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Working and specialized cardiac myocytes and their intercalated discs in adult human hearts without history of cardiac disease were examined by scanning electron microscopy. The NaOH/ultrasonication treatment of cardiac tissues resulted in the digestion of connective tissue and separation of intercellular junctions. Auricular and ventricular working cardiac myocytes were quasi-cylindrical in shape, bifurcated, and connected end-to-end at the intercalated discs. The intercalated discs in the working cardiac myocytes showed a stair-like profile, consisting of steps (plicate segments) and corresponding risers (interplicate segments). The ventricular myocytes, in particular, had many steps and risers. The plicate segments were filled with numerous finger-like microprojections. The strands of the myocytes in the sinoatrial node were oriented linearly, while those in the atrioventricular node formed a reticular network. The intercalated discs in both nodal cells were underdeveloped, having few microprojections. Myocytes in the atrioventricular bundle (His) and the right limb were arranged in parallel, and were characterized by the presence of slender branches. Purkinje cell strands formed reticular networks. The intercalated discs in the His-Purkinje system were irregular in appearance, and the microprojections were larger in size and smaller in number than those of working myocytes. The myocytes in the crista terminalis and surrounding the fossa ovalis resembled cells in the His-Purkinje system rather than auricular working myocytes in morphology, and may act as the internodal pathway. It is concluded that morphological differences in both the cytoarchitecture and intercalated discs were closely related with contraction and impulse propagation in the various regions of the human heart.

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