In situ morphology of the ductus venosus and related vessels in the fetal and neonatal rat.

In situ cross-sectional morphology of the ductus venosus and related vessels was studied after rapid whole-body freezing of the fetal and neonatal rat. In the fetus, the ductus venosus was open widely, connecting the umbilical sinus and the inferior vena cava. The diameter of the ductus venosus was 50% of the diameter of the umbilical sinus. The ductus venous joined the left dorsal side of the inferior vena cava. A thin, short, membrane-like edge was present at the inner junction of the ductus venosus and the inferior vena cava, presumably effecting laminar flow of the ductus venosus blood to the left side of the thoracic inferior vena cava. A very prominent eustachian valve was present at the junction of the inferior vena cava and the right atrium, presumably directing its flow to the opening of the foramen ovale. After birth, the ductus venosus narrowed rapidly and closed completely in 2 d. The closing ductus venosus was tubular, with the cranial end slightly wider than the caudal portion. Localized constriction was not present. These observations showed the structural substrate for preferential flow from the ductus venosus to the foramen ovale and left atrium in the fetus and did not support localized sphincter mechanism in postnatal closure of the ductus venosus.