The ductus venosus (DV) and the intrahepatic branches of the portal vein (BPV) play an important role in umbilical blood distribution to the fetal liver and the rest of the fetal circulation. Increased DV shunting is a major fetal survival mechanism during stress situations. The availability of a nonpregnant primate animal model with similar structure and function would greatly improve our understanding of DV function. However, the anatomic and histologic structure of the DV and the BPV have not been thoroughly investigated in any nonhuman primate species.

Anatomic and immunohistochemical (Masson's and ⍺-smooth actin stains) investigations were performed on 17 baboon fetuses at 173 ± 5 days' gestation (mean ± SEM, term=180 days) (Papio sp.) and 3 near term rhesus (Macaca mulatta) fetuses.

In both species the branchless, funnel-shaped DV coursed cranially, posteriorly, and slightly oblique to the left side. The DV and the efferent hepatic veins drained into a dilated ampullary area (the collectus venosus) that joined directly with the inferior cava. The length of the DV in baboons increased with gestational age (r=0.86, n=16). In 4 baboon fetuses, we observed an asymmetrical muscular lip at the isthmic portion of the DV. The media of intrahepatic BPV contained more smooth muscle cells than the media of the DV.

In nonhuman primate fetuses, the DV drains into a dilated ampullary area. An asymmetrical muscular lip forms a contractile element of the isthmic portion of the DV. The increased thickness of smooth muscle tissue in the DV isthmus and intrahepatic BPV in nonhuman primate fetuses support the concept of a general organization of a contractile apparatus that performs a sphincter-like function in the central venous hepatic system and plays a key role in blood flow redistribution.