Analysis of crystallite shape in rat incisor enamel

The hydroxyapatite crystallites of mammalian enamel appear as hexagons when seen in cross-sections examined with the transmission electron microscope. Using goniometric transmission electron microscopy, stereo-pair electron micrographs and freeze-fracture replicas, two models have been proposed to explain the hexagonal crystallite profile. The "hexagonal ribbon" model proposes that hexagonal profiles are true cross-sections of elongated hexagonal ribbons. The "rectangular ribbon"model proposes that crystallite profiles are three-dimensional rectangular segments (parallelepipeds), which are contained in the Epon sections and project as opaque hexagons in routine transmission electron micrographs. Morphological observations together with predictions from models indicate that the crystallites in rat incisor enamel are flat ribbons with rectangular cross-sectional profiles. The hexagonal images seen in electron micrographs of thin sections of enamel result from viewing parallelepiped-shaped segments of these crystallites as two-dimensional shadows.