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Biotechnology

Inside the race to make human sex cells in the lab

Scientists might soon be able to create eggs and sperm from skin and blood cells. What will that mean?

illustration of person with pipettes and a baby stroller
Amrita Marino

The way we make babies could be about to change. Maybe. 

An embryo forms when sperm meets egg. But what if we could start with other cells—if a blood sample or skin biopsy could be transformed into “artificial” sperm and eggs? What if those were all you needed to make a baby?

That’s the promise of a radical approach to reproduction. Scientists have already created artificial eggs and sperm from mouse cells and used them to create mouse pups. Artificial human sex cells are next.

The advances could herald the end of infertility—there’s no need to worry about a lack of healthy eggs or sperm if you can create new ones in the lab. It would open up alternative routes to parenthood as well. Same-sex couples could have genetically related children. If a cisgender woman could create her own sperm cells, she could use them to fertilize the egg of a partner. Likewise, a cisgender man could produce his own eggs to be fertilized by the sperm of his partner. And why stop there? The technology would allow four parents to make equal genetic contributions to a baby, for example. Or a single person could produce both the sperm and the egg that create an embryo. 

That’s the vision, at least, after a decade of tantalizing results in the lab. We know, more or less, how to do it. The problem is actually getting there and—maybe even harder—untangling the knot of ethical issues that will come up along the way. 

Human sex cells are proving far trickier to generate than mouse sperm and eggs. So tricky, in fact, that some researchers who have spent years trying to create them are starting to give up. The work is fiddly and requires an expert knowledge of how cells differentiate into sperm and eggs and how human embryos develop—the precise mechanisms of which are still poorly understood. 

The big question is whether we’ll ever be able to turn promising lab results into acceptable and safe changes in how we reproduce. In many ways, it’s still a deep mystery how sperm and eggs form. And without that knowledge, lab-made egg and sperm cells could carry risks of devastating diseases that might not become evident until the resulting babies are born, or even later in their lives. 

Optimists might argue that the same concerns were originally raised about in vitro fertilization; these days, around 73,000 babies are born each year as a result of assisted reproduction technologies like IVF in the US alone. If we can find a way to do it safely, the use of artificial sex cells could transform reproduction even more radically, and potentially redefine what it means to be a biological parent. 

But that is a big “if.” 

Recipes for sperm and eggs

Over the last decade or so, Mitinori Saitou, a developmental biologist at Kyoto University, has led and participated in the most groundbreaking research in the field of in vitro gametogenesis, as the creation of artificial egg and sperm cells in the lab is formally known. Much of the work relies on a Nobel Prize–winning technique developed in 2006 that allows scientists to turn adult cells into stem cells, which can form pretty much any specialized cell in the body: heart cell, liver cell, brain cell—you name it. The trick is working out how to encourage the stem cells to form egg or sperm cells. 

In mice, placing stem cells in a petri dish alongside cells taken from mouse embryos seems to work—the cells first turn into primitive precursor cells, which then eventually develop into egg cells. These eggs can even be fertilized with sperm to generate embryos. 

Back in 2012, Saitou, Katsuhiko Hayashi, and their colleagues were the first to use this approach to create primitive mouse egg cells in the laboratory; in 2016 the team generated mature egg cells. Some of the stem cells the team used were taken from mouse embryos, but others were created using cells from the animals’ tails. 

These eggs were matured and fertilized with sperm, and the resulting embryos were transplanted back into female mice, which gave birth to apparently healthy babies. The feat was a remarkable breakthrough by any standard, and news channels around the world were quick to state that human reproduction was about to change forever.

Saitou and Hayashi, as well as other teams, have had similar successes with mouse sperm. 

The race is now on to do the same with human cells. Researchers have managed to generate immature sex cells and are working on ways to push the cells further along, to a state where they can be used to create an embryo. Today, Saitou focuses on eggs. His team has pushed human cells to the oogonia stage—the stage before they become eggs—after culturing cells in the lab for four months. 

