In a previous study, Agarwal and his team found that men who used their cell phones more than four hours a day had significantly lower sperm quality than those who used their cell phones for less time.

CNN: Cell phones can affect sperm quality, researcher says

But he said the Kyoto paper was "quite a large step forward" in developing a process by which sperm could be made for infertile men, perhaps by taking as a starting point a cell from their skin or from something like bone marrow.

BBC: Stem cell sperm study leads to successful mouse births

Reproductive cloning, which created Dolly the sheep in 1997, involves transferring the nucleus of an adult cell into an egg, giving the resulting embryo a full set of genes without the normal sperm-meets-egg fertilisation step.

ECONOMIST: Endangered species