The transverse tubular system in mammalian myocardial cells

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Abstract
The fine structure of ox myocardial cells, as seen in transverse sections, is described. The presence of an extensive system of transverse tubules at Z region levels has been confirmed, the walls of the tubules being continuous with the sarcolemma.Preliminary observations in smaller animals indicate that similar tubules are readily seen in guinea‐pig myocardial cells and are present, but are more difficult to find, in the rat; communication between the tubules and the exterior is readily demonstrated in the guinea‐pig but less readily in the rat.Contact between the transverse tubules and the Z regions was not particularly close, and a thin delicate “circumferential Z tubule” was noted to lie round the myofibrils at most Z regions.The Z regions, on cross section, shows a tightly packed square array of filaments, double in number compared with the thin filaments seen elsewhere in the sarcomere; the appearances probably confirm for heart muscle the Z region structure which has been described for skeletal muscle.