The structure of the cardiac foramen ovale from eight genera of pinnipeds was studied using the scanning electron microscope. Specimens were obtained from fetuses or neonates of the Californian sea lion (Zalophus californianus), Antarctic fur seal (Arctocephalus gazella), walrus (Obenus rosmarus), grey seal (Halichoerus gryphus), ringed seal (Phoca hispida), bearded seal (Erignathus barbatus), Weddell seal (Leptonychotes weddelli), and crabeater seal (Lobodon carcinophagus). In each species, the structure that permits oxygenated blood from the placenta flowing in the caudal vena cava to pass directly into the left side of the heart, the foramen ovale, when viewed from the terminal part of the caudal vena cava had the appearance of the entrance to a short tunnel. A thin fold of tissue, the developed remains of the septum primum, projected from the caudal edge of the foramen ovale into the lumen of the left atrium. It constituted about 75% of the inner surface of the tunnel, and was generally unfenestrated. The wall of the interatrial septum contributed the "floor." The distal end of the tunnel was straight-edged. In most cases the septum primum was long enough to cover the foramen ovale. The siting of pulmonary veins in the roof of the left atrium appeared to be such that drainage from them after birth would press the septum primum over the foramen opening, thereby functionally closing it. Collapses of the tunnel was seen in all the neonatal seals, and in the 1-month-old neonate the fold of tissue was anchored to the interatrial septum along the surface of the crista dividens, which lay in the left atrium. Cellular protrusions and thread formation may play a role in the closure of the foramen ovale.
Malposition of Septum Primum