Meanwhile, in 2015, Kotaro Sasaki, a former mentee of Saitou based at the University of Pennsylvania, turned men’s blood cells into stem cells and generated the primordial cells that lead to sperm. “It’s kind of a recipe to make an early-stage [sex] cell,” says Sasaki. Since then, the team has been trying to encourage these primitive cells to mature into sperm in the lab. Most recently, the team managed to get what are known as spermatogonia—the immediate precursor of sperm cells. “We are one step closer now to making sperm in a dish,” says Sasaki. 

Scientists already know how to turn adult cells into stem cells, and—in mice—how to coax those into eggs and sperm. The race is now on to do the same with human cells.

But a critical final step is so far proving exceptionally elusive in the case of both eggs and sperm cells. Mature eggs and sperm have half as many chromosomes as other body cells. This is vital—it allows the two cells to fuse and form an embryo with a full set of chromosomes. Precursor cells must go through a special type of cell division called meiosis in order to halve their chromosomes. No one has managed to replicate this with human cells in the lab—yet.

Sasaki thinks he’s close. In unpublished work, he says he has managed to push the immature sperm cells one step further along the path to maturity, and that these cells have started to undergo meiosis. Once meiosis is completed, the sperm could be used to fertilize an egg, even if they are not fully matured. 

But there are other hurdles—some so challenging that many scientists have given up. For one thing, nudging the stem cells in the right direction requires, it seems, a unique touch and expertise. Not just anyone will be able to make egg and sperm cells in the lab, says Saitou. 

Top chef

Saitou and Hayashi, now at Kyushu University, lead world-renowned teams of extraordinary skill. Their achievements might not have been possible without the contributions of Hiroshi Ohta, for example. Ohta is an expert in anesthetizing newborn mice using ice, performing intricate surgery on them, and injecting cells into the animals’ miniature gonads. The entire procedure must be completed within five minutes or the animals die. Only a few people have such skills, which take months to develop. “I think our group was kind of lucky,” says Saitou. “It was a get-together of many talented scientists.”

The work is hampered by the lack of in-depth knowledge about how the primitive forms of egg and sperm cells develop naturally in the embryo—a process that is far from fully worked out in humans. Some of the embryo’s cells begin to differentiate into these primitive sex cells at around 14 days. But in some countries, it is illegal for researchers to even grow human embryos beyond 14 days. “They would send me to jail if I went beyond day 14,” says Azim Surani, who is working with precursors to artificial sex cells at the University of Cambridge in the UK.

The problem, from a research point of view, is that the 14-day rule “comes in just as the embryos start to get interesting,” says Surani. Without being able to easily study the critical process of how primitive cells begin forming egg and sperm cells, scientists are limited in their ability to mimic it in the lab. 


How it works

Cells are harvested from a skin biopsy or a blood sample.
Step 2: stem cells
Harvested cells are turned into stem cells.
Step 3: Petri Dish
Stem cells are grown in an embryo-like environment.

Step 4: Sperm and Egg
Precursor sex cells form, and eventually they mature into sperm or egg cells.
Step 5: IVF
The lab-grown cells are used to create an embryo in an IVF lab.
Step 6: embryo in utero
If the embryo is healthy, it can be transferred into a uterus, hopefully resulting in pregnancy.


Even if scientists were able to study embryos more freely, some mysteries would remain. Once the cells that make eggs and sperm are created, they are held in a kind of suspended animation until puberty or ovulation. What happens to them in the years in between? And how important is this phase for the health of mature eggs and sperm? “The honest answer is we don’t know,” says Surani.

The stem cells in the lab must also be generated and cared for under precise conditions. To survive, they must be bathed in a cocktail of nutrients that must be replaced every day. “It’s very time consuming and labor intensive … and it takes a lot of money,” says Bjorn Heindryckx at Ghent University in Belgium, one of the scientists who have given up on creating human eggs this way in the lab. “The outcome was too limited for the effort and the money that we spent on it,” he says.

Part of the challenge is that for the precursor stem cells to develop into fully matured egg or sperm cells, they must be placed in an environment mimicking that of newly developing ovaries or testes. Researchers studying mice use tissue taken from mouse embryos to induce the stem cells to differentiate into sex cells. But similarly using human tissue from discarded embryos is ethically and legally problematic. So scientists are working on ways to create the right environment without using tissue from embryos. 

