MORPHOLOGICAL CLASSIFICATION OF ATRIAL MUSCLE IN THE ATRIOVENTRICULARJUNCTIONAL AREA - 3-DIMENSIONAL RECONSTRUCTION OF SERIAL SECTIONS OF THE HUMAN HEART

To trace anatomical structures that might be associated with the dual atrioventricular (AV) nodal pathway and to investigate the morphologic characteristics of the cells that form these pathways, we examined se rial sections of the AV junctional area with a light microscope and re constructed them 3-dimensionally with a computer. Twelve hearts were o btained at autopsy from patients who had not shown AV conduction distu rbances or supraventricular tachycardia before death. The method of Le v et al was used to prepare serial sections. Fascicles of atrial muscl e contiguous with the AV node were examined with a light microscope an d were classified into 3 groups on the basis of morphologic characteri stics and myocyte diameter. A computer was used to reconstruct 3-dimen sionally the course of the fascicles and surrounding structures. At th e border of the AV node and bundle of His relatively large myocytes ex tended directly into the AV bundle from the anterosuperior interatrial septum. Morphologically, the course was considered to be consistent w ith the fast pathway. In contrast, small cells that entered the AV nod e from the inferoposterior interatrial septum resembled sinus node cel ls with few myofibrils and a winding shape. These cells extended from the coronary sinus ostium to the tricuspid valve annulus and are thoug ht to make up the slow pathway.