In a significant step toward combating male infertility, researchers at Yokohama City University have grown mouse sperm in a dish and used the sperm to produce pups that were themselves fertile in adulthood.
Researchers started with small fragments of tissue containing sperm stem cells, called spermagonia, collected from the testes of baby mice. They then grew those cells into functional sperm, using various chemicals to simulate the natural environment of the testes. The results of the study, published in this week’s issue of Nature, may eventually benefit infertile men and boys undergoing chemotherapy.
“When people with cancer undergo treatment, they are almost always rendered infertile,” says Martin Dym, a professor of biochemistry at Georgetown University. Dym was not involved in the study. “In men, you could freeze a sperm sample before treatment, but in [prepubescent] boys, you can’t. But they do have testes cells, and if you could develop those in culture, they could be used in in vitro fertilization down the line.”
Dym adds that the technique could also be used to redirect cells in the testes of infertile men to produce functional sperm. “These men don’t have normal sperm, but they will have normal spermagonial stem cells,” says Dym.
Takehiko Ogawa, a professor of urology, and his colleagues in Japan took small biopsies of testes tissue from baby mice that contained spermagonial stem cells but no mature sperm (The mice were too young to produce sperm.) To simulate the natural environment of the testes, Ogawa suspended the fragments on a semisolid support, partially bathing them in liquid.
The liquid contained a cocktail of chemicals called knockout serum replacement (KSR), a formulation that, counterintuitively, is used in cultures of embryonic stem cells to keep them in their undifferentiated state. Here, Ogawa found that KSR had the opposite effect, encouraging spermagonia to differentiate into mature sperm.
“We have not yet identified the key factors in KSR which really helped our system,” says Ogawa. “My next challenge is to identify those factors and make an even better culture media, to improve sperm quality, and make it applicable to other animals.”
While in vitro sperm counts were relatively low, the sperm produced were functional. The group inseminated adult mice with the cultured sperm, and found they were able to produce pups that, in turn, were able to mate naturally. “The final evaluation of sperm functionality was to make offspring and see the healthiness of offspring,” says Ogawa. “I’m still keeping the offspring, and they are about 14 months old, and they look very normal, comparable to other mice.”
In recent years, a handful of research groups have used various techniques to grow sperm from embryonic stem cells, adding growth factors to encourage stem cells to differentiate. While some efforts were able to successfully produce sperm, researchers have been unable to replicate these results, and none have been shown to produce fertile pups. Ogawa says the new system is a fairly simple one, and he hopes other researchers will attempt to replicate it.
The full health consequences in animals produced from in vitro sperm remains to be seen. Steve Krawetz, professor of obstetrics and gynecology at Wayne State University, says as cells differentiate into sperm, major changes occur in their DNA that make cells vulnerable to environmental factors. Depending on what is in the surrounding environment, factors could affect a cell’s DNA, creating defects that could be passed to future generations.
“We don’t know the long-term consequences or transgenerational effects,” says Krawetz. “But as a model system, this is fantastic. It gives us the opportunity to start looking at things in an in vitro setting and specifically alter and dissect cells in a much easier manner. It’s a great step forward.”
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