Cartilage in the Bulbus Arteriosus of Teleostean Fishes

In a comparative study of the cardiac outflow tract in teleostean fishes, eight specimens (two Sardina pilchardus, four Trachurus trachurus, one Coris julis and one Arnoglossus thori) displayed one or two cartilaginous foci in the wall of the bulbus arteriosus. The foci were located in the longitudinal bulbar ridges into which the distal portions of the leaflets of the right and left major bulbo-ventricular valves were attached. The cartilage was of hyaline nature; it consisted of sparsely distributed chondrocytes, embedded in a type II collagen-positive extracellular matrix. Each focus was surrounded by a collagenous capsule into which collagen fibres from the lateral attachments of the valve leaflets were inserted. Presence of cartilage in the attachments of the valve leaflets is not a requisite for normal function of the bulbo-ventricular valves. However, when cartilaginous foci occur at these sites, they may function as competent pivots in the context of the mechanical stimulation to which the lateral portions of the valve leaflets are subjected during cardiac cycle. Previous work carried out by other authors has demonstrated that in birds, the cardiac cartilage is of neural crest origin. The presence of cartilage in the bulbus arteriosus of teleosts points to the possibility that neural crest cells might enter this cardiovascular structure during embryonic development, however, the origin of the chondrocytes occurring in the bulbus arteriosus of teleosts remains an open question.