| Abstract |
Gastrulation is a crucial process of early morphogenesis that establishes the body plan of animals. It is a complex process that involves large-scale tissue deformations. This process is governed by the coordinated interactions of the mechanics of the tissues and biochemical signalling pathways that transform the initially unstructured embryo into a spatially organised, multilayered gastrula. This thesis studies two mechanical processes during the gastrulation of the beetle Tribolium and the fruit fly Drosophila. The first part of this thesis focuses on epiboly, the process during which an epithelial layer expands and encloses the embryo. Cell intercalations at the edge of the closing epithelial layer play an important role in epiboly. These cell intercalations are, mechanically, irreversible plastic rearrangements. I propose a continuum model of the interplay between the elasticity of the tissue and these plastic deformations. My continuum framework relates the energetics of intercalation events at the cell scale to tissue-scale dynamics. I discover that different macroscopic dynamics emerge in two different regimes, (i) the regime in which the energy barrier to intercalations swamps the elastic energy released by intercalations, and (ii) the fluidised regime in which this energy barrier vanishes. My results show that only the fluidised regime can reproduce the dynamics observed during the serosa closure process in Tribolium, thereby highlighting the importance of tissue fluidity for Tribolium epiboly. In the second part of this thesis, I model germ-band extension during Drosophila development to explain the observed variability of the shapes of the germ-band and to investigate the role of the attachment of the embryo to the surrounding vitelline envelope in this context. In my model, the dynamics of germ-band extension are reduced to dynamics of the germ-band midline, which is modelled as an elastic line. First, I analyse two models of a planar germ-band midline: a “pushed” model, in which the elastic line is pushed by a force representing the cell intercalations that drive germ-band extension, and a “growing” model, in which these intercalations lengthen the germ-band midline. Stability analysis reveals that only the “pushed” model reproduces the instabilities characteristic of the observed curved and twisted germ-band shapes. This highlights the importance of the pushing force for the mechanics of germ-band extension. Furthermore, this “pushed” model demonstrates that attachment of the embryo to the vitelline envelope stabilises germ-band extension. I conclude by outlining a three-dimensional extension of the “pushed” model to describe the role of the ellipsoidal geometry of the embryo in germ-band extension. |