The AV junction region of the heart: a comprehensive study correlating gross anatomy and direct three-dimensional analysis. Part II. Morphology and cytoarchitecture.

This "Part II morphology and cytoarchitecture" study is based on paraffin-embedded specimens in which the extracellular and intracellular matrix are preserved; single parallel, perpendicular, and transverse serial sections of the entire atrioventricular (AV) junction region (AVJR) and their correlation with photographs of the tissue blocks. As in Part I, the same major new findings are: 1) a coronary sinus fossa is formed by the superoposterior right medial atria wall (MAW), the left atrium, and the coronary sinus roof; 2) the posterior MAW forms two myocardial bridges and is isolated from the sinus venarum by the floor of the inferior vena cava; 3) the tendon of Todaro terminates in the superior lip of the coronary sinus ostium; 4) only ordinary myocardium contacts the annulus fibrosus, and there is little to no collagen separating its myofibers and tissues; 5) the ventricular septum shoulder is humped shaped, completely overlaid by annular myocardium, and joined by struts of papillary muscle; 6) the membranous septum joining the ventricular septum shoulder to the crista supraventricularis forms part of the aortic valve sinus walls; and 7) myocardium of the atrionodal bundles is aggregated into numerous small fascicles encased by collagen and is outside of the MAW as are the other specialized tissues. The proximal AV bundle and medial atrionodal bundle are aligned to the medial leg of Koch's triangle and the tendon of Todaro. These data show, therefore, that the AVJR contains two overlapping atrial circuits. In the MAW, acivation of the posterior region is delayed because of the two myocardial bridges. Puncture of the AVJR can produce communication with an extracardiac space, posteriorly and medially, and with the aorta, anteriorly.