Detection and analysis of a factor concerned with cell differentiation and growth during mouse molar tooth development by means of the grafting to a chick wing bud.

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Tooth development involves reciprocal epithelio-mesenchymal interactions and complex morphogenetic events. Because these processes bear similarities to those occurring in the developing limb, we asked whether signaling mechanisms found in the limb also occur in the tooth. We grafted mouse embryonic tooth germs to the anterior margin of the host chick embryonic wing buds and determined whether the tooth tissues had polarizing activity. Indeed the grafted tooth germs induced the formation of the ectopic digits as a miller image as well as the surface structures. These activity of the molar tooth germ increased from bud to cap stage and reached at the maximum at late bell stage. With further development, the polarizing activity began to decrease, however, once it became the highest level at neonates. And then the activity decreased to undetectable level at 3-week old.In situ hybridization demonstrated that the tooth germ grafts induced expression of chick HoxD cluster genes prior to induction of ectopic digits. We concluded that the factors produced by the tooth germs were similar to those in the zone of polarizing activity in the limb development and might regulate morphogenesis and differentiation in the molar tooth development.