Scientists say lab-grown human sex cells could be less than a decade from reality, with major implications for fertility and reproduction.

The technology could, in theory, allow anyone to have biological children, regardless of sex, fertility or age.

Researchers are making rapid progress towards turning adult skin or blood cells into eggs and sperm through a process known as in-vitro gametogenesis (IVG) – a method that reprogrammes cells genetically to become gametes.

The technique typically starts by converting adult cells into stem cells, which are then guided into becoming primordial germ cells – the precursors to eggs and sperm.

These are placed inside a lab-grown organoid (a miniature version of an organ), which provides the biological signals needed to develop them further.

Prof Katsuhiko Hayashi, a developmental geneticist at the University of Osaka, told a meeting of the European Society of Human Reproduction and Embryology in Paris this week that his lab is about seven years away from creating viable human sperm.

Other frontrunners include a team at the University of Kyoto and California-based startup Conception Biosciences, whose Silicon Valley backers include OpenAI founder Sam Altman.

Conception’s chief executive told the Guardian the company is focused on producing clinical-grade human eggs and believes the technology could reverse population decline and open the door to human gene editing.

Hayashi said: “I feel a bit of pressure. It feels like being in a race.

“On the other hand, I always try to persuade myself to keep to a scientific sense of value.”

Hayashi’s lab has already produced baby mice with two biological fathers, suggesting the technique could one day be used by same-sex couples.

He said the lab receives emails from prospective fertility patients about once a week.

He said: “We get emails from [fertility] patients, maybe once a week.

“Some people say: ‘I can come to Japan.’ So I feel the demand from people.”

Matt Krisiloff, chief executive of Conception Biosciences, said: “Just the aspect alone of pushing the fertility clock … to potentially allow women to have children at a much older age would be huge.

“Outside of social policy, in the long term this technology might be the best tool we have to reverse population decline dynamics due to its potential to significantly expand that family planning window.”

Hayashi presented his team’s latest progress at the event, including the creation of primitive mouse sperm inside a lab-grown testicle organoid measuring about 1mm across, and the development of a human ovary organoid – a step towards growing human eggs in the lab.

Inside the artificial testes, Hayashi’s team managed to grow spermatocytes – the precursors of sperm cells – before the cells died.

He said an updated version of the organoid with a better oxygen supply could allow the cells to reach maturity.

Hayashi estimated that viable lab-grown human sperm could be about seven years away.

Creating sperm from female cells, he said, would be “technically challenging, but I don’t say it is impossible.”

He also suggested that his former colleague Prof Mitinori Saitou, based at Kyoto University, or Conception Biosciences could be ahead in the race.

“But they [Conception] are really, really secretive,” he added.

Others agreed with Hayashi’s timescale.

“People might not realise how quickly the science is moving,” said Prof Rod Mitchell, research lead for male fertility preservation in children with cancer at the University of Edinburgh.

“It’s now realistic that we will be looking at eggs or sperm generated from immature cells in the testicle or ovary in five or 10 years’ time.

“I think that is a realistic estimate rather than the standard answer to questions about timescale.”

Prof Allan Pacey, professor of andrology and deputy vice-president at the University of Manchester, said: “I think somebody will crack it. I’m ready for it. Whether society has realised, I don’t know.”

While several labs have successfully produced baby mice from lab-grown eggs, producing viable human eggs has been far more technically difficult.

A recent breakthrough in understanding how eggs stay dormant in the ovary for years could prove key.

Krisiloff declined to share detailed updates but said the company is “making really good progress on getting to a full protocol”.

In the best case, he said, the technology could reach the clinic within five years – though it may take longer.

Most researchers believe years of testing would be required to ensure that lab-grown cells do not carry genetic mutations that could be passed to embryos and future generations.

Some mice created using lab-grown cells have lived normal lifespans and been fertile.

Hayashi said: “We really need to prove that this kind of technology is safe.

“This is a big obligation.”

In the UK, using lab-grown eggs or sperm in fertility treatment is currently illegal.

The Human Fertilisation and Embryology Authority is already considering how safety could be ensured and what testing would be needed before clinical use could be approved.

Mitchell said:“The idea that you can take a cell that was never supposed to be a sperm or an egg and make it into a sperm or an egg is incredible.

“But it does bring the problem of safety. We need to be confident that it’s safe before we could ever use those cells to make a baby.”