In mammals, the number of teeth that form an animal's dentition is strictly controlled. In mice, the diastema, a toothless region between the incisors and the molars, contains tooth primordia during embryonic development, but these primordia are suppressed by apoptosis. On , Paul Sharpe and colleagues surprisingly reveal that a defect in primary cilia results in increased hedgehog signalling and in the formation of ectopic teeth in the diastema. Mice mutant for the Ift88/polaris gene, which encodes a cilia intraflagellar transport protein, display ectopic tooth formation that correlates with ectopic sonic hedgehog (Shh) activity, and tissue-specific mutants reveal that polaris is required in the dental mesenchyme, but not the ectoderm, for normal tooth development. The authors also demonstrate that mice mutant for the Shh antagonist Gas1 display increased Shh activity and ectopic diastema teeth. Taken together, these date indicate that, contrary to prior reports, primary cilia negatively regulate Shh activity in the diastema mesenchyme, resulting in the suppression of tooth formation.
