Cells Of Different Sizes Research Articles

On the Ewald Artifacts in Computer Simulations. The Test-Case of the Octaalanine Peptide With Charged Termini

The treatment of electrostatic interactions in molecular simulations is of fundamental importance. Ewald and related methods are being increasingly used to the detriment of cutoff schemes, which are known to produce several artifacts. A potential drawback of the Ewald method is the spatial periodicity that is imposed to the system, which could produce artifacts when applied in the simulation of liquids. In this work we analyze the octaalanine peptide with charged termini in explicit solvent, for which severe effects due to the use of Ewald sums were predicted using continuum electrostatics. Molecular Dynamics simulations for a total of 158 nanoseconds were performed in cells of different sizes. From the comparison of the results of different system sizes, no significant periodicity-induced artifacts were observed. It is argued that in current biomolecular simulations, the incomplete sampling is likely to affect the results to a larger extent than the artifacts induced by the use of Ewald sums.

Efficient Monte Carlo methods for continuum radiative transfer

We discuss the efficiency of Monte Carlo methods in solving continuum radiative transfer problems. The sampling of the radiation field and convergence of dust temperature calculations in the case of optically thick clouds are both studied. For spherically symmetric clouds we find that the computational cost of Monte Carlo simulations can be reduced, in some cases by orders of magnitude, with simple importance weighting schemes. This is particularly true for models consisting of cells of different sizes, for which the run times would otherwise be determined by the size of the smallest cell. The use of importance weighting is extended to scattered photons, which is found to be useful in calculations of scattered flux and could be important for three-dimensional models when observed intensity is needed only for one general direction of observations. Convergence of dust temperature calculations is studied for models with optical depths TV = 10-10 4 . We examine acceleration methods where radiative interactions inside a cell or between neighbouring cells are treated explicitly. In optically thick clouds with strong self-coupling between dust temperatures, the run times can be reduced by more than one order of magnitude. Use of a reference field was also examined. It eliminates the need for repeating simulation of constant sources (e.g., background radiation) after the first iteration and it was found to significantly reduce sampling errors. We finally discuss the applicability of our methods to three-dimensional models.

LIFE CYCLE, SIZE REDUCTION PATTERNS, AND ULTRASTRUCTURE OF THE PENNATE PLANKTONIC DIATOM PSEUDO‐NITZSCHIA DELICATISSIMA (BACILLARIOPHYCEAE)1

Pseudo‐nitzschia delicatissima (Cleve) Heiden is a very common pennate planktonic diatom found in temperate marine waters, where it is often responsible for blooms. Recently, three distinct internal transcribed spacer types have been recorded during a P. delicatissima bloom in the Gulf of Naples (Mediterranean Sea, Italy), which suggests the existence of cryptic diversity. We carried out mating experiments with clonal strains belonging to the most abundant internal transcribed spacer type. Pseudo‐nitzschia delicatissima is heterothallic and produces two functional anisogametes per gametangium. The elongated auxospore possesses a transverse and a longitudinal perizonium. The sexual phase was observed to occur over a wide size spectrum, spanning 19–80 μm and corresponding to almost the whole range of cell length observed for P. delicatissima. We also investigated cell morphology, valve ultrastructure and morphometry of parental, F1‐generation strains, and the progeny of crosses between parental and F1 strains. Although ultrastructural features match those described for P. delicatissima, variability in cell shape was recorded in the largest cells of the F1 generation as well as in valves with an abnormal arrangement of poroids. As many other diatoms, P. delicatissima undergoes size reduction over its life cycle, and cells of different size showed differences in growth rates and the amount of size reduction per cell cycle. Cells between 60 and 30 μm in length showed the fastest growth and the slowest rates of size reduction per generation. In culture, P. delicatissima cells can decrease to 8 μm in length; however, such small cells (≤30 μm) are not recorded in the sea, and this raises interesting questions about the factors that control their survival in the natural environment.

