multinucleate osteoclasts in medaka as evidence of active bone remodeling putative sites of bone resorption in the acellular bony skeleton of the medaka fish oryzias latipes were investigated primarily by rna in situ hybridization and histological analysis numerous cells that displayed intense enzymatic activity of tartrateresistant acid phosphatase trap the main marker of osteoclasts were distributed in the pharyngeal region of this fish moreover these cells expressed cathepsin k an osteoclastspecific gene as well as the genes for trap and vacuolartype proton atpase vatpase some of the trappositive cells displayed all of the morphological characteristics equivalent to those of mammalian and birdtype osteoclasts these cells were associated primarily with the shedding teeth and their supporting bones pedicles where alkaline phosphatase alpasepositive osteoblasts were also located implying progressive bone remodeling associated with tooth replacement in these regions in contrast the inner aspects of the neural and hemal arches of the vertebral column which were the only sites of bone resorption other than the toothbearing bones showed sporadically aligned flat mononuclear trappositive cells without a ruffled border indicating a different mode of bone remodeling in these regions these results suggest the feasibility of medaka as a model animal for the investigation of bonerelated abnormalities and their genetic backgrounds