Extranodal specialized sinoatrial cells.

To define the occurrence of specialized pacemaker and transitional (slender) cells beyond the anatomic limits of the sinoatrial node, cytomorphometric evaluations were carried out in specimens from ten human hearts. The morphometric analysis showed significant histocytologic peculiarities of the extranodal slender cells, which were 60% to 70% smaller than those of the ordinary atrial myocardium, and their location at a mean distance of 12 to 14 mm from the posterior sinoatrial nodal edge, beneath the crista terminalis. These data can help in understanding the substrates for subsidiary pacemaking in sinoatrial disease.