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Biomedical subjects

P Schaaf

Publications and source records attributed to P Schaaf.

At least 55 records · Page 3Linked to original sources
PubMed1995

[Rheologic and biophysical aspects of cellular adhesion and aggregation: importance in hemorheology].

Interactions between blood cells in the form of cellular aggregates or adhesion are observed in a variety of normal and pathological conditions. Aggregation of erythrocytes or platelets, adhesion of platelets and leucocytes and immune agglutination of RBC are examples of interactions involving blood cells. Cell adhesion and aggregation are modulated by specific interactions (antigen-antibodies reactions, adhesive macromolecules interactions...) or non-specific (van der Waals forces, electrostatic interactions, molecular bridgings...). These interactions may result in morphological and structural changes, or polarization phenomena. At the dynamic level, cellular adhesion (or aggregation) can divided in 4 main steps: transport, cellular activation (endogenous or exogenous), morphological, physical or steric rearrangements, contact (intercellular or on an artificial surfaces). It will be the nature of the interactions involved in these steps that will determine the binding cohesion and kinetic. In this paper, different types of interactions and the regulation mechanisms of adhesion and aggregation phenomena involved in blood hemodynamics will be summarized and some examples (RBC or platelets aggregation; platelets or leucocytes adhesion) will illustrate the importance of these phenomena in clinical hemorheology.

Biophysical Phenomena↗
PubMed1994

Fluctuation of the number of particles deposited on a flat surface by a random sequential adsorption mechanism.

The problem of the fluctuations of the number n of particles adsorbed on surfaces through a random sequential adsorption process is discussed. Attention is paid, in particular, to the effect of the size of the adsorbing surfaces upon the variance sigma2(n) of this number. On the basis of computer simulations, it is shown that sigma2(n) is not proportional to the area a of the surface but can be written as a sum of three contributions, which are proportional to a, a1/2, and a0. A theoretical estimate based on the relation between the radial distribution function g(r) and the fluctuation is presented and provides a basis for these findings. This analysis is of general validity and can, in particular, also be applied to the equilibrium case (in the absence of a phase transition) and to the ballistic deposition process.

Journal Article↗
PubMed1994

Kinetics of exchange processes in the adsorption of proteins on solid surfaces.

The homogeneous exchange process whereby IgG molecules adsorbed onto latex particles are replaced by IgG molecules from the bulk solution was studied by means of 125I radiolabeling. The exchange mechanism was investigated on surfaces saturated with either labeled or unlabeled proteins in the presence of a solution of the opposite species in two sets of independent experiments. After rinsing of the surface by pure buffer followed by supplementary IgG adsorption, the exchange process followed a kinetic law of first order with respect to the IgG molecules from the bulk solution, and the apparent exchange rate constant was (2.3 +/- 0.4) x 10(-5) cm.hr-1.

Adsorption↗
PubMed1994

Development of the rabbit retina. V. The question of 'columnar units'.

A qualitative and quantitative description of the columnar units in the mammalian retina, and a discussion of their ontogeny and putative functions is given. Columnar arrangements of cells exist in the developing retina which can be observed by means of scanning electron microscopy. In the adult retina, each Müller cell ensheaths a columnar group of neuronal cells. Counting the number of cells in radial H/E stained sections at various developmental stages reveals a constant ratio of neuronal cells per Müller cell, independent of the developmental stage (after postnatal day 9), and independent of the retinal topography. Such groups of cells always consist of one Müller cell, 11 rod photoreceptor cells, about 2 bipolar cells, and 1 to 2 amacrine cells. Retinal ganglion cells, cone photoreceptor cells, and horizontal cells are more sparsely distributed in the retina than these units; since they are known to arise earlier in the ontogenesis than other cell types they are considered to exist independently of the columnar units. It is suggested that the units arise by migration of groups of preneurons along a common Müller (precursor) cell; these preneurons and the corresponding Müller cell may be clonally related. In the adult retina, such columns might constitute metabolic and functional units.

Animals↗
PubMed1994

Adhesion of hard spheres under the influence of double-layer, van der Waals, and gravitational potentials at a solid/liquid interface.

