The discovery that cellular development is not a one-way street has earned this year's Nobel Prize in physiology or medicine. John B. Gurdon, a developmental biologist at the Wellcome Trust/Cancer Research UK Gurdon Institute at the University of Cambridge in the United Kingdom, and Shinya Yamanaka, a stem cell researcher at Kyoto University in Japan and the Gladstone Institute at the University of California, San Francisco, have won the prize for their discovery that mature cells can be reprogrammed to resemble the versatile cells of a very early embryo. These so-called pluripotent cells have the ability to become any of the body's tissues. The pair's work, which bridges two eras of modern biology, “revolutionised our understanding of how cells and organisms develop,” the Nobel committee wrote in its award announcement.
…In normal development, cells mature from their pluripotent state into various, specialized cell types a neuron, muscle cell, or skin cell, for example. For many years developmental biologists thought that the cellular maturation process was irreversible. In 1962, however, John Gurdon, working at the University of Oxford, showed that under the right conditions, a mature cell nucleus could become developmentally young again. He replaced the nucleus of a frog egg with a nucleus taken from a cell in a tadpole's intestine. In a few cases, the egg cell was able to “reprogram” the DNA in the tadpole nucleus and the egg cell developed into an adult frog-the first animals cloned from mature cells*. Other researchers built on Gurdon's findings, most famously the team that cloned Dolly the sheep using a similar feat of nuclear transplantation. That breakthrough demonstrated that mammal cells could undergo the same transformation from mature to immature.
More here. (Note: Since the late 1970's when I first read the details of Gurdon's brilliant experiment, I have been waiting for him to win the Nobel Prize…and finally, I feel vindicated and extremely happy!)