Structure of molluscan thick filaments: a common origin for diverse appearances.

Thick filaments from the smooth adductor muscles of the oysters Ostrea edulis and Crassostrea angulata have been examined in the electron microscope after negative staining. The two well-known patterns of stain (whose origin and relation have been uncertain), one a series of transverse narrow lines at intervals of 144 Å along the filament axis and the other a regular two-dimensional arrangement of stained spots (Bear &; Selby, 1956), are found to be mutually interconvertible by rotating the grid around the filament axis. This is interpreted to mean that the spots are the projections of stained regions running through the filament in a common direction. Only when looking along this direction will the net pattern be seen with maximum clarity and sharpness. On rotation of the filament round its axis, the spots broaden transversely to the axis, overlapping and ultimately only the axial periodicity will remain. The structure is therefore not helical, but resembles a crystal lattice, although no period can be discerned normal to the net plane.The addition of 10 mm-EDTA to all solutions used in the filament preparation (except the stain), especially when ammonium molybdate is the stain employed, removes many puzzling appearances (probably caused by positive staining) which render the interpretation difficult. The appearance of the negatively stained filament can be related to the stain patterns in negatively stained paramyosin paracrystals (Cohen et al., 1971).