The upshot is that it will likely take a highly skilled team years of dedicated research. “It’s not impossible, but it would not be easy to do,” says Surani.

That hasn’t discouraged a handful of biotech companies from taking an interest in artificial sex cells.  

Conception, a company employing a team of around 30 scientists in Berkeley, California, aims to “turn stem cells into human eggs” to enable older or infertile women, as well as male couples, to have genetically related children. “I’m gay, and it’s something I was very personally interested in,” says Matt Krisiloff, the company’s CEO.

Krisiloff says his team has made “quite a bit” of progress, and that he’s “very excited” about his results. But he won’t say what they are. The company has not published its research, although Krisiloff says that he does plan to at some point. Krisiloff envisions that in the near future—he won’t say when—the company will be able to create egg cells from people’s blood cells. He expects to eventually partner with an IVF clinic, which would fertilize the eggs to produce embryos. 

When I told Heindryckx about the company, his response was: “Oy, you’re kidding.” 

Dangerous mutations

While everyone contacted by MIT Technology Review was confident that eventually we’ll be able to create artificial human egg and sperm cells in the lab, there is less certainty over whether we’ll ever be able to safely use them for reproduction. 

One worry is that our cells accumulate DNA damage as we get older—it’s thought to be one of the reasons many cancers are more likely to affect us later in life. And body cells are thought to have more mutations than germ cells that form eggs and sperm. A skin cell taken from a 50-year-old is going to have many more mutations than a typical egg or sperm cell from a 30-year-old. We don’t know if or how these might affect an embryo, or a baby.

Saitou reckons that if the technology were ever to become a clinical reality, you’d likely have to store your cells ahead of time—ideally at birth. 

Scientists also worry that the technique would influence how DNA works in babies born from artificial sex cells. Certain molecules can attach themselves to our DNA and change the way our genes are expressed—essentially changing the way they make proteins. These so-called epigenetic changes can switch genes on or off, or just turn them up or down. Epigenetic changes are made to our DNA throughout life and are thought to be influenced by what we eat, how much we exercise, whether or not we smoke, and other lifestyle factors.

But they might also be triggered when cells are grown in a dish. This is thought to occur in IVF, even if embryos are only left in culture for a few days. These epigenetic changes probably explain why babies conceived via IVF tend to have different birth weights from those conceived spontaneously—and this can vary depending on the brand of nutrient-rich liquid the cells are bathed in. 

If a few days in a dish can influence the way genes are expressed, what about weeks or months? The most advanced human sex cells generated so far have been cultured in the lab for four months. “It’s very long, and not natural,” says Heindryckx.

And while scientists have been able to generate mouse pups from stem cells, a closer look at the study results suggests that the vast majority of the embryos created that way were far from healthy.

Thousands of eggs need to be created in order to generate a few that are healthy enough to be successfully fertilized. Then almost all the embryos created with artificial eggs die—and they die in strange ways. The embryos look misshapen and appear to have many abnormalities, says Saitou. 

illustration of person with stylized genetic components
AMRITA MARINO

“The focus is always … on the one born mouse,” he says. “But if you have one live-born mouse, you have 999 dead embryos.” The success rate hasn’t improved in the last 10 years, either, says Saitou. 

“I don’t think there’s a fundamental biological reason why it wouldn’t work,” says Heidi Mertes, a medical ethicist at Ghent University who has scrutinized the ethical implications of deriving sex cells from body cells. “But I wouldn’t want to be the first patient to try this, let’s put it that way.”

Going rogue

All that might not stop someone from trying it. We all saw what happened when He Jiankui used CRISPR genome-­editing technology to alter the DNA of two embryos, resulting in the birth of gene-edited twin baby girls known as Lulu and Nana, as well as a third baby. He ostensibly set out to lower the twins’ risk of contracting HIV—but may have exposed them to other health risks that may only make themselves apparent later in the girls’ lives. He was widely condemned and was eventually sentenced to prison in China. 

