A histological study of the short-term effects of fluoride on enamel and dentine formation in hamster tooth-germs in organ culture in vitro

Buffering of protons released by mineral formation during amelogenesis in mice

Regulation of pH by ameloblasts during amelogenesis is critical for enamel mineralization. We examined the effects of reduced bicarbonate secretion and the presence or absence of amelogenins on ameloblast modulation and enamel mineralization. To that end, the composition of fluorotic and non-fluorotic enamel of several different mouse mutants, including enamel of cystic fibrosis transmembrane conductance regulator-deficient (Cftr null), anion exchanger-2-deficient (Ae2a,b null), and amelogenin-deficient (Amelx null) mice, was determined by quantitative X-ray microanalysis. Correlation analysis was carried out to compare the effects of changes in the levels of sulfated-matrix (S) and chlorine (Cl; for bicarbonate secretion) on mineralization and modulation. The chloride (Cl^- ) levels in forming enamel determined the ability of ameloblasts to modulate, remove matrix, and mineralize enamel. In general, the lower the Cl^- content, the stronger the negative effects. In Amelx-null mice, modulation was essentially normal and the calcium content was reduced least. Retention of amelogenins in enamel of kallikrein-4-deficient (Klk4-null) mice resulted in decreased mineralization and reduced the length of the first acid modulation band without changing the total length of all acidic bands. These data suggest that buffering by bicarbonates is critical for modulation, matrix removal and enamel mineralization. Amelogenins also act as a buffer but are not critical for modulation.

The impact of fluoride on ameloblasts and the mechanisms of enamel fluorosis

Intake of excess amounts of fluoride during tooth development cause enamel fluorosis, a developmental disturbance that makes enamel more porous. In mild fluorosis, there are white opaque striations across the enamel surface, whereas in more severe cases, the porous regions increase in size, with enamel pitting, and secondary discoloration of the enamel surface. The effects of fluoride on enamel formation suggest that fluoride affects the enamel-forming cells, the ameloblasts. Studies investigating the effects of fluoride on ameloblasts and the mechanisms of fluorosis are based on in vitro cultures as well as animal models. The use of these model systems requires a biologically relevant fluoride dose, and must be carefully interpreted in relation to human tooth formation. Based on these studies, we propose that fluoride can directly affect the ameloblasts, particularly at high fluoride levels, while at lower fluoride levels, the ameloblasts may respond to local effects of fluoride on the mineralizing matrix. A new working model is presented, focused on the assumption that fluoride increases the rate of mineral formation, resulting in a greater release of protons into the forming enamel matrix.

Fluoride down-regulates the expression of matrix metalloproteinase-20 in human fetal tooth ameloblast-lineage cells in vitro

Fluoride is associated with a decrease in the incidence of dental caries, but excessive fluoride intake during tooth enamel formation can result in enamel fluorosis. Fluorosed enamel has increased porosity, which has been related to a delay in the removal of amelogenin proteins as the enamel matures. This delay in protein removal suggests that fluoride may affect either the amount or the activity of enamel matrix proteinases. In this study, we investigated the role of fluoride in the synthesis and secretion of matrix metalloproteinase-20 (MMP-20), the proteinase primarily responsible for the initial hydrolysis of amelogenin during the secretory stage of enamel formation. Cultured human fetus tooth organ ameloblast-lineage cells were exposed to 10 microM fluoride and analyzed for synthesis of MMP-20. Immunoblotting showed that 10 microM NaF down-regulated the synthesis of MMP-20 by 21% compared with control cells, but did not alter the amount of amelogenin or kalikrein-4 (KLK-4) synthesized by the cells. Real-time polymerase chain reaction (PCR) showed that 10 microM NaF down-regulated MMP-20 mRNA expression to 28% of the levels found in the non-treated cells. These in vitro results suggest that fluoride can alter the expression of MMP-20 by ameloblasts, resulting in a disturbance of the balance between MMP-20 and its substrate that may contribute to the retention of amelogenins in the formation of fluorosed enamel.

