Morphogenesis of the Systemic Venous System Normal venous development is a process of progressive appearance of a series of paired venous structures, the development of anastomotic channels between them, and the eventual selective regression of certain components Initially, the systemic venous tributaries are bilaterally symmetric Three sets of channels join together on each side, opening to the developing heart tube through paired channels known as the sinus horns The sinus horns themselves open caudally into the atrial component of the developing heart tube When first recognized as draining to the heart tube, there are no anatomic landmarks that permit the distinction of a separate “sinus venosus,” or systemic venous sinus, such as that found in the hearts of lower vertebrates (see Chapter 3).1 Instead, the three major venous channels from each side of the developing embryo drain into each horn From medial to lateral, the channels are the omphalomesenteric or vitelline veins, the umbilical veins, and the common cardinal veins (Fig 27.1, left) The superior and inferior cardinal veins, which drain the cephalic and caudal regions of the embryo, respectively, join together initially to form bilaterally symmetric common cardinal veins Subsequent to the development of anastomotic channels throughout the embryo between the right- and left-sided venous channels, the flow is shifted to favor the developing right side of the atrial component of the heart tube Concomitant with this shift to right-sided dominance, the left sinus horn rotates within the developing atrioventricular junction, moving around the fulcrum provided by the dorsal mesocardium, which itself provides the portal of entry for the developing pulmonary vein (see Chapter 3) After the rotation, the left-sided venous channel, which has regressed significantly in size, opens inferiorly and caudally within the systemic venous sinus, itself now distinguished from the remainder of the developing right atrium by the formation of the so-called venous valves (Fig 27.2) A gradual regression of the left-sided vitelline and cardinal veins follows, with increasing prominence of the right sinus horn Eventually, the entirety of the systemic venous sinus within the confines of the valves becomes incorporated as the smooth-walled venous component of the morphologically right atrium, with the opening of the right common cardinal vein becoming the orifice of the superior caval vein, and that of the right vitelline vein becoming the entrance of the inferior caval vein The mouth of the left sinus horn persists as the orifice of the coronary sinus (see Fig 27.1, right) The initial anastomotic connection between the right and left superior cardinal veins enlarges and becomes the left brachiocephalic vein, often known as the innominate vein If development proceeds normally, there is regression of the component of the left superior cardinal vein initially extending from the anastomotic channel to the developing left atrioventricular junction Its remnant forms the oblique vein of the left atrium and the ligament of Marshall (Fig 27.3) FIG 27.1 How the bilaterally symmetric venous channels seen early during embryologic development (left) become converted into the postnatal situation in which the superior and inferior caval veins, along with the coronary sinus, drain to the systemic venous sinus of the morphologically right atrium (right) FIG 27.2 Scanning electron micrograph showing the developing right atrium subsequent to incorporation of the systemic venous sinus and formation of the venous valves The course of the left sinus horn through the developing left atrioventricular junction is seen The pulmonary ridges also can be seen at the base of the primary atrial septum, although the opening of the solitary pulmonary vein between them is not visible Note that the left sinus horn has retained its own walls within the left atrioventricular junction (Courtesy Professor Nigel Brown.)