Pharmacokinetics of chronic fluoride ingestion in growing pigs

The present study was undertaken to estimate bio-availability and biological half-life of fluoride and accumulation of fluoride in bone in the domestic pig. Eight animals receiving 2 mg F-/kg b.w. per day from age 8 to 14 months were compared with eight controls. Plasma fluoride concentrations just prior to the daily oral dose were measured at regular intervals. After 112 days post-dose, plasma fluoride levels were measured over a 48-hour period following the daily oral dose or a single intravenous dose. Mean bio-availability factor for the oral dose was 0.3 (range 0.2-0.4), and mean biological half-life was 59 days (range 49-72). Bone fluoride content calculated from the pharmacokinetic parameters derived from plasma data was similar to the content of fluoride measured in the bone at slaughter. The study showed that accumulation of fluoride in bone influences plasma fluoride levels during chronic administration of fluoride to growing pigs. The long biological half-life found showed that it was not possible to achieve steady-state plasma levels within the six-month experimental period used. This means that, for dose-response studies of dental fluorosis in this animal, it is not possible to achieve steady-state plasma concentrations as a basis for correlations to the degrees of pathological change observed in the teeth.     

Fluoride content of infant formulae in Australia

The prevalence of dental fluorosis in Australia and the United States of America has increased in both optimally fluoridated and non-fluoridated areas. This has been attributed to an increase in the fluoride level of food and beverages through processing with fluoridated water, inadvertent ingestion of fluoride toothpaste, and the inappropriate use of dietary supplements. A major source of fluoride in infancy is considered to be infant formula which has been implicated as a risk factor for fluorosis in a number of studies. In this study the fluoride content of the infant formulae commonly used in Australia was determined. The acid diffusible fluoride of each powdered formula was isolated by microdiffusion and measured using a fluoride ion-specific electrode. The fluoride content of milk-based formulae ranged from 0.23 to 3.71 μg F/g and for soy-based formulae from 1.08 to 2.86 μg F/g. When reconstituted, according to the manufacturer's directions, with water not containing fluoride, the formulae ranged in fluoride content from 0.031 to 0.532 ppm, with the average fluoride content 0.240 ppm. Using average infant body masses and suggested volumes of formula consumption for infants 1–12 months of age, possible fluoride ingestion per kg body mass was estimated. None of the formulae, if reconstituted using water containing up to 0.1 ppm F, should provide a daily fluoride intake above the suggested threshold for fluorosis of 0.1 mg F/kg body mass. However, W reconstituted with water containing 1.0 ppm F they should all provide a daily fluoride intake of above the suggested threshold for fluorosis with intakes up to 2–3 times the recommended upper ‘optimal’ limit of 0.07 mg/kg body mass. Under these conditions the water used to reconstitute the formulae would provide 65–97 per cent of the fluoride ingested. These figures are likely to be overestimates due to the intake of nutrients from other sources reducing formulae consumption and also due to the lower bioavallability of fluoride from milk-based formulae. Further, it is generally believed that the maturation stage of enamel formation is the critical period for fluorosis development by chronic, above-threshold fluoride exposure. The maturation stage for the anterior permanent teeth, however, is after the first twelve months of life where fluoride intake from infant formula consumption per kg body mass is highest. The level of fluoride in the commonly used Australian formulae would suggest that infant formula consumption alone is unlikely to be a risk factor for dental fluorosis in a non-fluoridated community, but could make a major contribution to an infant's daily fluoride intake. However, prolonged consumption (beyond 12 months of age) of infant formula reconstituted with optimally-fluoridated water could result in excessive amounts of fluoride being ingested during enamel development of the anterior permanent teeth and therefore may be a risk factor for fluorosis of these teeth.     

