Representative HE staining (up panel) and Masson’s trichrome staining (down panel) of the atrial myocardium.

<p>(A) Structurally normal atrial myocardium in the control group and severe fibrosis in the pacing group (×100). (B) Atrial myocardium with HE staining under ×400 light microscopy. (C) A small amount of collagen is visible within the atrial myocardium of the control group, and an increased amount of collagen is visible within the atrial myocardium following pacing (×100). The red areas represent myocytes, and the blue areas represent collagen. (D) Atrial myocardium with Masson’s trichrome staining under ×400 light microscopy. C, control group; P, pacing group. (n = 6 for Group C, n = 9 for Group P).