Signalling centre vortices coordinate collective behaviour in social amoebae

Self-sustaining signalling waves provide a source of information in living systems. A classic example is the rotating spiral waves of cAMP (chemoattractant) release that encode Dictyostelium morphogenesis. These patterns remain poorly characterised due to limitations in tracking the signalling behaviour of individual cells in the context of the whole collective. Here, we have imaged Dictyostelium populations over millimetre length scales and track the emergence, structure, progression and biological effects of cAMP waves by monitoring the signalling states and motion of individual cells. Collective migration coincides with a decrease in the period and speed of waves that stem from an increase in the rotational speed and curvature of spiral waves. The dynamics and structure of spiral waves are generated by the vortex motion of the spiral tip. Spiral tip circulation spatially organises a small group of cells into a ring pattern, which also constrains spiral tip motion. Both the cellular ring and tip path gradually contract over time, resulting in the acceleration of spiral rotation and change in global wave dynamics. Aided by mathematical modelling, we show that this contraction is due to an instability driven by a deflection in cell chemotaxis around the spiral tip cAMP field, resulting in a deformation of the cellular ring pattern towards its centre. That is, vortex contraction modulates the source of information which, upon dissemination (excitable signal relay) and decoding (chemotaxis), triggers morphogenesis. By characterising rotating spiral waves at this level of detail, our results describe a mechanism by which information generated by a self-sustaining signal, and disseminated across the population, is modulated at the organisational source.

On the Coriolis Effect for Internal Ocean Waves

A derivation of the Ostrovsky equation for internal waves with methods of the Hamiltonian water wave dynamics is presented. The internal wave formed at a pycnocline or thermocline in the ocean is influenced by the Coriolis force of the Earth's rotation. The Ostrovsky equation arises in the long waves and small amplitude approximation and for certain geophysical scales of the physical variables.

Epsilon-Near-Zero Metamaterials

This Element introduces the exotic wave phenomena arising from the extremely small optical refractive index, and sheds light on the underlying mechanisms, with a primary focus on the basic concepts and fundamental wave physics. The authors reveal the exciting applications of ENZ metamaterials, which have profound impacts over a wide range of fields of science and technology. The sections are organized as follows: in Section 2, the authors demonstrate the extraordinary wave properties in ENZ metamaterials, analyzing the unique wave dynamics and the resulting effects. Section 3 is dedicated to introducing various realization methods of the ENZ metamaterials with periodic and non-periodic styles. The applications of ENZ metamaterials are discussed in Sections 4 and 5, from the perspectives of microwave engineering, optics, and quantum physics. The authors close in Section 6 by presenting an outlook on the development of ENZ metamaterials and discussing the key challenges addressed in future works.

First and Second Wave Dynamics of Emergency Department Utilization during the COVID-19 Pandemic: a retrospective study

Background: Since the COVID-19 pandemic, there has been a decrease in emergency department(ED) utilization. Although this has been thoroughly characterized for the first wave(FW), studies during the second wave(SW) are limited. We examined the changes in ED utilization between the FW and SW, compared to 2019 reference periods. Methods: We performed a retrospective analysis of ED utilization in 3 Dutch hospitals in 2020. The FW and SW (March until June and September until December, respectively) were compared to the reference periods in 2019. ED visits were labeled as (non)COVID suspected. Findings: During the FW and SW ED visits decreased by 20.3% and 15.3%, respectively, when compared to reference periods in 2019. During both waves high urgency visits significantly increased with 3.1% and 2.1%, and admission rates (ARs) increased with 5.0% and 10.4%. Trauma related visits decreased by 5.2% and 3.4%. During the SW we observed less COVID-related visits compared to the FW (4,407 vs 3,102 patients). COVID related visits were significantly more often in higher need of urgent care and ARs where at least 24.0% higher compared to non COVID visits. Interpretation: During both COVID-19 waves ED visits were significantly reduced, with the most distinct decline during the FW. ED patients were more often triaged as high urgent and the ARs were increased compared to the reference period in 2019, reflecting a high burden on ED resources. These findings indicate the need to gain more insight into motives of patients to delay or avoid emergency care during pandemics and prepare EDs for future pandemics.