Circulation of excitation waves in a model medium containing unexcitable obstacles

The 3D structure of the heart tissue of Wistar rats from different age groups has been reconstructed by light microscopy of consecutive series of semithin sections. At the histological resolution level, connective tissue segments in the myocardium of aged (18 months) animals were found, while in the myocardium of young (2 months) animals no connective tissue segments were observed. A mathematical model and the results of the 3D reconstruction were used to simulate the formation of excitation wave circulation in the myocardium. The values of conductivity and the excitability threshold, and the pulse frequency rates were found at which the disturbances of the heart rhythm resulting from the excitation wave circulation are predicted by the mathematical modeling.     

Electromechanical model of excitable tissue to study reentrant cardiac arrhythmias

We introduce the concept of a contracting excitable medium that is capable of conducting non-linear waves of excitation that in turn initiate contraction. Furthermore, these kinematic deformations have a feedback effect on the excitation properties of the medium. Electrical characteristics resemble basic models of cardiac excitation that have been used to successfully study mechanisms of reentrant cardiac arrhythmias in electrophysiology. We present a computational framework that employs electromechanical and mechanoelectric feedback to couple a three-variable FitzHugh–Nagumo-type excitation-tension model to the non-linear stress equilibrium equations, which govern large deformation hyperelasticity. Numerically, the coupled electromechanical model combines a finite difference method approach to integrate the excitation equations, with a Galerkin finite element method to solve the equations governing tissue mechanics. We present example computations demonstrating various effects of contraction on stationary rotating spiral waves and spiral wave break. We show that tissue mechanics significantly contributes to the dynamics of electrical propagation, and that a coupled electromechanical approach should be pursued in future electrophysiological modelling studies.     

Microgravity dependence of excitable biological and physicochemical media

Neuronal tissue and especially the central nervous system (CNS) is an excitable medium. Self-organisation, pattern formation, and propagating excitation waves as typical characteristics in excitable media consequently have been found in neuronal tissue. The properties of such phenomena in excitable media do critically depend on the parameters (i.e., electromagnetic fields, temperature, chemical drugs) of the system and on small external forces to which gravity belongs. The spreading depression, a propagating excitation depression wave of neuronal activity, is one of the best described of the those wave phenomena in the CNS. Especially in the retina as a true part of the CNS it can be easily observed with optical techniques due to the high intrinsic optical signal of this tissue. Another of such waves in neuronal tissue is the propagating action potential in nerve fibres. In this paper, data from our laboratories concerning the influence of gravity on the velocity of propagating waves in excitable media are summarized mainly in terms of the retinal spreading depression and propagating action potentials. Additionally, we have used waves in gels of the Belousov-Zhabotinsky reaction as the physicochemical model system of biological activity as the properties of these waves follow the same theories as the spreading depression and action potentials and they have some striking similarities in wave behavior. Thus propagating Belousov-Zhabotinsky waves are described by their gravity dependence.     

Mechanisms of ventricular arrhythmias elicited by coexistence of multiple electrophysiological remodeling in ischemia: A simulation study

Myocardial ischemia, injury and infarction (MI) are the three stages of acute coronary syndrome (ACS). In the past two decades, a great number of studies focused on myocardial ischemia and MI individually, and showed that the occurrence of reentrant arrhythmias is often associated with myocardial ischemia or MI. However, arrhythmogenic mechanisms in the tissue with various degrees of remodeling in the ischemic heart have not been fully understood. In this study, biophysical detailed single-cell models of ischemia 1a, 1b, and MI were developed to mimic the electrophysiological remodeling at different stages of ACS. 2D tissue models with different distributions of ischemia and MI areas were constructed to investigate the mechanisms of the initiation of reentrant waves during the progression of ischemia. Simulation results in 2D tissues showed that the vulnerable windows (VWs) in simultaneous presence of multiple ischemic conditions were associated with the dynamics of wave propagation in the tissues with each single pathological condition. In the tissue with multiple pathological conditions, reentrant waves were mainly induced by two different mechanisms: one is the heterogeneity along the excitation wavefront, especially the abrupt variation in conduction velocity (CV) across the border of ischemia 1b and MI, and the other is the decreased safe factor (SF) for conduction at the edge of the tissue in MI region which is attributed to the increased excitation threshold of MI region. Finally, the reentrant wave was observed in a 3D model with a scar reconstructed from MRI images of a MI patient. These comprehensive findings provide novel insights for understanding the arrhythmic risk during the progression of myocardial ischemia and highlight the importance of the multiple pathological stages in designing medical therapies for arrhythmias in ischemia.     

