Visualization of changes in ciliary tip configuration caused by sliding displacement of microtubules in macrocilia of the ctenophore Beroë.

Macrocilia from the lips of the ctenophore Beroë consist of multiple rows of ciliary axonemes surrounded by a common membrane, with a giant capping structure at the tip. The cap is formed by extensions of the A and central-pair microtubules, which are bound together by electron-dense material into a pointed projection about 1.5 micron long. The tip undergoes visible changes in configuration during the beat cycle of macrocilia. In the rest position at the end of the effective stroke (+30 degrees total bend angle), there is no displacement between the tips of the axonemes, and the capping structure points straight into the stomach cavity. In the sigmoid arrest position at the end of the recovery stroke (-60 degrees total bend angle), the tip of the macrocilium is hook-shaped and points toward the stomach in the direction of the subsequent effective stroke. This change in tip configuration is caused by sliding displacement of microtubules that are bound together at their distal ends. Electron microscopy and two-dimensional models show that the singlet microtubule cap acts as if it were hinged to the ends of the axonemes and tilted to absorb the microtubule displacement that occurs during the recovery stroke. The straight and hooked shapes of the tip are thought to help the ctenophore ingest prey.