The two first cell fate decisions taken in the mammalian embryo generate three distinct cell lineages: one embryonic, the epiblast, and two extraembryonic, the trophoblast and primitive endoderm. miRNAs are essential for early development, but it is not known if they are utilized in the same way in these three lineages. We find that in the pluripotent epiblast they inhibit apoptosis by blocking the expression of the proapoptotic protein Bcl2l11 (Bim) but play little role in the initiation of gastrulation. In contrast, in the trophectoderm, miRNAs maintain the trophoblast stem cell compartment by directly inhibiting expression of Cdkn1a (p21) and Cdkn1c (p57), and in the primitive endoderm, they prevent differentiation by maintaining ERK1/2 phosphorylation through blocking the expression of Mapk inhibitors. Therefore, we show that there are fundamental differences in how stem cells maintain their developmental potential in embryonic and extraembryonic tissues through miRNAs.
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