Osteoclasts and the resorption of bone by transplanted mammary carcinoma in rats.

Rat mammary tumour cells were grafted to parietal bones as an experimental model to study the nature of bone resorption around metastatic carcinomas in the skeleton. After periods of growth of from 6 to 56 days bones and tumours were removed and embedded in epoxy resin. The appearances were compared with those found when whole parathyroid glands were grafted in similar positions. Tumours were evident in all animals at the time of death and some were palpable five days after grafting. In 15 of the 21 animals with tumour, osteoclasts and resorption were found, and in only two of these were the tumour cells not separated from the bone surfaces. In 6 animals killed between 6 and 12 days after grafting there was new bone formation without resorption. There were osteoclasts and resorption under the grafted parathyroid glands which were always separated from the resorbing cells by fibrous tissue. The appearances of the bone surfaces under the tumours and the parathyroid glands suggested that the resorption in both situations was similar, was brought about by the secretion of a locally active agent and mediated by osteoclasts. This is further support for the role of osteoclasts in bone resorption around metastatic carcinomas.