Changes in microtubule packing during the stretching of an extensible microtubule bundle in the ciliate Nassula

The cytopharyngeal sheath in the ciliate Nassula is a long hollow tube-shaped microtubule bundle that forms part of a large feeding organelle called the cytopharyngeal basket. During the initial stages of ingestion of algal filaments by Nassula the sheath is stretched, becomes approximately elliptical in cross-section, and its external cross-sectional perimeter increased by a factor of about two. The mean circumferential centre-to-centre spacing of radially oriented rows of sheath tubules increases from 57 to 137 nm during stretching but sheath thickness and the radial spacing of sheath tubules do not change appreciably. It is suggested that extensible circumferentially oriented intertubule links and relatively inextensible radial links may define the anisometric mechanical properties of this particular microtubule bundle which are related to its cytoskeletal role. The possibility that extensible links resist stretching elastically and provide the restoring forces for return of the sheath to its former shape and dimensions after stretching is considered.