The continuous eruption of mouse incisors throughout life is maintained by adult stem cells in the apical end.
In these teeth, the continuous expression of Fgf10 in the mesenchyme plays a role in the maintenance of the epithelial
stem cell compartment, referred to as the "apical bud." However, little is known about the epithelial signaling that induces
and maintains Fgf10 expression. Focusing on the epithelial-mesenchymal interaction during tooth development, we thoroughly
investigated candidates expressed in the apical bud. In situ hybridization and immunostaining showed that Fgf9
mRNA and protein were detected in the basal epithelium, stellate reticulum, and inner enamel epithelium of the apical
bud. Recombinant Fgf9 protein stimulated cell proliferation in cultures of apical end mesenchyme. Furthermore, Fgf9-
releasing beads inhibited apoptosis in mesenchymal tissue cultures and maintained the expression of Fgf10. On the other
hand, Fgf10-releasing beads induced Fgf9 expression in cultures of apical buds. Taken together, these results suggest that
the stem cell niche in growing incisors is maintained by an epithelial mesenchymal interaction via Fgf9 and Fgf10 signaling.
New perspectives on tooth development and the dental stem cell niche
