Coupled self-organization: Thermal interaction between two liquid films undergoing long-wavelength instabilities

The effects of thermal coupling between two thin liquid layers, separated by a gas layer, are discussed. The liquid layers undergo long-wavelength instabilities driven by gravitational and thermocapillary stresses. To study the dynamics, both a linear stability analysis and a full numerical solution of the thin-film equations are performed. The results demonstrate that the stability properties of the combined system differ substantially from the case where both layers evolve independently from each other. Most prominently, oscillatory instabilities, not present in single-liquid layer configurations, may occur.Comment: 12 pages, 9 figure

Label-free 3D visualization of cellular and tissue structures in intact muscle with second and third harmonic generation microscopy.

Second and Third Harmonic Generation (SHG and THG) microscopy is based on optical effects which are induced by specific inherent physical properties of a specimen. As a multi-photon laser scanning approach which is not based on fluorescence it combines the advantages of a label-free technique with restriction of signal generation to the focal plane, thus allowing high resolution 3D reconstruction of image volumes without out-of-focus background several hundred micrometers deep into the tissue. While in mammalian soft tissues SHG is mostly restricted to collagen fibers and striated muscle myosin, THG is induced at a large variety of structures, since it is generated at interfaces such as refraction index changes within the focal volume of the excitation laser. Besides, colorants such as hemoglobin can cause resonance enhancement, leading to intense THG signals. We applied SHG and THG microscopy to murine (Mus musculus) muscles, an established model system for physiological research, to investigate their potential for label-free tissue imaging. In addition to collagen fibers and muscle fiber substructure, THG allowed us to visualize blood vessel walls and erythrocytes as well as white blood cells adhering to vessel walls, residing in or moving through the extravascular tissue. Moreover peripheral nerve fibers could be clearly identified. Structure down to the nuclear chromatin distribution was visualized in 3D and with more detail than obtainable by bright field microscopy. To our knowledge, most of these objects have not been visualized previously by THG or any label-free 3D approach. THG allows label-free microscopy with inherent optical sectioning and therefore may offer similar improvements compared to bright field microscopy as does confocal laser scanning microscopy compared to conventional fluorescence microscopy

Electroosmotic flow in small-scale channels induced by surface-acoustic waves

Numerical simulations of the Navier-Stokes, Nernst-Planck, and the Poisson equations are employed to describe the transport processes in an aqueous electrolyte in a parallel-plate nanochannel, where surface-acoustic waves (SAWs) are standing or traveling along (piezo-active) channel walls. It is found that -- in addition to the conventional acoustic streaming flow -- a time-averaged electroosmotic flow is induced. Employing the stream function-vorticity formulation, it is shown that the Maxwell stress term causes an electroosmotic propulsion that is qualitatively identical to the one discussed in the context of alternating current (AC) electroosmosis (EOF). Differences arise mainly due to the high actuation frequencies of SAWs, which are in the MHz range rather than in the kHz regime typical for ACEOF. Moreover, the instantaneous spatial periodicity of the EOF in the travel direction of the SAW is intrinsically linked to the dispersion relation of the latter rather than a free geometric parameter. This leads to a specific frequency band where an EOF of sizable magnitude can be found. On the low frequency end, the ratio between the electric double layer (EDL) thickness and the SAW wavelength becomes extremely small so that the net force leading to a non-vanishing time-averaged flow becomes equally small. On the high frequency end, the RC time of the EDL is much larger than the inverse of the SAW frequency leading to a vanishing effective charge density of the EDL. For a parallel-plate channel, the EOF can be maximized by using two SAWs on both channel walls that have the same frequency but are phase-shifted by $180^\circ$. It appears that the SAW-EOF is the dominant pumping mechanism for such a scenario. The proposed actuation might be a viable alternative for driving liquid electrolytes through narrow ducts and channels, without the need for electric interconnects and electrodes.Comment: 16 pages, 6 figure

Two-Photon Microscopy Allows Imaging and Characterization of Cochlear Microvasculature In Vivo

Impairment of cochlear blood flow has been discussed as factor in the pathophysiology of various inner ear disorders. However, the microscopic study of cochlear microcirculation is limited due to small scale and anatomical constraints. Here, two-photon fluorescence microscopy is applied to visualize cochlear microvessels. Guinea pigs were injected with Fluorescein isothiocyanateor Texas red-dextrane as plasma marker. Intravital microscopy was performed in four animals and explanted cochleae from four animals were studied. The vascular architecture of the cochlea was visualized up to a depth of 90.0 +/- 22.7 mu m. Imaging yielded a mean contrast-to-noise ratio (CNR) of 3.3 +/- 1.7. Mean diameter in vivo was 16.5 +/- 6.0 mu m for arterioles and 8.0 +/- 2.4 mu m for capillaries. In explanted cochleae, the diameter of radiating arterioles and capillaries was measured with 12.2 +/- 1.6 mu m and 6.6 +/- 1.0 mu m, respectively. The difference between capillaries and arterioles was statistically significant in both experimental setups (P < 0.001 and P = 0.022, two-way ANOVA). Measured vessel diameters in vivo and ex vivo were in agreement with published data. We conclude that two-photon fluorescence microscopy allows the investigation of cochlear microvessels and is potentially a valuable tool for inner ear research

