The Mechanical Properties of Early Drosophila Embryos Measured by High-Speed Video Microrheology

Abstract

In early development, Drosophila melanogaster embryos form a syncytium, i.e., multiplying nuclei are not yet separated by cell membranes, but are interconnected by cytoskeletal polymer networks consisting of actin and microtubules. Between division cycles 9 and 13, nuclei and cytoskeleton form a two-dimensional cortical layer. To probe the mechanical properties and dynamics of this self-organizing pre-tissue, we measured shear moduli in the embryo by high-speed video microrheology. We recorded position fluctuations of injected micron-sized fluorescent beads with kHz sampling frequencies and characterized the viscoelasticity of the embryo in different locations. Thermal fluctuations dominated over nonequilibrium activity for frequencies between 0.3 and 1000 Hz. Between the nuclear layer and the yolk, the cytoplasm was homogeneous and viscously dominated, with a viscosity three orders of magnitude higher than that of water. Within the nuclear layer we found an increase of the elastic and viscous moduli consistent with an increased microtubule density. Drug-interference experiments showed that microtubules contribute to the measured viscoelasticity inside the embryo whereas actin only plays a minor role in the regions outside of the actin caps that are closely associated with the nuclei. Measurements at different stages of the nuclear division cycle showed little variation.

A toolbox to explore the mechanics of living embryonic tissues

“…Passive MR has been used to measure the intracellular mechanical properties in blastomeres of early C. elegans embryos [210] as well as in syncytial Drosophila embryos [211]. Active MR using OTs has recently been used to study the rheological behavior of the cytoplasm of mouse oocytes [103].…”

Relative distance between tracers as a measure of diffusivity within moving aggregates

“…Often, tracers move within domains of finite size, for example individual molecules inside single cells [7,17,31] or individual cells within cell aggregates [1,32].…”

Disentangling Random Motion and Flow in a Complex Medium

“…Subdiffusion can arise in a number of situations, including confinement, transient binding events, or motion in a viscoelastic medium (24). Here, we focus on the latter case, which has been shown to be applicable to the bacterial (4) and eukaryotic (16,17,25) cytoplasm, as well as the eukaryotic nucleoplasm (26). Particles in a viscoelastic medium undergo fractional Brownian motion, with negative velocity correlations that decay as a power law in time ½hṽðtÞ$ṽðt þ tÞi ¼ 2Daða À 1Þjt j aÀ2 (10).…”

“…Although previous single-particle tracking studies have demonstrated a viscous cytoplasm in some stationary cell types (8,14) and a viscoelastic one in others (15)(16)(17), the microscopic material properties within such rapidly moving cells remain uncharacterized. HL60 cells crawl on a two-dimensional surface, with significant changes in shape engendering internal flow and deformation of the cytoplasmic contents.…”