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

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Abstract
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 1.9–2.4×105 g for 1–1.5 hours. The yields of the fluid were 44.8±2.3 (mean ± standard error) μl/g of enamel tissue at 1.9×105g, and 53.9±1.9 μl/g at 2.4×105 g. A significant finding was that the total Ca concentration of the enamel fluid (3.9–6.0×10−4 M) was lower than that of porcine serum (2.9×10−3 M), reflecting a distinct, compartmentalized microenvironment, isolated from the circulating blood. Another significant finding was that the ionic calcium activity (5.3×10−5 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−], 5×10−3 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. The free CA2+, calculated from the Ca2+ activity obtained by a Ca electrode, was 0.153 mM, suggesting that substantial amounts of calcium were bound, presumably by proteins or other binding species. The degree of saturation (DS) of enamel apatite, and other related calcium phosphates, was calculated based on the analytical results. The DS of the fluid with respect to enamel apatite indicated that the separated fluid was essentially in equilibrium with the existing crystals. The results suggest that enamel mineralization in the secretory stage may be regulated by a limited calcium supply through the ameloblast layer and the low level of calcium activity resulting from binding by proteins derived from the original organic matrix.