The new method is based on Somatic Cell Nuclear Transfer
(SCNT), a well-established method for generating stem cells. SCNT removes the
nucleus from an egg cell, and then replaces it with the nucleus of a different
In a process that is incompletely understood, the
cytoplasm of the egg reprograms the implanted nucleus and triggers it to behave
as if it were the original egg cell, but with the genetic blueprint of the
donated cell's nucleus. When fertilised and
implanted in a surrogate mother, it can develop into offspring. This is
the technique that was used for the first cloned sheep, Dolly, in 1996.
Until now, using SCNT required immature non-fertilised
egg cells that needed to be arrested in a specific cell cycle stage, called metaphase; otherwise, the cells were thought to be unusable. But the scientists used fertilised eggs that had
already undergone the first cell division and then
used SCNT to replace the nuclei. Surprisingly, they were still able to produce
living mouse clones.
'It has always been thought that this capacity for
reprogramming ended with metaphase' confirmed study leader Dr
Shoukhrat Mitalipov, of Oregon
Health and Science University. 'Our study shows
that this reprogramming capacity remains in the later embryonic cell cytoplasm.
It looks like the factors continue working and they efficiently reprogram the
cells -just as they do in metaphase'.
A number of scientists have tried using cells during interphase -
the cell stage used here - in the past without success. The difference
between previous techniques and that used by Dr Mitalipov's team was that both the
egg and donor cells first had their cell cycles carefully synchronised.
'That was the secret', Dr Mitalipov said. 'When we did that matching, then everything
Embryonic stem cells are
capable of transforming into any type of cell in the body. Scientists have high hopes of using stem cell therapy in regenerative
medicine, but research is limited due to ethical, practical and legal
restrictions to stem cell resources. If the new technique is successful in
humans, it could allow the use of unused early embryos - routinely retrieved in
excess during IVF treatments -
'I think we're looking at tens of thousands, hundreds
of thousands, of embryos that are constantly being discarded. This paper now
unlocks the ability to use those embryos,' Dr James
Byrne, of University
of California, Los Angeles, who was not
involved in the research, told the Los Angeles Times.
The next step for Dr Mitalipov's team will be to test the
technique in rhesus macaques.