Calcium distribution in freeze-dried enamel organ tissue during normal and altered enamel mineralization

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on the rates at which matrix proteins break down and/or the rates at which the by-products from this degradation are withdrawn from the maturing enamel.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.

Calcium transport across the dental enamel epithelium

Dental enamel is the most highly calcified tissue in mammals, and its formation is an issue of fundamental biomedical importance. The enamel-forming cells must somehow supply calcium in bulk yet avoid the cytotoxic effects of excess calcium. Disrupted calcium transport could contribute to a variety of developmental defects in enamel, and the underlying cellular machinery is a potential target for drugs to improve enamel quality. The mechanisms used to transport calcium remain unclear despite much progress in our understanding of enamel formation. Here, current knowledge of how enamel cells handle calcium is reviewed in the context of findings from other epithelial calcium-transport systems. In the past, most attention has focused on approaches to boost the poor diffusion of calcium in cytosol. Recent biochemical findings led to an alternative proposal that calcium is routed through high-capacity stores associated with the endoplasmic reticulum. Research areas needing further attention and a working model are also discussed. Calcium-handling mechanisms in enamel cells are more generally relevant to the understanding of epithelial calcium transport, biomineralization, and calcium toxicity avoidance.

Localization of plasma membrane Ca2+ pump mRNA and protein in human ameloblasts by in situ hybridization and immunohistochemistry

The distribution of the plasma membrane Ca(2+)-pump (PMCA) proteins in human ameloblasts was examined immunohistochemically using monoclonal antibodies JA8 and 5F10. Further, the distribution of mRNA transcripts derived from two PMCA genes, PMCA-1 and PMCA-4 was examined using in situ hybridization. In rats, the PMCA-1 gene is purported to code for PMCA proteins with a role in maintaining the intracellular Ca2+ levels in nonepithelial cells. Other genes including the PMCA-4 gene may code for PMCA proteins characteristic of Ca2+ transporting epithelia. The present results show immunohistochemical staining in the Tomes processes and plasma membranes of human ameloblasts. Our studies also demonstrate a gradation of expression of the PMCA-1 and PMCA-4 mRNA transcripts which parallels the onset and progression of enamel mineralization. These studies suggest that PMCA proteins in human ameloblasts may function both in intracellular Ca2+ homeostasis and in regulating the vectorial Ca2+ influx into mineralizing enamel.

Developmental pattern and subcellular localization of parvalbumin in the rat tooth germ

The EF-hand calcium-binding protein parvalbumin has been extensively studied in nerve and muscle cells. Its possible role in biomineralization during tooth development was here investigated by determining its subcellular localization by immunogold cytochemistry. The developmental sequences of amelogenesis and dentinogenesis were studied in rat molars, and in continuously growing rat incisors. The findings confirm that parvalbumin is a nuclear and a cytosolic protein, not associated with any particular intracellular organelle. Epithelial and mesenchymal undifferentiated cells contained no specific parvalbumin immunolabelling. In differentiated ameloblasts, secretory-pole (Tomes' process) formation was associated with a proximal-distal gradient of parvalbumin labelling. But after the Tomes' process had formed, parvalbumin was evenly distributed throughout the cell. The parvalbumin contents of ruffle-ended and smooth-ended ameloblasts appeared to be very different. Differentiated odontoblasts were less heavily labelled than ameloblasts, and the label was restricted to the cell body during the whole of dentinogenesis. These data suggest that parvalbumin could contribute to membrane plasticity during differentiation, as shown during dendritic growth in the nervous cells. Moreover, as may occur in excitable cells, parvalbumin could buffer calcium specifically in the cells producing mineralized enamel and dentine during the later stages of tooth development.

