Electron microscope observations on thick filaments in striated muscle from the lobster Homarus americanus

The size and appearance of thick filaments in lobster claw and tail muscle have been determined from EM observations on thin sections and isolated filaments. The filaments from both types of tissue are about 180 Å in diameter, but those from claw muscle are approximately 6 μ long, whereas the filaments from tail muscle are only ca. 3.4 μ long. The ratio of thin to thick filaments in the two muscles is 12/1 and 3/1, respectively. Also noted was an apparent variation in cross-sectional density of the filaments. In tail muscle, the filaments appear to be solid in the central region whereas in the A-I overlap region they appear to be hollow.