Changes in dental fluorosis following an adjustment to the fluoride concentration of Hong Kong's water supplies

In June, 1978, the fluoride concentration in Hong Kong water supplies was reduced from 1.0 to 0.7 mg/L. The objectives of this study were (1) to determine whether, as a result of this minor adjustment, a consequent reduction in the prevalence and severity of dental fluorosis came about, and (2) to determine whether dental fluorosis develops during enamel secretion and primary mineralization or during the maturation stage of enamel development. Dental fluorosis was assessed by Dean's community fluorosis index (CFI) on upper central incisors in 2382 children aged from 7 (exposed to 0.7 mg/L only) to 13 years. The children were selected from four districts served with drinking water by four different water treatment stations. Differences in the distributions of dental fluorosis scores across ages were significant in all districts. The susceptibility to fluoride was assessed statistically through a series of analyses whereby the fluoride concentration in the drinking water (both coincident with enamel secretion and periods of enamel maturation) was correlated with CFI. It was concluded (1) that CFI values were reduced following a minor adjustment to the fluoride concentration in drinking water, (2) that dental fluorosis develops during the maturation stage of enamel development, (3) that the development of dental fluorosis may occur over a period of 16 to 24 months, commencing from 12 to 32 months following enamel secretion, and (4) that Dean's index is a suitable instrument for monitoring the effects on dental fluorosis of minor adjustments to the fluoride concentration in drinking water.     

不同质量浓度氟对大鼠切牙成釉细胞转化生长因子-β1表达的影响

目的研究不同质量浓度氟对大鼠切牙生长过程中转化生长因子-β1（TGF-β1）表达的影响,探讨氟斑牙的发病机制。方法40只Wistar大鼠随机分为3组,建立氟斑牙动物模型。3组分别为低剂量氟组（F-质量浓度60 mg·L-1,13只）、高剂量氟组（F-质量浓度120 mg·L-1,13只）和对照组（蒸馏水,14只）。10周后取材,采用苏木精-伊红（HE）染色和免疫组织化学染色的方法观察氟对大鼠切牙成釉细胞的形态及TGF-β1表达的影响。结果实验组大鼠切牙均出现典型的氟斑牙症状,牙面出现白垩色改变,釉质表面有横纹。HE染色结果显示成釉细胞形态发生改变,细胞排列紊乱,甚至成灶性堆积,可见空泡性变。免疫组织化学染色结果显示TGF-β1在分泌期和成熟期成釉细胞均为强阳性表达,在星网状层、中间层均为阳性表达,在新形成的釉基质中呈阳性表达。2个实验组TGF-β1的表达强度明显低于对照组（P<0.01）,2个实验组之间的差异无统计学意义（P＞0.05）。结论氟可能通过抑制TGF-β1的表达而干扰了成釉细胞的分化和基质分泌,造成釉质发育障碍。     

Distribution of dental fluorosis in the primary dentition

By the use of a new classification system of dental fluorosis, primary and permanent teeth were examined in samples of children born in areas with 3.5, 6.0 and 21.0 parts/10(6) F- in the water supplies. The severity of dental fluorosis was lower than in the corresponding permanent teeth, but a significant increase in severity was noted with increasing concentration of fluoride in the drinking water. The distribution of dental fluorosis within the dentition followed the same pattern irrespective of fluoride content of the water. Thus, a progressive increase in severity was noted from the anterior to the posterior teeth. This pattern is presumably determined by variations in enamel thickness. The much thinner enamel layer of primary teeth may also explain the lower degree of dental fluorosis in these teeth rather than a maternal barrier to fluoride. The degree of dental fluorosis observed in medium and high fluoride areas did not support the hypothesis that the rapidly growing skeleton of infants prevents fluoride-derived enamel changes.     

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.     