Histochemical, ultrastructural, and electron microprobe analytical studies on the localization of calcium in rat incisor ameloblasts at early stage amelogenesis

The enamel organs of rat incisors were separated from the enamel surface and processed for rapid freezing and freeze-substitution. A histochemical stain for calcium (GBHA) of thick Epon sections revealed intense calcium reactions in the secretory ameloblasts, exclusively in the tubulovesicular structures extending throughout their distal cytoplasm. Electron microscopy revealed a thin layer of amorphous material with clusters of electron-dense granules along the distal surface of secretory ameloblasts. In young secretory ameloblasts without typical Tomes' processes, a considerable number of mitochondria were located in the distal cytoplasm and contained numerous electron-dense granules. Similar dense granules as well as fine ribbon-like electron-dense figures, all containing significant amounts of calcium, were observed in some of the tubulovesicular structures at the distal end of these cells. A putative exocytotic figure of such dense granules was also observed. The electron-dense granules were rare in more differentiated ameloblasts with elongated Tomes' processes, which occasionally displayed ribbon-like figures in some of the tubulovesicular structures in the process region. No significant calcium peak was detected in the extracellular amorphous material, secretory granules, or along the lateral plasma membranes. These observations may imply high calcium concentrations in mitochondria and tubulovesicular structures in the distal cytoplasm of secretory ameloblasts relative to that of the cytosol and support the possible contribution of these organelles in secretory ameloblasts to cellular calcium regulation at least in the early stage of amelogenesis.

Changes in the mode of calcium and phosphate transport during rat incisal enamel formation

The distribution of 45Ca, 32PO4, 22Na, and calcein in the freeze-dried sections of rat lower incisor was examined. Also, the ratio of 45Ca to 32PO4 transported into the enamel at various developmental stages was studied after the simultaneous injection of 45Ca and 32PO4. The distribution of calcein fluorescence indicated the presence of an extracellular route from capillary to enamel in the areas of both the secretory and smooth-ended ameloblasts. Autoradiograms showed that the 45Ca incorporation into the enamel in the smooth-ended ameloblast region was higher than that into the secretory enamel, and a remarkably high incorporation was observed in the enamel of the apical two-thirds of the ruffle-ended ameloblast region. Although the 32P incorporation into the enamel of the smooth- and ruffle-ended ameloblast region was higher than in the secretory enamel, the differences between these two regions were not so evident as that observed in the case of 45Ca. The high labeling of 45Ca and 22Na was observed in the apical two-thirds of the ruffle-ended ameloblasts. The 45Ca/32PO4 ratio in the secretory enamel was significantly lower than that in the blood, but in the enamel of the smooth-ended ameloblast region the ratio was not significantly lower. Contrarily, the ratio in the enamel of the ruffle-ended ameloblast region was much higher than that in blood. These results indicate that the mode of transport of these ions into enamel is altered in relation to the morphological changes of the ameloblasts.

Effect of cobalt on Ca2+-Mg2+ ATPase in rat incisor maturation ameloblasts

Rat incisor maturation ameloblasts were studied to determine the effect of injected cobalt on the distribution and intensity of Ca2+-Mg2+ ATPase. The dosage of cobalt utilized temporarily inhibits enamel mineralization and alters ameloblast-associated calcium. A modified Wachstein-Meisel medium containing cerium as the capturing ion was used to localize Ca2+-Mg2+ ATPase cytochemically. The distribution and intensity of the reaction product in normal maturation ameloblasts was, as previously reported, primarily in association with the plasma membranes. The lateral cell membranes of both smooth-ended and ruffle-ended ameloblasts were reactive. The ruffled border region contained the heaviest concentration of reaction product. Although cobalt did not alter the general pattern of distribution of the reaction product in either cell type, in all regions of activity the intensity was noticeably increased. Cells modulating from smooth-ended to ruffle-ended ameloblasts and under the influence of cobalt exhibited an irregular dense layer along the enamel surface, and large focal accumulations of electron-dense material in the various extracellular compartments. This may indicate interference with a putative resorptive activity of these cells.

