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PubMedDate not supplied

Electrical signal transmission and gap junction regulation in a bone cell network: a cable model for an osteon.

A cable model is formulated to estimate the spatial distribution of intracellular electric potential and current, from the cement line to the lumen of an osteon, as the frequency of the loading and the conductance of the gap junction are altered. The model predicts that the characteristic diffusion time for the spread of current along the membrane of the osteocytic processes, 0.03 sec, is nearly the same as the predicted pore pressure relaxation time in Zeng et al. (Annals of Biomedical Engineering. 1994) for the draining of the bone fluid into the osteonal canal. This approximate equality of characteristic times causes the cable to behave as a high-pass, low-pass filter cascade with a maximum in the spectral response for the intracellular potential at approximately 30 Hz. This behavior could be related to the experiments of Rubin and McLeod (Osteoporosis, Academic Press, 1996) which show that live bone appears to be selectively responsive to mechanical loading in a specific frequency range (15-30 Hz) for several species.

Action Potentials↗
PubMedDate not supplied

Patterns of axon collateralization of identified supragranular pyramidal neurons in the cat auditory cortex.

Nine pyramidal neurons in layers II and III of cat primary auditory cortex (AI) were fully reconstructed after intracellular injections of horseradish peroxidase or biocytin. Each neuron was functionally characterized according to its position relative to an anteroposterior sequence of best frequency responses. All labeled somata were in layers II or III and gave rise to typical apical and basal dendritic arbors as well as to extensive systems of axon collaterals. The primary axon of all except 1 cell entered the white matter and was probably directed toward other cortical areas ipsi- or contralaterally. Two major intracortical collateral systems emerged from the main axon in AI, one ending in the vicinity of the cell and the second at a distance. (1) Many local and recurrent collaterals, given off in layers III and V, contributed terminal branches to the formation of a columnar pattern of terminations extending superficially and deeply into the soma. The column extended through layers I-V, with some constriction in the middle portion corresponding to layer IV. (2) The axon of each cell also gave rise to 2-5 thick, long-range collaterals in layers III and/or V. These ran parallel to the pial surface for several millimeters. At several points along these long horizontal collaterals, vertically directed branches emerged to form columnar terminations, again extending through layers I-V. These columns did not overlap with that formed in the vicinity of the cell, and were situated at distances 500-1200 microns from the cell body. When viewed in the tangential plane, horizontal collaterals were oriented, on the whole, dorsoventrally with respect to the surface of the cortex. This may correspond to the organization of isofrequency bands previously described in cats. The results suggest that the major spread of excitation in AI is mediated by horizontal collaterals of pyramidal cells and that it occurs along the lines of isofrequency domains. Within the latter the collaterals may link columns of cells with like properties and/or serve to coordinate activity patterns in spatially separated portions of AI.

Animals↗
PubMedDate not supplied

Performance characteristics of a real-time digital x-ray fluoroscopic system using an intensified charge injection device camera and CsI:Na crystal.

A digital x-ray photoelectronic imaging system has been constructed using an optically flat 152-mm-diam, 2.5-mm-thick CsI:Na scintillating crystal. X-ray images formed by the scintillator are viewed by a single microchannel plate intensified charge injection device (ICID) camera and digitized at a real-time rate by a computerized frame-grabbing system. Video images are recorded and selected image frames are subjected to image processing and analysis schemes. Parameters governing the performance characteristics of the system are determined accordingly. For a 152-mm field size at the crystal plane, a spatial resolution limit of 1.50 +/- 0.10 1p/mm (1 sigma) measured at the 4% level of the modulation transfer function of the system has been obtained. This result is consistent with the measurements of the full width at half-maximum of the line spread function which is found to be 645 +/- 35 microns (1 sigma). Similarly, the intrinsic resolution of the CsI:Na scintillator only was found to be 6.5 +/- 0.5 1p/mm (1 sigma). Contrast ratio measurements, which are mainly determined by the quality of phosphor, have indicated a value of 12.1 +/- 0.6, whereas minimum visible details are observed at radiation exposure rates of 100 microR/s. This limit has been reduced to 10 microR/s using the single-scan integrating option provided by the ICID camera. A maximum contrast resolution of 1% corresponding to 100 statistically significant meaningful gray levels is achieved at a maximum exposure rate of 1000 microR/s. Consequently, although the imaging capability of the present system compares favorably with that of conventional x-ray video-fluoroscopic systems, a better performance may be achieved by using a higher resolution cooled dual-microchannel intensified CID camera in conjunction with a thinner CsI:Na crystal and a real-time digital image processing subsystem.

