Fluoride Reduces Enamel Matrix in Tooth Organ Cultures

Objectives: To test the hypothesis that a one-day exposure of secretory stage ameloblasts to fluoride (F) eventually influences enamel matrix content and enamel mineralization in later stages of enamel formation, long after exposure to F has ceased. Methods: Hamster first molar tooth germs were radiolabeled in vitro with tritium proline the first day of culture, exposed to F during the second day of culture, followed by another 5-day culture in F-free culture media. The last day of culture the explants were labeled with ⁴⁵Ca and ³²P-orthophosphate. Media were collected at various time points and counted.Explants were analysed for remaining matrix proteins and uptake of mineral ions. Results: We found that exposure of secretory ameloblasts to F immediately increased the release of radioproline-labeled fragments into the media over controls. Removal of F from the media rapidly lowered the release of fragments to control levels. Five days after the exposure to F the explants contained less radioproline-labeled enamel matrix and took up less ⁴⁵Ca and ³²P than control explants. Conclusions: A brief in vitro exposure of secretory ameloblasts to F enhances intracellular degradation of newly synthesized enamel matrix proteins. This results in a slight reduction of the amount of enamel matrix in the extracellular space, implying that a brief exposure to F can potentially reduce the thickness of the enamel layer. The data also suggests that restoring matrix enamel turnover and ameloblasts function sets in as soon as F is removed from the environment but that in the long run mineralization defects remain. Thus, disruption of secretory amelogenesis by F is one potential mechanism by which a complete enamel mineralization in fluorotic teeth will not be achieved in vivo.