Hans Sautter Yoshiki Sasai, one of Japan’s top stem-cell researchers, died this morning (5 August) in an apparent suicide. He was 52.

Sasai, who worked at the RIKEN Center for Developmental Biology (CDB) in Kobe, Japan, was famous for his ability to coax embryonic stem cells to differentiate into other cell types. In 2011, he stunned the world by mimicking an early stage in the development of the eye — a three-dimensional structure called an optical cup — in vitro, using embryonic stem cells.

But lately he had been immersed in controversy over two papers, published in Nature in January, that claimed a simple method of creating embryonic-like cells, called stimulus-triggered acquisition of pluripotency (STAP). Various problems in the papers led to a judgement of scientific misconduct for their lead author, Haruko Obokata, also of the CDB. The papers were retracted on 2 July.

Sasai, who was a co-author of both papers, was cleared of any direct involvement in the misconduct. But he has been harshly criticized for failure of oversight in helping to draft the paper. Some critics, often on the basis of unsupported conjecture, alleged deeper involvement of the CDB. An independent committee recommended on 12 June that the CDB, where Sasai was a vice-director, be dismantled. Sasai had been instrumental in launching the CDB and helped it to develop into one of the world’s premier research centres.

Just after 9 a.m., Sasai was found hanging in a stairwell of the Institute of Biomedical Research and Innovation, next to the CDB, where he also had a laboratory. He was pronounced dead just after 11 a.m., according to reports by Japanese media. A bag found at the scene contained three letters: one addressed to CDB management, one to his laboratory members and one to Obokata.

In a brief statement released this morning, RIKEN president Ryoji Noyori mourned the death of the pioneering researcher. “The world scientific community has lost an irreplaceable scientist,” he said.