The role of macrophages in osteoclast neogenesis in vivo: quantitative analysis of osteoclasts and macrophages in endochondral bone development in rats treated with hydrocortisone or calcitonin.

Osteoclasts and macrophages in endochondral bone of rats were analyzed after long-term treatment with hydrocortisone (HC) or elcatonin (ECT). The growth plate-metaphysis junction ( GPMJ ) of the HC-treated bones showed increased numbers of osteoclasts, but the incidence of osteoclast profiles with ruffled borders was low and the cells had fewer nuclei. The number of macrophages in the GPMJ of HC-treated bone did not significantly differ from that in the control. In the metaphyseal trabecular (MT) region macrophages were numerous, but osteoclasts were depleted. Compared to controls the ECT-treated osteoclasts in the GPMJ had an increased number of nuclei per cell, and the incidence of osteoclast profiles with ruffled borders was reduced. There was a small increase in the number of osteoclasts and a large increase in the number of macrophages. The osteoclasts in the MT regions of ECT-treated bones were significantly larger compared to controls. The macrophages did not differ in number from the controls. This investigation supports the proposal that the GPMJ is the principal site of osteoclasts neogenesis. Discrepancies in features of osteoclasts in the GPMJ and MT regions of each treatment group were probably due to a difference in peroid when the osteoclasts were exposed to the drugs.