Body parts made to order

But there is a thicket of ethical and legal issues, as well as technical problems, to be tackled. The cells are obtained from embryos created at in-vitro fertilisation clinics and so far do not seem definably different from the handful of primordial cells from which an entire individual iscreated.

Though the scientists involved in the work consider use of the cells justified because they come from embryos that would otherwise have been discarded, others believe the cells have a special status in that they retain the potential to develop into an individual, and that the use of the cells may draw criticism if this status is not taken into account.

The new cells, known as human embryonic stem cells, have eluded capture until now because they exist in this state only fleetingly before turning into more specialised cells, and need special ingredients to be kept alive outside the body.

The cells have many possible uses, of which the most promising is to grow new tissue, of any kind, for transplant into a patient’s body. The cells may also offer effective routes to human cloning, although both the researchers and their sponsor deny any interest in this application. Another likely use is in gene therapy, the insertion of new or modified genes into body tissue.

Two forms of human embryoniccells have been developed, one by a team under Dr James A Thomson of the University of Wisconsin in Madison, the other by Dr John Gearhart and colleagues at the Johns Hopkins University School of Medicine in Baltimore.

Congress in 1995 banned federal financing of research on foetal cells, including those derived from embryos, and the university researchers whose work was announced were funded by the Geron Corporation of Menlo Park, California, a biotechnology company that specialises in anti-ageing research.

The research “has potential health benefits which I think are extremely promising, and I am sorry that the law prevented us from supporting it,” said Dr Harold Varmus, director of the US National Institutes of Health.

After an egg is fertilised, it divides several times and forms a blastocyst, a hollow sphere with a blob of 15 to 20 cells, known as the inner cell mass, piled up against one wall. It is from these cells that the embryo develops. The donors of the blastocysts granted permission forthem to be used in research.

If researchers are able to use the cells to grow new tissues, the work could alleviate the shortage of livers and other organs for transplant. Cultures of the cells in the laboratory could be nudged down different developmental pathways to become heart or bone marrow or pancreatic cells. Before reaching their final stages, the about-to-become heart cells, for example, could be injected into a patient’s ailing heart. Guided then by the body’s own internal regulatory signals, the cells would develop into new, young heart tissue, supplementing or replacing the heart cells already there. The same approach should in principle work with any tissue of the body. Human embryonic stem cells would thus serve as a universal spare parts system.

Geron, which has exclusive licences to use the cells, says it regards them as qualitatively different from other cells used in research. “Because these cells are derived from human blastocysts there is a moral authority here, so we take these cellsseriously,” says a spokesman for Geron.

Geron believes that use of the cells is justified because they are something less than a living embryo, and life-saving treatments may be derived from them. Dr Gearhart said he did not consider the cells that he and Dr Thomson have isolated to have a special moral status because “they cannot form a foetus — you cannot take one of these cells and form a being out of it.”Still, Dr Gearhart said he would not argue with the view at Geron that the cells had a different standing from ordinary cells. Dr Kevin Fitzgerald, a geneticist and Jesuit priest at Loyola University Medical School, said that if the human embryonic stem cells are totipotent, “then you are disrupting the viability of life and we are back to the question of how to justify destroying life for the purposes of scientific advancement”.