Guardian Unlimited | The Guardian | Scientists get to the root of colour loss: "The scientists traced the greying process to the demise of a type of stem cell in the skin. These make a continuous supply of melanocytes, cells which produce pigments in skin and hair. These stem cells not only get depleted over time, they also start making mistakes, turning into melanocytes in the wrong place in the hair follicle where they are unable to colour the hair."