Ventricle and outflow tract of the African lungfish Protopterus dolloi

We report a morphologic study of the heart ventricle and outflow tract of the African lungfish Protopterus dolloi. The ventricle is saccular and appears attached to the anterior pericardial wall by a thick tendon. An incomplete septum divides the ventricle into two chambers. Both the free ventricular wall and the incomplete ventricular septum are entirely trabeculated. Only a thin rim of myocardium separates the trabecular system from the subepicardial space. The outflow tract consists of proximal, middle, and distal portions, separated by two flexures, proximal and distal. The proximal outflow tract portion is endowed with a layer of compact, well-vascularized myocardium. This portion is homologous to the conus arteriosus observed in the heart of most vertebrates. The middle and distal outflow tract portions are arterial-like, thus being homologous to the bulbus arteriosus. However, the separation between the muscular and arterial portions of the outflow tract is not complete in the lungfish. A thin layer of myocardium covers the arterial tissue, and a thin layer of elastic tissue underlies the conus myocardium. Two unequal ridges composed of loose connective tissue, the spiral and bulbar folds, run the length of the outflow tract. They form an incomplete division of the outflow tract, but fuse at the distal end. The two folds are covered by endocardium and contain collagen, elastin, and fibroblast-like cells. They appear to be homologous to the dextro-dorsal and sinistro-ventral ridges observed during the development of the avian and mammalian heart. Two to three rows of vestigial arterial-like valves appear in the dorsal and ventral aspects of the conus. These valves are unlikely to have a functional role. The possible functional significance of the "gubernaculum cordis," the thick tendon extending between the anterior ventricular surface and the pericardium, is discussed.