April Flowers for redOrbit.com – Your Universe Online
A new study, from the Stanford University School of Medicine and Montana State University, demonstrates that, when implanted into the reproductive system of a mouse model, stem cells created from adult, infertile men will yield primordial germ cells. Primordial germ cells normally become sperm cells.
The findings, published in Cell Reports, help to further our understanding of a genetic cause of male infertility and basic sperm biology. The research team says that their approach holds considerable potential for clinical applications.
All of the infertile male participants suffer from a genetic mutation that prevents their bodies from producing mature sperm. The study suggests that the men with this condition — called azoospermia — might have produced germ cells at some point in their early lives, but these cells were lost as the men matured to adulthood.
“Our results are the first to offer an experimental model to study sperm development,” said Renee Reijo Pera of the Institute for Stem Cell Biology & Regenerative Medicine and Montana State University. “Therefore, there is potential for applications to cell-based therapies in the clinic, for example, for the generation of higher quality and numbers of sperm in a dish.”
“It might even be possible to transplant stem-cell-derived germ cells directly into the testes of men with problems producing sperm,” she added. Considerable study to ensure safety and practicality is needed, however, before reaching that point.
Infertility is a fairly common problem, affecting between 10 and 15 percent of couples in the US. The researchers say that many men are affected by genetic causes of infertility, most commonly due to the spontaneous loss of key genes on the Y sex chromosome. Until now, the causes of infertility at the molecular level have not been clear.
The fact that the research team was able to create primordial germ cells from the infertile men is very promising, but they note that these stem cells created far fewer of these sperm progenitors than the stem cells of men without the genetic mutations. They are sure, however, that this research provides a 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 Reijo Pera, 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.”
Humans share many cellular and physiological processes with common laboratory animals such as mice or fruit flies. In reproduction, however, there are significant variances, making it challenging to recreate the human reproductive processes in a laboratory setting. In addition, many crucial steps, such as the development and migration of primordial germ cells to the gonads, occur in the relatively short first days or weeks after conception.
Reijo Pera collaborated with Cyril Ramathal, PhD, a postdoctoral student at Stanford, to create what is known as induced pluripotent stem cells (iPSCs) from the skin samples of five men. The iPSCs closely resemble embryonic stem cells in their ability to become nearly any tissue in the body. Two of the participants were fertile, the remaining three carried a type of mutation on their Y chromosome known to prevent the production of sperm.
Regardless of the fertility or infertility of the men from whom they were derived, the primordial germ cells stopped differentiating in the mice before becoming mature sperm. The researchers believe this is because of the differences in the human and mouse reproductive processes. They were surprised, however, by the fact that the infertile men’s iPSCs could be used to create germ cells at all.
Prior studies using mice with a similar type of infertility discovered that the mice had germ cells as newborns, but that these cells were quickly depleted. The findings of the new study indicate that infertile human men might have had at least a few functioning germ cells as newborns or infants. The team says that more study is required, but they believe that their findings suggest these men might be able to have children of their own later in life by collecting and freezing some of this viable tissue during their childhood.
“This research provides an exciting and important step for the promise of stem cell therapy in the treatment of azoospermia, the most severe form of male factor infertility,” said Michael Eisenberg, MD, an assistant professor of urology at Stanford and director of the male reproductive medicine and surgery program. “While the study clearly demonstrates the importance that genetics play in spermatogenesis, it also suggests that some of these limitations could potentially be overcome.”
A previous study by Reijo Pera in 2009 demonstrated that it was possible to generate functional, sperm-producing germ cells from human embryonic stem cells grown in a laboratory under certain conditions. These cells are difficult to obtain, however, as they were derived from human embryos donated for research after in vitro fertilization procedures. Another challenge in using these cells for infertile couples wanting their own genetic children is that the child would inherit a portion of the genome from the embryo from which the cells were derived. Finally, the process required to create these cells required the stem cells to be engineered to overexpress several genes in order to drive the embryonic stem cells to become germ cells.
The stem cells created from the infertile men’s skin samples, however, required no artificial manipulation. These cells differentiated into what the team called “germ-cell-like cells” simply by virtue of the environment in which they were placed—in this case, the seminiferous tubules of mice, where the animals’ sperm production takes place. Without manipulation, the cells expressed many genes known to be expressed in primordial germ cells, and underwent a genetic reprogramming process called demethylation associated with sperm production.
“We found that the induced pluripotent stem cells were very efficiently driven down that lineage pathway,” said Reijo Pera. “This system has shown that the genetic background of the person from whom the stem cells are derived affects how many primordial germ cells are made. We’re now able to recapitulate that process and begin to study things like gene expression patterns and the distinct steps involved in this process.”
When the stem cells from the infertile men were compared to those of the fertile men, stark differences surfaced. The infertile men each carried a mutation in a region of the genome known as AZF1, which are associated with the production of few or no sperm. Their stem cells, according to the researchers’ estimations, were 50-to 100-fold less efficient in their ability to become primordial germ cells than the stem cells of the fertile men.
“Studying why this is the case will help us understand where the problems are for these men and hopefully find ways to overcome them,” said Reijo Pera.
“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,” she said. “This approach has great potential for treatment of individuals who have genetic/idiopathic causes for sperm loss or for cancer survivors who have lost sperm production due to gonadotoxic treatments.”
“In addition, it provides very intriguing possibilities for men rendered sterile after cancer treatments,” said Eisenberg. “Being able to efficiently convert skin cells into sperm would allow this group to become biologic fathers. Infertility is one of the most common and devastating complications of cancer treatments, especially for young boys and men.”
The research team intends to continue their studies using non-human primates for future implantation studies, in addition to exploring the effects of other types of infertility-associated mutations on germ cell production.