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Inter-ventricular septum: New observations on the structure and function coupling

Histological relationships and the distribution of junctions between myocardial cells and connective tissue were investigated in the inter-ventricular septum. Pig hearts were studied using 2 techniques: Picro-Sirius red-collagen-staining of frozen sections for connective tissue septa, and immune-staining with antibodies against Desmoplakins for intercalated discs. By overlaying serial sections 10 microns apart, the distribution of 1,010 intercalated discs in 11 base-to-apex fields was counted and scored in relation to connective tissue septa. The orientation of connective tissue structures was also observed in rat and rabbit hearts. The inter-ventricular septum is not separated into gross anatomical parts by connective tissue septa. Perimysial connective tissue septa run base-to-apex within the inter-ventricular septum following spiral patterns. Gap junctions are absent from connective tissue septa larger than twice the myocyte diameter. Only 8.36% of the total number of intercalated discs were co-localised with in the connective tissue using an overlay technique of immunocytochemical and connective tissue stains. Co-localisation was only 3.00% when the counts were made in the immune-stained sections, guided for connective tissue localisation by serial sections stained with Sirius red. Histology confirms the spiral pattern of ventricular myocardial structures. The reported ultrasound septal reflection is caused by changes in the direction of myocytes rather than the presence of connective tissue sheets. The near absence of intercalated discs in perimysial connective tissue tracts in the interventricular septum confirms the myocardial predisposition for longitudinal propagation of electrical excitation along the sheets of myocardium defined by the perimysial septa. These septa, not directly involved in impulse transmission between myocardial cells, modulate the propagation of electrical excitation. These aspects of structure and electrophysiological relationship within the inter-ventricular septum have important consequences for understanding ventricular function and dysfunction, and for choosing anisotropy factors to be used in mathematical models of impulse propagation in the heart.