Immunocytochemical localization of cathepsin D in the rat osteoclast.

We performed immunocytochemical localization of cathepsin D in osteoclasts of the proximal growth plate of the rat femurs using both the avidin-biotin-peroxidase complex method for cryo-semi-thin (1 micron) sections and the colloidal gold-labeled IgG method for K4M ultra-thin sections. At the light microscopic level, cathepsin D immunoreactivity in the osteoclasts appeared at the vesicles, granules, and/or small vacuoles. They were distributed throughout the cytoplasm of each cell and were relatively numerous close to the bone surface. This antigen could not be detected at the eroded bone surface. As for other cells, immunoreactivity was seen only in the lysosomes of osteoblast-like cells. Immunoreactivity in the osteoclasts was stronger and greater in the density and number than in osteoblast-like cells. At the electron microscopic level, osteoclasts with well-developed ruffled border possessed numerous cathepsin D-containing lysosomes, vacuoles, and coated vesicle-like structures. Cathepsin D-containing lysosomes fused with cathepsin-negative vacuoles and formed large secondary lysosomes. Osteoclasts with poorly developed ruffled border possessed fewer cathepsin D-containing lysosomes than those with well-developed ruffled border. No immunogold particles were seen in vacuole-like channel expansions of the ruffled borders, between the channels of the ruffled borders, or on the eroded bone surface. These findings demonstrate that osteoclasts contain a large amount of cathepsin D. They suggest that cathepsin D is necessary for osteoclastic bone resorption, that it plays an indirect rather than direct role.