Wnt signalling sustains an EpiSCs subpopulation similar to primitive streak with increased mesendodermal potency

During gastrulation, epiblast cells are pluripotent and their fate is thought to be constrained principally by their position. Cell fate is progressively restricted by localised signalling cues from areas including the primitive streak (PS). However, it is unknown whether this restriction accompanies, at the single cell level, a reduction in potency. Investigation of these early transition events in vitro is possible via the use of Epiblast Stem Cells (EpiSCs), self-renewing pluripotent cell lines equivalent to the postimplantation epiblast. Strikingly, EpiSCs express various early lineage-specific markers in self-renewing conditions. However, it is unknown whether cells that express these markers are pluripotent, spontaneously differentiated, or biased towards specific lineages. Here we show that EpiSC are inherently heterogeneous and contain two major and mutually exclusive subpopulations with PS and neural characteristics respectively. Using differentiation assays and embryo grafting we demonstrate that PS-like EpiSCs are biased towards mesoderm and endoderm differentiation but they still retain their pluripotent character. The acquisition of a PS character by undifferentiated EpiSC is mediated by paracrine Wnt signalling. Elevation of Wnt activity promotes further restriction into PS-associated cell fates which occurs via the generation of distinct clonal mesendodermal and neuromesodermal precursors. Collectively, our data suggest that primed pluripotency encompasses a range of reversible lineage-biased states reflecting the birth of pioneer lineage precursors from a pool of uncommitted EpiSCs similar to the earliest cell fate restriction events taking place in the gastrula-stage epiblast. Total RNA obtained (3 biological replicates) from flow sorted dsRed2+, dsRed2-, and +CHIRON treated for 48h cells was isolated and labelled/amplified using the Illumina® TotalPrep™ RNA Amplification Kit (Life Technologies)