Heart inflow tract of the African lungfish Protopterus dolloi

We report a morphologic study of the heart inflow tract of the African lungfish Protopterus dolloi. Attention was paid to the atrium, the sinus venosus, the pulmonary vein, and the atrioventricular (AV) plug, and to the relationships between all these structures. The atrium is divided caudally into two lobes, has a common part above the sinus venosus, and appears attached to the dorsal wall of the ventricle and outflow tract through connective tissue covered by the visceral pericardium. The pulmonary vein enters the sinus venosus and runs longitudinally toward the AV plug. Then it fuses with the pulmonalis fold and disappears as an anatomic entity. However, the oxygenated blood is directly conveyed into the left atrium by the formation of a pulmonary channel. This channel is formed cranially by the pulmonalis fold, ventrally by the AV plug, and caudally and dorsally by the atrial wall. The pulmonalis fold appears as a wide membranous fold which arises from the left side of the AV plug and extends dorsally to form the roof of the pulmonary channel. The pulmonalis fold also forms the right side of the pulmonary channel and sequesters the upper left corner of the sinus venosus from the main circulatory return. The AV plug is a large structure, firmly attached to the ventricular septum, which contains a hyaline cartilaginous core surrounded by connective tissue. The atrium is partially divided into two chambers by the presence of numerous pectinate muscles extended between the dorsal wall of the atrium and the roof of the pulmonary channel. Thus, partial atrial division is both internal and external, precluding the more complete division seen in amphibians. The present report, our own unpublished observations on other Protopterus, and a survey of the literature indicate that not only the Protopterus, but also other lungfish share many morphologic traits.