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PubMed1998

A dynamic model of proliferation and differentiation in the intestinal crypt based on a hypothetical intraepithelial growth factor.

A widely accepted model of the temporal and spatial organization of proliferation and differentiation in intestinal epithelial is based on a cellular pedigree with all cells descending from a few active stem cells and undergoing a sequence of transitory divisions until the non-proliferating maturing cell stages develop. Model simulations have shown that such a pedigree concept can explain a large variety of data. However, so far there is neither a direct experimental proof for the existence of an intrinsic age structure in the transitory proliferative cell stages nor for the distinction between stem and transitory cells. It is our objective to suggest an alternative model which is based on evidence for intercellular communications such as might be mediated through gap junctions. We consider the diffusion of a hypothetical intraepithelial growth factor in a chain of cells which are connected via gap junctions. Individual cells can divide if a critical growth factor concentration is exceeded. Simulation studies show that the model is consistent with many observed features of the small intestinal crypt in steady state and after perturbation.

Animals↗
PubMed1971

Coupling of solute and solvent flows in porous lipid bilayer membranes.

The present experiments were designed to evaluate coupling of water and nonelectrolyte flows in porous lipid bilayer membranes (i.e., in the presence of amphotericin B) in series with unstirred layers. Alterations in solute flux during osmosis, with respect to the flux in the absence of net water flow, could be related to two factors: first, changes in the diffusional component of solute flux referable to variations in solute concentrations at the membrane interfaces produced by osmotic flow through the unstirred layers; and second, coupling of solute and solvent flows within the membrane phase. Osmotic water flow in the same direction as solute flow increased substantially the net fluxes of glycerol and erythritol through the membranes, while osmotic flow in the opposite direction to glycerol flow reduced the net flux of that solute. The observed effects of osmotic water flow on the fluxes of these solutes were in reasonable agreement with predictions based on a model for coupling of solute and solvent flows within the membrane phase, and considerably in excess of the prediction for a diffusion process alone.

Amphotericin B↗
PubMed2004

Born in a follicle--a historical perspective.

A review of major studies of tetrapod skin development since the 1870s illustrates how knowledge of structure and mechanism progressed through phases emphasizing Natural History, morphology, endocrinology, and tissue manipulation prior to the prevailing "molecular era." Each successive phase of investigation, while suffering from its own limitations and constraints, has produced conceptual advances. At various times, different systems in various organisms have been research models of choice for practical and/or technical reasons. Comparative studies of scaled and non-scaled integuments and appendages thereof, e.g., nails, claws, glands, hair, and especially feathers, revealed data that suggested new directions for research programs. Some non-mammalian models still offer unique opportunities for pursuit of specific questions pertinent to studies of hair: arguments between American and British schools concerning feather development that originated in the 1930s remain unresolved and may thus affect interpretation of recent investigations. The current emphasis on the study of diffusible molecules involved in papilla-follicle interactions in hair development and replacement can only be understood in the context of the interwoven history of questions relating sequentially to evolutionary homology, physiological controls of tissue homeostasis, embryonic induction, and, most recently, molecular genetics.

Animals↗
PubMed1988

A diffusion model of the transient response of the cochlear inner hair cell synapse.

One class of models of hair cell synaptic function that has been investigated in recent years consists of one or more reservoirs of synaptic material connected to other reservoirs and/or the synaptic cleft by means of diffusion paths. One such general model is considered here, comprising two reservoirs and a global source of synaptic material connected in series and releasing material into the synaptic cleft by a diffusion path characterized by an intensity-dependent permeability. The explicit form of the solution of the model for a sudden onset of stimulation is derived: The solution comprises two exponentially decaying terms plus a constant. This solution is shown to have a unique inverse. This allows the determination of the parameters of the model directly from experimental data from auditory-nerve fibers in the Mongolian gerbil. The behavior of the derived model parameters with variation of stimulus intensity is demonstrated, and implications for synaptic function are discussed.

Acoustic Stimulation↗
PubMed2001

Time-resolved study of biofilm architecture and transport processes using experimental and simulation techniques: the role of EPS.

