Most, if not all, human embryonic stem cell lines available to publicly funded researchers in the US have been mixed with mouse cells and may not be suitable for clinical trials.
This came as a blow as George Bush announced on 9 August that federally funded researchers will only be able to work on existing stem cell lines.
Human embryonic stem cells are obtained by extracting a tiny mass of cells from an embryo about five days old. The vast majority of the cell lines all over the world were grown on a 'feeder' layer of embryonic mouse cells, which release growth factors that help sustain the human embryonic stem cells.
The problem is that the mouse cells may harbour endogenous viruses dangerous to humans, and transplanting the existing human embryonic stem cells into humans would constitute xeno-transplantation (the transplantation of tissue from an animal into a human), and be regarded as such by the US and Drug Administration.
Using existing human embryonic stem cells in transplants would not be impossible, but it would be difficult, say some US scientists.
"If the research looks great and we're ready to go to human trials, we will need more stem cells," Jeffrey Rothstein of Harvard University told the Washington Post.
California-based Geron Corp claims it has succeeded in growing human embryonic stem cells without using mouse cells, but it is not clear whether they have been created before Bush's announcement.
Source: "Stem cells face xenotransplantation glitch" by Emma Young, 24-8-01 www.newscientist.com
Article first published October 2001