A conceptual understanding of the events during venous embryology and development will be of great help in understanding the anatomy of the mature venous system as well as its common anatomic variations. Similarly, knowledge of the histologic structure of veins and their valves will facilitate an understanding of venous physiology. This chapter aims to present the development and structure of the venous system in an organized fashion so that its physiology is easily correlated.
EMBRYOLOGY OF THE VENOUS SYSTEM
The embryo has three major venous systems that have distinct functional roles during development and which ultimately differentiate into the adult venous system. The formation of these segments begins early in week 3 of development through a process called vasculogenesis, and by week 4, these three systems have formed (Figure 15-1). The vitelline system drains the yolk sac, developing gastrointestinal tract and foregut derivatives. The umbilical system carries oxygenated blood from the maternal placental vessels to the embryo, while the cardinal system drains blood from the developing body wall, head, neck, and limbs into the sinus horns of the embryonic heart. These three systems are bilaterally symmetric initially but undergo substantial remodeling and lose their symmetry during development. Far greater variation is present in the mature venous system compared to the arterial, which is a reflection of the highly variable organization in the networks of primitive venous channels as well as the extensive remodeling. Small and large disturbances in the remodeling process lead to variations in the mature venous system as well as commonly recognized anomalies.
Venous embryologic development at 4 and 5 weeks. (Reproduced with permission from Meghan Lark.)
The developing vitelline system originates in the capillary plexuses of the yolk sac and includes part of the vessels that drain the embryonic gastrointestinal tract and gut derivates. This system empties into the sinus horns of the developing heart through the paired vitelline veins. These veins are also connected to symmetric right and left vitelline plexuses that project into the developing liver parenchyma, which form the hepatic sinusoids. The left vitelline vein begins to diminish as the left sinus horn regresses to form the coronary sinus. These events preferentially shunt blood from the developing gastrointestinal tract and yolk sac through the right vitelline vein and liver via several transverse anastomoses. One of these transverse anastomoses within the liver becomes dominant and forms the ductus venosus, which serves to shunt oxygenated blood into the right atrium. By the end of the third month, the left vitelline vein adjacent to the sinus venosus has completely disappeared, and all blood from the vitelline system is directed through the liver and into the future right atrium via the right vitelline vein (Figure ...