Cancellous bone of the spine is greater in black women

Differences in osteocyte and lacunar density between Black and White American women

We examined the differences in osteocyte and lacunar density between Black and White women, using previously obtained iliac bone biopsies from 34 healthy Black women, aged 21-70 years, and 94 White women, aged 20-73 years. For each subject, the density of osteocytes (Ot.N/B.Ar), empty lacunae (EL.N/B.Ar), and total lacunae (Tt.L.N/B.Ar) and the proportion of osteocyte-occupied lacunae (Ot.N/Tt.L.N) were separately measured in whole trabeculae, superficial bone (<25 microm from the bone surface), and deep bone (>45 microm from the bone surface). Compared with White women, Black women had higher values for osteocytes, empty lacunae, and total lacunae and lower values for percent occupied lacunae in superficial bone and whole trabeculae (P < 0.01 to <0.001). In deep bone there were more osteocytes and total lacunae in Black women, but the other measurements did not differ significantly between the two groups. As in White women, there were fewer osteocytes and total lacunae and more empty lacunae in deep than in superficial bone. The regressions of osteocyte and total lacunar density on age were not significant in Black women, but postmenopausal Black women had fewer osteocytes than premenopausal Black women, and percent occupied lacunae declined significantly with age in whole trabeculae and deep bone, which could only have resulted from osteocyte death. In contrast to White women, there was no inverse relationship between bone formation rate and osteocyte density in superficial bone and the observed bone formation rate was lower than predicted by osteocyte density. We conclude the following: (1) Cancellous bone is made with more osteocytes in Black than in White women, most likely because of diminished apoptosis of osteoblasts; this could contribute to increased bone strength in Black women. (2) In Black women, as in White women, there are fewer osteocytes and total lacunae and more empty lacunae in deep than in superficial bone. (3) There was moderate age-related loss of osteocytes in deep bone in Black women, indicating that osteocyte density depends more on the age of the bone than on the age of the subject. (4) The higher osteocyte density in Black women was not responsible for their lower bone formation rate.

Ultrasound of the calcaneus and bone mineral density differs in older black and white women but is not impacted by current physical activity

Black women have lower fracture rates, primarily due to bone-dependent factors; however, the racial differences in quantitative ultrasound (QUS) for older black and white women are poorly characterized, especially in regard to the mediating effects of physical activity. The primary aim of this study was to determine if QUS measures differ in age- and body-size-matched older black and white women and to explore if the racial differences were mediated by differences in current habitual physical activity. We performed dual-energy X-ray absorptiometry (DXA) bone mineral density (BMD) measures and QUS measures [broadband ultrasound attenuation (BUA), speed of sound (SOS), quantitative ultrasound index (QUI)] of the calcaneus in 30 black and 30 white women matched in age (68.3 years) and body mass index (30.0 kg/m2). Black women had greater QUS measures of SOS (1,563.6+/-31.7 vs 1,541.2+/-23.4, p=0.003) and QUI (102.6+/-18.5 vs 90.4+/-15.4, p=0.008), with a strong trend for greater BUA (79.2+/-15.1 vs 71.9+/-15.1 m/sec, p=0.066) compared with white women. As expected, black women had greater BMD measures at all sites; whole body (6.8%, p<0.010), lumbar spine (10.4%, p=0.008), proximal femur (14.2%; p<0.001) and femoral neck (20.3%; p<0.001), compared with white women. Although white women reported expending 45% more energy (p=0.03) in moderate intensity physical activity than black women on a weekly basis, current physical activity did not attenuate the difference in QUS measures between the races. The relations between QUS and BMD measures were similar in black compared with white women. After controlling for BMD, the racial differences in QUS measures were not apparent, precluding the conclusion that bone quality independently contributes to the reduction in fracture risk in older black women. The impact of current physical activity on QUS measures in older black and white women is negligible.

Parathyroid hormone and rates of bone formation are raised in perimenopausal rural Gambian women

To investigate rates of bone turnover and calcium homeostasis in Gambian women, we recruited 103 peri- and postmenopausal women, aged 45 to 80+ years and 11 women of reproductive age. Fasting blood was analyzed for plasma osteocalcin, PTH, 25-hydroxyvitamin D [25(OH)D], 1,25-dihydroxyvitamin D [1,25(OH)(2)D], total- and bone-specific alkaline phosphatase. Plasma and urinary calcium, inorganic phosphate, sodium, potassium, creatinine, and albumin and urine free deoxypyridinoline (Dpd) was also measured. Samples from 20 premenopausal and 31 postmenopausal women from Cambridge, UK were analyzed, using the same methodology for comparison. For the Gambian women, peak calcium excretion occurred at around 50 years of age. For women aged > or =45 years, calcium excretion decreased by 3.0% per year of age (SE 1%; P < 0.005). In this age group, 25(OH)D also decreased with age (P < 0.005). Urinary sodium output, pH, and titratable acid output decreased (all P < 0.05) and total alkaline phosphatase (P < 0.005), osteocalcin (P < 0.005), and PTH (P < 0.05) increased with age. Comparisons were made between the following groups of Gambian and British women: premenopausal, early (age 55-64 years)- and late (age 65+ years)-postmenopausal. Gambian women of all ages were lighter (P < 0.001), shorter (P < 0.01), and had higher plasma bone-specific alkaline phosphatase activity (P < 0.05) and higher concentrations of osteocalcin (P < 0.05), PTH (P < 0.001), 1,25(OH)(2)D (P < 0.001), and 25(OH)D (P < 0.001). There were no consistent differences in calcitonin, while urinary free Dpd outputs were lower in the Gambians (P < 0.001). Plasma calcium, phosphate, and albumin (P < 0.01) were significantly lower. Urinary calcium, phosphate, sodium, and potassium excretion were lower, particularly in the postmenopausal group (P < 0.001). Although Gambian urine pH was more acidic, titratable acid output was lower (P < 0.01). These data show that Gambian women with low dietary calcium intakes and good vitamin D status have low urinary calcium excretion and that menopausal changes in calcium and bone metabolism among Gambian women are similar to those seen in other populations. In addition, they demonstrate that Gambian women of all ages have raised plasma PTH and 1,25(OH)(2)D concentrations and raised markers of osteoblast activity. We postulate that high endogenous PTH concentrations may be beneficial to bone health in Gambian women, removing fatigue damage and improving bone quality.

