Ultrastructure of the Sinoatrial Node in the Rabbit Heart, with Special Reference to the Cytoarchitecture

Abstract

In order to understand the correlation between the impulse and activation sequence, the cytoarchitecture and innervation of the sinoatrial (SA) node in the rabbit heart were investigated by scanning (SEM) and transmission electron microscopy. For SEM, tissue specimens including the SA node were treatred with 6N NaOH to remove both the endocardium and connective tissue.
The SA node consisted of small bundles of nodal cells. The nodal cells were smaller in size than atrial myocytes, spindle-shaped, and had a moderate number of myofibrils, small mitochanclria and many glycogen particles. They were contacted end-to-end or end-to-side with each other at the intercalated disks.
The nodal cell bundles were mostly oriented pararell to the crista terminalis, but some cell bundles with a reticular net were found. At the periphery of the SA node, nodal cells were combined with atrial myocytes. In addition, adrenergic and cholinergic nerve endings were in close proximity to the nodal cells.