Tissue Tectonics During Gastrulation In Bilaminar Disc Embryos

The mammalian body plan is established during gastrulation, where the basic cell layers from which all adult tissues derive are laid down for the first time. In humans, this process starts immediately after implantation, during the third week of development, and coincides with the period when most embryonic losses (~40%) occur during pregnancy. In this very early phase of development, the embryo establishes a spatial pattern of cellular organization controlled by genes and is, in part, influenced by the surrounding tissues contributing to the placenta. At this stage, the symmetry of the embryo breaks: the embryo is no longer a round disc, but instead cells on one end express different genes to those in the other end (in other words, the anterior-posterior axis emerges). How this break in symmetry is determined in non-rodent mammals (i.e. humans) is not yet clear. Our knowledge of gastrulation in humans is very limited because human embryos cannot be directly studied during this period. However, human and pig embryos (and most other mammals) start gastrulation from a flat disc epiblast. The flat disc is made of a bilaminar cell layer of epiblast and hypoblast cells and is surrounded by trophectoderm cells. Biochemical signals (known as morphogens) that orchestrate the onset of gastrulation in bilaminar disc embryos are not well known. Furthermore, the changes in the biophysical characteristics of epiblast cells in terms of size, shape, and movement during gastrulation in flat disc epiblasts are also unknown. In this proposal, we will study these processes using pig embryos as model system, due to their resemblance to human embryos at this stage. Furthermore, we will use human and pig stem cells model of gastrulation (gastruloids) to study the shared mechanisms of symmetry breaking and axial patterning across species. We will use a combination of in vivo, in vitro, and computational models to integrate biochemical, cell geometry and shape information across different developmental stages to establish fundamental principles of mammalian embryogenesis. The multi-scale (from individual cells to complex tissues), multispecies and multidisciplinary approaches selected for this project will provide a holistic new understanding of one of the key developmental processes of animals. The new knowledge of the mechanisms of gastrulation in bilaminar disc embryos gained in this project will provide a blueprint of the key principles governing the formation of a mammalian embryo. This work will impact our understanding of the development diversity and evolution of mammalian species; will have an impact in medicine by improving our understanding of the causes of early embryonic loss; and will enable the development of new methodologies for the generation of human organs in animals for xenotransplantation.

Regulative development is a feature of mammalian embryogenesis. The mechanisms that determine the formation of the animal body plan are determined by interactions at molecular, cellular and tissue levels within the conceptus. In this project we will investigate how interactions between embryonic (epiblast) and extraembryonic tissues define the onset of gastrulation in bilaminar disc embryos. We hypothesize that symmetry breaking in the mammalian flat disc epiblast is governed by morphogenetic processes influenced by boundary conditions established by extraembryonic tissues. In this grant we will build for the first time an atlas compiling spatial transcriptomics with the spatial distribution of cell sizes, shapes, and movements during gastrulation stages in the bilateral disc embryo. This novel spatiotemporal information will be crucial to disclose whether symmetry breaking leading to Epithelial to Mesenchymal Transition (EMT) is preceded by tissue fluidization in what is known as Unjamming Transition (UJT) and study how the process is controlled by the epiblast itself and its interplay with extraembryonic tissues. The goal of our project is to perform multiscale analysis of developing bilaminar disc embryos and integrate spatial gene expression with cell geometry and velocity during the onset of gastrulation to create a blueprint of the conditions that govern the establishment of the mammalian body plan. We will use a holistic approach combining spatial gene expression, computational modelling, and functional investigations in vitro (in 3D models of gastrulation) and in vivo in embryos to establish the common mechanisms of early gastrulation across species developing embryonic discs (pigs, humans, and cattle).