The organization of contractile filaments in a mammalian smooth muscle.
Ordered arrays of thin filaments (65 A diameter) along with other apparently random arrangements of thin and thick filaments (100-200 A diameter) are observed in contracted guinea pig taenia coli rapidly fixed in glutaraldehyde. The thin-filament arrays vary from a few to more than 100 filaments in each array. The arrays are scattered among isolated thin and thick filaments. Some arrays are regular such as hexagonal; other arrays tend to be circular. However, few examples of rosettes with regular arrangements of thin filaments surrounding thick filaments are seen. Optical transforms of electron micrographs of thin-filament arrays give a nearest-neighbor spacing of the thin filaments in agreement with the "actin" filament spacing from x-ray diffraction experiments. Many thick filaments are closely associated with thin-filament arrays. Some thick filaments are hollow circles, although triangular shapes are also found. Thin-filament arrays and thick filaments extend into the cell for distances of at least a micron. Partially relaxed taenia coli shows thin-filament arrays but few thick filaments. The suggestion that thick filaments aggregate prior to contraction and disaggregate during relaxation is promoted by these observations. The results suggest that a sliding filament mechanism operates in smooth muscle as well as in striated muscle.
D000199:Actins / Q000032:analysis; D000447:Aldehydes; D000818:Animals; D003594:Cytoplasmic Granules; D006168:Guinea Pigs; D002479:Inclusion Bodies; D008297:Male; D008854:Microscopy, Electron; D008870:Microtubules; D008931:Mitochondria, Muscle; D009119:Muscle Contraction; D009130:Muscle, Smooth / Q000032:analysis / Q000166:cytology*; D009210:Myofibrils; D014961:X-Ray Diffraction
D000199:Actins; D000447:Aldehydes