Evaluation Studies as Topic↗
PubMed2002

The mode of mechanical integrin stressing controls intracellular signaling in osteoblasts.

Following the idea that integrin receptors function as mechanotransducers, we applied defined physical forces to integrins in osteoblastic cells using a magnetic drag force device to show how cells sense different modes of physical forces. Application of mechanical stress to the beta1-integrin subunit revealed that cyclic forces of 1 Hz were more effective to stimulate the cellular calcium response than continuous load. Cyclic forces also induced an enhanced cytoskeletal anchorage of tyrosine-phosphorylated proteins and increased activation of the focal adhesion kinase (FAK) and mitogen activated protein (MAP) kinase. These events were dependent on an intact cytoskeleton and the presence of intracellular calcium. Analyses of the intracellular spatial organization of the calcium responses revealed that calcium signals originate in a restricted region in the vicinity of the stressed receptors, which indicates that cells are able to sense locally applied stress on the cell surface via integrins. The calcium signals can spread throughout the cell including the nucleus, which shows that calcium also is a candidate to transmit mechanically induced information into different cellular compartments.

Calcium↗
PubMed2002

Electrotonically mediated oscillatory patterns in neuronal ensembles: an in vitro voltage-dependent dye-imaging study in the inferior olive.

Spatiotemporal profiles of ensemble subthreshold neuronal oscillation were studied in brainstem slices using high-speed voltage-sensitive dye imaging. After local electrical stimuli, the overall voltage profile demonstrated coherent oscillatory waves that spread over the inferior olive (IO). These oscillations were also observed in concurrently obtained intracellular recordings from IO neurons. Over the first few seconds after the stimuli, the optically recorded oscillations clustered into coherent groups comprising hundreds of neurons. Statistical analysis of the spatial profiles of these clusters revealed size fluctuation around stable core regions that were surrounded by a rim the diameter of which varied in time during the oscillation period. The neuronal ensemble oscillations were calcium derived and had an average frequency range of 1-7 Hz. This rhythmic response demonstrated a different spatiotemporal distribution in the presence of picrotoxin, which induced the merging of neuronal clusters into larger areas of coherent activity. The possibility that such clustering is a consequence of intrinsic oscillations in ensembles of coupled neurons was tested using mathematical modeling.

Action Potentials↗
PubMed1995

[Performance of a multispect 2 gamma camera for imaging 511-keV gamma rays].

The performance of a modified Multispect 2 gamma camera, which was supplied with two pairs of collimators constructed for imaging positron emitters was tested. The measurements involved the determination of sensitivity and of spatial resolution. The septal penetration was calculated approximately using two different methods. The spatial resolution determined by FWHM as measured by planar imaging in water at 10 cm distance from the collimator was evaluated as 14.0 mm with collimator 511-A and 12.5 mm with collimator 511-B. Using a Gaussian fit of the line-spread function septal penetrations of 12% with collimator 511-A and of 36% with collimator 511-B were calculated. The absolute system sensitivity measured with 18F was 66 cpm with collimator 511-A and 98.7 cpm/37 kBq with collimator 511-B. The high sensitivity with collimator 511-B may be explained, at least partially, by the high septal penetration.

