Mechanical Aspects of Drosophila Gastrulation

Abstract

Tissue morphogenesis is a ubiquitous and fundamental process of Developmental Biology. During this process active forces drive the tissue to undergo complex spatial reorganization that transforms the tissue into structures with specific shapes and functions. A famous example of such a process is the formation of the ventral furrow and the invagination of the ventral mesoderm during Drosophila gastrulation. This is the first global morphogenetic movement during Drosophila development that transforms the one-layered cellular blastoderm into a multi-layered structure through the folding of the epithelial cellular layer of the ventral side of the embryo. This movement is driven by a contractile apical actin-myosin network, which constricts the cell's apex and drives them through a series of coordinated cell shape changes that internalizes the ventral epithelium of the blastoderm. In particular the mesodermal cells first elongate and then shorten back as the ventral furrow is formed. While it has been well documented that apical constriction is necessary for ventral furrow formation the passive mechanism through which apical constriction could transmit forces throughout the bulk tissue of the cell and drive the specific sequence of cell shape change that forms the ventral furrow remains poorly understood. In this thesis we present a thorough study of both the active and passive physical mechanisms involved in Drosophila gastrulation. We first present a mesoscopic description of the force generating machinery involved in the constriction of the apical surface areas of the cells. We then present an experimental technique to identify the viscous cytoplasmic flow as the primary mechanisms responsible for tissue lengthening. Finally we present a model that incorporates the elastic effects of the cellular cortex that demonstrates tissue shortening and the subsequent formation of the Drosophila ventral furrow.