Whole enamel scrapings from teeth of embryonic calves have been separated by density gradient centrifugation in organic solvents into fractions (1.6 < p < 2.4 g/cm3) representing progressive stages of mineral phase maturation. Single enamel particles or their small aggregates from such fractions were examined by transmission electron microscopy, electron diffraction, and high-spatial-resolution electron probe micro-analysis. The electron optical methods demonstrated the presence of poorly crystalline hydroxyapatite as the only detectable solid phase in all fractions. Octacalcium phosphate and brushite were not identified in the fractions. Changes in electron diffraction patterns were indicative of a progressive increase in apatite crystallinity with enamel maturation. Molar Ca/P ratios were found to range from 1.48 to 1.70, with a higher mean value obtained for lower-density fractions (p < 2.0 g/cm3). Lower-density fractions contained some particles with high ratios (~2.0-4.0) and a non-uniform distribution of Ca and P, as revealed by electron probe mapping. These characteristics are suggested as possibly being related to carbonate phases in early enamel.
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