Common themes and cell type specific variations of higher order chromatin arrangements in the mouse

BACKGROUND: Similarities as well as differences in higher order chromatin arrangements of human cell types were previously reported. For an evolutionary comparison, we now studied the arrangements of chromosome territories and centromere regions in six mouse cell types (lymphocytes, embryonic stem cells, macrophages, fibroblasts, myoblasts and myotubes) with fluorescence in situ hybridization and confocal laser scanning microscopy. Both species evolved pronounced differences in karyotypes after their last common ancestors lived about 87 million years ago and thus seem particularly suited to elucidate common and cell type specific themes of higher order chromatin arrangements in mammals. RESULTS: All mouse cell types showed non-random correlations of radial chromosome territory positions with gene density as well as with chromosome size. The distribution of chromosome territories and pericentromeric heterochromatin changed during differentiation, leading to distinct cell type specific distribution patterns. We exclude a strict dependence of these differences on nuclear shape. Positional differences in mouse cell nuclei were less pronounced compared to human cell nuclei in agreement with smaller differences in chromosome size and gene density. Notably, the position of chromosome territories relative to each other was very variable. CONCLUSION: Chromosome territory arrangements according to chromosome size and gene density provide common, evolutionary conserved themes in both, human and mouse cell types. Our findings are incompatible with a previously reported model of parental genome separation

Quantitative comparison of DNA detection by GFP-lac repressor tagging, fluorescence in situ hybridization and immunostaining

<p>Abstract</p> <p>Background</p> <p>GFP-fusion proteins and immunostaining are methods broadly applied to investigate the three-dimensional organization of cells and cell nuclei, the latter often studied in addition by fluorescence in situ hybridization (FISH). Direct comparisons of these detection methods are scarce, however.</p> <p>Results</p> <p>We provide a quantitative comparison of all three approaches. We make use of a cell line that contains a transgene array of lac operator repeats which are detected by GFP-lac repressor fusion proteins. Thus we can detect the same structure in individual cells by GFP fluorescence, by antibodies against GFP and by FISH with a probe against the transgene array. Anti-GFP antibody detection was repeated after FISH. Our results show that while all four signals obtained from a transgene array generally showed qualitative and quantitative similarity, they also differed in details.</p> <p>Conclusion</p> <p>Each of the tested methods revealed particular strengths and weaknesses, which should be considered when interpreting respective experimental results. Despite the required denaturation step, FISH signals in structurally preserved cells show a surprising similarity to signals generated before denaturation.</p

Simulation of the distribution of chromosome targets in cell nuclei under topological constraints

Abstract. Several models for the distribution of subchromosomal targets under topological constraints were developed which take into account that chromosomes occupy distinct, mutually exclusive territories in the cell nucleus. Nuclei and two pairs of chromosome territories of various size were modeled by spheres or ellipsoids under the simplified assumption that the entire set of chromosome territories present in a diploid cell nucleus completely fills the nuclear interior and that each territory occupies a fraction of the nuclear volume proportional to its DNA content. Monte Carlo simulations of the distribution of the territory gravity centers were performed taking into account the constraint of territory extension by the nuclear boundary and the constraint of territory self avoidance, i.e. territories should not intersect each other. In addition, various assumptions were made with regard to the location of point-like targets either within or at the surface of two &apos;homologous&apos; model territories. For each assumption the distance between the two point-like targets and between each target and the center of the model nucleus was calculated in Monte Carlo simulations and in part also analytically. The distribution of point-like targets in model nuclei under the influence of these topological constraints depends on the shape of the model nucleus and shows strong deviations from a model often applied in previous studies. In this model the random distribution of point-like targets was described under the assumption that such targets are distributed uniformly and independently from each other within the nuclear space without any constraints except for the nuclear boundary. All models were applied to experimentally measured distributions of chromosomal subregions delineated by fluorescence in situ hybridization with subregion specific probes. We demonstrate that a neglect of geometrical constraints in the simulation of target distributions can lead to erroneous conclusions of whether experimental target distributions occur in a random manner or not