Cellular material and extracellular polymeric substances are the basic structural elements in biofilm systems. The structure and role of EPS for biofilm development and metabolic processes have not been precisely determined and, therefore, have not yet been included as a necessary element in modelling and simulation studies. This is due to the difficulty of experimentally detecting the extracellular polymeric substances in situ and differentiating them from cellular material on the one hand, and to the subsequent uncertainty about appropriate models--e.g. rigid hindrances, porous microstructure or visco-elastic structure--on the other hand. In this work, we report on the use of confocal laser scanning microscopy to monitor the development of a monoculture biofilm of Sphingomonas sp. grown in a flow cell. The bacterial strain was genetically labelled resulting in strong constitutive expression of the green fluorescent protein. The development of extracellular polymeric substances was followed by binding of the lectin concavalin A to cell exopolysaccharides. The growth of the resulting strain was digitally recorded by automated confocal laser scanning microscopy. In addition, local velocity profiles of fluorescent carboxylate-modified microspheres were observed on pathlines throughout the biofilm. The CLSM image stacks were used as direct input for the explicit modelling and three-dimensional numerical simulation of flow fields and solute transport processes based on the conservation laws of continuum mechanics. At present, a strongly simplifying EPS-model is applied for numerical simulations. The EPSs are preliminarily assumed to behave like a rigid and dense hindrance with diffusive-reactive solute transport.

Biofilms↗
PubMed1980

Visual consequences of the foveal pit.

When viewing a uniform field through a small aperture moved circularly before the pupil of the eye, many observers report a diffuse dim spot subtending roughly 1 degree visual angle, which rolls around the fixation point in synchrony with the motion of the aperture. Measurements of the Stiles-Crawford maximum for different locations within the fovea confirm that small but systematic shifts in foveal directional sensitivity are responsible for this entoptic effect. A quantitative model is developed which accounts for the directional sensitivity shifts on the basis of refraction of light at the sloping sides of the foveal pit. Supporting evidence for the model is provided by a second entoptic phenomenon: fundal scatter from the image of a glare source on the eccentric retina is reflected by the foveal pit in such a way as to render it visible. Estimates of the slope of the foveal pit from this effect agree well with that predicted from the observed shifts in directional sensitivity.

Fixation, Ocular↗
PubMed2003

Estimation of temperature transients for biomass pretreatment in tubular batch reactors and impact on xylan hydrolysis kinetics.

A combined heat transfer/kinetic model was developed to quantify temperature variations in small tubular batch reactors and estimate the effect of deviations from isothermal operation on the kinetics of biomass pretreatment. Assuming that heat transfer was dominated by conduction in the radial direction, a classic parabolic time-dependent partial differential equation was applied to describe the temperature in the system and dedimensionalized to provide a single solution for application to all situations. A dimensionless expression for the reaction kinetics for xylan hydrolysis was then developed, and a single parameter expressed as the dimensionless ratio of the first-order rate constant times the tube radius squared divided by the thermal diffusivity was found to control the reaction rate. Three different characterizations of the deviation between the concentration profile predicted for isothermal xylan hydrolysis and that based on the transient temperature were directly related to this dimensionless rate constant parameter for both catalyzed and uncatalyzed hydrolysis kinetics. These results were then used to project the relationship between deviations in yield from isothermal results and the tube radius and reaction time.

Biomass↗
PubMed2005

Cost-effectiveness of rituximab (MabThera) in diffuse large B-cell lymphoma in The Netherlands.

OBJECTIVE: To determine the incremental cost-effectiveness ratio (ICER) of cyclophosphamide, doxorubicin, vincristine and prednisone (CHOP) vs. CHOP plus rituximab (R-CHOP) in diffuse large B-cell lymphoma (DLBCL) patients in the Netherlands. METHODS: A state transition model was developed to estimate the clinical course, costs and quality of life of patients with stage II, III or IV DLBCL receiving initial treatment with CHOP or R-CHOP to arrive at the ICER. The base year for the cost analysis was 2002 and was performed from the societal perspective. Only direct medical costs were included. The time horizon of the model was 15 yr and both costs and effects were discounted at 4%. Sensitivity analyses were performed to determine the effect of varying base-line assumptions of the model. RESULTS: The incremental gain in quality adjusted life years (QALYs) was 0.88 in both the younger and the older patient groups. The costs were 12 343 higher in the younger group of patients and 15 860 in the older patients. This resulted in an ICER of 13 983 for the younger and 17 933 for the older patients per QALY gained. These results were sensitive to the time horizon of the model, other variations had a marginal impact on the outcome. CONCLUSION: The addition of rituximab to standard therapy for DLBCL results in a gain of 0.88 QALYs. The ICER of 13 983 for younger and 17 933 for older patients per QALY gained should, seen in the light of disease severity, be considered acceptable by most policy makers in priority setting for budget allocation.

