| Abstract |
Epithelial tissues are one of the most abundant tissues in our body. They make up essential organs like the gut, heart and eyes. These organs take up their complex 3D shapes during normal development of the embryo. Our understanding of such large-scale 3D shape changes is limited mainly due to the technical difficulties of imaging and quantifying such developmental events. In this thesis, I study two events in which epithelial monolayers change their 3D shape. In both the projects, I use data from light-sheet microscopic images of developmental events. These data are provided by my collaborators. In this thesis, I further analyzed them using quantitative approaches and interpreted them using computational models. In the first project, I study a case of a developing tissue inside a rigid confinement. A perfect model system for this is the Drosophila embryo which consists of an epithelial monolayer (blastoderm) inside a rigid shell (vitelline membrane). During gastrulation, the blastoderm is under compressional stresses due to tissue proliferation and compression from the germband extension. During this time, an invagination separating the future head and the trunk region appears. This is known as the cephalic furrow (CF). As the CF disappears after some time, its relevance in the normal development of the embryo is unclear. To understand its role, my collaborators image the blastoderm in mutant embryos which lack CF. These mutant embryos have either of the genes even-skipped (eve) or buttonhead (btd) knocked down. In the absence of CF, temporary ectopic folds appear in the blastoderm in locations which vary between embryos. Unlike the CF, ectopic folds appear suddenly and hence look like buckling events. I hypothesize that ectopic folds appear because of the compressive stresses generated in the blastoderm during the germband extension or by the compression of tissues that are adjacent to mitotic domains. Moreover, in normal embryos, CF, which is a controlled invagination, acts as a sink for the compressive stresses and thus suppresses ectopic folds. To test this hypothesis, I modelled the blastoderm as a 2D elastic tissue which is confined inside a rigid boundary acting as the vitelline membrane. In my model, I show that the stresses generated by both the germband extension and the mitotic domains contribute to the formation of ectopic folds. I model the CF as a region with some preferred intrinsic curvature, thus acting as a programmed fold. I show that ectopic folds are inhibited in the presence of a CF. However, the efficiency of the CF depends on the strength of the CF and, interestingly, the timing of the CF. I observe that even a weak CF can inhibit ectopic folds if it appears before the appearance of mitotic domains. I speculate that this could explain why the CF appears before the mitotic domains in the Drosophila embryo. n the second project, I study a case of shape change associated with the development of the Drosophila wing. Here I focus on the wing disc pouch, an epithelial monolayer that forms the adult wing blade. During metamorphosis, the larval wing disc evaginates to form the pupal wing. This process is known as eversion. During late larval stage, the wing disc pouch looks like a spherical cap. I refer to this stage as wL3 (wandering larval stage 3) in this thesis. Four hours after pupariation (4hAPF), the spherical cap deforms to an asymmetric dome such that it has a higher curvature along one cross-section compared to its perpendicular cross-section. Using segmented outlines of the wing in the two cross-sections of multiple images at different developmental stages, I compute the mean shape and quantify the curvature along the arclength of these shapes. To model this shape change, I use a 3D spring lattice whose initial curvature is matched to the curvature of the wL3 stage. Next, using apical cell shape data, provided by my collaborator, I compute a quantity referred to as a “spontaneous deformation tensor”. This tensor quantifies the amount of deformation, at a specific location between two developmental stages, due to cell area changes, cell elongation changes, and neighbour exchanges. I input this deformation pattern in my model which then changes its 3D shape. I find that the deformation due to changes in cell area and elongation increase the size of the tissue globally without affecting its curvature. However, the deformation due to cell rearrangements enhances curvature along one cross-section more than its perpendicular cross-section. Overall, the quantifications and modelling shows how different cellular behaviours deform the tissue locally. Moreover, a spatial pattern of different cellular behaviours can explain essential aspects of the shape change observed during the development of the wing. |