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, according to researchers at the Stanford University School of Medicine and Montana State University.

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 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," said Renee Reijo Pera, PhD, former director of Stanford's Center for Human Embryonic Stem Cell Research and Education. "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."

A difficult process to study

Unlike many other cellular and physiological processes, human reproduction varies in significant ways from that of common laboratory animals like mice or fruit flies. Furthermore, many key steps, like the development and migration of primordial germ cells to the gonads, happen within days or weeks of conception. These challenges have made the process difficult to study.

Reijo Pera, who is now a professor of cell biology and neurosciences at Montana State University, is the senior author of a paper describing the research, published May 1 in Cell Reports. The experiments in the study were conducted at Stanford, and Stanford postdoctoral scholar Cyril Ramathal, PhD, is the lead author of the paper.

The research used skin samples from five men to create what are known as induced pluripotent stem cells, which closely resemble embryonic stem cells in their ability to become nearly any tissue in the body. Three of the men carried a type of mutation on their Y chromosome known to prevent the production of sperm; the other two were fertile.

The germ cells made from stem cells stopped differentiating in the mice before they produced mature sperm (likely because of the significant differences between the reproductive processes of humans and mice) regardless of the fertility status of the men from whom they were derived. However, the fact that the infertile men's cells could give rise to germ cells at all was a surprise.

Previous research in mice with a similar type of infertility found that although they had germ cells as newborns, these germ cells were quickly depleted. The Stanford findings suggests that the infertile men may have had at least a few functioning germ cells as newborns or infants. Although more research needs to be done, collecting and freezing some of this tissue from young boys known to have this type of infertility mutation may give them the option to have their own children later in life, the researchers said.

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Stem cells from some infertile men form germ cells when transplanted into mice