Professor Sasai was a stem cell biologist. He developed methods to guide human embryonic stem cells (hESCs) into forming brain cortex, eyes, and other organs in tissue culture. Yoshiki was one of the founding leaders of the RIKEN Center for Developmental Biology (CDB) in Kobe, a premier research institute for biology in Japan and Director of the Laboratory for Organogenesis and Neurogenesis at the research institute RIKEN. Sasai was best known for developing new methods to grow stem cells into organ-like structures, and for his studies which showed that diffusion of signaling factors (including Noggin, Chordin, Follistatin and their homologs) from organizing centers direct dorsal ectoderm, maintained a neural fate for these cells, and compete to initiate the embryonic nervous system. Using Xenopus laevis, as a model in molecular embryological studies, his group worked to clarify the structure and extent of the signaling networks involved in setting up the dorsal-ventral axis and determining neural fate in the ectoderm, which in turn determines the growth and differentiation of the brain in its earliest stages of development.