Veins

There are three venous systems in the early embryonic period.

Omphalomesenteric veins

The omphalomesenteric veins develop in the visceral mesoderm of the secondary umbilical vesicle. They run towards the caudal border of the septum transversum. Soon after, they form anastomoses around the duodenal loop before entering the septum transversum. Parts of these anastomoses degenerate. The portal vein develops from the remaining parts of the anastomoses. The omphalomesenteric veins also form anastomoses within the septum transversum. At S13, hepatic sinusoids arise with the growth of hepatic cords. To begin with, there are two hepato-cardiac channels between the liver and the sinus venosus. The right channel enlarges and becomes the terminal, or post-hepatic, segment of the inferior vena cava; the left channel disappears.