Doubts keep growing about the stunning discovery that super stem cells could be created merely by placing white blood cells from young mice in acid or otherwise stressing them, says Paul Knoepfler, a stem cell researcher at UC Davis.

Among other inconsistencies, Knoepfler referred to several unexplained anomalies in images of these STAP cells in two papers, published by the prestigious journal Nature on Jan. 29. One image appears to suggest signs that virtually all cells treated with an acid bath were being reprogrammed, a result that would be extraordinary. Stem cell reprogramming to date has been inefficient, with a low percentage of treated cells being reprogrammed.

"The more I look at these two STAP papers, the more concerned I get ... The bottom line for me now is that some level a part of me still clings to a tiny and receding hope this has all been overblown due to simple misunderstandings, but that seems increasingly unlikely," Knoepfler wrote Sunday on his blog, IPS Cell.

This undated image made available by the journal Nature shows a mouse embryo formed with specially-treated cells from a newborn mouse that had been transformed into stem cells. Researchers in Boston and Japan say they created stem cells from various tissues of newborn mice. If the same technique works for humans, it may provide a new way to grow tissue for treating illnesses like diabetes and Parkinson's disease. The report was published online on Wednesday, Jan. 29, 2014 in the journal Nature. (AP Photo/RIKEN Center for Developmental Biology, Haruko Obokata)

Nature is conducting its own investigation, Knoepfler noted. But in addition, the journal should release "unmodified, original versions" of the images and data in the papers, Knoepfler wrote.

The images contained "minor errors" that didn't change the basic findings, said Charles Vacanti, a Harvard University professor who is part of the scientific team reporting the discovery, according to a Feb. 22 article in a Japanese newspaper, the Asahi Shimbun.

Controversy is normal for any major scientific advance. Skeptics must be converted, and the only way to do that is to show the data. The 1997 announcement of the first mammalian clone, Dolly the sheep, was greeted with considerable doubt because it was believed that genetic imprinting made such cloning impossible. But others were eventually able to confirm the finding.

In this case, doubters say such an apparently easy method of reprogramming cells would generate pluripotent stem cells far too easily, because stress is common in animals. Such stem cells are known to cause tumors, so evolution should have selected against such a response.

Nature's own role has been criticized. The journal was taken to task for its handling of online journalism Feb. 20 by another stem cell blogger, Alexey Bersenev. He chided Nature for not linking to sources.

"In scientific journalism, every claim must be linked to appropriate original source," Berseney wrote. "Nature consistently refuses to acknowledge bloggers, online discussions and other web resources with valid credible information. This is not acceptable for sci journalism."

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STAP stem cell doubts keep proliferating