Apical Domain Behavior in Isolated Polar Cells of the Preimplantation Mouse Embryo

Abstract

Symmetry breaking events in development are key to establishing new cell fates. In mammalian embryos, the first cell fate decision, taking place during preimplantation development, gives rise to trophectoderm (TE) and inner cell mass (ICM) cells. Without TE formation, an embryo cannot implant into the uterus, leading to its arrest. The formation and inheritance of an apical domain plays a key role in the first decision and is necessary for TE formation. The inheritance of the apical domain has been previously explained using the polarity model, in which the apical domain can be inherited asymmetrically by daughter cells during division. Interestingly, recent work observed disassembly of the apical domain prior to cytokinesis, followed by de novo formation on exposed cell surfaces of daughter cells. Disassembly and reformation, however, is at odds with asymmetric apical domain formation taking place even when cells are allowed to divide in isolation (i.e. without any cell-cell contact). In this project, I used live imaging of isolated polar blastomeres with Ezrin-neonGreen-labeled-apical domains to understand apical domain behavior independent of cell-cell adhesions in both non-dividing and dividing cells. I found that isolated, non-dividing polar cells at the 8-cell, 16-cell, and 32-cell stages were all able to autonomously maintain an apical domain, meaning that polarity is a cell-intrinsic property at these stages of development. The apical domain exhibited spreading behaviors at the 8, but not 16 or 32 cell interphases. Furthermore, during the 8-16 and 16-32 cell divisions, the apical domain exhibited spreading behaviors towards the cytokinetic furrow, which coincided with re-localization of actin (labeled with Lifeact-mCherry) form the apical domain to the cytokinetic furrow. While this actin re-localization-induced spreading occurred at both the 8-16 and 16-32 divisions, it took place only briefly before the division and resulted in limited spreading of the apical domain. Consequently, most 16-32 divisions resulted in asymmetric divisions with only one daughter inheriting the apical domain. In contrast, 8-16 divisions were either asymmetric or symmetric, with symmetric divisions resulting from the apical domain spreading extensively during the 8-cell stage interphase. In summary, these findings indicate a relationship between the actin-cytoskeleton and apical domain behavior during cell divisions and highlight stage specific behaviors of the apical domain that influence its inheritance in the early preimplantation embryo.