Sarcotubules and subsarcolemmal caveolae and their continuity with the sarcolemma in frog striated muscle

After using tannic acid mordanting, T-tubules and subsarcolemmal caveolae show a dense staining with lead citrate, with some increase in contrast also of the sarcolemma, in frog muscle. While a few T-tubules show direct continuity with the extracellular space, the majority open indirectly via caveolae. Caveolae lie immediately beneath the sarcolemma, mainly in a single row, and are more numerous in relation to I-bands.