What if science, with a shake of a test tube, could circumvent the ethical objections to embryonic stem cell research? Several proposals would in principle let scientists obtain precious embryonic stem cells without harming embryos (equally precious to some) in the process. For eager biotechnologists, that arrangement would sound almost too good to be true--and indeed, it most likely is.

William B. Hurlbut of Stanford University, a member of the US President's Council on Bioethics who is a firm believer in the "implicit moral dignity" of the embryo, has attracted attention by suggesting a combination of genetic engineering and cloning called altered nuclear transfer. In one scheme, the nucleus of a mature cell would be extracted and altered to turn off one or more genes that are vital during an embryo's development. The nucleus would be injected into a prepared egg cell that is then zapped with electricity to activate it, as in cloning. If all goes as it should, this biological entity, which Hurlbut says "never rises to the level of what can properly be called a living being", would become at most an unorganised clump of embryonic cells suitable for scientific research and possibly clinical treatments.

Not all bioethicists share Hurlbut's enthusiasm for this plan. That cellular clump would bear a great likeness to a teratoma--a grotesque tumour mixing together different cell types, from hair to muscle to teeth. Although it may not be classifiable as an embryo, in the eyes of many, it certainly triggers what Leon Kass, the chairman of the council, has called the "yuck factor" for viscerally identifying unethical practices. Critics have also questioned whether intentionally creating a doomed abomination is morally superior to destroying embryos that already have no future. And yuckiness aside, to make successfully even one line of stem cells in this way, hundreds of human eggs might be needed, which itself entails ethical and technical problems.

Two Columbia University researchers have circulated a perhaps more pragmatic idea: pluck living ES cells from the many embryos produced in vitro that have died spontaneously. Donald W. Landry and Howard A. Zucker have begun work on tests for assessing markers such as the final arrest of cell division, which the scientists equate with "brain death" for embryos.

Production of what amounts to sacrificial monsters is unlikely to satisfy those who believe that any tinkering with the primordial stuff of life is wrong.

Ironically, the Landry/Zucker scheme would rescue nominally healthy cells from dead embryos, while healthy but unused IVF embryos would continue to be discarded. It also forgoes the dream of someday cloning ES cells from a patient's own body for use in treatments. Such bespoke stem cells would be safe from immune rejection; ones derived from dead embryos would not be. Hundreds of thousands of cell lines might therefore need to be cultured and stored to provide all patients with immunologically compatible cells.

Other would-be solutions include techniques for extracting individual stem cells without harming embryos and for using unfertilised human eggs coaxed into a short-lived process resembling embryo formation. Another straightforward strategy would avoid ever going near an embryo. Instead an adult stem cell would be forced to "dedifferentiate", or revert to its more embryonic pluripotent state. At the moment, however, such a concept borders more on alchemy than biochemistry. A US National Academy of Sciences report issued in April summarised these approaches as seeming to have numerous technical hurdles for now.

A critique in the New England Journal of Medicine specifically aimed at Hurlbut's proposal may further dampen all these ideas. Douglas Melton, George Daley and Charles Jennings of Harvard University argued that the switching off of a gene does not represent "a transition point at which a human embryo acquires moral status". No similar developmental or biochemical benchmark may ever lend ethical certitude to this field. Industrial-scale production of sacrificial monsters is unlikely to satisfy those who believe that any tinkering with the primordial stuff of life is wrong.