Mammalian Sperm Motility: Observation and Theory

Mammalian spermatozoa motility is a subject of growing importance because of rising human infertility and the possibility of improving animal breeding. We highlight opportunities for fluid and continuum dynamics to provide novel insights concerning the mechanics of these specialized cells, especially during their remarkable journey to the egg. The biological structure of the motile sperm appendage, the flagellum, is described and placed in the context of the mechanics underlying the migration of mammalian sperm through the numerous environments of the female reproductive tract. This process demands certain specific changes to flagellar movement and motility for which further mechanical insight would be valuable, although this requires improved modeling capabilities, particularly to increase our understanding of sperm progression in vivo. We summarize current theoretical studies, highlighting the synergistic combination of imaging and theory in exploring sperm motility, and discuss the challenges for future observational and theoretical studies in understanding the underlying mechanics.\ud Acronyms and Definitions\ud Acrosome: the cap of the sperm head containing enzymes allowing penetration of the zona pellucida via the acrosome reaction\ud Adenosine triphosphate (ATP): the currency unit of chemical energy transfer in living cells\ud Axoneme: a phylogenetically conserved structure within the eukaryotic flagellum consisting of a ring of nine microtubule doublets and a central pair, frequently referred to as 9 + 2\ud Bending moment density: the moment per unit length associated with flagellar bending; it can be divided into a hydrodynamic moment, an elastic moment (from the flagellar bending stiffness), an active moment (generated by dyneins exerting forces between adjacent microtubule doublets), and a passive moment resisting shear\ud Capacitation: the physiological state of a sperm required for fertilization, which is accompanied by the motility patterns associated with hyperactivation, characterized in saline by high-amplitude asymmetric beating\ud Central pair: a pair of microtubules along the length of the axoneme, symmetrically and slightly offset from the axoneme centerline\ud Cumulus oophorus: the outer vestment of the mammalian egg consisting of hundreds of cells radiating out from the egg embedded within a non-Newtonian hyaluronic acid gel\ud Dynein: a molecular motor within the axoneme, attached between adjacent microtubule doublets, that exerts a shearing force to induce axonemal bending\ud Flagellum: a motile cellular appendage that drives the swimming of sperm and other cells; this article focuses on the eukaryotic flagellum\ud Microtubule doublet: a pair of proteinaceous filament structures running the length of the axoneme; dyneins drive their bending, which induces flagellar motion\ud Mid-piece: the region of a sperm flagellum with a mitochondrial sheath, where ATP is generated\ud Oocyte: the egg\ud Outer dense fibers and fibrous sheath: accessory structures reinforcing the mammalian sperm flagellum; the combined axoneme and accessory structures are referred to as 9+9+2\ud Resistive-force theory: an approximation for the local drag of a slender filament element in Stokes flow (or a viscoelastic generalization thereof)\ud Rheotaxis: directed motility in response to the influence of fluid flow\ud Shear: in the context of the flagellum, the relative movement of adjacent microtubule doublets\ud Slender-body theory: an improved approximation for the local drag on a slender filament element in Stokes flow (or a viscoelastic generalization thereof)\ud Zona pellucida: a tough glycoprotein coat between the human egg and the cumulus oophorus, which a sperm must penetrate for successful fertilizatio

The care of chickens

The purpose of this article is to provide purchasers of young chickens with an out- -1- line of the procedures and problems associated with successful chicken raising, and perhaps to serve as a reminder to those poultry farmers who already have a good knowledge of the subject. The wide range of subject matter has made it necessary to present much of the material in a condensed form, and reference should be made to various departmental leaflets for an elaboration of many of the points mentioned in this article. The advisory service of the Department of Agriculture, Poultry Branch, is available to poultry farmers seeking information. Instruction on all matters relating to poultry husbandry will be freely given and visits to farms by field officers can be arranged

Projected Changes in the Seasonal Cycle of Surface Temperature

When forced with increasing greenhouse gases, global climate models project a delay in the phase and a reduction in the amplitude of the seasonal cycle of surface temperature, expressed as later minimum and maximum annual temperatures and greater warming in winter than in summer. Most of the global mean changes come from the high latitudes, especially over the ocean. All 24 Coupled Model Intercomparison Project phase 3 models agree on these changes and, over the twenty-first century, average a phase delay of 5 days and an amplitude decrease of 5% for the global mean ocean surface temperature. Evidence is provided that the changes are mainly driven by sea ice loss: as sea ice melts during the twenty-first century, the previously unexposed open ocean increases the effective heat capacity of the surface layer, slowing and damping the temperature response. From the tropics to the midlatitudes, changes in phase and amplitude are smaller and less spatially uniform than near the poles but are still prevalent in the models. These regions experience a small phase delay but an amplitude increase of the surface temperature cycle, a combination that is inconsistent with changes to the effective heat capacity of the system. The authors propose that changes in this region are controlled by changes in surface heat fluxes

