[Ultrastructural changes induced in rat ameloblasts and enamel by NaF administration, especially the stages of transition and maturation].

Single subcutaneous injections of sodium fluoride (84 mg/kg NaF) were administered to male Wistar rats weighing 100 g each. After 6, 12, and 24 hours and then after 2 and 5 days, the animals were fixed by perfusion with a mixture of 2.5% glutaraldehyde and 2.0% paraformaldehyde; and their upper incisors were subjected to optical microscopy, microradiography, and electron microscopy. 1. Changes in ameloblasts in the matrix-formation stage: Large vacuoles and dark globules, which frequently appeared to be stacked within the cell, could be seen in the distal one-third of the ameloblast 6 hours after NaF injection. These globules and vacuoles disappeared 24 hours after injection. Distortion of the Tomes' processes and separation of the ameloblasts from the enamel surface too could be seen. The separated areas gradually expanded to form cystic cavities, which developed toward the end of the formation stage of amelogenesis. But these cavities never extended to ameloblasts in the transitional stage. 2. Changes in ameloblasts in the transitional stage: Transitional ameloblasts may be divided into 2 stages: the early transitional stage, during which the proximal portion of the Tomes' process persists, and the late transitional stage, during which all trace of the Tomes' process has disappeared and a basement membrane-like structure has been produced. The appearances of ameloblasts in these two stages altered after NaF injection. In the early transitional stage, 6 hours after the injection, large vacuoles and dark globules appeared in the distal portion of the cell. Similar to vacuoles appearing during the matrix-formation stage, these vacuoles and globules disappeared 24 hours after injection. Traces of the Tomes' process, however, persisted and assumed an irregular, wavy form. The adjacent enamel adapted to and interdigitated with the cel surfaces without a structure resembling the basement membrane. NaF injection caused slight changes in the late transitional stage: small vacuoles appeared at the distal ends of the cell and disappeared 24 hours later. 3. Changes in the ameloblasts in the enamel-maturation stage: Six hours after the injection, small vacuoles appeared at the distal portion of the cell close to the striated border which was poorly developed. These vacuoles disappeared, and the striated border resumed its usual features 24 hours after the injection. 4. Changes in the enamel: In the forming enamel, a calciotraumatic line consisting of hypermineralized and hypomineralized layers could be seen. Another hypermineralized line appeared at the enamel surface adjacent to the transitional stage. Electron microscopy showed that this hypermineralized layer consisted of crowded, disoriented, needle-shaped crystals.