IMMUNOLOCALIZATION OF THE CATION-INDEPENDENT MANNOSE 6-PHOSPHATE RECEPTOR AND CATHEPSIN-B IN THE ENAMEL ORGAN AND ALVEOLAR BONE OF THE RAT INCISOR

In order to examine our hypothesis that maturation ameloblasts could d egrade the enamel matrix in a manner analogous to bone resorption medi ated by osteoclasts, we have assessed the distribution of lysosomal en zymes in the enamel organ by immunolocalizing the cation-in-independen t mannose 6-phosphate receptor (MPR) and the lysosomal enzyme cathepsi n B at all stages of amelogenesis. Secretory ameloblasts showed strong immunoreactivity for MPR in the supranuclear Golgi region and in the cytoplasm between the Golgi region and the distal junctional complexes . However, cathepsin B immunoreactivity was mainly seen in the distal portion of Tomes' process, which was unreactive for MPR immunogenicity . In maturation ameloblasts, the MPR was observed on the ruffled borde r of the ruffle-ended ameloblast (SA) but not on the distal cell membr ane of the smooth-ended ameloblast (SA), although both cell types demo nstrated strong immunoreactivity for MPR in the Golgi region. Immunore active cathepsin B was seen at the distal ends of both RA and SA. It i s postulated that the nascent lysosomal enzymes bind to the mannose 6- phosphate receptors which target them not only to intracellular lysoso mes, but also to the ruffled border of maturation ameloblasts where th ese enzymes are secreted into the enamel. Since MPR and lysosomal enzy mes were also detected on the ruffled border of osteoclasts (Ocl) adja cent to alveolar bone, our immunocytochemical approach provides strong evidence for a similarity between the maturation process in enamel, a s mediated by the ruffle-ended maturation ameloblasts, and bone resorp tion mediated by osteoclasts. This study has established that a common mechanism, based on MPR-targeted lysosomal secretion and matrix degra dation, is basic to the maturation process involved in calcified tissu es as different as bone and enamel.