Figure 3.

Histological Findings of the His Bundle Area and the LBB Area

(A and B) Histological sections (stained with Masson's trichrome) through the division of the bundle of His and its right and left branches at the level of the interventricular component of the membranous septum, being attached inferiorly to the crest of the muscular interventricular septum. Note the relationship of the left branch (always subendocardial) with the nadir of the noncoronary leaflet of the aortic valve. (B) The right branch is running intramyocardially 1 or 2 mm from the endocardium of the right ventricle. (C1, C2, C3) Histological sections of 3 specimens stained with Masson's trichrome of the left branch 12 to 20 mm distal from its origin. The left branch is located between the muscular septum and the endocardium of the left ventricle. The thickness of the muscle fibers of the left branch is very variable, small in C1 and abundant in C3; the endocardium has smooth muscle cells in its thickness; and surrounding the left branch there is connective tissue of variable thickness. C1: Green star indicates fibrous tissue; black arrow indicates septal artery. ∗Left bundle branch myocytes separated by adipose/connective tissue. HB = His bundle; LBB = left bundle branch; RBB = right bundle branch; TV = tricuspid valve.