Scientists have recovered stem cells from cloned human embryos in a breakthrough that could lead to new treatments for such illnesses as Parkinson's disease and diabetes.
US scientists said in Wednesday's edition of the journal Cell that they had harvested stem cells from six embryos created from donated eggs, in a technique using methods like those that produced Dolly the sheep in Britain.
Shoukhrat Mitalipov of the Oregon Health & Science University, who led the research, said that two embryos had been given DNA from skin cells of a child with a genetic in disorder and the others had DNA from fetal skin cells.
Mitalipov said the success came not from a single technical innovation, but from revising a series of steps in the process. He noted it had taken six years to reach the goal after doing it with monkey embryos.
He said that based on monkey work, he believed human embryos made with the technique could not develop into cloned babies, and that he had no interest in trying to do that.
However, opponents said it was unethical to experiment on human embryos and called for a ban.
Scientists have cloned more than a dozen kinds of mammals, starting with Dolly the sheep.
The new work was financed by the university and the Leducq Foundation in Paris.
Dr George Daley, a stem cell expert at Children's Hospital Boston who did not participate in the work, called the new results one landmark step in a very long journey towards creating DNA-matched transplant tissue.
Daley said that scientists must now compare the embryo-cloning approach with another technology that reprogrammed blood or skin cells directly into substitutes for embryonic stem cells.
This reprogramming approach is technically simpler and does not involve embryos or require the donation of human eggs.
It was widely acclaimed when it was reported in 2007 and its Japanese developer shared a Nobel Prize last year.
But these substitute cells show some molecular differences compared to embryonic ones, which has led to questions about whether they can safely be used for treating patients.
Daley said that this was another tool and it was essential to compare the cells from the two methods. "We have to learn more about this tool," he said.
Daley said he believed that scientists would prefer using the reprogramming approach unless it could be proven beyond any doubt that embryo cloning produced better cells for treating patients.
Mitalipov said that he believed his technique would present a particular advantage for treating patients with certain types of rare diseases. These are caused by mutations in genes of the mitochondria, the power plants of cells.
He noted his technique, unlike the cell-reprogramming approach, would supply tissue with new mitochondrial genes that could replace defective ones.
Those new genes would come from the egg.
The Reverend Tad Pacholczyk, director of education for National Catholic Bioethics Centre in Philadelphia, reiterated his opposition to embryo cloning and called the approach unethical.
"It involves the decision to utilise early human beings as repositories for obtaining desired cells,'' Pacholczyk said. "You're creating them only to destroy them."
Marcy Darnovsky, executive director of the Centre for Genetics and Society in Berkeley, California, said that she was glad that Mitalipov doubted the embryos could be used to clone babies.
She said the report still provided a good opportunity for the US government to ban the use of cloning for reproduction.