Mechanisms of Asymmetric Cell Division in the Early Mammalian Embryo

Abstract

Preimplantation development features the segregation of trophectoderm (TE) and inner cell mass (ICM) progenitors in the mammalian embryo. Differential activation of TE and ICM cell-fate-specific gene expression programs have been linked to the presence or absence of apicobasal polarity, respectively, in blastomeres of the 16-cell-stage embryo and beyond. Polarity differences are first set up at the 8-16 cell transition by asymmetric divisions of polarized 8-cell-stage blastomeres. This symmetry-breaking event was thought to proceed via the asymmetric inheritance of the apical domain (AD), though recent findings suggest the AD may disassemble prior to division. These observations have therefore raised many questions about the mechanistic basis for asymmetric polarity inheritance. Here in this thesis, I test the hypothesis that asymmetric inheritance of the basolateral domain ensures the asymmetric polarity of daughter cells. First, via the live imaging of mouse embryos, I confirmed that the AD is disassembled during the 8-16 cell transition. Then, by developing and validating reagents to visualize the dynamics of basolateral domain proteins, the localization and inheritance patterns of three conserved basolateral factors (SCRIB, LLGL1, and E-cadherin) were assayed. I discovered that SCRIB was retained at the membrane during mitosis, and that its overexpression inhibited establishment of apical polarity. This latter finding points to possible antagonism between regulators of apical and basolateral polarity. Furthermore, I revealed that E-cadherin is retained at basolateral regions during mitosis, and that it undergoes asymmetric inheritance at the 8-16 cell transition. Finally, I showed that AMOT, a key factor that links polarity to cell-fate-specific gene expression through regulation of the Hippo pathway, is symmetrically inherited at the 8-16 cell transition, despite its apically-polarized localization at the 8-cell stage. Together, this work outlines a novel model whereby asymmetric inheritance of basolateral domain components may differentially regulate the establishment of polarity in daughter cells at the 16-cell stage. Additionally, I propose that disassembly of the AD may be required for symmetric AMOT inheritance, which in turn may be necessary for timely activation of cell-fate-specific Hippo signaling. In summary, this work provides novel insights into the first symmetry-breaking event during mammalian development.