Development of the ductus venosus in the SD rat.

We used scanning electron microscopy to observe the development of the ductus venosus in the fetal rat liver. At day 13 of gestation, the vascular system in the liver was already formed and the umbilical vein had branched many capillaries to the parenchyma of the liver and was connected to the posterior vena cava directly by one small ductus venosus. At day 14 of gestation, the umbilical vein bulged at its terminal part and bifurcated into the ductus venosus, which joined the posterior vena cava, and a branch that anastomosed with the vitelline vein. The ductus venosus had no branches and subsequently enlarged and then degenerated just before birth. The bulging part of the umbilical vein and its branches degenerated in the later stages of gestation. The vitelline vein developed to form the capillaries of the liver and the intestinal venous system. In the SD rat liver, the ductus venosus was therefore established by development of the terminal part of the umbilical vein, which anastomosed directly with the posterior vena cava.