Epitaxial overgrowth of apatite crystals on the thin-ribbon precursor at early stages of porcine enamel mineralization

Abstract

The aim of the present work was to investigate changes in cross-sectional morphologies of enamel crystallites as a function of location in secretory porcine enamel. Enamel tissues were obtained from 5- to 6-month-old slaughtered piglets. For examination by electron microscopy, a portion of the secretory enamel was embedded in resin and ultrathin sections were prepared with a diamond knife. In parallel studies, compositional and structural changes of enamel mineral were assessed by chemical analysis and Fourier transform infrared (FTIR) spectroscopy. For this purpose, two consecutive layers of the outer secretory enamel, each approximately 30 μm thick, were separated from the labial side of permanent incisors. Using high-resolution electron microscopy, early events of enamel crystal growth were characterized as the epitaxial growth of small apatite units on the lateral surfaces of the initially precipitated thin ribbon. These apatite units had regular triangle or trapezoid cross-sections. After fusions of those isolated trapezoids on both lateral sides of the platy template, the resulting enamel crystallites had the well-documented flattened-hexagonal shapes in cross-sections. The initially precipitated thin plate was buried inside the overgrown apatite lamella and then retained as a central dark line. Similar morphological evidence for the epitaxial nucleation and overgrowth of carbonatoapatite on the platy template was obtainedin vitro. Chemical and FTIR analyses of the enamel layer samples showed that the characteristics of the youngest enamel mineral were distinct from those of enamel crystals found in older secretory enamel. The overall results support the concept that initial enamel mineralization comprises two events: the initial precipitation of thin ribbons and the subsequent epitaxial growth of apatite crystals on the two-dimensional octacalcium phosphate-like precursor.