Two kinds of thick filament in smooth muscle cells in the adductor of a clam, Chlamys nobilis

The ultrastructure of the opaque portion of the adductor muscle in the pecten Chlamys nobilis was investigated. The opaque portion was composed of smooth muscle cells that contained thin and thick filaments. The thick filaments were classified into two kinds, thinner and thicker, according to the statistical analysis of diameters. They were also classified as being shorter and longer, when isolated native filaments were examined. The thick filaments may consequently be classified into two kinds: thinner and shorter filaments, and thicker and longer ones. The thinner and shorter filaments were about 26.5 nm in diameter and 7.5 μm in length, and the thicker and longer ones were about 42.0 nm in diameter and 13.0 μm in length, respectively. A regular periodicity was apparent on the surface of the core after removal of myosin molecules from its surface. The periodicity seemed similar for the two kinds of thick filament.