Using SEM after EDTA etching, the mid-coronal inner enamel of human permanent teeth was classified into three regions of the 1st, 2nd, and 3rd zones. The 1st zone showing a highly negative birefringence was the innermost 10 - 15 μm enamel. This zone consisted of arcade and circular initial prisms, and the succeeding arcade prisms only. These initial prisms arising perpendicularly to the dentine surface resembled pseudoprisms because these prisms showed a somewhat centripetal arrangement of crystal lites and indistinct prism boundaries. The succeeding prisms were frequently bent following a faint slit within the prism. The 2nd zone adjacent to the 1st zone measured 20 - 40 μm in thickness. This zone was mainly composed of horseshoe-shaped prisms with EDTA-insoluble prism sheaths in the deep-etched prism boundaries, but the inner-half layer had dotted irregular prisms including circular, double marginal, and spiral shapes with the prism sheaths. Prismless structures were rarely seen in the 2nd zone. The 3rd zone was mainly occupied by horseshoe-shaped prisms without EDTA-insoluble prism sheaths in the deep-etched prism boundaries, although tuft prisms in the 3rd zone contained a large amount of EDTA-insoluble substances in the prisms, interprismatic regions, and the boundaries.
Kodaka, T.; Kuroiwa, M.; and Abe, M.
"Fine Structure of the Inner Enamel in Human Permanent Teeth,"
Scanning Microscopy: Vol. 4
, Article 12.
Available at: https://digitalcommons.usu.edu/microscopy/vol4/iss4/12