Geometrical confinement guides Brachyury self-patterning in embryonic stem cells

During embryogenesis, signaling molecules initiate cell diversification, sometimes via stochastic processes, other times via the formation of long range gradients of activity which pattern entire fields of cells. Such mechanisms are not insensitive to noise (Lander, 2011), yet embryogenesis is a remarkably robust process suggesting that multiple layers of regulations secure patterning during development. In the present study, we present a proof of concept according to which an asymmetric pattern of gene expression obtained from a spatially disorganised population of cells can be guided by the geometry of the environment in a reproducible and robust manner. We used ESC as a model system whithin which multiple developmental cell states coexist (MacArthur and Lemischka, 2013; Smith, 2017; Torres-Padilla and Chambers, 2014). We first present evidence that a reciprocal regulation of genes involved in the establishment of antero-posterior polarity during peri-implantation stages of mouse development is spontaneously occuring within ESC. We then show that a population of cells with primitive streak characteristics localise in regions of high curvature and low cell density. Finally, we show that this patterning did not depend on self-organised gradients of morphogen activity but instead could be attributed to positional rearrangements. Our findings unveil a novel role for tissue geometry in guiding the self-patterning of primitive streak cells and provide a framework to further refine our understanding of symmetry breaking events occuring in ESC aggregates. Finally, this work demonstrates that the self-patterning of a specific population of ESC, Brachyury positive cells in this case, can be directed by providing engineered external geometrical cues.