Catabolic and anabolic phases of osteodystrophy in aged rats.

The aims of this study were to investigate the incidence and characteristics of osteodystrophy that occurred in rats which were maintained until death without experimental intervention. We defined and characterized catabolic and anabolic phases of osteodystrophy, based on the morphology of the cortical bone. During the catabolic phase, large resorption cavities appeared within the cortical bone. During the anabolic phase, progressively increased amounts of new bone was formed, which partially or completely filled previous resorption cavities and also extended into the medullary space. Contrary to previous experimental studies of renal failure in rats, which documented suppression of bone formation, the present studies found that the majority of rats displayed evidence of an anabolic phase with extensive new bone formation. The new bone originated within and around fibers that had formed adjacent to bone trabeculae or within resorption cavities. Hence, osteitis fibrosa was a preliminary stage to de novo bone formation. There was an association between the incidence of osteodystrophy and kidney failure as the cause of death, and it is proposed that the observed osteodystrophy had been a consequence of secondary hyperparathyroidism due to renal failure.