Inductive changes in early tooth development: I. A study of mouse tooth development on the chick chorioallantois

The developing jaws of embryonic mice, 9-12 days postconception, were dissected out, and the presumptive tooth-bearing areas were separated into the mesodermal and ectodermal portions. Recombinations were grown on the chick chorioallantois. It ap-peared that the morphogenic fields are de-termined by day 10 for molars and by day 10.5 for incisors. The ectodermal com-ponent determined tooth type from day 10. In 1939, Butler ' proposed three potential fields that determine the morphology of the developing teeth in mammals (Fig 1): an anterior incisivation field, a posterior molari-zation field, and an intermediate caninization field. In various mammals the three fields were postulated to be modified or even sup-pressed. In cows and sheep, for instance, the upper anterior field was suppressed, re-sulting in no upper anterior teeth, whereas in the lower jaw, the canine field was par-tially suppressed and the tooth bud in-fluenced by the incisivation field to produce an incisiform canine tooth (Fig 1, D, E). In rodents, both anterior fields were modified and the canine fields entirely suppressed, re-sulting in the typical rodent dentition (Fig 1, F, G). Until recently, it has been thought that the morphology of teeth, as well as that of other organs derived from both ectoderm and mesoderm, was primarily determined by the ectodermal component. However, Sell-man2 and Horstadius3 demonstrated that the presence of neural crest cells-ecto-mesenchyme-determined whether teeth de-veloped in the larval forms of the urodeles. Sengel4 showed that the mesodermal com