Ultrastructure of the atrioventricular valve and ventricular endo- and myocardium of the tigerfish, Hydrocynus forskahlii

General ultrastructural properties of the atrioventricular valve and the endo- and myocardium of the tigerfish Hydrocynus forskahlii add further information to our knowledge of fish heart morphology. Small pieces of tissue from the heart of H. forskhalii were immersion-fixed in 3% glutaraldehyde in Sorensen's phosphate buffer. Samples were postfixed in 1% osmium tetroxide and processed for transmission electron microscopy using standard techniques. The atrioventricular valve consists of dense collagen and fibroblasts lying between two layers of squamous endothelial cells. The ventricular endothelium consists of squamous cells with large nuclei protruding into the lumen of the ventricle. These cells lie on a basal lamina which overlies loose collagen in the subendothelial space. The ventricular myocardium is built up of distinctive nucleated fibres. Mitochondria, ribosomes, and glycogen are abundant in the sarcoplasm. Less conspicuous intercalated discs with no typical, steplike pattern are found between the myocardial cells. The Z lines as well as the I and A bands are clearly visible. The M lines are prominent, but the most important feature is the N line which is found on either side of the Z line in the lateral parts of the I band. The current research clearly underlines the fact that cardiac ultrastructure presents considerable diversity in different fish species.