Measurement of cone dark adaptation: a comparison of four psychophysical methods

Purpose: Dark adaptometry is an important clinical tool for the diagnosis of a range of conditions, including age-related macular degeneration (AMD). In order to identify the most robust, clinically applicable technique for the measurement of cone dark adaptation, the repeatability and agreement of four psychophysical methods were assessed. Methods: Data were obtained from 31 healthy adults on two occasions, using four psychophysical methods. Participants’ pupils were dilated and 96% of cone photopigment was bleached before threshold was monitored in the dark using one of the techniques, selected at random. This procedure was repeated for each of the remaining methods. An exponential recovery function was fitted to all threshold recovery data. The co-efficient of repeatability (CoR) was calculated to assess the repeatability of the methods and a repeated measures analysis of variance (ANOVA) was used to compare mean recovery parameters. Results: All four methods demonstrated a similar level of inter-session repeatability for measurement of cone recovery, yielding CoRs between 1.18 and 1.56 minutes. There were no statistically significant differences in estimates of mean time constant of cone recovery (cone τ) between the four methods (p = 0.488), however significant differences initial and final cone thresholds were reported (p < 0.005). Conclusions: All of the techniques were capable of monitoring the rapid changes in visual threshold that occur during cone dark adaptation and the repeatability of the techniques was similar. This indicates that, despite the respective advantages and disadvantages of these psychophysical techniques, all four methods would be suitable for measuring cone dark adaptation in clinical practice

The effect of pre-adapting light intensity on dark adaptation in early age-related macular degeneration

Background This study aimed to identify the pre-adapting light intensity that generated the maximum separation in the parameters of dark adaptation between participants with early age-related macular degeneration (AMD) and healthy control participants in the minimum recording time. Methods Cone dark adaptation was monitored in 10 participants with early AMD and 10 age-matched controls after exposure to three pre-adapting light intensities, using an achromatic annulus (12° radius) centred on the fovea. Threshold recovery data were modelled, and the time constant of cone recovery (τ), final cone threshold, and time to rod-cone-break (RCB) were determined. The diagnostic potential of these parameters at all pre-adapting intensities was evaluated by constructing receiver operating characteristic (ROC) curves. Results There were significant differences between those with early AMD and healthy controls in cone τ and time to RCB (p < 0.05) at all pre-adapting ‘bleaching’ intensities. ROC curves showed that the diagnostic potential of dark adaptometry was high following exposure to all three pre-adapting intensities, generating an area under the curve in excess of 0.87 ± 0.08 for cone τ and time to RCB for all conditions. Conclusions Dark adaptation was shown to be highly diagnostic for early AMD across a range of pre-adapting light intensities, and therefore, the lower pre-adapting intensities evaluated in this study may be used to expedite dark adaptation measurement in the clinic without compromising the integrity of the data obtained. This study reinforces the suggestion that cone and rod dark adaptation are good candidate biomarkers for early AMD

Changing an impermissible LIFO method

Taxpayers utilizing an impermissible last-in, first-out (LIFO) inventory method may be able to change it into a “permissible” one at a modest tax cost. Rev. Proc. 97-27 provides that the IRS is not precluded from transforming an impermissible method in an earlier year just because a taxpayer alters a submethod within the impermissible method in a later year. As a result, taxpayers should examine their LIFO method and, if permissible, alter it before the IRS challenges it and tries to include the LIFO reserves in income. Moreover, taxpayers utilizing an impermissible LIFO method or submethod that has generated large prior advantages need not recognize them under a cut-off method in the form of a Sec. 481(a) adjustment. A recent Tax Court case, Mountain State Ford Truck Sales, Inc., is discussed to illustrate the significant negative tax consequences of utilizing impermissible LIFO inventory account procedures

Microbial competition in porous environments can select against rapid biofilm growth

Microbes often live in dense communities called biofilms where competition between strains and species is fundamental to both evolution and community function. While biofilms are commonly found in soil-like porous environments, the study of microbial interactions has largely focused on biofilms growing on flat, planar surfaces. Here we use novel microfluidic experiments, mechanistic models, and game theory to study how porous media hydrodynamics can mediate competition between bacterial genotypes. Our experiments reveal a fundamental challenge faced by microbial strains that live in porous environments: cells that rapidly form biofilms tend to block their access to fluid flow and redirect resources to competitors. To understand how these dynamics influence the evolution of bacterial growth rates we couple a model of flow-biofilm interaction with a game theory analysis. This shows that hydrodynamic interactions between competing genotypes give rise to an evolutionarily stable growth rate that stands in stark contrast with that observed in typical laboratory experiments: cells within a biofilm can outcompete other genotypes by growing more slowly. Our work reveals that hydrodynamics can profoundly affect how bacteria compete and evolve in porous environments, the habitat where most bacteria live

Cityscapes without figures: geophysics, computing and the future of urban studies

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