Three‐dimensional architecture of sarcoplasmic reticulum and T‐system in human skeletal muscle

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Abstract
A modified Golgi method combined with stereoscopy has been used to demonstrate the three‐dimensional architecture of the sarcoplasmic reticulum (SR) and the T‐system in human skeletal muscle. SR formed a continuous repeating network with a different structure dependent upon the sarcomere position. Intermyofibrillar SR contained three regions: 1) fenestrated collars overlying the M‐band region, 2) terminal cisternae overlying the A‐I region, and 3) a three‐dimensional anastomosed tubular network overlying the Z‐band region. Longitudinal and/or transverse SR tubules connected these regions. Subsarcolemmal SR was also composed of three regions: 1) transversely oriented polygonal meshes overlying the M‐band, 2) single‐layered tubules overlying the Z‐band region, and 3) a loose network between the two. In the subsarcolemmal sarcoplasm, where mitochondria were aggregated, SR anastomosed loosely and showed nonfenestrated cisternae beneath the plasma membrane. The T‐system was composed of transversely oriented networks overlying the A‐I region with occasional longitudinal tubules connecting these networks.