Effects of fluoride on human enamel and selachian enameloid in vitro: a high-resolution TEM and electron diffraction study.

The effects of fluoride on human enamel and selachian enameloid in vitro were visualized in TEM and analyzed with electron diffraction. It is demonstrated that under precise pH conditions, inducing concentration balance between F-ions and apatite, calcium fluoride is no longer formed, and crystalline changes occur instead. A secondary growing process, inducing a twofold increase in crystal size, involves all crystal faces, altering the hexagonal symmetry. It is suggested that the mechanism involved is not a dissolution/precipitation process but rather a secondary growth of residual crystallites induced by apatite dissolution.