Mechanochemical Control of Symmetry Breaking in the Caenorhabditis elegans Zygote

Cell polarity is the asymmetric organization of cellular components along defined axes. A key requirement for polarization is the ability of the cell to break symmetry and achieve a spatially biased organization. Despite different triggering cues in various systems, symmetry breaking (SB) usually relies on mechanochemical modulation of the actin cytoskeleton, which allows for advected movement and reorganization of cellular components. Here, the mechanisms underlying SB in Caenorhabditis elegans zygote, one of the most popular models to study cell polarity, are reviewed. A zygote initiates SB through the centrosome, which modulates mechanics of the cell cortex to establish advective flow of cortical proteins including the actin cytoskeleton and partitioning defective (PAR) proteins. The chemical signaling underlying centrosomal control of the Aurora A kinase–mediated cascade to convert the organization of the contractile actomyosin network from an apolar to polar state is also discussed.

Structure and Magnetodielectric Properties of Composite Ceramics Based on Lead Ferroniobate

In this research, the structural, electrodynamic, and magnetodielectric properties of composite ceramic materials based on lead ferroniobate Pb(Fe0.5Nb0.5)O3 have been studied. The experimentally obtained temperature and frequency dependences of the dielectric constant indicate the presence of a low-temperature polar state, large values of the dielectric constant in the microwave range, and high electrical conductivity of the materials studied.

Nature of polar state in 0.67BiFeO3–0.33BaTiO3

This study was conducted to further understand the nature of the polar state in 0.67BiFeO3–0.33BaTiO3 (0.67BF–0.33BT). A typical relaxor-type dielectric anomaly was observed (Tf = ~627 K, TB = ~820 K). The remnant polarization (Pr), maximum value of electrostrain (Sm) and magnitude strain at Ec in the bipolar mode (Sneg) increase clearly during heating (Pr, ~40 mC/cm2; Sm, 0.191 % under 40 kV/cm at 453 K). The first-cycle bipolar electrostrain loops indicate that the minimum strain on the negative side of the bipolar strain curves is negative. The slopes of the relative permittivity versus log frequency plots in unpoled (–21) and poled (–23) specimens are similar. The transition between the ergodic relaxor state and ferroelectric-like state does not involve a clear dielectric anomaly even in the poled specimen. Analyses based on the Rietveld refinement of XRD patterns, bright-field images and selected-area electron diffractions (SAED) demonstrate that the formation of the long-range ferroelectric domains is difficult under the poling field.

Polar alignment of a protoplanetary disc around an eccentric binary – III. Effect of disc mass

ABSTRACT An initially sufficiently misaligned low-mass protoplanetary disc around an eccentric binary undergoes damped nodal oscillations of tilt angle and longitude of ascending node. Dissipation causes evolution towards a stationary state of polar alignment in which the disc lies perpendicular to the binary orbital plane with angular momentum aligned to the eccentricity vector of the binary. We use hydrodynamic simulations and analytical methods to investigate how the mass of the disc affects this process. The simulations suggest that a disc with non-zero mass settles into a stationary state in the frame of the binary, the generalized polar state, at somewhat lower levels of misalignment with respect to the binary orbital plane, in agreement with the analytical model. Provided that discs settle into this generalized polar state, the observational determination of the misalignment angle and binary properties can be used to determine the mass of a circumbinary disc. We apply this constraint to the circumbinary disc in HD 98800. We obtain analytical criteria for polar alignment of a circumbinary ring with mass that approximately agree with the simulation results. Very broad misaligned discs undergo breaking, but the inner regions at least may still evolve to a polar state. The long-term evolution of the disc depends on the evolution of the binary eccentricity that we find tends to decrease. Although the range of parameters required for polar alignment decreases somewhat with increasing disc mass, such alignment appears possible for a broad set of initial conditions expected in protostellar circumbinary discs.