TOKYO, March 10 (Reuters) - A Japanese scientist called on Monday for withdrawing stem-cell research he had been involved in that had appeared to promise a new era of medical biology as doubts have arisen over the results.

The research, described as game-changing by experts at the time, was covered breathlessly in Japan after it was published in the journal Nature, with co-researcher Haruko Obokata becoming an instant celebrity.

But since then, there have been repeated reports that other scientists could not replicate the team's results and that there were problems with its data and images.

"It is no longer clear what is right," Teruhiko Wakayama told public broadcaster NHK of the research, published in January, which appeared to show a simple way to reprogramme mature animal cells back into an embryonic-like state that would allow them to generate many types of tissue.

The Japanese research institute Riken, where Obokata works, said last week it had "launched an independent inquiry into the content of the papers, conducted by a panel of experts from within and outside Riken, and will publish the results of this inquiry as soon as it is concluded".

Wakayama, a professor at the University of Yamanashi west of Tokyo, went a step further.

"When conducting the experiment, I believed it was absolutely right," he said.

"But now that many mistakes have emerged, I think it is best to withdraw the research paper once and, using correct data and correct pictures, to prove once again the paper is right. If it turns out to be wrong, we would need to make it clear why a thing like this happened."

A Riken spokesman declined to comment on the professor's call. He said it was not clear when the institution could unveil the results of the investigation.

The research appeared to offer promise that human cells could in future be reprogrammed into embryonic cell-like Stimulus-Triggered Acquisition of Pluripotency, or STAP, cells. This was thought to have the possibility of a simpler way to replace damaged cells or grow new organs for sick and injured people.

The Japanese researchers, joined by others from Brigham and Women's Hospital and Harvard Medical School in the United States, took skin and blood cells, let them multiply, then subjected them to stress "almost to the point of death", they explained, by exposing them to various events including trauma, low oxygen levels and acidic environments.

One of these "stressful" situations was simply to bathe the cells in a weak acid solution for around 30 minutes. Within days, the scientists said they had found that the cells had not only survived but had also recovered by naturally reverting into a state similar to that of an embryonic stem cell. (Reporting by Kiyoshi Takenaka; Additional reporting by Kate Kelland; Writing by William Mallard; Editing by Nick Macfie)

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