A study of the structure of the terminal part of the T-tubule system and the distribution of sub8arcolemmal caveolae was undertaken in guinea-pig psoas muscle. The results were correlated with the array of cell surface features as revealed by frozen-etched, shadowed replicas of the cell membrane. Freeze-etching revealed numerous small rounded objects overlying the Z-and I-bands and the A-I junction regions of the sarcomeres. These objects appeared as pits or excrescences, depending on whether

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... e cell membrane was viewed from outside or inside the cell, and were interpreted as apertures in the membrane. Conventional thin sections demonstrated the presence of numerous subsarcolemmal caveolae with a similar distribution to the rounded features seen in the replicas. Such sections also showed that the T-tubules, lying at the A-I junctions, seem to change direction when approaching the cell surface and may occasionally appear to branch in the subsarcolemmal region. The T-tubules often terminated in caveolae. Caveolae were sometimes seen in direct communication with the extracellular space. No simple direct communications of T-tubules with the cell surface were observed. After treatment of the muscle with lanthanum during fixation, thin sections revealed apparently continuous dense deposits from the cell surface, through the caveolae to the T-tubule proper. It thus appears that each T-tubule communicates indirectly with the extracellular space via one or more subsarcolemmal caveolae.