[Electron microscopic research on the structures of exposed cementum]

In this study the ultrastructure of exposed cementum just below the cemento-enamel junction was investigated. In a surface layer of exposed cementum measuring up to 4 microns in depth, newly formed crystallites were observed. These crystallites were 660-1800 A long and 130-260 A thick. They were thicker than those in the deeper zones of exposed cementum and in sound cementum. Electron diffraction studies showed that these crystallites were hydroxylapatite. In the zone of new crystallites at the tooth surface the crossbanding of collagen fibers was not discernible. At the tooth surface carious changes were seen only in minor areas. Another finding was the formation of large crystallites. They were found to have the following dimensions: length 3000 A, thickness 550 A.