Growing where they haven’t grown before

For the first time, researchers have found the right laboratory conditions for growing mouse precursor cells into sperm. The finding could be a boon to fertility research. Unlike female animals, which are born with a finite supply of eggs, males begin producing sperm with the onset of puberty from a group of stem cells in the testes.

To determine what lab-culture ingredients could nurture these so-called spermatogonial stem cells (SSCs) outside the body, Ralph Brinster and his colleagues of the University of Pennsylvania spent almost a decade on trial-and-error experiments.