X-ray micro-analysis of the mineralization patterns in developing enamel in hamster tooth germs exposed to fluoride in vitro during the secretory phase of amelogenesis

The developing enamel from three-day-old hamster first maxillary (M1) molar tooth germs exposed to fluoride (F-) in vitro was analyzed for its mineral content by means of the energy-dispersive x-ray microanalysis technique. The aim of this study was to obtain semi-quantitative data on the F(-)-induced hypermineralization patterns in the enamel and to confirm that the increase in electron density observed in micrographs of F(-)-treated enamel (Lyaruu et. al., 1986, 1987b) is indeed due to an increase in mineral content in the fluorotic enamel. The tooth germs were explanted during the early stages of secretory amelogenesis and initially cultured for 24 hr in the presence of 10 ppm F- in the culture medium. The germs were then cultured for another 24 hr without F-. In order to compare the ultrastructural results directly with the microprobe data, we used the same specimens for both investigations. The net calcium counts (measurement minus background counts) in the analyses were used as a measure of the mineral content in the enamel. The aprismatic pre-exposure enamel, deposited in vivo before the onset of culture, was the most hypermineralized region in the fluorotic enamel, i.e., it contained the highest amount of calcium measured. The degree of the F(-)-induced hypermineralization gradually decreased (but was not abolished) in the more mature regions of the enamel. The unmineralized enamel matrix secreted during the initial F- treatment in vitro mineralized during the subsequent culture without F-. The calcium content in this enamel layer was in the same order of magnitude as that recorded for the newly deposited enamel in control tooth germs cultured without F-.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium transport by a calmodulin-regulated Ca-ATPase in the enamel organ

Using aldehyde-fixed rat incisor enamel organ, we localized Ca-ATPase activity ultracytochemically in the plasma membranes, the mitochondrial inner membranes, and the Golgi membranes of secretory ameloblasts and the cells of stratum intermedium at the secretory stage and papillary layer cells at the maturation stage, but not in maturation ameloblasts. This Ca-ATPase activity was totally dependent on substrate ATP, the enzyme activator CaCl2, and also sensitive to the specific calmodulin blocker trifluoperazine (TFP) in the incubation media. Specific antigenic sites of endogenous calmodulin were demonstrated in polyribosomes, the nucleus, mitochondria, and the cytoplasmic matrix along the plasma membranes of secretory ameloblasts, by the protein A-immunogold technique using sheep antiserum against bovine testis calmodulin. All other enamel organ cells-such as stratum intermedium, papillary layer cells, and maturation ameloblasts-were also weakly immunoreactive. In control sections incubated with antiserum pre-absorbed with an excess of calmodulin and protein A-gold complex, only a few gold particles were observed to be randomly associated with the tissues. Daily intraperitoneal injection of TFP (1 and 5 mg per 100 g body weight) for one week resulted in prominent migration of mitochondria from the infranuclear to supranuclear regions of secretory ameloblasts but caused no other morphological alterations in the enamel organ cells. EDX analysis of ultrathin sections revealed significantly lower peaks of Ca and P in the forming enamel of TFP-injected rats than those in controls. However, little reduction in the Ca and P levels in the maturing enamel was observed in TFP-injected rats. When growing enamel surfaces were exposed with NaOCl and examined with SEM, a remarkable defect in the enamel matrix was observed in the forming enamel but not in the maturing enamel. These results suggest that early enamel mineralization is dependent upon an intact calmodulin-regulated Ca-transporting ATPase in secretory ameloblasts and that enamel maturation is controlled by different mechanism(s).

Distribution of calcium and phosphate in cells of the enamel organ in the rat lower incisor

The distribution of calcium and phosphate in the cells of the enamel organ of the rat lower incisors was investigated by autoradiography and energy-dispersive x-ray spectrometry (EDS).

