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Kent Academic Repository Full Text Document (Pdf) Kent Academic Repository Full text document (pdf) Citation for published version Aris, Christopher (2021) A Histological Examination of Enamel Microevolution over 2000 Years of Human History using British Populations. Doctor of Philosophy (PhD) thesis, University of Kent,. DOI Link to record in KAR https://kar.kent.ac.uk/87407/ Document Version UNSPECIFIED Copyright & reuse Content in the Kent Academic Repository is made available for research purposes. Unless otherwise stated all content is protected by copyright and in the absence of an open licence (eg Creative Commons), permissions for further reuse of content should be sought from the publisher, author or other copyright holder. Versions of research The version in the Kent Academic Repository may differ from the final published version. Users are advised to check http://kar.kent.ac.uk for the status of the paper. Users should always cite the published version of record. Enquiries For any further enquiries regarding the licence status of this document, please contact: [email protected] If you believe this document infringes copyright then please contact the KAR admin team with the take-down information provided at http://kar.kent.ac.uk/contact.html 1 A HISTOLOGICAL EXAMINATION OF ENAMEL MICROEVOLUTION OVER 2000 YEARS OF HUMAN HISTORY USING BRITISH POPULATIONS Christopher Aris Skeletal Biology Research Centre School of Anthropology and Conservation University of Kent A thesis submitted for the degree of Doctor of Philosophy in Anthropology June 2020 2 A HISTOLOGICAL EXAMINATION OF ENAMEL MICROEVOLUTION OVER 2000 YEARS OF HUMAN HISTORY USING BRITISH POPULATIONS Abstract The study of permanent enamel thickness and secretion rates of ameloblasts have yielded insights into the evolution of enamel when compared between hominin species. However, the study of modern human permanent tooth enamel has received far less attention, particularly regarding daily secretion rates (DSRs) and changes over time between populations. Moreover, enamel thickness and DSRs have rarely been analysed in conjunction for human samples. The research here uses dental histological thin sections to examine inner, mid, and outer DSRs for lateral and cuspal enamel, and the average (AET), relative (RET), cuspal (CT), and lateral (LT) thickness, for permanent first molar, upper canine, and upper first incisor crowns, from five British populations spanning the last 2000 years of history. The populations cover the Roman period (1-4AD) to modern day. A total of 265 teeth were analysed: first molar (n=115), upper canine (n=69), and upper first incisor (n=81). Results display consistent and significant trends towards decreasing DSRs from the ancient to modern populations. This was observed in all first molar cuspal regions (inner p<0.001, mid p<0.001, and outer p<0.001) and the inner and outer lateral regions (inner p=0.01 and mid p<0.001), in all upper first incisor cuspal and lateral regions (all p<0.001), and in all upper canine cuspal and lateral regions (all p<0.001). Enamel thickness features revealed less inter-population variation in first molars. The Early Anglo-Saxon RET was significantly larger than that of the Late Anglo-Saxon (p<0.001) and Medieval (p<0.001) populations. The Modern-day population LT was significantly thicker than the Roman populations (p=0.04). These differences allude to potential differences in diet between these populations. Alongside the differences seen between populations, the data provides the first major evidence for changes in the daily rate of enamel growth in human permanent dentition between populations. That similar variation was not observed in enamel thickness suggests thickness and growth can vary significantly and independently between human population. Word count: 53,398 (excluding Appendix C) Number of pages: 208 Year of submission: 2020 School of Anthropology and Conservation University of Kent i CONTENTS LIST OF TABLE AND FIGURES v Tables list v Figures list vii ACKNOWLEDGEMENTS viii CHAPTER 1: INTRODUCTION AND THESIS STRUCTURE 1 1.1 Introduction 1 1.2 Research questions 3 CHAPTER 2: BACKGROUND – DENTAL ANATOMY AND AMELOGENESIS 4 2.1 Enamel anatomy 4 2.1.1 Enamel structure and components 4 2.1.2 Enamel prisms 5 2.1.3 Aprismatic enamel 6 2.1.4 Incremental enamel lines 6 2.1.5 Other enamel structures 9 2.1.6 Enamel defects 11 2.2 Amelogenesis 12 2.2.1 Presecretory stage 12 2.2.2 Secretory stage 13 2.2.3 Transition stage 14 2.2.4 Maturation stage 15 2.2.5 Post-maturation stage 16 CHAPTER 3: LITERATURE REVIEW I – HISTORY OF ENAMEL HISTOLOGY AND VARIATION IN 17 HUMAN ENAMEL GROWTH AND DEVELOPMENT 3.1 Importance of enamel growth and earliest history of enamel histology 17 3.1.1 Early study of incremental lines 18 3.1.2 Early human histology 18 3.2 Modern history of enamel histology 19 3.2.1 inter-species and population analyses 19 3.2.2 Amelogenesis biomechanics 20 3.3 Intraspecific study of human enamel