The dorsal mesocardium and development of the pulmonary veins in human embryos

Pulmonary vein development was studied using serial histologic sections of normal human embryos of Carnegie stages 11 to 15. Three-dimensional models were created in the program Swivel 3D on a Macintosh IIfx computer. The position of the mesocardium was found to be an important factor in the placement of the vein. Since the vein grows through a gap in the myoepicardium of the dorsal atrial wall created by the mesocardium, the vein can only grow where the mesocardium is positioned. Displacement of the initially median pulmonary vein ostium into the left atrium appeared to be caused by the formation of the left valve of the sinus venosus. This latter structure displaces the mesocardium to the left from stage 14 and later, carrying the vein to the left as well. The subsequent development of several pulmonary veins from the original single pulmonary vein occurred later, as the apex of the heart rotated to the left and brought the left atrium into a dorsal midline position. The study shows that correct placement of the pulmonary vein in the left atrium is the consequence of the successful execution of a sequence of developmental events in cardiogenesis.