US researchers have cloned healthy mice from skin cells for the first time.
Many cloned mice survived into adulthood
Despite notorious difficulties in producing animals through cloning, nine of 19 mice who were born survived into adulthood.
The scientists replaced the nucleus from an unfertilised egg with the nucleus from an adult skin stem cell.
Embryos produced in this way may also be a useful source of stem cells, say the researchers in the Proceedings of the National Academy of Sciences.
Researchers have previously managed to clone mice using other kinds of adult cells, but it has been an inefficient process.
In this study, scientists from Rockefeller University and the Howard Hughes Medical Institute used a type of stem cell found in the skin, called keratinocytes, which are attractive to scientists because they are easily accessible.
The cells were found in hair follicles lying underneath the skin and are involved in hair growth and in repairing skin wounds.
The existence of these cells has been known about for some time, but scientists have only recently discovered that they can self-renew and produce many different types of cells - the hallmarks of stem cells.
Finding ways of isolating these cells has also proved problematic.
But the team have now been able to create mouse embryos by removing the nucleus from an unfertilised egg cell and replacing it with the nucleus from an adult skin cell.
These were then cultured in the lab to become early-stage embryos called blastocysts.
These were then put into the wombs of adult mice and pregnancies were allowed to develop.
Usually, just one to two per cent of such blastocysts survive to birth and many cloned mice are not healthy.
In this study, the success rate was only 1.6% using skin cells from female mice and 5.4% using cells from male mice.
They suggest the difference may be caused by the female cells undergoing more complex changes than the male cells.
As well as being a good starting point for cloning mice, the researchers from said their method could be useful for generating embryonic stem cells.
The blastocysts created could be used as a source of embryonic stem cells, they said.
In turn these cells could, in theory, be used to produce any other type of cell, such as nerve cells or muscle cells.
Researchers are hoping that embryonic stem cells could one day be a useful treatment for many conditions such as Alzheimer's and heart disease.
If they could be produced from individual patient's skin cells for use in their treatment, it would avoid problems of rejection, said the researchers.
Dr Elaine Fuchs, of the Howard Hughes Medical Institute who was one of the researchers on the study, said: "This work opens the door for generating embryonic stem cells, which is easier than cloning mice from adult skin stem cells.
"If researchers overcome the current technical hurdles of making human embryonic stem cells by nuclear cloning, it may one day be possible to generate tailor made embryonic stem cells from a patient's skin stem cells."
John Martin, professor of cardiovascular medicine at University College London, said: "It's an interesting result, but predictable.
"It's part of a process of understanding how we can achieve a practical solution by taking stem cell medicine into man.
"This is a long process that's going to go on for five to 10 years before we get anything therapeutic out of it."