Fig. 7
Aprismatic enamel at the tooth cusp surface. Notice that the aprismatic enamel is indistinguishable from the interrod, it just does not have any rods (previously termed prisms, whence the name of this aprismatic layer). See Supplementary Fig. 3 for the precise position of this region in the tooth. a PIC map of aprismatic enamel, showing that nearly all crystals are green, thus their c-axis is oriented at +30°. b–f SEM images of the same region after etching. b SEM image at precisely the same magnification as the PIC map in a, with magenta arrows indicating a hole in the tooth surface, infiltrated with epoxy, which resisted etching, and two rods. Arrows in a point to the same features before etching. c SEM image of the same region at lower magnification. The arrows were scaled down with the image, and indicate precisely the same features. d–f Increasingly magnified images of etched aprismatic enamel. The blue arrow in panels b–f indicate a feature visible in all SEM images and well resolved in d–f. Panels e and f clearly show that all crystals are aligned parallel to one another and perpendicular to the tooth surface. Panel e shows this as the elongation direction, which is −36° from the vertical. Their green color in b indicates a c-axis orientation of +30° from the vertical (also shown in e), thus the c-axes are 66° apart from the elongation direction, or 24° apart from the tooth surface. Thus, the crystalline c-axes are approximately parallel—not perpendicular—to the tooth surface. Supplementary Fig. 12 shows more PIC maps of the aprismatic layer in another tooth, confirming that crystalline c-axes are oriented randomly with respect to the tooth surface
