Histology and ultrastructure of the sinus node {12, 10}.   A. Gross specimen showing the junction of the superior vena cava with the right atrial wall. The crista terminalis is seen in cross-section. The artery to the SA node is very prominent in this specimen. However, it is more subtle in most hearts. B. Histologic section of the specimen shown in A. The sinus node is the pale-staining tissue surrounding the artery. It is oblong and it is commonly insulated from the working myocardium by fibrous and adipose tissue. (H&E, 4X) C. The SA node of this newborn shows a discrete accumulation of specialized cells in the periphery of the SA node artery. Note the small myocyte size and the thin endocardium of the crista terminalis as a reference to size and age of the heart. D . In older patients, larger amounts of collagen deposition and even amyloid deposits can be seen. This micrograph shows an increased amount of fibrous tissue (phosphotungstic acid hematoxylin, X 3) E. The myocytes of the sinus node are thin and irregular in shape and measure 5 to 6 micrometers in diameter. They have cytoplasmic projections that form a three-dimensional network. They tend to form clusters of several cells. Cross-striations are also fewer within these cells. They have irregular branching and are arranged haphazardly in an abundant stroma that contains collagenous and elastic fibers. (H&E X 400) F. Transitional cells form the connections between the SA node and the working myocardium. These cells are paler than the working myocytes and are embedded in abundant extracellular matrix. (H&E X 200) G. Transmission electron micrograph of a bundle of SA node cells surrounded by a single basal lamina. These myocytes have junctions formed by direct apposition of adjacent cells. They have irregularly scattered desmosomes and only a few gap junctions. They contain few mitochondria and no T-tubules. (8,000 X) H. At higher magnification this bundle of SA node cells is shown to be insulated by a layer of collagen. An adjacent fibroblast is also shown. The myocytes have irregular sarcomeres. The sarcoplasm of one cell is molded and directly in contact with that of an adjacent myocyte. (20,000 X)