A morphological study of the left bundle branch in the normal human heart.

To clarify the distribution pattern of the left bundle branch (LBB) in the human heart, the AV conduction system was studied in 13 autopsied hearts obtained from subjects aged 50 to 80 years. Vertical serial sections (7 micron) of the bundle of His and LBB were prepared and every 20th section was stained alternately with hematoxylin-eosin (HE) or by the elastica van Gieson (EVG) method and examined by light microscopy. Reconstruction was performed using a two-dimensional system in order to histologically differentiate the bundle cells from Purkinje cells. The LBB bifurcated into the anterior and posterior radiations and the cells in the septal portion were almost all Purkinje cells except in two cases showing a septal branch between the two radiations. The LBB usually branched widely from the bundle of His. An extremely anterior fascicle of the LBB was found in all cases. The distribution of the LBB at the top of the ventricular septum was divided into network and continuous types. Purkinje cells were present on both the atrial and apical sides of the two main radiations. It was suggested that these findings resulted from the fact that we morphologically differentiated the bundle cells from Purkinje cells by light microscopy.