Scientists have turned skin tissue from infertile men into early-stage sperm cells in a groundbreaking study that raises hopes for new therapies for the condition.

The unexpected success of the procedure has stunned some scientists, because it was thought to be impossible for the men to make any sperm.

The men who took part in the study had major genetic defects on their Y sex chromosomes, which meant they could not produce healthy adult sperm on their own.

About 1% of men cannot make any sperm, a condition known as azoospermia, while a fifth of men have low sperm counts. Male fertility is a concern for roughly half of couples who seek IVF treatment.

In the latest study, researchers took skin cells from three infertile men and converted them into stem cells, which can grow into almost any tissue in the body. When these cells were transplanted into the testes of mice, they developed into early-stage human sperm cells.

What we found was that cells from men who did not possess sperm at the time of clinical observation were able to produce the precursors for sperm, said Cyril Ramathal, of Stanford University.

Skin cells from infertile men grew into fewer early-stage sperm cells than cells taken from normally fertile men, the study found.

The research is at an early stage, but scientists suspect that the converted skin cells might have grown into mature sperm cells if they had been transplanted into the infertile mens testes.

If further work confirms the suspicion, it may be possible to restore male fertility by taking mens skin cells, turning them into stem cells, and injecting these into their testes. The same might be done for men who are left infertile after having chemotherapy for cancer.

Being able to efficiently convert skin cells into sperm would allow this group to become biologic fathers, said Michael Eisenberg, director of male reproduction and surgery at Stanford, who was not involved in the study. Infertility is one of the most common and devastating complications of cancer treatments, especially for young boys and men.

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Scientists turn tissue from infertile men into sperm cells