NEW YORK • Nearly 40 years after the world was jolted by the birth of the first test-tube baby, a new revolution in reproductive technology is on the horizon - and it promises to be far more controversial than in-vitro fertilisation ever was.

Within a decade or two, researchers say, scientists will likely be able to create a baby from human skin cells that have been coaxed to grow into eggs and sperm, and used to create embryos to implant in a womb. The process, known as in-vitro gametogenesis or IVG, has so far been used only in mice. But stem cell biologists say it is only a matter of time before it could be used in human reproduction.

With IVG, two men could have a baby by using skin cells from one to make an egg that would be fertilised by sperm from the other. Women with fertility problems could have eggs made from their skin cells, rather than go through the lengthy and expensive process of stimulating their ovaries to retrieve their eggs.

"It gives me an unsettled feeling because we don't know what this could lead to," said University of California, Davis, stem cell researcher Paul Knoepfler. "You can imagine one man providing both the eggs and the sperm, almost like cloning himself. You can imagine that eggs becoming so easily available would lead to designer babies."

"There are groups out there that want to reproduce among themselves," said law professor Sonia Suter at George Washington University, who began writing about IVG even before it had been achieved in mice. "You could have two pairs who would each create an embryo, and then take an egg from one embryo and sperm from the other, and create a baby with four parents."

Three prominent academics in medicine and law sounded an alarm earlier this year.

"IVG may raise the spectre of embryo farming on a scale currently unimagined, which might exacerbate concerns about the devaluation of human life," Dr Eli Adashi, a medical science professor at Brown University, Professor Glenn Cohen at Harvard Law School and Dr George Daley, dean of Harvard Medical School, wrote in the Science Translational Medicine journal.

Still, how soon IVG might become a reality in human reproduction is open to debate. "I wouldn't be surprised if it was five years, and I wouldn't be surprised if it was 25 years," said Scripps Research Institute researcher Jeanne Loring, who hopes to use IVG to increase the population of the nearly extinct northern white rhino.

However, "people are a lot more complicated than mice", said Ms Susan Solomon, chief executive of the New York Stem Cell Foundation. "And we've often seen that the closer you get to something, the more obstacles you discover."

IVG requires layers of complicated bioengineering. Scientists must first take adult skin cells and reprogramme them to become embryonic stem cells capable of growing into different kinds of cells.

Then, the same kind of signalling factors that occur in nature are used to guide those stem cells to become eggs or sperm.

NYTIMES