Ultra-high-resolution scanning electron microscopy of the sarcoplasmic reticulum of the rat atrial myocardial cells

BACKGROUND: The sarcoplasmic reticulum (SR) of mammalian ventricular and atrial muscles share common features but also differ because T-tubules are rare and extended junctional SR is exclusively seen in the atrium. This scanning electron microscope (SEM) study was undertaken to clarify the three-dimensional organization of the rat atrial SR system. Specific preparations were examined with an ultra-high-resolution SEM. METHODS: Fixed right rat atria were frozen, fractured, and macerated by the aldehyde-osmium-DMSO-osmium method to remove myofibrils and cytoplasmic matrix. Left exposed were mitochondria, SR, and sarcolemma. Dried specimens were then impregnated by osmium-hydrazine and examined without metal coating. RESULTS: In place of conventional T-tubules, a prominent type of sarcotubules, Z-tubules, were found at the Z-line level. Branches from these tubules joined the cisternal SR, which was 100-300 nm in diameter and localized near the Z-line, and formed extensive SR meshworks and polygonal patches. Bulbous swellings, the corbular SR, were also evident. Sarcotubular reticulum completely surrounded each myofibril. The intermyofibrillar SR, especially Z-tubules, joined the peripheral subsarcolemmal SR, which was also arranged as a meshwork and was closely apposed to the sarcolemma. CONCLUSIONS: These SEM observations confirm the organization of the rat atrial SR system and present new, detailed, three-dimensional images of Z-tubules, cisternal SR, extended junctional SR, and peripheral SR, which provide further structural insight.