Sarcolemma, T-tubules, and subsarcolemmal caveolae. Interrelationships and continuity demonstrated by tannic acid mordanting

Abstract

Muscle cells of rat rectus abdominus and diaphragm were mordanted in tannic acid and ultrathin sections, after staining with lead citrate, showed a variable but usually marked increase in electron density of T-tubules, subsarcolemmal caveolae and, to a lesser degree, the plasmalemma, basal lamina material, and associated collagen microfibrils. The technique permits clear demonstration of continuity of the membranes of T-tubules and caveolae with the sarcolemma. While some T-tubules open directly into the extracellular space, others do so indirectly via a caveola. The T-tubules show some variation in diameter and not all are transverse in orientation; they frequently branch, reunite, and run longitudinally, often connecting T-tubules of adjacent triads across a Z-line. Subsarcolemmal caveolae (approximately 100 nm diameter) line the sarcolemma in a single row and show frequent connections by narrow tubules to form dumbell, trefoil, and four-leafed profiles.The findings lead to the conclusion that both T-tubules and caveolae are open to the extracellular space and that T-tubules open both directly and indirectly via caveolae to the surface.