This cross-section shows striated muscle fibers about 10-100|im thick. Size and morphology depend on muscle function and workload. The nuclei HI are clearly located in the periphery of the cells. The plasmalemma envelops the cytoplasm (sarcoplasm). The crevices between the muscle fibers contain the loose connective tissue of the endomysium, which consists mostly of reticular fibers. The even, dense and only softly stained dots in this cross-section reflect the even distribution of myofibrils (cf. Fig.228). Occasionally, dependent on the fixation, the fibrils show a grouping pattern (Cohnheim's fields, groups). The plasmalemma is often called sarcolemma. This terminology was coined in light microscopy. In fact, the sarcolemma consists of the plasmalemma, the basal lamina and a network of delicate reticular fibers. These are not distinct components in light microscopic evaluations.
1 Nucleus of a skeletal muscle fiber
3 Nucleus of a connective tissue cell
Semi-thin section; stain: methylene blue-azure II; magnification: x 800
Longitudinal section through striated myofibrils with conspicuous sar-comere organization. A sarcomere extends from one Z-band □ to the next H. In Z-bands, the actin filaments of two successive sarcomeres twist together and are cross-linked. Actin filaments in the form ofI-bands (isotropic bands) flank both sides of the Z-band [2. The thicker myosin filaments form the Aband (anisotropic band) 13 of the myofibril. Actin and myosin filaments form a twisted helix and leave only the H-band HI (light zone) in the middle of the A-band. The H-band consists only of myosin filaments, the I-bands only of actin filaments. The dark centerline in the H-zone is the centermembrane. Cross-linking of the myosin filaments creates this line. Note the peripheral nucleus E close to the fiber surface under the sarcolemma (plasmalemma) ^ (see Figs. 226, 227).
q 3 A-bands (anisotropic)
"2 4 H-band (H-zone; Hensen's bands) with M-band (M-line)
U 5 Mitochondria
13 6 Nucleus of the muscle fiber g 7 Extracellular matrixwith collagen fibrils
Electron microscopy; magnification: x 6000
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