Radioactive calcium or phosphate was injected i.p. into seven-day-old rats of the Wistar strain. The animals were frozen 0.5, 1, and 10 min after injection, and embedded in 5% carboxymethyl cellulose. Sagittal sections of 10 μm thickness were made in which the lower incisor was included as a part of the whole-body section. For autoradiography, the sections were freeze-dried and placed in contact with dry thin films prepared from autoradiographic emulsion. For EDS, sections were mounted on carbon stubs, freeze-dried, coated with carbon, and examined by EDS in a SEM. 45Ca and 32P autoradiograms showed that the radioactivity was located over the papillary layer cells adjacent to the secretory stage ameloblasts and was much higher here than in the ameloblastic layer. On the other hand, there was no significant difference between the amount of radioactivity of these two cell layers in the maturation stage, although higher radioactivity was detectable in the maturation stage enamel than in the secretory stage enamel. Pronounced Ka x-ray peaks were obtained for P, S, Cl, and K originating from the cells of the papillary and ameloblastic layers in the secretory stage, but only very low peaks were obtained for Ca. On the other hand, in addition to these elements, remarkably high Ca and Fe peaks could be detected in the ameloblastic layer of the maturation stage.

The enamel fluid in the early secretory stage of porcine amelogenesis: chemical composition and saturation with respect to enamel mineral

The present study reports on the separation of fluid from soft, "cheeselike" enamel of porcine permanent teeth in the secretory stage, and the determination of its chemical composition. The enamel tissues were dissected from mandibles of 5 to 6-month-old piglets and pooled under mineral oil in centrifuge tubes, and then centrifuged at 1.9-2.4 X 10(5) g for 1-1.5 hours. The yields of the fluid were 44.8 +/- 2.3 (mean +/- standard error) microliter/g of enamel tissue at 1.9 X 10(5) g, and 53.9 +/- 1.9 microliter/g at 2.4 X 10(5) g. A significant finding was that the total Ca concentration of the enamel fluid (3.9-6.0 X 10(-4) M) was lower than that of porcine serum (2.9 X 10(-3) M), reflecting a distinct, compartmentalized microenvironment, isolated from the circulating blood. Another significant finding was that the ionic calcium activity (5.3 X 10(-5) M) in the enamel fluid was one order of magnitude lower than the total Ca concentration. The averaged results of other determinations were: pH, 7.26; total [P], 3.9 mM; [Mg2+], 0.8 mM; [Na+], 140 mM; [K+], 20 mM; [Cl-], 150 mM; [F-], 5 X 10(-3) mM; and osmolality of the fluid, 312 mosmol/kg H2O (in the same range as that of the serum, 310 mosmol/kg H2O). The apparent electrical unbalance of the analytical data, 8.65 meq excess of positive charges, was ascribed to the presence of HCO3- in the fluid; the computed ionic strength was 164 mM.(ABSTRACT TRUNCATED AT 250 WORDS)

Calmodulin blocker inhibits Ca++-ATPase activity in secretory ameloblast of rat incisor

The effects of the calmodulin blocker, trifluoperazine (TEP), on membrane-bound Ca++-ATPase, Na+-K+-ATPase (EC 3.6.1.3.) and the ultrastructure of the enamel organ were investigated in the lower incisors of normal and TFP-injected rats. The rats, of about 100 g body weight, were given either 0.2 ml physiological saline or 100 micrograms TFP dissolved in 0.2 ml physiological saline through a jugular vein and fixed by transcardiac perfusion with a formaldehyde-glutaraldehyde mixture at 1 and 2 h after TFP administration. Non-decalcified sections of the enamel organ less than 50 micron in thickness, prepared from dissected lower incisors, were processed for the ultracytochemical demonstration of Ca++-ATPase and Na+-K+-ATPase by the one-step lead method at alkaline pH. In control saline-injected animals the most intense enzymatic reaction of Ca++-ATPase was demonstrated along the plasma membranes of the entire cell surfaces of secretory ameloblasts. Moderate enzymatic reaction was also observed in the plasma membranes of the cells of stratum intermedium and papillary layer. Reaction precipitates of Na+-K+-ATPase activity were localized clearly along the plasma membranes of only the cells of stratum intermedium and papillary layer. The most drastic effect of TFP was a marked disappearance of enzymatic reaction of Ca++-ATPase from the plasma membranes of secretory ameloblasts, except for a weak persistent reaction in the basolateral cell surfaces of the infranuclear region facing the stratum intermedium. The cells of stratum intermedium and papillary layer, however, continued to react for Ca++-ATPase even after TFP treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultracytochemical demonstration of ATP-dependent calcium pump in ameloblasts of rat incisor enamel organ

