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Ultrastructural studies on the atrio-ventricular junction and ventriculo-bulbar junction in heart of teleosts.

The ultrastructure of the atrio-ventricular junction (A-V junction) and ventriculo-bulbar junction in the heart of a teleostean species, Gadus virens L., is described. The former junction is a narrow canal having a length of 0.35 to 0.60 mm and a wall thickness of 0.20 to 0.32 mm. At the lumenal side of the junctional wall occurs a layer of myocardial tissue which is in continuity with that of the atrium and ventricle. Further, at the atrial side of the junction this tissue is trabeculated, whereas it is untrabeculated at the ventricular side. A number of nerves occur near the atrium. The myocardial cells are tightly packed into bundles, which are surrounded by a thick external lamina. These cells are connected by numerous nexuses and desmosomes, and are rich in myofibrils. At the atrial side of the A-V junction the endocardial cells contain numerous bristle-coated vesicles, tubules of agranular endoplasmic reticulum and large inclusion bodies of moderate electron density, whereas at the ventricular side these cells are poor in bristle-coated vesicles and agranular endoplasmic reticulum, and the inclusion bodies are small and electron dense. The ventriculo-bulbar junction is a narrow canal; the wall is 0.5 to 1.0 mm thick and contains outermost high amounts of loose collagen tissue, and innermost a layer of dense collagen tissue. Between the 2 latter layers occurs a layer of myocardial tissue which is continuous with that of the ventricle. The endocardial cells in this junction is structurally similar to those located at the ventricular side of the A-V junction. The structure of the A-V junction in teleosts is compared with that of the A-V node and bundle of His in mammals, and the sino-atrial junction in teleosts. It is proposed that the myocardial tissue of the A-V junction in teleosts may conduct electrical impulses between atrium and ventricle.