Persistent free-running circannual reproductive cycles during prolonged exposure to a constant 12L:12D photoperiod in laboratory woodchucks (Marmota monax).

Serum levels of gonadal steroid were assayed at approximately 3-month intervals in groups of 5 to 8 male or female woodchucks which were exposed to a natural photoperiod for 1 year as yearlings or 3 years as adults (Study 1), or a constant photoperiod of 12L:12D from birth for 4.5 years (Study 2). After 4.5 years of 12L:12D, food intake was measured in November and compared with that in natural photoperiod animals (Study 3). Other groups of 11 males and 3 females were housed in 12L:12D for 2.5 years after capture at 2 months of age, and gonadal structure and serum steroid levels in November were compared with those of animals at selected times in the normal annual cycle (Study 4). All animals were provided food and water ad libitum and were not induced to hibernate. In Study 1, normal circannual breeding season elevations in testosterone in males and in progesterone in females were detected in most animals maintained in natural photoperiod. In Study 2, similar cycles persisted for 4.5 years in animals exposed to 12L:12D. However, based on quarterly blood samples, obvious asynchrony relative to natural light animals appeared to develop after 2, 3, or 4 years, with apparent free-running intervals of about 10 to 11 months. In Study 3, mean daily food consumption in late autumn for woodchucks in the 12L:12D group was 72% greater than animals in the natural photoperiod. In Study 4, some woodchucks exposed to 12L:12D for only 2.5 years had prematurely increased spermatogenic activity, Leydig tissue development, and elevated serum testosterone levels in November. They were similar in November to those in natural photoperiod animals in March, and significantly greater than those in natural photoperiod animals in November when normal regression and repair of the testis was complete. Likewise, females in the 12L:12D group had luteinized follicles and elevated progesterone in November which were not noted in natural photoperiod animals and which were similar to those observed during the spring in unbred females under normal conditions. The results suggest that circannual cycles of metabolic and reproductive activity in woodchucks persist in the absence of normal changes in photoperiod, are entrained to seasonal changes in the natural photoperiod, and can recede to a periodicity of less than 12 months within 2.5 to 4 years of laboratory maintenance in 12L:12D.     

Morpho-functional changes in the turtle midbrain tectum after enucleation

Morpho-functional changes in the tectum mesencephali during degeneration after enucleation were studied inEmys orbicularis L. Comparison of amplitude-time characteristics of evoked potentials of the visual center with degenerative changes in axon terminals and fibers of the optic nerve in the same animals revealed a "light" type of degeneration of the terminals of unmyelinated axons and a "dark" type in terminals of myelinated axons. During "dark" degeneration (4–5 months after enucleation) the low-amplitude presynaptic component of the evoked potential, reflecting excitation of large myelinated fibers, disappeared and changes occurred in the characteristics of the first high-amplitude component, the appearance of which is connected with excitation of myelinated fibers of medium diameter. The last component disappeared 7 months after the operation, along with disappearance of the "dark" degeneration. During "light" degeneration (2.5–3.5 months) changes took place in the characteristics of the second high-amplitude component of the evoked potential, which reflects excitation of thin fibers, both myelinated and unmyelinated, whose ranges of diameters overlap. This component disappeared after 6–7 months, almost simultaneously with disappearance of the first high-amplitude component, as the result of simultaneous completion of degeneration of myelinated fibers of medium and small diameter.     

