The mouse anterior-posterior (A-P) axis polarization is preceded by formation of the distal visceral endoderm (DVE). However, the mechanism of the emergence of DVE cells is not well understood. Here, we show by in vitro culturing of embryos immediately after implantation in micro-fabricated cavities (narrow; 90 micro-meter, wide; 180 miro-meter in diameter) that the external mechanical cues exerted on the embryo, i.e. cultured in the narrow cavity, are crucial for DVE formation as well as elongated egg cylinder shape. This implies that these developmental events immediately after implantation are not simply embryo-autonomous processes but require extrinsic mechanical factors. Further whole genome-wide gene expression profiles with DNA microarray revealed that no significant difference of transcripts were evident with or without mechanical cues except DVE-related markers. Thus, we propose that external mechanical cues rather than not specific molecular pathways can trigger the establishment of the A-P axis polarization, which is one of the fundamental proccesses of mammalian embryogenesis.