Developmental sequence of bone-resorbing cells induced by intramuscular implantation of mineral-containing bone particles into rainbow trout, Oncorhynchus mykiss

Abstract

Mineral-containing bone particles (BPs) were implanted intramuscularly into rainbow trout (Oncorhynchus mykiss) to investigate the sequence of appearance of bone-resorbing cells. A fibrous substance first surrounded the implanted BPs and was gradually replaced by connective tissue containing capillaries. Two weeks after BP implantation, relatively small multinucleated cells (type-1 cells), whose cytoplasm stained deeply with hematoxylin, appeared along the surfaces of the BPs. At later stages (after 4–8 weeks), the majority of cells which appeared to be resorbing the BPs were multinucleated cells whose cytoplasm stained deeply with eosin (type-2 cells). Type-2 cells contained more nuclei than type-1 cells. Electron-microscopical observations revealed that type-2 cells had the characteristic features of osteoclasts: the presence of numerous mitochondria, vacuoles and granules, and a differentiation of the cell membrane and cytoplasm into a ruffled border and clear zone, respectively. A tartrate-resistant acid phosphatase activity, which is an established characteristic of osteoclasts in terrestrial vertebrates, but which had not previously been examined in teleosts, was demonstrated histochemically in the type-2 cells. Development of type-2 cells was closely correlated with the development of connective tissue. These findings suggest that the development of a capillary network around the implanted BPs enables circulating osteoclast-progenitors to reach the surface of the BPs.