High resolution electron microscopy of young apatite crystals in human fetal enamel.

Apatite crystal growth in enamel has been followed during the secretory stage in 5 month-old human fetuses. The crystals were observed in a high resolution transmission electron microscope operating at 300 kV. This microscope has a Scherzer resolution of 0.19 nm and for each micrograph, the absence of astigmatism and drift was checked with optical diffractograms. Crystals in various stages of growth from approximately 3 to 10 unit cells in thickness could be observed along [0001] and mean value of 1010 zone axes. For growing crystals, a highly ordered atomic structure with an ionic distribution consistent with hydroxyapatite was noted as demonstrated by the correspondence of computer simulated and observed images. Moreover, most of the crystals showed structural defects such as edge dislocations and periodically repeated strain fields created by screw dislocations. The observed dislocations were described by Burgers vectors running parallel to mean value of 1120 axes.