Dynamic Physical Geometry and Intracellular Fluid Mechanics During Epithelial Remodeling

Abstract

Tissue morphogenesis requires epithelial cells to coordinate internal mechanical forces to drive complex structural rearrangements. The Drosophila melanogaster germband extension (GBE) is an ideal model for this process, where the tissue rapidly elongates along the anterior-posterior axis while narrowing along the dorsal-ventral axis. Canonically, the mechanics driving GBE have been constrained by rigid assumptions – analyzing the tissue using two-dimensional representations and static planar identities, restricting the focus of force generation exclusively to the apical domain. This dissertation addresses these fundamental biophysical limitations by demonstrating that morphogenesis is instead driven by dynamic physical geometry, advancing our understanding of both two-dimensional topological remodeling and three-dimensional volumetric forces. First, utilizing live-imaging microscopy and computational tracking, this research identifies a continuous, angle-dependent “interface rectification” mechanism. Rather than possessing fixed contractile or elongating identities, transverse cell-cell interfaces systematically rotate toward the vertical axis, dynamically recruiting Myosin II and continuously updating their contractile potential. This rotational recruitment replenishes the pool of contracting interfaces, facilitating multiple, temporally staggered type 1 (T1) transitions within a single cell to achieve macroscopic tissue elongation. Second, by transitioning to a fully three-dimensional framework, this work demonstrates that contractile forces are not strictly apical phenomena. Cytosolic area oscillations manifest as a complex superposition of bidirectional propagating waves and standing waves along the entire apical-basal axis, mechanically constrained by local volume conservation. Crucially, the nucleus acts as a moving mechanical discontinuity – a “passable piston” – within this fluid environment. The shape and mechanical stiffness of the nucleus heavily dictate the propagation of these cytosolic waves, with its wide equator damping fluid flow and anchoring the formation of standing wave nodes. Reciprocally, active cytosolic actomyosin contractions drive elastic nuclear deformations and direct short-term axial rigid-body displacements. Together, these findings establish that form and function are intricately linked across spatial dimensions. Tissue-wide planar polarity is sustained by continuous interface rotation, while three-dimensional intracellular fluid dynamics are governed by the physical shape and spatial positioning of the nucleus. By demonstrating that biology utilizes dynamic physical geometry to continuously update mechanical properties, this dissertation provides a unified physical framework for large-scale tissue morphogenesis.