Scientists in S Korea have cloned 30 human embryos. They hope to obtain cells that could eventually be used to treat disease.
The took the genetic material from normal cells in women donors and combined it with their eggs.
This was done at the Seoul National University by Suk Hwanf and his team.
The embryos were then developed to produce stem cells. These stem cells can divide into any tissue in the body.
These cells will (it is hoped) replace those which have failed, for example, in patients with Alzheimer's disease.
'Because these cells carry the nuclear genome of the individual, after differentiation they could be expected to be transplanted without immune rejection for treatment of degenerative disorders. Our approach opens the door for the use of these specially developed cells in transplantation medicine.' said Professor Hwang.
His research is being published online by the journal Science (Science Express web site).
Mice proof
There have been claims in the past for the creation of human embryo clones to study so-called stem cells - many of them disputed.
But no group has reported producing so many early-stage clones and seen their development progress to such an advanced stage.
The work has also been subjected to the rigorous scrutiny of independent scientists before publication in a major journal.
'These are the most advanced human embryo clones yet produced,' Professor Hwang told BBC News Online.
The team says it sought approval for its work from an ethical review board and obtained informed consent from its women donors before proceeding with the work.
Thirty embryos
The team tells Science Express how it used 242 eggs in its experiments taken from 16 women.
In each case, material was transferred from the nucleus of a non-reproductive (somatic) cell, containing the woman's genetic blueprint, into a nucleus-free egg from the same donor.
Following this transfer, factors within the host egg's exterior, or cytoplasm, are believed to have reprogrammed the new nuclear contents by activating versatile embryonic genes, while silencing the more limited adult somatic cell genes.
In total, 30 embryos - exact genetic copies of their female donors - were then cultured to the so-called blastocyst stage at which stem cells could be extracted.
These special cells were seen to divide into all three of the main tissue types found in the human body, the researchers report.
The cells were even transplanted into mice to show they could differentiate into still more specific cell types, offering further proof of their 'pluripotency'.
The stated intention is to study human embryonic stem cells to see how they could be used as a therapeutic tool to treat disorders, such as diabetes, osteoarthritis, and Parkinson's disease, among others, in which tissues in the body have begun to fail.
Non-egg future
Editor-in-chief of the journal Science, Donald Kennedy, said: 'The potential for embryonic stem cells is enormous, but researchers still must overcome significant scientific hurdles.'
And he added: 'These results seem promising. But, it's important to remember that cell and tissue transplantation and gene therapy are still emerging technologies, and it may be years yet before embryonic stem cells can be used in transplantation medicine.'
Addressing ethical concerns, he also called for a worldwide ban on activities which would seek to use this technology to create living children.
Professor Hwang, whose expertise has been developed in animal cloning, said any attempt to produce a baby would be 'crazy'.
'We will never try to produce cloned human beings,' he said.
'During animal cloning, we experienced so many difficulties and dangers with deformities, especially in the internal organs.'
Commenting on the Korean work, Roger Pedersen, professor of regenerative medicine, at the University of Cambridge, UK, told BBC News Online: 'The present work has substantially advanced the cause of generating transplantable tissues that exactly match the patient's own immune system.
'These researchers' findings also make it possible to learn how to reprogramme the human genome to an embryonic state.
'This will likely accelerate the development of alternative ways of reprogramming human cells, which could in the future diminish the need to use human eggs for this purpose.'
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