Exit from quiescence couples epithelial stress amplification to fluidization

Cellular quiescence is a state of reversible cell cycle arrest that is associated with tissue dormancy. Timely regulated entry into and exit from quiescence is important for processes such as tissue homeostasis, tissue repair, stem cell maintenance, developmental processes and immunity. Here we show that quiescent human keratinocyte monolayers contain an actinomyosin-based system that facilitates global viscoelastic flow upon serum-stimulated exit from quiescence. Mechanistically, serum exposure causes rapid amplification of pre-existing contractile sites leading to a burst in monolayer stress that subsequently drives monolayer fluidization. The stress magnitude after quiescence exit correlates with quiescence depth, and a critical stress level must be reached to overcome the cell sheet displacement barrier. The study shows that static quiescent cell monolayers are mechanically poised for motility and identifies global stress amplification as a mechanism for tissue fluidization.

Work-to-Family Conflict and Children’s Problems with School, Friends, and Health: Household Economic Conditions and Couple Relationship Quality as Contingencies

What is the relationship between work-to-family conflict (WFC) and children’s problems with school, friends, and health? And does that association depend on household economic conditions and couple relationship quality? Using four waves of longitudinal data from the Canadian Work, Stress, and Heath Study (2011–2017), the present study finds that—overall—both fathers’ and mothers’ levels of WFC are associated with elevated levels of children’s problems over time. However, we also discover that household income and spousal disputes moderate this focal relationship—and they do so differently for mothers and fathers. First, the positive association between WFC and children’s problems is stronger for mothers (but not fathers) in households with lower income. Second, the positive association between WFC and children’s problems is stronger for fathers (but not mothers) who report more frequent disputes with their spouse. We discuss the implications of these patterns for current theorizing about stress amplification dynamics and situate that discussion within broader ideas in the ecological model of human development.

Stress amplification around weak inclusions in the dry and strong subducting oceanic lithosphere

<p>Intermediate-depth subduction seismicity is still hiding most of its secrets. While plate unbending is recognised as the main stress loading mechanism, the processes responsible for earthquake nucleation are still unclear and depend upon the question whether failure occur in a wet dehydrating slab or in a completely dry lithosphere. The recent observation of subduction-related pseudotachylytes (quenched frictional melts produced during seismic slip along a fault) in the dry ophiolites of Moncuni (Lanzo Massif, W. Alps)<sup>1</sup>, an exhumed example of the actual intermediate-depth seismicity, and the interpretation of seismic data from various double-plane seismic zones in subducting slabs<sup>2</sup> suggest that the seismogenic portions of subducting oceanic slabs can be dominantly dry. In absence of a fluid-mediated embrittlement (i.e. dehydration embrittlement), a dry and strong slab requires extremely high differential stress for brittle failure to occur.</p><p>Here we investigate with numerical simulations the potential of a subducting dry oceanic slab of building up the high differential stress required for failure. We performed pseudo-2D thermo-mechanical simulations of free subduction of a dry slab in the asthenosphere considering a visco-elasto-plastic rheology. We tested both a homogeneous dry plate and a dry plate with scattered weak circular inclusions representing domains of partial hydration in the first 40 km of the slab.</p><p>The stress field in the unbending portion of the slab describes two arcs, the outer one in compression and the inner one in extension, in agreement with the two planes of seismicity. For the homogenous plate the maximum values of differential stress are around 1 GPa, i.e. not high enough for triggering earthquakes. The presence of weak inclusions induces a stress amplification, which can be of several folds if elastic properties of the inclusions are sufficiently degraded, but still maintaining a high viscosity. For inclusions with a shear modulus decreased by 60-70% relative to the surrounding material, but similar viscosity, stress values in excess of 4 GPa are obtained, high enough for brittle failure at 100 km of depth. This inclusion rheology is compatible with that of a slightly hydrated and serpentinized meta-peridotite. These meta-peridotite domains are likely to be found in the oceanic lithosphere around faults related to slab bending which represent the main pathways for fluid infiltration in the slab.</p><p>We conclude that extremely high deviatoric stresses can be achieved in dry and strong subducting plates in presence of scattered domains of meta-peridotite acting as local stress amplifiers. These previously unreported stress values may explain brittle seismic failure at intermediate depth conditions.</p><p> </p><p>References:</p><p>1: Pennacchioni et al., 2020, Record of intermediate-depth subduction seismicity in a dry slab from an exhumed ophiolite, Earth Planet. Sc. Lett. 548, 116490</p><p>2: Florez and Prieto, 2019, Controlling factors of seismicity and geometry in double seismic zones, Geophys. Res. Lett. 46, 4171-4181</p>

Is complex fault zone behaviour a reflection of rheological heterogeneity?

Fault slip speeds range from steady plate boundary creep through to earthquake slip. Geological descriptions of faults range from localized displacement on one or more discrete planes, through to distributed shearing flow in tabular zones of finite thickness, indicating a large range of possible strain rates in natural faults. We review geological observations and analyse numerical models of two-phase shear zones to discuss the degree and distribution of fault zone heterogeneity and effects on active fault slip style. There must be certain conditions that produce earthquakes, creep and slip at intermediate velocities. Because intermediate slip styles occur over large ranges in temperature, the controlling conditions must be effects of fault properties and/or other dynamic variables. We suggest that the ratio of bulk driving stress to frictional yield strength, and viscosity contrasts within the fault zone, are critical factors. While earthquake nucleation requires the frictional yield to be reached, steady viscous flow requires conditions far from the frictional yield. Intermediate slip speeds may arise when driving stress is sufficient to nucleate local frictional failure by stress amplification, or local frictional yield is lowered by fluid pressure, but such failure is spatially limited by surrounding shear zone stress heterogeneity. This article is part of a discussion meeting issue ‘Understanding earthquakes using the geological record’.