The shapes of elongating gastruloids are consistent with convergent extension driven by a combination of active cell crawling and differential adhesion

Gastruloids have emerged as highly useful in vitro models of mammalian gastrulation. One of the most striking features of 3D gastruloids is their elongation, which mimics the extension of the embryonic anterior-posterior axis. Although axis extension is crucial for development, the underlying mechanism has not been fully elucidated in mammalian species. Gastruloids provide an opportunity to study this morphogenic process in vitro. Here, we measure and quantify the shapes of elongating gastruloids and show, by Cellular Potts model simulations based on a novel, optimized algorithm, that convergent extension, driven by a combination of active cell crawling and differential adhesion can explain the observed shapes. We reveal that differential adhesion alone is insufficient and also directly observe hallmarks of convergent extension by time-lapse imaging of gastruloids. Finally, we show that gastruloid elongation can be abrogated by inhibition of the Rho kinase pathway, which is involved in convergent extension in vivo. All in all, our study demonstrates, how gastruloids can be used to elucidate morphogenic processes in embryonic development.Author summary: During embryonic development, a mammalian embryo develops from a single cell to a complete organism with a complex body plan. To ensure that tissues and organs are formed exactly in the right places, the embryo goes through a highly orchestrated series of events that establish the fundamental axes of the body, such as the anterior-posterior axis, which runs from the head (anterior) to the tail (posterior). Extensive elongation along this axis is crucial for proper development, but the underlying mechanisms are not completely understood, partially because experimentation with embryos is cumbersome. In this study, we used aggregates of mouse embryonic stem cells, known as gastruloids, which mimic elements of embryonic development, most importantly elongation, in a dish. We developed a computational approach to simulate aggregate shapes and used measured aggregate shapes to decide between multiple hypotheses for the mechanisms driving elongation. We found that the combination of two separate mechanisms can explain the measured shapes: 1. active crawling of cells that leads to a narrowing in one direction and extension in the perpendicular direction (like when squeezing a stress ball) and 2. differences in the stickiness (adhesion) between different types of cells.