Saroj Palnitkar

Effects of Ethnicity and Age or Menopause on the Remodeling and Turnover of Iliac Bone: Implications for Mechanisms of Bone Loss

We measured histologic indices of bone remodeling and turnover separately on the cancellous, endocortical, and intracortical subdivisions of the endosteal envelope, and on the combined total surface, in transiliac bone biopsies obtained after double tetracycline labeling in 142 healthy women, aged 20–74 years, 34 black and 108 white, 61 premenopausal and 81 postmenopausal. The data were analyzed by two‐way analysis of variance of the four groups defined by age/menopause and ethnicity and by linear regression of the major variables on age. None of the interaction terms was significant and none of the regression slopes on age differed between blacks and whites, indicating that, as for the previously reported structural indices, the effects of ethnicity and of age/menopause are independent. Accordingly, the data were also analyzed separately for the effect of ethnicity (pre‐ and postmenopausal combined) and age/menopause (blacks and whites combined). The analyses led to the following conclusions. (1) The geometric mean bone formation rate on the combined total surface was 25% lower in blacks than in whites; other histologic differences between ethnic groups were inconsistent between surfaces. (2) Serum osteocalcin (OC) but not bone‐specific alkaline phosphatase (BSAP) was lower by about 15% in blacks than in whites. (3) The lower bone turnover in blacks is most likely in the directed rather than in the stochastic component because of a higher bone mass and consequent reduced susceptibility to fatigue damage. (4) All Class 1 bone formation variables and the three resorption indices were significantly higher in the postmenopausal compared with the premenopausal subjects, reflecting a 33% increase in activation frequency. (5) BSAP, but not OC, was increased relatively more (66%) than the bone formation rate (BFR). Consequently, BSAP is more sensitive to the effects of menopause than OC, but OC is more sensitive to the effects of ethnicity than BSAP. (6) There were highly significant differences between the three subdivisions of the endosteal envelope for every non–cell‐related variable. All Class 1 formation variables were highest on the endocortical surface, but the magnitude and pattern of the differences otherwise was inconsistent between variables. The contributions of the different subdivisions to the total bone formation rate were cancellous 54%, endocortical 13%, and intracortical 33%. (7) The previously reported changes in bone surface location, together with the presently reported changes in activation frequency and wall thickness indicated that there was no significant effect of age/menopause on erosion depth on the cancellous and intracortical surfaces but a large increase in erosion depth on the endocortical surface. (8) The increase in bone turnover that results from hormonal changes is most likely in the stochastic rather than in the directed component because it serves no purpose but has harmful effects on skeletal integrity.

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.

Age and distance from the surface but not menopause reduce osteocyte density in human cancellous bone

Previous studies of osteocyte density in human cancellous bone have relied mainly on autopsy samples and have demonstrated an age-related decline in men, but there are insufficient data in women. Using previously obtained transiliac bone biopsies from 94 healthy white women, aged 20-73 years, 38 premenopausal and 56 postmenopausal, we measured osteocytes and lacunae in ten randomly selected areas using 5-microm-thick sections stained with Goldner trichrome. For each subject, the number of osteocytes (Ot.N/B.Ar), empty lacunae (EL.N/B.Ar), and total lacunae (Tt.L.N/B.Ar) per bone area, and the proportion of occupied lacunae (Ot.N/Tt.L.N), were calculated. In 92 cases the measurements were made separately in superficial bone (<25 microm from the surface) and in deep bone (>45 microm from the surface). Mean values and differences between extreme values (DEV) for each variable were computed from the ten measured areas. In addition, confocal microscopic examination was performed on 100 microm sections. We found that Ot.N/B.Ar, Tt.L.N/B.Ar, and Ot.N/Tt.L.N decreased, but EL.N/B.Ar increased significantly with age (p < 0.001). The rates of decline were most rapid initially, falling exponentially with increasing age; the linear regressions for all four variables were significant in premenopausal, but not postmenopausal, women. At all ages, there were significantly more osteocytes in superficial than in deep bone; there was no significant decline with age in superficial bone, but a steeper exponential decline in deep bone than in whole trabeculae. DEV did not change with age for any variable. Confocal images revealed that the morphology of the osteocyte network was heterogeneous in different regions and trabeculae. The trabeculae with lower osteocyte density contained acellular areas, especially in interstitial bone. We conclude: (1) osteocyte density declines with age in women as it does in men; (2) the decline occurs exclusively in deep bone, not in superficial bone, suggesting that it is the age of the bone rather than the age of the subject that is important; (3) the rate of age-related decline falls exponentially with age and is not significant in postmenopausal women alone; (4) except for the differences between superficial and deep bone, the pattern of osteocyte distribution within and between trabeculae was not affected by age or menopause; and (5) the data raise the possibility that one function of remodeling in iliac cancellous bone is to maintain osteocyte viability.

