Lineage mapping the pre-implantation mouse embryo by two-photon microscopy, new insights into the segregation of cell fates.
In the mouse embryo, asymmetric divisions during the 8-16 cell division generate two cell types, polar and apolar cells, that are allocated to outer and inner positions, respectively. This outer/inner configuration is the first sign of the formation of the first two cell lineages: trophectoderm (TE) and inner cell mass (ICM). Outer polar cells become TE and give rise to the placenta, whereas inner apolar cells become ICM and give rise to the embryo proper and yolk sac. Here, we analyze the frequency of asymmetric divisions during the 8-16 cell division and assess the relationships between cell polarity, cell and nuclear position, and Hippo signaling activation, the pathway that initiates lineage-specific gene expression in 16-cell embryos. Although the frequencyof asymmetric divisions varied in eachembryo, we found that more than six blastomeres divided asymmetrically in most embryos. Interestingly,manyapolar cells in 16-cell embryoswere located at outer positions, whereas only one or two apolar cells were located at inner positions. Live imaging analysis showed that outer apolar cells were eventually internalized by surrounding polar cells. Using isolated 8-cell blastomeres, we carefully analyzed the internalization process of apolar cells and found indications of higher cortical tension in apolar cells than in polar cells. Last, we found that apolar cells activate Hippo signaling prior to taking inner positions. Our results suggest that polarandapolarcells have intrinsic differences that establish outer/inner configuration and differentially regulate Hippo signaling to activate lineage-specific gene expression programs.