Introduction

The emphasis in this chapter is on the evolution of the gnathostome dentition, with developmentally reiterated units (individual tooth germs) organized in a specific temporal and spatial pattern in their initiation and for their replacement. The genetic and molecular controls of this patterning process are much less well known than those of the morphogenetic events of single tooth production that is well documented (Thesleff et al. 2007; Bei 2009). The individual morphogenetic units develop to become functional teeth in the phenotype and are organized along the jaws or pharyngeal arches for shape, size, spacing and timing of replacement, specific for each tooth in their proximal–distal position. Research has begun to identify the genes involved in patterning for tooth site and tooth replacement via in situ hybridization probes to active genes and by generic antibodies to the gene products. In particular, studies of mechanisms for regulation of tooth replacement have benefited by those extended beyond mammals to fishes (Fraser et al. 2004, 2006a, 2006b, 2008, 2009) and reptiles such as snakes and lizards (Buchtova et al. 2008; Vonk et al. 2008; Handrigan et al. 2010; Handrigan and Richman 2010a, 2010b).

As in all structures derived from epithelium, including feathers, scales and hair, teeth are formed initially by co-operative interaction between the epithelium and underlying mesenchyme, but for teeth the contribution of cranial neural crest to the process is critical, as reviewed by Smith and Hall (1993) and recently in mammals (Miletich and Sharpe 2004). Initially, in tooth formation the neural-crest-derived mesenchymal condensation is related to a thickened epithelial band, restricting the regions that can make teeth to this odontogenic band, as an early morphogenetic process. Both cell types are also identified by different localized, intensified gene expression at equivalent times to the morphological stages (examples are Otlx2, Pitx2, Fgf8, Bmp4 in the epithelium and Pax9, Barx1, Msx1, Msx2 in the mesenchyme). This topic, of the localization of the odontogenic cells and early signalling patterns before tooth formation, has been reviewed from comparison between mouse and both osteichthyan and chondrichthyan fish, to suggest that there is much conservation (Smith et al. 2009b; Fraser et al. 2010).