A comparative study of disturbed mineralization of rat incisor enamel induced by strontium and fluoride administration

Isotopic composition of sheep wool records seasonality of climate and diet

RATIONALE

Hair keratin is a very important material in ecological and archaeological studies because it grows continuously, can be obtained non-invasively, does not require extensive processing prior to analysis and can be found in archaeological sites. Only a few studies have examined seasonal variations in hair isotope values, and there is no published dataset examining the isotope variability recorded in the keratinous tissues of stationary (i.e., non-migrating) domestic mammals.

METHODS

Thirty-six Irish sheep were sampled in eight farms every three months between September 2006 and June 2007. A shearing strategy was adopted to sample only the most recently grown wool in order to represent an average of the summer, autumn, winter and spring conditions. The stable isotope ratios of the ground samples were measured using two different stable isotope mass spectrometers operated in dual-inlet (C, N) and continuous-flow (O, H) mode.

RESULTS

Wool O isotope ratios are a good proxy for seasonal variability in climate and can be used to anchor a chronology independently of other isotope records (C, N) that are influenced by diet or physiology. By contrast, interpretation of seasonal variations in hair H isotope composition in terms of climate is more complex probably due to the influence of dietary H. The C and N isotope values of grass-fed animals varied seasonally, probably reflecting the annual cycle of seasonal variation in grass isotope values. The highest δ(13) C values were measured in summer-grown wool, while the highest δ(15) N values were measured in winter-grown wool. Supplementation of the sheep diet with concentrates was detected easily and was marked by an increase in δ(13) C values and a decrease in δ(15) N values in winter-grown wool.

CONCLUSIONS

The present study demonstrates that time-resolved sampling and stable isotope ratio analysis of sheep wool can be used to identify short-term changes in diet and climate and therefore offer a tool to examine a wide variety of present and past husbandry practices.

Barrier formation: potential molecular mechanism of enamel fluorosis

Enamel fluorosis is an irreversible structural enamel defect following exposure to supraoptimal levels of fluoride during amelogenesis. We hypothesized that fluorosis is associated with excess release of protons during formation of hypermineralized lines in the mineralizing enamel matrix. We tested this concept by analyzing fluorotic enamel defects in wild-type mice and mice deficient in anion exchanger-2a,b (Ae2a,b), a transmembrane protein in maturation ameloblasts that exchanges extracellular Cl(-) for bicarbonate. Defects were more pronounced in fluorotic Ae2a,b (-/-) mice than in fluorotic heterozygous or wild-type mice. Phenotypes included a hypermineralized surface, extensive subsurface hypomineralization, and multiple hypermineralized lines in deeper enamel. Mineral content decreased in all fluoride-exposed and Ae2a,b(-/-) mice and was strongly correlated with Cl(-). Exposure of enamel surfaces underlying maturation-stage ameloblasts to pH indicator dyes suggested the presence of diffusion barriers in fluorotic enamel. These results support the concept that fluoride stimulates hypermineralization at the mineralization front. This causes increased release of protons, which ameloblasts respond to by secreting more bicarbonates at the expense of Cl(-) levels in enamel. The fluoride-induced hypermineralized lines may form barriers that impede diffusion of proteins and mineral ions into the subsurface layers, thereby delaying biomineralization and causing retention of enamel matrix proteins.

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.

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.

Disturbed enamel mineralization in a rat incisor model

Possession of full-thickness hard enamel appears to be one of the indispensable life-saving characteristics of rats. Previous studies by Suga and his colleagues and by others demonstrated that various types of malformation are evoked in continuously erupting rat incisors. In the current report, we directed our effort to oversee various types of enamel malformation caused experimentally in rat incisors. We surveyed the specimens collected by Suga and his colleagues, as well as specimens we obtained. From the results, it is conceivable that perturbation of the programmed sequential events during enamel development is a major factor in the establishment of enamel malformation. Animal studies with either 1-hydroxyethylidene-1,1-bisphosphonate (HEBP) or a multidentate phosphonic acid (EDTPO) confirmed that dentin mineralization provides a certain inductive effect on the secretion of enamel matrix and subsequent enamel crystallization. Our recent studies using anti-microtubular agents led to the conclusion that the acceleration of mineralization in outer enamel is a type of enamel malformation, most likely due to disruption of the cellular regulation of calcium transport under severe toxic regimens. In future work, experimental approaches combining measurements of kinetic factors with static observation of enamel lesions are required before we can gain a comprehensive understanding of the pathogenesis of disturbed enamel mineralization. The kinetic factors to be considered include the rates of tissue apposition and tooth eruption which determine the total volume of tooth substance formed, and the rate of mineral accretion. Furthermore, information as to the composition, crystallinity, solubility, and mechanical properties of enamel defects is needed before we can assess the susceptibility of teeth having those lesions to caries and other physico-chemical attacks in the oral environment.