The deposition process of colloidal particles or microorganisms on flat surfaces is analyzed by means of computer simulations. Interparticle interactions (double layer and van der Waals) and weak gravitational forces are taken into account; hydrodynamic interactions, on the other hand, are neglected. In particular, the deposition probability as a function of the deposition location of a particle in the presence of one or two identical fixed particles is discussed. It is shown, in particular, that the ratio of the adhesion probabilities at a given location r, for particles subject to weak gravitation, in the presence and in the absence of the interparticle interaction U(r) follows approximately a Boltzmann law exp[-U(r)/kT], even though the adsorption process is fully irreversible. This result validates, as far as the distribution function of particles on a surface is concerned, Adamczyk's assumption [Adamczyk, Z., Zembala, M., Siwek, B. & Warszynski, P. (1990) J. Colloid Interface Sci. 140, 123-137] that the adhesion process of Brownian particles can be modeled by a random sequential adsorption model with an adsorption probability equal to exp[-U(r)/kT].

Adsorption↗
PubMed1994

Resistance to insulin-stimulated glucose uptake and dyslipidemia in Asian Indians.

Persons from the Indian subcontinent have elevated coronary heart disease risk. We measured insulin resistance with the insulin suppression test in 22 Asian Indian men and women and an equal number of control subjects of European ancestry matched for age and body mass index. Asian men and women had increased glucose and insulin responses to oral glucose tolerance tests (P < .05 by ANOVA) and had approximately 60% higher steady-state plasma glucose levels during the insulin suppression test (P < .001 by ANOVA), consistent with insulin resistance. In response to mixed meals, Asian women had higher plasma free fatty acids and glycerol concentrations than women of European ancestry (P < .02 by ANOVA), whereas Asian Indian men had similar free fatty acid and glycerol levels compared with men of European ancestry despite higher plasma insulin levels. Thus, results in both sexes were consistent with resistance to insulin suppression of free fatty acid levels in Asian Indians. Asian Indians of both sexes had higher fasting plasma triglyceride (P < .01) and lower high-density lipoprotein cholesterol (P < .01) concentrations than men and women of European ancestry. Resistance to insulin-stimulated glucose uptake and to insulin suppression of free fatty acid levels in Asian Indians is associated with a number of metabolic abnormalities that are demonstrated risk factors for coronary heart disease, including increased glucose, insulin, and triglyceride concentrations and decreased high-density lipoprotein cholesterol concentrations.

Asia↗
PubMed1993

Influence of diffusion and gravity on the adhesion of a two-component mixture of hard spheres on a flat surface.

The adhesion of hard spheres, modeling particles of biological interest (proteins, bacteria, cells), on flat surfaces is investigated by means of Monte Carlo simulations. These computations include the Brownian diffusion of the particles in the bulk fluid, as well as the systematic displacement due to the gravitational field. The size of the particles influences directly both diffusion coefficient and net weight, with the consequence that the coverage at the jamming limit depends on this parameter. Results obtained in a former paper based on a lattice model are confirmed by the present continuous space model. In order to gain a better understanding of the adsorption competition of two types of particles, the proposed model is applied to the case of binary mixtures of spheres. For polydispersed suspensions, various parameters determine the final coverage, as well as the distribution of the small and large particles on the surface: the radii of the particles and the respective proportions of them in the infinitely large reservoir from which they are randomly selected. In this way, it is shown that the chronology of the adhesion of the small and large particles strongly influences the final number of each type of spheres fixed on the surface. Qualitatively, the present results resemble those obtained with disks placed by means of a classical random sequential adsorption mechanisms. Quantitatively, however, the number densities and coverage values determined in this way are significantly different due to the inclusion of the gravity and of the diffusion in the model.

Adhesiveness↗
PubMed1992

Configurations of adsorbed hard spheres after diffusion in a gravitational field.