Growth Rate and Cell Size Modulate the Synthesis of, and Requirement for, G1-Phase Cyclins at Start

In Saccharomyces cerevisiae, commitment to cell cycle progression occurs at Start. Progression past Start requires cell growth and protein synthesis, a minimum cell size, and G(1)-phase cyclins. We examined the relationships among these factors. Rapidly growing cells expressed, and required, dramatically more Cln protein than did slowly growing cells. To clarify the role of cell size, we expressed defined amounts of CLN mRNA in cells of different sizes. When Cln was expressed at nearly physiological levels, a critical threshold of Cln expression was required for cell cycle progression, and this critical threshold varied with both cell size and growth rate: as cells grew larger, they needed less CLN mRNA, but as cells grew faster, they needed more Cln protein. At least in part, large cells had a reduced requirement for CLN mRNA because large cells generated more Cln protein per unit of mRNA than did small cells. When Cln was overexpressed, it was capable of promoting Start rapidly, regardless of cell size or growth rate. In summary, the amount of Cln required for Start depends dramatically on both cell size and growth rate. Large cells generate more Cln1 or Cln2 protein for a given amount of CLN mRNA, suggesting the existence of a novel posttranscriptional size control mechanism.

Coupling gravitational and flow field-flow fractionation, and size-distribution analysis of whole yeast cells.

This work continues the project on field-flow fractionation characterisation of whole wine-making yeast cells reported in previous papers. When yeast cells are fractionated by gravitational field-flow fractionation and cell sizing of the collected fractions is achieved by the electrosensing zone technique (Coulter counter), it is shown that yeast cell retention depends on differences between physical indexes of yeast cells other than size. Scanning electron microscopy on collected fractions actually shows co-elution of yeast cells of different size and shape. Otherwise, the observed agreement between the particle size distribution analysis obtained by means of the Coulter counter and by flow field-flow fractionation, which employs a second mobile phase flow as applied field instead of Earth's gravity, indicates that yeast cell density can play a major role in the gravitational field-flow fractionation retention mechanism of yeast cells, in which flow field-flow fractionation retention is independent of particle density. Flow field-flow fractionation is then coupled off-line to gravitational field-flow fractionation for more accurate characterisation of the doubly-fractionated cells. Coupling gravitational and flow field-flow fractionation eventually furnishes more information on the multipolydispersity indexes of yeast cells, in particular on their shape and density polydispersity.

Multi-scale Tomography for Crustal P and S Velocities in Southern California

Seismic tomography is a viable tool in building depth-velocity models in the presence of strong lateral velocity variations. In this study 3-D P- and S-velocity models for the crust of southern California are constrained using more than 1,000,000 P-wave first arrivals and 130,000 S-wave arrivals from local earthquakes. To cope with the uneven distribution of raypaths, a multi-scale tomography is applied with overlapping model cells of different sizes. Within the 300 × 480 × 39 km3 model volume, the smallest cell size is 10 × 10 × 3 km3. During the iterations of velocity updating, earthquake hypocenters are determined using both P and S arrivals, and full 3-D ray tracing is implemented. Except near the edges and in the lower crust, the resultant models are robust according to various tests on the effects of reference models, resolution and signal-to-noise ratio. The tomographic velocities at shallow depths correlate very well with the regional geology of southern California. In the upper crust the P-wave and S-wave models exhibit slow velocities in major sedimentary basins and fast velocities in areas of crystalline rocks. Mid-crustal low velocity zones are present under the Coso Range, San Gabriel Mountains, and a large portion of the Mojave Desert. P- and S-velocity patterns maintain their similarity in the lower crust though the models are less reliable there.

