We have investigated the morphology of the sinus node of the human cardiac conduction system. Until today the sinus node (SN) is described as a heterogeneous system composed of 2 types of cells, namely, P or pale and T or transitional cells which are immersed in the matrix around the sinus nodal artery. T cells are said to share characteristics of P cells and of peripheral working atrial myocardial cells. This study was carried out on autoptic and explanted specimens using histochemical, immunohistochemical, and electron microscopic methods.
Our investigations show that SN tissue has a quite different cellular composition, ie, spherical and/or star-shaped cells organized in clusters with long cytoplasmic processes (type P), transitional cells, similar to myocytes but with a reduced number of sarcomeres (type T) and, finally, as yet not described in the literature, fibroblast-like cells with long bi-tripolar extensions contacting cells. Interestingly, SN is squared by connective and elastic fibers geometrically arranged. Immunohistochemistry shows that the 3 cell types of the SN node express mesenchymal markers revelatory of their embryological origin. Innervation appears to be more complex than previously thought; we identified a system of synaptophysin-positive cholinergic vesicles dependent on the sympathetic system and parasympathetic fibers expressing S100 protein.
Overall results indicate that the SN has an unexpected, systematic architecture.