Deoxyglucose↗
PubMedDate not supplied

[The characteristics of the spatial-temporal organization of the cortical potentials in the free-moving rabbit during training (a comparison of the stages in the generalization and specialization of the conditioned-reflex response)].

Analysis of successive topograms was applied to investigation of peculiarities of the spatio-temporal organization of cortical potentials of a rabbit at different stages of conditioning. Qualitative distinctions were shown between the stages in the character of regulation of the spatial synchronization (SS) related to variations in cortico-subcortical relations. Higher SS level at the stage of generalization changed for more complicated pattern of the spatio-temporal organization of cortical potentials. These complex spatio-temporal characteristics favoured facilitation of excitation spreading along the paths which provide specialization of the conditioned response under the action of the conditioned stimulus. Obtained results were correlated to specific membrane and synaptic modifications in the course of conditioning. The findings extend the notions of the cellular mechanisms which determine the features of the cortical SS at the stages of generalization and specialization of the conditioned reflex.

Animals↗
PubMed1989

Compensation of spatial system response in SPECT with conjugate gradient reconstruction technique.

A procedure for the determination of the system matrix in single photon emission tomography (SPECT) is described which uses the conjugate gradient reconstruction technique in order to take into account the variable system resolution of a camera equipped with parallel-hole collimators. The procedure involves the acquisition of the system line spread functions (LSF) in the region occupied by the object to be studied. Those data are used to generate a set of weighting factors based on the assumption that the LSFs of the collimated camera are of Gaussian shape with the full width at half maximum (FWHM) linearly dependent on the source depth in the span of image space. The factors are stored on a disc file for subsequent use in the reconstruction process. Afterwards the reconstruction is performed using the conjugate gradient method with the system matrix modified by the incorporation of these precalculated factors in order to take into account the variable geometrical system response. The set of weighting factors is regenerated whenever the acquisition conditions are changed (collimator, radius of rotation). In the case of an ultra high resolution (UHR) collimator 2000 weighting factors need to be calculated. The modification of the system matrix for the geometrical response allows the number of iterations to increase, considerably improving image definition without the appearance of noise artifacts. Moreover, phantom studies show that the number of iterations is less critical because of improved stability in the convergence to the solution. For brain studies of patients 10-15 iterations are usually performed. Studies with a single line source give a value between 7 and 8 mm for the FWHM of the point spread function (PSF) when the conjugate gradient method with modified system matrix is used on data acquired with a UHR collimator, whereas without the modification of the system matrix the result is 9 mm FWHM, if filtered backprojection (FBP) is used with the same filter as in the clinical studies the result is 15 mm FWHM. The results of this work show that proper definition of the system matrix using conjugate gradients influences the quality of the reconstruction remarkably. Nevertheless, further work has to be done in order to assess to what extent the system matrix is ill-conditioned and, eventually, to define a suitable regularization technique.

Humans↗
PubMed2003

A model of spatially evolving herpesvirus epidemics causing mass mortality in Australian pilchard Sardinops sagax.

In 1995 mass mortality of pilchards Sardinops sagax occurred along >5000 km of Australian coast; similar events occurred in 1998/99. This mortality was closely associated with a herpesvirus. The pilchard is an important food source for larger animals and supports commercial fisheries. Both epidemics originated in South Australian waters and spread as waves with velocities of 10 to 40 km d(-1). Velocity was constant for a single wave, but varied between the epidemics and between the east- and west-bound waves in each epidemic. The pattern of mortality evolved from recurrent episodes to a single peak with distance from the origin. A 1-dimensional model of these epidemics has been developed. The host population is divided into susceptible, infected and latent, infected and infectious, and removed (recovered and dead) phases; the latent and infectious periods are of fixed duration. This model produces the mortality patterns observed locally and during the spread and evolution of the epidemic. It is consistent with evidence from pathology. The wave velocity is sensitive to diffusion coefficients, viral transmission rates and latent period. These parameters are constrained using the local and large-scale patterns of epidemic spread. The relative roles of these parameters in explaining differences between epidemics and between east- and west-bound waves within epidemics are discussed. The model predicts very high levels of infection, indicating that many surviving pilchards recovered following infection. Control appears impracticable once epidemics are initiated, but impact can be minimised by protecting juvenile stocks.

