Abstract
The microscopic anatomy of the incisors was studied in 3 groups of rats:
A. 12 rats that were given 2 to 8 injections of .3 cc. of 2.5% sodium fluoride 24 or 48 hours apart. Age: 90-270 days.
B. 20 rats that were given single injections of .3 cc. of 2.5% sodium fluoride and allowed to live 1 to 48 hours after the administration. Age: 90-270 days.
C. 16 controls. Most of these were littermate controls.
Significant alterations were observed only in Groups A and B. The histoloigic changes in group A were:
1. Both the enamel and dentin show a pair of light (disturbed) and dark (recovery) incremental layers for each injection of sodium fluoride. The width of each pair is approximately 32μ for injections given 48 hours apart and approximately 16μ for injections given 24 hours apart.
2. The light layers represent the immediate response to the injections and are imperfect in formation and calcification.
3. The dark layers represent a recovery response and are normal in formation and normal or excessive in calcification.
4. This incremental pattern is a constant finding but shows disturbances when the administration is continued for more than 5 injections at 24-hour intervals.
The histologic changes in group B varied with the time interval between the injection and death:
1. 48-hour interval. One pair of light and dark incremental layers in the enamel and dentin.
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