Changes in topographical relation between the ductus arteriosus and left subclavian artery in human embryos: a study using serial sections.

At birth, the ductus arteriosus (DA) merges with the aortic arch in the caudal side of the origin of the left subclavian artery (ltSCA). Since the SCA (seventh segmental arteries) were fixed on the levels of the seventh cervical-first thoracic vertebral bodies, the confluence of the DA should migrate caudally toward the lower level. We aimed to describe the changing topographical anatomy of the DA and SCA using serial sections. First, we examined serial sagittal sections of 11 embryos (Carnegie stage 15-18), but the specimens were clearly divided into 2 groups with and without the lower confluence of the DA. Next, we examined serial horizontal sections of 40 specimens (Carnegie stage 14-16) and we chose 5 specimens (CRL 11 mm, 3 specimen; 1, 14 mm; 1, 15 mm) including the DA near (within 1-vertebral segment from) the ltSCA. The final approach of the DA occurred during the heart descent in which the apex of the heart migrated from the level of the first to the fourth thoracic vertebral body. Thus, the DA reached the SCA level before establishment of the heart descent. The right aortic arch maintained its entire course in 2 of the 5 specimens. Therefore, the positioning of the DA along the left aortic arch might occur independently of degeneration of the right arch. Notably, the tracheal bifurcation level was higher when the DA-ltSCA distance was greater. A contribution of the increased pulmonary volume was suggested for the final approach of the DA.