Human eggs can turn back time. And that miraculous property captivates stem cell researchers, who need human eggs to make cloned stem cells. But as scientists gear up to do cloning experiments, they are worried that the ethical and medical considerations surrounding egg donation will create a formidable obstacle to their work.

"Without eggs, there's no research," says Robert Lanza, vice president of medical and scientific development at Advanced Cell Technologies (ACT), a biotechnology company in Worcester, MA, that plans to clone stem cells. "It's been a bottleneck for this research from the get-go."

Scientists want to create cloned stem cells because these cells can be turned into different cell types, such as brain cells or insulin-producing cells, to treat patients with diseases such as Parkinson's or diabetes. Because the cloned stem cells are genetically matched to the patient, they are not subject to immune rejection. Stem cells created with DNA from a patient could also be used to generate cell lines that exhibit the genetic abnormalities of these diseases, giving scientists new models to study disease (see The Real Stem Cell Hope).

To make cloned embryonic stem cells, scientists insert the DNA of an adult cell into an egg stripped of its own genetic material. The egg, by an unknown mechanism, reverts the adult DNA back to its embryonic state and develops into an early-stage embryo, much like a normally fertilized egg, eventually generating stem cells whose DNA is identical to that of the adult.

This process, known as either therapeutic cloning or somatic cell nuclear transfer (SCNT), has been the source of major ethical debates. Critics, including President Bush, oppose the creation and destruction of human embryos for research purposes, while supporters say the technology will ultimately save human lives.

But even among proponents of stem cell research, a major ethical question remains: How can scientists get enough eggs without putting women at risk?

In 2005, discredited Korean researcher Hwang Woo Suk claimed to have efficiently generated 11 lines of cloned stem cells. His research was lauded in large part because he claimed to have used very few eggs, which meant the technology might be practical for clinical use. But investigations this year revealed that Hwang's human cloning research was a massive fraud, rife with both scientific fabrication and ethical violations.

For one thing, a report released this month by Korea's National Bioethics Committee concluded that Hwang's team did not adequately inform women of the risks associated with egg donation. The report also found that he used more than 2,000 eggs in his experiments, five times as many as originally reported.

That revelation alarms stem cell researchers in the United States because it adds to the uncertainty over how many eggs are required to clone a human cell. The human embryonic stem cell lines used in current research are generated using naturally fertilized embryos discarded from in vitro fertilization clinics. But SCNT will likely require fresh unfertilized eggs, which are not available from fertility clinics and therefore must be donated specifically for research.