Fluoride-induced changes to proteoglycan structure synthesised within the dentine–pulp complex in vitro

Fluoride is known to influence mineralisation patterns within dentine, where alterations in the post-translational modification of proteoglycans (PG) have been proposed as an implicating factor. In light of recent studies elucidating changing PG profiles in the transition of predentine to mineralised dentine, this study investigates the influence of fluoride on the major PG populations (decorin, biglycan and versican) within the pulp, predentine and dentine. Tooth sections from rat incisors were cultured for 14 days in the presence 0, 1 and 6 mM sodium fluoride and the PG extracted from the pulp, predentine and dentine matrices. PG species and corresponding metabolites were identified by their immuno-reactivity to antibodies against decorin, biglycan and versican. Component glycosaminoglycan chains were characterised with respect to their nature, chain length and disaccharide composition. Levels of PG extracted from pulp and predentine were reduced, particularly for biglycan. Fluoride did not influence levels of decorin or versican within predentine or dentine, although the processing of these macromolecules within pulp and predentine was affected, particularly at higher fluoride concentrations. Levels of dermatan sulfate were reduced within pulp and predentine, although the effect was less pronounced for predentine. Fluoride reduced sulfation of glycosaminoglycan chains within pulp and predentine tissues, with a notable reduction in Deltadi6S evident. In all three tissues, glycosaminoglycan chain length was reduced. Considering the various roles for PG in the dentine-pulp complex, either directly or indirectly in the mineralisation process, changes in the synthesis, structure and processing of the different PG species within the pulp, predentine and dentine matrices provides a further molecular explanation for the altered mineralisation patterns witnessed during fluorosis.

Fluoride enhances intracellular degradation of amelogenins during secretory phase of amelogenesis of hamster teeth in organ culture

Amelogenins are the major protein species synthesized by secretory ameloblasts and are believed to be involved in enamel mineralization. During enamel formation, amelogenins are progressively degraded into smaller fragments by protease activity. These amelogenin fragments are removed from the enamel extracellular space, thereby enabling full mineralization of the dental enamel. Enamel from fluorotic teeth is porous and contains more proteins and less mineral than sound enamel. In this study we examined the hypothesis that fluoride (F-) is capable of inhibiting the proteolysis of amelogenins in enamel being formed in organ culture. Hamster molar tooth germs in stages of secretory amelogenesis were pulse labeled in vitro with [3H]- or [14C] proline and subsequently pulse chased. The explants were exposed to F- at different days of chase (i.e., during secretory amelogenesis early after labeling, later after labeling or at stages just beyond secretory amelogenesis). Exposure of secretory stage explants to F- enhanced the release of radiolabeled fragments when F- was applied early after labeling but progressively less if applied later. In contrast, F- had no such effect in stages beyond secretion. The enhanced release of radiolabeled fragments in secretory stages was associated with a reduction of radioactivity in the soft tissue enamel organ indicating that fragmentation of enamel matrix proteins (mainly amelogenins) occurred intracellularly. Analysis by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) demonstrated that the fluorotic enamel contained less radiolabeled parent amelogenins (M(r) 28 kD and 26 kD) but more low-molecular-mass fragments than enamel from control explants. Our data indicate that F- promotes intracellular degradation of the newly synthesized parent amelogenins during secretory stage. Our in vitro data do not support the concept that F- impairs extracellular proteolysis of amelogenins, either in the secretory phase or in the stage just beyond the secretory phase.

The influence of fluoride on the cellular morphology and synthetic activity of the rat dentine-pulp complex in vitro

