This paper describes the effect of parathyroid extract on the fine structure of bone cells in the proximal metaphysis of the tibia. The extract was given to rats intraperitoneally, in single or repeated doses of 1–14 units (USP) per gram body weight, over short or extended periods, and the tissue was examined between 6 and 26 hr after the commencement of injection. During this period, osteoblasts showed the greatest changes. Their mitochondria became swollen and contained dense granules; both rough- and smooth-surfaced vesicles were distended; ribosomes were separated from the membranes; and various dense bodies appeared in the cytoplasm. Osteocytes were similarly but less affected, and osteoclasts differed little from those in normal animals. Eventually after repeated injections, the fine structure of many of the cells was disrupted. These changes were accompanied by erosion of the bony trabeculae without any great increase in numbers of osteoclasts. It is possible that the alterations in fine structure were the result of damage to the cells by the parathyroid extract. However, since the extract did not seem to stimulate classical osteoclastic activity, the mitochondrial swelling and dense bodies could have been related to the removal of mineral and matrix, respectively.

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