Polarized light microscopy of enamel structure in incisors from newborn infants.

Enamel structure in human primary incisors was histologically examined in order to compare the observations with those from recent studies on incipient carious and fluorosed enamel. Ground sections from mandibular incisors of five full-term human infants were examined in polarized light using aqueous media (water, Thoulet's media), alcohols with different refractive indices, and quinoline. Imbibition graphs were made along traverses from the amelodentinal junction (ADJ) to the enamel surface. Quantitative assessments of the birefringence in different media along the traverses indicated a change in the relative volumes of crystallites and surrounding medium with a continuous growth of crystallites toward the ADJ. The influence of organic material is discussed in relation to the differences in calculated form birefringence observed within the specimens. When examined in quinoline, a 30-40-mum-thick surface zone appeared distinct positively birefringent, indicating that the outermost enamel reacted as a molecular sieve. Because of the lower content of minerals the interpretation of this zone differed from that of similar "dark zones" observed in carious and fluorosed enamel.