The Rate of Osteoclastic Destruction of Calcified Tissues is Inversely Proportional to Mineral Density

This study examined the relative ease with which three dissimilar mineralized tissues from one individual organ were resorbed by osteoclasts in vitro. Cells released from the long bones of prehatch chicks by agitating fragments of the chopped bones in medium were cultured for 24 hours on slices cut from an Elephas maximus molar so that enamel, dentine, and coronal cementum were present in bands on the surface of the slice. The resultant pits were measured using a video-rate, line-confocal reflection light microscope system. Variations in tissue mineralization were characterized by analysis of digital backscattered electron images. The enamel pits were smaller than the dentine and the cementum pits, but the dentine and cementum pits were not significantly different from each other. The sizes of the pits correlated with the relative mineral densities of the three tissues, showing that the rate of osteoclastic destruction of calcified tissues is inversely proportional to mineral density. This indicates that the initial step in osteoclasis, the removal of the mineral phase, determines the volume removed and is the rate-limiting step.