Observation of the loss of the hydroxyapatite sixfold symmetry in a human fetal tooth enamel crystal.

Abstract

A deviation from the hydroxyapatite hexagonal symmetry of a human tooth enamel crystal observed by high-resolution electron microscopy is reported. This symmetry deviation is characterized by: (1) 'preferential' planes that can be indexed as (100) with an intensity that differs from the (300) and the other (100) hexagonal equivalent planes; and (2) streaking of higher order reflections in the optical diffractogram of the image of the crystal. Computer simulations show that similar 'preferential' planes can also be observed at specific crystal tilt angles (and/or beam tilt and/or objective aperture misalignment) and at crystal thickness/microscope defocus values in images of hydroxyapatite crystals observed along the [0001] or [2243] zone axes. The streaking of higher order reflections in the optical diffractogram is related to a deformation of the crystal itself and does indeed show a symmetry deviation of the crystal under observation.