Analysis of Vibration and Wave Propagation in Cylindrical Grid‐Like Structures

The wave propagation in and the vibration of cylindrical grid structures are analyzed. The grids are composed of a sequence of identical elementary cells repeating along the axial and the circumferential direction to form a two‐dimensional periodic structure. Two‐dimensional periodic structures are characterized by wave propagation patterns that are strongly frequency dependent and highly directional. Their wave propagation characteristics are determined through the analysis of the dynamic properties of the unit cell. Each cell here is modelled as an assembly of curved beam elements, formulated according to a mixed interpolation method. The combined application of this Finite Element formulation and the theory of two‐dimensional periodic structures is used to generate the phase constant surfaces, which define, for the considered cell lay‐out, the directions of wave propagation at assigned frequencies. In particular, the directions and frequencies corresponding to wave attenuation are evaluated for cells of different size and geometry, in order to identify topologies with attractive wave attenuation and vibration confinement characteristics. The predictions from the analysis of the phase constant surfaces are verified by estimating the forced harmonic response of complete cylindrical grids, obtained through the assembly of the unit cells. The considered analysis provides invaluable guidelines for the investigation of the dynamic properties and for the design of grid stiffened cylindrical shells with unique vibration confinement characteristics.

Foam Structure: From Soap Froth to Solid Foams

AbstractThe properties of solid foams depend on their structure, which usually evolves in the fluid state as gas bubbles expand to form polyhedral cells. The characteristic feature of foam structure—randomly packed cells of different sizes and shapes—is examined in this article by considering soap froth. This material can be modeled as a network of minimal surfaces that divide space into polyhedral cells. The cell-level geometry of random soap froth is calculated with Brakke's Surface Evolver software. The distribution of cell volumes ranges from monodisperse to highly polydisperse. Topological and geometric properties, such as surface area and edge length, of the entire foam and individual cells, are discussed. The shape of struts in solid foams is related to Plateau borders in liquid foams and calculated for different volume fractions of material. The models of soap froth are used as templates to produce finite element models of open-cell foams. Three-dimensional images of open-cell foams obtained with x-ray microtomography allow virtual reconstruction of skeletal structures that compare well with the Surface Evolver simulations of soap-froth geometry.

Taylor Averaging on Heterogeneous Foams

In this article we present a formulation to account for the effect of cell size distribution on the mechanical behavior of foams. The present approach averages the response of unit cells with different sizes via a Taylor averaging approach. This technique can be applied to different unit cell models where the cell size is explicitly given. In order to keep the averaging formulation analytically traceable, we select a unit cell model, bubble model (Ortiz, M., Gioia, G. and Cuitino,A.M. Manuscript in preparation), that considers the behavior of an homogeneous array of thin-walled spherical cells undergoing uniform finite deformation. This model relays on the computation of the strain energy density for the deformed configuration due to membrane stretching and bending. For this unit cell model, the effect of the cell size distribution is described only by the first three moments of the distribution, or the mean radius, variance and skeweness. The effect of these parameters on the foam behavior subjected to uniaxial compression is analyzed for two different values of the relative density of the foam. The model is compared against a unixial compression test of a ductile foam system with nearly spherical cells of different sizes. The cell size distribution is measured independenly and introduced to the model as a input. The prediction of load displacement response is in general good agreement with the experimental test.

A Fast Algorithm for Solving the Poisson Equation on a Nested Grid

We present a numerical method for solving the Poisson equation on a nested grid. A nested grid consists of uniform grids having different grid spacing and is designed to cover the space closer to the center with a finer grid. Thus, our numerical method is suitable for computing the gravity of a centrally condensed object. It consists of two parts: the difference scheme for the Poisson equation on the nested grid and the multigrid iteration algorithm. It has three advantages: accuracy, fast convergence, and scalability. First, it computes the gravitational potential of a close binary accurately up to the quadrupole moment, even when the binary is resolved only in the fine grids. Second, the residual decreases by a factor of 300 or more with each iteration. We confirmed experimentally that the iteration always converges to the exact solution of the difference equation. Third, the computation load of the iteration is proportional to the total number of the cells in the nested grid. Thus, our method gives a good solution at a minimum expense when the nested grid is large. The difference scheme is applicable also to adaptive mesh refinement, in which cells of different sizes are used to cover a domain of computation.

