Expression of fibroblast growth factors (Fgfs) in murine tooth development

Fgf signalling is known to play critical roles in tooth development. Twenty-two Fgf ligands have been identified in mammals, but expression of only 10 in molars and three in the incisor loop stem cell region have been documented in murine tooth development. Our understanding of Fgf signalling in tooth development thus remains incomplete and we therefore carried out comparative in situ hybridisation analysis of unexamined Fgf ligands (eight in molars and 15 in cervical loops of incisors; Fgf11-Fgf14 were excluded from this analysis because they are not secreted and do not activate Fgf receptors) during tooth development. To identify where Fgf signalling is activated, we also examined the expression of Etv4 and Etv5, considered to be transcriptional targets of the Fgf signalling pathway. In molar tooth development, the expression of Fgf15 and Fgf20 was restricted to the primary enamel knots, whereas Etv4 and Etv5 were expressed in cells surrounding the primary enamel knots. Fgf20 expression was observed in the secondary enamel knots, whereas Fgf15 showed localised expression in the adjacent mesenchyme. Fgf16, Etv4 and Etv5 were strongly expressed in the ameloblasts of molars. In the incisor cervical loop stem cell region, Fgf17, Fgf18, Etv4 and Etv5 showed a restricted expression pattern. These molecules thus show dynamic temporo-spatial expression in murine tooth development. We also analysed teeth in Fgf15(-/-) and Fgf15(-/-) ;Fgf8(+/-) mutant mice. Neither mutant showed significant abnormalities in tooth development, indicating likely functional redundancy.