Elucidation of molecular mechanisms on transition from crown to root in tooth development

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In development of human-teeth, mouse-and rat-molars, root formation with periodontal tissue starts after morphogenesis of crown is finished. However, the molecular mechanism for the transition from crown to root in the development has not been elucidated. Fibroblast growth factor (Fgf)-10, which is expressed in the dental mesenchyme, plays an important role in both cell-proliferation and differentiation of dental epithelium in the stage of crown morphogenesis. But, the expression of Fgf-10 disappears in the stage of root formation. On the other hand, in continuously growing teeth such as rodent incisor, vole-, rabbit-and gineapig-molar, the eternal formation of crown analog (labial side in case of incisors), which consists of enamel and dentin, resulting from the continuous expression of Fgf-10 at the apical end. To clarify significance of disappearance of Fgf-10 in the stage of root formation, we observed the growth of incisors of Fgf-10 deficient mice (lethal at postnatal 0 day) by implantation underneath the kidney capsule. In mutant type formation of enamel finished on the way of the development of incisors and only dentin continued to be formed. The incisors of Fgf-10 deficient mice mimicked the development of mouse-molars exactly. For example, Hertwig' epithelial sheath was observed not only in root analog of the lingual side but also in apical end of labial sides and both of the root sheath were fragmented concomitantly with the formation of root dentin. These results suggest that the disappearance of Fgf-10 is a key in formation of Hertwig' epithelial sheath and subsequently root formation with periodontal tissue.