Effects of Ethnicity and Age or Menopause on Osteoblast Function, Bone Mineralization, and Osteoid Accumulation in Iliac Bone

We measured histologic indices of osteoblast function, bone mineralization, and osteoid accumulation separately on the cancellous, endocortical, and intracortical subdivisions of the endosteal envelope and on the combined total surface in transiliac biopsies obtained after double tetracycline labeling in 142 healthy women, aged 20–74 years, 34 who were black (19 pre‐ and 15 postmenopausal) and 108 white (42 pre‐ and 66 postmenopausal). The data were subjected to two‐way analysis of variance of the four groups defined by age/menopause and ethnicity. Also, linear regressions of selected variables on age and between functionally related but independently measured variables were examined. None of the interaction terms was significant, and none of the regression slopes on age differed between blacks and whites, indicating that, as for the previously reported structural and remodeling indices, the effects of ethnicity and of age/menopause are independent. Accordingly, the data were analyzed separately for the effects of ethnicity (pre‐ and postmenopausal combined) and age/menopause (blacks and whites combined). The analyses led to the following conclusions (1) Osteoid surface and volume were higher and adjusted apposition rate and osteoid mineralization rate lower in postmenopausal than in premenopausal subjects, but none of the indices of osteoid accumulation differed between blacks and whites. (2) Each index of osteoid accumulation was significantly correlated with its primary independently measured kinetic determinant (osteoid thickness with adjusted apposition rate, osteoid surface/bone surface with activation frequency, and osteoid volume/bone volume with bone formation rate/bone volume). None of the regression parameters differed significantly between blacks and whites. (3) The ratio of mineralizing surface to osteoid surface (MS/OS) was substantially lower in all demographic groups than could be accounted for by the later onset of mineralization than of matrix apposition at individual bone forming sites. (4) The low values for MS/OS can be explained by terminal mineralization being too slow to trap enough tetracycline molecules to produce detectable fluorescence, and do not require that mineralization be interrupted. (5) MS/OS was about 25% lower in blacks than in whites on all surfaces with corresponding differences in derived indices based on MS/OS, including adjusted apposition rate, mineralization lag time, and formation period. (6) The lower values for MS/OS in blacks are most likely due to slower terminal mineralization. This could not be accounted for by a lower serum level of calcidiol, but is consistent with the reported effect of reduced bone blood flow. (7) All differences in bone cell function between blacks and whites that we have observed could be the result of the ethnic, and presumably genetic, difference in bone accumulation during growth. Higher bone mass would result in less fatigue microdamage, less need for repair by directed bone remodeling, lower bone turnover, lower bone blood flow, and slower terminal mineralization. (8) If this explanation is correct, there are no fundamental differences in the biology of bone remodeling between ethnic groups.

Relationships between osteocyte density and bone formation rate in human cancellous bone

Iliac cancellous osteocyte density decreases with age in deep bone but not in superficial bone, most likely because of remodeling. It has been suggested that osteocytes can inhibit bone remodeling. Accordingly, we examined the relationship between osteocyte density and bone formation rate in 92 healthy women. In superficial bone (<25 microm from the surface), we found a weak but significant (p < 0.03) inverse correlation between BFR/BS and Ot. N/B.Ar that was unaffected by menopause and independent of age. A weaker positive relationship with empty lacunar density improved significance. The data appear to suggest a negative feedback loop, but osteocytes explain only 10% of the variance in BFR/BS, and 97% of the variance in osteocyte density is explained by total lacunar density. This measure of initial osteocyte density during bone formation has a high coefficient of variation (20%) indicating large individual differences. We conclude that: (1) our data support the proposal that osteocytes can inhibit bone remodeling; (2) osteocyte density in superficial bone depends mainly on initial osteocyte density during bone formation and is maintained but not regulated by bone remodeling; and (3) the inverse relationship between BFR/BS and osteocyte density may reflect the homeostatic need to maintain calcium exchangeability in the lining cell-osteocyte syncytium.

Reduced vitamin D receptor expression in parathyroid adenomas: implications for pathogenesis

Parathyroid adenomas discovered fortuitously grow very slowly and their cell birth rate greatly declines, features explicable by an initial increase in secretory set‐point. In the nodules of severe uraemic parathyroid hyperplasia, there is an increased set‐point and decreased expression of both the calcium sensing receptor (CaSR) and the vitamin D receptor (VDR). Accordingly, we examined VDR and CaSR expression in parathyroid adenomas.

The basal rate of cell proliferation in normal human parathyroid tissue: implications for the pathogenesis of hyperparathyroidism.