Experimental osteo-fluorosis in the domestic pig: a histomorphometric study of vertebral trabecular bone

The experiment aimed at describing effects of fluoride on structure and remodeling of vertebral trabecular bone in pigs. Eight animals receiving a supplement of 2 mg F-/kg b.w. per day from age eight to 14 mo were compared with eight controls. Plasma fluoride increased from 0.7 +/- 0.1 microM/1 to 12.7 +/- 2.0 microM/1 in pigs receiving fluoride. At slaughter, the concentration of fluoride in dry fat-free bone was 149.3 +/- 10.5 mM/kg for fluorotic animals and 9.5 +/- 0.9 mM/kg for controls. Morphologic changes were assessed in un-decalcified specimens of the fourth lumbar vertebra by quantitative histology using fluorochromes as intra-vital tissue time markers. The volume of trabecular bone tissue (bone + marrow) was unchanged in fluorotic animals, but the volume density of bone was increased by 17%. Surface densities of cancellous bone were almost unchanged, whereas the thickness of trabeculae increased in fluorosis. Fluoride enhanced remodeling of trabecular surfaces: The fraction of surface occupied by resorption lacunae increased 40%, and the formative surface approximately 30%. No changes were demonstrated at surface points undergoing formation: Osteoid thickness, calcification rate, lamellar thickness, and completed wall thickness were normal. It is concluded that the observed findings cannot be explained by fluoride-induced changes in a single cell. Fluoride appears to affect all cells involved in remodeling by direct or indirect mechanisms.     

Radiologic assessment of bone maturity and cortical thickness in experimental osteo-fluorosis in the young pig

Radiographs of the left forelimb were obtained after slaughter in 16 14-month-old pigs. From age 8-14 months, eight pigs in the experimental group received 2 mg F-/kg body weight per day. Bone maturity in F-animals was the same as in controls. Cortical thickness was increased by 10 per cent in the fluorotic animals (p less than 0.01) and their plasma-fluoride levels increased throughout the experimental period to approximate those reported for man in endemic fluorosis areas. Thus fluoride given in the dose used and over that period, did not affect maturation in the long bones but increased cortical bone mass in the diaphyses.     

Effect of prenatal and postnatal fluoride on the human deciduous dentition. A literature review

Fluoride passes from the mother to fetal teeth. Much of the fluoride is taken up in secretory enamel, probably by the forming mineral apatite crystals. Some is retained with residual proteins. The low concentration of fluoride in the inner enamel is incorporated mainly during the secretory stage, while the enhanced concentration in the surface enamel is produced during the much longer maturation stage. Mature, hard enamel is generally absent during fetal life. The clinical question is whether prenatal fluoride imparts an additional benefit to the universally accepted effect of postnatal fluoride. In general, surface enamel fluoride levels of deciduous teeth increase with increasing pre- and postnatal fluoride administration. A consistent level of caries protection has been reported with pre- and postnatal administration of fluoride unrelated to the acquisition of fluoride in the surface enamel. Many children develop enamel opacities in their deciduous dentition related by various factors to enamel mineralization disturbances in drinking water areas even low in fluoride. Accumulation of fluoride due to an increased fluoride intake is a feature of fluorosed enamel in the deciduous as well as permanent dentition. The resulting mature fluorosed enamel retains a relatively high proportion of immature matrix proteins onto the crystal surface. The degree of fluorosis of the deciduous dentition is less compared with that of the permanent dentition, due probably to a partial protection afforded by the maternal loss of fluoride, formerly known as the "placental barrier".     

Scanning electron microscopy of trypsin-treated enamel from fluorosed rat molars

Fluoride-induced pitting and porosity of teeth have long been observed, but little progress has been made in determining their origin. We have observed, in the trypsin-treated surfaces of enamel, pits that disappear on completion of maturation, following the removal of the protein matrix and full mineralization. Since these pits were considered to be similar to those seen in fluorotic teeth, this scanning electron microscope (SEM) study was undertaken to determine the effect of fluoride on these transient developmental pits during enamel matrix maturation. A group of 20 eight-day-old rats was given daily intraperitoneal injections of NaF (20 mg/kg [9 mg F-/kg] body weight) for five days. Twenty control animals received intraperitoneal injections of isotonic saline. Maxillary and mandibular molars were dissected from the 13-day-old animals, washed in HEPES buffered (Ca2+/Mg2+) free basal medium, Eagle's (BME), incubated in 3% trypsin/BME for 5-10 min at room temperature, then indirectly sonicated in BME for 2-4 min. Clean crowns were fixed in 2.5% glutaraldehyde for three hr, dehydrated, critical-point-dried, and sputter-coated for SEM. Pits in the surfaces of developing enamel were observed in all groups. In control teeth, the pitting was restricted to the cervical margin, whereas in teeth from the fluoride-treated animals, pits were observed on some cuspal surfaces in addition to the cervical margin. These results confirmed that pits in trypsin-treated surfaces of developing enamel are a transient developmental event and showed that, in the presence of a high dose of fluoride, the maturation of enamel is modified with retention of the pits.     

