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In a surprising advance that sidesteps the ethical debates surrounding stem cell biology, researchers have come much closer to a major goal of regenerative medicine, the conversion of a patient's cells into specialized tissues that might replace those lost to disease.

The advance is an easy-to-use technique for reprogramming a skin cell of a mouse back to the embryonic state. Embryonic cells can be induced in the laboratory to develop into many of the body's major tissues.

If the technique can be adapted to human cells, it would let scientists use a patient's skin cell to generate new heart, liver or kidney cells that might be transplantable and would not be rejected by the patient's immune system.

Previously, the only way scientists knew they were likely to get such cells is by nuclear transfer, the insertion of an adult cell's nucleus into an egg whose own nucleus has been removed. The egg somehow reprograms the nucleus back to embryonic state.

The new technique, developed by Shinya Yamanaka of Kyoto University, depends on inserting just four genes into a skin cell. These accomplish the same reprogramming task as the egg, or at least one very similar.

The technique is much easier to apply than nuclear transfer, does not involve the expensive and controversial use of human eggs, and should avoid all or almost all of the ethical criticism directed at the use of embryonic stem cells.

"From the point of view of moving biomedicine and regenerative medicine faster, this is about as big a deal as you could imagine," said Irving Weissman, a leading stem cell biologist at Stanford University.

David Scadden, a stem cell biologist at the Harvard Medical School, said the finding that cells could be reprogrammed with simple biochemical techniques "is truly extraordinary and frankly something most assumed would take a decade to work out."

The new technique seems likely to be welcomed by many who have opposed human embryonic stem cell research. It "raises no serious moral problem, because it creates embryonic-like stem cells without creating, harming or destroying human lives at any stage," said Richard Doerflinger, a spokesman on stem cell issues for the U.S. Conference of Catholic Bishops. In themselves, embryonic stem cells "have no moral status," and the bishops' objections to embryonic stem cell research rest solely on the fact that human embryos must be harmed or destroyed to obtain them, he said.

Ronald Green, an ethicist at Dartmouth College, said it would be "very hard for people to say that what is created here is a nascent form of human life that should be protected." The new technique, if adaptable to human cells, "will be one way this debate could end," he said.

Ever since the creation of Dolly, the first cloned mammal, scientists have sought to lay hands on the mysterious chemicals with which an egg will reprogram a mature cell nucleus injected into it and set the cell on the same path of embryonic development as when egg and sperm combine.