Microtubule rearrangement and bending during assembly of large curved microtubule bundles in mouse cochlear epithelial cells.

Mature inner pillar cells in the mammalian organ of Corti are curved through about 60 degrees, where they arch over adjacent epithelial cells and the apex of an intercellular space called the tunnel of Corti. This report deals with changes in microtubule organization that are associated with cell bending and tunnel formation during morphogenesis of the mouse organ of Corti. A large bundle of up to 3,000 microtubules assembles in each inner pillar cell. Microtubule rearrangement occurs about 5 days after bundle assembly begins. The lumen of each initially straight hollow tube-shaped microtubule bundle is occluded as the bundle becomes more compact and elliptical in cross section. This event anticipates the once-only bending which subsequently occurs between particular levels (about 9-19 microns) below the top of a bundle as it curves into its final shape about 2 days later. Microtubule rearrangement presumably facilitates bending which is effected in the plane of least mechanical resistance parallel to the short axis of a bundle's elliptical cross-sectional profile. Precocious bending of bundles has been induced about 1.5 days in advance of the natural event. Abnormal positioning of these prematurely curved bundles indicates that bending is effected by a contractile mechanism located within bundles rather than being a response to externally applied forces. The potential importance of such microtubule-associated contractions for active modulation of the vibratory response in the cochlea during hearing is considered.