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 fertilization 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, in vitro gametogenesis, or I.V.G., so far has 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 opening up mind-boggling possibilities.

With I.V.G., two men could have a baby that was biologically related to both of them, by using skin cells from one to make an egg that would be fertilized 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 dont know what this could lead to, said Paul Knoepfler, a stem cell researcher at UC Davis. 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.

Some scientists even talk about what they call the Brad Pitt scenario when someone retrieves a celebritys skin cells from a hotel bed or bathtub. Or a baby might have what one law professor called multiplex parents.

There are groups out there that want to reproduce among themselves, said Sonia Suter, a George Washington University law professor who began writing about I.V.G. 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 about the possible consequences in a paper published this year.

I.V.G. may raise the specter of embryo farming on a scale currently unimagined, which might exacerbate concerns about the devaluation of human life, Dr. Eli Y. Adashi, a medical science professor at Brown; I. Glenn Cohen, a Harvard Law School professor; and Dr. George Q. Daley, dean of Harvard Medical School, wrote in the journal Science Translational Medicine.

Still, how soon I.V.G. might become a reality in human reproduction is open to debate.

I wouldnt be surprised if it was five years, and I wouldnt be surprised if it was 25 years, said Jeanne Loring, a researcher at The Scripps Research Institute in La Jolla who, with the San Diego Zoo, hopes to use I.V.G. to increase the population of the nearly extinct northern white rhino.

Loring said that when she discussed I.V.G. with colleagues who initially said it would never be used with humans, their skepticism often melted away as the talk continued. But not everyone is convinced that I.V.G. will ever become a regularly used process in human reproduction even if the ethical issues are resolved.

People are a lot more complicated than mice, said Susan Solomon, chief executive of the New York Stem Cell Foundation. And weve often seen that the closer you get to something, the more obstacles you discover.

I.V.G. is not the first reproductive technology to challenge the basic paradigm of baby-making. Back when in vitro fertilization was beginning, many people were horrified by the idea of creating babies outside the human body. And yet, I.V.F. and related procedures have become so commonplace that they now account for about 70,000, or almost 2 percent, of the babies born in the United States each year. According to the latest estimate, there have been more than 6.5 million babies born worldwide through I.V.F. and related technologies.

Of course, even I.V.F. is not universally accepted. The Catholic Church remains firm in its opposition to in vitro fertilization, in part because it so often leads to the creation of extra embryos that are frozen or discarded.

I.V.G. requires layers of complicated bioengineering. Scientists must first take adult skin cells other cells would work as well or better, but skin cells are the easiest to get and reprogram them to become embryonic stem cells capable of growing into different kinds of cells.

Then, the same kind of signaling factors that occur in nature are used to guide those stem cells to become eggs or sperm. (Cells taken from women could be made to produce sperm, the researchers say, but the sperm, lacking a Y chromosome, would produce only female babies.)

Last year, researchers in Japan, led by Katsuhiko Hayashi, used I.V.G. to make viable eggs from the skin cells of adult female mice, and produced embryos that were implanted into female mice, who then gave birth to healthy babies.

The process strikes some people as inherently repugnant.

There is a yuck factor here, said Arthur Caplan, a bioethicist at New York University. It strikes many people as intuitively yucky to have three parents, or to make a baby without starting from an egg and sperm. But then again, it used to be that people thought blood transfusions were yucky, or putting pig valves in human hearts.

Whatever the social norms, there are questions about the wisdom of tinkering with basic biological processes. And there is general agreement that reproductive technology is progressing faster than consideration of the legal and ethical questions it raises.

We have come to realize that scientific developments are outpacing our ability to think them through, Adashi said. Its a challenge for which we are not fully prepared. It would be good to be having the conversation before we are actually confronting the challenges.

Some bioethicists take the position that while research on early stages of human life can deepen the understanding of our genetic code, tinkering with biological mechanisms that have evolved over thousands of years is inherently wrongheaded.

Basic research is paramount, but its not clear that we need new methods for creating viable embryos, said David Lemberg, a bioethicist at National University in California. Attempting to apply what weve learned to create a human zygote is dangerous, because we have no idea what were doing, we have no idea what the outcomes are going to be.

Lewin writes for The New York Times.

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Growing an entire baby from skin cells could happen in a decade, scientists say - The San Diego Union-Tribune