Jun Wu, from the Izpisua Belmonte lab at the Salk Institute of Biological Studies, also spoke briefly of recent difficulties in generating viable chimeras following injection of naive human iPSCs tagged with a GFP reporter (hiPSC-GFP). Fortuitously, these studies led to media formulations that allowed his group to expand and propagate mouse epiblast stem cell cells (mEpiSCs) from embryonic day 5.75 (E5.75) embryos. When cultured with both FGF2 and WNT inhibition (IWR1), in the absence of serum, these mouse epiblast stem cells showed high cloning efficiency, comparable to that observed in mESCs. Careful characterization revealed a surprising regional specification of these cells (now termed rsEpiSCs); upon transplantation into mouse embryos, although they could contribute to all three germ layers, they could only incorporate into the posterior of the embryo, but not the distal or anterior regions (Wu et al., 2015). Similar culture conditions yielded human rsPSCs, which also contributed to all three germ layers, exclusively in the posterior region, of chimeric mouse embryos (Wu et al., 2015). This is in sharp contrast to conventional human PSCs, which failed to incorporate in E7.5 mouse epiblast; global genome-wide expression analysis confirmed that these stem cell states have unique molecular signatures.
