Simon, C. S.; McCarthy, A.; Woods, L.; Staneva, D.; Huang, Q.; Linneberg-Agerholm, M.; Faulkner, A.; Papathanasiou, A.; Elder, K.; Snell, P.; Christie, L.; Garcia, P.; Shaikly, V.; Taranissi, M.; Choudhary, M.; Herbert, M.; Brickman, J. M.; Niakan, K. K.

Studies in the mouse demonstrate the importance of fibroblast growth factor (FGF) and extra-cellular receptor tyrosine kinase (ERK) in specification of embryo-fated epiblast and yolk-sac-fated hypoblast cells from uncommitted inner cell mass (ICM) cells prior to implantation. Molecular mechanisms regulating specification of early lineages in human development are comparatively unclear. Here we show that exogenous FGF stimulation leads to expanded hypoblast molecular marker expression, at the expense of the epiblast. Conversely, we show that specifically inhibiting ERK activity leads to expansion of epiblast cells functionally capable of giving rise to naive human pluripotent stem cells. Single-cell transcriptomic analysis indicates that these epiblast cells downregulate FGF signalling and upregulate molecular markers associated with naive pluripotency. Our functional study demonstrates for the first time the molecular mechanisms governing ICM specification in human development, whereby segregation of the epiblast and hypoblast lineages occurs during maturation of the mammalian embryo in an ERK signal-dependent manner.