Morphological analysis of the hagfish heart. II. The venous pole and the pericardium

Abstract

The morphological characteristics of the venous pole and pericardium of the heart were examined in three hagfish species, Myxine glutinosa, Eptatretus stoutii, and Eptatretus cirrhatus. In these species, the atrioventricular (AV) canal is long, funnel-shaped and contains small amounts of myocardium. The AV valve is formed by two pocket-like leaflets that lack a papillary system. The atrial wall is formed by interconnected muscle trabeculae and a well-defined collagenous system. The sinus venosus (SV) shows a collagenous wall and is connected to the left side of the atrium. An abrupt collagen-muscle boundary marks the SV-atrium transition. It is hypothesized that the SV is not homologous to that of other vertebrates which could have important implications for understanding heart evolution. In M. glutinosa and E. stoutii, the pericardium is a closed bag that hangs from the tissues dorsal to the heart and encloses both the heart and the ventral aorta. In contrast, the pericardium is continuous with the loose periaortic tissue in E. cirrhatus. In all three species, the pericardium ends at the level of the SV excluding most of the atrium from the pericardial cavity. In M. glutinosa and E. stoutii, connective bridges extend between the base of the aorta and the ventricular wall. In E. cirrhatus, the connections between the periaortic tissue and the ventricle may carry blood vessels that reach the ventricular base. A further difference specific to E. cirrhatus is that the adipose tissue associated with the pericardium contains thyroid follicles. J. Morphol. 277:853-865, 2016. © 2016 Wiley Periodicals, Inc.

Introducing a novel mechanism to control heart rate in the ancestral pacific hagfish

“…Icardo et al (2016) were unable to observe specialized nodal tissue in the sinus wall or at the sinoatrial junction. Yet, electrocardiogram recordings in E. cirrhatus (Davie et al, 1987) showed a depolarization specific to only the central region of the sinus venosus, something not seen, however, in M. glutinosa (Arlock, 1975;Satchell, 1991).…”

Development and evolution of the metazoan heart

“…In squid, vascular endothelial growth factor (VEGF) and FGF receptors have been found in embryonic vessels, pointing to conserved signaling pathways; however, more studies are necessary to establish the gene regulatory networks in molluscan cardiogenesis. The circulation system in hagfish, the most ancestral of extant vertebrates, is special as it includes several accessory pumps to the branchial heart, which is equivalent to the heart in other vertebrates . These hearts exhibit limited looping and absent OFT elements.…”

“…As the sinus venosus is myocardialized in many species, it can be considered a true cardiac chamber. It will remain recognizable as a separate structure in hagfish connected to the left side of the common atrium, but in birds and most mammals (except monotremes) it will become incorporated in the dorsal atrial wall (Figure ). In lungfish, a separate pulmonary channel traverses the main atrial compartment and delivers oxygenated blood directly to the ventricle .…”

The vertebrate heart: an evolutionary perspective

“…Small amounts of myocardium can be noted inside the canal, which incorporates a two‐leafed AV valve without any defining papillary muscles (Icardo et al. ).…”

“…Underneath the atrium is a single ventricle, which is attached via an elongated atrioventricular (AV) canal. Small amounts of myocardium can be noted inside the canal, which incorporates a two-leafed AV valve without any defining papillary muscles (Icardo et al 2016).…”