Bulbus arteriosus of the antarctic teleosts. II. The red-blooded Trematomus bernacchii

The structure of the bulbus arteriosus of the Antarctic teleost, Trematomus bernacchii, has been studied by light, scanning, and transmission electron microscopy. The wall of the bulbus arteriosus is divided into endocardial, subendocardial, middle and external layers. The endocardial endothelium co vers the inner surface of the bulbus wall and invaginates into the subendoc ardium to form solid epithelial cords that show secretory activity. The sub endocardial tissue is divided into finger-like ridges. Ridge cells located under the endocardium appear in niches limited by collagen fibers and thin cell extensions. Away from the endocardium ridge cells cluster into small g roups, show some of the characteristics of smooth muscle cells, and appear enmeshed in a filamentous meshwork that lacks collagen and elastin fibers. The middle bulbus layer is formed by typical smooth muscle cells that are e nmeshed in a filamentous meshwork similar to that observed in the ridges. T he ridges and the middle layer appear to be formed by the same cell type, s mooth muscle, with a gradient of differentiation from the endocardium towar d the middle layer. In the absence of elastin fibers the filamentous meshwo rk should confer elastic properties to the bulbus wall. The stretching of t he meshwork along the main axis of the middle layer cells, and between diff erent cellular layers, suggests the existence of tensile stress and, hence, the involvement of smooth muscle cells in bulbus wall dynamics. The extern al layer is formed by numerous cellular types embedded in a collagenous mat rix. Among these cellular types, myofibroblasts, macrophages, granulocytes, lymphocytes, dendrite-like cells, degenerating cells, and plasma cells can be recognized. The subepicardial tissue appears to be a specialized site i nvolved in the production of the humoral immune response and displays many of the morphological characteristics of a germinal center. The outer limiti ng layer of the bulbus, the visceral pericardium, is formed by epithelial c ells that show desmosomes and tight junctions. This suggests a close contro l of permeability with respect to the pericardial fluid. Anat Rec 256:116-1 26, 1999. (C) 1999 Wiley-Liss, Inc.