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This review describes the three classical models (mosaic, positional, and polarization) proposed to explain blastocyst formation and summarizes the evidence concerning them. It concludes that the polarization model incorporates elements of the other two models and best explains most known information. I discuss key requirements of a molecular basis for the generation and stabilization of polarity and identify ezrin/E-cadherin, PAR proteins, and Cdx2 as plausible key molecular players. I also discuss the idea of a network process operating to build cell allocations progressively into committed differences. Finally, this review critically considers the possibility of developmental information being encoded within the oocyte and zygote. No final decision can be reached on a mechanism of action underlying any encoded information, but a cell interaction process model is preferred over one that relies solely on differential inheritance.
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Download Supplemental Tables 1–2 (PDF).
Download Supplemental Video (MOV). Video showing in the left panel a time-lapse record of the development of a 2-cell mouse embryo to an early expanding blastocyst. Note the progressive size reduction (cleavage) in blastomeres as the cells divide from 2 to 8 cells; the flattening that occurs at compaction during the 8-cell stage; and the appearance and expansion of the blastocoel at the 32-cell stage. In a parallel track, the nuclei of the embryo have been color-coded to show the disposition of the descendants from each 2-cell blastomere in the ECB. Note that division is asynchronous, and that there is coherent clonal growth. In this embryo the blue-derived descendant cells are largely mural trophoblast, with one subclone of 8-cells in the ICM and polar trophoblast (top right).