The relationship between the sinus node and the right atrial appendage.

BACKGROUND

The developmental anatomy of the right atrium, particularly the union between its sinus venarum (sinus) and atrial portions, has not been completely settled. Invagination of the embryonic sinus into the atrium brings about electrical and structural union between these primitive chambers but results in the formation of the terminal crest, a seemingly bulky and intrusive band of muscle separating sinus from atrium. However, it is unlikely that a structure that can be found in all mammalian species is merely a functionless remnant. Structural differences between the smooth sinus and the deeply trabeculated right atrial appendage emphasize the two-part shape of the right atrium, but it is hypothesized that the sinus and atrium form a single, well-knit chamber shaped for the supply of nutrient blood to atrial conducting tissues rather than for downstream bloodflow.

METHODS AND RESULTS

The morphology of the right atrium was studied grossly and microscopically in 54 adult hearts. The distribution of Thebesian sinusoids arising from the right atrial appendage was identified by ink perfusion and clearing of several additional adult and fetal hearts. The sinus node was found to be precisely co-extensive with the undercut portion of the terminal crest and with pectinate muscles lining the pyramidal portion of the right atrial appendage. Sinusoids and interpectinate spaces related to the undercut portion of the terminal crest are limited to the area of the sinus node.

CONCLUSION

These findings support the hypothesis that the right atrial appendage, pectinate muscles and terminal crest evolved to supply nutrient blood to conducting myocardium of the sinus portion of the right atrium, and that this chamber, like the right ventricle, is structured as a single and completely finished unit. Interpectinate spaces and Thebesian sinusoids offer clues to the location of conducting pathways, including the sinus node.