Built to Last: The Structure, Function, and Evolution of Primate Dental Enamel

Enamel is the strongest tissue in the body. It is not only hard (resistant to permanent surface deformation), but tough (resistant to crack propagation and brittle fracture). Its durability means that enamel can potentially be preserved, unaltered, for literally millions of years. Moreover, because it is not remodeled during life, enamel is unique among mammalian tissues in that it preserves a permanent record of its ontogeny. Enamel is composed of fiber-like mineral crystals and a small nonmineral fraction of water and protein that holds the mineral fibers together. The resulting composite material is much tougher than mineral alone. The mineral and nonmineral components are organized in a complex fabric that dissipates forces traveling through teeth and protects them from fracture. Mineral is the dominant component of enamel, which is much more highly mineralized than the two other calcified dental tissues, dentine and cementum. The enamel mineral is a form of carbonate hydroxyapatite [Ca10(PO4)6(OH)2]. The Ca:P ratio of enamel apatite is slightly lower than that of hydroxyapatite.1–3 In human teeth, enamel is densest (96% apatite) close to the outer surface and less dense (84% apatite) near the enamel-dentine junction.4 Though water and protein comprise only a minor part of mature enamel, they are crucial to its development (see Box 1) and are important in understanding its structural organization and physical properties. Abundant hydrophobic proteins called amelogenins dominate developing enamel, but as the mineral crystals grow in size during maturation, amelogenins disappear so that only the less abundant, acidic proteins, enamelins and tuftelins remain.5–8 This remnant protein fraction is contained in the spaces between mineral crystals, where it serves as a ‘‘glue’’ between crystallites.9 Most of the water within enamel is tightly bound to the mineral phase10 and serves to influence enamel’s compressibility and permeability.11,12 Although the high mineral content accounts for enamel’s hardness, it is the arrangement and organization of its mineral and nonmineral constituents (enamel structure) that modulates the way enamel responds to stress. Enamel structure can be visualized through a simple hierarchical model of increasing complexity and scale.13–16 The least complex and smallest structural units are apatite crystallites. In most mammal teeth, crystallites are grouped into more complex and largerscale structures known as prisms. The arrangement of groups of prisms determines the next most complex unit, enamel types. On a still larger and more complex scale, the different enamel types within a tooth define its schmelzmuster; this German term is preferred over its literal English translation, ‘‘enamel pattern,’’ which is a specialized description of the arrangement of enamel types on occlusal surfaces of hypsodont teeth.15 The most complex and largest-scale hierarchical level is that of the dentition, which describes the variation of schmelzmusMary Carol Maas is Postdoctoral Fellow in the Department of Anatomy at Northeastern Ohio Universities College of Medicine and Research Fellow in the Department of Anthropology and Laboratory of Vertebrate Paleontology at the University of Texas, Austin. Her interest in the biogeographic origins of modern mammals has taken her to Paleocene and Eocene fossil localities in North America and, most recently, the Eocene of Turkey. Her investigations of enamel microstructure and tooth wear have led her to electron microscopes in England, Germany, and the United States. At other times she lives near Mérida, Mexico.