It has no brain, heart or nervous system. So how can researchers tell when an embryo has died? That question is likely to become a focus of debate, as scientists search for ways to create 'ethical' human embryonic stem cells.

Many researchers are investigating methods for deriving such cells without destroying human embryos. Now, one group claims to have developed human embryonic stem cells from a very young embryo that had already died.

Miodrag Stojković, who led the project at the University of Newcastle upon Tyne, UK, believes his technique may be acceptable to governments in countries such as Germany and the United States. Laws in these countries restrict the use of embryonic stem-cell lines owing to concerns about killing potential humans.

Whether or not the method is judged as harmless will probably depend on whether the lives of such embryos have indisputably ended. “It will intensify the debate over how to define the death of an embryo, because now there is more at stake,” says Thomas Murray, president of the Hastings Center, a bioethics research institute in Garrison, New York.

During in vitro fertilization (IVF), well over half of human embryos arrest (stop dividing naturally), and are therefore unsuitable for transfer into women. Stojković, now a deputy director of the Prince Felipe Research Centre in Valencia, Spain, used 161 embryos donated by women in two local IVF clinics in his study, published online last week (X. Zhang et al. Stem Cells doi:10.1634/stemcells.2006-0377; 2006). Of these embryos, 29 were developing normally until cells were extracted, 119 arrested 3–5 days after fertilization, and 13 arrested 6–7 days after fertilization.

Arrested embryos were monitored for 24 to 48 hours, to check that none of the cells they contained started dividing again. In an IVF clinic, such embryos would meet the standard embryologist's criteria for having terminally arrested, and would typically be thrown away. Even in Italy, which has particularly restrictive laws stating that embryos cannot be destroyed and that all embryos created during IVF must be implanted, embryos which have arrested in this way are discarded.

The team derived healthy human embryonic stem-cell lines from eight of the normally developing embryos, one of the late-arrested embryos and none of the early-arrested embryos. Although many of the cells in the arrested embryos had distorted shapes or damaged chromosomes, a few cells remained healthy. To coax these into growing, the researchers removed the 'zona pellucida' that sheaths the embryo, and used different culture conditions.

A six-day-old embryo nestles in the womb, but which of its features mean it is alive? Credit: Y. NIKAS/WELLCOME PHOTO LIBRARY

The cell line from the arrested embryo seems to behave normally and can generate other tissue types, although “scientists will have lingering doubts about the 'health' of the cell line, if it's derived from a poor-quality embryo,” cautions stem-cell researcher George Daley of the Children's Hospital Boston in Massachusetts.

I don't see how anyone could attack such cell lines as unethical.

Some stem-cell scientists are excited by Stojković's achievement. “There is no destruction of an embryo involved,” says Hans Schöler, a director at the Max Planck Institute for Molecular Biomedicine in Münster, Germany. “If everything is confirmed, I don't see how anyone could attack such cell lines as unethical.”

But others say the technique will be dogged by ambiguity over what constitutes a dead embryo. In theory, the approach is similar to harvesting organs from dead people — but in the latter case there are clearly defined criteria for proclaiming someone irreversibly brain dead. There are no such rules for an embryo. And even the remote possibility that an embryo could revive would make this procedure unacceptable to some.

Donald Landry at Columbia University Medical Center, New York, is working to establish “ironclad” criteria to define embryo death. He and co-workers have observed 444 arrested human embryos; of these, 142 had fewer cells at the five-day mark and never developed into a normal blastocyst (D. W. Landry et al. Regen. Med. 1, 367–371; 2006). “They were truly dead,” he says. Landry is now planning to study embryo death at other stages of development and to find molecules that are only produced by terminally arrested embryos.

Stojković's technique is just the latest proposal for creating 'ethical' human embryonic stem cells. A recent Nature paper by Robert Lanza and colleagues from Advanced Cell Technology in Worcester, Massachusetts, sparked criticism after claiming to have made two such lines by taking single cells from embryos; they proved the principle, but did in fact destroy the embryos they used. Earlier this month, the body that operates the US National Stem Cell Bank agreed to distribute Lanza-style cell lines — if the federal government will fund research on them.

But without a change in current restrictions, most US scientists are unlikely to get their hands on 'ethical' lines. Legislation known as the Dickey Amendment prevents federal money from being spent on research that harms human embryos. Even if new techniques are deemed to leap this hurdle, President George W. Bush has banned federally funded studies on any human embryonic stem-cell lines derived after 9 August 2001.

James Battey, who heads the National Institutes of Health (NIH) Stem Cells Task Force, says the institutes won't seek legal advice on whether new lines could be funded until they receive a grant application for such work. But Landry hopes that embryonic stem cells derived from dead embryos might finally persuade the Bush administration to change its mind. “I think it would pass muster,” he says.