Genetically based influences on the site-specific regulation of trabecular and cortical bone morphology

The degree of site-specificity by which genes influence bone quantity and architecture was investigated in the femur of three strains of mice. Morphological indices were highly dependent on both genetic makeup as well as anatomical location showing that the assessment of bone structure from a single site cannot be extrapolated to other sites even within a single bone.

INTRODUCTION

The identification of genes responsible for establishing peak BMD will yield critical information on the regulation of bone quantity and quality. Whereas such knowledge may eventually uncover novel molecular drug targets or enable the identification of individuals at risk of osteoporosis, the site-specificity by which putative genotypes cause low or high bone mass (and effective bone morphology) is essentially unknown.

MATERIALS AND METHODS

microCT was used to determine morphological and microarchitectural features of the femora harvested from three genetically distinct strains of 4-month-old female mice, each with distinct skeletal mass (low: C57BL/6J [B6], medium: BALB/cByJ [BALB], high: C3H/HeJ [C3H]). Two trabecular regions (distal epiphysis and metaphysis) were considered in addition to four cortical regions within the metaphysis and diaphysis.

RESULTS AND CONCLUSIONS

Comparing morphological properties of the different trabecular and cortical femoral regions between the three strains of mice, it was apparent that high or low values of specific parameters of bone morphology could not be consistently attributed to the same genetic strain. Trabecular metaphyseal bone volume, for instance, was 385% larger in C3H mice than in B6 mice, yet the two strains displayed similar bone volume fractions in the epiphysis. Similarly, BALB mice had 48% more trabecular bone than C3H mice in the epiphysis, but there were no strain-specific differences in cortical bone area at the diaphysis. These data suggest that the genetic control of bone mass and morphology, even within a given bone, is highly site-specific and that a comprehensive search for genes that are indicative of bone quantity and quality may also have to occur on a very site-specific basis.

Genetically linked site-specificity of disuse osteoporosis

The genetic influence on bone loss in response to mechanical unloading was investigated within diaphyseal and distal femoral regions in three genetically distinct strains of mice. One mouse strain failed to lose bone after removal of function, whereas osteopenia was evident in multiple regions of the remaining two strains but in different areas of the bone.

INTRODUCTION

It is well recognized that susceptibility to osteoporosis is, in large measure, determined by the genome, but whether this influence is systemic or site-specific is not yet known. Here, the extent to which genetic variations influence regional bone loss caused by disuse was studied in the femora of adult female mice from three inbred strains.

MATERIALS AND METHODS

Adult C57BL/6J (B6), C3H/HeJ (C3H), and BALB/cByJ (BALB) mice were subjected to 15-21 days of disuse, achieved by hindlimb suspension, and six distinct anatomical regions of the femur were analyzed by high-resolution microCT.

RESULTS AND CONCLUSIONS

In B6 mice, the amount of disuse stimulated bone loss was relatively uniform across all regions, with 20% loss of trabecular bone and 10% loss of cortical bone. The degree of bone loss in BALB mice varied greatly, ranging from 59% in the metaphysis to 3% in the proximal diaphysis. In this strain, the nonuniformity of bone loss was directly related to the nonuniform distribution of baseline bone morphology (r2 = 0.94). In direct contrast with BALB and B6, disuse failed to produce significant losses of bone in any of the analyzed regions of the C3H mice. Instead, these animals displayed a unique compensatory mechanism to disuse, where the large loss of calcified tissue from the endocortical surface (-24%) was compensated for by an expansion of the periosteal envelope (10%). These data indicate a strong, yet complex, genetic dependence of the site-specific regulation of bone remodeling in response to a powerful catabolic signal. Consequently, the skeletal region of interest and the genetic make-up of the individual may have to be considered interdependently when considering the pathogenesis of osteoporosis or the efficacy of an intervention to prevent or recover bone loss.

Genetic predisposition to low bone mass is paralleled by an enhanced sensitivity to signals anabolic to the skeleton

The structure of the adult skeleton is determined, in large part, by its genome. Whether genetic variations may influence the effectiveness of interventions to combat skeletal diseases remains unknown. The differential response of trabecular bone to an anabolic (low-level mechanical vibration) and a catabolic (disuse) mechanical stimulus were evaluated in three strains of adult mice. In low bone-mineral-density C57BL/6J mice, the low-level mechanical signal caused significantly larger bone formation rates (BFR) in the proximal tibia, but the removal of functional weight bearing did not significantly alter BFR. In mid-density BALB/cByJ mice, mechanical stimulation also increased BFR, whereas disuse significantly decreased BFR. In contrast, neither anabolic nor catabolic mechanical signals influenced any index of bone formation in high-density C3H/HeJ mice. Together, data from this study indicate that the sensitivity of trabecular tissue to both anabolic and catabolic stimuli is influenced by the genome. Extrapolated to humans, these results may explain in part why prophylaxes for low bone mass are not universally effective, yet also indicate that there may be a genotypic indication of people who are at reduced risk of suffering from bone loss.