Infertile men have received new hope of fathering children after scientists grew mammalian sperm in a laboratory for the first time.
A team from Japan developed sperm from fragments of testes from mice and used them to fertilise eggs from which healthy, fertile young were born.
"I want to apply our method to other species including humans. The sperm produced in our system should be safe," said Dr Takehiko Ogawa of Yokohama City University.
The research team, reporting their results in the journal Nature, said the success of the mouse sperm held out the promise of discovering new techniques for male infertility.
Dr Allan Pacey, of the University of Sheffield, said success in mice was no guarantee it could be matched in humans but added: "This study is a small but important step in understanding how sperm are formed which may, in time, lead to us being able to routinely grow human sperm in the laboratory."