Abstract

The molecular and cellular mechanisms by which tissues take shape are fundamental to many biological processes. While the genetic pathways controlling tissue morphogenesis have been intensively analyzed, its mechanical principles are poorly understood. An excellent assay system to study the biophysical basis of tissue morphogenesis is zebrafish gastrulation, where within a few hours major morphogenetic changes result in the formation of germ layers and the establishment of the body axis. To obtain insight into the biophysical basis of tissue morphogenesis during gastrulation, we study enveloping cell layer (EVL) epiboly, the spreading of a squamous epithelium over the yolk cell. A circumferential actomyosin band within the yolk syncytial layer (YSL) has been proposed to act as a purse string pulling on the EVLmargin. However, direct evidence supporting this hypothesis has been missing. Using laser ablation to measure tension within the actomyosin band, we find an anisotropic tension distribution with highest tension parallel to the EVL margin. Notably, there is also tension perpendicular to the EVL margin, indicating that the actomyosin band is not free to constrict in this direction. To understand how anisotropic tension within the actomyosin band controls EVL epiboly, we have developed a hydrodynamic description of this process, modeling the actomyosin cortex as an active, viscoelastic gel. Quantifications of cortical flows within the EVL and YSL are consistent with predictions from our theory, supporting the general plausibility of our theoretical approach. Based upon experimental and theoretical results, we propose a new mechanism for the actomyosin band in EVL epiboly: in addition to function as a geometry-dependent purse string, it exerts a friction-based pulling force. Martin Behrndt, Guillaume Salbreux, Stephan Grill, Carl-Philipp Heisenberg http://www.ist.ac.at/research/research-groups/heisenberg-group/