Model for the examination of evolutionary trends in tooth development

Abstract

Through the use of serial computerized tomography (C-t) scans, two distinct developmental stages can be identified in mature teeth. C-t scans thus provide a non-destructive method for assessing growth within individual teeth, as well as for comparison of the development of modern and fossil teeth. The second deciduous molar (DM2) and first permanent molar (M1) resemble one another morphologically, despite differences in size and developmental rates. Thus, they provide an excellent model for studying variation in growth within an individual. To test the C-t method, we first examined a recent archaeological sample and then examined teeth from Skhul I. Serial C-t scans were used to compare two distinct developmental stages represented by the dentine-enamel junction (DEJ) and outer enamel surface (OES), respectively, in mandibular DM2 and M1 of 31 archaeological specimens. The difference in form and size between these two surfaces in and between teeth was calculated from intercusp distances measured at the DEJ and OES using the form distance matrix. Intercusp distances at the DEJ and OES of these teeth were then compared to their counterparts in the DM2 and M1 of Skhul I, taken here as representative of early anatomically modern Homo sapiens sapiens. Form differences between paired DM2 and M1 at the DEJ were smaller than those at the OES, supporting the hypothesis that differences between the two teeth increase throughout development. The increase in intercusp distances from the DEJ to OES was found to reflect the angulation of cusps relative to one another, rather than enamel thickness. Form differences between the Skhul DM2 and M1 were smaller than those observed in the recent series, and the recent M1 differed more than the DM2 from its fossil counterpart. The similarities found between the Skhul permanent and deciduous teeth and the recent DM2, may reflect a similar growth pattern. This would contribute to earlier crown completion in the fossil M1.

Molar Intercuspal Dimensions: Genetic Input to Phenotypic Variation

“…When these findings are considered together with those for individuals with Turner and Klinefelter syndromes, it seems that there is a definite trend in the relationship of the buccal and lingual cusps of premolar teeth, with intercuspal distances diverging as enamel thickness increases in association with additional X chromosomes (Townsend and Alvesalo, 1999). Three-dimensional analyses of occlusal surfaces and dentino-enamel junctions of extracted human teeth have confirmed that molar cusp tips tend to diverge as enamel thickness increases (Kanazawa et al, 1987), as have more recent computerized tomography studies of fossil hominid teeth (Smith et al, 1997).…”

Morphological comparison of the enamel–dentine junction and outer enamel surface of molars using a micro-computed tomography technique

“…Smith et al [9] quantified the differences in EDJ and OES sizes and shapes of the mandibular second deciduous molars and first permanent molars by calculating intercusp distances, although the resolution was not high (1.2 mm). They noted that the increase in intercusp distance at the OES was caused not by an increase in enamel thickness but rather a difference in cusp angulation, which has been confirmed using high-resolution mCT [10].…”

Intercusp differences in enamel prism patterns in early and late stages of human tooth development

“…From these studies, Butler suggested that cell proliferation and furrow formation continue in adjacent regions, so that the relative height and distance between cusps may change and additional cusps and invaginations appear following the onset of calcification. More recently, Smith et al (1997) demonstrated differences in angulation between cusps and proposed that their angulation reflects the spatial and temporal ordering of cell division and differentiation.…”

“…It appears that deciduous teeth grow faster than permanent teeth but have smaller, less densely arranged enamel prisms (Shellis, 1984). This rapid pattern of growth is also seen in fossil teeth (Bromage and Dean, 1985;Beynon and Wood, 1986;Dean et al, 1986;Beynon and Dean, 1987;Ramirez Rozzi, 1993;Smith, 1991;Smith et al, 1997).…”