Denver

Only about 275 of the roughly 400,000 frozen embryos currently stored in the United States are likely to be of any use in stem-cell research, according to the first comprehensive inventory of the embryos.

The low number gives the lie to the notion that fertility clinics are awash with spare embryos that are suitable for deriving new lines of human embryonic stem cells, opponents of the research say. But its supporters counter that there are not many embryos because so few fertility clinics arrange for patients to donate embryos to research.

According to a survey by Jacob Mayer, an embryologist at the Jones Institute for Reproductive Medicine in Norfolk, Virginia, and colleagues, donors have approved only 11,000 of the embryos for use in research (D. I. Hoffman et al. Fertil. Steril. 79, 1063–1069; 2003).

Many of the 11,000 embryos would not survive freezing and thawing, and only 25% of those that do are expected to develop to the blastocyst stage. Mayer and co-authors estimate that the process is so inefficient that just 275 would lead to new stem-cell lines.

“The American public had the impression that hundreds of thousands of embryos were out there that patients were eager to have destroyed in research,” says Richard Doerflinger, a lobbyist at the US Conference of Catholic Bishops. “This shows that this is clearly not the case.”

“Fertility clinics may not contain the plethora of material they were expected to,” says Mayer, whose institute conducted the survey of 340 fertility clinics in association with the Society for Assisted Reproductive Technology and RAND, a private non-profit research institution.

Researchers relying on frozen embryos from infertile couples are in for a shock, not only because of the scarcity but also because embryos from infertile couples may be less likely to develop normally, says Amy Sparks, director of the University of Iowa Reproductive Testing Laboratory in Iowa City. Her clinic, where about 300 embryos have been donated to research in the past 15 years, is one of just a handful in the country that has protocols for the use of donated embryos in stem-cell research.

Even a small number of suitable embryos is viewed by researchers as a potentially useful addition to the 11 stem-cell lines already sanctioned by the government for use in research. “That 275 is 20 times what we have now,” says George Daley, a stem-cell researcher at the Whitehead Institute in Cambridge, Massachusetts. Even though thousands of embryos have been designated for research, he says, “there's no effective way of getting them into researchers' hands”.

Some researchers predict that donations to research will rise as public awareness of stem-cell and other human embryo research increases. But opponents say that the numbers reflect the public's rejection of the use of embryos in research. “There's a widespread intuition in the public that embryos belong in uteruses,” says Ben Mitchell of the non-profit Center for Bioethics and Human Dignity in Bannockburn, Illinois.