A scanning electron microscope study of the three-dimensional extent of Retzius lines in human dental enamel

A method involving carefully planned sectioning and grinding of human maxillary first premolars, allowing enamel structure to be studied with the SEM in more than one plane at the same time, was employed to demonstrate the extent of Retzius lines within the confines of the three-dimensional enamel cap. It was found that the Retzius lines behaved as could be predicted by the theory stating that the striae represent one-dimensional cuts of two-dimensional, curved growth planes. Retzius lines in adjoining planes were continuous across the edge where the two planes met. In a supernumerary tooth, the surface of its hypoplastic tooth tip was continuous with distinct Retzius lines. The present observations provide for the first time direct morphologic verification of the supposed nature of Retzius lines, with reference to extent and continuity in three-dimensional space.