Comparative dental development and microstructure of ... - UCL

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170 A. D. BEYNON ET AL. Figure 4. Crown outlines, drawn from ground sections, of the teeth used in the histological part of this study. One section only from each tooth is represented even though several posterior teeth were sectioned more than once (see text). From top left to right through rows 1 and 2: dm 1 ,dm 2 ,I 1 ,I 2 ,M 1 ,M 2 (juvenile P. heseloni) andM 1 . Row 3: canine P 4 ,M 3 (adult Proconsul heseloni). Bottom row: M 1 and M 2 (P. nyanzae). Four tooth sections were reconstructed over the cusps in order to estimate enamel cap area and EDJ length used for the calculations of relative enamel thickness. (None of the linear enamel thickness measurements that appear in Table 1 were made on reconstructed outlines.) Representations of the high power reconstructions are shown with dashed lines as appropriate.
DENTAL DEVELOPMENT IN PROCONSUL 171 prepared from 13 teeth. Ground sections were first examined in polarized transmitted light, then in reflectance mode with a Leica laser confocal microscope at key locations. Measurements in this study were made both from high power photomontages of the tooth sections and also directly using a Zeiss Filar micrometer eyepiece. Enamel thickness For several unworn anterior and posterior teeth, of both P. heseloni and P. nyanzae, it was possible to make linear measurements of enamel thickness. For some other teeth, as in previous studies on enamel thickness, minor reconstructions on tracings of crown outlines were possible, either at the cervix or at the cusp tips to correct for damage or wear. It was then possible to make additional estimates of the area of the enamel cap, the dentine cap and the length of the EDJ in four slightly worn or damaged teeth. Measurements of enamel thickness were made in several ways that reflect previous studies on enamel thickness and which therefore allow comparison with the results of these studies. Linear measurements of enamel thickness were made on teeth where there was no occlusal wear, in the way detailed by Beynon & Wood (1986) and in Figure 1 of Macho & Berner (1994), but were made here on both mandibular and one maxillary tooth. Measurements 1 and 8 were omitted as they are not directly comparable in upper and lower teeth. Andrews & Martin (1991) present data for enamel thickness in P. africanus and P. major. Two measurements of enamel thickness as defined by Martin (1983) were therefore included to facilitate comparisons that are derived from measurements of the enamel and dentine cap area and from the length of the enamel dentine junction. These were: average enamel thickness (the area of the enamel cap ‘‘c’’ divided by the length of the enamel–dentine junction ‘‘e’’ as measured from longitudinal sections of teeth) and relative enamel thickness (the average enamel thickness value, c/e, corrected as a dimensionless index relative to ‘‘b’’, the area of the dentine cap). Enamel cross striations and striae of Retzius Evidence supporting the fact that enamel cross striations represent circadian increments of growth has been reviewed previously (Bromage, 1991; Dean, 1987, 1989, 1995a). Counts of cross striations can be used to estimate the time of cuspal enamel formation. Measurements of the distance (spacing) between cross striations provide an estimate of the daily rate of enamel secretion. It is also well established that counts of regular striae of Retzius in enamel or of surface perikymata can be used to calculate the time of lateral enamel formation when the number of cross striations, or days, between them is known (Bromage & Dean, 1985; Dean, 1987; Beynon & Dean 1998). In many places in the sections of the permanent teeth of P. heseloni and P. nyanzae it was possible to see enamel cross striations and regular striae of Retzius. However, these were less clear in the ground sections of the deciduous teeth. In the cuspal enamel of the P. nyanzae M 2 , enamel cross striations were exceptionally well preserved and could be tracked continuously from the dentine horn to the outer surface of the enamel along paths of groups of prism. This tooth was therefore chosen to make a more careful comparative study of cross striations in cuspal enamel in Proconsul and other primates. Figures 5(a) and 5(b) are confocal reflected light images of cross striations at the EDJ and at the surface of the cuspal enamel in the M 2 of P. nyanzae. Cuspal cross striations Measurements of cross striations were made in zones, or bands, of enamel spaced approximately 30 days apart (e.g., at roughly monthly intervals) through the cuspal enamel of the mesiobuccal cusp of second
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