The functional architecture of skeletal compared to cardiac musculature; myocyte orientation, lamellar unit morphology, and the helical ventricular myocardial band.
AbstractHow the cardiomyocytes are aggregated within the heart walls remains contentious. We still do not fully understand how the end-to-end longitudinal myocytic chains are arranged, nor the true extent and shape of the lamellar units they aggregate to form. In this review, we show that an understanding of the complex arrangement of cardiomyocytes requires knowledge of 3-dimensional cell orientation (helical and intrusion angle), and appreciation of cell packing within the connective tissue matrix. We show how visualisation and segmentation of high resolution 3-dimensional image data can accurately identify the morphology and orientation of the myocytic chains, and the lamellar units. Some maintain that the ventricles can be unwrapped in the form of a "helical ventricular myocardial band". Implying the ventricular muscle is arranged comparable to that of skeletal muscles, in an ordered fashion, as a compartmentalised band with selective regional innervation and deformation, and a defined origin and insertion. In contrast to the simpler interpretation of the helical ventricular myocardial band, we provide insight as to how the complex myocytic chains, the heterogeneous lamellar units, and connective tissue matrix form an interconnected meshwork, which facilitates the complex internal deformations of the ventricular wall. We highlight the dangers of disregarding the intruding cardiomyocytes. Preparation of the band destroys intruding myocytic chains, and thus disregards the functional implications of the antagonistic auxotonic forces they produce. We conclude that the ventricular myocardium is not analogous to skeletal muscle, but is a complex 3-dimensional meshwork, with a heterogeneous branching lamellar architecture.