Pioneering work by Japanese stem-cell researchers two years ago has taken a major step forward, helping the quest for versatile, grow-in-a-dish transplant tissue, according to papers published yesterday.

Two teams have combined ideas to devise a safer technique for reprogramming skin cells so that they become "pluripotent" stem cells - fundamental cells that then grow into specialized organs.

Their effort builds on an award-winning breakthrough in 2007 by Shinya Yamanaka of Kyoto University. He and his team introduced four genes into skin cells, reprogramming them so that they became indistinguishable from embryonic stem cells.