Introduction Enamel structure and development and its application in

Introduction

Enamel structure and development and its application in hominid evolution and taxonomy

Fernando Ramirez Rozzi

Laboratoire de Pale´oanthropologie, Station Marcelin Berthelot, 30 av. Marcelin Berthelot, 92360 Meudon la Foreˆt, France Journal of Human Evolution (1998) 35, 327–330 Article No. hu980257

Partout où quelque chose vit, il y a, ouvert quelque part, un registre où le temps s’inscrit.1 H. Bergson The assumption that dental development in fossil hominids was close to that in modern humans is based on the pioneering work of Weidenreich (1937) and Mann (1975), and has prevailed for many years. In 1985, Bromage & Dean published results on dental development in fossil hominids based on a new methodology for the analysis of the microstructure of the enamel. Bromage & Dean’s work casts doubt on previous conclusions and suggests that dental development in Plio-Pleistocene hominids was closer to that in extant great apes than to that in modern humans. Enamel is secreted by ameloblasts and each ameloblast is responsible for a prism in mature enamel (Figure 1). Enamel secretion starts on the dentine horn and proceeds in two directions. From the enamel-dentine junction, individual ameloblasts secrete enamel and thereby increase enamel thickness towards the ultimate surface of the crown. New ameloblasts also become active from the dentine horn to the cervix. Cross-striations appear as slight striations perpendicular to the axis of the prism under a light microscope (Boyde, 1965; Osborn, 1973) or as successive dark and light bands along prisms in a scanning electron microscope (SEM) (Risnes, 1986). The striae of 1 Wherever there is a living organism, there is somewhere on an open register a record of time.

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Retzius correspond to a two-dimensional representation of successive bend planes in the enamel (Risnes, 1985). The crown of a tooth can be divided into two stages based on the striae arrangement (Beynon & Wood, 1987). The cuspal (appositional) stage is comprised of striae which have not reached the enamel surface and involves successive layers of appositional enamel. In the imbricational stage the enamel layers are in imbricational position (as in partiallyoverlapping shingles on a roof), with striae reaching the enamel surface to produce slight depressions known as perikymata (Risnes, 1985; Dean, 1987). The methodology used by Bromage & Dean (1985) had been previously employed by oral biologists (Boyde, 1964; Massler & Schour, 1946; Schour & Hoffman, 1939; Schour & Poncher, 1937; Schour & Steadman, 1935). It is founded on the assumption that cross-striations and striae of Retzius correspond to incremental lines forming with a regular and systematic periodicity, cross-striations showing a circadian rhythm and the striae of Retzius an approximately circaseptan rhythm with a variation from 6 to 10 days between individuals in modern humans. Counting incremental lines in enamel results in an estimate of crown formation time. Criticism of Bromage & Dean’s work has focused principally on the methodology. Some anthropologists doubt that lines in enamel correspond to incremental markers. The assumption of the presence of  1998 Academic Press

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Figure 1. Schematic section of a tooth showing incremental lines in enamel. The number of crossstriations between adjacent striae is constant for one individual. The striae of Retzius form perikymata when they arrive at the enamel surface. The first stria arriving at the enamel surface determines the limit between the cuspal (appositional) stage and the imbricational stage. Hsb: Hunter-Schreger bands.

incremental lines in enamel is also not accepted by all scholars working on oral biology. In fact, the debate between anthropologists reflects the debate between oral biologists. Bromage & Dean’s work offered a new method to approach dental development in fossil hominids. Within a few years the original aim of the new methodology, the establishment of age-at-death of individuals, expanded to include work on, aspects of enamel secretion and tooth formation (Beynon & Wood, 1987; Beynon et al., 1991; Shellis, 1984), the relationships between enamel microstructural characteristics and environmental changes (Macho et al., 1996) and evolutionary processes in crown formation (Ramirez Rozzi, 1997). The knowledge of enamel formation and tooth development in extant and extinct hominoids has grown considerably in the last few years. However, the debate over the interpretation of lines in enamel as incremental markers persists. Studies involving dental microanatomy are sometimes excessively criticised because of confusion over the interpretation of enamel microstructures, and by misunderstandings in relation to the types of questions that can be validly answered by these data.

An international workshop on ‘‘Enamel Structure and Development, and its Application in Hominid Evolution and Taxonomy’’ was held in Paris, France, on 20–24 May, 1996. For the first time, scholars in oral biology and palaeoanthropologists met together in order to discuss the interpretation of enamel structures and to share current views on ongoing research into different aspects of the microstructure of the enamel. Twenty-two participants representing 19 universities world-wide were invited (Figure 2). The workshop was divided in seven sections and each section was structured around one question. Four participants addressed the question and one discussant led the debate. The questions were: (1) Is ameloblast activity periodic? Are there incremental lines in enamel or are they artefacts of material preparation? (2) What information on developmental biology can be obtained from tooth microstructure? (3) Enamel thickness: measurement, ontogeny, phylogeny, tooth form and function. (4) How can we characterize dental development in extant hominoids? What do we know? What don’t we know? What must be done in the near future? (5) What information can be inferred about dental

