Colonies of tiny cells flourishing in petri dishes in the US and Japan are reshaping the political and ethical landscape surrounding human stem-cell research.

In the process, these diminutive colonies also may level the playing field in stem-cell research – internationally and domestically.

These are some of the effects analysts say they see coming out of this week's announcements that two teams have genetically reprogrammed skin cells so that they take on the traits of embryonic stem cells.

Embryonic stem cells are the subject of intense medical interest because of their ability to develop into any of the major cell types in the human body. Over the long term, these stem cells could become the foundation for therapies for a range of diseases, scientists say. This week's announcement suggests it will be possible for scientists to study these cells without the ethical and political difficulties of harvesting them from unused human embryos.

For the emerging field of stem-cell research, "this is enormous," says Jesse Reynolds, a policy analyst at the liberal Center for Genetics and Society, based in Oakland, Calif. "I can't think of another development "that has been this big,"

"This is a paradigm shift," agrees Rev. Tad Pacholczyk, director of education at the National Catholic Bioethics Center in Philadelphia. "This will have a huge impact on the ethical debate."

That debate has centered on the sources for human embryonic stem cells – especially those that have the potential to be patient-specific. For research purposes, scientists have turned to fertility clinics where patients either have donated their nascent embryos to research or no longer need them to start a family. But the process of extracting the stem cells destroys these soon-to-be embryos, technically called blastocysts. The destruction is abhorrent to those who hold that human life begins at conception.

The ethical debate grows more heated when cloning – the most controversial idea for generating patient-specific stem cells – enters the picture. In 1997, a team in Scotland led by Ian Wilmut cloned Dolly the sheep from adult tissue by extracting the DNA from nucleus of adult cells and injecting it into the emptied nuclei of unfertilized sheep eggs. The eggs were fertilized, then implanted into ewes.

The approach is banned in humans. Last week, however, scientists from the Oregon National Primate Research Center in Beaverton, reported for the first time that they had used the technique to generate embryonic stem cells cloned from an adult primate – a macaque monkey. This strongly hinted that eventually the approach could work with humans.

But the technique, which in principle could draw on a patient's own cells to generate new tissue for treatments, is highly inefficient – requiring many eggs to yield one successful clone from which stem cells can be drawn and nurtured. It implies generating nascent embryos exclusively as stem-cell factories. And it raises the concern among many people that the approach will lead eventually to cloning humans as a means of reproduction.

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