Scientists in China have finally succeeded in creating functioning sperm¹ from mice in the laboratory. To accomplish this feat², the researchers coaxed³ mouse embryonic⁴ stem cells to turn into functional⁵ sperm-like cells, which were then injected into egg cells to produce fertile mouse offspring. The work, reported February 25 in Cell Stem Cell, provides a platform for generating sperm cells that could one day be used to treat male infertility⁶ in humans. "Reproducing germ cell development in vitro has remained a central goal in both reproductive biology and reproductive medicine," says co-senior study author Jiahao Sha of Nanjing Medical University. "We established a robust⁷, stepwise approach that recapitulates⁸ the formation of functional sperm-like cells in a dish. Our method fully⁹ complies with the gold standards recently proposed by a consensus¹⁰ panel of reproductive biologists, so we think that it holds tremendous promise for treating male infertility."

 

Infertility affects up to 15% of couples, and about one-third of cases can be traced to the man. One major cause of male infertility is the failure of precursor¹¹ germ cells in the testes to undergo a type of cell division called meiosis to form functional sperm cells. Several studies have reported the successful generation of germ cells from stem cells, but they did not fully evaluate the functionality of the germ cells or provide proof for all key hallmarks of meiosis.

 

Recently, a panel of reproductive biologists proposed gold standard criteria¹² to prove that the major events of meiosis have taken place in engineered germ cells. For example, researchers must show evidence of the correct nuclear DNA¹³ content at specific meiotic stages, normal chromosome¹⁴ number and organization, and the capacity of germ cells to produce viable¹⁵ offspring. Until now, the recapitulation of all of the essential steps of meiosis has remained a major obstacle to the production of functional sperm and egg cells in a dish. 

 

To overcome this hurdle¹⁶, Sha teamed up with co-senior study authors Qi Zhou and Xiao-Yang Zhao of the Institute of Zoology¹⁷ at the Chinese Academy of Sciences to develop a stem cell-based method that fully recapitulates meiosis and produces functional sperm-like cells. The first step was to expose mouse embryonic stem cells (ESCs) to a chemical cocktail¹⁸, which coaxed the ESCs to turn into primordial¹⁹ germ cells. Next, the researchers mimicked²⁰ the natural tissue environment of these precursor germ cells by exposing them to testicular cells as well as sex hormones²¹ such as testosterone.

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