Scientists have replicated one of the most significant accomplishments in stem cell research by creating human embryos that were clones of two men.

The lab-engineered embryos were harvested within days and used to create lines of infinitely reproducing embryonic stem cells, which are capable of growing into any type of human tissue.

The work, reported Thursday in the journal Cell Stem Cell, comes 11 months after researchers in Oregon said they had produced the world's first human embryo clones and used them to make stem cells. Their study, published in Cell, aroused skepticism after critics pointed out multiple errors and duplicated images.

In addition, the entire effort to clone human embryos and then dismantle them in the name of science troubles some people on moral grounds.

The scientists in Oregon and the authors of the new report acknowledged that the clones they created could develop into babies if implanted in surrogate wombs. But like others in the field, they have said reproductive cloning would be unethical and irresponsible.

The process used to create cloned embryos is called somatic cell nuclear transfer, or SCNT. It involves removing the nucleus from an egg cell and replacing it with a nucleus from a cell of the person to be cloned. The same method was used to create Dolly the sheep in 1996, along with numerous animals from other species.

Human cloning was a particular challenge, in part because scientists had trouble getting enough donor eggs to carry out their experiments. Some scientists said SCNT in humans would be impossible.

Dr. Robert Lanza, the chief scientific officer for Advanced Cell Technology Inc. in Marlborough, Mass., has been working on SCNT off and on for about 15 years. He and his colleagues finally achieved success with a modified version of the recipe used by the Oregon team and skin cells donated by two men who were 35 and 75.

After swapping out the nucleus in the egg cell, both groups used caffeine to delay the onset of cell division a technique that has been called "the Starbucks effect." But instead of waiting 30 minutes to prompt cell division, as was done in the Oregon experiment, Lanza and his team waited two hours.

It remains unclear exactly how the egg causes the cells in previously mature tissues in this case, skin to transform into a more versatile, pluripotent state.

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Results are a leap for embryonic stem cells