Diffusion of fluoride through the rat enamel organ in vitro

Fluoride ingestion from toothpaste and diet in 1- to 3-year-old Brazilian children

OBJECTIVE

This study estimated the total daily fluoride intake of 1- to 3-year-old children from diet and dentifrice. The constituents of the diet were divided into solids, water, milk, and other beverages, which were analyzed separately. The correlation between fingernail fluoride concentrations and the total daily fluoride intake by children was also investigated.

METHODS

Thirty-three children, living in a fluoridated area, participated in the study. Fluoride intake from diet was monitored by the 'duplicate plate' method, investigating the different constituents of the diet. Fluoride ingested from dentifrice was determined by subtracting the amount of fluoride recovered after brushing from the amount originally placed onto the child's toothbrush. Fingernails were clipped and collected on three occasions. Fluoride was analyzed with the ion-specific electrode, after hexamethyldisiloxane-facilitated diffusion. Data were tested by anova and Tukey's post hoc tests, Student's t-tests and linear regression (P < 0.05).

RESULTS

Mean (+/-SD) fluoride intake from diet and dentifrice was 0.025 +/- 0.013 and 0.106 +/- 0.085 mg/kg body weight/day, respectively, totaling 0.130 mg/kg body weight/day. A strong positive correlation (r = 0.971, P < 0.0001) was seen between the amount of dentifrice loaded onto the brush (0.49 +/- 0.30 g) and the amount of fluoride ingested during each tooth brushing (0.59 +/- 0.45 mg). Among the constituents of the diet, water and milk had a significantly higher contribution to the fluoride intake (0.18 +/- 0.11 mg/day, P < 0.0001), when compared with solids (0.07 +/- 0.05 mg/day) and other beverages (0.07 +/- 0.04 mg/day). Mean (+/-SD) fingernail fluoride concentration on the three dates of collection was 3.11 +/- 1.14, 2.22 +/- 1.47 and 3.53 +/- 1.40 mug F/g. There was no significant correlation between fingernail fluoride concentration and the total fluoride intake.

CONCLUSIONS

Most of the children are exposed to a daily fluoride intake above the suggested threshold for dental fluorosis. The dentifrice alone is responsible for an average of 81.5% of the daily fluoride intake, while among the constituents of the diet, water and milk are the most important contributors. In addition, small variations in daily fluoride intake cannot be detected in fingernails.

Dental fluorosis: chemistry and biology

This review aims at discussing the pathogenesis of enamel fluorosis in relation to a putative linkage among ameloblastic activities, secreted enamel matrix proteins and multiple proteases, growing enamel crystals, and fluid composition, including calcium and fluoride ions. Fluoride is the most important caries-preventive agent in dentistry. In the last two decades, increasing fluoride exposure in various forms and vehicles is most likely the explanation for an increase in the prevalence of mild-to-moderate forms of dental fluorosis in many communities, not the least in those in which controlled water fluoridation has been established. The effects of fluoride on enamel formation causing dental fluorosis in man are cumulative, rather than requiring a specific threshold dose, depending on the total fluoride intake from all sources and the duration of fluoride exposure. Enamel mineralization is highly sensitive to free fluoride ions, which uniquely promote the hydrolysis of acidic precursors such as octacalcium phosphate and precipitation of fluoridated apatite crystals. Once fluoride is incorporated into enamel crystals, the ion likely affects the subsequent mineralization process by reducing the solubility of the mineral and thereby modulating the ionic composition in the fluid surrounding the mineral. In the light of evidence obtained in human and animal studies, it is now most likely that enamel hypomineralization in fluorotic teeth is due predominantly to the aberrant effects of excess fluoride on the rates at which matrix proteins break down and/or the rates at which the by-products from this degradation are withdrawn from the maturing enamel. Any interference with enamel matrix removal could yield retarding effects on the accompanying crystal growth through the maturation stages, resulting in different magnitudes of enamel porosity at the time of tooth eruption. Currently, there is no direct proof that fluoride at micromolar levels affects proliferation and differentiation of enamel organ cells. Fluoride does not seem to affect the production and secretion of enamel matrix proteins and proteases within the dose range causing dental fluorosis in man. Most likely, the fluoride uptake interferes, indirectly, with the protease activities by decreasing free Ca(2+) concentration in the mineralizing milieu. The Ca(2+)-mediated regulation of protease activities is consistent with the in situ observations that (a) enzymatic cleavages of the amelogenins take place only at slow rates through the secretory phase with the limited calcium transport and that, (b) under normal amelogenesis, the amelogenin degradation appears to be accelerated during the transitional and early maturation stages with the increased calcium transport. Since the predominant cariostatic effect of fluoride is not due to its uptake by the enamel during tooth development, it is possible to obtain extensive caries reduction without a concomitant risk of dental fluorosis. Further efforts and research are needed to settle the currently uncertain issues, e.g., the incidence, prevalence, and causes of dental or skeletal fluorosis in relation to all sources of fluoride and the appropriate dose levels and timing of fluoride exposure for prevention and control of dental fluorosis and caries.

