Utilization of periodic markings in enamel to obtain information on tooth growth

Enamel retains two fundamental periodic markings: the prism cross-striations, marking a diurnal rhythm of prism elongation, and the striae of Retzius, a longer-term marker of formation of the enamel surface. It is argued that, while artefacts superficially resembling prism cross-striations can be observed, these should not mislead an experienced histologist and that true cross-striations are reliable markers of the daily incremental apposition of enamel. The incremental markings can be used to elucidate several aspects of tooth crown growth. (1) Rate of enamel apposition. In some teeth, e. g., hominoid permanent teeth, the enamel apposition rate increases progressively between the inner and outer enamel, while in others the apposition rate appears to be uniform. (2) Crown formation time. This can be estimated on intact teeth, using counts of perikymata (surface traces of the striae), together with information on the time interval between successive striae. Much better estimates can be obtained from sections, using counts of cross-striations. (3) Pattern of crown formation. The rate at which new enamel formation extends over the presumptive enamel-dentine junction (extension rate) can be estimated from knowledge of the cross-striation interval together with angular measurements of prisms and striae. Multiple measurements of the extension rate within a tooth can be used to provide an estimate of crown formation time. The main application of extension rate measurements is to give quantitative information on the pattern of crown formation. In large primate teeth, the extension rate is high early in crown formation and slower later in crown formation. In smaller, more rapidly-formed teeth, a gradient in extension rate appears to be absent. Enamel thickness is negatively correlated with extension rate among primates. This seems to be a mechanism ensuring that there is sufficient time for enamel maturation to be completed before eruption takes place. Among primates it appears that variation in the prism pattern (i.e., the extent to which the outline of the pattern 3 prisms is closed or open) is correlated with the extension rate. It is postulated that the extension rate controls prism shape by influencing the morphology of the Tomes process pits during their initial formation.