Chapter 10 - Ontogeny of the Dentition and Tooth Replacement

Abstract

The dentition is established by initiation of teeth in a specific sequence and pattern. Except for a few taxa which have only one generation of teeth, non-mammalian vertebrates are polyphyodont, replacing the teeth continually throughout life. Polyphyodonty has several functions. (1) Increases in number, size or both of successive generations of teeth is an essential component of the growth of the dentition. (2) Morphological changes in successive generations can allow exploitation of different food at different stages of the life cycle. (3) 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. The number and pattern of functional teeth depends on the rapidity with which the waves propagate along the dentition. The molecular processes underlying the phenomenon of tooth replacement waves are, however, so far unresolved. In bony fishes, 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. In mammals, tooth number and position are controlled by the interaction between growth factors such as sonic hedgehog, ectodysplasin, fibroblast growth factor, wingless-related protein, and bone morphogenetic protein, and their respective antagonists. Expression of genes for growth factors is in turn regulated by differential expression of homeobox genes in different parts of the mouth. It is becoming clear that establishment of the dentition in non-mammalian vertebrates is controlled by a suite of growth factors similar to that in mammals. Repeated tooth replacement is supported by populations of stem cells.