Animals↗
PubMed2005

Spatiotemporal distribution of insecticide resistance in Anopheles culicifacies and Anopheles subpictus in Sri Lanka.

The malaria situation in Sri Lanka worsened during the 1990s with the emergence and spread of resistance to the drugs and insecticides used for control. Chloroquine resistance has increased rapidly over this period, but adverse changes in malaria transmission are more closely associated with insecticide use rather than drug resistance. Insecticide susceptibility tests were routinely carried out in key anopheline vectors across the country for more than a decade. These sentinel data were combined with data collected by other research programmes and used to map the spatial and temporal trends of insecticide resistance in the main vectors, Anopheles culicifacies and A. subpictus, and to examine the relationship between insecticide resistance, changes in national spraying regimens and malaria prevalence. Both species had widespread resistance to malathion, the insecticide of choice in the early 1990s. Both species were initially susceptible to the organophosphate and pyrethroid insecticides used operationally from 1993, but some resistance has now been selected. The levels of malathion and fenitrothion resistance in A. subpictus were higher in some ecological regions than others, which may be related to the distribution of sibling species, agricultural pesticide exposure and/or environmental factors. The study highlights that the emergence and spread of insecticide resistance is a constant threat and that active surveillance systems are vital in identifying key vectors and evidence of resistance.

Animals↗
PubMedDate not supplied

Processing biological images from very low light emissions.

As opposed to strong light signals (> 10(8) photons mm-2 s-1), the construction of images from object sources with low level signals (< 10(2) photons mm-2 s-1) involves a probabilistic transfer- or point-spread function. The resulting images carry considerable uncertainty or spread, restricting resolution and quantification. In this paper we propose various strategies how to reconstruct object characteristics from very low light emissions by unfolding the imaging equation. Further, calibration techniques which help to associate light emissions with the decomposition of luminogenic substrates in a spatially selective way will be discussed.

Animals↗
PubMed2005

Endothelial network formed with human dermal microvascular endothelial cells in autologous multicellular skin substitutes.

A human skin equivalent from a single skin biopsy harboring keratinocytes and melanocytes in the epidermal compartment, and fibroblasts and microvascular dermal endothelial cells in the dermal compartment was developed. The results of the study revealed that the nature of the extracellular matrix of the dermal compartments plays an important role in establishment of endothelial network in vitro. With rat-tail type I collagen matrices only lateral but not vertical expansion of endothelial networks was observed. In contrast, the presence of extracellular matrix of entirely human origin facilitated proper spatial organization of the endothelial network. Namely, when human dermal fibroblasts and microvascular endothelial cells were seeded on the bottom of an inert filter and subsequently epidermal cells were seeded on top of it, fibroblasts produced extracellular matrix throughout which numerous branched tubes were spreading three-dimensionally. Fibroblasts also facilitated the formation of basement membrane at the epidermal/matrix interface. Under all culture conditions, fully differentiated epidermis was formed with numerous melanocytes present in the basal epidermal cell layer. The results of the competitive RT-PCR revealed that both keratinocytes and fibroblasts expressed VEGF-A, -B, -C, aFGF and bFGF mRNA, whereas fibroblasts also expressed VEGF-D mRNA. At protein level, keratinocytes produced 10 times higher amounts of VEGF-A than fibroblasts did. The generation of multicellular skin equivalent from a single human skin biopsy will stimulate further developments for its application in the treatment of full-thickness skin defects. The potential development of biodegradable, biocompatible material suitable for these purposes is a great challenge for future research.

Base Sequence↗
PubMed2004

Formation and function of the polar body contractile ring in Spisula.

