Estimating intracellular conductivity tensors from fluorescent labeling, three-dimensional scanning confocal microscopy data of normal and infarcted tissue including fibroblasts
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 junction 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 currentsinks. 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 the infarcted rabbit heart. The microscopic data consist of fluorescence images of cardiac tissue slices. The cell nucleus, gap junction connexin43, fibroblasts and collagen are labeled with fluorescence dyes of different spectra. In a previous work, these data were used to quantify the volume fractions of myocytes, fibroblast and the extracellular space. Additionally, the extracellular conductivity tensor was estimated and the amount of coupling gap junctions in the vicinity of fibroblasts was quantified. In this work, the intracellular conductivity tensors should be estimated from the confocal microscopic data. Therefore, the myocytes need to be segmented and the gap junction density between myocytes and fibroblasts extracted. With assuming conductivities for intracellular liquid and gap junction resistance, a numerical field calculation is performed for three principal directions in order to extract intracellular conductivity tensors. In the simulation part, the previously developed multidomain model representing the electrical properties of cardiac tissue in different domains like myocytes, fibroblast and the extracellular space will be used with the extracted data and the data from the previous work. A rabbit ventricular cell model in conjunction with a fibroblast model should be used in order reconstruct propagation.