The relation of ultrastructural changes in osteoclasts to resorption in bone cultures stimulated with parathyroïd hormone

Abstract

Changes in the size of osteoclasts, osteoclast nuclei, ruffled borders and clear zones in bones cultured with and without parathyroid hormone (PTH) were quantitated as a function of time. These data were correlated with the release of calcium into the medium, the numbers of osteoclasts in the bones and the number of nuclei per osteoclasts. Cell size increased significantly after three hours of treatment with PTH as compared to controls, ruffled borders increased significantly after 1–6 hours, and clear zones after three hours. The mean size of the cells and the clear zones increased at a similar rate and reached a plateau after 24 hours. Ruffled borders, however, increased at a higher rate and continued to increase up to 48 hours, occupying increasingly larger proportions of the cell. These early changes were followed by a significant increase in the numbers of osteoclasts' and a significant increase in calcium release by 6–12 hours. Hence, PTH has an early effect on osteoclast activity followed later by an effect on the differentiation of preosteoclasts to osteoclasts. The early effects on osteoclast activity may be responsible for rapid osteoclast-mediated changes in calcium homeostasis.