Nodal/activin signaling is indispensable for embryonic development. We examined what activin does to the embryoid bodies (EBs) produced from mouse embryonic stem cells (mESCs) expressing an epiblast marker. The EBs were produced by culturing mESCs by the hanging drop method for 24 hours. The resulting EBs were transferred onto gelatin-coated dishes and allowed to further differentiate. The 24-hour EBs showed a stronger expression of fibroblast growth factor (FGF)5 and Brachyury (specific to the epiblast) in comparison with mESCs. Treating the transferred EBs with activin A maintained transcript levels of FGF5 and Oct4, while inhibiting definitive endoderm differentiation. The activin A treatment reversed the endoderm differentiation induced by retinoic acid (RA), while the inhibition of nodal/activin signaling promoted RA-induced endoderm differentiation. Inhibition of nodal/activin signaling in EBs, including epiblast-like cells, promotes differentiation into the endoderm, facilitating the transition from the pluripotent state to specification of the endoderm.
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