Abstract
The effect of aging on long bone mechanical properties and bone formative capacity was characterized in the male Fisher 344 rat. The femurs of rats from three age groups (4 mo., 12 mo. and 28 mo.) were tested in three-point bending to determine their structural properties. The apparent material properties were then calculated by adjusting for bone geometry. Bone formation was assessed by dynamic histomorphometry of both cortical and cancellous bone as well as by Northern blot analysis for the expression of the osteoblast phenotypic proteins osteopontin (OP), osteocalcin (OC), type I collagen (COL) and alkaline phosphatase (AP). Aging resulted in a decline in the apparent material properties that was associated with a compensatory alteration of bone geometry that preserved structural strength and stiffness. Histomorphometric analysis revealed significant age-related decreases in cancellous bone volume, trabecular number and increased trabecular separation suggesting the existence of senile osteopenia in the proximal tibia of the male Fisher 344 rat. A significant decline in bone formation rate (BFR), but not mineral apposition rate, suggests that a reduction in osteoblast number, but not osteoblast activity, contributes to age-related bone loss. The decline in BFR with aging was reflected in a decreased mRNA expression for OP, OC and COL but not AP. Further, the pattern of mRNA expression was consistent with reduced osteoblast differentiation with aging. The present study indicates the age-related decline in material properties of long bones is paralleled by a decrease in osteogenesis.
Published Version
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