Total cell number influences the timing of Hippo/YAP activation during preimplantation development in mouse...

Total cell number may determine the timing of Hippo/YAP activity during first lineage specification. We combined mouse tetraploid embryos (reduced cell numbers) with a 2 × 4n aggregation approach (increasing cell numbers) to test whether YAP nuclear entry in inner cell mass (ICM) cells is temporally shifted. Tetraploids reached the blastocyst stage with approximately half the cells of diploids and exhibited delayed nuclear YAP in ICM at mid-blastocyst, despite overall ICM/trophectoderm patterning and pluripotency markers (Oct4, Nanog, Sox2) being comparable. Increasing total cell number by aggregating two tetraploids restored YAP timing to that of diploids. By the late-blastocyst stage, most tetraploid ICM cells displayed nuclear YAP, indicating that this delay is transient. These findings support a model in which total cell number -not ploidy per se-is associated with the timing of Hippo/YAP activation in the preimplantation embryo.