The transverse tubular (T) system of rat cardiac muscle fibers as demonstrated by tannic acid mordanting

T-tubules and subsarcolemmal caveolae in rat cardiac muscle fibres have been studied using the tannic acid mordanting technique. While T-tubules form an extensive and relatively regular meshwork with triads at Z-lines in ventricular fibers, in atrial fibers the tubules are often absent, or very sparse and with an irregular distribution. There appears to be a relation to fiber size, the meshwork being more extensive in fibers of larger diameter, and an inverse relationship to the number of couplings found between sarcoplasmic reticulum and plasmalemma. In larger atrial fibers, while the T-tubule meshwork was extensive, it did not show the regularity of distribution found in ventricular fibers. T-tubule continuity with the extracellular space was visualized clearly, both directly and indirectly via subsarcolemmal caveolae.