Researchers used stem cells to grow a replacement tooth for an adult mouse, the first time scientists have developed a fully functioning three-dimensional organ replacement, according to a report in the Proceedings of the National Academy of Sciences. The researchers at the Tokyo University of Science created a set of cells that contained genetic instructions to build a tooth, and then implanted this “tooth germ” into the mouse’s empty tooth socket. The tooth grew out of the socket and through the gums, as a natural tooth would. Once the engineered tooth matured, after 11 weeks, it had a similar shape, hardness and response to pain or stress as a natural tooth, and worked equally well for chewing. The researchers suggested that using similar techniques in humans could restore function to patients with organ failure.