A new study has found that stem cells made from the skin of adult, infertile men yield primordial germ cells - cells that normally become sperm - when transplanted into the reproductive system of mice.

The infertile men in the study each had a type of genetic mutation that prevented them from making mature sperm - a condition called azoospermia.

The research at the Stanford University School of Medicine and Montana State University suggests that the men with azoospermia may have had germ cells at some point in their early lives, but lost them as they matured to adulthood.

Although the researchers were able to create primordial germ cells from the infertile men , their stem cells made far fewer of these sperm progenitors than did stem cells from men without the mutations.

The research provides a useful, much-needed model to study the earliest steps of human reproduction.

"We saw better germ-cell differentiation in this transplantation model than we've ever seen," Renee Reijo Pera, PhD, former director of Stanford's Center for Human Embryonic Stem Cell Research and Education said.

"We were amazed by the efficiency. Our dream is to use this model to make a genetic map of human germ-cell differentiation, including some of the very earliest stages," Pera said.

The research is set to be published in Cell Reports.