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Modern Human Molar Enamel Thickness and Enamel—Dentine Junction Shape

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Modern Human Molar Enamel Thickness and Enamel—Dentine Junction Shape Archives of Oral Biology (2006) 51, 974—995 www.intl.elsevierhealth.com/journals/arob Modern human molar enamel thickness and enamel—dentine junction shape T.M. Smith a,*, A.J. Olejniczak a,b, D.J. Reid c, R.J. Ferrell d, J.J. Hublin a a Human Evolution Department, Max Planck Institute for Evolutionary Anthropology, Deutscher Platz 6, D-04103 Leipzig, Germany b Interdepartmental Doctoral Program in Anthropological Sciences, Stony Brook University, Stony Brook, NY 11794-4364, USA c Department of Oral Biology, School of Dental Sciences, University of Newcastle upon Tyne, Framlington Place, Newcastle upon Tyne NE2 4BW, UK d Center for Population and Health, Georgetown University, 313 Healy Hall, Box 571197, Washington, DC 20057-1197, USA Accepted 28 April 2006 KEYWORDS Summary This study examines cross-sections of molar crowns in a diverse modern Average enamel human sample to quantify variation in enamel thickness and enamel—dentine junc- thickness; tion (EDJ) shape. Histological sections were generated from molars sectioned bucco- Relative enamel lingually across mesial cusps. Enamel cap area, dentine area, EDJ length, and bi- thickness; cervical diameter were measured on micrographs using a digitizing tablet. Nine Enamel cap area; landmarks along the EDJ were defined, and X and Y coordinates were digitized in Dentine area; order to quantify EDJ shape. Upper molars show greater values for the components of Enamel—dentine enamel thickness, leading to significantly greater average enamel thickness than in junction (EDJ) length; lower molars. Average enamel thickness increased significantly from M1 to M3 in both EDJ shape; molar rows, due to significantly increasing enamel cap area in upper molars, and Fossil hominids; decreasing dentine area in lower molars. Differences in EDJ shape were found among Hominoids maxillary molars in combined and individual populations. Sex differences were also found; males showed significantly greater dentine area, EDJ length, and bi-cervical diameters in certain tooth types, which resulted in females having significantly thicker average enamel. Differences in enamel thickness and EDJ shape within molars were also found among populations, although few consistent trends were evident. This study demonstrates that enamel thickness and EDJ shape vary among molars, between sexes, and among populations; these factors must be considered in the categorization and comparison of ape and human molars, particularly when isolated teeth or fossil taxa are included. Human relative enamel thickness encompasses most * Corresponding author. Tel.: +49 341 355 0362; fax: +49 341 355 0399. E-mail address: [email protected] (T.M. Smith). 0003–9969/$ — see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.archoralbio.2006.04.012 Modern human molar enamel thickness and enamel—dentine junction shape 975 values reported for fossil apes and humans, suggesting limited taxonomic value when considered alone. # 2006 Elsevier Ltd. All rights reserved. Introduction permanent teeth come from studies of human molars.5,7,28,29 However, sample sizes in each of Enamel thickness studies these studies were generally about ten or less per tooth type, resulting in limited statistical capability Many anthropological studies have focused on for the detection of differences using conventional aspects of tooth enamel thickness due to its pur- parametric techniques such as Analysis of Variance ported taxonomic and phylogenetic value for the (ANOVA).3,30,31 For example, Grine5,7 found that interpretation of human evolution.1—4 Other enamel thickness increases along the molar row in studies have examined the functional implications modern human molars, but significant differences of enamel thickness and enamel distribution.5—9 among permanent molars were not detected. Related morphological variation, such as trends in Smith et al.3 employed non-parametric trend ana- enamel thickness throughout the molar row, dif- lyses in a similarly sized sample of chimpanzee and ferences between sexes, and differences among orangutan molars, and found significant increasing populations, have also been considered.3,5,7,10,11 trends in average enamel thickness in mesial sec- These studies facilitate refined interpretation of tions of both taxa, thus confirming statistically that the distribution of dental tissues, and ultimately tooth position must be considered in comparative they suggest that a number of factors must be studies of enamel thickness. considered when making comparisons of enamel Although studies of enamel thickness variation thickness between or among apes and humans are becoming more common, little is known about (Primates: Superfamily Hominoidea). variation in enamel thickness between sexes or Enamel thickness has been previously assessed among con-specific populations.2,10,11,29 Hlusko using linear measurements of exposed enamel in et al.2 did not find a significant difference in linear worn or naturally fractured teeth.2,12,13 Martin14 enamel thickness between male and female baboon