Different Cell Types Within the Sinoatrial Node
To the Editor:
In a recent article by Verheijck et al,1 4 types of myocytes were reported within the rabbit sinoatrial node. The nodal cells presented spontaneous diastolic depolarization and could be classified in 3 types according to their shape and size after enzymatic dispersion of the sinoatrial node. Differently from working atrial myocytes, nodal cells reacted with the antineurofilament monoclonal antibody. The fourth type of myocyte was reported as “atrial cells.” They did not present neurofilament immunoreactivity, were quiescent, and presented a shape similar to working atrial myocytes. There was a gradual increase in the density of “atrial cells” from the central area of the sinoatrial node toward the crista terminalis. The authors suggest that “atrial cells” of the sinoatrial node are identical to working atrial cells of the crista terminalis.
The immunohistochemical search for atrial natriuretic peptide (ANP) may also help to distinguish between nodal cells and working atrial myocytes. Nodal cells are usually devoid of ANP, but this peptide is abundant within working atrial myocytes.2 3 The few cells that presented a low level of ANP immunoreactivity in the rat and human sinoatrial node2 3 probably correspond to the “atrial cells” described by Verheijck et al.1 These ANP-positive cells are principally found at the periphery of the sinoatrial node. Both their diameter and ultrastructural features are intermediate between typical nodal cells and working atrial myocytes.2 Additionally, the ANP-positive cells within the sinoatrial node are smaller and present weak ANP immunoreactivity compared with working atrial myocytes from the crista terminalis.3
In conclusion, the “atrial cells” of the sinoatrial node reported by Verheijck et al1 probably correspond to the ANP-positive transitional nodal cells described by other authors. They are not identical to working atrial myocytes, presenting an intermediate phenotype between typical nodal cells and working atrial myocytes.
- Copyright © 1999 by American Heart Association