Cystic embryoid bodies with visceral yolk-sac-like structure (cystic EB-Vs) are used as a model for the study of early extraembryonic tissue formation containing visceral endoderm-like derivatives. In this study, we optimized the cell density of embryonic stem (ES) cells for developing cystic EB-Vs in a low-adherence 96-well plate. When ES cells were seeded at a density of 4000 cells/well, the cystic EB-Vs were most efficiently developed from ES cells via forming multicellular spherical aggregates called embryoid bodies (EBs). The suspension culture in the low-adherence plate was preferable for developing EBs into cystic EB-Vs rather than the attachment culture in the plate coated with 0.1% gelatin. The seeding cell density of 4000 cells/well was always superior to 1000 cells/well in the efficiency of cystic EB-V development. Because the high-cell density culture generally raises the limitation of oxygen and nutrient supplies, we investigated the effects of low-oxygen and low-nutrient conditions on the development of cystic EB-Vs. It was found that low oxygen tension was not a factor for promoting the development of cystic EB-Vs. It was suggested that a low-nutrient medium is preferred for developing cystic EB-Vs rather than a sufficient-nutrient medium. In conclusion, the suspension culture in the low-adherence 96-well plate seeded with 4000 ES cells/well was optimum for developing cystic EB-Vs. The low-nutrient condition may be one of the factors for promoting the development of cystic EB-Vs. (c) 2008, The Society for Biotechnology, Japan. All rights reserved.