Three-dimensional imaging reveals a hierarchical organisation of the myocardial mesh in mammalian hearts

Despite intense investigation through centuries, there is still no scientific consensus on how to describe myocardial microarchitecture. This is mainly because the myocardium must be visualised in three dimensions, and with sufficiently high resolution to fully appreciate its complex nature. X-ray microtomography allows for exactly such visualisation. Using this technique, we herein provide a description of the ventricular mural architecture in a wide range of mammals. Myocardial biopsies from the left ventricle of a human, pig, rabbit, giraffe, elephant, and sei whale were imaged using X-ray microtomography after iodine staining. Aggregations of cardiomyocytes were segmented and visualised in three dimensions, permitting accurate assessment of their shape and orientation. It was possible to segment individual components of the overall mesh using the three-dimensional images in all the studied species. The myocardium is most accurately described as a complex hierarchical meshwork, with the cardiomyocyte as the smallest working unit. The cardiomyocytes are bound together by endomysium to form aggregates, which are themselves compartmented by perimysium. A high degree of variation in the shape of aggregation was found within each biopsy, but, most remarkably, significant differences were observed between species. Some bundles of aggregated cardiomyocytes stand out from the adjacent myocardium within the mesh due to a clear change in their orientation. We provide evidence that the mammalian ventricular myocardium is a complex meshwork of cardiomyocytes. This mesh, although continuous along its direction of contraction, is separated by perimysial clefts into aggregated entities most appropriately described as aggregates. When comparing between species, there is remarkable heterogeneity in this anatomical appearance. We found no evidence of the myocardium being ordered into a large individual band as previously described elsewhere.