Bilateral cellular flows initiate left-right asymmetry during early gastrulation in amniotes.

Animals with bilateral symmetry have a body plan with an external left-right (L-R) symmetry but with an asymmetrical internal organization. The initiation of this L-R asymmetry, and the role of physical and biological processes underlying this are not well understood. Cells first start flowing inside a developing embryo during the process of early Gastrulation. This process is crucial in patterning animal body plans. Out of several model systems to study gastrulation, the chicken embryo is a good model system due to its transparency, easy availability, and close similarity to human gastrulation. The early stage of gastrulation is accompanied by a bilateral cellular flow known as “Polonaise Movements” forming two counter-rotating vortices merging at the midline.  Previous studies indicated that this initial phase of cellular flows seems to have L-R symmetry accompanied by the bilateral movement of cells. The first sign of L-R asymmetry was observed at a later stage when genetic regulatory programs took over and at the beginning of organogenesis. In this work, we investigate these early-stage gastrulation cell flows before the activation of laterality genes and quantify the cellular speeds and vorticity using the Particle Image Velocimetry (PIV) technique. We carry out a detailed visualization and quantification of time-varying speeds and vorticity for the initial 10 hours after the cell flows begin in the embryo. Our biophysical analysis shows that the bilateral counter-rotating flows show L-R asymmetry with a right dominance. In addition, the cellular flows also begin asymmetrically with a left dominance and shift towards a right dominance at the end.  This appearance of L-R asymmetry occurs before the expression of the L-R laterality genes and is purely due to the emergent mechanics resulting from the bilateral cell flows. When the cell division is inhibited, the primitive streak does not form, and we observe that the flow asymmetry is further amplified with a single vortex that is right dominant at the end.  In summary, we present a detailed analysis of flow patterns in the early-stage development in chick embryos. Our results challenge the current paradigm assuming that L-R symmetry holds in bilateral cell flows during early gastrulation before the activation of laterality genes.