Venous System

In the fifth week, three pairs of major veins can be distinguished: (a) the vitelline veins, or omphalomesenteric veins, carrying blood from the yolk sac to the sinus venosus; (b) the umbilical veins, originating in the chorionic villi and carrying oxygenated blood to the embryo; and (c) the cardinal veins, draining the body of the embryo proper (Fig. 11.41).

Cardinal Vein
Figure 11.40 A. Obliteration of the fourth aortic arch on the right and left and persistence of the distal portion of the right dorsal aorta. B. Case of interrupted aortic arch. The aorta supplies the head; the pulmonary artery, by way of the ductus arteriosus, supplies the rest of the body.

Anterior cardinal vein

Common cardinal vein

Aortic arches (II and III)

Internal carotid artery

Dorsal aorta

Posterior cardinal

Anterior cardinal vein

Common cardinal vein

Aortic Sac

Chorionic villus

Aortic sac

Umbilical vein and artery

Vitelline vein

Vitelline artery

Figure 11.41 Main components of the venous and arterial systems in a 4-mm embryo (end of the fourth week).

Chorionic villus

Aortic sac

Umbilical vein and artery

Vitelline vein

Vitelline artery

Figure 11.41 Main components of the venous and arterial systems in a 4-mm embryo (end of the fourth week).

Vitelline Veins

Before entering the sinus venosus, the vitelline veins form a plexus around the duodenum and pass through the septum transversum. The liver cords growing into the septum interrupt the course of the veins, and an extensive vascular network, the hepatic sinusoids, forms (Fig. 11.42).

With reduction of the left sinus horn, blood from the left side of the liver is rechanneled toward the right, resulting in an enlargement of the right vitelline

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Persistence Left Vitelline Vein
Figure 11.42 Development of the vitelline and umbilical veins during the (A) fourth and (B) fifth weeks. Note the plexus around the duodenum, formation of the hepatic sinusoids, and initiation of left-to-right shunts between the vitelline veins.

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