The first lineage determination in mammals

This review describes in detail the morphological, cytoskeletal and gene expression events leading to the gene regulatory network bifurcation point of trophoblast and inner cell mass cells in a variety of mammalian preimplantation embryos. The interrelated processes of compaction and polarity establishment are discussed in terms of how they affect YAP/WWTR activity and the location and fate of cells. Comparisons between mouse, human, cattle, pig and rabbit embryos suggest a conserved role for YAP/WWTR signalling in trophoblast induction in eutherian animals though the mechanisms for, and timing of, YAP/WWTR activation differs among species. Downstream targets show further differences, with the trophoblast marker GATA3 being a direct target in all examined mammals, while CDX2-positive and SOX2-negative regulation varies. [Display omitted] •Onset of compaction relative to polarisation varies among mammalian embryos.•De novo polarisation in embryos transits from apical domain to apical ring stage.•Restriction of nuclear YAP correlates to apical ring stage and trophoblast specification.•Hippo/Lats signals regulate YAP activity in mammalian embryos but not exclusively.•Variations in YAP/TEAD regulation of trophoblast lineage factors CDX2 and GATA3.