Microtubules and associated microfilaments in the tentacles of the suctorian Heliophrya erhardi matthes

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Abstract
At the ultrastructural level length changes accompanying linear movements of resting (nonfeeding) tentacles of the suctorian Heliophrya involve not only altered microtubule numbers, but also marked changes in the specific microtubule pattern of cross-sectioned tentacles. These changes in number and pattern indicate a sliding between axonemal microtubules. The visualization of microfilaments in the cytoplasm at the tentacle base and in the knob region could shed new light on the problem of whether microtubular sliding is an active or passive process. At the tentacle base, microfilaments are either arranged in a ring-shaped configuration around the axoneme, or they run parallel to the axonemal microtubules, whereas at the tentacle tip during the resting state, microfilaments are closely associated with the plasma membrane of the knob. They form a filamentous reticular layer, which is continuous at the anchorage site of axonemal microtubules with the dense epiplasmic layer of the tentacle shaft. Obviously, this filamentous layer is engaged in positioning the haptocysts at the plasma membrane and in holding the membrane itself under tension. The putative contractile nature of microfilaments and the epiplasmic layer is argued from ATP-sensitive glycerol models of tentacles and from the results of halothane treatment of native tentacles. Halothane treatment of resting tentacles also gave indications of the presence of differentially stable intermicrotubule-bridges. The role of microfilaments and halothane-resistant dynein-like inter-row bridges in tentacle movement is discussed. As soon as the plasma membrane of the knob is ‘sealed’ with the prey pellicle during feeding, the microtubules of the sleeve region slide into the knob where they bend back and outwards. The microtubules now appear decorated and sometimes cross-connected by microfilaments which adhere closely to the plasma membrane — now acting as a peritrophic membrane — lining the prey cytoplasm against the microtubules of the inner tube. These microfilaments which show a close association with the microtubules of the active knob area, are thought to be engaged in microtubular bending and stretching during feeding. They may also be involved in the transport of the peritrophic membrane in distal tentacle regions. Microcinematographically recorded oscillations in tentacle diameter in these regions are in agreement with the electron-microscopic findings of various states of collapsed tentacle axonemes. These observations, as well as the occurrence of helically twisted tentacles during feeding, suggest microfilament-mediated sequential back and forth movements of sleeve microtubules in the knob region which generate a proximally migrating helical wave.