South Korean scientists have reported the first successful cloning of a human embryo and, along with it, the production of stem cells that are theoretically capable of turning into every cell type in the human body.
The achievement, scheduled for publication in tomorrow's issue of the journal Science, is viewed as a crucial milestone in the longstanding effort to replace cells damaged by Parkinson's, heart disease and a host of other ailments.
Though the scientists kept the embryos alive for only a week - just long enough to develop a stock of stem cells - the experiment is likely to spark heated debate over the ethics of creating life to destroy it.
The experiment was couched as an example of "therapeutic cloning," aimed at treating diseases, but critics were voicing concern about the technique slipping into hands of those who would use the embryos to produce human clones.
"Anyone who is irresponsible enough could put them in a womb," said Richard Doerflinger, deputy director of pro-life activities at the U.S. Conference of Catholic Bishops.
The scientists, led by Woo Suk Hwang of Seoul National University, said they were working on a method of providing cells that could be transplanted into an ailing patient without sparking a fatal bout of rejection.
Rejection, a problem encountered in many organ transplants, is the body's way of ridding itself of anything that doesn't belong.
Using technology already employed to produce cloned animals ranging from mice to sheep and cows, the scientists in Korea implanted DNA from non-reproductive cells into eggs emptied of their own genetic material.
In this way, the eggs were given all the genetic information supplied by a sperm and egg during fertilization. Scientists used chemicals that stimulated the eggs to begin dividing and, eventually, to reach the stage where they produced the stem cells that serve as master cells for every part of the human body.
"Our approach opens the door for use of these specially developed cells in transplantation medicine," Hwang said in a prepared statement.
Before publishing the results, Science subjected the report to peer review by a panel of outside experts. In recent years, other scientific teams have claimed to have cloned embryos by the method known as "nuclear transfer," but none submitted its work to peer review. The claims were widely discredited.
Dr. John Gearhart, a stem cell researcher at the Johns Hopkins School of Medicine, cautioned that the Korean scientists didn't show that the cells could be turned into therapies.
Rather, they demonstrated a "proof of concept" - showing that nuclear transfer can be used to produce human embryos and that stem cells can be derived from them.
"The next step would be to show that tissues derived from the stem cells would not be rejected," Gearhart said, explaining that the necessary experiment could be performed in a laboratory dish.
Donated cells, eggs
In the Korean experiment, scientist recruited 16 healthy women who donated a total of 242 eggs. The volunteers were unpaid and were told they would not benefit personally from the research.
Scientists extracted DNA from each subject's cumulus cells - adult cells that provide nourishment for the woman's eggs - and injected it into her eggs. The scientists were able to get 19 of 66 donated eggs to develop to the blastocyst stage, in which an embryo develops a pocket of stem cells.
Dr. Rudolf Jaenisch, a stem cell researcher from the Whitehead Institute in Cambridge, Mass., said future experiments should fuse DNA from one person into an egg from another.
In this way, scientists can be sure that the resulting stem cells truly came from the implanted DNA, not from the host egg.
"And then - and this has nothing to do with cloning - you need to show if you can derive from embryonic stem cells functional cells that can be used for transplantation," Jaenisch said.
Dr. Allan Spradling, a stem cell biologist with the Carnegie Institution in Baltimore, noted that cloned animals have been troubled with genetic defects that have caused many to die early.
Though tissues derived from stem cells are less complicated than fully developed clones - and thus, less prone to error - problems could still arise.
"We don't know that these cells can function normally," he said. "This is research."
Researchers say most U.S. scientists won't be able to experiment with the Seoul researchers' new stem-cell line. Culling stem cells from embryos kills them, and President Bush has forbidden any federally funded research on stem cell lines created after Aug. 9, 2001 - making the South Koreans' recently developed line too new.
Dr. Laurie Zoloth, a bioethicist from Northwestern University in Evanston, Ill., said societies should be able to draw the line between using cloning technology for good and evil.
"These blastocysts are not intended to be inside the human body," she said. "They don't have the equivalent moral status to a child.
"Once you turn away from thinking about them as fully ensouled human babies, then the duty to heal becomes the overriding consideration."
The Associated Press contributed to this article.