Antibodies, Monoclonal↗
PubMed1984

Receptive fields of intrapulmonary chemoreceptors in the Pekin duck.

Reflex experiments indicate a uniform distribution of CO2 chemosensitivity in avian lungs, but neural recording experiments suggest a non-uniform distribution of intrapulmonary chemoreceptor (IPC) endings. To reconcile these observations, blood gases and PECO2 were measured while recording discharge frequencies of 32 IPC innervating the unidirectionally ventilated lungs of 14 Pekin ducks. IPC discharge frequencies, recorded from the left vagus, were determined while ventilating the perfused left lung with caudocranial and craniocaudal flows of 1% CO2 in air, and then while ventilating the unperfused left lung with known levels of CO2 in air. Lung PCO2 profiles were predicted using an eight-compartment computer model of cross-current gas exchange with log-normal ventilation-perfusion inequality and shunt. The PCO2 profiles and IPC discharge frequencies were used to calculate receptor location. At the 99% confidence limit, estimates of IPC location changed significantly in all but 7 IPC when the direction of ventilation was reversed, indicating many IPC have multiple endings. Eighteen of 32 IPC had receptive fields extending at least 50% of the parabronchial length, which may explain the uniform reflex chemosensitivity to intrapulmonary CO2 noted by others.

Animals↗
PubMed1990

Artificial matrix barriers: a diffusion study utilizing dextrans and microspheres.

Eight artificial matrices (AMs) were evaluated for the ability to restrict the passage of diffusion probes. Three AMs were composed exclusively of interstitial type I collagen (Col I) and differed from each other in thickness only. Four AMs consisted of reconstituted basement membrane (RBM) -coated polycarbonate filters (containing 10 microns diameter pores) and also only differed in thickness. One AM consisted of an uncoated 10 microns pore polycarbonate filter. The diffusion probes were uncharged fluorescein isothiocyanate-labeled dextrans, having molecular weights of 17,900, 42,000, 71,200, and 148,900 and negatively charged latex microspheres, having diameters of 0.08, 0.30, and 0.95 microns. Probes were applied to the AMs, incubated for 72 hr at 37 degrees C, and then analyzed spectrophotometrically. Dextran passage was increasingly restricted for Col I matrices as either molecular weight or collagen thickness increased (range 7% to 0.7%). Thin RBM-coated filters were more permeable to dextrans (range 100% to 30%) than Col I matrices. The diffusion rate of microspheres for Col I matrices (range 3.5% to 0) was similar to both thick and thin RBM-coated filters (range 4% to 0). The uncoated filter permitted the most diffusion for both dextrans and microspheres (range 100% to 7%). These data demonstrate that the AMs presented in this study will allow direct observation of the degradative and migratory potential of cells in vitro as they interact with various extracellular matrices.

Animals↗
PubMed1994

Self-diffusion in CNS tissue by volume-selective proton NMR.

We have used a volume-selective NMR pulse sequence to measure the diffusion coefficient (D) inside a selected cuboid volume. The sequence is a variant of the stimulated echo method for volume-selective spectroscopy, and allows sequential measurement of D in any of three orthogonal directions and for a range of diffusion times. The echo attenuation from which D is calculated is free from the cross terms that can arise because of interactions between volume selection and diffusion-sensitizing gradients. Anisotropic and diffusion-time-dependent behavior is observed in brain structures known to have axon fibers aligned primarily in one direction.

Axons↗
PubMed2005

Relapse in the addictive behaviors: integration and future directions.

This paper identifies the major consistencies in substantive and methodological findings across the review papers in this special issue on relapse in the addictive behaviors. The papers were consistent in suggesting that there have been major methodological advances which have helped to move the field forward. Furthermore, the papers show the need for taking a biopsychosocial approach to the study of relapse and the major difficulty across addictive behaviors in creating an acceptable operational definition of relapse. Suggestions for future research directions that follow from the papers include deriving and evaluating relapse definitions, systematically developing and testing models and theories of relapse, and understanding and narrowing the relapse research-clinical practice gap.

Alcoholism↗
PubMed2006

Leaching of different elements from subbase layers of alternative aggregates in pavement constructions.

