Fine structure of the M fibres in Stentor before and after shortening

The fine structure of the filamentous M fibres of Stentor coeruleus was examined in contracted specimens and in stentors relaxed in a solution of raised ionic strength with a high monovalent/divalent ion ratio, prior to fixation. In both, the microfilaments constituting the fibres were of two types, the great majority being about 8 nm in diameter, and a smaller proportion of microfilaments measuring 3–5 nm. The microfilaments of contracted M fibres were closely aggregated whereas those of uncontracted M fibres were more widely spaced. No appreciable change in size appeared to have occurred in the diameter of the filaments. These results were taken to indicate that the microfilaments do not undergo an internal conformational change, but rather shorten the M fibre by aggregating laterally and longitudinally.