For the first time Japanese scientists have accomplished a feat thought to be impossible - they have produced fertile mammalian sperm in a test tube.
The technique holds significant promise for male infertility, reports Nature.
Biologists have been trying to make sperm outside the body for almost a century.
So, a team led by Takehiko Ogawa of Yokohama City University designed a way to culture sperm and allow them to mature outside of the body.
They found that the key to getting sperm through meiosis lay in a simple change to standard culture conditions.
Ogawa and his colleagues took tissue fragments from neonatal mouse testes to mature.
They then soaked the testes tissue in a mixture called KnockOut Serum Replacement, often used to grow embryonic stem cells.
To track sperm development, they used a fluorescent protein that marked cells undergoing - or that had undergone - meiosis.
After several weeks in the mix, the testes looked normal and were producing sperm. Nearly half of the samples contained cells with sperm-like tails.
Finally, the researchers injected the sperm into egg cells. A few weeks later, surrogates delivered a dozen live, fertile offspring.