Ultrastructural evaluation of the effects of calcitonin on bone in thyroparathyroidectomized rats administered vitamin D.

Synthetic salmon calcitonin produced ultrastructural alterations in osteoclasts of thyroparathyroidectomized rats administered vitamin D. Osteoclasts in calcitonin-treated rats lacked brush borders and transitional zones, had a uniform peripheral distribution of mitochondria and vesicles and frequently were separated from the bone surface by a layer of osteoblasts. These changes were associated with a significant decrease in serum calcium and phosphorus. Electron microscopic changes were not detected in osteoblasts and osteocytes of rats receiving calcitonin. These findings were interpreted to suggest that the site of action of calcitonin in bone may be principally on osteoclasts.