It is known that the sinoatrial (SA) node consists of a heterogeneous population of cells in terms of morphology and electrical activity. However, it is not known how the cells are assembled to produce the intact SA node. Two models have been proposed: the mosaic model, which proposes a varying mixture of atrial and SA node cells from the centre to the periphery of the SA node (Verheijck et al. 1998), and the gradient model, which proposes no atrial cells within the SA node but the SA node cell type changes from the centre to the periphery of the SA node (Boyett et al. 2000). To investigate the make up of the intact SA node, we used immunocytochemistry to map the expression of atrial natriuretic peptide (ANP), neurofilament (NF; a protein found in nerve cells and cardiac pacemaker/conducting tissues), and connexin43 (Cx43; a component of gap junctions responsible for electrical coupling). Rabbits were killed humanely (UK Home Office Schedule 1 method) and the SA node isolated. Ten micrometre serial sections through the SA node were cut at the level of the normal position of the leading pacemaker site and processed for immunocytochemistry. Results were obtained using a Leica confocal microscope. We have previously investigated the expression of Cx45 (another component of gap junctions) as well as Cx43 in tissue sections through the SA node. The results of this study (and our previous results) are summarised in Fig. 1. Figure 1 shows a schematic diagram of a section cut through the atrial muscle of the crista terminalis and the SA node. The atrial muscle (black) expresses ANP but not NF, and the whole SA node (light and dark grey) expresses NF but not ANP. The atrial muscle also expresses Cx43 but not Cx45. The periphery of the SA node (light grey) expresses both Cx43 and Cx45, whereas the centre of the SA node (dark grey) expresses Cx45 but not Cx43.

On the basis of these immunocytochemical markers, three cell types can be identified: ANP-positive/NF-negative/Cx43-positive/Cx45-negative (black), ANP-negative/NF-positive/ Cx43-positive/Cx45-positive (light grey), and ANP-negative/ NF-positive/Cx43-negative/Cx45-positive (dark grey). Our results show that the use of ANP, NF, Cx43 and Cx45 as markers provides no evidence for atrial cells within the SA node – instead in any one area the cells are uniformly of one type. We conclude that immunocytochemistry supports the gradient model to produce the intact SA node.

All procedures accord with current UK legislation.