Performance of a CDMA-based HCS network with hybrid speed∕overflow-sensitive handover strategy

In a mixed cell architecture, in which cells of different sizes co-exist, the interaction between layers has a major impact on radio resource management. Such interaction can be in terms of directing the traffic overflow from the lower layer to the overlaying layer (overflow-sensitive handover) and/or of classifying the users based on their speed (speed-sensitive handover). In this study, the implementation of a hybrid speed/overflow-sensitive layer assignment strategy in a CDMA-based hierarchical cell structure (HCS) network is studied and its impact on the signalling load and capacity of the system is compared with three other assignment strategies. A new approach to controlling the signalling load in such a network is also proposed. The results indicate that by using the proposed strategy, the signalling load can be controlled even in an area in which the mobile stations have a time-varying speed distribution.

Leukocyte Fractionation by Lateral Migration

Lateral migration has been used to fractionate sheep leukocytes. The results obtained suggest that this method could be used to fractionate biological cells of different sizes. These experimental results are presented here and the important parameters that will affect the recovery and purity of the desired cells are discussed. Lateral migration is most suitable for the removal of larger cells which contaminate a dilute suspension of smaller cells.

Muricauda ruestringensis Has an Asymmetric Cell Cycle

Muricauda ruestringensis B 1 is a GRAM-negatives marine bacterium and a member of the Flavobacteriaceae family. It is characterized by long appendages, which appear at different stages of growth. At the outer end of these appendages there is a bulbous structure. Investigating the cell morphology of strain B I during batch growth revealed a high diversity of cell types and sizes. Apart from small rod-shaped cells and rods with appendages, there were large rods and spherical cells of different sizes as well as spherical cells which had fimbriae. To be able to study the cell cycle events, it was essential to monitor the population dynamics of the involved individuals. For this purpose, fluorochromising techniques, multi-parametric flow cytometry, image analysis and fluorescence microscopy were used. It was demonstrated that all cell types displayed a broad variation in DNA content; the precise number of chromosomes varied depending on the growth phase. The assortment was testified to hold 16S rDNA sequence identity. The cultures consisted of subpopulations whose density within a Percoll gradient varied considerably, ranging from 1.028 to 1.070. Consolidating the results of the morphological data, the chromosome content and the density of the subpopulations at different growth stages enabled us to construct an asymmetric cell cycle for the growth of strain B I under the specific culture conditions of our experiments.

Diagnosis failure in preimplantation genetic diagnosis (PGD): poor technique or bad embryos?

Objective: To investigate the causes of diagnosis failure observed in the course of PGD. Design: In the course of 41 PGD cycles performed by the Béclère-Necker hospitals in Paris within a one year period, we correlated the efficiency of diagnosis with embryo grades and their developmental potential. Materials/Methods: We started our newly created PGD program in November 1999 and have performed 41 cases in just over a one year period. Patients were hyperstimulated using GnRh agonist and Recombinant FSH. The embryos were cultured in IVF (MediCult) until day 2 and placed in M3 (MediCult) until day 3. One or two cells were removed from each embryos by cleavage stage embryo biopsy on day 3 using a fertilase laser for zona drilling in G2.2 medium (JCD). The embryos were then placed in G2.2 until the transfer procedure on day 4. PGD was performed by PCR (23 cases) for a range of single gene defects or by FISH for X-linked recessive diseases (5 cycles) and translocations (13 cycles). Embryo grades on day 3 and signs of compaction on day 4 were recorded for each embryo and correlated with the availability of diagnosis. Grade 1 embryos had cells of even size with no or little fragmentation. Grade 2 had cells of even size with less than 20% fragments and grade 3 were highly fragmented embryos associated or not with cells of different size. Statistical significance of the results was calculated using a χ2 test. Results: Overall, a diagnosis could be obtained for 187 of the 237 embryos analysed (79%). From the 50 embryos for which no diagnosis could be obtained, 40 were of grade 3 (80%) on day 3 and only 7 showed signs of compaction (14%) on day 4. These data were significantly different from those obtained from the group of embryos for which a diagnosis was obtained in which 40% were of grade 3 and 40% subsequently compacted. Embryo grade on day 3 and signs of compaction on day 4 depending on whether a diagnosis was obtained. ∗TotalNumber of embryos on day 3 (%)Number of embryos on day 4 (%)Grade 1Grade 2Grade 3CompactedArrestedDiagnosis18741 (22)71 (38)75 (40)∗p < 0.001.75 (40)∗p < 0.001.112 (60)No diagnosis503 (6)7 (14)40 (80) ∗p < 0.001.7 (14) ∗p < 0.001.43 (86)Total23744 (19)78 (33)115 (48)82 (35)155 (65)∗ p < 0.001. Open table in a new tab Conclusions: No diagnosis could be obtained from 21% of the embryos biopsied, however our study demonstrates that most of these embryos had a poor morphology and a poor developmental potential. We can speculate that the blastomeres analysed from theses embryos were arrested with degraded DNA which was not longer a suitable target for PCR or FISH analysis. We conclude that most untyped embryos would not have been good candidates for transfer. Supported by: This work was supported in part by the AFM.

