Contribution of foregut endoderm to tooth initiation of mandibular incisor in rat embryos.

Classical transplantation experiments with various amphibian tissues have shown that tooth development requires not only oral ectoderm and neural crest but also foregut endoderm. In addition, histological observation of oral membrane showed that the tooth germs are initiated in some ectodermal cells and neural crest cells adjacent to foregut endoderm. These studies suggest that tooth initiation requires the presence and cooperation of these three components. In mammals, however, there is no direct evidence that tooth formation is involved in the region of oral ectoderm adjacent to foregut endoderm. In order to elucidate the contribution of foregut endoderm to tooth formation, we established a new type of endodermal cell tracing system with a recombinant adenovirus called Adex-lacZ, and performed endodermal cell tracing in a long-term culture system. Cells labelled with Adex-lacZ were seen next to non-labelled thickening epithelium, presumptive incisor epithelium. These findings show the first direct evidence in mammals that tooth formation takes place in the specified part of oral ectoderm adjacent to foregut endoderm, suggesting that foregut endoderm plays a role in tooth initiation.