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. This year, they finally hit on a specific combination of sugars, proteins, and growth factors that keep SSCs alive and multiplying.

To test whether these cells could still produce sperm, the researchers grew SSCs from mice genetically engineered to glow green under fluorescent light and implanted them into infertile mice. Once mated, the mice produced glowing offspring, a result that could only have occurred if the SSCs developed into sperm cells, the researchers report in an upcoming Proceedings of the National Academy of Sciences.

Brinster and his colleagues suggest that men facing chemotherapy, which often causes infertility as a side effect, could preserve their fertility by having their SSCs kept alive in a lab for later reimplantation.--C.B.