A Study of Mouse Peri-Implantation Morphogenesis
Implantation constitutes a pivotal process during mammalian embryo development during which the blastocyst adheres to and implants into the maternal endometrium. This process is accompanied by major remodelling events in the embryonic and extraembryonic lineages from tissue level to single cell scale. With successful implantation as requirement for subsequent development, implantation poses a bottleneck stage for embryo survival, still the mechanisms driving peri-implantation embryogenesis are poorly understood. During my PhD, I aimed to decipher the morphogenetic events and the mechanisms driving peri-implantation development focussing on each of the three lineages of the blastocyst, the embryonic epiblast and the two extraembryonic lineages, the trophectoderm and primitive endoderm. My first goal was to characterise the blastocyst to egg cylinder transition in mouse embryos and understand its mechanism through live imaging and inhibitor treatments (1). Then, I aimed to examine how the interplay between the polar trophectoderm and epiblast shapes the epiblast tissue comparing mouse and human embryos upon implantation (2). Following the study of epiblast tissue shape acquisition, I investigated epiblast morphogenesis on the single cell level, which sees a massive remodelling as the individual cells polarise and form a monolayered epithelium surrounding a central lumen while exiting from naïve pluripotency. I specifically concentrated on the role of Integrin signalling during these stages taking advantage of Integrin β1 deficient mouse embryonic stem cells (mESCs) (3). Subsequently, I decided to shed light into the dynamics of rosette formation in mESCs developing in 3D as model for epiblast morphogenesis through time-lapse imaging (4). Following these studies, I then turned to the primitive endoderm to assess the primitive to visceral endoderm maturation that takes place upon implantation (5). Through single cell sequencing analysis and assessment of hepatocyte nuclear factor 1β knock-out embryos, that fail to mature the primitive endoderm, I could uncover a novel, direct signalling interaction between epiblast and visceral endoderm required for epiblast survival following egg cylinder formation. Taken together, this Dissertation focusses on various aspects of peri-implantation development ranging from comparative tissue shape and - force analyses to a live-imaging based analysis of single cell dynamics upon the exit from naïve pluripotency.