Antagonistic actions of Msx1 and Osr2 pattern mammalian teeth into a single row.

Mammals have single-rowed dentitions, whereas many nonmammalian vertebrates have teeth in multiple rows. Neither the molecular mechanism regulating iterative tooth initiation nor that restricting mammalian tooth development in one row is known. We found that mice lacking the transcription factor odd-skipped related-2 (Osr2) develop supernumerary teeth lingual to their molars because of expansion of the odontogenic field. Osr2 was expressed in a lingual-to-buccal gradient and restricted expression of bone morphogenetic protein 4 (Bmp4), an essential odontogenic signal, in the developing tooth mesenchyme. Expansion of odontogenic field in Osr2-deficient mice required Msx1, a feedback activator of Bmp4 expression. These findings suggest that the Bmp4-Msx1 pathway propagates mesenchymal activation for sequential tooth induction and that spatial modulation of this pathway provides a mechanism for patterning vertebrate dentition.