Long-Term Cultures of Human Cornea Limbal Explants Form 3D Structures Ex Vivo – Implications for Tissue Engineering and Clinical Applications

Long-term cultures of cornea limbal epithelial stem cells (LESCs) were developed and characterized for future tissue engineering and clinical applications. The limbal tissue explants were cultivated and expanded for more than 3 months in medium containing serum as the only growth supplement and without use of scaffolds. Viable 3D cell outgrowth from the explants was observed within 4 weeks of cultivation. The outgrowing cells were examined by immunofluorescent staining for putative markers of stemness (ABCG2, CK15, CK19 and Vimentin), proliferation (p63α, Ki-67), limbal basal epithelial cells (CK8/18) and differentiated cornea epithelial cells (CK3 and CK12). Morphological and immunostaining analyses revealed that long-term culturing can form stratified 3D tissue layers with a clear extracellular matrix deposition and organization (collagen I, IV and V). The LESCs showed robust expression of p63α, ABCG2, and their surface marker fingerprint (CD117/c-kit, CXCR4, CD146/MCAM, CD166/ALCAM) changed over time compared to short-term LESC cultures. Overall, we provide a model for generating stem cell-rich, long-standing 3D cultures from LESCs which can be used for further research purposes and clinical transplantation.     

Quantitative morphological changes in endocrine pancreas of rats with spontaneous diabetes mellitus

The present study describes the cytopathology of the pancreatic islets in 18-old male eSS rats with spontaneous diabetes mellitus as compared to aged-matched normal animals. Light-microscopic immunocytochemical and morphometric techniques were used to study islet-cell populations, while quantitative methods were employed specifically for the analysis of B-cell ultrastructure. The diabetic rats showed disruption of the islet structure and fibrosis in the stroma. The volume density (Vvi) of endocrine tissue and the Vvi and percentage of B cells were diminished, whereas the Vvi of exocrine tissue and the Vvi and percentage of D cells were increased. The number of medium and large islets as well as their mean volume (micron3) decreased in these animals. Pancreatic B cells from eSS rats showed an increase in the Vvi of endoplasmic reticulum, immature secretory granules and lysosomes. Conversely, the Vvi of total secretory granules and microtubules appeared diminished. The current observations contribute to our understanding of this useful animal model of diabetes mellitus, in the attempt to clarify the pathogenesis of the disease.     

Gravity sensing in the central nervous system

For human based space research it is of high importance to understand the influence of gravity on the properties of the central nervous system (CNS). Untill now it is not much known about how neuronal tissue can sense gravity. The aim of this study was to find out weather and how the CNS, as a complex system, can percept and react to changes in gravity. Neuronal tissue and especially the CNS fulfils all the requirements for excitable media. Consequently, self-organisation, pattern formation and propagating excitation waves as typical events of excitable media have been observed in such tissue. The Spreading Depression (SD), an excitation depression wave is the most obvious and best described of these phenomena in the CNS. In our experiments we showed that the properties of the SD and therefore the CNS in its properties as an excitable medium reacts very sensitive to changes in gravity.     

Model for computer simulation of bone tissue

The paper deals with the dependence of the torsional moment on the angle of the compact bone torsion in laboratory animals and humans. Based on the data for laboratory animals obtained by measurements, the data on dependence of the torsional moment and the angle of torsion were predicted for humans. The measurements were carried out in four groups of laboratory animals. One was the control group, and the other three groups were treated by various vitamin D3 metabolites. The same measurements were performed also in only one group of humans, due to the impossibility to treat humans with vitamin D3 metabolites. The functional relationship between the angle of torsion and the torsional moment for all the groups of animal bone tissue were determined by measurements, and results were used to predict the reaction of the human compact bone tissue if treated by vitamin D3 metabolites.     

Cardioelectric field on the atrial surface in pigs

The form and distribution of extracellular cardioelectric potentials and the sequence of the excitation wave propagation on epicardium of the pig atria were studied by the method of multichannel synchronous cardioelectrotopography. The studies have shown that in pig the excitation wave breaks on epicardium of the right atrium at the base of the upper vena cava. Negative initial atrial complexes are registered in this area. Two fronts of excitation wave spread from the zone of initial epicardial activation: one--to upper segments of dorsal and ventral sides of the right atrium, the second--to inter-atrial septum. The excitation wave comes to the left atrium with a delay relative to the beginning of depolarization of the right atrium. On account of the successive movement of the front of the excitation wave from pacemaker the two-phase potentials are formed on greater part of the epicardium of the pig atria. The lower part of the auricle of the left atrium is depolarized in atrial epicardium in the last turn. The sequence of excitation wave propagation in atrial epicardium close to ravenous (dog) and ungulate (sheep) animals and man is typical for the pig, but herewith the differences in time of covering the atria with excitation do exist.     

