Expandable pluripotent cell lines from STAP cells STAP cells have a limited self-renewal capacity under the conditions used for establishment (Fig. 2g and Extended Data Figs 2e and 5a). However, in the context of the embryonic environment, a small fragment of a STAP cell cluster could grow even into awhole embryo (Fig. 4f). With this in mind, we next examined whether STAP cells have the potential to generate expandable pluripotent cell lines in vitro under certain conditions.STAP cells could not be efficiently maintained for additional passages in conventional LIF1FBS-containing medium or 2i medium20 (most STAP cells died in 2i medium within 7 days; Extended Data Fig. 8a). Notably, an adrenocorticotropic hormone (ACTH)1LIFcontaining medium (hereafter called ACTH medium) known to facilitate clonal expansion of ES cells36 supported outgrowth of STAP cell colonies. When cultured in this medium on a MEF feeder or gelatin, a portion of STAP cell clusters started to grow (Fig. 5a, bottom; such outgrowth was typically found in 10–20% of wells in single cluster culture using 96-well plates and in .75% when 12 clusters were plated per well). These growing colonies looked similar to those of mouse ES cells and expressed a high level of Oct4-GFP.
After culturing in ACTH medium for 7 days, this growing population of cells, unlike parental STAP cells, could be passaged as single cells (Fig. 5a, bottom, and Fig. 5b), grow in 2i medium (Extended Data Fig. 8a) and expand exponentially, up to at least 120 days of culture(Fig. 5c; no substantial chromosomal abnormality was seen; Extended Data Fig. 8b, c). Hereafter, we refer to the proliferative cells derived from STAP cells as STAP stem cells.