Spatial distribution and network morphology of epicardial, endocardial, interstitial, and Purkinje cell-associated elastin fibers in porcine left ventricle.

Cardiac extracellular matrices (ECM) play crucial functional roles in cardiac biomechanics. Previous studies have mainly focused on collagen, the major structural ECM in heart wall. The role of elastin in cardiac mechanics, however, is poorly understood. In this study, we investigated the spatial distribution and microstructural morphologies of cardiac elastin in porcine left ventricles. We demonstrated that the epicardial elastin network had location- and depth-dependency, and the overall epicardial elastin fiber mapping showed certain correlation with the helical heart muscle fiber architecture. When compared to the epicardial layer, the endocardial layer was thicker and has a higher elastin-collagen ratio and a denser elastin fiber network; moreover, the endocardial elastin fibers were finer and more wavy than the epicardial elastin fibers, all suggesting various interface mechanics. The myocardial interstitial elastin fibers co-exist with the perimysial collagen to bind the cardiomyocyte bundles; some of the interstitial elastin fibers showed a locally aligned, hinge-like structure to connect the adjacent cardiomyocyte bundles. This collagen-elastin combination reflects an optimal design in which the collagen provides mechanical strength and elastin fibers facilitate recoiling during systole. Moreover, cardiac elastin fibers, along with collagen network, closely associated with the Purkinje cells, indicating that this ECM association could be essential in organizing cardiac Purkinje cells into "fibrous" and "branching" morphologies and serving as a protective feature when Purkinje fibers experience large deformations in vivo. In short, our observations provide a structural basis for future in-depth biomechanical investigations and biomimicking of this long-overlooked cardiac ECM component.