On Propagation of Excitation Waves in Moving Media: The FitzHugh-Nagumo Model

Background

Existence of flows and convection is an essential and integral feature of many excitable media with wave propagation modes, such as blood coagulation or bioreactors.

Methods/Results

Here, propagation of two-dimensional waves is studied in parabolic channel flow of excitable medium of the FitzHugh-Nagumo type. Even if the stream velocity is hundreds of times higher that the wave velocity in motionless medium (), steady propagation of an excitation wave is eventually established. At high stream velocities, the wave does not span the channel from wall to wall, forming isolated excited regions, which we called “restrictons”. They are especially easy to observe when the model parameters are close to critical ones, at which waves disappear in still medium. In the subcritical region of parameters, a sufficiently fast stream can result in the survival of excitation moving, as a rule, in the form of “restrictons”. For downstream excitation waves, the axial portion of the channel is the most important one in determining their behavior. For upstream waves, the most important region of the channel is the near-wall boundary layers. The roles of transversal diffusion, and of approximate similarity with respect to stream velocity are discussed.

Conclusions

These findings clarify mechanisms of wave propagation and survival in flow.     

Sequential use of human-derived medium supplements favours cardiovascular tissue engineering

For clinical application of tissue engineering strategies, the use of animal-derived serum in culture medium is not recommended, because it can evoke immune responses in patients. We previously observed that human platelet-lysate (PL) is favourable for cell expansion, but generates weaker tissue as compared to culture in foetal bovine serum (FBS). We investigated if human serum (HS) is a better human supplement to increase tissue strength. Cells were isolated from venous grafts of 10 patients and expanded in media supplemented with PL or HS, to determine proliferation rates and expression of genes related to collagen production and maturation. Zymography was used to assess protease expression. Collagen contraction assays were used as a two-dimensional (2D) model for matrix contraction. As a prove of principle, 3D tissue culture and tensile testing was performed for two patients, to determine tissue strength. Cell proliferation was lower in HS-supplemented medium than in PL medium. The HS cells produced less active matrix metallo-proteinase 2 (MMP2) and showed increased matrix contraction as indicated by gel contraction assays and 3D-tissue culture. Tensile testing showed increased strength for tissues cultured in HS when compared to PL. This effect was more pronounced if cells were sequentially cultured in PL, followed by tissue culture in HS. These data suggest that sequential use of PL and HS as substitutes for FBS in culture medium for cardiovascular tissue engineering results in improved cell proliferation and tissue mechanical properties, as compared to use of PL or HS apart.     

Analysis of bone tissue mechanical properties

This paper deals with the torsional moment depending on the angle of torsion of the compact bone in laboratory animals and humans. Based on the data from laboratory animals, obtained by measurement, the data on dependence of the torsional moment and the angle of torsion were assumed for humans. Measurements were carried out on four groups of compact bone in laboratory animals. One was the control group, and three other groups were treated by various vitamin D3 metabolites. Equal measurements were performed in only one group of compact bone in humans, due to the impossibility to treat humans with vitamin D3 metabolites. Functional relations between the angle of torsion and the torsional moment for all groups of animal body tissue were determined by measurements, and the results were used to assume the reaction of human compact bone tissue if treated by vitamin D3 metabolites.     

Multiplex FISH and three-dimensional DNA imaging with near infrared femtosecond laser pulses

We report on a novel technology for multicolor gene and chromosome detection as well as for three-dimensional (3D) DNA imaging by multiphoton excitation of multiple FISH fluorophores and DNA stains. Near infrared femtosecond laser pulses at 770 nm were used to simultaneously excite the visible fluorescence of a wide range of FISH fluorophores, such as FITC, DAC, Cy3, Cy5, Cy5.5, rhodamine, spectrum aqua, spectrum green, spectrum orange, Jenfluor, and Texas red as well as of DNA/chromosome stains, for example Hoechst 33342, DAPI, SYBR green, propidium iodide, ethidium homodimer, and Giemsa. In addition to the advantage of using only one excitation wavelength for a variety of fluorophores, multiphoton excitation provided the intrinsic possibility of 3D fluorescence imaging. The technology has been used in human genetics for the diagnosis of numerical chromosome aberrations and microdeletions. In particular, multicolor 3D images of the intranuclear localization of FISH-labeled chromosome territories in interphase nuclei of amniotic fluid cells have been obtained. Using the high light penetration depth at 770 nm, optical sectioning of Hoechst 33342-labeled DNA within living culture cells and within tissue of living tumor-bearing mice was performed.     

