Chapter 10 - Tooth Replacement and Ontogeny of the Dentition

Abstract

Nearly all non-mammalian vertebrates are polyphyodont: that is, the teeth are replaced continually throughout life. Polyphyodonty has several functions. First, increases in number, size, or both of successive generations of teeth is an essential component of the growth of the dentition. Second, morphological changes in successive generations can allow exploitation of different food at different stages of the life cycle. Third, continual replacement compensates for wear. The teeth of elasmobranchs are connected to a common fibrous band which moves forward continuously, carrying all the teeth with it. Among amphibians and reptiles, a common pattern of replacement involves “waves” of replacement passing along the tooth row, and affecting alternate tooth loci. The number and pattern of functional teeth depends on the rapidity with which the waves propagate along the dentition. In bony fish, replacement sometimes occurs in waves, but other patterns, ranging from apparently random to near-simultaneous replacement of all teeth in a jaw quadrant, also occur. The number of replacing teeth at a tooth locus is directly related to the rate of tooth replacement. The development of the dentition requires initiation of teeth in a specific sequence and pattern and this is related to the pattern of tooth replacement. In mammals, it appears that these are controlled by differential expression of homeobox genes in different parts of the mouth. Less is known about non-mammalian vertebrates but research on reptiles and amphibians indicate that WNT is important in regulating the activities of stem cells in the dental lamina, and hence the rate of replacement.