One hurdle is ethical and political. Human embryos have to be destroyed to produce stem cells. That has aroused opposition to human embryonic stem cell research, and it led the Bush administration to restrict federal funding for it. Scientists say that has slowed science in this effort.
Another hurdle is the inefficiency of the process. Even if the method described by scientists Wednesday works in humans, it would demand too much of a precious resource - women's unfertilized eggs.
The promise of producing stem cells by cloning is that they can be genetically matched to a particular patient. So theoretically, doctors should be able to transplant tissue created from them into that person without tissue rejection. And presumably, such transplants could help treat such conditions as diabetes and spinal cord injury.
The process used in the new experiment is "quite inefficient," Shoukhrat Mitalipov of the Oregon National Primate Research Center in Portland told reporters Wednesday.
He and his colleagues reported getting two batches of stem cells that required using about 150 monkey eggs apiece. That is far too many if one hopes to use human unfertilized eggs, which are cumbersome to obtain from women.
If further work can get that down to maybe five to 10 eggs per stem cell batch, "we will be closer to clinical applications," Mitalipov said.
"I am quite sure it will work in humans," he added.
But then there is another issue - showing that such stem cells really can be used to treat diseases safely. Mitalipov said he plans to do diabetes studies in monkeys.
For now, he and other scientists said, the new work is valuable for showing that stem cells can be produced through cloning in monkeys. It has been done in mice, but scientists had long been frustrated in their attempts in primates, where the research would be more relevant to humans.
The new work was published online Wednesday by the scientific journal Nature. The success was reported earlier this year at a research meeting in Australia, where it received limited media coverage. The results were given new attention Tuesday by a London newspaper, The Independent.
Dr. George Daley of the Harvard Stem Cell Institute, who was familiar with the work, told The Associated Press in an e-mail that it was a "a very important demonstration" that the process is feasible in primates.
Mitalipov's team merged skin cells of a 9-year-old rhesus macaque male with unfertilized monkey eggs that had the DNA removed. The eggs, now operating with DNA from the skin cells, grew into early embryos in the laboratory. Stem cells were recovered from these embryos.
The researchers have applied for a patent on their procedure.
Mitalipov said separate experiments obtained monkey stem cells from a different process called parthenogenesis, in which an egg grows into an early embryo without any genetic contribution from a male. The stem cells were genetic matches to the females that produced the eggs, he said, and early experiments suggest stem cells derived this way may someday prove useful for treating disease in women.
Nature took the unusual step of asking a separate group of scientists to verify Mitalipov's cloning results, and it published the verification along with Mitalipov's paper.
In an e-mail, the journal cited the highly publicized 2004 fraud that came out of South Korea, where researchers led by Hwang woo-Suk claimed to have produced stem cells from a cloned human embryo.
The journal said the research to verify Mitalipov's findings did not signal mistrust, but noted that questions would likely be raised, and "we view this as a relatively straightforward way of putting these questions to rest."
The verification study, by David Cram and others at the Monash University in Australia, used DNA analysis of the male macaque, the two monkeys whose eggs were used to create embryos, and the stem cells. The result "demonstrates beyond any doubt" that the stem cells came from cloned embryos, the Australians wrote in their Nature paper.