Fig. 1

A schematic diagram depicting the relationship between in vitro and in vivo pluripotent state progression. The diagram depicts the location of pluripotent cells (red) within the developing mouse embryo from embryonic day (E) 3.5 to E7.5. Prior to E3.5, cells of the pre-implantation embryo are ‘totipotent’, capable of generating both embryonic and extraembryonic cell types. At E3.5, cells in the ICM of the blastocyst are a heterogeneous mix of epiblast (Epi) and primitive endoderm (PrE) precursors. Epi cells are pluripotent and will generate all cells of the embryo-proper, including the germ cells, proper while PrE cells will generate extraembryonic cell types such as the yolk sac. The outer trophectoderm (TE) cells will generate extraembryonic cell types including the fetal portion of the placenta. One day later, at E4.5, the Epi and PrE cells are specified and become physically segregated into two distinct layers and the embryo implants into the uterus. At early post-implantation stages (E5.5) the Epi is in an entirely undifferentiated pluripotent state. At E6.5, cells within the proximal posterior of the embryo are exposed to differentiation-promoting signals from both embryonic and extraembryonic lineages that stimulate the onset of gastrulation and differentiation of cells as they enter the primitive streak (PS) region (yellow). By E7.5, the PS has extended distally and PS derivatives including extraembryonic mesoderm, embryonic mesoderm and definitive endoderm are being generated. The anterior Epi has also started to differentiate into anterior neurectoderm (NE). Pluripotency is lost at approximately E8.0. Pluripotent stem cell lines can be maintained in vitro and appear to resemble various embryonic stages of pluripotency. While embryonic stem cells (ESCs) can be derived from embryos from E3.5 and E7.5 and epiblast stem cells (EpiSCs) can be derived from embryos between E3.5 and E8.0, ESCs resemble the naïve pluripotent state (blue) existing in the early pre-implantation embryo while EpiSCs resemble primed pluripotent cells (green) of the late post-implantation Epi during gastrulation. Intermediate or formative states of pluripotency (orange), between the naïve and primed states likely exist in the embryo. While a number of potential states have been isolated, Epi-like cells (EpiLCs), generated from ESCs in vitro, have been most clearly defined in relation to the embryo and are more transcriptionally similar to E5.75 Epi than EpiSCs are. Representative brightfield images of ESC, EpiLC and EpiSC cultures are shown. Extraembryonic lineages are depicted in gray; dark gray lineages are TE-derived and light gray lineages PrE-derived. A = anterior, P = posterior, Pr = proximal, D = distal