Researchers hope to one day use stem cells to heal burns, patch damaged heart tissue, even grow kidneys and other transplantable organs from scratch. This dream edges closer to reality every year, but one of the enduring puzzles for stem cell researchers is how these remarkable cells know when it's time for them to expand in numbers and transform into mature, adult cells in order to renew injured or aging tissue. The answer to this crucial decision-making process may lie in a most remarkable organ: the front tooth of the mouse. Constantly growing incisors are the defining feature of all rodents, which rely on these sharp, chisel-like gnashers for burrowing and self-defense, as well as gnawing food. Inside the jaw, a mouse's incisors look more like a walrus's tusks or the teeth of a saber-toothed tiger, with only the sharpened tips showing through the gums at the front of the mouth. As the front of the tooth gets ground down, a pool of stem cells deep inside the jaw, at the very inner part of the tooth, is constantly building up the back of each incisor and pushing the growing tooth forward -- a bit like the lead of a mechanical pencil. "As we grow older, our teeth start to wear out, and in nature, once you don't have your teeth anymore, you die.
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