with clinical vertebral fracture and 56 healthy women. The fracture patients had 34% fewer osteocytes but no reduction in percent occupied lacunae. Some patients destined to sustain vertebral fracture make cancellous bone with fewer osteocytes.

INTRODUCTION

Patient's with vertebral fracture have less bone than appropriate healthy controls, but other factors may contribute to bone fragility. Iliac cancellous osteocyte density declines with age in healthy women; we asked whether this variable differed between fracture patients and healthy controls.

METHODS

Two groups of women were assembled. Forty-four (mean age, 66.2 years) had unequivocal evidence of bone fragility manifested as painful nontraumatic vertebral fracture, and 56 (mean age, 62.2 years) were skeletally healthy. All subjects underwent iliac bone biopsy. From archival embedded biopsy cores, new sections were stained with Goldner's trichrome, in which we enumerated osteocyte-occupied lacunae (stained), empty lacunae (unstained), and total lacunae per bone area.

RESULTS

Cancellous osteocyte density was 34% lower in the fracture group than in the controls (p < 0.001); this difference was not a consequence of higher turnover, having less bone, or the small difference in age. The area under the receiver operating characteristic (ROC) curve for discrimination between the groups was >90% for osteocyte density and <75% for bone volume/tissue volume (BV/TV). The disease-related osteocyte deficit was accompanied by a proportionate reduction in empty lacunae and no change in percent occupied lacunae; therefore, it was not the result of premature death. Both superficial bone (<25 microm from the surface) and deep bone (>45 microm from the surface) were affected. In contrast, the age-related deficit is accompanied by an increase in empty lacunae and fall in percent osteocyte-occupied lacunae and occurs only in deep bone, but not in superficial bone.

CONCLUSIONS

In some patients destined to sustain spontaneous vertebral compression fracture, iliac cancellous bone is made with fewer osteocytes than normal; the mechanism of osteocyte incorporation into bone needs more detailed study. Osteocyte deficiency could contribute to bone fragility, either by impairing the detection of fatigue microdamage or by reducing canalicular fluid flow. Current practices of defining vertebral fracture based on morphometry alone regardless of symptoms, and diagnosing osteoporosis based on bone densitometry alone regardless of fracture history, should be reexamined.

Long-term potentiation in bone--a role for glutamate in strain-induced cellular memory?

BACKGROUND

The adaptive response of bone cells to mechanical strain is a primary determinant of skeletal architecture and bone mass. In vivo mechanical loading induces new bone formation and increases bone mineral density whereas disuse, immobilisation and weightlessness induce bone loss. The potency of mechanical strain is such that a single brief period of loading at physiological strain magnitude is able to induce a long-lasting osteogenic response that lasts for days. Although the process of mechanotransduction in bone is incompletely understood, observations that responses to mechanical strain outlast the duration of stimulation necessitate the existence of a form of cellular memory through which transient strain episodes are recorded, interpreted and remembered by bone cells. Recent evidence supports the existence of a complex multicellular glutamate-signalling network in bone that shares functional similarities to glutamatergic neurotransmission in the central nervous system. In neurones, these signalling molecules coordinate synaptic communication required to support learning and memory formation, through a complex process of long-term potentiation.

PRESENTATION OF THE HYPOTHESIS

We hypothesise that osteoblasts use a cellular mechanism similar or identical to neuronal long-term potentiation in the central nervous system to mediate long-lasting changes in osteogenesis following brief periods of mechanical strain.

TESTING THE HYPOTHESIS

N-methyl-D-aspartate (NMDA) receptor antagonism should inhibit the saturating response of mechanical strain and reduce the enhanced osteogenicity of segregated loading to that of an equivalent period of uninterrupted loading. Changes in alpha-amino-3-hydroxy-5-methyl-isoxazole propionate (AMPA) receptor expression, localisation and electrophysiological responses should be induced by mechanical strain and inhibited by modulators of neuronal long-term potentiation.

IMPLICATIONS OF THE HYPOTHESIS

If true, this hypothesis would provide a mechanism through which the skeleton could be pharmacologically primed to enhance or retrieve the normal osteogenic response to exercise. This would form a basis through which novel therapies could be developed to target osteoporosis and other prevalent bone disorders associated with low bone mass.

