Video Force Microscopy (VFM): A New Technique that Allows Cell-Level Driving Forces to Be Determined from Time-Lapse Images

Abstract

For purposes of video force microscopy (VFM), the forces in the cells of an embryonic epithelium are assumed to consist of active forces, which can be mathematically resolved into equivalent forces along cell boundaries, and passive forces associated with deformation of the cytoplasm and its contained organelles and intermediate filament network, which are represented by an equivalent viscosity, mu. All triple junctions in the time-lapse images are tracked over time and finite element techniques are used to estimate the forces that must act on the passive components of each cell to deform them as observed. A mathematical inverse method is then used to determine the forces that must act along each cell edge in order to produce the net forces needed at each triple junction to drive the observed deformations. The technique has been successfully applied to multi-photon cross-sections of Drosophila embryos undergoing ventral furrow formation. There, it revealed that the ventral furrow is produced by contractions that vary smoothly with time and position in the apical surface of the presumptive mesoderm, by apical-basal contractions in the cells of this tissue and, surprisingly, by spatially more uniform basal contractions in the ectoderm. It was also able to quantify the sometimes subtle force modifications present in mutants that generate abnormal phenotypes. The indicated force alterations are consistent with known genotype-specific structural protein changes. When applied to wound healing in embryonic epithelia, it was able to quantify the forces generated in the purse string that closes the wound and the surrounding cells. It both of these contexts, VFM was able to quantify the forces that drive observed morphogenetic movements and to do so with sub-cellular spatial detail and sub-minute temporal resolution.