Mammal Teeth: Origin, Evolution, and Diversity

Dental Histology and Development

2 Dental Histology and Development Teeth, the armed ægis of life! Like the shield given to Pallas, which, when Medusa’s head was placed on it, turned all who viewed it, into stones: so teeth . . . spring from their embryonic beds, to be converted into crystal stones. — mosely, 1862 DENTAL HISTOLOGY IS the study of the microscopic structure of teeth. Dental microstructure is an important part of the story of the adaptive radiation of mammalian tooth form. Teeth are subject to considerable load forces, and the strength of a tooth depends in large part on the microscopic arrangement of its tissues. This arrangement reflects both the magnitude and the directions of forces that act on a tooth during food processing. It also reflects the phylogeny, or evolutionary history, of a mammal. This review of dental histology takes a developmental approach because tooth construction is an important key to understanding microstructure. The chapter ends with a brief consideration of an exciting new field, evolutionary developmental biology, whose practitioners are beginning to discover the genetic mechanisms that control tooth shape and its modification through evolution. FRACTURE MECHANICS AND TOOTH DESIGN It is a very bad thing for a mammal to break a permanent tooth. Teeth do not “heal” in the same manner that many other tissues do, and unlike many reptiles, mammals cannot simply replace them. On the other hand, teeth can be stressed by considerable or repeated forces as they go about the business of chewing, so these structures must be built to fracture foods without themselves being fractured. As stated in the introduction, teeth are, in a sense, in a death match with the foods they chew. If we are to understand tooth structure and, ultimately, tooth function, we need to know something about basic fracture mechanics and the physics of material failure. The relevant concepts are presented in chapter 3, and a more comprehensive review of fracture mechanics as they apply to teeth can be found in the literature (see, e.g., P. W. Lucas 2004; Popowics and Herring 2006; and P. W. Lucas et al. 2008). In order to understand dental structure, we need to consider how solids react when forces act on them. An object can either deform elastically or fail (Fig. 2.1). Elastic deformation occurs when a strained solid returns to its original shape after a load is removed from it (think of a rubber band being stretched, released, and returned to its original shape). Failure can involve either fracture (think of that rubber band breaking if stretched too far) or plastic deformation (the band deformed permanently and not returned to its original shape after being stretched). The strength of a tooth depends on its ability to resist failure (see Fig. 2.1). Because of the forces that act on teeth during food processing, occlusal surfaces must d e n t a l h i s t o l o g y a n d d e v e l o p m e n t 17 drical in shape, much like bunches of dried spaghetti strands (Fig. 2.2). The crystallites within each rod run roughly parallel to one another, but some within a rod may change their orientation and run a short distance until they meet the crystallites of another rod. The area where they meet forms a boundary or sheath between the rods, where protein and water tend to accumulate. These boundaries determine the shape of individual prisms. Prisms vary in diameter both between and within species and even within individual teeth, from about 2 μm to about 10 μm (see, e.g., Dumont 1996a, 1996b; and Maas and Dumont 1999). Prisms are packed together and run from the EDJ to the surface of the tooth (see Fig. 2.2). Packing patterns vary and can be divided into three types. Pattern 1 enamel prisms are cylindrical in shape with nearly circular cross sections and packed in alternating rows. Pattern 2 enamel prisms resemble cylinders open on the bottom and are U- or horseshoe-shaped in cross section. These are arranged in vertical rows separated by sheets of interprismatic enamel. Pattern 3 prisms are similar to Pattern 2 prisms but are arranged in horizontal rows and staggered vertically, which gives them a keyhole shape (Boyde 1965). Prism packing patterns also vary within and between species and within the teeth of an individual (see, e.g., Dumont 1996a; Maas and Dumont 1999; and Zeygerson, Smith, and Haydenblit 2000). Despite variation within and between individuals, evolutionary...