Estrogen deposits extra mineral into bones of female rats in puberty, but simultaneously seems to suppress the responsiveness of female skeleton to mechanical loading

To first test the possible effect of gender on the responsiveness of growing rat skeleton to mechanical loading, 5-week-old littermates of 25 male and 25 female rats were subjected to either free-cage activity or treadmill training for a period of 14 weeks (experiment 1). Using peripheral quantitative computed tomography (pQCT) and mechanical testing of the femoral neck, we observed female rats exhibiting a clearly lower responsiveness to external loading than male rats (+3.0% vs +25% in cross-sectional area (CSA), +4.2% vs +27% in the bone mineral content (BMC), -0.6% vs +10% in volumetric bone mineral density (BMD), and +4.7% vs +28% in fracture strength (F(max)) of the femoral neck). Also, relative to the mechanical demands placed on the skeleton, the bones of the young female rats were considerably denser (>50%) than those of the males. In the subsequent experiment 2, we repeated the above-noted first experiment with 33-week-old rats and observed virtually identical exercise-induced benefits (+2.1% vs +10% in CSA, +3.4% vs +18% in BMC, +2.5% vs +23% in BMD, and -1.1% vs +27% in F(max) in females vs males, respectively) and the growth/puberty-related condensation of mineral into female bones. Finally, in experiment 3, 60 littermates of 3-week-old female rats were first subjected to sham operation or ovariectomy and then further randomized to exercise or control groups, respectively, to study whether the condensation of mineral into female bones and their lower responsiveness to loading were attributable to the effects of estrogen. At the end of the 16-week intervention, our pQCT and mechanical testing analysis showed not only the anticipated effect of reduced bone density in the ovariectomized rats ( approximately -20%) but also the hypothesized better responsiveness to mechanical loading in these estrogen-depleted rats (-3.5% vs +9.1% in CSA, -0.4% vs +12% in BMC, +4.4% vs +9.6% in BMD, and -4.2% vs +16% in F(max) in SHAM vs OVX, respectively). In conclusion, the results of our series of three experiments suggest that as such estrogen seems to have very little primary effect on the sensitivity of female bone to respond to external loading, but rather deposits extra stock of mineral into female bones in puberty. This estrogen-driven extra condensation of the female skeleton seems to persist into adulthood, simultaneously damping the responsiveness of the female skeleton to mechanical loading.

Effects of gender, anthropometric variables, and aging on the evolution of hip strength in men and women aged over 65

Although gender differences in fall rates may partly explain the higher prevalence of fractures in elderly women than men, male bones may also be intrinsically stronger or suffer less structural degradation with age than those of women. We used hip structural analysis (HSA) to study gender differences in hip geometry and bone mineral density (BMD) as they evolved over time in elderly white men and women with the aim of identifying candidate biological pathways leading to heightened risk of hip fracture. We recruited 443 women and 439 men aged 67-79 years from a diet and cancer prospective population-based cohort study to a study of hip bone loss. Hip BMD was measured on two occasions 2-5 years apart by dual-energy X-ray absorptiometry and HSA software used to derive BMD and structural parameters at the narrow neck (NN), the intertrochanter (IT), and the shaft (S) regions. Structural indices calculated in each region were cross-sectional area (CSA)-amount of bone surface area in the cross section after excluding soft tissue space; section modulus (Z)-an index of bending resistance, subperiosteal width, endocortical width, cortical thickness; and cortical buckling ratio (CBR)-a measure of cortical instability. Compared to men, women had lower values of BMD, CSA, Z, subperiosteal width, endocortical width, and cortical thickness in all regions, except S endocortical width, after adjusting for weight, height, and age (P < 0.0001). CBR was higher in women than in men (P < 0.0001) in all regions. Longitudinal analysis of rates of change revealed faster rates of BMD decline in women than in men at the Hologic total hip, Hologic femoral neck, and IT regions (P < 0.029). Women had faster rates of subperiosteal and endosteal expansion than men at the NN (P < 0.011) and IT (P < 0.049) and faster increase in Z at the NN (P = 0.029). At the IT region, cortical thinning was faster in women than in men (P = 0.037) and CBR increased at a faster rate in women (P = 0.011). In conclusion, Z is lower in women than in men and expansion of the proximal femur occurs in both sexes, being faster in women than in men. Z does not decline at the same rate as BMD, implying that part of the effect of aging on BMD is due to expansion of the bony envelope without loss of bone mineral content. Faster expansion in the female femoral neck may in turn lead to greater fragility if wider diameter and thinner cortices become locally unstable.