Exposure to high fluoride concentrations in the immediate environment of the tissue is recognized to result in the post-translational modification of non-collagenous dentine extracellular matrix (ECM) components, potentially altering dentine mineralization. However, less is known about the effects of fluoride exposure on the morphology or metabolism of the cells associated with the dentine-pulp complex. This study examined the effects of fluoride exposure at defined concentrations on the cellular morphology and ECM synthetic activities of odontoblasts and pulpal fibroblasts by the culture of tooth sections from male Wistar rat incisors in Trowel-type cultures for up to 14 days, in the presence and absence of 6mM sodium fluoride. Histomorphometric analysis of the dentine-pulp complex of sodium fluoride-exposed tooth sections demonstrated no obvious gross morphological differences with respect to the odontoblasts and pulpal fibroblasts throughout the 14-day culture period, in comparison with unexposed tooth sections. No significant differences in odontoblast and pulpal fibroblast cell numbers were determined in the absence and presence of fluoride. Image analysis examination of odontoblast cytoplasmic:nuclear (C/N) ratios also showed no significant differences in fluoride-exposed and unexposed tooth sections, although reductions in the C/N ratios of pulpal fibroblasts were evident in fluoride-exposed sections at days 10 and 14. No significant differences in predentine width were observed in fluoride-exposed and unexposed tooth sections over the 14-day culture period. Autoradiography following [3H]proline incorporation into the dentine-pulp complex demonstrated inhibition of collagen synthesis, particularly by the odontoblasts in tooth sections exposed to 6mM sodium fluoride. These findings, in association with those from previous studies, imply that dentine ECM alterations may contribute to the altered mineralization of dentine during fluorosis, rather than secretory-related changes in odontoblast morphology.

Mechanism and timing of fluoride effects on developing enamel

Fluoride appears to specifically interact with mineralizing tissues, causing an alteration of the mineralization process. In enamel, fluorosis results in a subsurface hypomineralization. This hypomineralized enamel appears to be directly related to a delay in the removal of amelogenins at the early-maturation stage of enamel formation. The specific cause for this delay is not known, although existing evidence points to reduced proteolytic activity of proteinases that hydrolyze amelogenin. This delay in hydrolysis of amelogenins could be due to a direct effect of fluoride on proteinase secretion or proteolytic activity, or to a reduced effectiveness of the proteinase due to other changes in the protein or mineral of the fluorosed enamel matrix. The formation of dental fluorosis is highly dependent on the dose, duration, and timing of fluoride exposure. The early-maturation stage of enamel formation appears to be particularly sensitive to the effects of fluoride on enamel formation. Although the risk of enamel fluorosis is minimal with exposure only during the secretory stage, this risk is greatest when exposure occurs in both secretory and maturation stages of enamel formation. The risk of fluorosis appears to be best related to the total cumulative fluoride exposure to the developing dentition.

Biological mechanisms of dental fluorosis relevant to the use of fluoride supplements

Fluorosis occurs when fluoride interacts with mineralizing tissues, causing alterations in the mineralization process. In dental enamel, fluorosis causes subsurface hypomineralizations or porosity, which extend toward the dentinal-enamel junction as severity increases. This subsurface porosity is most likely caused by a delay in the hydrolysis and removal of enamel proteins, particularly amelogenins, as the enamel matures. This delay could be due to the direct effect of fluoride on the ameloblasts or to an interaction of fluoride with the proteins or proteinases in the mineralizing matrix. The specific mechanisms by which fluoride causes the changes leading to enamel fluorosis are not well defined; though the early-maturation stage of enamel formation appears to be particularly sensitive to fluoride exposure. The development of fluorosis is highly dependent on the dose, duration, and timing of fluoride exposure. The risk of enamel fluorosis is lowest when exposure takes place only during the secretory stage, but highest when exposure occurs in both secretory and maturation stages. The incidence of dental fluorosis is best correlated with the total cumulative fluoride exposure to the developing dentition. Fluoride supplements can contribute to the total fluoride exposure of children, and if the total fluoride exposure to the developing teeth is excessive, fluorosis will result.

The influence of fluoride on proteoglycan structure using a rat odontoblast in vitro system

