Odontogenic epithelium induces similar molecular responses in chick and mouse mandibular mesenchyme

Previous observations have shown that, during the initiation phase of odont ogenesis, signals from mouse odontogenic epithelium can elicit teeth in non -odontogenic but neural crest-derived mesenchyme isolated from ectopic site s including chick mandibular mesenchyme. In the present study the formation of ectopic tooth buds and dental mesenchyme in chick mandibular mesenchyme was examined using heterospecific recombinations between E11 mouse odontog enic epithelium and stage 23 chick lateral mandibular mesenchyme. Both morp hological criteria and chick-specific probes for Msx-1, Msx-2, and Bmp-4 mR NAs were used as markers for early dental mesenchyme. Our results demonstra ted that interactions of mouse odontogenic epithelium with chick mandibular mesenchyme induce early changes in the chick mandibular mesenchyme includi ng the appearance of a translucent zone, cell proliferation, and induction of expression of Msx-1, Msx-2, and Bmp-4, which have been shown to be assoc iated with the formation of dental mesenchyme. In addition, tooth bud-like structures that resemble E13 tooth buds in vivo both morphologically and in their patterns of gene expression formed after 6 days in the heterospecifi c recombinations. The tooth bud-like structures consist of invaginated mous e mandibular epithelium and condensed chick mandibular mesenchyme expressin g high levels of Msx-1 and Bmp-4, but undetectable levels of Msx-2. Unlike the induction of Msx-1, Msx-2, and Bmp-4 in the underlying mesenchyme, whic h is specific for signals derived from odontogenic epithelium, the inductio n of a translucent zone and cellular proliferation in the underlying mesenc hyme may be related to the growth-promoting potential of embryonic epitheli a and not be specific to signals derived from the odontogenic epithelium. S imilar to mouse odontogenic epithelium, agarose beads soaked in recombinant BMP-4 induced a translucent zone, cellular proliferation, and expression o f Msx-1, Msx-2, and Bmp-4 in chick mandibular mesenchyme after 24 hours. Th ese observations together showed that avian mandibular mesenchyme has odont ogenic potential that is expressed upon interactions with inductive signals from mouse odontogenic epithelium. Similar to odontogenesis in vivo, forma tion of dental mesenchyme in chick mandibular mesenchyme is mediated by the activation of Msx-1, Msx-2, and Bmp-4 Dev. Dyn. 1998;213:386-397. (C) 1998 Wiley-Liss, Inc.