Density of myosin filaments in the rat anococcygeus muscle, at rest and in contraction. II.

Rat anococcygeus muscles were fixed at rest or in contraction by conventional methods and prepared for electron microscopy. Myosin filaments were counted on cross sections and their density expressed per unit cytoplasmic area. In contracted muscles, the mean density increased from 86 to 168 filaments per micron 2 (1.95 times), while the density of intermediate (10 nm) filaments increased by 1.25 times. Cell cross sections from the same muscles were measured. Contraction produced a shrinkage which explains the apparent increased density of the 10 nm filaments; however an excess of 61 myosin filaments per micron 2 cannot be explained in this way. These findings provide the structural basis which quantitatively explains the birefringence changes observed in living contracted muscle (Godfraind-De Becker & Gillis, 1988). Our optical and electron optical results provide evidence for a reversible formation of myosin filaments during contraction of the rat anococcygeus muscle.