Ultrastructural examination of dentine formation in rat incisors following multiple fluoride injections

Rats were divided into two experimental and one control groups. In the first experimental group, two doses were injected each day at 6 hr intervals. The high dosages of sodium fluoride were administered daily for 5 days. These fluorotic animals were killed as the fluoride was exerting an effect on mineralization. In the other experimental group, the rats were allowed a recovery period by killing the rats 3 days after the last fluoride injections. Control animals received equivalent dosages of sodium chloride and were killed on the 5th day.Each day's injections induced a consistent paired response in the dentine in both experimental groups—a zone of hypermineralization followed by a zone of hypomineralization. The hypermineralized component was consistently visible only on microradiographs as bands of increased density, while the hypomineralized component was evident only on electron micrographs. The fine structure of the hypomineralized zone consisted of patchy areas with the appearance of predentine devoid of crystals and interspersed with calcospherites of mineralized dentine. These globules acquired more mineral with time to fill most areas, leaving unmineralized patches adjacent to odontoblast processes. In the recovery animals, a few small regions of unmineralization persisted and a layer of normal dentine formed over the fluoride response zones.Neither fluorotic nor recovery odontoblasts manifested visible ultrastructural alterations.