I thought this mousey was chewing gum, but it turns out that green lump in its mouth is a fully-functional, bioengineered tooth, the result of "tooth regenerative therapy" research at Tokyo University (of the ear-pulling navigation system fame).
Basically that little mouse, let's call him Jerry, lost a tooth and grew a new one in its place with the help of some scientists:
To create the new tooth, the researchers [...]took epithelial cells and mesenchymal cells (about 50,000 each) from a mouse embryo and cultivated them together in a collagen-based medium to create a tiny tooth bud — a mass of tissue that has the potential to develop into a tooth.
This mass of tissues was implanted in the spot where Jerry's old tooth used to be and after 50 days that mouse could nibble on cheese properly again. The new tooth grew to the same height as the surrounding ones, is just as hard, and has all the same blood vessels and nerves.
Researchers are hoping that this bioengineering process will one day make dentures and false teeth obsolete, but I think that anyone watching Shark Week is secretly imagining growing a mouthful of chompers like that.