Light and electron microscopy of the bulbus arteriosus of the European eel(Anguilla anguilla)

The bulbus arteriosus of teleost fish acts as an elastic reservoir that dil ates during ventricular systole to store a large part of the cardiac stroke volume. Despite its functional importance, the knowledge of the structure of the bulbus wall is still fragmentary. We have undertaken a series of stu dies in order to establish a general morphological plan of the teleost bulb us. The bulbus arteriosus of the European eel is studied here by means of c onventional light and transmission and scanning electron microscopy. The in ner surface of the bu[bus wall is irregular due to the presence of branchin g ridges that flatten and disappear toward the ventral aorta. The ridge sur face is covered by flattened endocardial cells that show moderately dense b odies. In the ridge tissue, cells near the endocardium are mostly undiffere ntiated and appear isolated in a loose filamentous matrix. Ridge cells prog ressively cluster toward the middle layer, become surrounded by a dense mat rix, and adopt characteristics typical of smooth muscle cells. This suggest s the existence of a differentiation gradient. The middle layer is formed b y typical smooth muscle cells embedded in a meshwork matrix that contains t hin and thick filaments. Stretching of this meshwork suggests an active rol e of smooth muscle cells in bulbus wall dynamics. Furthermore, large areas of the extracellular space are occupied by elastin-like material. The amoun t of this material decreases toward the external layer. Collagen is demonst rated across the entire thickness of the bulbus wall, its amount and organi zation increasing from the inner toward the outer bulbus surface. The exist ence of matrix gradients should progressively increase wall strength, maint aining bulbus dilation within safe physiological parameters. The epicardium is formed by flattened cells that contain numerous pinocytotic vesicles, s uggesting an active interchange of solutes with the pericardial cavity. Cop yright (C) 2000 S. Karger AG, Basel.