Localization of cellular calcium in differentiating ameloblasts and its relationship to the early mineralization process in mantle dentin and enamel in hamster tooth germs in vitro

Determinants and mechanisms of enamel fluorosis

Enamel fluorosis occurs when fluoride concentrations in or in the vicinity of the forming enamel are excessive during its pre-eruptive development. Fluoride concentrations in plasma, enamel and other tissues reflect the difference between intake and excretion, i.e. fluoride balance. In addition to the diet, modern sources of ingested fluoride include a variety of dental products, some of which have been identified as risk factors for fluorosis. Fluoride absorption is inversely related to dietary calcium which, at high concentrations, may cause net fluoride secretion into the gastrointestinal tract. The excretion of absorbed fluoride occurs almost exclusively via the kidneys, a process which is directly related to urinary pH. Thus, fluoride balance and tissue concentrations and the risk of fluorosis are increased by factors such as high protein diets, residence at high altitude, and certain metabolic and respiratory disorders that decrease pH. Factors that increase urinary pH and decrease the balance of fluoride include vegetarian diets, certain drugs and some other medical conditions. Although several other fluoride-induced effects might be involved in the aetiology of fluorosis, it now appears that inhibition of enzymatic degradation of amelogenins, which may delay their removal from the developing enamel and impair crystal growth, may be of critical importance. In addition to the effects of fluoride, disturbances in enamel formation that can be confused with fluorosis are caused by chronic acidosis and hypoxia independently of the level of fluoride exposure.

Structure and organization of odontoblasts

Differentiation of odontoblasts involves cell-to-cell recognition, contact stabilization involving the formation of attachment specializations, cytoplasmic polarization, development of the protein synthetic and secretory apparatus, and the active transport of mineral ions. The secretory odontoblast is characterized by an extensive rough-surfaced endoplasmic reticulum, a highly developed Golgi complex, and the presence of specific secretion granules. Type I collagen, a major constituent of dentin matrix, appears to be secreted by the odontoblast into predentin at the proximal portion of the odontoblast process, the major cytoplasmic process extending from the odontoblast cell body into the dentin. The odontoblast process contains a rich network of microtubules and microfilaments. The proximal portion of the process is also a site of fluid-phase endocytosis. Adjacent odontoblasts are held together by numerous macula adherens junctions and a well-developed distal junctional complex adjacent to be predentin. Junctional strands of the occludens type have been observed to be a component of this junctional complex. Tracer studies employing horseradish peroxidase indicate that this junctional complex does not form a tight barrier to the diffusion of tissue fluid from the interodontoblast spaces into the predentin. Many well-developed gap junctions are formed between adjacent odontoblasts and between odontoblasts and the fibroblasts that make up the subodontoblastic layer. Ca-ATPase activity is demonstrated in the Golgi complex and mitochondrial cristae and along the distal plasma membranes of odontoblasts. ALPase activity is also intense along the entire odontoblast cell surface. The osmium tetroxide-pyroantimonate technique for calcium localization demonstrates prominent reaction precipitates in mitochondria of odontoblasts. Energy-dispersive x-ray microanalysis of anhydrously fixed and processed odontoblasts detected Ca and P peaks throughout the cytoplasm. A sulfur peak is noted in the distal cytoplasm of odontoblasts and in matrix vesicles. Together, these results demonstrate the complexity and variety of cell functions involved in dentinogenesis.

Degradation of hamster amelogenins during secretory stage enamel formation in organ culture

Increasing amelogenin heterogeneity during pre-eruptive enamel formation has been explained by proteolytic cleavage of a parent amelogenin, differences in posttranslational modifications, translation of multiple alternative spliced mRNA transcripts or combinations of these possibilities. We investigated the possibility of proteolytic degradation of amelogenins during secretory amelogenesis by pulse-labelling amelogenins with [3H]proline followed by a pulse chase, all under organ culture conditions. The results indicate that during pulse chase, hamster molar tooth explants rapidly released substantial amounts of the radioactivity into the culture medium, as non-trichloroacetic-acid precipitable, noncollagenous 3H-activity at the expense of radioactivity associated with the proteins in the enamel space. Simultaneously, there was a continuous mineralization of the forming enamel in vitro as shown by an increase in total calcium content of the explants. Western blotting, microdissection studies and fluorography of radiolabelled matrix proteins after SDS-PAGE indicated that after an 8-h labelling, three radioactive amelogenin species could be extracted from forming enamel, one prominent species of molecular mass 26 kDa and two less prominent ones of 28 and 22 kDa. During pulse chase more amelogenin bands with lower molecular mass became apparent, a pattern similar to that observed in vivo. Examination of amelogenin blots with the glycan assay showed that none of the hamster amelogenins stained for carbohydrate. We conclude that changes in the amelogenin profiles during enamel development of cultured hamster explants are similar to those observed in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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)

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.