A system of opposing genetic forces determines why mammals develop a single row of teeth, while sharks sport several, according to a study published in the journal Science. When completely understood, the genetic programme described in the study may help guide efforts to re-grow missing teeth and prevent cleft palate, one of the most common birth defects.

Researchers discovered that turning off a single gene in mice resulted in development of extra teeth, next to and inside of their first molars. While the study was in mice, past studies have shown that the involved biochemical players are active in humans as well.

'This finding is exciting because extra teeth developed from tissue that normally does not give rise to teeth,' said Rulang Jiang, PhD, Associate Professor of Biomedical Genetics at the University of Rochester Medical Center, USA and an author of the article (Science 2009; 323(5918): 1232–1234).

'It was amazing to find that deleting one gene caused the activation of a complete tooth developmental programme. Finding out how the extra teeth developed will reveal how nature makes a tooth from scratch, which will guide tooth regeneration research.'