Scientists Clone Human Embryos

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Back in the 1990s, when the words "cloning" and "stem cells" sounded magically futuristic, any progress at all on either of these biological fronts was greeted with screaming front-page headlines, involving promise or peril or — most often — both.

Nowadays, thanks in large part to an understanding of how difficult both technologies actually are — and partly also to the human-cloning fraud perpetrated by the Korean scientist Hwang Woo Suk in 2004 — scientists are a lot more skeptical about the significance of each new claim. That's why there hasn't been so much excitement about a report published online Thursday by the journal Stem Cells. The authors claim to have created cloned human embryos that they believe are capable of producing stem cells — the raw material for all of the body's specialized tissues, from heart to muscle to liver and more.

You'd think that would be a huge deal. The scientists, from three private labs in La Jolla, Calif., and Detroit made the clones by a technique called somatic cell nuclear transfer: They took the nuclei from skin cells of adult men, inserted the genetic material into enucleated eggs from female donors, and got the eggs to begin dividing, forming embryos containing stem cells. In principle, replacement tissues grown from those cells would be genetically identical to the men they came from. So they could, in principle, be used to fix failing organs, without any fear of rejection.

To repeat: in principle.

The problem, as with every recent stem-cell advance so far, is that the principle and the practice are still awfully far apart. No one doubts that the scientists created cloned embryos — largely because they documented their techniques so carefully and performed confirming tests, but also because they aren't the first to have made clones (British scientists did it two years ago).

But while they're confident they can create stem-cell lines, they haven't done so yet. That reduces the excitement considerably — and indeed, Doug Melton of the Harvard Stem Cell Institute told the Associated Press, "I found it difficult to determine what was substantially new."

With dozens of labs around the world working on the problem, the creation of stem-cell lines from embryos like these will happen sooner or later. But even when they do, scientists will have to learn how to coax them into producing useful tissues. They'll also have to make certain that both the cloning and the coaxing don't damage the cells in a way that make them not just ineffective but lethal. That's the danger with a different form of stem-cell production, announced this past November, in which skin cells are simply genetically reprogrammed to revert to stem cells. The reprogramming, however, can trigger cancerous growth — so that technique, too, is far from ready for clinical use.

The reprogramming technique at least has the advantage of not requiring the creation and ultimate destruction of embryos, which poses huge ethical problems for some people.

But it's all just hand-waving for now. If a long string of "ifs" can be laid to rest, today's achievement may be seen a real step forward — but one that's only visible in hindsight.