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Figure 2. Participants at the Paris International Workshop. Back row from left to right: Michelle Lampl, Janet Monge, Steiner Risnes, David Gantt, Charles Fitzgerald, Alan Boyde, David Beynon, Don Reid, Kevin Kuykendall, Alan Mann, Hershey Warshawsky. Front row from left to right: Chris Dean, Gabriele Macho, Gary Schwartz, Tim Bromage, Wendy Dirks, Fernando Ramirez Rozzi. Absent from the picture are Bob Anemone, Yves Coppens, Michel Goldberg, Peter Shellis.

development in extinct hominids based on patterning of dental development and enamel and dentine microstructure? Can differences be distinguished between species? (6) What changes in tooth microstructure and development have occurred during the last 5 Ma? What external and internal factors influenced these changes? (7) Where are we at in dental microstructural studies? What are the limitations and potentials for future work in this field? This volume regroups papers originating from this meeting with the purpose of clarifying the fundamental aspects of the microstructure of enamel from a palaeoanthropological point of view. Some questions have a definitive answer whereas others need more research. It is clear, however, that enamel microstructure gives the

possibility of studying growth and development in fossil hominids directly. The papers in this special issue demonstrate the importance of the study of the microanatomy of hard tissues in paleoanthropology. Acknowledgements I am deeply indebted to Chris Dean for his enormous contribution to the preparation of this volume, it would have been impossible without his help. I am very grateful to Leslie Aiello who gave me the opportunity to produce this volume as a special issue in Journal of Human Evolution and for her continuous editorial assistance. I would also like to thank Gabriele Macho, the referees and associate editors that so much improved this volume. I thank Yves Coppens who

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helped me with the preparation of the meeting in Paris. The meeting ‘‘Enamel Structure and Development, and its Application in Hominid Evolution and Taxonomy’’ was funded by Singer-Polignac Foundation, Hugot Foundation (Collège de France), CNRS, and Leica. Thanks are also given to A. Statian, M. Tersis, and M.-F. Leroy for their secretarial assistance. References Beynon, A. D., Dean, M. C. & Reid, D. J. (1991). Histological study on the chronology of the developing dentition in Gorilla and Orangutan. Am. J. phys. Anthrop. 86, 189–203. Beynon, A. D. & Wood, B. A. (1987). Patterns and rates of molar crown formation times in East African fossil hominids. Nature 326, 493–496. Boyde, A. (1964). The structure and development of mammalian enamel. Ph.D. Thesis, University of London. Boyde, A. (1965). The structure of developing mammalian dental enamel. In (M. V. Stack & R. W. Fearnhead, Eds) Tooth Enamel, pp. 163–167. Bristol: J. Wright. Bromage, T. G. & Dean, M. C. (1985). Re-evaluation of the age at death of Plio-Pleistocene fossil hominids. Nature 317, 525–528. Dean, M. C. (1987). Growth layers and incremental markings in hard tissues; a review of the literature and some preliminary observations about enamel structure in Paranthropus boisei. J. hum. Evol. 16, 157–172. Macho, G., Reid, D., Leakey, M., Jablonski, N. & Beynon, D. (1996). Climatic effect on dental devel-

opment of Theropithecus oswaldi from Koobi Fora and Olorgesailie. J. hum. Evol. 30, 57–70. Mann, A. E. (1975). Paleodemographic aspects of the South African australopithecines. Philadelphia: University of Pennsylvania. Massler, M. & Schour, I. (1946). The appositional life span of the enamel and dentin-forming cells. J. dent. Res. 25, 145–150. Osborn, J. W. (1973). Variations in structure and development of enamel. Oral Sci. Rev. 3, 3–83. Ramirez Rozzi, F. V. (1997). Heterocronic process in dental development in Plio-Pleistocene hominids (abstract). J. hum. Evol. 32, A16–A17. Risnes, S. (1985). A scanning electron microscopy study of the three dimensional extent of Retzius lines in human dental enamel. Scand. J. dent. Res. 93, 145–152. Risnes, S. (1986). Enamel apposition rate and prism periodicity in human teeth. Scand. J. dent. Res. 94, 394–404. Schour, I. & Hoffman, M. M. (1939). The rate of apposition of enamel and dentin in man and other animals. J. dent. Res. 18, 161–175. Schour, I. & Poncher, H. C. (1937). The rate of apposition of human enamel and dentin as measured by the effects of accute fluorosis. Am. Assoc. Dis. Child. 54, 757–776. Schour, I. & Steadman, S. R. (1935). The growth pattern and daily rhythm of the incisor of the rat. Anat. Rec. 63, 325–332. Shellis, R. P. (1984). Variations in growth of the enamel crown in human teeth and a possible relationship between growth and enamel structure. Archs oral Biol. 29, 697–705. Weidenreich, F. (1937). The dentition of Sinanthropus pekinensis: a comparative odontography of the hominids. Palaeontol. Sin. 1, 120–180.