Using an in vitro rat incisor odontoblast system, the effect of fluoride on proteoglycans was investigated at both the metabolic and structural level. Incisors were removed from 4-week-old rats, split longitudinally, and the pulps removed. Teeth were incubated at 37 degrees C, 5% CO2 in Eagle's Minimum Essential Medium containing 35S-sulfate for 7 hours in the presence of 0 mM, 3 mM, or 6 mM sodium fluoride. Teeth were demineralized in EDTA, proteoglycan was extracted from the residue with 4 M guanidinium chloride, and further purified by anion exchange chromatography. Uptake of radiolabel was monitored by liquid scintillation counting. The resultant products were examined by cellulose acetate electrophoresis, SDS-PAGE, chondroitinase digestion, and amino acid analysis. Differential effects of fluoride were observed in both metabolism and biochemical characterization of proteoglycans following incubation at the two concentrations. Fluoride decreased uptake of the radiolabel but led to an accumulation of glycosaminoglycan within the proteoglycan of the matrix. Chondroitin sulfate was the predominant glycosaminoglycan identified, with the additional presence of dermatan sulfate and heparan sulfate identified. Dermatan sulfate levels increased in 3 mM-treated teeth. Fluoride-treated proteoglycans had a reduced molecular weight (200-90K to 180-79K); this reduction is primarily a result of smaller glycosaminoglycan chains, with limited reduction in the size of the core protein of 6 mM-treated teeth occurring. Such alterations in the biochemical metabolism and hence structure and function of proteoglycan may be implicated in the hypomineralization seen in fluorosis.

Biological mechanisms of fluorosis and level and timing of systemic exposure to fluoride with respect to fluorosis

Enamel fluorosis can occur following either an acute or chronic exposure to fluoride during tooth formation. Fluorosed enamel is characterized by a retention of amelogenins in the early-maturation stage, and by the formation of a more porous enamel with a subsurface hypomineralization. The mechanisms by which fluoride affects enamel development include specific effects on both the ameloblasts and on the developing enamel matrix. Maturation-stage ameloblast modulation is more rapid in fluorosed enamel as compared with control enamel, and proteolytic activity in fluorosed early-maturation enamel is reduced as compared with controls. Secretory enamel appears to be more susceptible to the effects of fluoride following acute fluoride exposure, such as may occur with the use of fluoride supplements. However, both human and animal studies show that the transition/early-maturation stage of enamel formation is most susceptible to the effects of chronic fluoride ingestion at above-optimal levels of fluoride in drinking water.

The effect of fluoride on the developing mineralized tissues

The work described considers the effects on calcified tissues of those concentrations of fluoride which are not overtly cyto-toxic, i.e., in the general region of up to 1-2 mumol/L. Plasma fluoride concentrations or those of the cellular environment are considered rather than dietary levels. The effect of fluoride ion on specific stages of tooth and bone development is discussed. Little effect has been observed on the modulation of gene expression as far as odontogenesis is concerned, although there is evidence that fluoride could be osteogenic in both embryonic and adult tissues. Expression of extracellular matrix protein genes seems not to be impaired, but subtle changes detected in the enamel matrix could be due to selective alterations in amino-acid uptake or interference with subsequent protein processing. This could also be due to an extension of the secretory period without concomitant changes in post-secretory matrix processing. Removal of matrix is apparently impaired, with concomitant incomplete maturation. While existing mineral phases can be affected, it is more likely that matrix and or mineral-matrix interaction is the site of action. Explant studies suggest that the effect may be reversible. Inhibition of proteolysis during enamel maturation may account for the reported inhibition of enamel crystal growth. This is supported by the finding that the normally incomplete maturation of porcine enamel is associated with a somewhat greater residual protein content. The use of animal models in the investigation of enamel dysplasia (fluoride-induced or otherwise) should therefore be viewed with caution.

Antagonism of fluoride toxicity by high levels of calcium but not of inorganic phosphate during secretory amelogenesis in the hamster tooth germ in vitro

