Development and septation of the atria and venous pole

Abstract

The posterior most part of the heart tube is commonly known as the sinus venosus which receives the venous blood from the right and left side of the embryo. It consists of two horns connected to the rest of the heart tube via the sinu-atrial foramen [Steding et al., 1990]. These horns receive blood from the vitelline, umbilical and common cardinal veins. Reconstructions of the venous pole of the heart in mouse embryos, based on molecular expression patterns in the surrounding myocardium, have shown that, at least in mice, a sinus venosus sensu stricto does not exist at any time in development. Instead, the systemic venous tributaries, i.c. the right and left sinus horns, drain separately and directly into the atria [Soufan et al., 2004]. Whether this situation is comparable to what occurs in human embryos remains to be proven. In this atlas we will therefore conform to the current opinion regarding development of the venous pole in man. In contrast to what is seen in most vertebrate embryos, asymmetry of the venous pole in human embryos, with the left sinus horn being deviated to the right, appears to be present from the beginning onward [Knauth et al., 2002]. When viewed from the luminal side of the heart tube, a bifurcation is seen where the two horns meet [Vernall, 1962; Steding, 1990], like the crotch in a pair of trousers [Webb, 1998]. This structure is called the sinus septum and is situated caudal to the dorsal mesocardium.