Age-Related Inequality between Rates of Formation and Resorption in Various Whole Bones of Rats

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Abstract
Variations in rates of bone turnover, consisting of bone formation and bone resorption, were characterized as a function of age and types of bone. Bone formation and bone resorption were quantified and compared in four ages of growing male rats (newborn, 0-2 weeks; weaning, 4-7 weeks; adolescence, 10-14 weeks; and mature, 15-23 weeks) for cephalic bone (calvaria), appendicular bone (femur), axial bones (sixth lumbar vertebrae and sternum), and the pelvis. In all four ages studied, inequality between the rates of bone formation and resorption existed in all bones, and the magnitude of the differences in metabolic imbalance turnover was age and bone type dependent. Formation was consistently higher than resorption in all five bones throughout the entire experimental periods. The ratio of resorption to formation was the lowest at birth (under 0.10:1) for all five bones and then increased. The most rapid bone growth in terms of bone calcium mass occurred at the weaning age (the net gain of calcium per bone per day was the highest) for all bones except the lumbar vertebrae, which occurred at adolescence. The magnitude of the inequality in the rates of bone formation and resorption was greatest in the newborn and diminished with age towards equality. At maturity, the ratio of resorption to formation was under 0.50:1 in the most dense or calcified bones such as the calvarium (0.40:1) and femur (0.49:1), and over 0.50:1 in the relatively thin cortex and well-trabeculated axial bones (0.60:1 in lumbar vertebrae and 0.75:1 in the sternum) and the pelvis (0.91:1). Additionally, large bones (calvaria, long bones, and pelvis) seemed to play an important role in regulating calcium homeostasis starting form the weanling age, since the amount of calcium released from each whole bone per day was high in these bones due to their large mass.

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