The ultrastructure of frog semitendinosus muscle was explored using the freeze-fracture, deep-etch, rotary-shadowing technique. Mechanically skinned fibers were stretched to decrease or eliminate the overlap of thick and thin filaments before rapid freezing with liquid propane. In relaxed, contracting, and rigor fibers, a significant number of bridgelike interconnections, distinct from those observed in the M-region, were observed between adjacent thick filaments in the non-overlap region. Their half-length and diameter corresponded approximately to the known dimensions of the cross-bridge (or myosin S-1). The interconnection may thus be formed by the binding of two apposed cross-bridges projecting from adjacent thick filaments. Fixation with 0.5% glutaraldehyde for 5-10 min before freezing effectively preserved these structures. The results indicate that the interconnections are genuine structures that appear commonly in stretched muscle fibers. They may play a role in stabilizing the thick filament lattice, and possibly in the contractile process.
Bridgelike interconnections between thick filaments in stretched skeletal muscle fibers observed by the freeze-fracture method.
S Suzuki, G H Pollack; Bridgelike interconnections between thick filaments in stretched skeletal muscle fibers observed by the freeze-fracture method.. J Cell Biol 1 March 1986; 102 (3): 1093–1098. doi: https://doi.org/10.1083/jcb.102.3.1093
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