Osteoclastic features of cells that resorb bone implants in rats

The subcutaneous implantation of devitalized, mineral-containing bone particles in rats elicits the recruitment and differentiation of multinucleated cells and the rapid resorption of the bone. Cytochemical and ultrastructural characteristics were studied 12 days after implantation of bone. Tartrate-resistant acid phosphatase activity was demonstrated in these cells, especially at the cell/bone interface. Mast cells were seen vicinal to, but not in contact with the osteoclasts and the bone particles. Electron microscopic evaluation revealed that many multinucleated cells in lacunae on the surfaces of the bone particles displayed features characteristic of in osso osteoclasts, including clear zones of attachment to the bone substrate, and ruffled borders overlying bone with frayed collagen fibers. Many cells contained multiple giant centrospheres; this finding suggests that fusion can occur between multinucleated cells. This model may be useful to characterize the differentiation and regulation of bone-resorbing cells.