Sinus node and atrium cells from the rabbit heart: a quantitative electron microscopic description after electrophysiological localization.

Electrophysiologically identified cell groups in the sinus node from the rabbit have been compared with atrial fibers with the electron microscope. The point counting method has been used to estimate the volume density of the following structures: nucleus, mitochondria, myofilaments, sarcoplasmic reticulum tubules and subsarcolemmal vesicles. These data were collected in leading pacemaker cells, latent pacemaker cells and atrium cells from the crista terminalis.It has been found that organized structures in leading pacemaker cells occupy about 50% of the cell volume, as compared with over 90% in atrial fibers. Leading pacemaker cells consequently appear extremely “empty”. It has also been found that the group of cells which show the characteristic features of leading pacemaker cells at the ultrastructural level as observed in the correlated experiments is larger than the leading center found in electrophysiology and thus it seems impossible with the actual observation methods to delineate the leading pacemaker center using only cytological criteria.