Changes in the plasma electrolytes and metabolites of the rat following acute exposure to sodium fluoride and strontium chloride

Acute exposure of rats to strontium or fluoride by i.p. injection of sodium fluoride or strontium chloride resulted in a systemic response in which changes occurred in the plasma electrolytes and metabolites. Strontium resulted in a rapid but temporary hypercalcaemia while fluoride produced a temporary hypocalcaemia. There was no significant hypophosphataemia after fluoride and only a transient hypophosphataemia with strontium. There was some indication of kidney damage and a general stress response following fluoride injection. These results do not support the hypothesis that interglobular dentine is associated with hypophosphataemia or hypoplastic enamel with hypocalcaemia and are in conflict with the observation that the formation of interglobular dentine following the injection of lead acetate is associated with hyperphosphataemia and hypercalcaemia.

Iron in the enameloid of perciform fish

It is known that a high concentration of iron is deposited in the enameloid of some teleostean fish. Previously, Suga et al. (1989) pointed out that the iron concentration in the enameloid is related to the phylogeny of fish rather than to the feeding habits, according to the results of quantitative iron analyses on the teeth of marine teleost fish of the Tetraodontiformes. In the present study, in order for the previous idea to be verified, quantitative iron analysis was made with an electron microprobe on the enameloid of fish belonging to the Perciformes, which is the largest group of teleostean fish in the world and consists of both marine and freshwater species. The enameloid of all the fish examined (57 species) contained high iron concentrations ranging from 0.2% to 10.2% at the surface or middle layer, whereas that of an advanced suborder, Tetraodontoidei, of the Tetraodontiformes was very low in iron, at a level which could not be discriminated from the background value of the emission intensity. The distribution pattern of iron in the enameloid was classified into at least two types, namely, type A, in which a high iron concentration was observed mainly in the surface layer, and type B, in which iron was deposited throughout the entire layer, although there were differences in concentration. There were some differences in the concentration and distribution of iron in the enameloid for the families; for example, those of the Scaridae had a type A distribution, with about 0.2% iron only at the surface layer, whereas those of the Cichlidae, Centrarchidae, and Acanthuridae, which showed a type B distribution, contained iron ranging from 2.9% to 10.5% at the surface or middle layer of enameloid. Such differences seemed to be associated with the difference in timing of the commencement of the iron deposition into the developing enameloid, which is probably related to the phylogeny of fish. There was no evidence to support the idea that the iron concentration in the enameloid is associated with the feeding habits of fish, as proposed by previous investigators.

Enamel hypomineralization viewed from the pattern of progressive mineralization of human and monkey developing enamel

Microradiograms and their computer-aided image analysis of ground sections of the developing enamel of human permanent third molars and monkey permanent teeth (Macaca fuscata) indicate that the mode of progressive mineralization of enamel is completely different between the matrix formation and maturation stages. During the former stage, the enamel matrix is slightly mineralized. During the latter stage, which takes a much longer period than the previous stage, the increase in the secondary mineralization takes place first slightly, from the surface toward the inner layer, and then heavily, from the inner layer toward the surface. The narrow outer layer mineralizes very slowly during the middle and late stages of maturation, but finally achieves the highest mineralization of the entire enamel layer. The very narrow innermost layer mineralizes slowly without expanding its width. The former three processes seem to be under the direct control of the ameloblasts. Hypoplastic areas which appear during the matrix formation stages are not necessarily accompanied by hypomineralization. Dysfunction of the cells immediately after the completion of matrix formation appears to cause hypomineralization throughout the entire width of matrix except for the innermost layer. Disorders of the cells occurring during the middle and/or the late stage of maturation--due to chronic metabolic disturbances, such as fluorosis--induced hypomineralization localized mainly at the outer layer. The hypomineralized enamel is not necessarily accompanied by hypoplasia. The process of enamel mineralization is not necessarily fully synchronized with that of tooth eruption. Therefore, the narrow outer layer, especially in the fissure and cervical regions, is sometimes hypomineralized even after the teeth have erupted normally.

Cyclical incorporation of 33P into rat incisor enamel in vivo as visualized by whole-mount radioautography

Phosphorus uptake during amelogenesis was investigated in the continuously erupting rat incisor. Five minutes after intravenous injection of 33P-labelled ortho phosphoric acid, whole-mount radioautography of entire incisors revealed heavy labelling in the form of bands and narrow parallel stripes at the surface of the enamel in the maturation zone. There was relatively little labelling over enamel in the secretion zone and over pigmented enamel. Thus 33P is incorporated cyclically into maturing enamel and is visualized as (1) a banded pattern that reflects the modulation of ruffle-ended and smooth-ended maturation ameloblasts and (2) a striped pattern that reflects the distribution of newly-formed protein secreted by maturation ameloblasts. Presumably these P incorporation patterns are closely related to other cyclical events known to occur during enamel maturation.