Microstructural Architecture of Atrial Septal Pouches and Smooth Interatrial Septum

This study aimed to characterize the microstructural architecture of left-sided (LSSP) and right-sided (RSSP) atrial septal pouches and the smooth interatrial septum. Seventy-five formalin-fixed, paraffin-embedded specimens (38 LSSP, 17 RSSP, 20 smooth septum) were serially sectioned and stained with H&E, orcein, Sirius Red, and Gomori trichrome, and their structural composition and histoarchitecture were evaluated by light microscopy. The LSSP comprised a free wall derived from primary septum and the opposing segment of left atrial wall, with a lumen that opens superiorly and tapers inferiorly toward an apex located above the septal fusion point; its free wall is typically bilaminar, formed by two endocardial layers enclosing transversely oriented myocardium embedded in fibroelastic connective and adipose tissue. In contrast, the RSSP is formed by a free wall derived from secondary septum and an opposing septal wall, opening inferiorly and narrowing superiorly toward an apex situated just below the fusion point; although both pouches share an endocardium-lined bilaminar free wall with a fibroelastic-myocardial core, the myocardial component is generally less prominent in the RSSP. In both pouch variants, the free wall tapered from apex to ostium, while the opposing atrial/septal wall was consistently thicker and increased toward the pouch ostium (p = 0.001). The fusion point showed fibroelastic tissue accumulation and was significantly thicker in LSSP than in RSSP (p = 0.018). In the smooth septum, oval fossa floor thickness differed across levels (superior > inferior > middle; p = 0.008). Quantitative H&E analysis demonstrated inter-variant differences in relative myocardial layer thickness (p = 0.044), driven by RSSP versus smooth septum (p = 0.037), whereas elastic (p = 0.427) and collagen content (p = 0.460) did not differ significantly. Gomori trichrome revealed differences in the myocardium-to-fibrosis ratio (p = 0.042), most pronounced between RSSP and smooth septum (p = 0.03). In conclusion, this study is the first to provide a comparative histological and morphometric characterization of the LSSP and RSSP, linking macroscopic anatomy with reproducible microstructural patterns and providing a foundation for further investigations of septal pouches.