One characteristic of the jaws of all birds is that they lack teeth. The ancestors of birds and crocodiles had teeth, and modern crocodiles have teeth, but birds lack these structures. Instead, bird beaks have a keratinized epithelium.
Transplantation studies have shown that the oral epithelium of birds retains the ability to form teeth: transplanting mouse neural crest-derived tooth mesenchyme allows the chick oral epithelium to form toothlike structures (Kollar and Fisher 1980; Mitsiadis et al. 2003). Indeed, if BMP-coated beads are placed into the Fgf8-expressing region of chick jaw epithelium, the beads induce the neural crest-derived mesenchyme to synthesize Msx1 and Msx2 transcription factors and form toothlike structures in the chick jaw (Chen et al. 2000). Harris and colleagues (2006) demonstrated that the talpid2 mutation of the chick forms teeth, and that these teeth look just like the first set of crocodile teeth. The reason teeth can form in this mutant is that the boundary between the oral and non-oral has changed. In those animals having teeth, the oral ectoderm signaling center (formed by the interaction of Fgf8, BMP4, and Sonic hedgehog) overlies the neural crest mesenchyme that is capable of forming teeth. In birds, the oral ectoderm signaling center is not positioned over the competent mesenchyme, and so the signal is not received. In the talpid2 mutant, the oral/non-oral ectoderm boundary has again shifted, so that the oral ectoderm signaling center once more meets the competent mesenchyme. Obviously, such mutants don’t occur often. In fact, they are “as rare as hen’s teeth.”
Chen, Y.-P. and 9 others. 2000. Conservation of early odontogenic signaling pathways in Aves. Proc. Natl. Acad. Sci. USA 97: 10044–10049.
Harris, M. P., S. M. Hasso, M. W. J. Ferguson and J. F. Fallon. 2006. The development of archosaurian first-generation teeth in a chicken mutant. Curr. Biol. 16: 371–377.
Kollar, E. J. and C. Fisher. 1980. Tooth induction in chick epithelium: Expression of quiescent genes for enamel synthesis. Science 207: 993–995.
Mitsiadis, T. A., Y. Chéraud, P. Sharpe and J. Fontaine-Pérus. 2003. Development of teeth in chick embryos after mouse neural crest transplantations. Proc. Natl. Acad. Sci. USA 100: 6541–6545.