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DENTAL FLUOROSIS: CHEMISTRY AND BIOLOGY

¹ The Nippon Dental University, Department of Pathology, 1-9-20 Fujimi, Chiyoda-ku, Tokyo 102, Japan; ² The Royal Dental College, Faculty of Health Sciences, Aarhus University, Vennelyst Boulevard, 8000 Aarhus C, Denmark;

View larger version (44K): [in a new window]   Figure 1. Schematic illustration of the events relevant to early enamel mineralization. The ameloblasts regulate mass transport (e.g., ions and organic matter) from blood circulation to the extracellular space and vice versa. The mineralizing environment is comprised of the secreted matrix proteins and proteases, and varieties of ions and soluble moieties. Post-secretory processing of the matrix proteins yields soluble moieties prior to removal from the forming enamel. Tissue organization is guided by the molecular architecture of the matrix proteins, while the driving force for precipitation is determined by activities of common ions in the enamel fluid. The resulting crystal formation is characterized by the initial precipitation of acidic precursors, thin ribbons in morphology, and the consecutive epitaxial overgrowth of carbonato-apatite.

View larger version (53K): [in a new window]   Figure 2. Schematic illustration of ion transport through the enamel organ layer and associated extracellular events taking place in the forming enamel. Ions transported through the cell layer are incorporated into the lattice positions. The precipitation rate is determined by the degree of supersaturation, fluid volume, crystal surface areas, protein coating, and the presence of inhibitors (e.g., carbonate and Mg2+) and promoter, i.e., fluoride. As a result, the Ca2+ concentration in the fluid ([Ca]s) surrounding the forming crystals is determined in principle by the balance between the rates of Ca2+ supply (RD) and consumption for crystal precipitation (RC). Proteolytic processing of the amelogenins and other proteins, possibly in a form of nanosphere aggregates, may depend on the [Ca]s levels.

View larger version (106K): [in a new window]   Figure 3. Schematic illustration of experimental set-up utilizing a diffusion chamber, which was developed to assimilate the in situ situation as depicted in Fig. 2. The reaction chamber was divided into two (exterior and interior) compartments (each 1.5 mL in volume) by an ultrafiltration membrane (Spectra/por 3, M.W. cut-off 3500). Hydroxyapatite seed crystals, with or without protein coating, were placed in the interior (lower) compartment, which was separated from the exterior compartment by an ultrafiltration membrane. The concentrations of the total PO4 (3 mM) and NaCl (160 mM) as a background electrolyte were the same between the exterior and interior solutions. The pH value of both solutions was adjusted at 7.3 ± 0.2. Fluoride was added to the solution to yield concentrations of 0.05 to 1.0 ppm, as well as 0 ppm by no addition of NaF. The supersaturated solution contained 1 mM CaCl2 and was delivered at constant rates in the exterior compartment. No calcium was added initially to the experimental solution placed in the interior compartment. In this set-up, Ca2+ ions were allowed to diffuse through the membrane into the interior compartment according to the concentration gradient.

View larger version (18K): [in a new window]   Figure 4. Changes in Ca concentration ([Ca]in) in the solution surrounding the seed crystals as a function of time after delivery of the supersaturated solution. Following the Ca transport along the concentration gradient between the exterior and interior compartments ([Ca]ex > [Ca]in), a plateau of the [Ca]in was attained after 12 hrs of equilibration. The plateau levels, [Ca]s, are markedly lowered by the presence of seed crystals and the addition of fluoride.

View larger version (19K): [in a new window]   Figure 5. Plots of the steady-state Ca concentration ([Ca]s), which was determined analytically after 30 hrs of equilibration in systems with various fluoride concentrations ([F]ex). The [Ca]s decreased almost linearly when [F]ex was increased in both systems with and without protein coatings of the seed crystals. In both systems, increasing the fluoride concentration resulted in the retention of [Ca]s at lower levels. The data with deviation bars were obtained from triplicate experiments, while the data without the bars were obtained from duplicate experiments. Note that enamel proteins substantially modified the [Ca]s, by reducing the Ca consumption for precipitation.