Whether the interference by fluoride (F-) with secretory amelogenesis in vitro could be modulated by altering the levels of calcium (Ca) and inorganic phosphate (P) in the medium was investigated. Hamster first upper molar tooth germs in the secretory phase of amelogenesis were exposed to 10 microM-1.31 mM (0.2-25 parts/10(6)) of F- in vitro for 2 days in the presence of either low (1.2 mM), moderate (2.1 mM) or high (4.1 mM) levels of Ca, or moderate (1.6 mM) and high (3.6 mM) levels of P. The biosynthesis and secretion of enamel matrix proteins under each of the experimental conditions were examined by labelling with [3H]-proline during the last 24 h of culture, and mineralization by labelling with 45Ca and [32P]-orthophosphate. With moderate levels of Ca and P (control medium), F- increased the uptake of 45Ca and 32P in a dose-dependent manner; F- did not inhibit the synthesis of matrix proteins but to a moderate extent impaired their secretion. In explants grown in the presence of 52 microM of F- the superficial layers of enamel matrix deposited in vitro (fluorotic matrix) failed to mineralize. Increasing P levels in the medium had no clear histological effect, whereas lowering Ca levels sometimes seemed to aggravate the F- effect. Raising Ca levels improved the histological pattern: in spite of the presence of F-, high Ca levels allowed a limited mineralization of the superficial layer of fluorotic matrix along with a strong rise in mineralization of the deeper layers of pre-exposure enamel. High Ca levels also considerably reduced the cellular changes in secretory ameloblasts induced by 52 microM of F- and slightly counteracted the inhibition of matrix secretion, as measured biochemically. Some of the effects of F- on secretory amelogenesis in vitro can thus be reversed by raising Ca levels in the medium. Therefore, the effect of F- on secretory amelogenesis in vitro seems to be primarily interference with the enamel mineralization process per se and, secondarily, an impairment of matrix secretion.

Autoradiographic, ultrastructural and biosynthetic study of the effect of colchicine on enamel matrix secretion and enamel mineralization in hamster tooth germs in vitro

First upper molar tooth germs of two to three days old hamsters were exposed in vitro to colchicine in concentrations ranging between 10(-7) and 10(-4) M in the presence of 45Ca and/or [3H]-proline for various times up to 18 h. Enamel mineralization was determined by chemical extraction of in vitro incorporated 45Ca and verified ultrastructurally. Quantitative autoradiography compared with water extracts from total explants radiolabelled with [3H]-proline showed a dose-dependent decrease of grain counts over the extracellular enamel to the similar extent as the decrease in radiolabelled amelogenins in water-extracts. It was concluded that water-extracts from total explants represent amelogenins from the extracellular compartment. Enamel matrix secreted in vitro during exposure to high doses of colchicine failed to mineralize and the complete loss was provoked of the distal parts of the secretory ameloblasts including the distal junctional complexes. Nevertheless, the mineralizing pre-exposure enamel neither hypermineralized nor increased uptake of 45Ca. These data do not support the hypothesis that secretory ameloblasts restrict transepithelial calcium transport by directing most of the calcium ions away from the mineralization front. The biosynthetic data furthermore suggest that enamel matrix proteins, only extractable with guanidine-HCl-EDTA, change their physico-chemical nature during secretory amelogenesis in vitro either during secretion or upon their extracellular mineralization.

Ultrastructure of in-vitro recovery of mineralization capacity of fluorotic enamel matrix in hamster tooth germs pre-exposed to fluoride in organ culture during the secretory phase of amelogenesis

The recovery of mineralization capacity of fluorotic enamel matrix was investigated in 3-day-old hamster first molar tooth germs already pre-exposed in organ culture to 10 parts/10(6) F- for 24 h during the secretory phase. The germs were then cultured for another 24 h in a fresh medium without F-. The unmineralized fluorotic enamel matrix secreted in vitro eventually mineralized in the absence of F- but the orientation of the crystals compared to those in the fluorotic enamel was disturbed, especially in the younger regions of the enamel nearest cervical-loop in which the underlaying fluorotic enamel was most hypermineralized; but least disturbed in the more mature parts of the enamel organ in which the fluorotic enamel was less hypermineralized. The subsequent culture in F(-)-free medium did not abolish or reduce the degree of hypermineralization induced by F- treatment during the initial 24 h of culture. It seems that in vitro the inhibitory effect of F- on enamel matrix mineralization during the secretory phase is completely reversible when the ion is removed from the matrix environment, i.e. F(-)-induced synthesis and secretion of defective enamel matrix is not the cause of the lack of matrix mineralization. The F(-)-induced hypermineralization seems to be irreversible.

