Expression and Function of FGFs-4, -8, and -9 Suggest Functional Redundancy and Repetitive Use as Epithelial Signals During Tooth Morphogenesis

The study analyzed the expression patterns of fibroblast growth factors Fgf-4, -8, and -9 in mouse molar and incisor tooth germs to understand their roles in tooth morphogenesis. Fgf-8 and Fgf-9 were initially expressed in the oral epithelium and later localized to the dental epithelium, with Fgf-8 disappearing after the start of epithelial budding. Fgf-4 and Fgf-9 were upregulated in the primary enamel knot and later in secondary enamel knots, influencing tooth shape. Fgf-9 persisted in the inner enamel epithelium and cervical loops of incisors. FGFs stimulated Msx-1 expression in dental mesenchyme, suggesting their role as epithelial signals in tooth formation. The study indicated that FGF-8 and FGF-9 are involved in tooth development initiation, while FGF-4 and FGF-9 regulate tooth shape, with FGF-9 also potentially involved in odontoblast differentiation. The coexpression with other signaling molecules suggests FGFs' participation in odontogenesis signaling networks.