The deposition and adhesion of particles on a solid surface are governed by a great number of interplaying forces. In this paper we analyze, by means of computer simulations, the influence of (i) the short-range repulsive forces, modeled by hard sphere interactions, (ii) the gravitational forces, and (iii) the diffusion process of the particles in the fluid on the structure of the surface covered by the particles. In particular, the evolution of the limiting coverage, [symbol, see text] infinity (where [symbol, see text] is the reduced relative surface coverage), and the radial distribution, g(r), at the jamming limit, are determined as a function of the gravitational forces. These forces play an important role in many experiments performed on latex beads. Our results should stimulate new experiments in this field and, thus, be directly experimentally tested. It is shown, for example, that for polystyrene particles [symbol, see text] infinity is constant and equal to the random sequential adsorption jamming limit value for radii R not larger than 1 micron. It increases for 1 </= R </= 3 micron and tends, for higher R, to a plateau, whose value is approximately equal to 0.61. The tendency to a closer packing when R is large, and thus large gravitational forces, is confirmed by the shape of the radial distribution function. This phenomenon occurs not only for jammed surfaces but also for unsaturated surfaces.

Journal Article↗
PubMed1992

Competitive adsorption of human immunoglobulin G and albumin: consequences for structure and reactivity of the adsorbed layer.

The affinity of polyclonal anti-IgG for human IgG adsorbed on silica surfaces was investigated by two complementary techniques, scanning angle reflectometry and 125I radiotracing. Special attention was paid to compare the reactivity of IgG adsorbed directly or by exchange with already adsorbed albumin. In particular it was shown that (i) in the first case (direct adsorption) the reaction between anti-IgG and adsorbed IgG was in the ratio 1:1 and (ii) in the second case (adsorption by exchange) there was no reaction.

Adsorption↗
PubMed1992

Neuronal ectopia in tiger retina.

Ocular pathology in a ten week-old tiger is described. In the retinae of both glaucomatous eyes, venous congestion and scattered hemorrhages were observed; rare perivascular infiltrates were seen. The basal lamina of the inner limiting membrane was considerably thickened, and serous exudates were widely distributed within the vitreous body. The retina was studied with a series of specific antibodies. Müller (glial) cells were well developed and could be immuno-labelled by antibodies against both vimentin and glial fibrillary protein (GFAP). Neurofilament-specific antibodies revealed the presence of ganglion and horizontal cells, and of a nerve fibre layer which was unusually distant from the inner limiting membrane. Within both plexiform layers, scattered neuronal cells were found. The most important finding was the presence of cells between the nerve fibre layer and the inner limiting membrane, which could be immuno-labelled by neuron-specific antibodies, and which expressed thy typical morphology of migrating neuroblasts. It is suggested that this neuronal ectopia might be due to an inflammatory related reactive change in Müller cells which, in turn, might have lost their orderly guiding function for migrating neuroblasts.

Animals↗
PubMed1991

Adsorption/desorption of human serum albumin on hydroxyapatite: a critical analysis of the Langmuir model.

We studied the adsorption of human albumin onto synthetic hydroxyapatite, using a radiotracer technique and a special flow cell. Adsorption was studied under various conditions corresponding to different thermodynamic paths. It appears that (i) as is the usual case, the isotherms obtained within a short time range (a few hours) do not correspond to a true equilibrium situation; (ii) when the adsorption process is followed for longer times, which is necessary at low bulk concentrations, one always reaches the plateau surface adsorption; (iii) this plateau value is independent of the "history" of the adsorption process and corresponds well to the jamming limit predicted by the random sequential adsorption model; and (iv) surface denaturation, leading to enhanced surface binding and thus decreasing desorption constants, is the important phenomenon that can partly and qualitatively explain our observations. Its time dependence, however, remains to be clarified.

Adsorption↗
PubMed1990

Primary neurulation in teleosts--evidence for epithelial genesis of central nervous tissue as in other vertebrates.

Early teleostian embryos were studied by scanning electron microscopy. Transversal and longitudinal views of the neural anlage clearly demonstrate that it has an ordinary epithelial organization consisting of parallel columnar cells. Unless it has been supposed earlier, there is no solid thickened neural plate but the neural ectoderm is tightly folded forming a very narrow neural groove. Thus, primary neurulation in teleosts is shown to occur similar to that in other vertebrates viz. by folding of the neural plate to get the neural tube. That means that the neuroepithelial cells retain their polarity instead to become organized from a randomly oriented mass of unpolarized cells, as it is thought to occur in secondary neurulation.

Animals↗