A probabilistic estimation of fluoride intake by infants up to the age of 4 months from infant formula reconstituted with tap water in the fluoridated regions of Ireland

Two probabilistic models were developed to estimate the acute and chronic exposure to fluoride of exclusively formula-fed infants aged 0-4 months as a result of the consumption of infant formula reconstituted with fluoridated tap water in Ireland. The estimates were based on calculated infant formula consumption and accepted body weight standards, together with reported concentrations of fluoride in infant formula powder and measured values for the fluoride content of water in Ireland. The mean acute exposure of infants to fluoride on any single day in areas served by 387 fluoridated water supplies was estimated to be between 0.11 and 0.14 mg/kg body weight depending on age group (95th percentiles 0.2 and 0.26 mg/kg b.w., respectively). These predicted intakes were well below the intake of fluoride associated with acute toxic effects, which is considered to be 5 mg fluoride (F(-))/kg body weight. The mean chronic exposure of infants to fluoride was estimated to be between 0.106 and 0.170 mg/kg b.w./day depending on body weight (95th percentiles 0.108 and 0.172 mg/kg b.w./day, respectively). This estimate described the average daily fluoride intake of infants during the first 4 months of life residing in the areas served by 226 water supplies that achieved an average yearly fluoride concentration below 1.03 mg/l. Dental fluorosis may be considered to be the only risk at these low doses and from our work it is estimated that there is a very low risk of moderate dental fluorosis of the permanent dentition in infants exposed to fluoride at these levels.     

氟化物作用下体外培养人牙胚内釉上皮细胞中Smad1、5表达的变化

目的观察不同浓度的氟化物对体外培养的人牙胚内釉上皮细胞中Smad1、5表达的影响,从细胞内信号转导水平观察氟对骨形成蛋白(BMP)信号转导的影响。方法Trowell法体外培养人牙胚,分别用10mg/L和25mg/L的氟化物刺激培养的牙胚,免疫组化观察结果。结果经图像分析显示,10mg/L组和25mg/L组从第4天开始,内釉上皮细胞中Smad1、5的表达均低于对照组。结论氟化物能抑制体外培养的人牙胚内釉上皮细胞中Smad1、5表达。提示氟可能通过抑制Smad1、5分子干扰上皮和间充质之间BMP正常的信号转导,进而使釉质的分化发育受到影响,可能是氟斑牙发生的细胞内机制之一。     

Fluoride metabolism and fluorosis

OBJECTIVES: This paper is primarily concerned with the only proven risk associated with water fluoridation: enamel fluorosis. Its purpose is to review current methods of measuring enamel fluorosis, its aetiology and metabolism. A further objective is to identify risk factors to reduce the prevalence of enamel fluorosis and employ methods to manage such risk factors. DATA: The prevalence of enamel fluorosis is increasing in Ireland and internationally. A critical period has been identified at which teeth are most at risk of developing enamel fluorosis: 15-24 months of age for males and 21-30 months of age for females. The data included took these two factors into account. SOURCE: A thorough narrative review of published literature was conducted to identify studies concerning the aetiology and metabolism of enamel fluorosis. Risk factors for fluorosis were identified from these studies. STUDY SELECTION: As it is the pre-eruptive phase of enamel development which represents the greatest risk to developing enamel fluorosis, studies examining sources of fluoride ingestion for young children were selected. These included studies on ingestion of fluoride toothpaste by young children, fluoride supplementation and infant formula reconstituted with fluoridated water. CONCLUSIONS: There is evidence that the age at which tooth brushing with fluoride toothpastes is commenced and the amount of fluoride placed on the brush are important risk factors in the incidence of dental fluorosis. It is recommended that brushing should not commence until the age of 2 and that a pea-sized amount (0.25 g) of toothpaste should be placed on the brush.     

