Changes in Acid-phosphate Content in Enamel Mineral during Porcine Amelogenesis

Abstract

The present study was undertaken to investigate changes in the acid-phosphate content of porcine enamel mineral during its development and to assess separately the HPO₄ ^2- pools in labile and stable forms. Enamel samples at the secretory and maturing stages of amelogenesis were obtained from the permanent incisors of five- to six-month-old slaughtered piglets. Human enamel from erupted, extracted teeth, synthetic hydroxyapatite, and carbonatoapatite containing acid phosphate were included as references. The acid-phosphate content of each sample was determined chemically through its pyrolytic conversion to pyrophosphate. The assessment of HPO₄ ^2- in labile forms was made by analysis of samples pre-equilibrated with solutions containing 3 mmol/L phosphate at pH 11 1 (to de-protonate the HPO₄ ^2- species on crystal surfaces). The analytical results of porcine enamel samples showed that: (a) the outermost secretory (youngest) enamel contained the highest HPO₄ ^2-, corresponding to about 16% of the total phosphate; (b) the acid-phosphate content decreased gradually to 10% in the inner (older) secretory and to 6% in the maturing tissue; (c) a substantial part of the HPO₄ ^2- in developing enamel tissue (50-60% of the HPO₄ ^2- for the secretory enamel) was in labile forms; and (d) the pool of the labile HPO₄ ^2- decreased with the growth of enamel mineral. In parallel studies with mature human enamel, it was ascertained that the total acid phosphate was only about 3% of the total phosphate, much lower than in developing porcine enamel, and that the labile pool of HPO₄ ^2- was also small, corresponding to about 15% of the total acid phosphate determined. The overall results indicate that acid phosphate is one of the major constituents of the early enamel crystals formed during amelogenesis and that the surfaces of the growing enamel crystallites are rich in HPO₄ ^2-.