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After the blastocyst stage, once an embryo implanted in endometrium (in case of rodent), the inner cell mass (ICM) of a fertilized embryo segregates into two layers: hypoblast and epiblast. The epiblast cells are the functional progenetors of soma and germ cells which later differentiate into 3 layers, definitive endoderm, mesoderm and ectoderm. Stem cells derived from epiblast are pluripotent. These cells are called epiblast-derived stem cells (EpiSC) and have several different cellular and molecular characteristics with Embryonic Stem Cells (ESC) (De-Miguel et al., 2009). Pluripotency in EpiSC is essentially different from that of embryonic stem cells. The pluripotency of EpiSC is primed pluripotency: primed to differentiate into specific cell linages. Naïve pluripotent stem cells (e.g. ESC) and primed pluripotent stem cells (e.g. EpiSC) not only sustain the ability to self-renew but also maintain the capacity to differentiate (Nichols and Smith, 2009). Since the cell status is primed to differentiate in EpiSC, however, one copy of the X chromosome in XX cells (female cells) in EpiSC is silenced (XaXi). EpiSC is unable to colonize and is not available to be used to produce chimeras. Conversely, XX cells in ESC are both active and can produce chimera when inserted into a blastocyst. Both ESC and EpiSC induce teratoma when injected in the test animals (skid mice) which proves pluripotency. EpiSC display several distinctive characteristics distinct from ESC (table 1). The celluar status of human ESC (hESC) is similar to primed state mouse stem cells rather than Naïve state (Hanna et al., 2010). Table 1. Comparison of Naïve and Primed pluripotent states (Nichols and Smith, 2009) Differentiating Naïve pluripotent Stem cells into primed pluripotent stem cells (e.g. adding activin and Fgf in the culture medium) can be accomplished but reprogramming of Primed cells into Naïve cells is more difficult. Several approaches to reprogramming EpiSC to achieve Naïve pluripotency have been applied. One of those methods is transfecting the primed pluripotent stem cell with a reprogramming factor, Klf4 (Guo et al., 2009). ==References== 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Epiblast-derived stem cells」の詳細全文を読む スポンサード リンク
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