The objective of this study was to analyze the accumulated effects of leaching in two test roads were municipal solid waste incineration (MSWI) bottom ash and aggregate from a railway embankment, respectively, were used as subbase aggregates. Solid samples from the subbase and the subgrade were collected in trenches, which were excavated perpendicular to the road extension. The samples were analyzed with respect to pH, water content, electrical conductivity and extractable fractions of macro and trace constituents. To conclude, spatial distribution patterns of different constituents in subbase and subgrade layers confirms the existence of two major transport processes in a road with permeable shoulders: diffusion underneath surface asphalt layers driven by a concentration gradient directed horizontally towards the shoulder of the road where the dissolved elements are carried away by advection.

Calcium↗
PubMed2007

Fluid transport across the isolated porcine ciliary epithelium.

PURPOSE: To quantify spontaneous fluid transport across the isolated porcine ciliary epithelium and determine its sensitivity to the electrolyte transport inhibitors ouabain and bumetanide, as well as bath Cl(-) and HCO(3)(-) levels. METHODS: A complete annulus of ciliary body was mounted in a custom-designed chamber appropriate for quantifying net fluid movement, as well as the transepithelial potential difference (PD) across the in vitro ciliary epithelium. RESULTS: A spontaneous and stable fluid flow (FF) in the blood-to-aqueous direction was measured over a 4-hour period. This flux solely reflected the secretory activity of the isolated ciliary epithelium (CE), given the absence of externally applied osmotic or pressure gradients. In contrast to FF, the PD declined during the 4 hours in vitro, suggesting that the integrity of the tight junctions may have been compromised during this time so that an increased movement of counter ions via the paracellular pathway could have shunted the PD, while at the same time transcellular fluid transport remained unaffected. The FF in the blood-to-aqueous direction (2.3 +/- 0.2 muL/hr; n = 7) was eliminated by a unilateral reduction in the bath Cl(-) levels on the blood side of the preparation and restored on reintroducing the anion to the bathing medium. This linkage between FF and blood side [Cl(-)] is consistent with the existence of a net Cl(-) flux across the porcine CE in the same direction as the fluid transport. Addition of bumetanide to the blood-side bath inhibited FF by approximately 40%, whereas the removal of CO(2)/HCO(3)(-) from the blood-side bathing solution elicited a approximately 50% reduction in FF. Ouabain inhibited the FF from either side of the preparation, although the effects were more rapid when the glycoside was applied to the blood side of the tissue. Overall, these findings indicate the dependence of FF on active ionic transport by the isolated CE. CONCLUSIONS: Isolated porcine ciliary epithelial preparations transport fluid in the blood-to-aqueous direction, indicating that measurements of volumetric fluid flow across this preparation may serve as a suitable model for future studies directed toward the pharmacological control of secretion.

Animals↗
PubMed1982

A study of macrophage-mediated initiation of fibrosis by asbestos and silica using a diffusion chamber technique.

Several cellular interactions have been identified as potentially important in fibrogenesis induced by mineral dusts. Evaluation of their relative importance in vivo remains a problem. Sealed diffusion chambers limited by Nuclepore membranes and implanted into mouse peritoneal cavities provide a means of assessing different stages of fibrogenesis by separating initiating mechanisms (dust-macrophage-lymphocyte combinations inside the chamber) from the target tissue. Fibrous reactions surrounding the chambers were quantitated by macroscopic and histological scoring, and by measurement of 14C glycine incorporated at the reaction site. Using this model the fibrogenicity of Rhodesian A chrysotile asbestos, DQ12 quartz and haematite were compared. Whereas asbestos-macrophage ratios of between 6 . 6 and 900 micrograms/10(6) mouse peritoneal macrophages (MPM) produced fibrosis, an equivalent response was obtained with 0.05 micrograms silica/10(6) MPM. Silica in amounts greater than this produced macrophage cytotoxicity without fibrogenesis. Haematite-macrophage combinations produced no significant fibrosis. It was confirmed that a direct dust-macrophage interaction forms the essential first step in fibrogenesis by both asbestos and silica and that the fibrogenicity is mediated by diffusible factor(s). Prior stimulation of host mice with Freund's complete adjuvant modified the fibrogenic response to some dust-cell combinations, suggesting an important role for host responses in determining the outcome of fibrogenic stimuli.

