Localization of cathepsin D in human odontoclasts. a light and electron microscopical immunocytochemical study

Odontoclasts are dentine and cementum resorbing cells whose relationship to bone resorbing osteoclasts is not clear. Like osteoclasts, they possess different cathepsins which are involved in mineralized tissue degradation during the tooth root resorption process in deciduous teeth. Whether cathepsin D, which in osteoclasts probably functions as an activator of other cathepsins, can be found in odontoclasts, has, however, not been investigated before. In order to determine its occurrence and localization, cathepsin D immunocytochemistry was applied to paraffin-embedded sections from 30 human deciduous tooth roots undergoing resorption. Using immunogold postembedding immunocytochemsitry on LR-Gold embedded specimens, the distribution of cathepsin D was investigated at the ultrastructural level. We identified tartrate-resistent acid phosphatase-positive mono- and multinuclear odontoclasts near and on the periodontal surfaces of tooth roots. Nearly all of these cells showed cytoplasmic granular cathepsin D immunoreactivity. At the electron microscopical level, gold labelling was seen on vacuoles and vesicles of the odontoclasts, which were identified as secondary lysosomes and phagosomes. Extracellularly it was seen along the ruffled border and in neighboured resorption areas of dentine and cementum. These findings indicate that cathepsin D is secreted into the resorbing area of human odontoclasts in order to participate in degradation of mineralized tooth matrix, but may also function as an activator of other proteases in lysosomal organelles.