How the male body produces sperm has long been hidden inside its genitalia. Trying to recreate the process in a lab for humans and other mammals has led to many failures; sperm development appeared to be dependent on unique conditions in the testes. But a new technique may finally be bringing that process into the open: A team of Chinese researchers reports turning a dish of a certain type of mouse stem cell into spermlike cells, which then were used to fertilize eggs and produce healthy mouse pups. The approach could help researchers study mammalian sperm development more directly, and it could spur efforts to develop treatments for male infertility in people.

“It is, I think, truly the first time any lab has been able to go all the way up to a live pup in vitro,” says Niels Geijsen, a stem cell biologist at the Hubrecht Institute in Utrecht, the Netherlands, “which is quite amazing, if this is indeed what happened.” The dramatic result—from a relatively straightforward approach—comes after years of unsuccessful or incomplete attempts, and it leaves some researchers wary about whether it can be replicated.

If it can, the technique offers researchers their first full glimpse of mammalian meiosis, the process by which cells in the testes and ovaries become sperm and eggs. And if a similar technique could produce human sperm cells, “the impact would be huge,” says Kyle Orwig, a stem cell biologist focused on male infertility treatments at the Magee-Womens Research Institute at the University of Pittsburgh in Pennsylvania. The advance would be particularly relevant to men with certain types of infertility—for example, those who become unable to produce functional sperm after chemotherapy. These men could still conceive biological children if other cells from their bodies are reprogrammed into stem cells and then prompted to develop into sperm.

Several labs have tried to create sperm in a dish from stem cells derived from early embryos, because they can differentiate into any kind of cell in the body. But the multistage process of meiosis, which involves a complex pairing up and separation of a cell’s DNA, “always seems to peter out as you approach the later stages,” Geijsen says. In 2011, a research group at Kyoto University in Japan working with mouse embryonic stem (ES) cells managed to turn them into cells that resemble primordial germ cells (PGCs), from which sperm and eggs arise. But to complete their maturation into sperm, which have half the number of chromosomes of nongerm cells, these PGC-like cells were transplanted back into mouse testes. That meant that biologists couldn’t observe all of meiosis—and that any future clinical application would require returning cells to a man’s body, which could cause tumors.

In the new study, led by researchers at the Chinese Academy of Sciences in Beijing and Nanjing Medical University in China and published today in Cell Stem Cell, no transplantation was necessary. The team first used the Kyoto University group’s approach to make PGC-like cells from mouse ES cells. They then combined these in a dish with testicular cells from newborn mice in various culture conditions—a process that took hundreds of trials, says author Xiao-Yang Zhao, a stem cell biologist now at Southern Medical University in Guangzhou, China. They finally landed on a cocktail of sex hormones and growth factors that includes a hormone-rich extract from the pituitary glands of cows. “God knows what’s in there,” says reproductive biologist Mary Ann Handel of the Jackson Laboratory in Bar Harbor, Maine, “but it’s probably good stuff.” Under those conditions, the cells go through a division process that has “all the hallmarks of meiosis,” says Handel, who helped develop a set of standards for documenting that process in a dish. The researchers then injected the resulting spermlike cells—which couldn’t swim—directly into eggs and implanted them in surrogate mouse mothers. “The final gold standard: They make babies!” Handel says.

By observing complete sperm development in vitro, researchers can tackle some fundamental questions. What triggers meiosis? How do surrounding cells in the testes direct that process? And how do a cell’s chromosomes manage to pair up and separate during division?

The clinical applications are much more remote, because mouse and human germs cells develop differently and may require different conditions. Still, the proof-of-concept is encouraging: “If it works in the mouse, there’s no biological reason to think it wouldn’t be effective in humans,” says stem cell researcher George Daley of Harvard Medical School in Boston. “But one has to actually define the [culture] conditions and walk the cells through this very careful choreography.”

Others are more skeptical about the results. “You have to be very cautious about the implications of this paper,” says stem cell biologist Mitinori Saitou, who led the Kyoto University team that first created PGC-like cells. He says several details in the paper struck him as strange: The cells were reportedly cultured at 37°C—about 3°C hotter than is typical, and potentially hot enough to hinder sperm development. He also notes that the fluorescence imaging meant to demonstrate the presence of PGC-like cells in the dish doesn’t seem to show proteins necessary for that type of cell, meaning the authors may not really have created the PGC-like cells they would need to generate sperm.

Zhao contends that the protein expression is consistent with PGC-like cells, and that the team observed similar results at 34°C and 37°C. But many in the field will be waiting for the next steps: testing whether the resulting mouse pups are genetically normal, trying out the technique in other animals, and using other and less controversial types of cells—such as stem cells that can be extracted and cryopreserved from adult testicular tissue—as the starting point. Zhao says his group plans to further optimize the culture conditions and eventually attempt to repeat the feat with human cells.