Scientists in Spain said on Wednesday they had made mature cells in living mice revert to their youthful, versatile state, in a step toward the goal of tissue regeneration by stem cells.

Right now, the technique is at its earliest stage and is hedged with safety questions, which makes it impossible to envisage in humans.

But, said the researchers, it opens up a new strategy leading to a beguiling end: that one day damaged tissue will be healed by simply reprogramming nearby adult cells into replacements for the lost or diseased area. A transplant would not be needed.

Stem cells have excited huge interest in medical research.

They are immature cells that differentiate into the specialized cells that comprise and maintain the human body.

In 2006, a team led by Shinya Yamanaka in Japan announced a breakthrough.

A clutch of four genes introduced into adult cells in a lab dish rewound these cells back to their baby state.

These so-called induced pluripotent stem cells -- known by their acronym of iPS -- have since become the most closely-followed innovation in the field.

Despite many hurdles, they are seen by some as being even more promising than embryonic stem cells, the "gold standard" for versatility but a source hotly opposed by moral conservatives.

Reporting in the journal Nature, a team led by Manuel Serrano and Maria Abad of the Spanish National Cancer Research Center created genetically-modified mice that carried the four "Yamanaka genes."

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Adult Stem Cells Programmed Into Embryonic State