Neuronal cells and neurotrophins in odontogenesis.

There is evidence from lower animals that in addition to oral ectoderm and cranial neural crest, tooth formation depends on neuronal cells. To analyze the possible neural influence on mammalian tooth formation, peripheral nerve fibers and neuronal cells were localized in the area of the developing rat first molar tooth germ. Moreover, to study whether factors needed for neuronal development might be involved in the regulation of tooth formation and innervation, expression of NGF-related neurotrophic factors and their receptors were localized by in situ hybridization. The data suggest that although peripheral nerve fibers appear not to be required for odontogenesis, neuronal cells associated with the embryonic rat tooth may participate in the regulation of tooth formation. Localization of neurotrophins and their receptors suggests that besides their apparent roles in the regulation of tooth innervation, they may serve non-neuronal, organogenetic functions during tooth formation. Moreover, it is possible that neuronal characteristics of the dental mesenchymal cells and the presence of neuronal cells in the tooth germs may explain the specific ability of neural crest-derived, but no other mesenchymal, cells to contribute to mammalian tooth formation.