BACKGROUND The basal rate of cell proliferation in the human parathyroid gland is generally believed to be low, but has never previously been measured directly, although, as in other tissues, it is relevant to the pathogenesis of neoplasia.

Dependence of bone yield (volume of bone formed per unit of cement surface area) on resorption cavity size during osteonal remodeling in human rib: implications for osteoblast function and the pathogenesis of age‐related bone loss

It is both a necessary and a sufficient condition for bone to be lost with age at any surface location that during remodeling the replacement of resorbed bone is incomplete. In both the ilium and the rib, the degree of such focal imbalance is smaller on the intracortical than on the endocortical or cancellous surfaces that are adjacent to bone marrow. The reason for this difference is unknown. To further examine this question, we measured various geometric variables in 1263 osteons in rib cross sections from 65 persons, including both sexes and age ranges 20 to 30 years and 60 to 70 years (four groups). Haversian canal (HC) area did not differ significantly between sexes or age groups. Percent osteonal refilling was close to 95% in all groups and did not differ between sexes but fell slightly with age. There was a very highly significant linear relationship between osteon bone area and (osteon area + HC area) in all groups, with coefficients of determination (r2) greater than 0.98. The regression slopes declined slightly with age in women but not in men. There was a very highly significant quadratic relationship between osteon bone area and osteon perimeter in all groups, with r2 values greater than 0.97. The ratio osteon bone area:osteon perimeter, an index of bone yield—the volume of bone deposited on each unit area of cement surface—was strongly related to osteon area and did not differ between sexes but was slightly less in the older groups. We conclude the following: (1) The high efficiency of intracortical remodeling in the rib is confirmed, with only trivial effects of age. (2) For HC area to be maintained within narrow limits and bone balance preserved, either initial osteoblast density or osteoblast capacity (the two determinants of bone yield) or, most likely, both must increase progressively with the size of the resorption cavity, suggesting that osteoblast recruitment (relative to available surface) and osteoblast lifespan increase with the volume of bone resorbed. (3) Intracortical remodeling in the rib is more efficient than marrow‐adjacent remodeling at any site, possibly because of the different relationships to the circulation. In osteonal remodeling, all molecules released from resorbed bone must travel past the sites of osteoblast recruitment and operation, but in hemiosteonal remodeling, some molecules may not be subject to this constraint. (4) If marrow‐adjacent remodeling became as efficient as rib intracortical remodeling, age‐related bone loss would cease to be an important medical problem.

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.

Abnormal bone remodeling in patients with spontaneous painful vertebral fracture.

The application of tetracycline‐based iliac bone histomorphometry to the study of the pathogenesis of osteoporosis has given conflicting results. Accordingly, we performed this procedure in 78 postmenopausal white women with one or more vertebral fractures identified according to rigorous criteria that excluded other causes of vertebral deformity and 66 healthy postmenopausal white women recruited from the same geographic region; the groups did not differ in age or weight. In each subject, measurements were made separately on the cancellous (Cn), endocortical (Ec), and intracortical (Ct) subdivisions of the endosteal envelope. In the fracture patients, osteoblast surface was reduced substantially on each subdivision, most markedly on the Cn surface, where about 25% of the deficit was in cuboidal (type II) osteoblasts, suggesting impaired recruitment; the remaining 75% of the deficit was in intermediate (type III) cells, suggesting earlier transition from type III to type IV (flat) cells. On the Ec and Ct surfaces, the deficit was exclusively in type III cells. Mean bone formation rate was reduced by about 18% on the Cn but not on the Ec or Ct surfaces. The deficit was more significant in subjects matched for Cn BV/TV when adjusted for the inverse regression on osteocyte density and after logarithmic transformation. The difference in bone formation rate resulted from a corresponding reduction in wall thickness without a change in activation frequency. The frequency distribution of bone formation rate was more skewed to the left in the fracture patients than in the controls. Osteoclast surface was significantly lower on each subdivision. The variation in osteoblast surface, bone formation rate, and osteoclast surface was significantly greater in the fracture patients than in the controls, with more abnormally low and abnormally high values. The data suggest the following conclusions: (1) The histologic heterogeneity of postmenopausal osteoporosis is reaffirmed; (2) the different subdivisions of the endosteal envelope, although in continuity, behave differently in health and disease; (3) a combination of defective osteoblast recruitment and premature osteoblast apoptosis would account for the deficit in type II and III cells and the reductions in wall thickness and bone formation rate on the Cn surface and the previously reported osteocyte deficiency in Cn bone; (4) premature disaggregation of multinuclear to mononuclear resorbing cells could account for the osteoclast deficit; and (5) some patients with vertebral fracture have one or another disorder of bone remodeling that at present cannot be identified by noninvasive means.