Today, more than 200 million people in the United States, Europe and Japan are missing at least one tooth, and in the U.S. alone, some 200,000 implant procedures are performed annually. But even state-of-the-art prosthetics are a poor substitute for authentic pearly whites: The titanium structure is often rejected by the surrounding tissues. Moreover, implants are rigid and can't adapt as a person's bite changes over time. "When implants go wrong, it's just a mess," says Yelick. "Dentists are thrilled that there will be an alternative." That alternative is being developed in a lab two floors above Yelick's fish tanks--the lab where her team is growing teeth.