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

At birth, the ductus arteriosus (DA) merges with the aortic arch in the slightly caudal side of the origin of the left subclavian artery (SCA). Since the SCA (seventh segmental arteries) were fixed on a level of the seventh cervical-first thoracic vertebral bodies, the confluence of DA should migrate caudally. We aimed to describe timing and sequence of the topographical change using serial sagittal sections of 36 human embryos and fetuses (CRL 8-64 mm; 5-10 weeks) those made easy evaluation of the vertebral levels possible in a few section. The DA or sixth pharyngeal arch artery seemed to slide down in front of the sympathetic nerve trunk along 1.0-1.2 mm from the second cervical vertebral level at 5-6 weeks and, at 6 weeks (CRL14-17 mm), the DA confluence with aorta reached the seventh cervical level. Because of the highly elongated common carotid artery, the sliding of DA confluence seemed to be much shorter than the growing cervical vertebrae growing from 1 mm to 2.4 mm. At the final topographical change at 6-7 weeks, the DA confluence further descended to a site 1-vertebral length below the left SCA origin. From 6 to 9 weeks, a distance from the top of the aortic arch to the left SCA origin was almost stable: 0.3-0.5 mm at 6 weeks and 0.4-0.6 mm at 9 weeks. The heart descent and the caudal extension of the trachea and bronchi, those occurred before the DA sliding, were likely to be a major driving force for the sliding.