Essential features of endocardial and myocardial morphology: SEM and TEM studies.

The conducting pathway of the ferret's myocardium and endocardium was studied under the electron and scanning microscope. Comparisons between the two methods showed that the scanning microscope is well suited for those dimensional demonstration of biological material. Contrary to a relative absence of interspecific differences in endocardial morphology, there is a strong variation of this morphology related to the intracardiac localization of the endocardial cells. The following findings were obtained. S.e. microscopically, it was observed that the endocardium of the sino-atrial node region is not smooth, and that, more likely, it shows rough surfaced profiles. The electron microscopic study shows that the cells of the S-A node are elongated. The S-A node is located at the junction of the superior vena cava with the right atrial wall. It consists of nodal fibres which are embedded in a richinterstitial connective tissue (Figs 1-8). The Purkinje fibres originate from large bundles in the region of the right and left atrioventricular valve in the area where heart muscle fibres were originally described by Purkinje (Purkinje, 1845); these fibres, meanwhile, have become synonymous with cells of the generalized conducting system. The Purkinje fibres consist of a poorly developed contractile apparatus and contain unorganized, fine, filamentous material (Illustration 1). The SR is poorly developed, transverse tubules are absent. S.e. microscopically, one can visualize the trabecular system and the sinusoids. The trabeculae obtain muscle fibres rich in contractile material and transverse tubules. The trabeculae appear to be tendonous (chordae tendineae), especially when they freely traverse the ventricular cavity (Fig. 16). The interventricular septum (the muscle fibres from this region) takes its origin from large bundles in the region of the right and left atrioventricular valves. The endocardium of the interventricular septum is filled with large numbers of plasma-lemma folds (Figs 17, 18). The endocardium which covers the papillary muscle has a thickness of 0.5 micron. The endocardial cells lie on the myocardium so close and so thin that the surface relief and part of the atriation of the myocardium are visible (Figs 13-15).