Fig. 4

Histological sections of the interatrial septum using Masson’s trichrome staining at levels 1 and 2, as shown in Fig. 1A. Right is dorsal, and top is towards the right atrium. (A) Level 1, located 1 cm caudal to the limbus, illustrates the layered structure of the interatrial septum, featuring two subendocardial fibrous layers (yellow double arrows), with a myocardial layer (black double arrow) between them. (B) Level 2, positioned at the entrance of the right-sided septal pouch, exhibits a similar structural organization, with clearly defined fibrous (yellow double arrows) and myocardial (black double arrow) layers. In (A) and (B), the black boxes indicate the area for (C) and (D), respectively. (C,D) Magnified view of the myocardial layer at levels 1 and 2, revealing an undulating (wave-like) arrangement of myocardial fibers. In (C) and (D), the black boxes indicate the area for (E) and (F), respectively. (E–F) A detailed view of the myocardial fibers at Level 1 and Level 2 is shown in longitudinal cut (l) (showing transverse orientation), cross-section (c) (with fibers oriented in a craniocaudal direction), and oblique cut (o) (with fibers oriented obliquely within the interatrial septum). Adipose tissue (a) was observed interspersed between the cells in level 1. LA, left atrium; RA, right atrium.