Li ion batteries for aerospace applications

Rechargeable Li ion batteries are perceived as likely substitutes for conventional nickel systems in an effort to minimize the mass and volume of the power subsystems in aerospace applications. The on-going consortium of NASA and DoD, after 2 years of existence, has propelled the advancement of aerospace Li ion technology in the US. Prototype cells of different sizes have been built by domestic manufacturers and are being evaluated both by NASA and Air Force. The early versions of these prototypes catered to needs of imminent NASA missions, i.e. Mars landers and rovers. Developmental efforts are underway to further improve the technology to meet the demands of long calendar life, as in the geosynchronous earth orbit (GEO) and the outer planets missions, and long cycle life as in the low earth orbit (LEO) missions. In this paper, we will briefly describe the objective and progress of this joint effort.

Simulation of the radon and radon progeny detection by silicon photodiodes.

The use of silicon photodiodes for field radon measurements requires the utilization of measuring cells for protection against environmental electromagnetic fields. It is very important to study and optimize the impact of geometry conditions (dimensions of the measuring cell) on the photodiode detection efficiency that can be reduced more than 10-30 times. Two models (for volume-distributed nuclides around the detector and for radon progeny deposited on the diode surface radon progeny) were applied for simulation of the photodiode detection in measuring cells of different sizes. Their use allows an optimal choice of the most appropriate cell regarding the measurement conditions. Thus, for soil gas measurements cells we use a cell of 3.6 cm in diameter by 20 cm in height; for determination of the radon exhalation rate we use a cell of 16 cm in diameter by 14 cm in height; and for measurements in dwellings and large spaces we use cells larger than 20 cm and lower than 4 cm.

Pod-2, along with pod-1, Defines a New Class of Genes Required for Polarity in the Early Caenorhabditis elegans Embryo

The asymmetric division of the one-cell Caenorhabditis elegans zygote gives rise to two cells of different size and fate, thereby establishing the animal's anterior–posterior (a-p) axis. Through genetics, a number of genes required for this polarity have been characterized, but many components remain unidentified. Recently, our laboratory discovered a mutation in the pod-1 gene (for polarity and osmotic defective) that uniquely perturbed polarity and osmotic protection. Here, we describe a new C. elegans polarity gene identified during screens for conditional embryonic lethals. Embryos in which this gene has been mutated show a loss of physical and developmental asymmetries in the one-cell embryo, including the mislocalization of PAR and POD-1 proteins required for early polarity. Furthermore, mutant embryos are osmotically sensitive, allowing us to designate this gene pod-2. Thus, pod-2, along with pod-1, defines a new class of C. elegans polarity genes. Genetic analyses indicate that pod-2 functions in the same pathway as pod-1. Temperature-shift studies indicate that pod-2 is required during oogenesis, indicating that aspects of embryonic polarization may precede fertilization. pod-2 mutant embryos also exhibit a unique germline inheritance defect in which germline identity localizes to the wrong spot in the one-cell embryo and is therefore inherited by the wrong cell at the four-cell stage. Our data suggest that pod-2 may be required to properly position an a-p polarity cue.

