The post-inner cellmass intermediate: implications for stem cell biology and assisted reproductive technology.

background: Until recently, the temporal events that precede the generation of pluripotent embryonic stem cells (ESCs) and their equivalence with specific developmental stages in vivo was poorly understood. Our group has discovered the existence of a transient epiblast-like structure, coined the post-inner cell mass (ICM) intermediate or PICMI, that emerges before human ESC (hESCs) are established, which supports their primed nature (i.e. already showing some predispositions towards certain cell types) of pluripotency. methods: The PICMI results from the progressive epithelialization of the ICM and it expresses a mixture of early and late epiblast markers, as well as some primordial germ cell markers. The PICMI is a closer progenitor of hESCs than the ICM and it can be seen as the first proof of why all existing hESCs, until recently, display a primed state of pluripotency. results: Even though the pluripotent characteristics of ESCs differ frommouse (naıve) to human (primed), it has recently been shown in mice that a similar process of self-organization at the transition fromICM to (naıve) mouse ESCs (mESCs) transforms the amorphous ICM into a rosette of polarized epiblast cells, a mouse PICMI. The transient PICMI stage is therefore at the origin of both mESCsand hESCs. In addition, several groups have now reported the conversion from primed to the naıve (mESCs-like) hESCs, broadening the pluripotency spectrum and opening new opportunities for the use of pluripotent stem cells. conclusions: In this review, we discuss the recent discoveries of mouse and human transient states from ICM to ESCs and their relation towards the state of pluripotency in the eventual stemcells, being naïve or primed. We willnowfurther investigatehowthese intermediate and/or different pluripotent stages may impact the use of human stem cells in regenerative medicine and assisted reproductive technology. [ABSTRACT FROM AUTHOR]