A "milestone" breakthrough in human cloning has been revealed by scientists, who successfully used skin to generate embryonic cells.
Scientists in the United States said that could mean avoid using fertilised human embryos for stem cell therapies, stressing they had no interest in creating cloned humans, but the development is sure to spark renewed debate on the ethics of cloning human cells.
The "Brave New World" process the scientists have developed could also be used as a starting point for cloning humans. Human embryonic stem cells are normally derived from "unwanted" fertilised human embryos left over from IVF treatment, which raises ethical concerns.
It is the first time scientists have managed to create human embryos through cloning developed enough to provide stem cells.
In the new study, reported in the journal Cell, scientists transferred nuclei from human skin cells into human egg cells. The scientists used caffeine to keep the eggs stable during the procedure after it was shown to be effective in monkeys.
"The importance of the egg donor is again illustrated in this paper. Only high quality human eggs had the potential to reprogram somatic cells. It is remarkable that adding caffeine was the key that resulted in ES cell lines from all three donors.” Professor Alison Murdoch, professor of reproductive medicine at Newcastle University told HuffPost UK.
The same somatic cell nuclear transfer (SCNT) technique was employed by researchers at the Roslin Institute in Edinburgh to produce Dolly the Sheep, the first mammal to be cloned from an adult cell.
Scientists have previously cloned monkey embryos and mined them for stem cells, but until now been frustrated in their attempts to do the same with humans.
Generally it has not proved possible to create human embryos that develop further than the eight cell stage - too early to yield stem cells.
A key problem has been that human egg cells appear more fragile than those of other species.
Lead researcher Professor Shoukhrat Mitalipov, from Oregon Health and Science University, said: "Our finding offers new ways of generating stem cells for patients with dysfunctional or damaged tissues and organs.
"Such stem cells can regenerate and replace those damaged cells and tissues and alleviate diseases that affect millions of people."
The stem cells demonstrated an ability to convert into several different cell types, including nerve, liver and heart cells.
Dr Mitalipov added: "Furthermore, because these reprogrammed cells can be generated with nuclear genetic material from a patient, there is no concern of transplant rejection.
"While there is much work to be done in developing safe and effective stem cell treatments, we believe this is a significant step forward in developing the cells that could be used in regenerative medicine."
Scientists had never cloned a monkey using SCNT despite several years of trying, he pointed out.
Cloning humans would be even more difficult, especially given the fragility of human eggs.
Dr Mitalipov added: "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."
Josephine Quintavalle, director of Comment on Reproductive Ethics, told HuffPost UK she had questions about the use of cloning in such a procedure: "If Dr Mitalipov's objective in cloning human embryos is to create stem cells for therapeutic purposes, we know that this particular goal has already been long achieved without ethical controversy by Nobel Prize Winner Prof Shinya Yamanaka, and his work with induced pluripotent stem cells.
"Not to mention the numerous successful adult stem cell cures, such as those involving umbilical cord blood for example, which go back many years. There is no scientific justification whatsoever for human cloning."
British stem cell and cloning academics have warmly welcomed the findings.
"Provided that the experiments are reproducible in the hands of others, the findings offer the potential to accelerate progress towards patient-specific embryonic stem cells for the treatment of degenerative disease," Professor Mary Herbert of Newcastle University's International Centre for Life told The Huffington Post UK.
"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 and neurogenetics at King's College London.
"It also provides an alternative and more physiological method of reprogramming. Like many good experiments, caffeine has made an invaluable contribution."