Dissecting the Role of Morphogenesis in the Origins of the First Two Cell Lineages in the Mouse Embryo

• Main Author: Stephenson, Robert
• Other Authors: Rossant, Janet
• Language: en_ca
• Published: 2011
• Subjects: mouse; epithelium; polarity; polarization; blastocyst; cell adhesion; E-cadherin; Cdx2; aPKC; ROCK; Yap; embryo; PKC zeta; Lats; Oct4; 0307; 0379; 0369
• Online Access: http://hdl.handle.net/1807/31947

Although the mechanisms underlying the divergence of the first cell types in the mouse, the trophectoderm (TE) and the inner cell mass (ICM) have received considerable attention, the upstream signals stimulating their divergence are not well understood. The work presented here examines the roles that morphogenetic factors such as cell adhesion and polarization play in the development of these cell types. I show here that in embryos completely lacking both maternal and zygotic E-cadherin, the normal epithelial morphology of outer cells is disrupted but individual cells still initiate TE and ICM-like fates. A larger proportion of cells than normal expressed TE markers like Cdx2 (a homeodomain containing transcription factor), suggesting that formation of an organized epithelium is not necessary for TE-specific gene expression. Individual cells in such embryos still generate an apical-like domain that correlates with elevated Cdx2 expression. I also show that repolarization can occur in isolated early ICMs from both wild type and Cdx2 mutant embryos, indicating that Cdx2 is not required to initiate polarity. Importantly, I demonstrate a critical role for the Rho-associated kinase ROCK in apical-basal polarization of preimplantation blastomeres. Loss of apical-basal polarization leads to a reduction of Cdx2 expression in outer blastomeres due to activation of Lats1/2 kinase and reduced nuclear Yap1. The influence of polarization upon Lats1/2 kinase is stage-dependent however, as apolar 8-cell blastomeres retain nuclear Yap1. Cell position appears to serve as an additional cue for nuclear localization of Yap and Cdx2 expression from the 8-cell stage to E3.5. Cell polarization plays an additional role in the embryo of maintaining cells in consistently outer or inner positions, thus ensuring that Cdx2 is expressed exclusively in the developing TE. The results of this work demonstrate important links between morphogenesis, cell fate and patterning in the preimplantation embryo. Both cell polarization and cell position act as critical cues to determine gene expression and to pattern this expression within the embryo.