Cellular communications using aerial platforms

This paper is devoted to the study of cellular communications using aerial platforms (APs). A set of key equations is derived that quantify the coverage area on the ground as a function of AP elevation, the operation of the adaptive multibeam antenna on the AP, and the formulation of contiguous terrestrial cells and their shapes. Specifically, we consider the deployment of an AP to provide terrestrial mobile radio communications using the universal mobile telecommunication system operating in its wide-band code-division multiple-access mode. Calculations are made of the number of users versus E/sub b//N/sub 0/ for different service rates. Multitiered cellular structures having cells of different size that are steerable with the offered teletraffic are examined. The array structure to achieve this is identified. The preliminary results shows that an AP at a height of 21 km covers an area of radius 517 km. Up to 21 users per cell with a service rate of 8 kb/s can be accommodated in the 3.2-GHz band. These services can be provided within an area of radius 70 km with transmitted powers of less than 1 W. High system capacity is proved to be possible by constructing cells of radius as small as 100 m using square planar arrays with dimensions of less than 12 m /spl times/12 m. The AP system provides high capacity and Doppler frequency shifts that only originate from roving mobiles.

Polarity controls forces governing asymmetric spindle positioning in the Caenorhabditis elegans embryo.

Cell divisions that create daughter cells of different sizes are crucial for the generation of cell diversity during animal development. In such asymmetric divisions, the mitotic spindle must be asymmetrically positioned at the end of anaphase. The mechanisms by which cell polarity translates to asymmetric spindle positioning remain unclear. Here we examine the nature of the forces governing asymmetric spindle positioning in the single-cell-stage Caenorhabditis elegans embryo. To reveal the forces that act on each spindle pole, we removed the central spindle in living embryos either physically with an ultraviolet laser microbeam, or genetically by RNA-mediated interference of a kinesin. We show that pulling forces external to the spindle act on the two spindle poles. A stronger net force acts on the posterior pole, thereby explaining the overall posterior displacement seen in wild-type embryos. We also show that the net force acting on each spindle pole is under control of the par genes that are required for cell polarity along the anterior-posterior embryonic axis. Finally, we discuss simple mathematical models that describe the main features of spindle pole behaviour. Our work suggests a mechanism for generating asymmetry in spindle positioning by varying the net pulling force that acts on each spindle pole, thus allowing for the generation of daughter cells with different sizes.

Cell cycle-related changes in regulatory volume decrease and volume-sensitive chloride conductance in mouse fibroblasts

Cell cycle-related changes in the ability to regulate cell volume following hyposmotic swelling were studied in mouse fibroblasts using videomicroscopy and the whole-cell patch clamp technique. Regulatory volume decrease (RVD) and volume-sensitive Cl- conductance (G(Cl,vol)) were measured: (1) in proliferating cells of different sizes; (2) in cells arrested in defined phases of the cell cycle (G1, G1/S, S, and M phases) using mevastatin, mimosine, hydroxyurea, aphidicolin, cytosine beta-D-arabinofuranoside, and taxol; and (3) in serum-starved cells (G(0) state). Cells in all groups were able to undergo RVD, although the cells approaching mitosis (i.e., the largest cells in proliferating cultures and the taxol-treated cells) had the lowest rates of shrinkage during RVD. In agreement with this finding, the density of G(Cl,vol) was stable in proliferating and cell cycle-arrested cells for most of the cell cycle, with the exception of the cells approaching mitosis and the new daughter cells where the density was decreased to half. The impairment of RVD was greatest in serum-starved cells which also had the lowest density of G(Cl,vol). We conclude that proliferating cells maintain an ability to recover from osmotic swelling as they progress through the cell cycle, although this ability may be compromised during mitosis.