Variation in thin filament size in the skeletal muscle of the frog

The sartorius muscle of Rana esculenta was fixed and dehydrated by different methods and cross-sections of sarcomeres of different lengths were examined by electron microscopy. The area within the outlines of the thin filaments in negatives was measured using a high-resolution television system and a graphics tablet or by using a microdensitometer and computer processing the images with a density threshold above the background level. In all specimens the area of the thin filaments in the I-bands was found to be larger than in the S-zones of the A-bands where the thick and thin filaments interpenetrate. The difference was statistically significant and independent of the length of the sarcomere. The results are in agreement with previous observations in glycerol-extracted fibres. It is suggested that the change in size of the thin filaments might be accounted for by some interaction with the thick filaments.