Dental variables associated with differences in severity of fluorosis within the permanent dentition

The aim of this study was to assess the influence of age at start, duration, and completion of enamel formation, as well as of tooth-eruption age and enamel thickness on the severity of dental fluorosis within the permanent dentition. The material comprised Ugandan children (n = 219), aged 10-14 years, with 28 teeth and at least 1 tooth with fluorosis. The children were permanent residents in districts with either 0.5 mg or 2.5 mg fluoride/l in the drinking water. Fluorosis was assessed on the vestibular surfaces of all teeth using the modified Thylstrup and Fejerskov (TF) index. In order to relate fluorosis to the dental variables, the material was divided into a test group (n = 103), with fluorosis on all teeth, and a reference group (n = 116), with fluorosis on up to 27 teeth. The reference group was used to confirm or refute the findings in the test group. Paired comparisons showed significantly higher median TF scores for the late than for the early mineralizing and erupting teeth. In multiple regression analyses, the age at start, duration, and completion of enamel formation as well as tooth eruption was significantly related to the severity of fluorosis after controlling for enamel thickness (P < 0.05, n = 14). The effect (R2change) of the dental variables on the variation in severity of fluorosis within the dentition was in decreasing order: the duration of enamel formation, age at completion of enamel formation, tooth-eruption age, and the start of enamel formation.     

Timing of fluoride intake in relation to development of fluorosis on maxillary central incisors

OBJECTIVES: Several studies have focused on the timing of fluoride intake relative to the development of dental fluorosis. This study reports the relationships of fluoride intake during the first 48 months of life with fluorosis on early-erupting permanent teeth. METHODS: Subjects were followed from birth to 48 months with questionnaires every 3-4 months. Questionnaires gathered data on intakes from water, diet, supplements, and dentifrice to estimate total fluoride intake. Early-erupting permanent teeth of 579 subjects were assessed for fluorosis using the Fluorosis Risk Index (FRI) at approximately age 9. Fluorosis cases were defined as having FRI definitive or severe fluorosis on both maxillary central incisors. Individuals with FRI questionable fluorosis were excluded. The importance of fluoride intake during different time periods was assessed using t-tests and logistic regression. RESULTS: One hundred and thirty-nine (24%) subjects had fluorosis on both maxillary central incisors. Mean fluoride intake per unit body weight (bw) ranged from 0.040 to 0.057 mg/kg bw, with higher intake during earlier time periods and relative stability after 16 months. In bivariate analyses, fluoride intakes during each of the first 4 years were individually significantly related to fluorosis on maxillary central incisors, with the first year most important (P < 0.01), followed by the second (P < 0.01), third (P < 0.01), and fourth year (P = 0.03). Multivariable logistic regression analyses showed that, after controlling only for the first year, the later years individually were still statistically significant. When all four time periods were in the model, the first (P < 0.01) and second years (P = 0.04) were still significant, but the third (P = 0.32) and fourth (P = 0.82) were not. CONCLUSIONS: The first two years of life were most important to fluorosis development in permanent maxillary central incisors; however, this study also suggests the importance of other individual years.     

The macroscopic and scanning electron-microscopic appearance and microhardness of the enamel, and the related histological changes in the enamel organ of erupting sheep incisors resulting from a prolonged low daily dose of fluoride

