A Constitutive Model for a Maturing Fibrin Network

Blood clot formation is crucial to maintain normal physiological conditions but at the same time involved in many diseases. The mechanical properties of the blood clot are important for its functioning but complicated due to the many processes involved. The main structural component of the blood clot is fibrin, a fibrous network that forms within the blood clot, thereby increasing its mechanical rigidity. A constitutive model for the maturing fibrin network is developed that captures the evolving mechanical properties. The model describes the fibrin network as a network of fibers that become thicker in time. Model parameters are related to the structural properties of the network, being the fiber length, bending stiffness, and mass-length ratio. Results are compared with rheometry experiments in which the network maturation is followed in time for various loading frequencies and fibrinogen concentrations. Three parameters are used to capture the mechanical behavior including the mass-length ratio. This parameter agrees with values determined using turbidimetry experiments and is subsequently used to derive the number of protofibrils and fiber radius. The strength of the model is that it describes the mechanical properties of the maturing fibrin network based on it structural quantities. At the same time the model is relatively simple, which makes it suitable for advanced numerical simulations of blood clot formation during flow in blood vessels.     

A Neurophysiologically Plausible Population Code Model for Feature Integration Explains Visual Crowding

An object in the peripheral visual field is more difficult to recognize when surrounded by other objects. This phenomenon is called “crowding”. Crowding places a fundamental constraint on human vision that limits performance on numerous tasks. It has been suggested that crowding results from spatial feature integration necessary for object recognition. However, in the absence of convincing models, this theory has remained controversial. Here, we present a quantitative and physiologically plausible model for spatial integration of orientation signals, based on the principles of population coding. Using simulations, we demonstrate that this model coherently accounts for fundamental properties of crowding, including critical spacing, “compulsory averaging”, and a foveal-peripheral anisotropy. Moreover, we show that the model predicts increased responses to correlated visual stimuli. Altogether, these results suggest that crowding has little immediate bearing on object recognition but is a by-product of a general, elementary integration mechanism in early vision aimed at improving signal quality.     

Quantitative aspects of the histochemical tetrazolium salt reaction on monoamine oxidase activity in rat liver

Summary The tetrazolium method for the histochemical detection of monoamine oxidase (MAO) activity in rat liver cryostat sections has been tested for its specificity and its possible use in quantification. The tetrazolium salt tetranitro blue tetrazolium is recommended for the localization of MAO activity, rather than nitro blue tetrazolium or BPST [2-(2-benzothiazolyl)-3(4-phthalhydrazidyl)-5-styryl-tetrazolium]. Hardly any formazan was produced in the absence of the substrate tryptamine and Marsilid, a specific inhibitor of MAO activity, prevented formazan production almost completely. A linear relationship between the integrated absorbance measured with a microdensitometer and either the incubation period or section thickness was obtained. We conclude that the method described in this paper can be used for the quantitative analysis of MAO activity in tissue sections of rat liver. MAO activity was found to be 20–25% higher in the periportal zone of rat liver than in the perivenous zone.     

Comparison of the Reconciliation Behavior of Stumptail and Rhesus Macaques

Observations were made following spontaneous aggressive incidents in a breeding group of captive stumptail macaques (Macaca arctoides). Participants were observed both during the first 10 min following the aggressive incident, and during matched-control observations. Data on 670 pairs of former opponents were collected, and compared with a sample of 573 such pairs of rhesus macaques (M. mulatta), which had been observed with identical methods in a previous study. Selective attraction between opponents was confirmed for the stumptail monkeys, i.e. both the absolute and the relative rate of nonagonistic body contact between individuals increased after aggression between them. The species was found to be considerably more conciliatory than rhesus monkeys, and to have a remarkably rich repertoire of reassurance gestures. The most characteristic conciliatory behavior is the hold-bottom ritual, in which one individual (usually the subordinate) presents its hindquarters, and the other (usually the dominant) clasps the other's haunches. The high rate of reconciliation among stumptail monkeys extended to all relationship classes, and a correlation with the closeness of social bonds (measured as time spent in association) could not be demonstrated. Because of the generality of the species' high peacemaking tendencies it is assumed that group cohesiveness is of great survival value in the wild.     

Tissue-specific expression directed by an Arabidopsis thaliana pre-ferredoxin promoter in transgenic tobacco plants

We have isolated and analyzed a pre-ferredoxin gene from Arabidopsis thaliana. This gene encodes a 148 amino acid precursor protein including a chloroplast transit peptide of 52 residues. Southern analysis shows the presence of a single copy of this ferredoxin (Fd) gene in the A. thaliana genome. Its expression is tissue-specific and positively affected by light. Response times, both to dark and light conditions, are remarkably rapid.A chimeric gene consisting of a 1.2 kb Fd promoter fragment fused to the -glucuronidase reporter gene was transferred to tobacco. This fusion gene is expressed in a tissue-specific way; it shows high levels of expression in green leaves, as compared to root tissue.     

Interaction between des-Tyr1-γ-endorphin and HLA class I molecules: Serological detection of an HLA-A2 subtype

Preincubation of lymphocytes with des-Tyr¹--endorphin (DTE) inhibits the reaction between some HLA alloantisera and their corresponding antigens. One HLA-A2-specific antiserum was found which could detect a subtype of the HLA-A2 antigen on DTE-treated lymphocytes from some donors. Comparison with the HLA-A2 subtypes as defined by a combination of cytotoxic T lymphocyte typing and biochemistry showed a complete correlation with the previously described HLA-A2.3 subtype.