[Dynamics and ultrastructural characteristics of osteoclast formation]

Using electron microscopy and 3H-thymidine autoradiography it has been shown that osteoclasts of rats and rabbits are formed by fusion of cell precursors which become members of the osteoclasts 14 hours following S-phase. Monoblasts and promonocytes are considered as DNA-synthesizing forms. A single injection of parathormone in a dose of 10 units/100 g of body weight leads to an increase in the fractions of DNA-synthesizing cells. Monocytes and macrophages appear to be direct osteoclast precursors in resorption zones. As a result of their fusion there appear "unfixed" polynuclear macrophages (young osteoclasts). The number of osteoclast nuclei increases also at the expense of little-differentiated phagocytes, the inclusion of the latter takes place at different stages of the life span of osteoclasts depending on the intensity of bone resorption processes. The structural characteristics of fusion of cell precursors are considered.