Interstitial potentials and their change with depth into cardiac tissue.

The electrical source strength for an isolated, active, excitable fiber can be taken to be its transmembrane current as an excellent approximation. The transmembrane current can be determined from intracellular potentials only. But for multicellular preparations, particularly cardiac ventricular muscle, the electrical source strength may be changed significantly by the presence of the interstitial potential field. This report examines the size of the interstitial potential field as a function of depth into a semi-infinite tissue structure of cardiac muscle regarded as syncytial. A uniform propagating plane wave of excitation is assumed and the interstitial potential field is found based on consideration of the medium as a continuum (bidomain model). As a whole, the results are inconsistent with any of the limiting cases normally used to represent the volume conductor, and suggest that in only the thinnest of tissue (less than 200 micron) can the interstitial potentials be ignored.     

Different sensitivities of two mechanisms of excitation waves in heart tissue to anti-arrhythmia agents--blockers of fast sodium currents

In experiments with rabbit ventricular tissue sensitivity of two kinds of spiral wave sources of excitation to fast sodium current inhibition was compared. These spiral wave sources were circulation in a ring around an obstacle and circulation in tissue without an obstacle (reverberator). It was observed that after application of antiarrhythmic drugs lidocaine or mexiletine there was a prominent growth of cycle length of reverberator during first few seconds of circulation and a little change of cycle length for the circus movement around obstacle. It is proposed that different sensitivity of both kinds of spiral wave sources to antiarrhythmic drugs can be used for their distinguishing in clinical practice.     

Vulnerability in an excitable medium: analytical and numerical studies of initiating unidirectional propagation.

Cardiac tissue can display unusual responses to certain stimulation protocols. In the wake of a conditioning wave of excitation, spiral waves can be initiated by applying stimuli timed to occur during a period of vulnerability (VP). Although vulnerability is well known in cardiac and chemical media, the determinants of the VP and its boundaries have received little theoretical and analytical study. From numerical and analytical studies of reaction-diffusion equations, we have found that 1) vulnerability is an inherent property of Beeler-Reuter and FitzHugh-Nagumo models of excitable media; 2) the duration of the vulnerable window (VW) the one-dimensional analog of the VP, is sensitive to the medium properties and the size of the stimulus field; and 3) the amplitudes of the excitatory and recovery processes modulate the duration of the VW. The analytical results reveal macroscopic behavior (vulnerability) derived from the diffusion of excitation that is not observable at the level of isolated cells or single reaction units.     

The hypothalamo-pancreatic axis: evidence for a neurohormonal pathway in the control of the release of insulin and glucagon.

Segments of freshly dissected rat hypothalamic tissue corresponding to the ventrolateral (VLH) and ventromedial (VMH) regions were incubated in Gey and Gey medium at 37 C under 95% O2 and 5% CO2 for 30 minutes. Groups of male rats which had been fasted for 6 hours received injections (i.v.) of either VMH medium or VLH medium while a third group received control medium only. Blood samples were taken from the aorta 3 minutes post-injection and circulating levels of insulin and glucagon were determined by RIA. The medium from the incubation of the VMH tissue significantly elevated glucagon levels and significantly lowered plasma concentrations of insulin compared to the levels in animals receiving injections of control medium. The hormone levels in animals receiving an injection of medium in which VLH tissue had been incubated did not differ significantly from the controls. In another type of experiment VMH medium, but not VLH medium, was able to overcome the somatostatin-induced inhibition of glucagon release. These observations suggest that hypothalamic factors may be involved in the regulation of the endocrine pancreas.