Evidence that apatite crystals of rat incisor enamel have hexagonal cross sections

Images of nearly cross sectioned enamel crystals obtained with the transmission electron microscope were examined in detail and correlated with various possible cross-sectional shapes of the crystals. The following images were seen: 1) elongated hexagons with sharp outlines, maximal density, and minimal size (type A profile); 2) elongated hexagons with one pair of long sides of high contrast, two pairs of short sides of low contrast, high density in the center of the image, and low density towards the low contrast sides (type B profile); 3) slanted hexagons with one pair of high contrast sides, two pairs of low contrast sides (one pair of long, one pair of short sides), and the density decreasing from the center towards the low contrast sides (type C profile); this profile was seen more frequently than the type B profile; and 4) octagons with low contrast edges all around, the density decreasing from the center towards the edges (type D profile). Types B, C, and D profiles were larger than type A profiles. Similar crystal images have been described in the literature. When tilting a specimen in increments of 6 degrees, the transformation of type A to type C and type C to type D profiles was observed. Many crystal images did not change their shape greatly when tilting the section through 6 degrees. No rectangular or rhomboidal profiles were seen. It is argued that types A, B, C, and D profiles cannot be generated by apatite crystals with a parallelepiped cross section. They are easily explained, however, on the basis of crystals with hexagonal cross section.