University of Wisconsin-Madison scientists have made another breakthrough in stem cell research.

Stem cell pioneer James Thomson and his colleagues have developed a safer way of turning cells from newborns' foreskins into something similar to human embryonic stem cells.

Thomson was the first to isolate human embryonic stem cells in 1998.

Previous efforts to convert cells left behind viruses and outside genes, possibly leading to mutations, impairing the cells' growth or creating tumors.

Thomson and his team reported Thursday in the journal Science that they solved this problem by delivering special genes with a plasmid, which is a small, stable circle of DNA. This material reprogrammed the skin cells before it was removed. Cells that appear to have embryonic stem cells' healing potential remained.

Stephen Duncan, a stem cell researcher at the Medical College of Wisconsin, said the technique holds promise because any basic molecular biology lab can use it. He was not involved in the UW-Madison research but has been working along similar lines.

"I think we're getting to a position where we can start to think about using these cells therapeutically," Duncan said.

Alan Trounson, president of the California Institute of Regenerative Medicine, predicted the new method "will be recognized as another important step forward on reprogramming cells."

Cell reprogramming has proved one of the most dynamic areas of biology in recent years as a growing number of researchers pursue alternatives to using human embryonic stem cells.

Some people object to using those cells because human embryos are destroyed in their production.

Thomson and other scientists have returned cells to an embryonic state, offering a potential solution to the controversy. However, their methods rendered the cells unsafe for use in humans.

That led to a stream of research on improving the safety and efficiency of reprogramming. Several methods have been developed.

"At this point, there is no way to see which one is going to be superior over the others," said Andras Nagy, a senior scientist at Toronto's Mount Sinai Hospital. "Most likely there will be no single winner of this race."

Nagy developed a method that delivers reprogramming genes with a mobile genetic element called piggyBac, which can be popped out of a cell's genome when its work is done.

Thomson said the method used to send the cells back to their embryonic state matters less than how normal the cells are when they get there.

He said his lab will now analyze the reprogrammed cells carefully to determine how closely they match human embryonic stem cells. So far, there appear to be subtle differences between the two, he said.