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Scientists have made a significant breakthrough by using cloning to create human embryonic stem cells, a move towards developing replacement tissue that could treat disease, according to a study published Wednesday.

The study, published in the journal Cell, used methods like those that produced Dolly the sheep, the first cloned animal.

The cloned embryos were used as a source of stem cells, which can make new heart muscle, bone, brain tissue or any other type of cell in the body.

But few experts think that production of stem cells through cloning is likely to be medically useful soon, or possibly ever.

"An outstanding issue of whether it would work in humans has been resolved," said Rudolf Jaenisch, a biologist at MIT's Whitehead Institute in Cambridge, Mass., who added that he thinks the achievement "has no clinical relevance."

"I think part of the significance is technical and part of the significance is historical," said John Gearhart, head of the Institute for Regenerative Medicine at the University of Pennsylvania. "Many labs attempted it, and no one had ever been able to achieve it."

Meanwhile, opponents say it is unethical to experiment on human embryos and have called for a ban.

Stem cells are one of the great hopes for medicine. Being able to create new tissue might be able to heal the damage caused by a heart attack or repair a severed spinal cord.

"It's been a holy grail that we've been after for years," says Dr. John Gearhart, a stem cell pioneer at the University of Pennsylvania.

But every previous attempt ended in failure or fraud, leading many scientists to wonder if the goal might be impossible to reach.