Comparative Ultrastructure of the Sinus

MYOCARDIUM was one of the first tissues to be studied with the electron microscope, but the ultrastructure of the cardiac conduction system has heretofore been described only in subhuman species. Electron microscopy has been employed to examine the conduction system of the dog,'rabbit,5' 6 frog,7 sheep8,9 rat'O, 11 and steer.'2 13 Interest in the specialized tissue of the heart which comprises the conduction system has been intensified in recent years by experiments which record membrane action potentials of single cells of the sinus node, A-V (atrioventricular) node and bundle, and Purkinje fibers. There are several reasons for the relative paucity of published observations on the fine structure of the conduction system. The regions to be studied are small, and with current methods of preparing tissue for electron microscopy one must utilize samples of a size (usually about 1-mm cubes) that it is difficult to be certain the sample obtained is actually from the area intended. A second problem concerns the nature and extent of postmortem changes in conduction system tissue, but this can be circumvented in the experimental animal by obtaining samples in vivo. Because of the likelihood of significant destruction of the critical centers involved by such sampling