Development and maturation of the fibrous components of the arterial roots in the mouse heart

Abstract

The arterial roots are important transitional regions of the heart, connecting the intrapericardial components of the aortic and pulmonary trunks with their ventricular outlets. They house the arterial (semilunar) valves and, in the case of the aorta, are the points of coronary arterial attachment. Moreover, because of the semilunar attachments of the valve leaflets, the arterial roots span the anatomic ventriculo‐arterial junction. By virtue of this arrangement, the interleaflet triangles, despite being fibrous, are found on the ventricular aspect of the root and located within the left ventricular cavity. Malformations and diseases of the aortic root are common and serious. Despite the mouse being the animal model of choice for studying cardiac development, few studies have examined the structure of their arterial roots. As a consequence, our understanding of their formation and maturation is incomplete. We set out to clarify the anatomical and histological features of the mouse arterial roots, particularly focusing on their walls and the points of attachment of the valve leaflets. We then sought to determine the embryonic lineage relationships between these tissues, as a forerunner to understanding how they form and mature over time. Using histological stains and immunohistochemistry, we show that the walls of the mouse arterial roots show a gradual transition, with smooth muscle cells (SMC) forming the bulk of wall at the most distal points of attachments of the valve leaflets, while being entirely fibrous at their base. Although the interleaflet triangles lie within the ventricular chambers, we show that they are histologically indistinguishable from the arterial sinus walls until the end of gestation. Differences become apparent after birth, and are only completed by postnatal day 21. Using Cre‐lox‐based lineage tracing technology to label progenitor populations, we show that the SMC and fibrous tissue within the walls of the mature arterial roots share a common origin from the second heart field (SHF) and exclude trans‐differentiation of myocardium as a source for the interleaflet triangle fibrous tissues. Moreover, we show that the attachment points of the leaflets to the walls, like the leaflets themselves, are derived from the outflow cushions, having contributions from both SHF‐derived endothelial cells and neural crest cells. Our data thus show that the arterial roots in the mouse heart are similar to the features described in the human heart. They provide a framework for understanding complex lesions and diseases affecting the aortic root.

Krox20 defines a subpopulation of cardiac neural crest cells contributing to arterial valves and bicuspid aortic valve

“…In particular, NCCs expressing Krox20 contribute to the insertion zone of the valve leaflets and the interleaflet triangle between each leaflet. Consistently, a recent study showed that the interleaflet triangle fibrous tissues derived from the outflow cushions region have contribution from NCCs (Richardson et al, 2017).…”

A novel source of arterial valve cells linked to bicuspid aortic valve without raphe in mice

“…However, at E11.5, prior to expression of CS56, and also at E12.5, Versican was expressed within the bulk of the IC ( Figure 2J–O ). At E10.5 Sox9, a transcription factor necessary for the formation of endocardial cushions and maintained in mature valve leaflets ( Akiyama et al, 2004 ; Richardson et al, 2018 ), was abundantly expressed in the circumferential layer of cardiac jelly where the main cushions form, but only in the area immediately adjacent to the endothelium overlying the IC that was labelled by CS56 ( Figure 2P,S – compare to 2 D,G); there was no Sox9 in the main part of the IC at this stage. At E11.5 Sox9 was also expressed throughout the IC and this was maintained at E12.5 ( Figure 2Q,R,T,U ).…”

“…The dorsal pericardial wall and the pharyngeal mesoderm are composed of SHF cells that directly generate the outflow tract myocardium, much of the smooth muscle and fibrous tissue in the proximal and distal outflow vessel walls, and also the endocardium that lines these structures ( Waldo et al, 2005 ; Harmon and Nakano, 2013 ; Verzi et al, 2005 ; Richardson et al, 2018 ). Moreover, the SHF through endothelial EndMT, indirectly populates the outflow tract cushions ( Verzi et al, 2005 ); Figure 3—figure supplement 1 ).…”

“…We have previously shown that NCC are essential for positioning the main outflow tract endocardial cushions and patterning the arterial valve leaflets, but make only a minor contribution to these IC ( Phillips et al, 2013 ). More recently, we have also shown that the arterial roots ( Figure 1A ) that connect the ventricles with the great arteries and house the arterial valves, have a complex origin from both SHF-derived cells and NCC ( Richardson et al, 2018 ). However, the contribution of each lineage to the different valve leaflet primordia, and how these relationships affect normal and abnormal leaflet patterning including BAV, remains obscure.…”

New Concepts in the Development and Malformation of the Arterial Valves

“…The aortic and pulmonary valves, which sit within the arterial roots between the myocardium of the left and right ventricular outflow tracts and the aorta and pulmonary trunk, respectively, are structurally similar to one another, and are closely comparable in humans and mice [ 1 , 2 , 3 ]. Thus, in the mature heart both the aortic and pulmonary valves have three superficially similar leaflets that form the moving parts of the valve.…”