Actin and myosin dynamics during epithelial remodeling in avian gastrulation

Epithelial remodeling is powered by contractile forces exerted by the actomyosin cytoskeleton. In invertebrates, pulsatile contractile flows of the medio-apical actomyosin cortex have been shown to be critical in promoting junction contraction that ultimately drive cell rearrangements, apical constriction, and cell extrusion. However, how actomyosin dynamics drives epithelial remodeling in amniotes remains poorly understood. In this study, we generated transgenic quail lines reporting actin and myosin and investigate their dynamics in gastrulating embryos. We show that during this process, epithelial remodeling events are closely associated with the contraction of junctional myosin. Although we observe medio-apical contractile flows, those appear to contribute to junction contraction by promoting junctional myosin recruitment. Notably, by characterizing live and apoptotic cell extrusions and their associated actomyosin dynamics, we provide new insights into the cellular processes underlying primitive streak formation and the emergence of germ layers.