Revisiting trophectoderm-inner cell mass lineage segregation in the mammalian preimplantation embryo

In the first days of life, cells of the mammalian embryo segregate into two distinct lineages, trophectoderm and inner cell mass. Unlike nonmammalian species, mammalian development does not proceed from predetermined factors in the oocyte. Rather, asymmetries arise de novo in the early embryo incorporating cues from cell position, contractility, polarity, and cell-cell contacts. Molecular heterogeneities, including transcripts and non-coding RNAs, have now been characterized as early as the 2-cell stage. However, it's debated whether these early heterogeneities bias cells toward one fate or the other or whether lineage identity arises stochastically at the 16-cell stage. This review summarizes what is known about early blastomere asymmetries and our understanding of lineage allocation in the context of historical models. Preimplantation development is reviewed coupled with what is known about changes in morphology, contractility, and transcription factor networks. The addition of single-cell atlases of human embryos has begun to reveal key differences between human and mouse, including the timing of events and core transcription factors. Furthermore, the recent generation of blastoid models will provide valuable tools to test and understand fate determinants. Lastly, new techniques are reviewed, which may better synthesize existing knowledge with emerging data sets and reconcile models with the regulative capacity unique to the mammalian embryo. Graphical Abstract Lineage segregation requires crosstalk between cell position, polarity, and mechanical signaling and is defined by the activation of core transcriptional networks. ZGA, zygotic genome activation.