The effect of fluoride on apatite structure and growth

Fluoride participates in many aspects of calcium phosphate formation in vivo and has enormous effects on the process and on the nature and properties of formed mineral. The most well-documented effect of fluoride is that this ion substitutes for a column hydroxyl in the apatite structure, giving rise to a reduction of crystal volume and a concomitant increase in structural stability. In the process of enamel mineralization during amelogenesis (a unique model for the cell-mediated formation of well-crystallized carbonatoapatite), free fluoride ions in the fluid phase are supposed to accelerate the hydrolysis of acidic precursor(s) and increase the driving force for the growth of apatitic mineral. Once fluoride is incorporated into the enamel mineral, the ion likely affects the subsequent mineralization process by reducing the solubility of the mineral and thereby modulating the ionic composition in the fluid surrounding the mineral, and enhancing the matrix protein-mineral interaction. But excess fluoride leads to anomalous enamel formation by retarding tissue maturation. It is worth noting that enameloid/enamel minerals found in vertebrate teeth have a wide range of CO3 and fluoride substitutions. In the evolutionary process from elasmobranch through enameloid to mammalian enamel, the biosystems appear to develop regulatory functions for limiting the fluoridation of the formed mineral, but this development is accompanied by an increase of carbonate substitution or defects in the mineral. In research on the cariostatic effect of fluoride, considerable emphasis is placed on the roles of free fluoride ions (i.e., preventing the dissolution and accelerating the kinetics of remineralization) in the oral fluid bathing tooth mineral. Fluoride also has been used for the treatment of osteoporosis, but much still remains to be learned about maximizing the benefit and minimizing the risk of fluoride when used as a public health measure.

Anion translocation through the enamel organ

The objective of this study was to determine whether cells of the secretory- and maturation-stage enamel organ of rats contain anion translocation mechanisms similar to those found in other ion-regulating epithelia. Sodium bromide (Br) was used to localize the distribution of anions in the enamel organ. Furosemide, an inhibitor of the Na-K-2Cl co-transporter and other anion transporters, was administered with NaBr or sodium fluoride (F) to investigate if halogens other than Cl can use these transport mechanisms. We obtained the data by using freeze-fracture and freeze-drying methodology in conjunction with scanning and transmission electron microscopy (SEM, TEM) and energy-dispersive x-ray spectroscopy (EDS). The secretory- and maturation-stage enamel organ prevented Br from entering the enamel matrix. Br was localized in the Tomes' processes, but not in the enamel matrix, strongly suggesting that the distal intercellular junctions of ameloblasts are "tight". Furosemide disrupted anion transport to allow not only Cl but also Br to enter the forming enamel matrix. Periodic administration of high F doses promoted the formation of bands of disrupted enamel, reflecting the periodicity of F administration. The same concentration of F administered with furosemide increased the severity of disrupted enamel, resulting in "blisters" and pits in the maturing enamel. The enamel "blisters" contained pools of small, disorganized enamel crystallites. The group receiving furosemide only displayed normal enamel structure but had increased Cl in the enamel matrix. This study provides evidence that anion transporters, possibly the Na-K-2Cl co-transporter, function to regulate anion translocation, including F, to the enamel matrix in secretory- and maturation-stage enamel organ. These mechanisms may explain why the ionic composition on the cellular side of the anion barrier is different from that of the enamel matrix.

