Preimplantation mouse embryo development involves temporal–spatial specification and segregation of three blastocyst cell lineages: trophectoderm, primitive endoderm and epiblast. Spatial separation of the outer-trophectoderm lineage from the two other inner-cell-mass (ICM) lineages starts with the 8- to 16-cell transition and concludes at the 32-cell stages. Accordingly, the ICM is derived from primary and secondary contributed cells; with debated relative EPI versus PrE potencies. We report generation of primary but not secondary ICM populations is highly dependent on temporal activation of mammalian target of Rapamycin (mTOR) during 8-cell stage M-phase entry, mediated via regulation of the 7-methylguanosine-cap (m 7 G-cap)-binding initiation complex (EIF4F) and linked to translation of mRNAs containing 5′ UTR terminal oligopyrimidine (TOP-) sequence motifs, as knockdown of identified TOP-like motif transcripts impairs generation of primary ICM founders. However, mTOR inhibition-induced ICM cell number deficits in early blastocysts can be compensated by the late blastocyst stage, after inhibitor withdrawal; compensation likely initiated at the 32-cell stage when supernumerary outer cells exhibit molecular characteristics of inner cells. These data identify a novel mechanism specifically governing initial spatial segregation of mouse embryo blastomeres, that is distinct from those directing subsequent inner cell formation, contributing to germane segregation of late blastocyst lineages.

Preimplantation stages of mouse embryo development involve temporal and spatial specification and segregation of three late blastocyst cell lineages; trophectoderm (TE), primitive endoderm (PrE) and epiblast (EPI). Spatial separation of the outer TE lineage from the two inner cell mass (ICM) lineages (PrE and EPI) starts with the 8- to 16-cell transition and concludes following transit through the 16- to 32-cell stages. This results in an early blastocyst ICM derived from descendants of primary founding inner cells and a secondarily contributed population, of which subsequent relative EPI versus PrE potencies are subject to debate. Here, we report generation of primary but not the secondary ICM populations is highly dependent on temporally discreet activation of the mammalian target of Rapamycin (mTOR, specifically mTORC1) during M-phase entry at the 8-cell stage. This role is mediated via regulation of the 7-methylguanosine- (7mG) cap binding initiation complex (EIF4F), linked to translation of a subset of key mRNAs containing 5[prime] UTR terminal oligopyrimidine (TOP-) or TOP-like sequence motifs; as knockdown of identified TOP-like motif containing transcripts also impairs generation of 16-cell stage primary ICM founders. However, mTOR inhibition induced ICM cell number deficits at the early blastocyst stage can be compensated by the late blastocyst stage, in the absence of inhibition. This compensation is likely initiated at the 32-cell stage when supernumerary outer cells in mTOR-inhibited embryos exhibit molecular characteristics of inner cells. Collectively, the data identify a novel mechanism specifically governing initial spatial segregation of blastomeres in the mouse embryo, that is distinct from those directing subsequent inner cell formation and contributes to germane segregation of late blastocyst lineages.