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At least 487 records · Page 27Linked to original sources
PubMed1993

Crossbridges in the complete unit cell of rigor insect flight muscle imaged by three-dimensional reconstruction from oblique sections.

We have computed two types of 3-D reconstructions from single images of oblique transverse sections through rigor insect flight muscle (IFM) that permit simultaneous examination of all myosin crossbridges within the unit cell. One type, crystallographic serial section reconstruction (CSSR), utilizes primarily real space image manipulations of the periodic crossbridge lattice to obtain a 3-D reconstruction from a single image. The CSSRs, which do not average successive unit cells along the filament axis, reveal variations in the rigor double chevrons within the 116 nm long axial repeat and in particular show that specific crossbridges are absent. CSSRs establish that in rigor, the 116 nm period contains nine 12.9 nm repeats of attached crossbridges rather than the eight 14.5 nm repeats of myosin head origins observed in the relaxed state. This indicates that dominance of the actin repeat on myosin head form enforces axial and azimuthal changes on the crossbridge origins on the thick filament. The second type, superlattice reconstruction (SLR), is carried out entirely in Fourier space and produces an averaged reconstruction with the symmetry of the unit cell enforced. SLRs measure the 3-D transform of the complete unit cell, permitting direct comparison with X-ray diagrams from native muscle. Averaging several SLRs together has produced the highest resolution reconstruction of IFM to date. Oblique section reconstructions made by both methods confirm in greater detail the presence of two-headed lead crossbridges and single-headed rear crossbridges implying heads with differing angles and conformation. Reduced twist in the thin filament coincident with the lead crossbridge is also apparent. We have modeled several interpretations of this reduced twist in terms of structural changes in both myosin and actin at the lead bridge. In addition, these 3-D images resolve a feature just Z-ward of the rear crossbridge where antibody labeling has identified part of the large troponin complex of IFM.

Actins↗
PubMed2001

Geometrical constraints on the development of a diatom.

In a plane hexagonal array of pores such as occurs in Thalassiosira oestrupii and other related species of marine diatom, the presence of a single, exceptional pore near the center of the array can cause the distortion in the pattern in such a way that the pores tend to diminish in diameter D with increasing distance R from the center. By a simple mathematical argument it is shown that for large values of R/D the pore diameter D varies as R(-1/6). This decrease in pore diameter with distance may have some developmental consequences. The law is generalized to other types of pattern. The second type of constraint, related to the presence of outer boundaries, is also discussed.

Animals↗
PubMed1997

Computer simulation of inspiratory airflow in all regions of the F344 rat nasal passages.

Data from laboratory animal experiments are often used in setting guidelines for safe levels of human exposure to inhaled materials. The F344 rat has been used extensively in laboratory experiments to determine effects of exposure to inhaled materials in the nasal passages. Many inhaled materials induce toxic responses in the olfactory (posterior) region of the rat nasal passages. The location of major airflow routes has been proposed as playing a dominant role in determining some olfactory lesion location patterns. Since nasal airflow patterns differ significantly among species, methods are needed to assess conditions under which these differences may significantly affect extrapolation of the effects of local dose in animals to potential disease outcome in humans. A computational fluid dynamics model of airflow and inhaled gas uptake has been used to predict dose to airway walls in the anterior F344 rat nasal passages (Kimbell et al., Toxicol. Appl. Pharmacol., 1993; 121, 253-263). To determine the role of nasal airflow patterns in affecting olfactory lesion distribution, this model was extended to include the olfactory region. Serial-step histological sections of the nasal passages of a F344 rat were used to construct the computer model. Simulations of inspiratory airflow throughout the rat nasal passages were consistent with previously reported experimental data. Four of the five major simulated flow streams present in the anterior nose (dorsal lateral, middle, ventral lateral, and ventral medial streams) flowed together to exit ventrally at the nasopharyngeal duct, bypassing the ethmoid recesses. The remaining dorsal medial stream split to flow both medially and laterally through the olfactory-epithelium-lined ethmoid recesses in a Z-shaped pattern when viewed sagitally. Simulated flow in the ethmoid recesses was more than an order of magnitude slower than flow in the anterior and ventral parts of the nasal passages. Somewhat higher volumes of flow were predicted in the dorsal medial stream when the nasal vestibule was reshaped to be upturned, and more flow was allocated to the dorsal medial stream with increased inspiratory airflow rate, suggesting that rats may be able to allocate more airflow to this stream by both modifying the shape of the nasal vestibule and increasing inhaled air velocity during sniffing. The present study provides the first description of flow in the complex olfactory region of the nose of the F344 rat. This model will be used to evaluate the role of airflow patterns in determining the distribution of xenobiotically induced olfactory mucosal lesions. This information, combined with models of disposition in the airway lining, will provide comprehensive dosimetry models for extrapolating animal response data to humans.

