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, ultimately driving cell rearrangements, apical constriction, and cell extrusion. However, how actomyosin dynamics drive epithelial remodeling in vertebrates, especially amniotes, remains poorly understood. In this study, we generated transgenic quail lines reporting actin and myosin and investigated 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, these are irregular, produce only transient or subtle cell and junctional deformations, and are not consistently associated with junctional or cellular contraction during convergent extension of the primitive streak or during mesendodermal cell ingression. 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.