Somatic cells make up most of the part in the mammalian body such as internal organ, connective tissue, blood and bones through cellular differentiation process. However, Obokata and her colleagues discovered an unexpected reprogramming phenomenon, known as stimulus-triggered acquisition of pluripotency (STAP). Genetic manipulation or transfer of nucleus were unnecessary in this reprogramming procedure, in which mouse somatic cells such as splenic CD45+ lymphocytes regained pluripotency after exposure to low pH medium.  Besides that, injected progeny of embryonic stem (ES) cells seldom appeared in the placental lineages but can be found extensively in the embryonic part of the chimaera. Surprisingly, in blastocyst injection assay, STAP cells not only contributed to embryo but presented around 60% of placental tissues and fetal membrane in the chimaera whereas those tissues did not appear in the ES cells. Hence, STAP cells included pluripotent cells (Oct41Cdx22) and trophoblast-stem-like cells (Oct42Cdx21). In addition, by treatment with adrenocorticotropic hormone (ACTH) and leukaemia inhibitory factor (LIF), mouse STAP stem cell unable to differentiate into placental lineages and only expressed little trophoblast marker genes. When exposed to the Fgf4 medium, STAP cells converted into trophoblast-stem-like cells that were unusual with ES cells or STAP stem cells. These cells known as Fgf4-induced stem cells with an enhanced trophoblastic characteristic. Fgf4-induced stem cells contributed to embryonic and placental tissues in vivo as opposed to blastocyst-derived trophoblast stem cells, meanwhile, those cells can transform into ES-like cells while cultured in the LIF+FBS-containing medium. From the analysis of the molecular features (in vitro cell conversion and chimaera formation), it can be concluded that conversion of STAP cells was distinctive and went beyond the pluripotent state that involved differentiation of trophoblast.

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