[Ultrastructural changes in rat-incisor odontoblasts and dentin caused by administration of sodium fluoride]

Subcutaneous injections of 8.4 mg of sodium fluoride were administered to young male Wistar rats weighing 100 g each. After 1, 3, 6, 12, and 24 hours and then after 2 and 4 days, the rats were fixed by perfusion; and their upper incisors were subjected to optical microscopy, microradiography, and electron microscopy. In some instances, 0.1% ruthenium red (RR) was added to the fixing or demineralizing solution for the sake of observing acid-mucopolysaccharide distribution. 1. The following changes were observed in odontoblasts: swelling of the rough endoplasmic reticulum, vacuole formation, increased numbers of autophagic vacuoles and Golgi vacuoles, and abnormal accumulation of cytosome-4. All of these changes tended to have recovered by 24 hours after administration. 2. In the predentin, abnormal increases in amounts of amorphous substance occurred 1 hour after and continued until 12 hours after administration. Thereafter, increases gradually retarded; and the level observed in controls was restored by the fourth day after administration. RR staining showed this amorphous substance to contain large amounts of acid mucopolysaccharides. 3. Two types of abnormal collagen--segmental collagen aggregate (SCA) and fibrous long-spacing collagen (FLS)--were observed in the amorphous substance mentioned in the preceding paragraphy. In longitudinal sections, SCAs were 320 nm long and 200 nm wide; symmetrically arranged striations were visible within them. FLSs were observed as long, thick fibrils with periodic striations of about 160 nm. With the passing of time, these abnormal collagen aggregates and fibrils shifted from the distal end of the odontoblast toward the dentin, in which they became embedded. 4. Mineralization in the abnormal collagen commenced with deposition of needle-shaped or thin platelike crystals. As time passed, the collagen came to be completely covered with the crystals. Loosening of collagen molecular binding seems to occur following advances in mineralization and crystal growth. 5. Abnormality in dentin mineralization resulted in the formation of a hypermineralized layer and a hypomineralized layer beneath the hypermineralized layer. The hypermineralized layer contained densely distributed crystals. Its boundary with the hypomineralized was clearly observable. The boundary with teh regular dentin, however, was shifting and indistinct.