Rolling controls sperm navigation in response to the dynamic rheological properties of the environment

Mammalian sperm rolling around their longitudinal axes is a long-observed component of motility, but its function in the fertilization process, and more specifically in sperm migration within the female reproductive tract, remains elusive. While investigating bovine sperm motion under simple shear flow and in a quiescent microfluidic reservoir and developing theoretical and computational models, we found that rolling regulates sperm navigation in response to the rheological properties of the sperm environment. In other words, rolling enables a sperm to swim progressively even if the flagellum beats asymmetrically. Therefore, a rolling sperm swims stably along the nearby walls (wall-dependent navigation) and efficiently upstream under an external fluid flow (rheotaxis). By contrast, an increase in ambient viscosity and viscoelasticity suppresses rolling, consequently, non-rolling sperm are less susceptible to nearby walls and external fluid flow and swim in two-dimensional diffusive circular paths (surface exploration). This surface exploration mode of swimming is caused by the intrinsic asymmetry in flagellar beating such that the curvature of a sperm’s circular path is proportional to the level of asymmetry. We found that the suppression of rolling is reversible and occurs in sperm with lower asymmetry in their beating pattern at higher ambient viscosity and viscoelasticity. Consequently, the rolling component of motility may function as a regulatory tool allowing sperm to navigate according to the rheological properties of the functional region within the female reproductive tract.

How do crystals grow? Steno’s approach

Steno (1638-1686) operated in a historical context rich of discoveries and observations done by previous scientists such as Vannoccio Biringucci, Georg Bauer (Agricola), Johannes von Kepler, Robert Hooke, Christiaan Huyghens, Erasmus Bartholin, and others. Steno also had to fight against some irreducible dogmatic and “mythological” beliefs, such as the vis formativa and succus lapidescens, supported by e.g. Michele Mercati and Anselmo Boetius de Boot, respectively. In De solido intra solidum naturaliter contento dissertationis prodromus Steno deals with almost all aspects of Earth Sciences and not just "solid inclusions" as it might seem from the full title of the Prodromus. This contribution deals only with aspects related to crystallography and minerals in general. The most famous is highlighted by the sentence “non mutatis angulis” which is a clear reference to the fact that interfacial angles of quartz crystals do not change regardless of the size and the number of the faces. This observation was then generalized as a law for all minerals by Jean-Baptiste Romé de l’Isle a century later. Less well known but of great importance is Steno’s assertion that the crystals grow thanks to the addition of particles that come from an external fluid and are not “fed” from the inside like in vegetables; moreover, the speed of growth is not the same for all faces. For example, the faces of the “pyramid” in quartz can grow more or less rapidly than those of the prism (giving rise to either squat or elongated crystals). It can therefore be argued that Steno has greatly contributed to the concept of anisotropy in the solid state, typical of all crystals. Stenonite, Sr2Al(CO3)F5, is a new mineral dedicated to his memory about sixty years ago.

