Professor Harry Moore, of the Centre for Stem Cell Biology at the University of Sheffield, UK, said there were a number of key differences between mice and human testes cells.

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To be fair, all this was also true of the cells described in the Nature paper, which was written by Shoukhrat Mitalipov, of the Oregon Stem Cell Centre in Beaverton, and his colleagues.

ECONOMIST: Human embryonic stem cells

Dr Lorenz Studer, head of the Stem Cell and Tumor Biology Laboratory at the centre, and the lead researcher on the study, told BBC News Online that the study was "proof of principle" that cloned embryonic stem cells could be reliably transformed into a variety of useful cell types.

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The German Aerospace Centre has tested this idea in an aircraft using a fuel cell to power two electric motors built into the hub of an aircraft nose wheel.

ECONOMIST: A cheaper, cleaner way for the long taxi to the runway