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

Dental hypomineralization treatment: A systematic review

INTRODUCTION

Defects in the maturation stage of amelogenesis result in a normal volume of enamel but insufficient mineralization, called hypomineralization. Molar-incisor hypomineralization (MIH), amelogenesis imperfecta and dental fluorosis (DF) are examples of such defects.

OBJECTIVE

To evaluate the effectiveness of the treatments applied to the different forms of dental hypomineralization.

MATERIALS AND METHODS

PubMed, Scopus, Cochrane Library, Web of Science, and Embase were screened. The research was limited to studies published in English, Spanish, and Portuguese, until May 30, 2018. The research question was formulated following the Population, Intervention, Comparison, Outcome strategy. The quality of the methodology of each article was evaluated employing the Cochrane Handbook for Systematic Reviews.

RESULTS

From the initial research, 7895 references were obtained, of which 33 were included in the systematic review. The following treatments were reported: desensitizing and remineralizing products, resin infiltration, restorations, fissure sealants, tooth bleaching, enamel microabrasion and calcium, and vitamins supplements.

CONCLUSIONS

Although the results are suggestive, there is a clear need for a greater uniformity of the methodologies, thus allowing for the development of clinical guidelines. Nevertheless, it was possible to identify several effective treatments for teeth with MIH (arginine pastes or fluoride varnishes) and DF (tooth bleaching and/or enamel microabrasion).

CLINICAL SIGNIFICANCE

Because MIH, amelogenesis imperfecta, and DF are commonly seen in dental daily practice, it is extremely important to analyze the literature regarding its treatment.

Combined effect of fluoride and 2,3,7,8-tetrachlorodibenzo-p-dioxin on mouse dental hard tissue formation in vitro

Fluoride interferes with enamel matrix secretion and mineralization and dentin mineralization. The most toxic dioxin congener, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), also impairs dental hard tissue formation and mineralization in vitro and in vivo. Our aim was to investigate in vitro whether the combined effect of sodium fluoride (NaF) and TCDD on dental hard tissue formation is potentiative. For this purpose, mandibular first and second molar tooth germs of E18 mouse embryos were cultured for 5-12 days with NaF and TCDD alone at various concentrations (2.5, 5, 10, 12.5, 15, and 20 μM and 5, 10, 12.5, and 15 nM, respectively) to determine the highest concentrations, which alone cause no or negligible effects. Morphological changes were studied from the whole tooth photographs and histological tissue sections. The concentrations found were 15 μM for NaF and 10 nM for TCDD. While at these concentrations, the effects of NaF and TCDD alone were barely detectable, the effect of simultaneous exposure on dentin and enamel formation was overt; mineralization of predentin to dentin and enamel matrix secretion and mineralization were impaired. Immunohistochemical analysis revealed that the combined exposure modified amelogenin expression by odontoblasts. Morphology of ameloblasts and the expression of amelogenin indicated that ameloblasts were still secretory. The results show that NaF and TCDD have potentiative, harmful effects on the formation of dental hard tissues. Since children can be exposed to subclinical levels of fluoride and dioxins during early childhood, coincidently with mineralization of the first permanent teeth, this finding may have clinical significance.

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.

Effect of calcium, given before or after a fluoride insult, on hamster secretory amelogenesis in vitro

We tested the hypothesis that high-calcium medium given prior to or immediately after exposure to fluoride (F) reduces the negative effects of F on secretory amelogenesis. Hamster molar tooth germs were grown in organ culture in media with different calcium levels. Deposition of enamel matrix and matrix mineralization were monitored by incorporation of [3H]proline and uptake of 45Ca and acid-soluble 32PO4. Ameloblast structure and the occurrence of a fluorotic enamel matrix were examined by light and electron microscopy. A preculture of explants in high-calcium medium partially prevented the formation of fluorotic (non-mineralizing) enamel matrix, increased matrix secretion but could not prevent F-induced hypermineralization of the pre-exposure enamel. High-calcium medium, applied after F insult, accelerated the recovery of fluorotic matrix, improved ameloblast structure, enhanced amelogenin secretion, and increased enamel thickness. The data indicate that it might be the balance between the amount of mineral deposition and that of matrix secretion which is critical for the mineralization of newly secreted enamel. Exposure to F disturbs this balance by enhancing mineralization of the pre-exposure enamel, probably generating an excess of protons. High calcium may protect against F exposure by enhancing amelogenin secretion into the enamel space, thereby increasing the local buffering capacity at the mineralization front.

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 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.

Effects of vincristine on the developing hamster tooth germ in vitro

Vincristine is one of the cytostatic drugs present in cocktails commonly used for the treatment of cancer in children. The aim of this study was to evaluate biochemically and histologically the toxic effects of this drug on the developing tooth in vitro using the organ culture model in order to be able to predict what damage the drug can induce in the developing teeth from children undergoing anti-neoplastic chemotherapy. The most profound effect of the drug (10(-8)M-10(-4)M vincristine) on the developing tooth germ was the induction of mitotic arrests at the cervical loop and in the inter-cuspal regions. The 10(-4)M-10(-6)M vincristine doses were cytotoxic to most cells in the developing tooth germ. The 10(-7)M vincristine dose apart from induction of mitotic arrests, did not appear to be cytotoxic to the mature differentiated secretory cells. However, this dose induced incomplete nuclear polarization of the differentiating ameloblasts and odontoblasts. At 10(-8)M vincristine, the only effect observed were mitotic arrests; the secretory cells did not appear to have been affected at all. On the other hand, mineralization (TCA-soluble 45Ca and 32P uptake) was dose-dependently decreased from 10(-7)M vincristine upwards. 10(-9)M vincristine, the lowest dose tested, did not induce any changes in the developing tooth germ. The organ culture data indicate that 10(-9)M vincristine is the highest (safe) dose which does not induce any toxic effects in the developing hamster tooth germ.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Use of fluoride by young children and prevalence of mottled enamel