The truth is that there will always be someone willing to bend the rules to be the first to achieve some scientific feat—even if it is an ethically dubious one. And when it comes to reproductive medicine, a dangerous mix of huge sums of money and limited regulation enables new, experimental treatments to be speedily tested on willing, and often desperate, would-be parents. “There are always mavericks,” says Surani. 

Academic researchers must get approval from an ethics committee before they undertake any significant research involving people. And any individual who carries out an experimental treatment at a hospital will need approval from that hospital’s ethics committee, too. But people who work outside these institutions may not be held to the same ethical standards.

Human artificial gametes are set to become a scientific reality in the coming years. Just how soon depends on who you ask. 

Last year, the International Society for Stem Cell Research—an international group of researchers working in the field—published updated guidelines for research and treatments. The guidelines explicitly forbid the use of eggs or sperm generated from stem cells to enable people to have babies. The procedure is designated “not allowed; currently unsafe.” But the ISSCR guidelines are just that—they are not laws. 

Reproductive medicine is poorly regulated in the US as it is, and a person who uses artificial sex cells to help someone get pregnant might argue that it’s not breaking any laws. When I asked a representative of the American Society for Reproductive Medicine how the use of artificial gametes might be controlled in the US, his response was: “By asking that question, you have put yourself ahead of pretty much every policymaker in the US.” 

“It’s a little unclear, honestly,” says Krisiloff, who has been in conversations with consultants about how to run FDA-approved clinical trials with artificial eggs. “I think that this might be the type of thing where … they might get a cease-and-desist letter after the fact,” he says. “But there may not be a totally clear legal framework to say it’s illegal to do so in the first place.”

It’s easy to draw comparisons with IVF; that technology was also hyped as offering an end to infertility and decried by others as unnatural. IVF appears to be safe for babies so far. Millions of healthy babies have been born as a result of the technology. 

But some argue that we still don’t really know if IVF has long-term effects. The first person born using the technique, Louise Brown, is now 44—we don’t know if there are health risks that will only become apparent in later life. “No one has really considered these long-term effects,” says Saitou. “And these would probably be more profound if you start with artificial gametes.”

Mertes, the medical ethicist at Ghent, asks if we should even be trying, given the safety risks. Having genetically related children is not the only path to parenthood—there are other options for people who are unable to conceive with their own sperm and eggs. “We shouldn’t keep reinforcing the idea that genetic parenthood is something that justifies a lot of risks,” she says.

But the topic is sensitive, and Mertes has been subject to backlash for airing her thoughts. Why shouldn’t everyone have the same parenthood options as cisgender, heterosexual, fertile men and women?

“I don’t know if it should be done, and we don’t know if it’s safe,” says Sasaki. “But it’s possible, so from an ethical and legal perspective, you need to discuss it intensely.” 

Now is the time for these discussions. Human artificial gametes are set to become a scientific reality in the coming years. Just how soon depends on who you ask. “To be very honest, I don’t like to guess,” says Saitou. “With some unexpected roadblocks, suddenly the progress stops, but with some unexpected breakthroughs, it suddenly speeds up.”

Saitou is acutely aware of the implications of his work. In Japan, he says, the general public regards the technology with awe, but some fellow scientists aren’t as convinced. Some have argued that perhaps the genes of people with infertility should not be passed to the next generation. The process is too artificial, they argue; the resulting embryos—and babies—may struggle to survive. 

The argument reminds Saitou of a legendary manga comic book by Osamu Tezuka, called Hi No Tori, or Phoenix, which is set in the future. “There’s a story about how all mammals are created … in some artificial ways,” he says. In the story, the animals are kept alive in what are essentially test tubes. As soon as the animals leave this protective environment, they die: “They are just so unfit and artificial.” 

Saitou wonders if embryos—or potentially babies—generated from artificial sex cells may suffer a similar fate, given the low odds of success seen in mice. “This time may come,” he says. “It’s a comic, but … [as science progresses], I somehow see that our society is gradually approaching what [Tezuka] depicted.” 

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