Ever wonder why sharks get several rows of teeth and people only get one? Some geneticists did, and their discovery could spur work to help adults one day grow new teeth when their own wear out.

A single gene appears to be in charge, preventing additional tooth formation in species destined for a limited set. When the scientists bred mice that lacked that gene, the rodents developed extra teeth next to their first molars — backups like sharks and other non-mammals grow, University of Rochester scientists reported Thursday.

If wondering about shark teeth seems rather wonky, consider: Tooth loss from gum disease is a major problem, here and abroad, and dentures or dental implants are far from perfect treatments. If scientists knew exactly what triggers a new tooth to grow in the first place, it's possible they could switch that early-in-life process on again during adulthood to regenerate teeth.

"It's exciting. We've got a clue what to do," said Dr. Songtao Shi of the University of Southern California School of Dentistry, who said the Rochester discovery will help his own research into how to grow a new tooth from scratch.

Also intriguing: All the mice born without this gene, called Osr2, had cleft palates severe enough to kill. So better understanding of this gene might play a role in efforts to prevent that birth defect, the Rochester team reported in the journal Science.

Teeth may not be visible until long after birth, but they start to form early in embryo development. Teeth ultimately erupt from a thickened band of tissue along the jaw line called the dental lamina, a band that forms in a top layer of the gum called the epithelium. Scientists have long thought the signals for tooth formation must lie in that tissue layer as well.

Not so, the Rochester team found: All the action takes place instead in a deeper cell layer called the mesenchyme.

Think of the Osr2 gene as a control switch, a kind of gene that turns on and off the downstream actions of other genes and proteins. In that mesenchymal tissue, the Osr2 gene works in concert with two other genes to make sure budding teeth form in the right spot, said lead researcher Dr. Rulang Jiang, a geneticist at Rochester's Center for Oral Biology.

"It's almost a self-generating propagation of the signal" that leads to one tooth after another forming all in a row, he explained.

Knocking that molecular pathway out of whack causes either missing or extra teeth to result, Jiang showed in a series of mouse experiments.

(Agencies via China Daily February 28, 2009)