Ligamentum arteriosum: Structure, innervation, neurochemistry and function

Abstract

Introduction: The ligamentum arteriosum is generally considered a mere remnant of the embryonic bypass (ductus arteriosus) from the pulmonary circulation to the aortic arch, obliterating soon after childbirth. However, there have been anecdotal contradictory reports about this structure. This study set out to elucidate the morphology, innervation and neurochemistry of LA innervation. Method: LA of human, pig, wild-type and transgenic mice were studied using routine and special histological staining methods, single- and double-immunofluorescence labeling using antibodies directed against structural markers, neuropeptides, and transmitter synthesizing enzymes such as α-smooth muscle actin (αSMA), neuropeptide Y (NPY), and tyrosine hydroxylase (TH) respectively. Ultrastructural studies of pig and mice LA using Transmission electron microscopy (TEM) was employed. Pig LA was subjected to varying Hz of electrical field stimulation, increasing cumulative doses of noradrenalin and alpha 1 adrenergic blockers (tamsulosin and prazosin). Results: Contrary to a canonical ligament, the ligamentum arteriosum was mainly made up by αSMA-positive cells in all three species as observed from results of immunofluorescence labelling and histological staining. TEM confirmed the presence of vascular smooth muscle cells within LA tunica media. Additionally, myofilaments, dense bodies and bands, all relevant in the formation of the contractile apparatus of vascular smooth muscles were observed. The LA received a noticeable amount of chrna3-eGFP. PGP 9.5, TH, NPY, SP, CGRP -positive fibers were observed in all three species. TEM revealed nerve terminals in close proximity to smooth muscle cells. The LA had a frequency and dose dependent contraction in response to electrical field stimulation and exogenous noradrenalin respectively. There was also a dose dependent relaxation upon administration of tamsulosin and prazosin . Conclusion: It is fact that the LA no longer serves in its original capacity as a foetal shunt connecting the two great vessels. However, the presence of contractile smooth muscle is present long after obliteration and until senescence. These muscular elements have noticeable sympatheic and sensory innervation. It may be postulated that the contractile abilities of LA myocytes may act on the two great vessels to which it is attached causing a change in their distensibility.