In?Vitro Measurement of Particle Margination in the Microchannel Flow: Effect of Varying Hematocrit

It has long been known that platelets undergo margination when flowing in blood vessels, such that there is an excess concentration near the vessel wall. We conduct experiments and three-dimensional boundary integral simulations of platelet-sized spherical particles in a microchannel 30 μm in height to measure the particle-concentration distribution profile and observe its margination at 10%, 20%, and 30% red blood cell hematocrit. The experiments involved adding 2.15-μm-diameter spheres into a solution of red blood cells, plasma, and water and flowing this mixture down a microfluidic channel at a wall shear rate of 1000 s⁻¹. Fluorescence imaging was used to determine the height and velocity of particles in the channel. Experimental results indicate that margination has largely occurred before particles travel 1 cm downstream and that hematocrit plays a role in the degree of margination. With simulations, we can track the trajectories of the particles with higher resolution. These simulations also confirm that margination from an initially uniform distribution of spheres and red blood cells occurs over the length scale of O(1 cm), with higher hematocrit showing faster margination. The results presented here, from both experiments and 3D simulations, may help explain the relationship between bleeding time in vessel trauma and red blood cell hematocrit as platelets move to a vessel wall.     

CFD predicted pH gradients in lactic acid bacteria cultivations

The formation of pH gradients in a 700 L batch fermentation of Streptococcus thermophilus was studied using multi-position pH measurements and computational fluid dynamics (CFD) modeling. To this end, a dynamic, kinetic model of S. thermophilus and a pH correlation were integrated into a validated one-phase CFD model, and a dynamic CFD simulation was performed. First, the fluid dynamics of the CFD model were validated with NaOH tracer pulse mixing experiments. Mixing experiments and simulations were performed whereas multiple pH sensors, which were placed vertically at different locations in the bioreactor, captured the response. A mixing time of about 46 s to reach 95% homogeneity was measured and predicted at an impeller speed of 242 rpm. The CFD simulation of the S. thermophilus fermentation captured the experimentally observed pH gradients between a pH of 5.9 and 6.3, which occurred during the exponential growth phase. A pH higher than 7 was predicted in the vicinity of the base solution inlet. Biomass growth, lactic acid production, and substrate consumption matched the experimental observations. Moreover, the biokinetic results obtained from the CFD simulation were similar to a single-compartment simulation, for which a homogeneous distribution of the pH was assumed. This indicates no influence of pH gradients on growth in the studied bioreactor. This study verified that the pH gradients during a fermentation in the pilot-scale bioreactor could be accurately predicted using a coupled simulation of a biokinetic and a CFD model. To support the understanding and optimization of industrial-scale processes, future biokinetic CFD studies need to assess multiple types of environmental gradients, like pH, substrate, and dissolved oxygen, especially at industrial scale.     

Climate amenity and BMI

Objectives: Our goal was to examine the relationship between BMI and climate amenable for physical activity at the county level in the U.S. Research Methods and Procedures: Using Geographic Information Systems tools and 6‐year National Oceanic and Atmospheric Administration station hourly weather records, an index of amenable climate was derived for all U.S. counties. This index was linked to individual BMI in a multi‐level analysis that accounted for other individual characteristics from the 2002 survey of the Behavioral Risk Factor Surveillance System. Results: There was an inverse relationship between climate amenable to physical activity and BMI at the county level after controlling for individual risk factors, county road density, and median household income and unemployment rate. Residents in high climate‐amenity counties tended to have a lower BMI. Discussion: The contribution of less amenable climate to overweight and obesity in the U.S. is likely to be substantial because it cuts across wide geographic areas. Health promotion strategies that promote mixed land use or other urban design conducive to walking and other physical activities should consider broader environmental disamenities to mitigate their influence. Strategies for outdoor physical activity should also be tailored for people of different racial groups and educational backgrounds due to observed differences in their response to climate amenity.     