histology 20 3.3.1 Intraspecific permanent and deciduous patterns 21 3.3.2 Intraspecific patterns of permanent enamel growth and thickness 22 3.3.3 Intraspecific geographic variation between populations 23 3.3.4 Temporal studies of modern human enamel 24 3.4 Concluding remarks I 25 LITERATURE REVIEW II – ENAMEL MEASURES 26 3.5 Growth measures 26 3.5.1 Daily enamel secretion rate (DSR) 26 ii 3.6 Thickness measures 27 3.6.1 Average enamel thickness (AET) 27 3.6.2 Relative enamel thickness (RET) 28 3.6.3 Linear enamel thicknesses: cuspal (CT) and lateral (LT) thickness 29 3.7 Concluding remarks II 30 CHAPTER 4: MATERIALS AND METHODS 31 4.1 Materials – dental samples 31 4.1.1 Roman samples 33 4.1.2 Early Anglo-Saxon samples 33 4.1.3 Late Anglo-Saxon samples 34 4.1.4 Medieval samples 34 4.1.5 Modern-day samples 34 4.2 Methods – sample preparation 36 4.2.1 Epoxy resin embedding 36 4.2.2 Cross sectioning 37 4.2.3 Initial Slide Production 39 4.2.4 Grinding and polishing 40 4.2.5 Cleaning and cover slipping 40 4.2.6 Health and safety 41 4.2.7 Preparation time 41 4.3 Measurements taken 42 4.3.1 Daily secretion rates 42 4.3.2 Average enamel thickness 43 4.3.3 Relative enamel thickness 44 4.3.4 Cuspal thickness 44 4.3.5 Lateral thickness 44 4.3.6 Inter-observer error 46 CHAPTER 5: RESEARCH CHAPTER – ENAMEL THICKNESS AND GROWTH RATES IN MODERN 47 HUMAN PERMANANENT FIRST MOLARS OVER A 2000 YEAR PERIOD IN BRITAIN 5.1 Introduction 48 5.1.1 Amelogenesis and enamel secretion rates 49 5.1.2 Enamel thickness 49 5.1.3 Inter-specific studies of permanent enamel thickness and diet 50 5.1.4 Studies of enamel thickness within human permanent molars 51 5.1.5 Relationship between enamel thickness and underlying growth mechanisms 52 5.2 Materials and methods 53 5.2.1 Dental sample 53 5.2.2 Enamel thickness 53 5.2.3 Daily secretion rates of enamel 53 5.2.4 Statistical analysis 54 5.3 Results 55 5.3.1 Tooth size and maxillary and mandibular molars 55 5.3.2 Enamel thickness measures 59 iii 5.3.3 Cuspal daily secretion rates 62 5.3.4 Lateral daily secretion rates 65 5.4 Discussion 68 5.4.1 Relative enamel thickness 68 5.4.2 Linear measures of enamel thickness 70 5.4.2.1 Cuspal enamel thickness 71 5.4.2.2 Lateral enamel thickness 71 5.4.3 Daily secretion rates 72 5.4.3.1 Daily secretion rates compared between enamel regions 72 5.4.3.2 Cuspal daily secretion rates compared between populations 73 5.4.3.3 Lateral daily secretion rates compared between populations 73 5.4.4 Relationship between DSRs and linear thicknesses 74 5.4.5 Conclusions 75 CHAPTER 6: RESEARCH CHAPTER – ENAMEL GROWTH RATES OF ANTERIOR TEETH IN MALES 76 AND FEMALES FROM MODERN AND ANCIENT BRITISH POPULATIONS 6.1 Introduction 77 6.1.1 Intraspecific study of human enamel secretion rates 77 6.2 Materials and methods 78 6.2.1 Dental sample 78 6.2.2 Estimating sex 78 6.2.3 Daily secretion rates 79 6.2.4 Statistical analysis 81 6.3 Results 82 6.3.1 Differences between tooth types 82 6.3.2 Differences in DSRs between biological sex groups 84 6.3.3 Differences in DSRs between biological sex groups and between populations 86 6.4 Discussion 92 6.4.1 Daily secretion rates compared between the sexes within each populations 92 6.4.2 Daily secretion rates compared between the sexes, from ancient to modern populations 92 6.4.3 Daily secretion rates between populations 92 6.4.4 Daily secretion rates compared to posterior teeth 93 6.4.5 Conclusions 94 CHAPTER 7: DISCUSSION 96 7.1 Enamel growth variation across the tooth row and between groups 96 7.1.1 Roman DSRs 96 7.1.2 Early Anglo-Saxon DSRs 97 7.1.3 Late Anglo-Saxon DSRs 97 7.1.4 Medieval DSRs 98 7.1.5 Modern-day DSRs 99 7.1.6 Enamel growth variations between the biological sexes 100 7.2 First molar enamel thickness compared to known ranges 100 7.2.1 Roman upper molar enamel thickness 101 7.2.2 Early Anglo-Saxon first molar enamel thickness 101 iv 7.2.3 Late Anglo-Saxon first molar enamel thickness 102 7.2.4 Medieval first molar enamel thickness 103 7.2.5 Modern-day first molar enamel thickness 104 7.3 Discussion – concluding remarks 106 7.3.1 Variation in enamel growth 106 7.3.2 Molar enamel thickness variation between populations 107 7.3.3 Summary 108 7.4 Limitations 109 7.4.1 Limited tooth types and only one tooth analysed per individual 109 7.4.2 Disparity in sample sizes 111 7.4.3 Low number of dental growth measures 111 CHAPTER 8: CONCLUSIONS 113 8.1 First molar enamel 113 8.2 Upper anterior tooth enamel 114 8.3 Enamel sexual dimorphism 114 8.4 Enamel growth and thickness 115 8.5 Future research directions 115 REFERENCES 117 APPENDIX A: GLOSSARY 154 A.1 Glossary of relevant terms 154 A.1.1 Directional and surface definitions 154 A.1.2 Numbering and descriptive systems 156 A.1.3 Enamel specific abbreviations and acronyms 156 A.2 Dental anatomy 157 A.3 Tooth types 162 APPENDIX B: SEX DETERMINATION SHEET 164 APPENDIX C: PUBLISHED ARTICLES 166 v TABLES AND FIGURES Tables list Table 4.1: Descriptive information of the British populations.
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