Animals↗
PubMed2005

An in vitro patient-tailored model of human cerebral artery for simulating endovascular intervention.

An in vitro patient-tailored reproduction model of cerebral artery, a hardware platform for simulating endovascular intervention for making diagnoses and surgical trainings is presented. 3-D configuration of vessel lumen is reproduced as vessel model with 13 microm modeling resolution, using CT and MRI information. Physical characteristics of cerebral artery, such as elastic modulus and friction coefficient, are also reproduced. We also propose a novel method to visualize stress condition on vessel wall using photoelastic effect. Consequently, it should be helpful for clinical applications, academic researches and other various purposes.

Cerebral Arteries↗
PubMed2005

Bone enhancement filtering: application to sinus bone segmentation and simulation of pituitary surgery.

We present a novel multi-scale bone enhancement measure that can be used to drive a geometric flow to segment bone structures. This measure has the essential properties to be incorporated in the computation of anatomical models for the simulation of pituitary surgery, enabling it to better account for the presence of sinus bones. We present synthetic examples that validate our approach and show a comparison between existing segmentation techniques of paranasal sinus CT data.

Algorithms↗
PubMed2005

Fully automatic shape modelling using growing cell.

In this paper, we present a new framework for shape modelling and analysis: we suggest to look at the problem from a pattern recognition point of view, and claim that under this prospective several advantages are achieved. The modelling of a surface with a point distribution model is seen as an unsupervised clustering problem, and tackled by using growing cell structures. The adaptation of a model to new shapes is studied as a classification task, and provides a straightforward solution to the point correspondence problem in active shape modelling. The method is illustrated and tested in 3D synthetic datasets and applied to the modelling of brain ventricles in an elderly population.

Cerebral Ventricles↗
PubMed2005

New ratios for the detection and classification of CJD in multisequence MRI of the brain.

We present a method for the analysis of deep grey brain nuclei for accurate detection of human spongiform encephalopathy in multisequence MRI of the brain. We employ T1, T2 and FLAIR-T2 MR sequences for the detection of intensity deviations in the internal nuclei. The MR data are registered to a probabilistic atlas and normalised in intensity prior to the segmentation of hyperintensities using a foveal model. Anatomical data from a segmented atlas are employed to refine the registration and remove false positives. The results are robust over the patient data and in accordance to the clinical ground truth. Our method further allows the quantification of intensity distributions in basal ganglia. sCJD patient FLAIR images are classified with a more significant hypersignal in caudate nuclei (10/10) and putamen (6/10) than in thalami. Defining normalised MRI measures of the intensity relations between the internal grey nuclei of patients, we robustly differentiate sCJD and variant CJD (vCJD) patients, as an attempt towards the automatic detection and classification of human spongiform encephalopathies.

Algorithms↗
PubMed2005

Automated surface matching using mutual information applied to Riemann surface structures.

Many medical imaging applications require the computation of dense correspondence vector fields that match one surface with another. To avoid the need for a large set of manually-defined landmarks to constrain these surface correspondences, we developed an algorithm to automate the matching of surface features. It extends the mutual information method to automatically match general 3D surfaces (including surfaces with a branching topology). First, we use holomorphic 1-forms to induce consistent conformal grids on both surfaces. High genus surfaces are mapped to a set of rectangles in the Euclidean plane, and closed genus-zero surfaces are mapped to the sphere. Mutual information is used as a cost functional to drive a fluid flow in the parameter domain that optimally aligns stable geometric features (mean curvature and the conformal factor) in the 2D parameter domains. A diffeomorphic surface-to-surface mapping is then recovered that matches anatomy in 3D. We also present a spectral method that ensures that the grids induced on the target surface remain conformal when pulled through the correspondence field. Using the chain rule, we express the gradient of the mutual information between surfaces in the conformal basis of the source surface. This finite-dimensional linear space generates all conformal reparameterizations of the surface. We apply the method to hippocampal surface registration, a key step in subcortical shape analysis in Alzheimer's disease and schizophrenia.

Algorithms↗
PubMed2005

Shape modeling using automatic landmarking.

This paper describes a novel approach to automatically recover accurate correspondence over various shapes. In order to detect the features points with the capability in capturing the characteristics of an individual shape, we propose to calculate the skeletal representation for the shape curve through the medial axis transform. Employing this shape descriptor, mathematical landmarks are automatically identified based on the local feature size function, which embodies the geometric and topological information of the boundary. Before matching the resulting landmarks, shape correspondence is first approached by matching he major components of the shape curves using skeleton features. This helps in keeping the consecutive order and reducing the search space during the matching process. Point matching is then performed within each pair of corresponding components by solving a consecutive assignment problem. The effectiveness of this approach is demonstrated through experimental results on several different training sets of biomedical object shapes.