Initial studies suggested that spatial organization of the putative polar body contractile ring was determined by the peripheral aster in Spisula [Biol. Bull. 205 (2003) 192]. Here we report detailed supporting observations, including testing of aster and ring function with inhibitors. The metaphase peripheral aster was confirmed to spread cortically in an umbrella-like pattern, with microtubule-poor center. The aster disassembled during anaphase, leaving the spindle docked at the F-actin-poor center of a newly generated cortical F-actin ring that closely approximated the aster in location, measured diameter range, and pattern. Cytochalasin D and latrunculin-B permitted all events except ring and polar body formation. Nocodazole disassembly or taxol stabilization of the peripheral aster produced poorly defined rings or bulging anaphase asters within the ring center, respectively, inhibiting polar body formation. Polar body extrusion occurred at the ring center, the diameter of which diminished. Ring contractility-previously assumed-was verified using blebbistatin, a myosin-II ATPase inhibitor that permitted ring assembly but blocked polar body extrusion. The data support the hypothesis that peripheral aster spreading, perhaps dynein-driven, is causally related to polar body contractile ring formation, with anaphase entry and aster disassembly also required for polar body biogenesis. Previously reported astral spreading during embryonic micromere formation suggests that related mechanisms are involved in asymmetric somatic cytokinesis.

Actins↗
PubMed2003

Experimental study of the immiscible displacement of shear-thinning fluids in pore networks.

The pore scale mechanisms and network scale transient pattern of the immiscible displacement of a shear-thinning nonwetting oil phase (NWP) by a Newtonian wetting aqueous phase (WP) are investigated. Visualization imbibition experiments are performed on transparent glass-etched pore networks at a constant unfavorable viscosity ratio and varying values of the capillary number (Ca), and equilibrium contact angle (theta(e)). Dispersions of ozokerite in paraffin oil are used as the shear-thinning NWP, and aqueous solutions of PEG colored with methylene blue are used as the Newtonian WP. At high Ca values, the tip splitting and lateral spreading of WP viscous fingers are suppressed; at intermediate Ca values, the primary viscous fingers expand laterally with the growth of smaller capillary fingers; at low Ca values, network spanning clusters of capillary fingers separated by hydraulically conductive noninvaded zones of NWP arise. The spatial distribution of the mobility of shear-thinning NWP over the pore network is very broad. Pore network regions of low NWP mobility are invaded through a precursor advancement/swelling mechanism even at relatively high Ca and theta(e) values; this mechanism leads to irregular interfacial configurations and retention of a substantial amount of NWP along pore walls; it becomes the dominant mechanism in displacements performed at low Ca and theta(e) values. The residual NWP saturation increases and the end WP relative permeability decreases as Ca increases and both become more sensitive to this parameter as the shear-thinning behavior strengthens. The shear-thinning NWP is primarily entrapped in individual pores of the network rather than in clusters of pores bypassed by the WP. At relatively high flow rates, the amplitude of the variations of pressure drop, caused by fluid redistribution in the pore network, increase with shear-thinning strengthening, whereas at low flow rates, the motion of stable and unstable menisci in pores is reflected in strong pressure drop fluctuations.

Journal Article↗
PubMed1998

The slow wave does not propagate across the gastroduodenal junction in the isolated feline preparation.

Detailed spatial analysis of the propagation of individual slow waves was performed in the isolated gastroduodenal preparation of the cat. Use was made of a system that allowed the simultaneous recordings from 240 extracellular electrodes, which were positioned across the gastroduodenal region. Reconstructions of the spread of propagation (n = 31) revealed that (a) the antral slow wave never propagated into the duodenum but was blocked at the pyloric ring, (b) the duodenal slow wave did not activate the antral tissue, and (c) a quiescent zone in which no slow waves could be recorded was always present at the most proximal part of the duodenum immediately distal to the pyloric ring. Furthermore, phase density distributions of duodenal cycles revealed that antral activity had no influence on the rate of discharge of duodenal pacemakers. Light microscopic study of sections of the duodenum close to the pyloric ring and further away did not show any structural differences between the quiescent zone and the active areas. In conclusion, slow waves do not propagate across the gastroduodenal junction in the isolated feline preparation and therefore do not seem to play a role in the electro-mechanical integration between the stomach and the duodenum.