The enamel organ of the growing rat incisor was fixed with a mixture of formaldehyde and glutaraldehyde and processed for ultracytochemical demonstration of Ca- and Mg-activated membrane ATPase by a one-step lead technique at alkaline pH. To inhibit nonspecific alkaline phosphatase, 5 mM levamisole was added to the incubation media. Intense Ca- and Mg-ATPase activity was demonstrated in the cell surfaces of the secretory ameloblasts, except at the proximal and distal junctional complexes and the gap junctions in the lateral and basal cell surfaces. Deep plasma membrane invaginations at the proximal and distal parts of Tomes processes facing interrod- and rod-enamel growth regions exhibited the strongest enzymatic reaction. Mg-ATPase activity was also shown to be present in the plasma membranes of secretory ameloblasts but it was less intense than Ca-ATPase. Except for a slight reaction in the Golgi membranes, all other cell organelles of the secretory ameloblasts and the adjacent enamel matrix were free of enzymatic reaction. However, when the tissues were incubated in media lacking levamisole, a prominent enzymatic reaction was observed in the newly secreted enamel matrix of the rod and interrod growth regions as well as on the plasma membranes of the cells. In maturation ameloblasts of both ruffle-ended and smooth-ended types, a weak reaction for Ca- and Mg-ATPase was restricted to basal cell surfaces facing the papillary cell layer. In tissues incubated in media lacking levamisole, a variable deposition of reaction products was observed in the Golgi membranes, mitochondrial membranes, tubular elements of smooth endoplasmic reticulum in the ruffled border zone, and along the plasma membranes of the ruffled border. Throughout the secretory and maturation stages, a moderate and/or weak enzymatic reaction for both Ca- and Mg-ATPase was seen in the plasma membranes of the cells of the stratum intermedium and the papillary layer when incubated in media with levamisole. Omission of substrate ATP and/or the enzyme activator CaCl2 from the incubation media for Ca-ATPase produced a negative reaction in the tissues examined. When the calmodulin blocker trifluoperazine was administered to the rats intravenously, Ca-ATPase activity was almost completely abolished from the plasma membranes of secretory ameloblasts, but not of other cell types.

Ultrastructure of the dental epithelium during enameloid mineralization in a teleost fish, Cichlasoma cyanoguttatum

Secretory-stage inner dental epithelial cells (IDE) of tooth buds deposited an unmineralized, ectodermally-derived, enameloid collagen matrix. Pharyngeal plates bearing tooth buds were fixed: some were demineralized, others treated with guanidine-EDTA, then fixed and post-fixed in osmium tetroxide with potassium ferricyanide. Thin Epon sections were viewed in a Jeol 100B TEM. Nascent enameloid crystals were orientated parallel to the collagen fibres and attained widths of 200 nm. Enameloid collagen was absent in demineralized mature enameloid. The outer dental epithelial plasma membrane was deeply invaginated forming extensive channels associated with elongated fuzzy-coated vesicles. Four configurations of IDE cells were characterized by cellular constituents, including elongated granules, Golgi complexes, multivesicular bodies, large electron-dense granules and extracellular amorphous material which was also adjacent to cells containing few organelles associated with protein synthesis, within infoldings of ruffled apical membranes and multivesicular bodies. This material was considered to be resorbed enameloid collagen, not a secretory product.