The discovery of suspicious images in two papers published in January in the British scientific journal Nature has cast doubt on the results of ground-breaking stem cell research led by scientist Haruko Obokata.

Riken, the research institute based in Kobe that hosted the research, has opened an investigation into whether the photos, which depict the placental development of a mouse, were intentionally altered, Nature said Monday.

The research, which explored the use of physical stress to induce adult cells in mice to return to an embryonic state, appeared to provide a simple solution to the complicated problem of producing stem cells for use in medical treatments.

However, some images included in the two papers seem to have been altered, while others bear an unusually strong resemblance to each other, Nature reported.

"The matter has been brought to Nature's attention and we are investigating," a spokesperson for Nature Publishing Group said.

Teruhiko Wakayama, a professor at Yamanashi University and a co-author of the papers, told Nature he was able to reproduce the experiment's results independently and "the results are absolutely true."

Critics of the study also found problems in a 2011 paper Obokata published in another scientific journal. Charles Vacanti, a doctor at Harvard Medical School in Boston who worked with Obokata on the research, said he had already requested that the journal correct the mistake.

He told Nature the error was "honest" and "did not affect any of the data, the conclusions or any other component of the paper."