The approach they used — called somatic cell nuclear transfer — effectively strips an egg of its chromosomes and packs it instead with DNA from a donor.
Nurtured by a stew of nourishing chemicals and zapped with two jolts of electrical current, many of the eggs began to divide and grew for five to six days. At that point, the embryos had 64 to 200 cells, including a dense inner cell mass from which stem cells were extracted.
In past efforts to coax such an assemblage of components to life, researchers have burned through dozens of donor eggs without getting any embryos even to the 16-cell stage at which stem cells become a remote possibility.
This time, the researchers said their methods were so efficient that they could create at least one embryonic stem cell line from each batch of eggs donated by 10 female volunteers. In one case, a single donor produced eight eggs of such exceptional quality that researchers were able to derive four embryonic stem cell lines.
The volunteers, between the ages of 23 and 31, donated their eggs anonymously and were “financially compensated for the time, effort, discomfort and inconvenience associated with the donation process,” the study authors wrote.
The success of the experiments rekindled debate among bioethicists, who have long anticipated that human cloning would become a reality. In 2002, a commission of bioethicists established by then-President George W. Bush unanimously urged a ban on reproductive cloning. But the panel was deeply divided about the propriety of “therapeutic cloning” for research and medical treatment.
Dr. Robert Lanza, the chief scientific officer of Advanced Cell Technology Inc., said it remained to be seen whether embryonic stem cells generated this way were more useful for studying and treating diseases than stem cells created by reprogramming adult cells to an embryonic state.