Ultrastructural study of fluoride-induced in-vitro hypermineralization of enamel in hamster tooth germs explanted during the secretory phase of amelogenesis

The effects of fluoride (5, 10 and 20 parts/10(6) F-) were studied in vitro with light and electron microscopy in 5-day-old hamster maxillary second molar tooth germs explanted when most of the ameloblasts are in the secretory phase, and cultured for 24 h in the presence of F-. F- at all doses investigated induced hypermineralization of that enamel which had been secreted in vivo just prior to exposure to F-. The most intense hypermineralization was in the aprismatic enamel near the cervical loop region, where the in-vivo enamel layer was thinnest and gradually decreased (but was not abolished) with the increasing thickness of in-vivo formed enamel in the more mature parts of the enamel organ. The fluoride-induced hypermineralization in the aprismatic enamel layer did not stain at all with dilute toluidine blue solution and was therefore indistinguishable from the underlying dentine in light micrographs. The hypermineralization was due to growth in thickness of the enamel crystals, which in the aprismatic enamel layer resulted in a lateral fusion of all the enamel crystals. Thus fluoride administered during the secretory phase of enamel formation decontrols or even abolishes enamel crystal growth in length and promotes crystal growth in thickness so producing the hypermineralization of the pre-fluoride enamel. Enamel matrix secreted in the presence of fluoride did not mineralize.

Short-term effects of fluoride on biosynthesis of enamel-matrix proteins and dentine collagens and on mineralization during hamster tooth-germ development in organ culture

The effect of various concentrations of fluoride (F-) on cell proliferation, matrix formation and mineralization was examined in hamster molar tooth germs in premineralizing and mineralizing stages. The exposure lasted 16 h (mineralizing stages) and 24 h (premineralizing stages) and the F- levels ranged from 2.63 microM to 2.63 mM; [3H]-thymidine, [3H]-proline, 45Ca and 32PO4 were used as markers for cell proliferation, matrix formation and mineralization, respectively. The proline-labelled amelogenins were isolated by sequential extraction with water and formic acid and their nature examined by SDS-urea-polyacrylamide electrophoresis. Digestion by collagenase was used to assess the amount of proline incorporated into collagens. F- in concentrations up to 1.31 mM inhibited neither biosynthesis of DNA and amelogenins, nor synthesis of collagens and their hydroxylation. Amelogenins extracted from F- induced, non-mineralizing enamel matrix had the same electrophoretic mobility and the same degree of phosphorylation as amelogenins from normal, mineralizing enamel. However, F- increased the uptake of 45Ca and TCA-soluble 32P dose-dependently, starting with 52 microM. Thus, interference with secretion of enamel matrix by F- takes place at much lower concentrations than required to inhibit biosynthesis of enamel matrix.

Long-term (8 days) effects of exposure to low concentrations of fluoride on enamel formation in hamster tooth-germs in organ culture in vitro

Second maxillary molars of 3-4-day-old hamsters were cultured for 7-8 days in the continuous presence of fluoride (F-) or chloride in concentrations between 2.63 microM and 1.31 mM. For biochemical study, explants were labelled during the last 24 h of culture with a triple label of [3H]-proline, 45Ca and 32PO4. The 3H-labelled presumptive amelogenins were separated from the 3H-labelled dentine collagens by a three-step extraction procedure. Histologically, chronic exposure to F- had no obvious effects below 26.3 microM; at 26.3 microM of F-, a non-mineralizing enamel matrix was observed besides that of a normal mineralizing enamel. From 52 microM of F- onwards, only a non-mineralizing enamel matrix was found in decreasing amounts extracellularly as F- concentrations increased. Except for the presence of globular dentine, dentinogenesis was not obviously affected by F-. Biochemically, total synthesis of presumptive amelogenins was hardly disturbed, but their solubility was changed by chronic F- treatment; more amelogenins became formic-acid soluble at the expense of water-soluble amelogenins. Chronic exposure to F- decreased the water-soluble amelogenin fraction according to a logarithmic function of the medium F- concentration. By extrapolation, it was calculated that concentrations higher than 1-2 microM of F- affect amelogenesis in vitro. Synthesis of dentine collagen was not affected by chronic exposure to F- in vitro. Chronic exposure to F- decreased uptake of 45Ca and to a less extent trichloroacetic acid-soluble 32PO4. Chronic F- exposure may inhibit energy production in the enamel organ resulting in an impairment of enamel matrix secretion as well as that of a trans-epithelial transport mechanism for calcium.