Localization of nerve cells in the developing rat tooth.

Earlier studies have shown that mammalian tooth formation can take place in the absence of peripheral nerve fibers. This has been taken to indicate that neurons are not needed for mammalian tooth development. However, our recent localization of peripherin, which is a neuronal cell marker, has suggested that neuronal cell bodies may be associated with developing teeth. In this study, we have analyzed in vivo and in vitro the presence of neuronal cells in developing rat tooth germs. When E14 and E16 rat first molars (thickening of presumptive dental epithelium and bud-stage tooth germ, respectively) were cultured in vitro, peripheral trigeminal axons degenerated. However, with antibodies against peripherin and L1 neural cell adhesion protein, we detected neuronal cell bodies and their axons in the explants. Next, the expression of neurofilament light-chain (NF-L) mRNAs was studied by in situ hybridization of embryonic E12 first branchial arches and tooth germs from initiation to completion of crown morphogenesis (E13, five-day post-natal teeth). NF-L transcripts were first seen at the bud stage (E15) next to the dental epithelium at the buccal side of the tooth germ. At the cap stage (E18), NF-L mRNAs were located under the oral epithelium at some distance from dental epithelium. These expression patterns correlate to the previous localization of peripherin-positive cells and suggest that NF-L expression also revealed neuronal cells. Taken together, these results demonstrate that, in addition to projections of peripheral neurons, neuronal cells are associated with the developing teeth. Hence, it is possible that neuronal cells may participate in the regulation of mammalian tooth formation.