Anatomy of the sinus node of domestic cats (Felis catus)

Anatomy of the sinus node was studied in 10 domestic cats (5 males and 5 females). In the feline heart, the sinus node lied 0.06mm-0.11 mm beneath the epicardium, at terminal sulcus, near the junction between the cranial vena cava and the right atrium, its shape was almost triangular. Its cranial end was near the myocardium of the atrial wall and it was narrow, while the caudal end was near the myocardium of the right auricle and it was wide. The dimensions of the sinus node in male cats were 2.78 mm x 2.80 mm x 0.54 mm, and in the female cars 2.75 mm x 2.64 mm x 0.45 mm. Histologically, the sinus nr,de of cats contained normally a dense collagen frame and the cells with perinuclear clear zone (P) and transitional cells (T) were the principal cells of parenchyma. Since these cells contained less myofibrils than working myocardium, therefore, they were distinctively paler. The "P" cells (7-9.75 mu) were bigger than "T" cells (5-6 mu) and smaller than the ordinary myocardial cells (9-11.87 mu). At the junction between "T" cells and "P" cells intercalated discs were not seen. While the glycogen content of the "P" cells was very little, it was more in "T" cells almost at the same level as ordinary atrial myocardial cells.