demonstrated, however, that it is difficult to accu- molars, but when scaled for tooth size, female rately measure enamel thickness based on exposed baboons had relatively more enamel than males. enamel. Martin14,15 measured thickness in buccolin- Among humans, Schwartz and Dean11 examined gual molar sections cut through mesial cusp tips, enamel and dentine proportions in canines and third which were scaled for body size (using a surrogate molars, and found that males showed a greater measurement local to each molar) to make compar- dentine area than females, which was significantly isons across taxa, resulting in a measure of relative different in third molars. Sex differences in enamel enamel thickness (RET). Other scholars have quan- thickness are likely to be taxon-specific, however, tified dental tissue proportions using lateral (flat- evidence available at present does not facilitate plane) radiography16,17 and medical computed testing whether sexual dimorphism in human and tomography,18,19 although it has been demonstrated baboon enamel thickness is comparable. Moreover, that these methods of visualization do not result in little is known about population-level enamel thick- accurate measurements of enamel thickness.20,21 ness variation. Grine5,7 studied human molars from More recently, multiple forms of high-resolution geographically diverse populations, and found simi- computed microtomography have yielded accurate lar values between groups and low levels of varia- measurements of dental tissue thicknesses.22,23 tion in the mixed population sample. Nonetheless, Despite recent advances in three-dimensional med- samples of molars from geographically diverse popu- ical imaging, the most widely-available enamel lations have not been compared on a large scale, thickness data result from recent large histological including analyses of variation within the molar row. studies of dental development.24—27 In addition to enamel thickness and its variation An emerging trend in recent studies is an empha- throughout the molar row, several studies suggest sis on documenting changes in enamel thickness that the dentine component of the molar crown, or throughout the molar row (i.e., metameric varia- the shape of the enamel—dentine junction (EDJ), tion), which is important for the interpretation may be indicative of species-level affiliation.32—35 of data on isolated fossil teeth where only one or Although it has been shown that EDJ shape in max- a few teeth, but not an individual’s entire dentition, illary molars may distinguish species from one are available for study.2,3,7 The most widely avail- another with relatively high resolution,35 it remains able data for interpretation of trends in hominoid unknown whether this aspect of morphology, like 976 T.M. Smith et al. enamel thickness, may differ between molar types, Table 1 Modern human molars included in this study between sexes, or between populations within a species. A large sample of human molars represent- Row Tooth SA NA NE Dan Combined ing different populations thus presents an opportu- Max M1 25 — 10 2 37 nity to study EDJ shape in light of these possible M2 13 — 9 3 25 sources of variation. M3 13 24 13 1 51 It is also unclear whether enamel thickness and Mand M1 17 — 10 28 55 the shape of the EDJ may be correlated, and the M2 11 — 15 19 45 sample studied here may illuminate the relationship M3 5 5 13 21 44 between these two aspects of morphology. For Totals 84 29 70 74 257 example, Martin14 suggested that relatively tall Row: maxillary or mandibular molar positions. SA, South dentine horn tips might be associated with thin African population; NA, North American population; NE, enamel in Gorilla molars, as both characters are Northern England population; Dan, medieval Danish popula- tion. related to folivory (see also Shimizu36 and Ulhaas et al.37 for discussions of these characters in cerco- pithecoid primates). Because the EDJ begins to take Denmark (Table 1). Sex was known for all South shape earlier in development than enamel or den- African and North American individuals, and also tine deposition (i.e., the zones of maturation at the for a few individuals from Northern England. future dentine horn tips are identifiable before Approximately 530 histological sections were gen- the commencement of hard tissue secretion), the erated from 435 molars of 372 individuals; 257 of macromorphology of a tooth is likely to be influ- these sections were chosen for the study of enamel enced by the locations of the dentine horn tips. thickness and 247 of these were used for the study of Examination of tissue thickness and shape simulta- EDJ shape. Teeth that had not completed crown neously in a large sample of a single taxon will formation of the mesial cusps or that showed evi- facilitate exploration of the relationships of the dence of marked enamel hypoplasia were excluded coronal dentine and enamel components of the from the study because measurements of enamel molar crown. thickness are likely to be affected. This study aims to examine variation in enamel thickness among molar types, between sexes, and Components of enamel thickness among populations in a large sample of regionally diverse human molars.
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