Pathogenesis of osteoporosis

There are many pathways that might lead to decreased bone mass, skeletal fragility, and increased fracture risk in osteoporosis. Some of these have been clearly identified, such as estrogen deficiency. Others that were conceived on the basis of experimental findings and recent scientific discoveries such as abnormalities of cytokines, bone growth factors, and osteoblast transcription factors remain interesting but speculative. The recent revolution in genomics and proteomics opens new avenues for pursuing in great depth the pathways leading to osteoporosis. Animal models developed largely in rodents can suggest specific factors that can be further studied in primate models and in osteoporotic patients. Identification of specific pathogenetic mechanisms should lead to new approaches to the diagnosis and management of this disorder.

The effect of in vivo mechanical loading on estrogen receptor alpha expression in rat ulnar osteocytes

The presence of estrogen receptor alpha (ER alpha) in osteocytes was identified immunocytochemically in transverse sections from 560 to 860 microm distal to the midshaft of normal neonatal and adult male and female rat ulnas (n = 3 of each) and from adult male rat ulnas that had been exposed to 10 days of in vivo daily 10-minute periods of cyclic loading producing peak strains of either -3000 (n = 3) or -4000 microstrain (n = 5). Each animal ambulated normally between loading periods, and its contralateral ulna was used as a control. In animals in which limbs were subject to normal locomotor loading alone, 14 +/- 1.2% SEM of all osteocytes in each bone section were ER alpha positive. There was no influence of either gender (p = 0.725) or age (p = 0.577) and no interaction between them (p = 0.658). In bones in which normal locomotion was supplemented by short periods of artificial loading, fewer osteocytes expressed ER alpha (7.5 +/- 0.91% SEM) than in contralateral control limbs, which received locomotor loading alone (14 +/- 1.68% SEM; p = 0.01; median difference, 6.43; 95% CI, 2.60, 10.25). The distribution of osteocytes expressing ER alpha was uniform across all sections and thus did not reflect local peak strain magnitude. This suggests that osteocytes respond to strain as a population, rather than as individual strain-responsive cells. These data are consistent with the hypothesis that ER alpha is involved in bone cells' responses to mechanical strain. High strains appear to decrease ER alpha expression. In osteoporotic bone, the high strains assumed to accompany postmenopausal bone loss may reduce ER alpha levels and therefore impair the capacity for appropriate adaptive remodeling.

Sex steroids and the construction and conservation of the adult skeleton

Here we review and extend a new unitary model for the pathophysiology of involutional osteoporosis that identifies estrogen (E) as the key hormone for maintaining bone mass and E deficiency as the major cause of age-related bone loss in both sexes. Also, both E and testosterone (T) are key regulators of skeletal growth and maturation, and E, together with GH and IGF-I, initiate a 3- to 4-yr pubertal growth spurt that doubles skeletal mass. Although E is required for the attainment of maximal peak bone mass in both sexes, the additional action of T on stimulating periosteal apposition accounts for the larger size and thicker cortices of the adult male skeleton. Aging women undergo two phases of bone loss, whereas aging men undergo only one. In women, the menopause initiates an accelerated phase of predominantly cancellous bone loss that declines rapidly over 4-8 yr to become asymptotic with a subsequent slow phase that continues indefinitely. The accelerated phase results from the loss of the direct restraining effects of E on bone turnover, an action mediated by E receptors in both osteoblasts and osteoclasts. In the ensuing slow phase, the rate of cancellous bone loss is reduced, but the rate of cortical bone loss is unchanged or increased. This phase is mediated largely by secondary hyperparathyroidism that results from the loss of E actions on extraskeletal calcium metabolism. The resultant external calcium losses increase the level of dietary calcium intake that is required to maintain bone balance. Impaired osteoblast function due to E deficiency, aging, or both also contributes to the slow phase of bone loss. Although both serum bioavailable (Bio) E and Bio T decline in aging men, Bio E is the major predictor of their bone loss. Thus, both sex steroids are important for developing peak bone mass, but E deficiency is the major determinant of age-related bone loss in both sexes.