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Ultrastructure of the internodal myocardium in the rat.

Ultrastructural studies of the rat atrial myocardium reveal that the specialised myocardial cells can be distinguished from the usual atrial myocytes by their lack of specific atrial granules. On the basis of this distinction it has been shown that there is interposition of atrial myocardial tissue in the pathway between the two nodes and that there are no morphologically specialised internodal pathways. The atrial myocardial cells themselves exhibit variations in their content of cytoplasmic organelles such as myofibrils, T-tubules and sarcoplasmic reticulum but they all contain membrane-bound specific atrial granules.