Histological, histochemical and ultrastructural studies on the bulbus arteriosus of the sticklebacks, Gasterosteus aculeatus and Pungitius pungitius (Pisces: Teleostei)

The bulbar wall has three layers. Its lining consists of squamous-columnar endothelial cells that store neutral mucopolysaccharides and are PAS-positive. They do not contain large amounts of acid phosphatase, acid mucopolysaccharides, glycogen or lipids. A morphometric analysis shows that 32% of the cell volume in Pungitius and 12% in Gasterosteus is occupied by specific granules, 100–600 nm in diameter. According to X-ray probe micro-analysis, these granules bind chromium ions, even though the endothelial cells do not contain catecholamines. Rootlets, packed with plasmalemmal vesicles, extend from the endothelial cells into the middle layer of the bulbus. Here, smooth muscle cells alternate with elastic fibres. The staining reactions of bulbar elastica are compared with those in the mammalian aorta and the ligamentum nuchae. The outer layer of the bulbus is visceral pericardium and beneath its covering mesothelial cells are numerous collagen fibres, non-myelinated nerves, occasional fibroblasts and melanocytes. Scanning electron microscopy shows that the bulbar lining is thrown into longitudinal folds, but that there are no trabeculae subdividing the lumen.
Many features of the bulbus arteriosus may be related to the low systolic pressures of teleosts and to the proximity of their heart and gills. In contrast to mammals, only a small part of the arterial system can act as a windkessel. The bulbus is thus more distensible than the mammalian aorta and must lie within the pericardial cavity so that its greater excursions can be accommodated. Perhaps because the bulbus is so distensible, it has elastic fibres rather than lamellae. This in turn may affect the organization of the smooth muscle cells which do not form "span muscles" as in some mammalian aortae. Like most cells in the bulbus, they are joined to others by desmosomes. Evidently, firm cohesion is important in highly distensible vessels.