Comparison of rates of enamel synthesis in impeded and unimpeded rat incisors.

Periodic intubations of rats with solutions of fluoride (F) lead to the appearance of bands of disrupted pigmentation in continuously erupting incisors. Distances between fluorotic bands reflect time intervals between intubations. In this experiment, the periodicity of fluorotic banding was used for estimation of the rate of enamel synthesis in impeded and unimpeded rat incisors. Rats kept on a low-F diet and distilled water were intubated two or four times per week with 2 mg NaF/150 g body weight. In a group of rats, one of the mandibular incisors was cut at the gingival margin after two weeks, and intubations were continued for an additional two weeks. In another group of F-intubated rats, incisors were cut or notched at the gingival margin twice, six days apart. Control rats either received the same periodic F intubations or were maintained on the low-F diet without intubation. Measurements of spacing between fluorotic bands were identical in impeded and unimpeded teeth, even though the latter erupted at a faster rate. In an unimpeded mandibular incisors, there was a significant elongation of the secretory zone and a shortening of the pigmentation zone, resulting in reduced pigmentation intensity of the erupted portions of the teeth. The results show that the rate of enamel synthesis is independent of the eruption rate.