A top scientist using stem cells from human embryos to cure disease and repair injuries will proceed with his work, he said in Tucson on Tuesday, despite a recent breakthrough showing the controversial embryos may no longer be needed.

That blockbuster breakthrough was announced last week by researchers in Wisconsin and Japan, who have discovered how to genetically program human skin cells to behave like embryonic stem cells.

That means the skin cells can develop into any cell in the human body — in the brain, heart, liver, muscle or bones — where they potentially can be used for lifesaving repairs or cures. Until now, only stem cells from human embryos — each a potential human life — could do that.

But scientists deeply involved in human embryonic stem-cell research are unlikely to scrap years of work, however controversial, to start over with the new skin-cell technology, said one noted for cutting-edge achievements in this field.

"I do think a great deal of this work could be done with the skin-cell-derived stem cells. But we'd have to start completely over, from scratch, and we are not going to slow down to do that, not at this point," said Hans Keirstead, a neurobiologist and stem-cell researcher at the University of California-Irvine.

Speaking at University Medical Center, Keirstead outlined his progress, which has taken off in the past three years — after California sidestepped President Bush's ban on federal funding for research using human embryos.

In 2004, California voters overwhelmingly passed Proposition 71, allocating $3 billion in state funds to develop new embryonic stem-cell lines, attracting some of the best and the brightest researchers in the world to relocate there.

Since then, Keirstead and his team at the Reeve-Irvine Research Center (named for the late actor Christopher Reeve, who suffered a devastating spinal-cord injury) have used these new stem-cell lines to restore full mobility to rats paralyzed by spinal cord injuries.

To do that, he had to find a way to purify and differentiate human embryonic stem cells to develop into a type of nervous-system cell needed to restore spinal cell function. He is using similar techniques to transform embryonic cells into Motor neurons to restore muscle function.

These breakthroughs have created a blueprint that can be used to create cells to replace those damaged by such human scourges as Multiple Sclerosis, Parkinson's disease, heart disease, stroke, ulcerative colitis, muscle Atrophy in newborns, Lou Gehrig's disease and diabetes.

Embryonic stem cells — taken from 5-day-old embryos discarded by fertility clinics — "have the potential to address every single human disorder," Keirstead said. "They are the greatest single scientific advance in human history, with the potential to develop into any kind of cell in the human body."

Keirstead's work is expected to set the stage for the first human clinical trial in the world using embryonic stem cells, possibly as early as next year.

Only 40 years old, Keirstead is as passionate about defending the use of human embryos for this purpose as he is for the work itself.

"The ethical controversy starts with the fact that if the embryo were implanted in the uterus, it would develop into a human," he said. "But these embryos were destined to be destroyed.

"It is my personal feeling it's a very ethical decision to use this tissue to end human suffering, to better human life, than to destroy it.

"If you think this is a divine entity with tremendous spiritual value, I don't disagree with you. I value them no less."

Keirstead's unwillingness to abandon progress in embryonic stem-cell research, to begin again with non-controversial human skin cells, actually is shared by the skin-cell researchers at the University of Wisconsin.

They've warned that work with embryonic stem cells must continue, that their research remains in its early stages and that comparison studies must be done to ensure the genetically engineered skin cells do not behave in unexpected ways.

There also are concerns that the techniques used might disrupt human DNA and trigger cancer in patients — questions that must be answered before they can be approved for human experiments.

"It is my personal feeling it's a very ethical decision to use this tissue to end human suffering, to better human life, than to destroy it."

Hans Keirstead, neurobiologist and stem-cell researcher at the University of California-Irvine