Forces driving epithelial spreading in zebrafish epiboly

A variety of developmental and disease related processes depend on epithelial cell sheet spreading. In order to gain insight into the biophysical mechanism(s) underlying the tissue morphogenesis we studied the spreading of an epithelium during the early development of the zebrafish embryo. In zebrafish epiboly the enveloping cell layer (EVL), a simple squamous epithelium, spreads over the yolk cell to completely engulf it at the end of gastrulation. Previous studies have proposed that an actomyosin ring forming within the yolk syncytial layer (YSL) acts as purse string that through constriction along its circumference pulls on the margin of the EVL. Direct biophysical evidence for this hypothesis has however been missing. The aim of the thesis was to understand how the actomyosin ring may generate pulling forces onto the EVL and what cellular mechanism(s) may facilitate the spreading of the epithelium. Using laser ablation to measure cortical tension within the actomyosin ring we found an anisotropic tension distribution, which was highest along the circumference of the ring. However the low degree of anisotropy was incompatible with the actomyosin ring functioning as a purse string only. Additionally, we observed retrograde cortical flow from vegetal parts of the ring into the EVL margin. Interpreting the experimental data using a theoretical distribution that models the tissues as active viscous gels led us to proposen that the actomyosin ring has a twofold contribution to EVL epiboly. It not only acts as a purse string through constriction along its circumference, but in addition constriction along the width of the ring generates pulling forces through friction-resisted cortical flow. Moreover, when rendering the purse string mechanism unproductive EVL epiboly proceeded normally indicating that the flow-friction mechanism is sufficient to drive the process. Aiming to understand what cellular mechanism(s) may facilitate the spreading of the epithelium we found that tension-oriented EVL cell divisions limit tissue anisotropy by releasing tension along the division axis and promote epithelial spreading. Notably, EVL cells undergo ectopic cell fusion in conditions in which oriented-cell division is impaired or the epithelium is mechanically challenged. Taken together our study of EVL epiboly suggests a novel mechanism of force generation for actomyosin rings through friction-resisted cortical flow and highlights the importance of tension-oriented cell divisions in epithelial morphogenesis.