Fertile sperm are rare in men with an extra sex chromosome Dennis Kunkle Microscopy/SPL

Turning skin cells into sperm may one day help some infertile men have babies. Research in mice has found a way to make fertile sperm from animals born with too many sex chromosomes.

Most men have two sex chromosomes – one X and one Y – but some have three, which makes it difficult to produce fertile sperm. Around 1 in 500 men are born with Klinefelter syndrome, caused by having an extra X chromosome, while roughly 1 in 1000 have Double Y syndrome.

James Turner of the Francis Crick Institute in London and his team have found a way to get around the infertility caused by these extra chromosomes. First, they bred mice that each had an extra X or Y chromosome. They then tried to reprogram skin cells from the animals, turning them into induced pluripotent stem cells (iPS), which are capable of forming other types of cell.


To their surprise, this was enough to make around a third of the skin cells jettison their extra chromosome. When these cells were then coaxed into forming sperm cells and used to fertilise eggs, 50 to 60 per cent of the resulting pregnancies led to live births.

Cancer risk

This suggests that a similar technique might enable men with Klinefelter or Double Y-related infertility to conceive. But there is a significant catch.

We don’t yet know how to fully turn stem cells into sperm, so the team got around this by injecting the cells into mouse testes for the last stages of development. While this led to fertile sperm, it also caused tumours to form in between 29 and 50 per cent of mice.

“What we really need to make this work is being able to go from iPS cells to sperm in a dish,” says Turner.

“It has to be done all in vitro, so only normal sperm cells would be used to fertilise eggs,” says Zev Rosenwaks of the Weill Cornell Medical College in New York. “The danger with all iPS cell technology is cancer.”

Journal reference: Science, DOI: 10.1126/science.aam9046