Algorithms↗
PubMed2006

Mechanisms of sleep apnea at altitude.

At altitude normal people often develop periodic breathing in sleep--regularly recurring periods of hyperpnea and apnea. This phenomenon is probably explained by instability of the negative feedback system for controlling ventilation. Such systems can be modeled by sets of differential equations that describe behavior of key components of the system and how they interact. Mathematical models of the breathing control system have increased in complexity and the accuracy with which they simulate human physiology. Recent papers by Zbigniew Topor et al. (5,6) describe a model with two separate feedback loops, one simulating peripheral and the other central chemoreceptor reflexes, as well as accurate representations of blood components, circulatory loops and brain blood flow. This model shows unstable breathing when one chemoreceptor loop has high gain while the other has low gain, but not when both have high gain. It also behaves in counter-intuitive way by becoming more stable when brain blood flow is reduced and unresponsive to blood, gas changes. Insights from such models may bring us closer to understanding high altitude periodic breathing.

Altitude↗
PubMed2006

Neural crest delamination and migration: integrating regulations of cell interactions, locomotion, survival and fate.

During the entire process of neural crest development from specification till final differentiation, delamination and migration are critical steps where nascent crest cells face multiple challenges: within a relatively short period of time that does not exceed several hours, they have to change drastically their cell- and substrate-adhesion properties, lose cell polarity and activate the locomotory machinery, while keeping proliferating, surviving and maintaining a pool of precursors in the neural epithelium. Then, as soon as they are released from the neural tube, neural crest cells have to adapt to a new, rapidly-changing environment and become able to interpret multiple cues which guide them to appropriate target sites and prevent them from distributing in aberrant locations. It appears from recent studies that, behind an apparent linearity and unity, neural crest development is subdivided into several independent steps, each being governed by a multiplicity of rules and referees. Here resides probably one of the main reasons of the success of neural crest cells to accomplish their task.

Animals↗
PubMed2006

Neural crest contribution to the cardiovascular system.

Normal cardiovascular development requires complex remodeling of the outflow tract and pharyngeal arch arteries to create the separate pulmonic and systemic circulations. During remodeling, the outflow tract is septated to form the ascending aorta and the pulmonary trunk. The initially symmetrical pharyngeal arch arteries are remodeled to form the aortic arch, subclavian and carotid arteries. Remodeling is mediated by a population of neural crest cells arising between the mid-otic placode and somite four called the cardiac neural crest. Cardiac neural crest cells form smooth muscle and pericytes in the great arteries, and the neurons of cardiac innervation. In addition to the physical contribution of smooth muscle to the cardiovascular system, cardiac neural crest cells also provide signals required for the maintenance and differentiation of the other cell layers in the pharyngeal apparatus. Reciprocal signaling between the cardiac neural crest cells and cardiogenic mesoderm of the secondary heart field is required for elaboration of the conotruncus and disruption in this signaling results in primary myocardial dysfunction. Cardiovascular defects attributed to the cardiac neural crest cells may reflect either cell autonomous defects in the neural crest or defects in signaling between the neural crest and adjacent cell layers.

Animals↗
PubMed2006

The genetic regulation of pigment cell development.

Pigment cells in developing vertebrates are derived from a transient and pluripotent population of cells called neural crest. The neural crest delaminates from the developing neural tube and overlying ectoderm early in development. The pigment cells are the only derivative to migrate along the dorso-lateral pathway. As they migrate, the precursor pigment cell population differentiates and expands through proliferation and pro-survival processes, ultimately contributing to the coloration of organisms. The types of pigment cells that develop, timing of these processes, and final destination can vary between organisms. Studies from mice, chick, Xenopus, zebrafish, and medaka have led to the identification of many genes that regulate pigment cell development. These include several classes of proteins: transcription factors, transmembrane receptors, and extracellular ligands. This chapter discusses an overview of pigment cell development and the genes that regulate this important process.

Animals↗
PubMed1992

Computer simulation of erythrocyte transit in the cerebrocortical capillary network.

The relationship between erythrocyte flow path length and transit time was studied using a probabilistic computer simulation of erythrocyte transit in morphometrically reconstructed cerebrocortical microvascular network of the rat. The results suggest (1) a near-cubic relationship between erythrocyte flow and vessel diameter, (2) preferential pathways of erythrocyte flow, (3) an exponential relationship between erythrocyte transit time and flow path length.

Animals↗