Recent observations on enamel crystal formation during mammalian amelogenesis

BACKGROUND

Enamel mineralization taking place during amelogenesis is a unique model to investigate carbonatoapatite formation in vivo. The abundance of proteinaceous crystal growth inhibitors, in particular amelogenins, contributes significantly to the mineralization process. Their putative roles are to prevent random proliferation of crystal nuclei and to regulate the growth kinetics and orientation of the formed enamel crystals.

METHODS

The enamel fluid surrounding the forming enamel crystals contains high concentrations of carbonate and magnesium ions, both of which seem to modulate the mineralization process. Particularly, Mg ions can adsorb onto enamel crystal surfaces in a manner to compete with Ca ions. Enamel mineral formed during amelogenesis is featured as calcium-deficient, acid phosphate-rich carbonatoapatites. Currently the most putative stoichiometry model for enamel mineral is (Ca)5-x(HPO4)v(CO3)w(PO4)3-x (OH)1-x.

RESULTS

Very significant changes in the morphology, stoichiometry, and solubility of enamel crystals occur during the various stages of amelogenesis. The early enamel mineralization comprises two events: the initial precipitation of the well-documented thin ribbons and the subsequent overgrowth of apatite crystals on those templates. The thin ribbons precipitated in the vicinity of the secretory ameloblasts have the highest contents of acid phosphate, particularly in the form of exchangeable species, whereas both the exchangeable and unexchangeable acid phosphate decrease concomitantly with the progress of the apatite overgrowth and the appearance of elongated hexagonal crystals in the late secretory stages.

CONCLUSIONS

Those morphological and compositional features seem to be consistent with the formation of precursors, such as octacalcium phosphate.

Enameloid formation in two tetraodontiform fish species with high and low fluoride contents in enameloid

Forming teeth of parrotfish and pufferfish were viewed by transmission electron microscopy to correlate cytological features of the enameloid organ with the species' fluoride (F) content in mature enameloid. Secretory-stage inner dental epithelial cells (IDE) of parrotfish (high F) and pufferfish (low F) secreted procollagen granules into the enameloid collagen matrix. The odontoblasts of both species, less numerous than IDE cells, also contained procollagen granules at the enameloid matrix formation stage. After the full thickness of enameloid matrix collagen had been deposited, enameloid crystallites formed parallel to the long axis of the enameloid collagen fibres. Concurrently, the plasma membranes of the outer dental epithelial cells (ODE) became invaginated in both species, but to a much greater extent in parrotfish. Highly undulating parrotfish ODE cells surrounded numerous fenestrated capillaries. In contrast, pufferfish ODE cells remained straight with few adjacent capillaries. Extensive tight junctions formed between ODE and IDE cells of both species, sealing the extracellular space. With increased mineralization, enameloid collagen fibres were no longer discernible. A thin layer of amorphous material, which subsequently mineralized, was secreted on to the enameloid surface by IDE cells in both species. Pufferfish odontoblasts secreted a mineralizing amorphous layer on the pulpal aspect of the enameloid. The results suggest that at the mineralization stage, a triad of cytostructural features, highly invaginated ODE cells, highly vascularized ODE cells, and extensive tight junctions are strongly correlated with high fluoride content of mature enameloid mineral. Species without any one of these features have lower fluoride in the enameloid.

Possible function of matrix proteins in fluoride incorporation into enamel mineral during porcine amelogenesis

The present study was undertaken to elucidate the mechanism of fluoride incorporation into secretory enamel mineral, with porcine enamel used as a model. Although the fluoride content in the enamel varied greatly among the animals, we observed that the fluoride-to-calcium ratio in the enamel tissue was maximal at the beginning of the secretory stage; the F/Ca ratio decreased (and leveled off) with the advancement of mineralization. In vitro work showed that some of the fluoride in the secretory enamel tissue was removed with the extraction of organic matter, mostly amelogenins. Furthermore, coating hydroxyapatite crystals with enamel matrix proteins resulted in a retardation of fluoride incorporation into the crystals when exposed to fluoride solutions, as a result of an inhibition of apatite reprecipitation. We also confirmed that the growth kinetics of fluoridated apatite onto HA seeds decreased with increasing coverage of the seed surface with the enamel proteins. All the results of the present study strongly suggest that the fluoride incorporation into enamel mineral during the secretory stage may be regulated by the kinetics of mineralization, which is highly dependent on the driving force for precipitation and the presence of proteinaceous inhibitors, mainly amelogenins.