Animals↗
PubMed2006

In vivo determination of diffusive transport parameters in a superfused tissue.

To address the hypothesis that functional changes in tissue transport can be related to structural alterations, we combined mathematical modeling with in vivo experimentation. The model concept includes interstitial diffusion and removal by a distributed microvasculature. Transport of solute and water across the peritoneum is measured via a plastic chamber affixed to the abdominal wall of anesthetized Sprague-Dawley rats. Solutions containing [(14)C]mannitol, with or without vasoactive compounds [control (C; n = 10), C + nitroprusside (NP; n = 10), C + norepinephrine (NE; n = 10)], were infused into the chamber, and the volume and tracer concentrations were determined over 60 min to calculate the mass transfer coefficient (MTC) and the water flux. At 60 min, FITC-dextran (500 kDa) was given to mark the perfused vasculature. After euthanasia, the tissue under the chamber was frozen, dried, sliced with a cryomicrotome, and examined with fluorescent microscopy and quantitative autoradiography. The microvessel density (x10(3)/cm(2): NE, 50 +/- 10; C, 180 +/- 7.0; NP, 225 +/- 15) resulted in marked differences (P < 0.05) in water flux (mul.min(-1).cm(-2): NE, 0.1 +/- 0.1; C, 1.6 +/- 0.4; NP, 1.0 +/- 0.2) and in mannitol MTC (x10(3) cm/min: NE, 0.9 +/- 0.3; C, 3.8 +/- 0.3; NP, 3.6 +/- 0.6). Concentration profiles and calculated capillary permeability and tissue diffusivity were significantly different among the groups. These results demonstrate a direct correlation of mass transfer, diffusion, capillary permeability, and water flux with peritoneal vascular density and validate a method by which mechanistic changes in transport may be measured.

Animals↗
PubMed1992

Internal and interfacial structure of membranes studied using X-ray standing waves.

The X-ray standing wave (XSW) method developed in the mid-Sixties was used then to determine the position of heavy atoms in and on crystals of silicon and germanium with sub-Angström resolution. The advent of layered synthetic microstructures, used primarily as wide-bandpass X-ray monochromators, heralded a new era in the use of XSW to study biologically relevant structures with a length scale of the order of tens of Angströms. The original measurements were performed on model membrane Langmuir-Blodgett (LB) films and served to establish the utility of the XSW approach in determining heavy-atom location in such systems with sub-Angström resolution and in tracking the heavy-atom layer as it moves during a thermotropic transition. Recent measurements show that the XSW is well defined at close to 1000 A from the XSW generating surface. Thus, the useful probing distance of XSW is of this length scale also without a compromise in resolution. In addition to the above measurements on well ordered systems the XSW method is being used to profile ion distribution 'directly' at the membrane/aqueous interface. Recent results show that the diffuse double layer can be established reversibly by suitably adjusting the pH of the aqueous phase next to a phospholipid membrane. The advantages and disadvantages of this new surface technique as applied to the study of membrane structure and interfacial phenomena are discussed.

Crystallography, X-Ray↗
PubMed1996

Brownian dynamics simulation of the lateral distribution of charged membrane components.

Brownian dynamics simulations were performed to study the contribution of electric interactions between charged membrane components to their lateral distribution in a two-dimensional viscous liquid (bilayer lipid membrane). The electrostatic interaction potential was derived from an analytical solution of the linearized Poisson-Boltzmann equation for point charges in an electrolyte solution--membrane--electrolyte solution system. Equilibrium as well as dynamic quantities were investigated. The lateral organization of membrane particles, modelled by mobile cylinders in a homogeneous membrane separating two electrolyte solutions was described by spatial distribution functions, diffusion coefficients and cluster statistics. Disorder, local order and crystal-like arrangements were observed as a function of the particle charge, the closest possible distances between the charges and the particle density. The simulations revealed that the system is very sensitive to the position of the charges with respect to the electrolyte solution-membrane interface. Electrostatic interactions of charges placed directly on the membrane surface were almost negligible, whereas deeper charges demonstrated pronounced interaction. Biologically relevant parameters corresponded at most to local and transient ordering. It was found that lateral electric forces can give rise to a preferred formation of clusters with an even number of constituents provided that the closest possible charge-charge distances are small. It is concluded that lateral electrostatic interactions can account for local particle aggregations, but their impact on the global arrangement and movement of membrane components is limited.

Cell Membrane↗