Comparative Ultrastructure of the Sinus Node in Man and Dog

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Abstract
Ultrastructure of the sinus node was studied in human and canine hearts and found to be similar. The principal cell of the sinus node is a small round pale cell with randomly distributed sarcosomes and sparse myofibrils. These cells have been designated as P cells. They occur in elongated clusters and make contact with each other in all directions but do not make direct contact with ordinary working myocardium. Transitional cells, which have features intermediate between P cells and working myocardium, serve as the connections between P cells and the rest of the heart. Each P cell is surrounded by a plasma membrane and groups of them are bound by a basement membrane. The only specialized junction between P cells is the desmosome, which occurs singly and seldom, with most contact being between apposing plasma membranes. P cells exhibit unusually active pinocytosis but have only a sparsely developed sarcoplasmic reticulum. The sarcosomes of P cells are much simpler in internal structure than are those of adjacent working myocardium obtained at the same time in the same heart in the same way, by direct perfusion with glutaraldehyde into the beating sinus node. Correlation of the electron microscopic appearance of the sinus node with that based on light microscopy is discussed, and some of the possible functional significances of the fine structure are considered.