title: Determining Physical Properties of the Cell Cortex.
abstract: Actin and myosin assemble into a thin layer of a highly dynamic network underneath the membrane of eukaryotic cells. This network generates the forces that drive cell- and tissue-scale morphogenetic processes. The effective material properties of this active network determine large-scale deformations and other morphogenetic events. For example, the characteristic time of stress relaxation (the Maxwell time τM) in the actomyosin sets the timescale of large-scale deformation of the cortex. Similarly, the characteristic length of stress propagation (the hydrodynamic length λ) sets the length scale of slow deformations, and a large hydrodynamic length is a prerequisite for long-ranged cortical flows. Here we introduce a method to determine physical parameters of the actomyosin cortical layer in vivo directly from laser ablation experiments. For this we investigate the cortical response to laser ablation in the one-cell-stage Caenorhabditis elegans embryo and in the gastrulating zebrafish embryo. These responses can be interpreted using a coarse-grained physical description of the cortex in terms of a two-dimensional thin film of an active viscoelastic gel. To determine the Maxwell time τM, the hydrodynamic length λ, the ratio of active stress ζΔμ, and per-area friction γ, we evaluated the response to laser ablation in two different ways: by quantifying flow and density fields as a function of space and time, and by determining the time evolution of the shape of the ablated region. Importantly, both methods provide best-fit physical parameters that are in close agreement with each other and that are similar to previous estimates in the two systems. Our method provides an accurate and robust means for measuring physical parameters of the actomyosin cortical layer. It can be useful for investigations of actomyosin mechanics at the cellular-scale, but also for providing insights into the active mechanics processes that govern tissue-scale morphogenesis.
authors.affiliation: Max Planck Institute for the Physics of Complex Systems, Dresden, Germany; Max Planck Institute of Molecular Cell Biology and Genetics, Dresden, Germany.
authors.forename: Arnab
authors.initials: A
authors.lastname: Saha
authors.affiliation: Max Planck Institute for the Physics of Complex Systems, Dresden, Germany; Max Planck Institute of Molecular Cell Biology and Genetics, Dresden, Germany; Biotechnology Center, Technical University Dresden, Dresden, Germany.
authors.forename: Masatoshi
authors.initials: M
authors.lastname: Nishikawa
authors.affiliation: Max Planck Institute of Molecular Cell Biology and Genetics, Dresden, Germany; Institute of Science and Technology Austria, Klosterneuburg, Austria.
authors.forename: Martin
authors.initials: M
authors.lastname: Behrndt
authors.affiliation: Institute of Science and Technology Austria, Klosterneuburg, Austria.
authors.forename: Carl-Philipp
authors.initials: CP
authors.lastname: Heisenberg
authors.affiliation: Max Planck Institute for the Physics of Complex Systems, Dresden, Germany.
authors.forename: Frank
authors.initials: F
authors.lastname: Jülicher
authors.affiliation: Max Planck Institute for the Physics of Complex Systems, Dresden, Germany; Max Planck Institute of Molecular Cell Biology and Genetics, Dresden, Germany; Biotechnology Center, Technical University Dresden, Dresden, Germany. Electronic address: stephan.grill@biotec.tu-dresden.de.
authors.forename: Stephan W
authors.initials: SW
authors.lastname: Grill
mesh_terms: D000205:Actomyosin / Q000378:metabolism; D000818:Animals; D055592:Biophysical Phenomena; D017173:Caenorhabditis elegans / Q000166:cytology* / Q000196:embryology; D004548:Elasticity; D054262:Gastrulation; D053685:Laser Therapy; D014783:Viscosity; D015027:Zebrafish / Q000196:embryology / Q000378:metabolism*
chemical_list: D000205:Actomyosin