This breakthrough brings researchers closer to developing individualized disease treatments, but could also be used by rogue scientists to advance research into cloning a human.
The embryonic stem cells that the researchers culled have the potential to develop into any type of tissue or cell in the body. Embryonic stem cell research is controversial because harvesting the cells destroys the embryo.
The research could, in theory, eventually allow scientists to take a plug of skin or blood sample from a patient and use it to grow perfectly matched tissue, organs or batches of cells.
"Our approach opens the door for the use of these specially developed cells in transplantation medicine," Woo Suk Hwang of Seoul National University in Korea, who led the study that will be published Friday in the journal Science, said in a statement.
Stem-cell researcher Dr. Rudolf Jaenisch of the Whitehead Institute for Biomedical Research in Cambridge, Mass., hailed the advance, telling the Associated Press the work provides proof that human therapeutic cloning is possible.
Still, "it's not of practical use at this point," Jaenisch said, adding that years of additional research are required before embryonic stem cell transplants could be considered for people.
In their experiments, the Seoul researchers used 242 eggs collected from 16 unpaid volunteers. These women also donated some cells from their ovaries.
Using the same process used to clone animals, they removed the gene-containing nucleus of each egg and replaced it with the nucleus from the donor's ovarian cell.
The scientists then activated the egg cells using a chemical process that caused the eggs to grow as if they had been fertilized by sperm. Through this process, the scientists succeeded in getting 30 embryos to grow to a stage when stem cells could be extracted.
The researchers were eventually able to harvest just one stem cell line from those embryos. Those stem cells began forming muscle, bone and other tissues in test tubes and when implanted into mice, the Seoul team reported.
Outside experts on cloning praised the work, that was unveiled Thursday at a meeting of the American Association for the Advancement of Science.
"It is a very impressive study. It obviously represents a major medical milestone," Dr. Robert Lanza, who has helped lead cloning experiments at Advanced Cell Technology, told Reuters.
"I think it could help spur a medical revolution," he added.
Lanza said it is now important that laws be passed to ban using cloning to make a human baby. The early stages of the development of these stem cells would also be used to create a cloned baby.
He noted that some researchers, notably Kentucky fertility expert Dr. Panos Zavos, have been trying to clone a baby.
"He's got the cookbook now. It's scary. We really need to move as soon as possible," Lanza said.
That possibility concerns those opposed to the research.
"The how-to instructions have been posted," Richard Doerflinger of the U.S. Conference of Catholic Bishops, told the AP. "If you can bring an embryo to the one-week-old stage, you can implant that embryo in the womb. Once you do, no government can stop you unless they want to coerce abortions."
Some also consider it unethical to destroy embryos as part of this stem cell research.
Responding to a reporter's question in 2001, President Bush said, "The use of embryos to clone is wrong. We should not, as a society, grow life to destroy it."
The Bush administration policy forbids any federally funded research on stem cells from embryos destroyed after Aug. 9, 2001.