Optogenetic dissection of the roles of actomyosin in the mechanics underlying tissue fluidity

Rapid epithelial tissue flows are essential to building and shaping developing embryos. However, it is not well understood how the mechanical properties of tissues and the forces driving them to flow are jointly regulated to accommodate rapid tissue remodeling. To dissect the roles of actomyosin in the mechanics of epithelial tissue flows, here we use two optogenetic tools, optoGEF and optoGAP, to manipulate Rho/Rho-kinase signaling and actomyosin contractility in the germband epithelium, which flows via convergent extension during Drosophila body axis elongation. The ability to perturb actomyosin across the tissue allows us to analyze the effects of actomyosin on cell rearrangements, tissue tensions, and tissue mechanical properties. We find that either optogenetic activation or deactivation of Rho1 signaling and actomyosin contractility at the apical surface of the germband disrupts cell rearrangements and tissue-level flows. By probing mechanical tensions in the tissue using laser ablation and predicting tissue mechanical properties from cell packings, we find that actomyosin influences both the anisotropic forces driving tissue flow and the mechanical properties of the tissue resisting flow, leading to complex relationships between actomyosin activity and tissue-level flow. Moreover, our results indicate that changes in the distribution of medial and junctional myosin in the different perturbations alter tissue tension and cell packings in distinct ways, revealing how junctional and medial myosin have differential roles in promoting and orienting cell rearrangements to tune tissue flows in developing embryos.

Corrosion resistance of valve steel in diesel exhaust gas containing 5, 10 and 20 % of FAME

The operating properties of exhaust valves under heavy thermal and mechanical loads are dependent on the strength of steel i.e. resistance to mechanical deformations under increased and high temperatures and high temperature corrosive action of hot exhaust gas. Long term operation of exhaust valves under the conditions of cyclic thermal and mechanical tensions in the environment of diesel exhaust gas where the main oxidizing components are oxygen, carbon dioxide and overheated steam leads to a corrosion of the steel surface and diffusion processes on the thin surface layer of steel, which, under extreme conditions, may lead to a deformation of the valve, an increased leakage of the combustion chamber and a damage or destruction of the engine. The investigations of the corrosion resistance of valve steel in diesel exhaust gas containing 5, 10 and 20 % of FAME were conducted on the durability test stand of BOSMAL Automotive Research and Development Institute in Bielsko-Biała in the oxidation catalyst chamber under the conditions simulating the operation of exhaust valves in diesel engines. The tests have confirmed that the corrosion resistance of valve steel in diesel exhaust gas decreases with an increased content of FAME in the fuel and as the content of Cr, Si, Ni is reduced in the tested valve steel and the content of Mn is increased in the tested valve steel.

GEODYNAMICS

Experimental investigations of fault zones in the polymetallic indications zone had shown the effectiveness of rocks deflected mode measurements control through observations and analysis of natural electric fields (NF) variations. Application of 4-electrodes compensated method of NF registration upraises sensitivity and noise reduction during measurements of local signals on the background of significant natural and artificial disturbances. Areas with increased non-stable mechanical tensions can be defined with this method.

Abstracts of papers presented at the sixteenth Genetics Society's Mammalian Genetics and Development Workshop held at the Institute of Child Health, University College London on 21 and 22 November 2005

Prior to cranial neural tube closure, the neural folds adopt a biconvex morphology which is thought to be due to expansion of the underlying mesenchyme. Dorso-lateral hinge points (DLHPs) then form, which allow the dorsal tips of the neural folds to ‘flip around’ resulting in apposition of the tips and facilitating subsequent fusion. Cranial closure is particularly prone to perturbation, leading to exencephaly in many mouse mutants and as a result of a variety of teratogenic influences. This may reflect mechanical tensions affecting the closing cranial neural folds. For example, the presence of ventral flexures of the body axis at the mid- and forebrain levels mechanically opposes the formation of DLHPs. Several processes have been implicated as important in overcoming these mechanical tensions, thereby assisting in cranial neural tube closure. These include contraction of actin microfilaments at the luminal surface of the neuroepithelium and apoptosis in the dorsal and dorsolateral neuroepithelium. The latter may act to increase flexibility in the dorsal neural folds, enhancing DLHP formation. Neural crest cells (NCC) originate in the dorsal tips of the neuroepithelium and undergo an epithelial-to-mesenchymal transition, allowing them to delaminate, exit the neuroepithelium and migrate extensively throughout the embryo to form numerous derivatives. We hypothesized that delamination of the NCC from the neuroepithelium may enhance the mechanical flexibility of the dorsal tips of the neural folds, allowing the ‘flip around’ event to occur.

The roles of node regression and elongation of the area pellucida in the formation of somites in avian embryos

Experiments have been carried out on explanted chick embryos to test certain widely accepted concepts about the role of Hensen's node in somite formation. The relationship between elongation of the area pellucida and regression of Hensen's node has also been investigated. We conclude from these experiments that: (a) The timing of somite formation is not controlled by the regression of Hensen's node, nor by the shearing of the mesoderm into right and left halves. (b) Somite size and shape are probably controlled by local conditions in the chick embryo. (c) Elongation and regression are two different events. (d) The position of the somites probably depends on mechanical tensions in the area pellucida. (e) The notochord is not required for the stability of somites in vivo.

Effects of extracellular calcium depletion on membrane topography and occluding junctions of mammary epithelial cells in culture.

Ca2+ dependence of occluding junction structure and permeability, well documented in explanted or cultured epithelial sheets, presumably reflects inherent control mechanisms. As an approach to identification of these mechanisms, we induced disassembly of zonulae occludentes in confluent monolayers of mouse mammary epithelial cells by exposure to low concentrations of the chelators, EGTA or sodium citrate. Stages in disassembly were monitored during treatment by phase-contrast microscopy and prepared for transmission and scanning electron microscopy. Cellular response included several events affecting occluding junctions: (a) Centripetal cytoplasmic contraction created tension on junction membranes and displaced intramembrane strands along lines determined by the axis of tension. (b) Destabilization of junction position, probably through increased membrane fluidity, augmented tension-induced movement of strands, resulting in fragmentation of the junction belt. (c) Active ruffling and retraction of freed peripheral membranes remodeled cell borders to produce many filopodia, distally attached by occluding-junction fragments to neighboring cell membranes. Filopodia generally persisted until mechanically ruptured, when endocytosis of the junction and adhering cytoplasmic bleb ensued. Junction disassembly thus resulted from mechanical tensions generated by initial centripetal contraction and subsequent peripheral cytoskeletal activity, combined with destabilization of the junction's intramembrane strand pattern.