Human cloning has been used to create stem cells from adults for the first time, in a breakthrough which could lead to tissue and organs being regrown.
Using the cloning technique that produced Dolly the sheep in 1996, researchers were able to turn adult human skin cells into stem cells, which can grow into any type of tissue in the body. They even used the cells of a 75-year-old man, raising the prospect of body parts being regenerated in old age.
The advance could lead to tissue-transplant operations for a range of debilitating disorders, such as Parkinson's disease, multiple sclerosis, heart disease and spinal cord injuries. Last year, a team created stem cells from the skin cells of babies, but it was unclear whether it would work in adults.
However, a team of scientists from the Research Institute for Stem Cell Research at CHA Health Systems in Los Angeles and the University of Seoul said they had achieved the same result with two men, one aged 35, the other 75 years old.
"The proportion of diseases you can treat with lab-made tissue increases with age. So if you can't do this with adult cells it is of limited value," said Robert Lanza, co-author of the research, which was published in the journal Cell Stem Cell.
The technique works by removing the nucleus from an unfertilised egg and replacing it with the nucleus of a skin cell. An electric shock causes the cells to divide until they form a "blastocyst", a small ball of a few hundred cells.
In IVF, a blastocyst is implanted into the womb, but with the new technique the cells would be harvested to create other organs or tissues.
The breakthrough is likely to reignite the debate about the ethics of creating human embryos for medical purposes and the possible use of the same technique to produce cloned babies.
Although the embryos created may not produce a human clone even if implanted in a womb, the prospect is now closer. However, scientists have tried for years to clone monkeys and have yet to succeed.
Lanza admitted that without strong regulations, the early embryos produced in therapeutic cloning "could also be used for human reproductive cloning, although this would be unsafe and grossly unethical". However, he said it was important for the future of regenerative medicine that research into therapeutic cloning should continue.
Shoukhrat Mitalipov, a reproductive biologist from Oregon Health and Science University, who developed the technique last year, said: "The advance here is showing that [nuclear transfer] looks like it will work with people of all ages. I'm happy to hear our experiment was verified and shown to be genuine."
However, the high cost of creating stem cells means that at present "only a few wealthy old men could do it", said Lanza.
A big barrier to producing patient-specific stem cell lines for tens of millions of people using this technique is that few women want to donate eggs, a sometimes painful process. But many people have genetically similar immune systems, Lanza said, so just "100 human embryonic stem cell lines would generate a complete match for over half the American population".