Sequences of three closely related variants of a complex satellite DNA diverge at specific domains

Major differences among the sequences of the repeat units of a very complex satellite DNA are located in domains which are sensitive to S1 nuclease under torsional stress, indicating that the domains assume unusual secondary or tertiary structures. Repeat units of the satellite, which accounts for 3% of the DNA of a land crab, have been inserted into pBR322 and the primary sequences of three cloned variants determined. The variants selected for sequencing include 1) RU (2089 base pairs (bp) ), representative of the average size of repeat units of cellular satellite; 2) TRU (1674 bp), truncated at an extra EcoRI site; and 3) EXT (2639 bp), extended by a 5-fold amplification of a 142-bp segment, one copy of which is present in RU and TRU (Bonnewell, V., Fowler, R.F., and Skinner, D.M. (1983) Science 221, 862-865). It appears that every copy of the satellite may be different and that the variants do not arise from cloning accidents. Extensive domains, as long as approximately 560 bp, are greater than 95% homologous among RU, TRU, and EXT; these conserved domains are composed of DNA whose base composition and sequences do not have remarkable features. By contrast, the sequences that comprise the divergent domains are unusually rich in 1) tracts of (dG X dC) 13-23 and arrangements of similar but not identical repetitive oligonucleotides or 2) alternating purines and pyrimidines (pu/py).     

Tubulin oligomers and microtubule assembly studied by time-resolved X-ray scattering: separation of prenucleation and nucleation events

This paper describes a time-resolved X-ray scattering study of microtubule assembly by synchrotron radiation. The method is complementary to light scattering but allows a better distinction between oligomeric and polymeric assembly states. With an improved rapid temperature jump device, it is shown that temperature-induced microtubule assembly is preceded by prenucleation and nucleation events involving oligomers of tubulin, in analogy with earlier results from near-equilibrium temperature scans. In general, the two phases closely overlap, but in certain conditions they can be observed separately. The prenucleation events seen by X-rays can be described as a rapid temperature-dependent equilibrium, with ring oligomers dissociating into smaller oligomers and subunits at elevated temperature. Different solution conditions affect mainly the time lag between the prenucleation and nucleation phases; this in turn determines the apparent magnitude of the prenucleation steps. By contrast, the temperature dependence of the equilibrium between the prenucleation oligomers shows little influence on solution conditions. The results suggest that the ring-forming and tubule-forming assembly modes of tubulin are governed by different interactions between subunits, although they may be based on a pool of similar intermediates.     

Relation of fatty acid composition in lead-exposed mallards to fat mobilization,lipid peroxidation and alkaline phosphatase activity

The increase of n-6 polyunsaturated fatty acids (PUFA) in animal tissues has been proposed as a mechanism of lead (Pb) poisoning through lipid peroxidation or altered eicosanoids metabolism. We have studied fatty acid (FA) composition in liver and brain of mallards (Anas platyrhynchos) feeding for 3 weeks on diets containing combinations of low or high levels of vitamin E (20 or 200 UI/kg) and Pb (0 or 2 g/kg). Saturated FA, n-6 PUFA and total concentrations of FA were higher in livers of Pb-exposed mallards, but not in their brains. The percentage of n-6 PUFA in liver and brain was slightly higher in Pb-exposed mallards. The increase of n-6 PUFA in liver was associated with decreased triglycerides and increased cholesterol in plasma, thus could be in part attributed to feed refusal and fat mobilization. The hepatic ratios between adrenic acid (22:4 n-6) and arachidonic acid (20:4 n-6) or between adrenic acid and linoleic acid (18:2 n-6) were higher in Pb exposed birds, supporting the existing hypothesis of increased fatty acid elongation by Pb. Among the possible consequences of increased n-6 PUFA concentration in tissues, we found increased lipid peroxidation in liver without important histopathological changes, and decreased plasma alkaline phosphatase activity that may reflect altered bone metabolism in birds.