He developed methods to guide human embryonic stem cells (hESCs) into forming brain cortex, eyes (optic cups), and other organs in tissue culture.

Sasai worked at the Riken Center for Developmental Biology (CDB) in Kobe, and was Director of the Laboratory for Organogenesis and Neurogenesis.

In 1993 Sasai was awarded a PhD from the Kyoto University School of Medicine, and served a residency at Kobe Municipal General Hospital.

[12] In 2014 Sasai was a co-author on two papers published in Nature, shortly thereafter retracted, that described stimulus-triggered acquisition of pluripotency or "STAP" cells.

[16][17][18] In response to the Riken investigation, Sasai described himself as "overwhelmed with shame",[19] and following a month of hospitalization was found dead from an apparent suicide by hanging on August 5, 2014.