Cytodifferentiation and Degeneration of Odontoclasts in Physiologic Root Resorption of Kitten Deciduous Teeth
Authors: Sasaki Takahisa , Shimizu Teruo , Suzuki Hiroshi , Watanabe Chiaki 
Published: July 2008

To investigate the cytodifferentiation and degeneration of odontoclasts in physiologic root resorption, we studied deciduous incisors undergoing resorption in 6-month-old kittens by electron microscopy of ultrathin sections. The endogenous peroxidase activity within the cells was also examined by incubating the tissue slices in diaminobenzidine-H<sub>2</sub>0<sub>2</sub> medium. The resorbing tissues consisting of multinucleated giant cells macrophages granular leukocytes fibroblasts and many blood vessels were observed at the resorbing surface of the root dentine. Macrophages and granular leukocytes exhibited endogenous peroxidase activity, but mononuclear and multinucleated preodontoclasts and multinucleated odontoclasts did not. These preodontoclasts contained abundant mitochondria, a moderate amount of rough endoplasmic reticulum, stacks of Golgi membranes, lysosomes and numerous polyribosomes scattered throughout the cytoplasm. Many cellular processes extended from their cell surfaces by which the preodontoclasts appeared to fuse to one another during their multinucleation. Concomitant with the multinucleation process, the preodontoclasts developed numerous pale vacuoles throughout the cytoplasm. These vacuoles seemed to arise from some smooth endoplasmic reticula, perhaps representing Golgi-endoplasmic reticulum-lysosome, and the Golgi saccules. However, the preodontoclasts did not yet form a ruffled border and clear zones. When these preodontoclasts came into direct contact with resorbing dentine surfaces, they began to form the clear zones against dentine surfaces. Characteristically, numer--ous pale vacuoles were accumulated in the cytoplasm adjacent to the clear zone then they penetrated into the cytoplasm of the clear zone, and with this, ruffles of the plasma membranes appeared. Through a further movement of more pale vacuoles towards the ruffled plasma membranes the odontoclasts developed typical ruffled borders against the resorbing dentine surfaces. At this differential phase, little pale vacuoles appeared in the Golgi area, but the cisterns of the Golgi apparatus themselves reached their greatest extent during cellular differentiation. Fully differentiated odontoclasts frequently extended long broad cellular processes into the dentinal tubules exposed to the resorption lacunae. Although some odontoclastic processes penetrating the dentinal tubules contained vacuoles and lysosomal structures, most processes lacked any cytoplasmic organelles, and their cytoplasm resembled that of the clear zone. But these processes never exhibited ruffled-border-like structures. When the odontoclasts terminated their resorptive function, the ruffled borders lost their structural complexity, and excess plasma membranes derived from the ruffled borders were withdrawn within the cytoplasm as pale vacuoles. Finally, the odontoclast cytoplasm was occupied by numerous pale irregularly shaped vacuoles and autophagosomes containing cellular debris. These degenerated odontoclasts frequently fused with neutrophils. These findings are thought to indicate the life cycle of odontoclasts in the physiologic root resorption of deciduous teeth.