Fine incremental lines observed on the enamel surfaces with a prismless structure in human teeth after phosphoric acid etching.

In human tooth enamel, the surface prismless layer contains liner laminas homologous to the cross-striations of enamel prisms other than the Retzius lines, while these lines terminate on the perikymata or run in parallel with the enamel surface. In our present scanning electron microscope observations after phosphoric acid etching, many fine incremental lines (FIL) running transversely were found on the inner occlusal enamel with a prismless structure in a young permanent premolar. Almost the same structures were also found on the upper-coronal enamel with scattered prismless and indistinct prism structures in an exfoliated deciduous molar, and the FIL occasionally went across the indistinct prisms or caused the lower half of the prisms to disappear. Their intervals ranged from about 0.5 to 2 microns; less than the diameter of the prisms. Such results suggest that the FIL are not the termination of the laminate striations in the surface prismless layer, but that the previously unnoticed patterns sometimes formed by the sequential completion of the activity of ameloblasts which interact with each other in a transverse row at the final stage of matrix formation.