Scientists in Massachusetts said Sunday they had succeeded in creating the world's first cloned human embryos, a controversial advance intended to speed the development of new medical therapies but which also could hasten the arrival of the first baby produced by cloning.

The cloned human embryos, made by researchers at Advanced Cell Technology in Worcester, grew for only a few hours long enough to form microscopic balls containing four to six cells each. The creations made from the fusion of a single skin cell and a human egg are still so unformed that some ethicists and scientists remain divided over whether they should be called embryos and to what extent they may deserve special moral standing.

Nonetheless, the work broke enough new scientific and ethical ground to reignite a debate over human cloning. It's now illegal for federally supported scientists to conduct research on human cloning, but privately funded scientists such as those at ACT are under no such restriction. Several members of Congress on Sunday vowed to place legal questions about cloning on the top of their agendas.

Michael West, ACT's chief executive, has said he has no interest in making cloned human babies. Rather, the goal is to coax cloned embryos to grow for just a few days, then isolate from them embryonic stem cells, which have the capacity to grow into all kinds of human tissues a process called therapeutic cloning.

Experiments in animals have suggested that cloned human embryonic stem cells could launch an era of regenerative medicine in which replacement tissues and perhaps organs could be grown for transplantation. "Human therapeutic cloning could be used for a host of age- related diseases," West said in a statement.

In August, President Bush announced a policy that allows federally financed scientists to conduct research on human embryonic stem cells. But those cells must be retrieved from embryos that were created by in-vitro fertilization and had been scheduled for destruction at fertility clinics. Federally funded stem-cell research involving cloned embryos is precluded under the Bush policy.

Some scientists have argued, however, that the best way to make stem cells for transplantation into patients is to grow them from embryos that are clones of those patients. That way, the stem cells and the tissues made from those stem cells would be genetically identical to the patient and, in theory, less likely to be rejected by the patient's immune system.

That approach has raised ethical concerns because it would require the creation of human embryos with the intent of destroying them. The latest work doesn't show that stem cells can be derived from cloned human embryos, because the clones didn't live past the six- cell stage. Typically, an embryo must grow to a mass of a few hundred cells before it gives rise to stem cells.

But the work breaks new scientific ground by demonstrating that a single cell taken from a human adult can be coaxed to turn into what appears to be a healthy young embryo a feat until now accomplished only in farm animals and mice. And it breaks new ethical ground by creating the beginnings of a human being from a single parent a step that many people have said is, at a minimum, morally precarious.

The only previous report of such an experiment was by South Korean scientists in 1998. But that work was never published in a scientific venue and some experts have questioned whether it was as successful as the scientists there had reported.

Scientists involved in the latest work said it was justified because they and the company's ethics advisory board had concluded that the creations did not have the same moral standing as conventional human embryos and because the creations had such great potential to reduce human suffering. "This work sets the stage for human therapeutic cloning as a potentially limitless source of immune-compatible cells for tissue engineering and transplantation medicine," Robert Lanza, vice president in charge of medical and scientific development at ACT, said in a news release. "Our intention is not to create cloned human beings, but rather to make lifesaving therapies for a wide range of human disease conditions, including diabetes, strokes, cancer, AIDS, and neurodegenerative disorders such as Parkinson's and Alzheimer's disease."

Some spoke out Sunday in favor of the work. "We believe that reproductive cloning should be prohibited but therapeutic cloning should be allowed to go forward with oversight" by the Food and Drug Administration, said Carl Feldbaum, president of the Biotechnology Industry Organization.

But members of Congress and others on Sunday warned that they would work to preclude further such studies in this country. "I find it very, very troubling and I think most of the Congress would," said Sen. Patrick Leahy, D-Vt.

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