EM of part of a skeletal muscle fiber showing myofibrils in transverse section. Tubular components of the sarcoplasmic reticulum (SR) with intervening mitochondria (Mi) encircle myofibrils. The section passes through different parts of A bands of sarcomeres and shows an orderly arrrangement of myofilaments in each region (A, the thick and thin filament overlap zone; H; M). 63,000×. The inset shows the square lattice pattern of the Z band (Z) and associated thin filaments in nearby I band (I). 45,000×.