Mechanics of Epithelial Tissue Morphogenesis

Morphogenesis is the fundamental and remarkable biological process that produces elaborate and diverse tissues and organs from simple groups of cells, which can happen on timescales as short as minutes or as long as days. One of the biggest challenges in understanding morphogenesis is the gap between our knowledge of the molecular-scale activities of genes and proteins, and the large-scale behaviors of cells and tissues. To fill this gap, a complete understanding of both biochemical and mechanical factors involved in morphogenesis is needed. Morphogenesis is naturally a mechanical process in which tissues are physically sculpted by mechanical stress, strain, and movements of cells that are induced by these genetic and molecular programs. However, many of the mechanical factors involved in morphogenesis remain poorly understood partially due to the strong coupling of mechanical factors and biological factors, the active responses of living tissues to the environment, and the lack of experimental methods to study the mechanics of tissues in vivo. Epithelial tissues play crucial roles in shaping early embryos and are widely spread in mature animals to serve as boundaries and barriers. They are robust tissues that not only support the structure of embryos and organs, but also actively change shape and structure, displaying a fluid behavior during morphogenesis. Contractile tension and cell-cell adhesion are thought to be the main mechanical factors involved in epithelial tissue morphogenesis, but how the balance between these two determines epithelial tissue mechanics remains unclear. To build a fundamental understanding of the mechanical mechanisms underlying epithelial tissue morphogenesis, this dissertation studies the germband epithelial tissue in the early Drosophila melanogaster embryo and addresses two important open questions in the field of mechanics in morphogenesis: (1) what mechanical factors are involved in the morphogenesis of epithelial tissues; (2) how does a cell control these factors to tune tissue mechanical behaviors. In this dissertation, we developed a systematic, quantitative, in vivo experimental approach to explore mechanics of epithelial tissue morphogenesis in the Drosophila embryo by integrating molecular genetics approaches, live confocal fluorescence imaging, and quantitative image analysis. Combining our experimental studies in the Drosophila embryo with our collaborators’ theoretical modeling approaches, we showed that the shapes and alignment of cells within tissues can help us understand and predict epithelial tissue mechanical behaviors, such as tissue fluidity, during morphogenesis and how defects in these processes can result in abnormalities in embryo shape. We also observed that the Drosophila germband tissue transitions from more solid-like to more fluid-like behavior to help accommodate dramatic tissue flows during convergent extension, which indicates that the mechanical properties of developing tissues might be tuned during morphogenetic events. To elucidate molecular mechanisms underlying how tissue mechanical properties may be regulated during morphogenesis, this dissertation explores the role of cell-cell adhesion in controlling epithelial tissue mechanics. By systematically modulating cell-cell adhesion levels in the Drosophila germband tissue and combining live imaging and quantitative image analysis, we studied the effects of cell-cell adhesion levels on cellular and tissue behaviors. We found biphasic dependencies of cell rearrangements, cell shape, and tissue fluidity on cell-cell adhesion levels, which are surprisingly linked to each other by cell patterns in the tissue. In particular, tissues comprising cells with either lower or higher cell-cell adhesion levels tend to rearrange faster and show cell patterns indicating more fluid-like tissue behaviors. Further studies suggested that cell-cell adhesion works with cytoskeletal molecules to achieve these effects. The experimental approaches developed for exploring mechanics in 2-D in the Drosophila germband epithelial tissue are expanded upon in order to investigate germband tissue mechanics in 3-D. These approaches are also used to study mechanics in the inner ear round window membrane of the guinea pig for clinical application. This dissertation advances our understanding of mechanics of epithelial tissue morphogenesis in vivo and provides a practical, quantitative, and appealing platform for exploring mechanics in living tissues during morphogenesis. This helps fill the gap in our knowledge of molecular-scale activities and tissue-level behaviors, provides insight into building tissues with precise shapes and structures in the lab, and sheds light on human diseases associated with improper regulation of tissue mechanics such as birth defects, aberrant wound healing, and cancer metastasis