Epidote U–Pb ages vs. fluid–mineral interaction

<p>Recently, the application of LA–ICP–MS has enabled U–Pb dating of epidote minerals within the epidote–clinozoisite solid solution series (Peverelli et al., 2020). Epidote crystallization ages can provide an absolute time frame of deformation sequences when combined with detailed microstructural and metamorphic P–T analysis. Epidote deformation occurs in a brittle manner over a wide range of conditions below its closure temperature for Pb diffusion (685–750 °C; Dahl, 1997); hence, such deformation will not affect its formation U–Pb age. Nevertheless, the possibility of isotopically resetting epidote via fluid–mineral interaction has to be taken into account even at low deformation temperatures.</p><p>We investigated the geochemical and Sr–Pb isotopic characteristics of epidote in one hydrothermal vein in the Aar Massif (central Swiss Alps). The vein is associated with an Alpine shear zone and it is composed of aggregates of 0.1–1 mm anhedral to subhedral epidote grains (epidote-A) + green biotite within a quartz matrix. This quartz dynamically recrystallized by subgrain rotation at temperatures above 400 °C (Stipp et al., 2002) along with crystallization of a second epidote generation (epidote-B) made of tiny (< 0.1 mm) anhedral epidote grains in part mantling epidote-A and defining a fold. We address whether interaction with the fluid that precipitated epidote-B chemically affected epidote-A, i.e. whether the U–Pb age measured by LA–ICP–MS in epidote-A still dates its crystallization upon vein formation or displays age disturbance.</p><p>LA–ICP–MS Sr and Pb concentration data overlap between epidote-A and epidote-B, as do their REE patterns, with (La/Yb)<sub>N</sub> ratios of 0.03–0.92. Lead and Sr isotopic signatures were measured respectively by solution MC–ICP–MS and by TIMS in epidote-A and in separates mixing different proportions of epidote-A and -B (no pure mechanical separates of epidote-B possible), and they are different. This requires open-system conditions during deformation, i.e., introduction of an external fluid with higher <sup>87</sup>Sr/<sup>86</sup>Sr and <sup>208</sup>Pb/<sup>206</sup>Pb ratios during crystallization of epidote-B. Despite the presence of an external fluid and the incorporation of external Sr and Pb in epidote-B, LA–ICP–MS U–Pb isotopic data for epidote-A define a regression in a Tera–Wasserburg plot indicating an age of 19.2 ± 4.3 Ma, consistent with epidote-A crystallization during original vein opening. The preservation of the crystallization age in epidote-A indicates that interaction with the fluid that formed epidote-B did not geochemically and isotopically affect epidote-A. The consistency in trace element contents between epidote-A and -B hints that the epidote-forming cations were inherited by the fluid from epidote-A, and thus suggests dissolution-precipitation as the formation process for epidote-B.</p><p> </p><p>Dahl, Earth Planet. Sci. Lett. 150, 277–290, 1997.</p><p>Peverelli et al., Geochronology Discuss. [preprint], https://doi.org/10.5194/gchron-2020-27, in review, 2020.</p><p>Stipp et al., Geological Society, London, Special Publications, 200(1), 171-190, 2002</p>

Internal friction controls active ciliary oscillations near the instability threshold

Ciliary oscillations driven by molecular motors cause fluid motion at micron scale. Stable oscillations require a substantial source of dissipation to balance the energy input of motors. Conventionally, it stems from external fluid. We show, in contrast, that external fluid friction is negligible compared to internal elastic stress through a simultaneous measurement of motion and flow field of an isolated and active Chlamydomonas cilium beating near the instability threshold. Consequently, internal friction emerges as the sole source of dissipation for ciliary oscillations. We combine these experimental insights with theoretical modeling of active filaments to show that an instability to oscillations takes place when active stresses are strain softening and shear thinning. Together, our results reveal a counterintuitive mechanism of ciliary beating and provide a general experimental and theoretical methodology to analyze other active filaments, both biological and synthetic ones.

On the Drag parameter of ICME propagation models

<p>ICME (Interplanetary Coronal Mass Ejection) are violent phenomena of solar activity that affect the whole heliosphere and the prediction of their impact on different solar system bodies is one of the primary goals of the planetary space weather forecasting. The travel time of an ICME from the Sun to the Earth can be computed through the Drag-Based Model (DBM), which is based on a simple equation of motion for the ICME defining its acceleration as a=-Γ(v-w)v-w, where a and v are the CME acceleration and speed, w is the ambient solar-wind speed and Γ is the so-called drag parameter (Vršnak et al., 2013).<br>In this framework, Γ depends on the ICME mass and cross-section, on the solar-wind density and, to a lesser degree, on other parameters. The typical working hypothesis for DBM implies that both Γ and w are constant far from the Sun. To run the codes, forecasters use empirical<br>input values for Γ and w, derived by pre-existent knowledge of solar-wind condition and by solving the “inverted problem” (where the ICME travel time is known and the unknowns are Γ and/or w). In<br>the 'Ensemble' approaches (Dumbovich et al., 2018; Napoletano et al. 2018), the uncertainty about the actual values of such inputs are rendered by Probability Distribution Functions (PDFs), accounting for the values variability and our lack of knowledge. Among those PDFs, that of Γ is poorly defined due to the relatively scarce statistics of recorded values. </p><p>Employing a list of past ICME events, for which initial conditions when leaving the Sun and arrival conditions at the Earth are known, we employ a statistical approach to the Drag-Based Model to determine a measure of Γ and w for each case. This allows to obtain distributions for the model parameters on experimental basis and, more importantly, to test whether different conditions of relative velocity to the solar wind influence the value of the drag efficiency, as it must be expected for solid objects moving into an external fluid. In addition, we perform numerical simulations of a solid ICME-shaped structure moving into the solar-wind modelled as an external fluid. Outcomes from these simulations are compared with our experimental results, and thus employed to interpret them on physical basis.</p>