But challenges still exist. Tumors developed frequently in mice that were cloned from the altered cells, because the genes that make them pluripotent also cause cancer later in life. The researchers agreed that the technique would have to be modified before it could be used on human cells.
Harvard's Hochedlinger, publishing in Cell Stem Cell, demonstrated that the reprogrammed cells retained crucial "epigenetic markers" that are important for the cell's function.
"There should be no fear," he stated, that the reprogramming itself would make cellular processes go awry. His team also found that these markers showed up in subsequent generations of the unique mouse cell line, and that the faux "stem cells" could be coaxed into "therapeutically relevant cell types, like heart tissue and blood."
Dr. George Daley of the Harvard Children's Hospital expressed concern about the political implications of the work. "It is important to note that the opponents of stem cell research will seize upon these studies as cause to stop funding human [embryonic stem] cell research...that these alternatives obviate the need to use embryos in research."
Meanwhile, Dr. Kevin Eggan of Harvard University's Stem Cell Institute has a paper in Nature that debunks the scientific belief that only unfertilized eggs can have their genetic material swapped out in place of new DNA.
This means that scientists may one day use fertilized human eggs to create clones, from which stem cells could be harvested for research.
In a phone interview, Eggan explained that, for over a year, he had had funding and review board approval to create genetic clones -- and human stem cell lines -- from human eggs, just like Dolly the sheep was created by plucking the gene-containing nucleus out of an egg and replacing it with an adult sheep's DNA.
But he needed human eggs. While women readily respond to advertisements for egg donation in newspapers of elite colleges -- "$10,000 if you're blond and blue-eyed with a high SAT score" -- medical researchers cannot compensate women in the same way that infertile couples can.
Despite thousands of dollars in ads running in paid Boston-area publications, Eggan couldn't find a single donor. When they heard they wouldn't be paid for the months of hormonal treatment and sometimes painful egg retrieval, all the women declined.
"That left us with a lot of time to get back to first principles, to think about why we needed unfertilized eggs in the first place," said Eggan.
At a conference, it clicked -- human frozen fertilized eggs are plentiful. In the lab, mouse fertilized eggs, halted at a crucial step in cell division, received new nuclei from other mouse cells. The new embryos -- clones -- continued to develop into baby mice.
The paper notes that even eggs with severe defects accepted the new genetic material and developed.
"[On their own] these eggs would never have made a viable fetus," said Eggan, adding that he hopes that fertility clinics that would normally throw away such defective cells will instead donate them to research.
"There are a half million fertilized eggs created every year in the U.S., and 3-10 percent of those will not develop. The embryologist can see that they don't have the right number of chromosomes, even at the one-celled stage."