Animals↗
PubMed1994

Investigation of the imaging properties of an x-ray film scanner.

In this note, the imaging properties of a CCD x-ray film scanner were investigated. The relationship between optical density and the logarithm of output pixel value provides information on the linearity of the system. The resolution properties of the scanner can be characterized by the presampling modulation transfer function which was calculated from a set of line spread functions with various alignments relative to the sampling grid. Our results show that the scanner is linear up to 2.0 optical density units, which leads to restrictions in digitizing x-ray films with higher optical density. The measured MTF shows the good spatial resolution of the CCD scanner which is demonstrated on an example.

Equipment Failure Analysis↗
PubMed2000

Beam alignment and related problems of spherical aberration corrected high-resolution TEM images.

For spherical aberration corrected transmission electron microscopes recently developed, the Scherzer resolution in proportion to C(s) 1/4 lambda 3/4 is still inevitably limited by the influence of some other electron optical factors, such as chromatic aberration coefficient of objective lens (C(c)), beam divergence and alignment, incident electron energy spread (deltaE), and the instability of accelerating voltage (deltaV) and of lens current (deltaI). Depending on the image resolution guaranteed by C(s) correction, the defocus spread caused by C(c), deltaV, deltaI and deltaE would need to be reduced. The effect of beam alignment as a phase shift in contrast transfer function is also studied for the C(s) corrected microscopes with different values of C(s), defocus and spatial frequency. There is a relationship between the phase shift and defocus that allows us to find a series of 'alignment-free' focal conditions. A triangular relation among C(s), defocus and lattice spacing is established for proper image contrast and 'alignment-free' imaging with the C(s) corrected microscope.

Journal Article↗
PubMed1999

Differential distribution of inositol trisphosphate receptor isoforms in mouse oocytes.

In mammalian fertilization, inositol 1,4,5-trisphosphate receptor (IP3R)-dependent Ca2+ release is a crucial signaling event that originates from the vicinity of sperm-egg interaction and spreads as a wave throughout the egg cytoplasm. While it is known that Ca2+ is released by the type 1 IP3R in the egg cortex, the potential involvement of other isoform types responsible for the Ca2+ rise in the mouse egg (interior) and their spatial distribution are not known. In addition, the biochemical basis has not been definitively established for the development of increased sensitivity to inositol 1,4,5-trisphosphate (IP3) during meiotic maturation. Using specific antibodies to the type 1, 2, and 3 IP3R, we tested the hypotheses that different IP3R isoforms are responsible for the internal Ca2+ elevation and that they contribute to the maturation-associated acquisition of IP3 sensitivity. In both preovulatory oocytes and ovulated eggs of CF-1 mice, immunofluorescence revealed that types 1 and 2 isoforms were present in the cell cortex and interior. Type 1 was observed throughout the cytoplasm, and Western analysis indicated a 1.9-fold maturation-associated increase. In contrast, the signals detected for the type 2 (high-affinity) isoform and type 3 were present to a lesser extent, with type 2 restricted to isolated islands (similar to aggregates of vesicles detected by electron microscopy), which, in the cortex, may amplify early sperm-egg signaling events. The cortical-to-perinuclear localization of the receptor and cortical vesicle aggregates imply an efficient mechanism for propagating Ca2+ release from the cortex into the interior of the egg to activate development, and the isoform localization analysis indicates a clear spatial and biochemical heterogeneity. Types 1 and 2 isoforms were also present in granulosa cells.

Amino Acid Sequence↗