A sporadic super state

Abstract

It seems that embryonic stem cells regularly pass through a transient state during which they can generate all the cell types of an animal, including those of the placenta. See Article p.57 Mouse embryos progressively lose totipotency — the ability to develop into all embryonic and extraembryonic cell types, and to develop as a live animal — after the two-cell embryo stage. Embryonic stem (ES) cells, derived from the inner cell mass of the later blastocyst stage, are thought to be unable to contribute to extraembryonic tissue. Now, Samuel Pfaff and colleagues report a rare population in cultured ES cells that expresses transcripts previously found only in two-cell embryos and that has the potential to develop into extraembryonic tissue. Almost all ES cells transiently enter this privileged two-cell-like state, regulated in part by histone-modification enzymes. Interestingly, many of the two-cell-like-embryo transcripts are initiated by endogenous retrovirus-like elements, suggesting that placental mammals have hijacked foreign sequences for cell-fate regulation.