Stem cells have been hailed by scientists as the great hope to one day prevent, halt and even reverse damage from diabetes, spinal cord injuries and degenerative diseases like Parkinson's and Alzheimer's.

Stem cells obtained from human embryos seem to offer the best chance of new therapies, because unlike other stem cells they have the ability to morph into almost any type of tissue. But researchers complain that political roadblocks are keeping them from determining the full potential of these cells.

Six years ago, President Bush limited federally funded research to about 20 viable lines of cells that had been extracted from embryos prior to August 9, 2001. The stem cell community has repeatedly called for the restrictions to be lifted, charging that the designated cell colonies have been compromised or corrupted and that failure to ease the ban is hobbling U.S. efforts to discover new life-saving treatments. But opponents say such research is immoral, because embryos must be destroyed to obtain their cells.

Last week, Congress sent legislation to Bush that would allow federally funded scientists to study cells from frozen embryos that fertility clinic patients no longer need and have chosen to donate instead of discard. The Stem Cell Research Enhancement Act of 2007 also spells out ethical guidelines governing such research.

Bush immediately renewed his pledge to veto the bill (as he did similar legislation that reached his desk last year)—and it is questionable whether the Senate can muster the votes to override it. The Senate passed the measure in April by a 63-to-34 margin—just four votes shy of the two thirds majority needed to nix a veto—on a day when two supporters of the bill were absent.

In vetoing the measure, Bush said it would be "a grave mistake" for "American taxpayers [to] be compelled to support the deliberate destruction of human embryos."

Critics argue further that embryonic stem cell research is unnecessary because stem cells from adults are purportedly just as promising. In fact, just a day before the House vote last week, researchers announced that they had coaxed plain old skin cells taken from adult mice to mimic embryonic cells, which—if they learn to do the same for humans—could offer a simple and seemingly less controversial way to create scores of stem cells.

Although researchers said the finding did not negate the need for studies on true embryonic stem cells, critics seized on it as further proof that such research is superfluous: If adult cells could be transformed into embryolike cells, they argued, then why not use them instead of destroying embryos?

The claim echoes those made many times in the emotional debate over federal funding of new human embryonic work. In particular, opponents of embryonic stem cell research have repeatedly pointed to the supposed power of stem cells extracted from the adult body, which in the hands of at least one laboratory seemed to nearly match that of embryonic stem cells.

In contrast to last week's study other laboratories have never reproduced the dramatic findings on which such arguments are based. And in the past six months investigations have exposed strange flaws in the data from one lab that claimed to have successfully manipulated adult stem cells to act like embryonic tissue.

At first blush, a flurry of unreproducible results might suggest problems within the field itself.

In recent interviews top stem cell experts said that the failures reflect the field's high public profile and its politicization, which obscure real progress. "I hate it when [big claims go unreplicated], because it makes our field look like a field full of shams and frauds," says stem cell biologist Irving Weissman of Stanford University.

Conventional wisdom has long held that adult stem cells are only capable of forming their tissue of origin. Researchers have isolated stem cells from only some of the body's organs and tissues, including the blood, brain, skeletal muscle, heart muscle and most recently from skin. They hope to discover whether stem cells also exist for other key organs such as the pancreas, liver and spinal cord. Much adult stem cell research focuses on identifying the genes and molecules that define such cells and allow them to replenish themselves indefinitely as they produce the various cell types of their organs—the defining characteristics of a stem cell. Learning to grow and manipulate adult stem cells in the lab might also allow researchers to create tissue regenerating treatments from them.

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