The hypothesis that diffuse opacities in enamel result from a chronic, mild disturbance to ameloblast activities was tested using fluoride. Three sheep (HF) were dosed orally with 0.5, and 3 (LF) with 0.2 mg fluoride/kg body weight daily for 6 months. A control sheep (C) received no additional fluoride. The 7 sheep were killed at or close to the time of emergence of their permanent central incisors. One tooth from each sheep was sectioned longitudinally. The enamel related to the secretory (S) and maturation (M) phases of ameloblast activity at the start of fluoride dosing was determined from a tetracycline marker. The pattern of mineralization of the outer 150 micron of the cut labial enamel was assessed using microhardness testing. The SEM appearance of the acid-etched outer enamel was compared in S and M zones in 5 teeth. The enamel of the C tooth was translucent. Diffuse opacities, similar in appearance to human fluorosis, were present in all fluoride-treated teeth. Hardness values in the outer 70 micron of the enamel decreased as the fluoride dose increased and, in the HF teeth, were lower in the S zone than in the M zone. Fluoride given during the M phase induced a surface hypomineralization which increased in degree and depth when fluoride was also given during the S phase. The SEM appearance of M and S enamel was similar in 2 LF and 1 HF teeth but, in the other HF tooth, S enamel but not M enamel had a disordered prism structure and loosely-packed crystals in an abnormal organic matrix. Histological examination revealed that ameloblasts remained in only 4 of the 7 teeth and that their regression and the formation of the cementum adjacent only to the labial enamel were progressing abnormally.     

Fluoride and enamel defects

The concentration of fluoride in drinking water is the major determinant of the prevalence and severity of dental fluorosis in a community. Fluorosis is more prevalent and discernible in permanent teeth than in primary teeth; the intensity can range from barely perceptible, whitish striations in enamel to confluent pitting and dark staining. The traditional belief is that fluorosis is produced only during the secretory stages of ameloblastic activity. Some recent reports suggest that the maturation stages of enamel development are as important as or even more important than the secretory stages as the time when fluorosis can be produced. The question of timing remains unresolved. Many questions also remain about general and individual physiologic variations in relation to susceptibility to dental fluorosis. Good criteria for differential diagnosis exist to distinguish dental fluorosis from non-fluoride enamel opacities. An increasing number of reports indicates that the prevalence of fluorosis may be increasing among children in fluoridated and non-fluoridated communities. Reasons for the increases may relate to misuse of dietary fluoride supplements, ingestion of fluoride toothpastes, or increasing amounts of fluoride in foods or the atmosphere. The intensity of the increased fluorosis is in the milder categories and is not generally unsightly. It should be recognized that a small amount of fluorosis may be an alternative to a greater prevalence of dental caries, a disease that may produce cosmetic problems and sequelae worse than those produced by fluorosis.     

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.     

Fluoride concentrations in unerupted fluorotic human enamel.

Unerupted fluorotic human enamel was obtained from teeth surgically removed from patients with dental fluorosis. Fluoride was measured in samples produced by serial acid etching from the surface to the interior of blocks of buccal and lingual enamel. The severity of fluorosis, according to the TF index, was determined from the macroscopic and microradiographic appearance of the specimens. The shape of the fluoride profiles was not affected by the degree of severity of fluorosis, but the fluoride concentrations increased with increasing severity of lesions. Fluoride concentrations were similar to those previously recorded in erupted fluorotic enamel and were not related to the length of time the teeth had been present in the jaws. It was concluded that the fluoride content of erupted fluorotic enamel represents fluoride acquired during tooth formation and that further uptake prior to eruption may be negligible.     

Changes in the fluoride-induced modulation of maturation stage ameloblasts of rats

The maturation stage of enamel development is characterized by a cyclic modulation of the ameloblasts between bands of smooth-ended cells and longer bands of ruffle-ended cells. There are cyclic patterns of calcein staining of and 45Ca uptake in the enamel associated with this cellular modulation. Rats were given 0, 75, 100, or 150 ppm fluoride in their drinking water. Fluoride disrupted the cyclic patterns of the maturation stage, resulting in fewer bands of smooth-ended ameloblasts, fewer calcein-stained stripes, and fewer cycles of 45Ca uptake. When animals were given water containing 0 ppm fluoride following ingestion of water containing 100 ppm fluoride, the pattern of calcein staining returned to that of the control enamel. The disruption of the cyclic patterns in the maturation stage and the increased protein content of maturation enamel seem to be among the early events in the development of fluorosis.