Japanese scientists are exploring the possibility of resurrecting extinct animals by cloning.
Why are we asking this now?
Scientists in Japan have refined a cloning technique that has enabled researchers to clone apparently healthy mice from the corpse of a mouse that had been frozen for 16 years.
They employed the same techniques that scientists used to create Dolly the sheep but added an extra stage where they effectively repeated part of the process which enabled them to produce cloned offspring.
The scientists believe it might be possible to use the same techniques to clone creatures from the frozen tissue of animals buried in permafrost regions - for example, mammoths.
Who carried out this work and why?
The team was led by Teruhiko Wakayama of the Riken Centre for Developmental Biology in Kobe, Japan. He was interested in adapting the Dolly method of cloning, which involves taking the nucleus of a cell and inserting it into an unfertilised egg cell that has had its own nucleus removed.
After a jolt of electricity the unfertilised egg develops into an early embryo.
However, unlike Dolly, the mouse embryos in the Japanese study were not inserted into surrogate mothers but broken apart to retrieve individual embryonic stem cells.
These cells were inserted into another batch of unfertilised egg cells and, after another jolt of electricity, the result was cloned embryos. Only at this stage were the embryos implanted into surrogate mothers, which gave birth to the clones of the original frozen mouse.
Is this the first time scientists have produced clones from dead animals?
No. Dolly was cloned from a sheep that had died long before. In Dolly's case, scientists had taken tissue from Dolly's clone and frozen it using chemicals called "cryopreservatives" which prevent the formation of damaging ice crystals inside the cells.
Therefore Dolly was the clone of a frozen animal that had died many years before she was born. Indeed, when one newspaper learnt that Dolly was the clone of a dead animal, it famously asked: "Can we now raise the dead?" on its front page.
Can we now raise the dead?
It depends. Dolly was the clone of a dead animal and scientifically it means the dead animal's genome was "resurrected" in the body of another individual. But if this were to happen to a human being, few people would consider the deceased person had been raised from the dead. It's no more true than saying that when a person dies, they have been "raised from the dead" if they have a living identical twin.
So will it be possible to resurrect extinct animals?
This is what Wakayama and his colleagues tentatively propose in their paper published in the Proceedings of the National Academy of Sciences. They point out that all clones so far produced from dead animals have been created from quick-frozen tissue that has been meticulously kept at cold temperatures without thawing, using cryopreservation. This does not happen in real life.
Has anyone tried to resurrect extinct animals?
Scientists have tried, with some success, to extract DNA from various extinct mammals, such as mammoths and the Tasmanian tiger, but cloning poses a new set of problems.
It is difficult to extract cells with perfectly preserved DNA since it degrades over time. Corpses frozen in permafrost for several thousand years are likely to have suffered repeated thawing and freezing that will damage the cells and their DNA.
It is also hard to find non-extinct animals to act as surrogate egg donors and mothers. For the mammoth, the African elephant might be the best choice, although there may be differences that pose insuperable barriers.
Another problem has to do with mitochondrial DNA, which is passed down the maternal line through the egg cell. An elephant's mitochondria may not work in a mammoth clone.
Could cloning be used to protect living species from extinction?
Some scientists are suggesting this as a last-ditch measure to safeguard threatened animals. But cloning is never going to be the panacea to extinction.
Cloning animals on the verge of extinction could help a species hang on in zoos and parks, but it does little to generate the genetic diversity needed for long-term survival.
It also does nothing to address the root causes of extinction.
Will this latest development ever be used on people?
One can never say never in science. But cloning from the grave is still a long way from reality for all sorts of ethical, legal and scientific reasons.