Dr.-Ing. Gunnar Seemann

Bettina Schwab

Cardiac tissue can be considered as a composite material consisting of cells and fluids. These cells include amongst others myocytes and fibroblasts. Myocytes and fibroblasts make up the majority with respect to cell number with myocytes dominating in their volume fraction. Myocytes are the cell types that both generate the electrical activation of the heart and do the mechanical work. Fibroblasts on the other hand play a role in healing processes and have been suggested to be sinks for electrical charge. Therefore, fibroblasts can influence the electrophysiological properties of cardiac tissue.
Myocytes are electrically coupled via gap junction channels. In studies of cell culture, electrical coupling via gap junction channels has also been reported between myocytes and fibroblasts as well as in-between fibroblasts. Due to gap junctions channels of low conductance,the electrical coupling between myocytes and fibroblasts as well as in-between fibroblasts appears to be of lesser degree than in-between myocytes. However, fibroblasts are abundant in cardiac tissue and their ratio (by number) to myocytes ranges between 0.5 and 2.4. Significantly larger ratios can be found in cardiac disease such as infarction.
Nevertheless, contribution of fibroblasts to conduction is still not well understood and their role in cardiac diseases is controversially discussed. A salutary role in cardiac conduction was suggested to result from increased electrical coupling by fibroblasts, in particular in regions, where myocytes are isolated by collagenous septa and scars. It was also suggested that fibroblasts might take a malignant arrhythmogenic role by forming electrical bridges or current sinks. Furthermore, a pathological increase of fibroblasts (fibrosis) can lead to a heterogeneous repolarization, which also might favor arrhythmogenic events. Resolving these controversies has been impeded because direct measurement of electrical coupling of fibroblasts in cardiac tissue in situ and assessment of the effects is difficult.
Mathematical descriptions of the electrophysiology of cardiac fibroblasts have been developed and applied in computational simulations to gain insights into electrical interactions between fibroblasts and myocytes. In these simulations, established mathematical models of ventricular myocytes have been electrically coupled to membrane models of cardiac fibroblasts. Gap junction channels between myocytes and fibroblasts have been represented as ohmic resistors. Simulations have been performed to study interactions of a single myocyte with a varying number of coupled fibroblasts and to characterize conduction in cable-like cell strands and two-dimensional tissue slices.
In this work, confocal microscopic data of rabbit tissue are the basis to develop an in-silico model of conduction in infarcted rabbit heart. The microscopic data consist of fluorescence images of cardiac tissue slices. The cell nucleus, gap junction connexin 43, fibroblasts and collagen are labeled with fluorescence dyes of different spectra. These data are used to quantify the fibroblast-myocyte ratio and amount of coupling gap junctions between the cells. Microscopic imaging is known to exhibit artefacts, for instance, blurring and cross-talk. Blurring can be distinguished by the so-called point spread function, the image of a single point. Reassigning the blurred light to its origin is done by a process called deconvolution. Also, the excitation and emission spectra of fluorophores are distributed and overlap to some extent with spectra of other fluorophores. Therefore, there is crossalk between the channels. A 4-channel deconvolution and cross-talk correction method will be developed in order to attenuate these imaging artefacts.
In the simulation part, the previously developed multi-domain model representing the electrical properties of cardiac tissue in different domains like myocytes, fibroblast and the extracellular space will be used. A rabbit ventricular cell model should be used in order reconstruct the measurements. If new measurement data of rabbit fibroblasts exists this data will be integrated in the previously developed model of cardiac fibroblasts. Within this model, the fibroblast-myocyte ratio and the coupling should be varied in order to investigate the effects on the cells electrophysiology and conduction e.g. conduction block. It should also be quantified which thickness a fibroblast layer must have in order to build a barrier for conduction. Furthermore, reentry should be investigated in a 2-dimensional model. As a next step, a schematic 2D description of infarct with islands of surviving myocytes has to be developed. This will be used to validate the hypothesis that isolated myocytes in the vicinity of normal tissue might lead to so-called reflected re-entry. Based on the microscopic data, the amount and coupling of fibroblasts should be quantified in the physiological case and infarction. A homogenized 3D model of the infarct tissue will be developed in order to validate the simulation against optical fluorescence measurement of the electrical propagation in the same tissue.