Chiral flows can induce neck formation in viscoelastic surfaces

During division in animal cells, the actomyosin cortex has been found to exhibit counter-rotating cortical flows, also known as chiral flows, along the axis of division. Furthermore, such chiral surface flows were shown to influence cellular rearrangements and drive the left-right symmetry breaking in developing organisms. In spite of this prospective biological importance, at the current state, no numerical simulations have been done to study the influence of chiral flows on the cell cortex shape. To deepen the insight on that matter, we present here a numerical study of an axi-symmetric viscoelastic surface embedded in a viscous fluid. To investigate the influence of a chiral flow field on the surface shape and material transport, we impose a generic counter-rotating force field on this surface which induces a chiral flow field. Notably, we find that the building of a neck, as is observed during cell division, occurs if there is a strong shear elastic component. Furthermore we find that a large areal relaxation time results in flows towards the equator of the surface. These flows assist the transport of a surface concentration during the forming of a contractile ring. Accordingly, we show that chiral forces by themselves can drive pattern formation and stabilise contractile rings at the equator.Comment: 13 pages, 5 pages for appendix, 6 figure

Haidinger’s brushes elicited at varying degrees of polarization rapidly and easily assesses total macular pigmentation

Macular pigments (MPs), by absorbing potentially toxic short-wavelength (400–500 nm) visible light, provide protection against photo-chemical damage thought to be relevant in the pathogenesis of age-related macular degeneration (AMD). A method of screening for low levels of MPs could be part of a prevention strategy for helping people to delay the onset of AMD. We introduce a new method for assessing MP density that takes advantage of the polarization-dependent absorption of blue light by MPs, which results in the entoptic phenomenon called Haidinger’s brushes (HB). Subjects were asked to identify the direction of rotation of HB when presented with a circular stimulus illuminated with an even intensity of polarized white light in which the electric field vector was rotating either clockwise or anti-clockwise. By reducing the degree of polarization of the stimulus light, a threshold for perceiving HB (degree of polarization threshold) was determined and correlated (r2=0.66) to macular pigment optical density assessed using dual-wavelength fundus autofluoresence. The speed and ease of measurement of degree of polarization threshold makes it well suited for large-scale screening of macular pigmentation

Improving the repeatability of heterochromatic flicker photometry for measurement of macular pigment optical density

Background: Heterochromatic flicker photometry (HFP) is a psychophysical technique used to measure macular pigment optical density (MPOD). We used the MPS 9000 (MPS) HFP device. Our aim was to determine if the repeatability of the MPS could be improved to make it more suitable for monitoring MPOD over time. Methods: Intra-session repeatability was assessed in 25 participants (aged 20-50 years). The resulting data was explored in detail, e.g., by examining the effect of removal and adjustment of data with less than optimal quality parameters. A protocol was developed for improved overall reliability, which was then tested in terms of inter-session repeatability in a separate group of 27 participants (aged 19-52 years). Results: Removal and adjustment of data reduced the intra-session coefficient of repeatability (CR) by 0.04, on average, and the mean individual standard deviation by 0.004. Raw data observation offered further insight into ways of improving repeatability. The proposed protocol resulted in an inter-session CR of 0.08. Conclusions: Removal and adjustment of less than optimal data improved repeatability, and is therefore recommended. To further improve repeatability, in brief we propose that each patient perform each part of the test twice, and a third time where necessary (described in detail by the protocol). Doing so will make the MPS more useful in research and clinical settings. © 2012 Springer-Verlag

The development of a multidisciplinary fall risk evaluation tool for demented nursing home patients in the Netherlands

