Diffusion on random site percolation clusters. Theory and NMR microscopy experiments with model objects

Quasi two-dimensional random site percolation model objects were fabricate based on computer generated templates. Samples consisting of two compartments, a reservoir of H$_2$O gel attached to a percolation model object which was initially filled with D$_2$O, were examined with NMR (nuclear magnetic resonance) microscopy for rendering proton spin density maps. The propagating proton/deuteron inter-diffusion profiles were recorded and evaluated with respect to anomalous diffusion parameters. The deviation of the concentration profiles from those expected for unobstructed diffusion directly reflects the anomaly of the propagator for diffusion on a percolation cluster. The fractal dimension of the random walk, $d_w$, evaluated from the diffusion measurements on the one hand and the fractal dimension, $d_f$, deduced from the spin density map of the percolation object on the other permits one to experimentally compare dynamical and static exponents. Approximate calculations of the propagator are given on the basis of the fractional diffusion equation. Furthermore, the ordinary diffusion equation was solved numerically for the corresponding initial and boundary conditions for comparison. The anomalous diffusion constant was evaluated and is compared to the Brownian case. Some ad hoc correction of the propagator is shown to pay tribute to the finiteness of the system. In this way, anomalous solutions of the fractional diffusion equation could experimentally be verified for the first time.Comment: REVTeX, 12 figures in GIF forma

The Phylogenetic Origin of oskar Coincided with the Origin of Maternally Provisioned Germ Plasm and Pole Cells at the Base of the Holometabola

The establishment of the germline is a critical, yet surprisingly evolutionarily labile, event in the development of sexually reproducing animals. In the fly Drosophila, germ cells acquire their fate early during development through the inheritance of the germ plasm, a specialized maternal cytoplasm localized at the posterior pole of the oocyte. The gene oskar (osk) is both necessary and sufficient for assembling this substance. Both maternal germ plasm and oskar are evolutionary novelties within the insects, as the germline is specified by zygotic induction in basally branching insects, and osk has until now only been detected in dipterans. In order to understand the origin of these evolutionary novelties, we used comparative genomics, parental RNAi, and gene expression analyses in multiple insect species. We have found that the origin of osk and its role in specifying the germline coincided with the innovation of maternal germ plasm and pole cells at the base of the holometabolous insects and that losses of osk are correlated with changes in germline determination strategies within the Holometabola. Our results indicate that the invention of the novel gene osk was a key innovation that allowed the transition from the ancestral late zygotic mode of germline induction to a maternally controlled establishment of the germline found in many holometabolous insect species. We propose that the ancestral role of osk was to connect an upstream network ancestrally involved in mRNA localization and translational control to a downstream regulatory network ancestrally involved in executing the germ cell program

Preventie van astma

Primary prevention of asthma is not easy to achieve. - Preventive measures which are useful in all children are prevention of passive smoking exposure in utero and/or in childhood and the use of breast-feeding during at least the first six months of life. - Single interventions towards reduction of house dust mite allergens are not effective in preventing asthma; only a combined intervention with breast feeding, stopping of passive smoking, and reduction of allergens of pets and house dust mite appeared to be effective in young children at risk for asthma. - In children with already existing asthma, a combination of (secondary) preventive measures with sanitation of the house, reduction of allergens of pets and house dust mite, and optimal ventilation is useful. - Removal of pets from homes of children in the general population before the onset of atopy and allergic disorders is not effective. - Children visiting day-care report more airway infections and respiratory complaints in the first years of life. However, the chance on asthma or atopy development in later life is not influenced. Therefore, there is no reason to avoid day-care attendance in the first years of life. - There is no evidence for the use of probiotics in order to prevent asthma. Their use is not advised. - The Dutch vaccination program does not increase the risk on atopy or asthma. - There is no evidence for the benefit of influenza vaccination in children with asthma. - Exposure to a dog or cat in already sensitised children with asthma is associated with more severe asthma. Therefore, pets are not advised with these children