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In 2016, two Japanese reproductive biologists, Katsuhiko Hayashi and Mitinori Saitou, made an announcement in the journal Nature that read like a science fiction novel. The researchers had taken skin cells from the tip of a mouse's tail, reprogrammed them into stem cells and then converted those stem cells into eggs. The eggs, after fertilization, were transferred to the uterus of female mice, which produced 10 offspring. Some of those offspring even had young of their own afterwards.

Gametes are the cells, such as eggs and sperm, that are essential for sexual reproduction. With their experiment, Hayashi and Saitou provided the first evidence that what is known as in vitro gametogenesis, or I.V.G. - the production of gametes outside the body, starting with non-reproductive cells - is possible in mammals. The mice that had hatched from the laboratory-made oocytes were described as "completely normal".

The Japanese experiment could change the science of human reproduction. The first successful in vitro fertilization, in 1978, made it possible to conceive an embryo outside the body.

Today, about two per cent of all babies in the United States are conceived in a laboratory, via I.V.F. - last year, analysts estimated the global I.V.F. market at more than US$23 billion.

Oocytes have become commodities that are harvested, bought, donated and stored. But oocytes, some of the most complex cells in the body, and large enough to be visible to the naked eye, are difficult to obtain; as a woman ages, their number and quality decline. "If mature human oocytes could be derived from a person's skin cells, it would avoid most of the cost, almost all the inconvenience and all the risk of IVF," wrote Stanford bioethicist Henry Greely in his 2016 book, "The End of Sex and the Future of Human Reproduction," in which he discussed new techniques for making stem cells that had won the Nobel Prize in 2012. He predicted that in the next 20 to 40 years, sex will no longer be the method by which most people make babies ("among people with good health coverage," he qualified).

A hundred years ago, many Americans died in their mid-50s. Today, we can expect to live into our seventies and eighties. In the US and many other countries, women are giving birth for the first time at an older age than a few decades ago, but the age at which women lose fertility has not changed: By forty-five, the chances of pregnancy without assisted reproductive technology are extremely low.

Biologists have theories, none of them conclusive, as to why women's fertility declines so much in middle age and why the ovaries age at least twice as fast as other organs in the body. Deena Emera, evolutionary geneticist and author of a forthcoming book on evolution and the female body, says the vast majority of female mammals, including chimpanzees, retain the ability to conceive for most of their lives. Elephants, which can live up to 70 years, can become pregnant and give birth until their sixth decade. Human females share their long post-reproductive lifespan with only a few other mammals, mainly toothed whale species. We are joined in this strange and frustrating reality by narwhals, belugas and orcas.

Why science invests millions in R&D to manufacture artificial human egg cells and why not, also artificial wombs that can deliver designer babies on demand ? Because there is a lot of money to be made. It's a business model that will also come in handy if in a near future women turn out to be infertile.

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