ELECTRON MICROSCOPY: MUSCLE: Skeletal muscle, myofibrils. (EM-labeled); RCH/AMC1681, Tangential sections of tubules of the sarcoplasmic reticulum, triads (at level of A-I junction), rat tissue. 1-mitochondria in interfibrillar space, 2-nucleus of muscle cell, 3-Z lines of sarcomeres, 4-M lines of sarcomeres, 5