The curious case of ligamentum arteriosum: It is more than a ligament

Abstract

IntroductionThe ligamentum arteriosum is generally considered to be a mere a remnant of the embryonic bypass (ductus arteriosus) from the pulmonary circulation to the aortic arch, obliterating soon after childbirth. This study set out to elucidate the morphology, innervation, and neurochemistry of this structure.MethodThe mediastinum of wildtype mice (C57BL/6J, n=5), reporter mouse strains indicating the expression of the nicotinic receptor subunit α3 (chrna3‐eGFP, n=4) and of choline acetyltransferase (chat‐eGFP, n=4), human (n=10) and pig (n=9) ligaments obtained from body donors (4 male, 6 female, aged 69–97 years) and commercial abattoir, respectively, were subjected to single‐ and double‐labeling immunofluorescence using antibodies against structural markers such as α‐smooth muscle actin (αSMA), protein gene product 9.5 (PGP 9.5), neurofilament 68 (NF68), against neuropeptides such as neuropeptide Y (NPY), vasoactive intestinal peptide (VIP), substance P (SP), and calcitonin gene‐related peptide (CGRP), and against transmitter synthesizing enzymes such as tyrosine hydroxylase (TH; nordrenergic), choline acetyltransferase (ChAT; cholinergic), and nitric oxide synthase (NOS). Paraffin embedded human samples were stained with hematoxylin and eosin, Masson‐Goldner trichrom and elastica‐van Gieson dyes.ResultsIn mice and human ligaments, remnants of a lumen and of an internal elastic membrane were observed. The lumen of the “ligament” was not completely obliterated, even though it was not patent from one side to the other. Contrary to a canonical ligament, the ligamentum arteriosum is mainly made up of collagen and abundant αSMA‐positive cells in all three species. Some human samples contained a fibrous core, sometimes calcified. Blood vessels and nerve bundles were abundant in the adventitia. In mice, the point of attachment of the ligament to both the aortic arch and pulmonary trunk received a noticeable amount of chrna3‐eGFP+ but no cholinergic innervation. PGP 9.5‐, TH‐, NPY‐, CGRP‐, and SP‐immunoreactive nerve fibers were observed in this rank order of frequency. In pig, VIP+ fibers were additionally observed. In human ligaments, PGP 9.5+, Nf68+, TH+, NPY+, VIP+, CGRP+, and SP+ fibers were observed.ConclusionThe so‐called ligamentum arteriosum retains a continuous smooth muscular wall until senescence with noticeable, presumably postganglionic sympathetic (TH+, noradrenergic) and to a lesser extent sensory C‐fiber innervation (CGRP+, SP +) long after childbirth. It is clearly not a passive structure and the dense innervation could influence impedance/compliance (reported to be the best prognostic parameter for patient survival in pulmonary hypertension (Hunter et al., Am Heart J. 155:166–74, 2008) of the vascular segments to which it is attached.