Human skin from infertile men has been used to create sperm for the first time, in a scientific development which could offer hope to those who cannot have children.
Skin samples from three men were genetically engineered, turning back the developmental clock, so that they assumed the properties of embryonic stem cells - which can grow into virtually any kind of body tissue.
After being implanted into the testes of mice, the samples generated into early-stage sperm cells, in the US trials.
Scientists said that although the "cell precursors'' were insufficient to support conception, the breakthrough suggests that in future, the method could be used to grow fertile sperm from infertile men.
Infertility affects at least 10 per cent of couples, and in at least one third of cases relates to male fertility problems, which are often genetic.
The most common defect is missing regions of male Y chromosomes, which is associated with the production of few or zero sperm.
The trials by Stanford University involved three men suffering from such defects. When their tissue samples were genetically engineered and then implanted into the testes of mice, cells were successfully generated.
The findings, published in the journal Cell Reports, indicate that Y chromosome infertility occurs relatively late in the maturing process of sperm cells.
Lead researcher Dr Reijo Pera, from the Institute for Stem Cell Biology and Regenerative Medicine, said: "Our results are the first to offer an experimental model to study sperm development.
"It might even be possible to transplant stem-cell-derived germ cells directly into the testes of men with problems producing sperm."
In 2012, US scientists from the University of Pittsburgh found that it was possible to generate sperm cell precursors from skin tissue from fertile men.
However, the trials are the first time such methods have succeeded on men with fertility problems.
Researchers suggested the findings could bring hope to men who suffer from genetic infertility problems and from those who become infertile after treatment for diseases such as cancer.
Dr Reijo said: "Our studies suggest that the use of stem cells can serve as a starting material for diagnosing germ cell defects and potentially generating germ cells."
- Daily Telegraph