In experiments that could open a new era in stem cell biology, scientists have found a simple way to reprogram mature animal cells back into an embryonic-like state that allows them to generate many types of tissue. The research, described as game-changing by experts in the field, suggests human cells could in future be reprogrammed by the same technique, offering a simpler way to replace damaged cells or grow new organs for sick and injured people. The experiments were reported in two papers in the journal Nature on Wednesday.