Early lineage segregation in mouse development results in two, either CDX2- or OCT4/NANOG-positive, cell populations. CDX2-positive cells form the trophectoderm (TE), OCT4/NANOG-positive cells the inner cell mass (ICM). In a second lineage decision ICM cells segregate into Epiblast (EPI) and primitive endoderm (PE). EPI and PE formation depend on the activity of the transcription factors Nanog and Gata4/6. A role for Nanog, a crucial pluripotency factor, in preventing PE differentiation has been proposed, as outgrowths of mutant ICMs result in PE, but not EPI derivatives. We established Nanog-mutant mouse lines and analyzed EPI and PE formation in vivo. Surprisingly, Gata4 expression in mutant ICM cells is absent or strongly decreased, thus loss of Nanog does not result in precocious endoderm differentiation. However, Nanog-deficient embryos retain the capacity to form PE in chimeric embryos and, in contrast to recent reports, in blastocyst outgrowths. Based on our findings we propose a non-cell autonomous requirement of Nanog for proper PE formation in addition to its essential role in EPI determination
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