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The atrioventricular (AV) node of the golden hamster is situated unusually high in the interatrial septum when compared to other species such as the rat. 2 main cell types, characterized by electron-lucent or electron-dense cytoplasm respectively, are found in the node; although both types contain numerous myofilaments these are irregularly arranged and sarcomeric banding is poor. A third variety comparising transitional cells, with features intermediate between the main nodal cells and general atrial myocardial cells, are found at the periphery of the node. Similar electron-lucent and electron-dense cells are also found in the bundle and the mean diameter of bundle cells increases as one passes from the node to the bundle bifurcation. In the node, specific heart granules (SHG) identified by ANP-28 immunoreactivity are found only in transitional cells and even here they are very sparse, unlike general atrial myocytes in which they are plentiful. Numerous nerve varicosities are present throughout the node and bundle and 5-hydroxydopamine (5-OHDA) labelling demonstrates that most of them have features of either noradrenergic or cholinergic terminals; a few non-cholinergic, non-adrenergic varicosities are also present.