Embryonic stem cells generated in this way could be used to treat people with degenerative diseases such as Parkinson's, the researchers hope. Because the cells are genetically identical to the patient, they should not be rejected in the way that transplanted organs can be.
'Our finding offers new ways of generating stem cells for patients with dysfunctional or damaged tissues and organs', said Dr Shoukhrat Mitalipov, from Oregon Health and Science University, USA, who ran the study.
The researchers used a technique known as somatic cell nuclear transfer (SCNT), which involved removing the genetic material from a donor egg and replacing it with the genetic material taken from skin cells. Using just two eggs, the scientists were able to develop early-stage embryos that contained enough cells to generate embryonic stem cell lines in the lab, which were genetically identical to the donor skin cells.
SCNT has been used successfully to generate embryonic stem cells in several animals, but has not previously been successful in humans. Other labs have had some success in generating human embryos by SCNT, but none of the embryos survived long enough to be used to generate embryonic stem cells.
'This is an important advance because it is feasible – one embryonic stem cell line was generated from just two eggs', said Christopher Shaw, Professor of Neurology at King's College London.
However, the use of human embryos in research is a controversial issue. Although allowing babies to develop from cloned embryos, 'reproductive cloning', is illegal, there are concerns that this type of research into therapeutic cloning could be used to create human clones.
Professor Ian Wilmut, who led the team that created Dolly the sheep, spoke to the Guardian, saying: 'The new work may encourage some people to attempt human reproductive cloning but the general experience is that it still results in late fetal loss and the birth of abnormal offspring'.
Professor Robin Lovell-Badge, from the MRC National Institute for Medical Research, praised the team for their advances in therapeutic cloning techniques, but added that reproductive cloning 'is an unsafe procedure in animals and it will similarly be an unsafe procedure in humans. For this reason alone it should not be attempted'.
Dr Mitalipov said: 'Our research is directed toward generating stem cells for use in future treatments to combat disease. While nuclear transfer breakthroughs often lead to a public discussion about the ethics of human cloning, this is not our focus, nor do we believe our findings might be used by others to advance the possibility of human reproductive cloning'.
An alternative to embryonic stem cells are induced pluripotent stem (iPS) cells. These are created by taking adult cells and exposing them to conditions under which they can revert to stem cells, without needing to use embryos. Mitalipov plans to compare the benefits of the new embryonic stem cells and iPS cells, according to Nature.