The prevalence of mottled enamel in the permanent dentition of children participating in a fluoride (F-) program at the dental school of the Vrije Universiteit (Amsterdam) was investigated in a study utilizing the Thylstrup-Fejerskov (TF) index. The randomly chosen children received a F- regime considered optimal by the Dutch Advisory Committee for Prevention of Oral and Dental Diseases. From the children examined (n = 83; 49 boys and 34 girls; mean age, 13 years and 5 months), 74% exhibited mottled enamel in a slight to moderate degree. More teeth were affected and the degree of mottling was higher when children started to use F- at an earlier age. Unintentional ingestion of toothpaste containing 0.15% F- during frequent toothbrushing in combination with the daily intake of F- tablets before the age of four may explain the high prevalence of mottled enamel. After these treatments, F- concentrations in plasma of young children can reach values which can directly affect the developing tooth germ.

A western-blotting study of enamel glycoproteins in rat experimental fluorosis

Experimental fluorosis was induced in order to get information on enamel protein glycosylation, using Western-blotting methodology with peroxidase-labelled concanavalin A. Fluoride inhibited amelogenin degradation, especially the production of intermediate forms. Within the non-amelogenin family of proteins there were changes in both the conventionally stainable components and the glycoconjugates revealed by lectin only. Fluoride influenced the whole extracellular processing of enamel proteins including movement between the mineral and non-mineral compartments. A different degradation scheme of enamel proteins, which also affects the glycoconjugates, might be of importance in the properties of the fluorosed enamel surface and its interactions with the oral environment.

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.

Effects of calcium and phosphate on secretion of enamel matrix and its subsequent mineralization in vitro

We examined the effects of various calcium (Ca) and phosphate (P) levels on enamel matrix synthesis, secretion, and its subsequent mineralization in vitro. Second maxillary molar tooth germs of three-day-old hamsters were cultured for nine days in vitro in media containing low (0.9 mmol/L), moderate (2.6 mmol/L), or high (4.5 mmol/L) medium levels of Ca, with either moderate (1.65 mmol/L) or high (3.65 mmol/ L) medium levels of P. Explants were then examined histologically. For examination of matrix synthesis and mineralization, explants were labeled during the last 24 hr of culture with a triple label of 3H-proline, 45Ca, and 32PO4.

At low Ca levels, tooth germs failed to deposit enamel matrix and dentin, and no mineralization took place, regardless of the levels of P. Low levels of Ca, however, did not prevent deposition of pre-dentin. At moderate and high levels of Ca, considerable amounts of enamel and dentin were deposited in vitro, and both matrices mineralized. At high Ca levels, however, the forming enamel hypermineralized, was more irregular, and tended to be thinner. Increasing P concentrations at moderate and high Ca levels resulted in formation of a more regular enamel and dentin and a better-controlled mineralization. Biochemically, high levels of Ca tended to decrease enamel matrix secretion but significantly enhanced the uptake of 45Ca. This Ca-stimulated increase of 45Ca uptake could be reduced to below control levels by increases in P medium levels.

We conclude that: (1) a minimum medium Ca concentration is required to induce enamel matrix deposition and mineralization of both enamel and dentin; (2) high levels of medium Ca induce hypermineralization of enamel and give rise to deposition of a more irregular enamel than at moderate Ca levels; and (3) high levels of P are not able to induce mineralization when Ca levels are low but seem to moderate effects of moderate and high levels of Ca.

Light and transmission electron microscopy of the effects of calcium, magnesium and phosphate on dentine and enamel formed by rat molars in vitro

The effect of addition of eight different combinations of Ca, Mg and P supplements (control, Ca, Mg, P, CaMg, CaP, PMg and CaMgP) on three-day-old rat maxillary second molars, explanted at the premineralizing stage and cultured for two weeks, was studied. Light-microscopy sections, cut parallel to the occlusal plane, were divided into four sectors and given a score according to an ordinal scale for dentine and enamel depending on the regularity of these matrices. An analysis of variance on these scores revealed a significant favourable effect of Mg, CaMg and CaMgP and an adverse effect of Ca on enamel. A favourable effect on dentine regularity was obtained after addition of Ca or Mg. Ultrastructurally, enamel changes such as amorphous enamel matrix, voids and disturbance in rod-interrod pattern were seen after addition of Ca, P, CaP. Thin enamel with less tight packing of crystals was observed after CaMg addition. A thick layer of enamel with highly-organized rod-interrod pattern was seen with Mg, PMg and CaMgP addition. It is suggested that Mg plays an important role in the interaction with Ca and P for the harmonious development of enamel and dentine in vitro.

Effects of fluoride on protein secretion and removal during enamel development in the rat

Enamel maturation consists of a loss of the early secreted matrix proteins and an increase in mineralization. This study investigated the changes in enamel proteins of the rat incisor, caused by the ingestion of fluoride at various stages of enamel formation. Rats were given 0, 10, 25, 50, or 100 ppm fluoride in drinking water for five weeks. Changes in the protein composition of the secretory, early-maturation, and late-maturation enamel were investigated by means of gel filtration chromatography and polyacrylamide gel electrophoresis. No differences were found between fluorosed and control enamel proteins in secretory enamel. In fluorosed early-maturation enamel, amelogenins were retained in larger quantities than in control enamel in animals ingesting 25 ppm fluoride or greater. At the late-maturation stage of enamel formation, only enamel from animals ingesting 100 ppm fluoride in drinking water contained more protein, when compared with control enamel. This study suggests that fluoride ingestion levels resulting in enamel fluorosis inhibit the mechanisms involved in the removal of proteins during enamel maturation.

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.