ABSTRACT
Experiments using pigs and dogs have confirmed a circadian rhythm during both enamel (Mimura, 1939) and dentine formation (Yilmaz et al., 1977), and there is overwhelming circumstantial evidence that points to a daily rhythmic slowing of matrix secretion during the formation of enamel in modern humans (Asper, 1916; Komai, 1942; Boyde, 1963, 1964, 1976). Enamel prisms are formed with alternating constrictions and varicosities along their lengths which reflect this daily rhythm. Enamel cross-striations are another manifestation of this rhythm visible when ground sections of teeth are viewed with light microscopy. Regular density variations in atomic number contrast along the length of the enamel prisms have the same periodicity as cross-striations (Boyde, 1976; Boyde and Jones, 1983) and so demonstrate that cross-striations are not likely to be optical artifacts. Scanning electron microscopy (SEM) of replicas of fractured surfaces of fossil hominid teeth occasionally reveal alternating enamel prism varicosities and constrictions (Fig. 2.1), and when these can be counted over the whole of an axially fractured surface it is possible to estimate the time of enamel formation in these teeth (Beynon and Dean, 1987). When more than one tooth from a single individual can be sectioned for light microscopy, counts of cross-striations can be continued from one growing tooth to another and used to estimate an age at death (Boyde, 1963, 1964).
