Embryonic stem cells are harvested from three sources: aborted fetuses, so-called cadaveric stem cells; embryos left over from in-vitro fertilisation efforts, so-called discarded embryos; and embryos created in the laboratory solely for the purpose of producing stem cells, so-called research embryos. Opposition to the use of embryonic stem cells from any of these sources comes mainly from those who hold that human life begins at conception and that destroying an embryo at any stage of development is tantamount to infanticide. Advocates of embryonic stem-cell research hold that while embryos certainly deserve respect they are not yet fully human and that the good that may result from medical research studies with their cells justifies their use.
Some stem cells, however, have also been isolated from adult tissues, and opponents of human embryonic stem-cell research argue that research should be limited to such cells. But the general view of scientists working in this area is that adult stem cells, while they may one day prove useful for treatment, are simply not as versatile as their embryonic counterparts, because they are already partly differentiated. In the USA, where there is a bitter fight over federal funding for stem-cell research (see page 217), more than 80 Nobel Laureates signed a letter to President George Bush arguing against regulations that would limit research to those stem cells derived from adult tissues. Indeed, the prospect of restrictions on the use of embryos as a source of stem cells has led to fears of a US brain drain. One leading US stem-cell researcher, Roger Pedersen from the University of California, San Francisco, announced this week that he was moving to the UK-"to get some work done", he was quoted as saying.
In Europe, a report by members of the High Level Expert Group of the European Science Foundation last month arrived at a similar conclusion to their US colleagues, saying that it "is essential to proceed with research on stem cells derived from embryos, fetal tissues and adults, in parallel". And in the UK, the Chief Medical Officer's Expert Group on Therapeutic Cloning in a report published last year also raised concerns about the limitations of stem cells extracted from adult tissues. "It may in the future become possible to reprogramme adult cells to behave like stem cells but at the moment this remains largely hypothetical."
As a review in the current issue of Cell points out, the concept of the stem cell is changing. Stem cells might be better thought of not as fixed cellular entities, but rather in terms of their biological function. Such functions might be induced in different cell types, both embryonic and adult. The focus on function instead of cell type helps take us away from emotive and ultimately irreconcilable debate about the status of the embryo.
Given the potential benefits of human stem-cell research, use of cadaveric cells and embryos surplus to fertility treatments seems justified on pragmatic grounds. For the present, the potential for adult stem cells is at this time too uncertain. However, the creation of embryos solely for the purpose of creating stem cells is more difficult to accept, because, for all practical purposes, they are being treated as a product to be exploited. Although the creation of human embryos for research has been allowed in the UK since 1990, the practice has been tightly regulated. At this time, given the large supply of discarded embryos that is available-more than 100 000 in the USA alone, according to some estimates-the creation of embryos solely for the purpose of producing human stem cells is not only unnecessary but also a step too far.