Structural and calcification patterns of the neonatal line in the enamel of human deciduous teeth

The neonatal line of the enamel in human deciduous teeth was observed by scanning electron microscopy using secondary and backscattered electron signals. The neonatal line containing irregular structures of enamel prisms with disordered crystal arrangements was basically formed by the abrupt bending of the prisms towards the root. Usually, the prisms gradually bent back again to regain their previous orientation, but the prisms in the inner and the surface layer sometimes ran straight ahead after bending. The prism sheath regions showing hypocalcification contained a relatively large amount of organic material as shown by treatment with chromium sulfate. When etched with ethylenediaminetetraacetic acid (EDTA), such prism sheath regions were extensively eroded due to the lower density of crystals caused by the abrupt bending of the prisms. Abnormally shaped prisms and small prismless areas were occasionally present. The neonatal line formed by the double bending of the prisms is likely to represent a particular type of rhythmic Retzius line rather than a pathologic Retzius line, whereas the hypocalcified sheath regions may be similar to those of some pathologic lines of Retzius.