Sinus venosus incorporation: contentious issues and operational criteria for developmental and evolutionary studies

Abstract

The sinus venosus is a cardiac chamber upstream of the right atrium that harbours the dominant cardiac pacemaker. During human heart development, the sinus venosus becomes incorporated into the right atrium. However, from the literature it is not possible to deduce the characteristics and importance of this process of incorporation, due to inconsistent terminology and definitions in the description of multiple lines of evidence. We reviewed the literature regarding the incorporation of the sinus venosus and included novel electrophysiological data. Most mammals that have an incorporated sinus venosus show a loss of a functional valve guard of the superior caval vein together with a loss of the electrical sinuatrial delay between the sinus venosus and the right atrium. However, these processes are not necessarily intertwined and in a few species only the sinuatrial delay may be lost. Sinus venosus incorporation can be characterised as the loss of the sinuatrial delay of which the anatomical and molecular underpinnings are not yet understood.

Quantified growth of the human embryonic heart

“…4 . The inflow tract myocardium, which surrounds the two superior caval veins and may act as a chamber in early human development ( Faber et al, 2019 ), initially grows exponentially, but its growth levels off halfway through embryonic development. It remains to be shown whether this change in growth rate coincides with the so-called atrialisation of this myocardium, whereby the sinus venosus loses its identity as a separate chamber.…”

Development and evolution of the metazoan heart

“…As the sinus venosus is myocardialized in many species, it can be considered a true cardiac chamber. It will remain recognizable as a separate structure in hagfish connected to the left side of the common atrium, but in birds and most mammals (except monotremes) it will become incorporated in the dorsal atrial wall (Figure ). In lungfish, a separate pulmonary channel traverses the main atrial compartment and delivers oxygenated blood directly to the ventricle .…”

Reptiles as a Model System to Study Heart Development

“…Facilitated by the sinuatrial delay in electrical activation, the SV activates and contracts before the atria and thus functions as a separate chamber (Jensen et al 2017b). In mammals, the embryo develops an SV, which later becomes the caval vein myocardium that activates together with the atrial myocardium (Spach et al 1972;Opthof 1988;Steding et al 1990;Faber et al 2019). The walls of the atrial and ventricular chambers of reptiles are readily distinguishable from the chamber walls of mammals and birds by their very extensive layer of fine trabecular muscle (Jensen et al 2016aBoukens et al 2019;Kroneman et al 2019).…”

“…The entry of the SV to the right atrium is regulated by the myocardial sinuatrial valve, which has two leaflets (Keith and Flack 1907;Faber et al 2019). Between the atria and the ventricle(s), there is a single atrioventricular junction divided into left and right halves by the atrial septum (Benninghoff 1933;Webb 1979;Farrell et al 1998).…”