BACKGROUND: Demented nursing home patients are at high risk for falls. Falls and associated injuries can have a considerable influence on the autonomy and quality of life of patients. The prevention of falls among demented patients is therefore an important issue. In order to intervene in an efficient way in this group of patients, it is important to systematically evaluate the fall risk profile of each individual patient so that for each patient tailor-made preventive measures can be taken. Therefore, the objective of the present study is to develop a feasible and evidence based multidisciplinary fall risk evaluation tool to be used for tailoring preventive interventions to the needs of individual demented patients. METHODS: To develop this multidisciplinary fall risk evaluation tool we have chosen to combine scientific evidence on the one hand and experts' opinions on the other hand. Firstly, relevant risk factors for falling in elderly persons were gathered from the literature. Secondly, a group of Dutch experts in the field of falls and fall prevention in the elderly were consulted to judge the suitability of these risk factors for use in a multidisciplinary fall risk evaluation tool for demented nursing home patients. Thirdly, in order to generate a compact list of the most relevant risk factors for falling in demented elderly, all risk factors had to fulfill a set of criteria indicating their relevance for this specific target population. Lastly the final list of risk factors resulting from the above mentioned procedure was presented to the expert group. The members were also asked to give their opinion about the practical use of the tool. RESULTS: The multidisciplinary fall risk evaluation tool we developed includes the following items: previous falls, use of medication, locomotor functions, and (correct) choice and use of assistive and protective devices. The tool is developed for the multidisciplinary teams of the nursing homes. CONCLUSION: This evidence and practice based multidisciplinary fall risk evaluation tool targets the preventive interventions aimed to prevent falls and their negative consequences in demented nursing home patients

Pattern formation on deforming active viscoelastic surfaces

Pattern formation on self deforming materials is playing an increasingly vital role in the study of many biological processes. An example is the cell cortex, a network of interlinked actin filaments connected to the inside of the cell membrane. The stiffness of these filaments makes the cortex rigid and gives the cortex a strong influence in the shape of the cell. Additionally, molecular turnover and motor proteins allow shape changes necessary for the cell to divide and to adapt to its environment. Due to the incredible number of proteins that make up the cell cortex, simulation of the molecular dynamics for the entire cortex is impossible. However, when considering a larger scale, these dynamics can be approximated, making it possible to model the cortex as an active viscoelastic surface. To implement this, we use the surface equivalent of a Maxwell material, additionally distinguishing between shear and dilational stress. The motor proteins are modelled by an advection diffusion equation on the surface combined with a concentration dependent surface tension. Surrounding the surface is a fluid modelled by the Navier-Stokes equations. We study the formation of patterns both analytically and numerically. In the analytical study the 2-dimensional curved surface is reduced to a flat 2-dimensional surface with periodic boundary conditions and no surrounding fluid. We then show, that despite the minimal complexity of this model, spatiotemporal patterns can develop on a viscoelastic surface if the relaxation times are different. For the numerical study the Arbitrary Lagrangian Eulerian method (ALE) is applied. The equations for the surface and bulk are solved separately, but this partial method is numerically unstable for dominantly viscous surfaces. With that in mind, we also implemented a monolithic model for viscous surfaces, solving the bulk and surface equations simultaneously. As a special case the influence of a chiral flow field on a sphere is studied. These chiral flows have been observed in biological experiments and are hypothesised to originate from the small scale torques caused by the helix structure of the actin filaments. We found that a high shear elastic modulus in combination with a chiral force field can induce neck formation. Additionally, for a concentration dependent chiral force field combined with active surface tension and a high dilational elastic modulus, the chiral forces can stabilise a contractile ring. These results provide mechanistic evidence that chiral flows can play a role during cell division

Optical diagnostic techniques in ophthalmology

Roy de Kinkelder beschrijft en evalueert optische technieken die dagelijks in oogklinieken gebruikt worden om aandoeningen van het netvlies (retina) vast te stellen. Deze technieken geven informatie over de structuur en de functionele staat van het netvlies. De Kinkelder ontwikkelde een model dat de nauwkeurigheid en precisie vaststelt van systemen voor optische coherentie-tomografie (OCT) die de dikte van de zenuwvezellaag kunnen bepalen. Daarnaast analyseerde hij bewegingsartefacten die optreden tijdens het driedimensionaal afbeelden van het netvlies met OCT, en beschreef hij een methode om deze kleine oogbewegingen te detecteren en te corrigeren. Verder testte en evalueerde De Kinkelder methoden die de hoeveelheid pigment in de macula vaststellen (heterochromatische flikker-fotometrie) en een methode die de functionele staat van het netvlies test (Macubit) om leeftijdsgerelateerde maculadegeneratie in een vroeg stadium vast te stellen