Researchers Discover Why Teeth Form in a Single Row

In 1975, Steven Spielberg ruined America’s summer vacation with the release of his man-versus-shark classic Jaws. Universal Pictures promoted Spielberg’s blockbuster in cinema lobbies with a signature poster featuring the word JAWS in blood red, an innocent young woman swimming in the water below, and the open mouth of a gigantic shark rising to the surface with sinister intentions. To highlight the gruesome attack to come, the artist accurately depicted the shark as possessing multiple rows of teeth. This artistic flourish has led many a spooked moviegoer to wonder then and now: Why do most mammals manage fine with a single row of teeth, while sharks and other creatures of the deep look like they’re chewing barbed wire?

In the February 27 issue of the journal Science, NIDCR grantees reported they may have solved the mystery. Teeth normally form early in embryonic development from a well-recognized mode of tissue induction in which a cell layer called the mesenchyme interacts with another known as the epithelium. This dynamic give-and-take activates the needed genetic programs within each layer to precisely pattern a tooth. Orchestrating the process are DNA-binding proteins called transcription factors. They determine which genes get turned on and off. In the current paper, the scientists found in mice that the transcription factor Osr2 restricts the spatial field in which a tooth can form in the mesenchyme. Remove Osr2, and extra teeth form adjacent to the molars. The scientists found that when present, Osr2 counters the field-expanding actions of the Msx1-Bmp4 pathway in mesenchyme. “In mammals, Osr2 suppresses this pathway along the buccolingual axis [straight across from cheek to tongue] to restrict molar development to one tooth row,” the researchers concluded. “Diversity in dentition patterns in other vertebrates is likely due, at least in part, to evolutionary changes in antagonistic interactions regulating this pathway across the tooth morphogenetic field.”


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