609B Poster - 08. Patterning, morphogenesis and organogenesis
Friday April 08, 2:00 PM - 4:00 PM

Authors:
Konstantin Doubrovinski ¹; Joel Tchoufag ²; Mohamad Ibrahim Cheikh ¹; Kranthi Mandadapu ²; Amanda Goldner ¹

Affiliations:
1) UT Southwestern; 2) UC Berkeley

Keywords:
q. epithelial sheets; q. other (Tissue mechanics measurements)

Epithelial morphogenesis is a process through which epithelia develop complex shapes. A common mode of epithelial morphogenesis is folding, where an initially flat sheet of cells bends out of plane to create a fold. A popular system to study folding is ventral furrow formation during gastrulation in Drosophila melanogaster. Despite a long-standing effort to understand the mechanism of tissue folding, the mechanism of ventral furrow formation remains unclear.
We argue that the key missing information required to understand tissue folding is the knowledge of tissue material properties. Indeed, as is known from basic physics considerations, the knowledge of tissue material properties together with the knowledge of tissue deformation dynamics determine the forces driving the dynamics uniquely. Hence, determining material properties of embryonic tissues is absolutely required to arrive at a predictive model of morphogenetic dynamics.
Here, we present a novel technique based on micron-sized bendable cantilevers to directly probe material properties of epithelia in a developing embryo. Our data characterizes how embryonic tissues respond to localized nanonewton range forces. In particular, we show that (1) the tissue restores its shape incompletely after an external pulling force is removed, (2) the tissue restores its shape to a lesser extent if the applied force is spread over a larger area, (3) when constant force is applied, the displacement of the point being pulled increases according to a characteristic 1/2 power law. To interpret our data, we developed a novel 3D computational model of epithelia. Based on a very small set of simple assumptions, our model quantitatively explains all the above as well as several other key features of our measurements.
Our modeling approach can be used further to explain key aspects of tissue dynamics during Drosophila gastrulation. Specifically, we characterize gastrulation dynamics in a genetic background (anillin knockdown) where cells remain completely open to the yolk-sack. Despite cells being completely open on their basal site, ventral furrow formation proceeds (almost) completely unperturbed, with ventral furrow forming to the full extent. Our computational model readily accounts for this unexpected observation. In this way, our in vivo measurements together with computational modeling strongly constrain the relative contributions of the various physical effects involved in ventral furrow dynamics.