2025 Theses Doctoral

Measuring and Manipulating the Mechanics of Epithelial Cells and Tissues

Cupo, Christian

Mechanical forces are largely responsible for shaping sheets of epithelial cells into tissues and organs. During epithelial morphogenesis, cells dynamically tune their mechanical forces to locally promote or resist shape changes. Our current understanding of how these mechanical forces coordinate and drive tissue shape changes is lacking. In particular, it is not well understood why some epithelial cell sheets remodel and flow like fluids, while others stretch and bend like elastic solids, or at what length scale mechanical behaviors of individual cells give rise to tissue-level behaviors. In this work, I develop a framework to analyze tissue structure and dynamics through quantification of cell packings and protein localization patterns. I then combine this framework with optogenetic perturbations and nanoengineering techniques to measure and manipulate the mechanical properties and forces of cultured cells with high spatiotemporal precision.

In chapter 2, I address these challenges by studying how cells respond to different mechanical cues in the fruit fly, Drosophila melanogaster, during early development. Specifically, I quantify global system disorder arising from internal and external stresses by using order parameters that describe the area and stretch distortion between the center of cells. Using these order parameters, we find that structural disorder is synchronized with internal and external stresses imposed on the germband. Furthermore, we dissect the contribution of each stress on the embryo using different mutants that inhibit one or more of these stresses.

In chapter 3, I refine this analysis to identify the cellular and supracellular myosin networks that tune cellular mechanics and forces, connect them to local cell packings, and determine their overall impact on tissue movement. Using cell geometries to infer mechanical properties, I analyze the cell packings during the active processes of fluid-like germband extension (GBE) and the solid-like bending during ventral furrow formation (VFF). We find that the local cell packings match the global cell packings in the ventral furrow, while the local cell packings match the regional behavior in the germband when averaging over N = 3 nearest neighbors.

In chapter 4, I investigate how the mechanical forces of epithelial tissue sheets can be altered using an in vitro culture system. Using optogenetic tools, I manipulate myosin activity in cultured cells and quantify the induced forces by measuring the deflection of an array of micropillars on which the tissue is grown. I show that the traction forces resulting from activation of the optogenetic tool are increased up to 3-fold higher than baseline during activation and decrease to near-baseline levels 30 minutes after activation is stopped.

These studies improve our understanding of the mechanisms and mechanics underlying cellular self-organization and clarify how perturbations of these mechanisms can lead to disease states like congenital anomalies and cancer.