The epiblast is the first cell type that forms apical-basal polarity de novo as the mouse embryo implants into the maternal uterus, while the extraembryonic neighbours of the epiblast - trophectoderm and primitive endoderm - retain their pre-established polarity beyond implantation [1]; however, it is still unclear how the epiblast establishes apical-basal polarity de novo. Here, we focused on Rap1 signaling pathway, which is activated during the transition of the epiblast from the naïve to primed state of pluripotency during implantation [2]. Through the preestablished in vitro three-dimensional culture system [3], genetic knockouts and proximity-biotinylation analyses, we found that Rap1 integrates multiple signals that contribute to de novo formation of apical-basal polarity. Importantly, formation of apical-basal polarity in the epiblast is essential for its correct patterning and proper communication with the extraembryonic lineages. Altogether, these results not only dissect molecular details of de novo apical-basal polarity formation, but also have broader implications for epithelial polarity and development.