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.