MICROTUBULE REARRANGEMENT AND BENDING DURING ASSEMBLY OF LARGE CURVEDMICROTUBULE BUNDLES IN MOUSE COCHLEAR EPITHELIAL-CELLS

Mature inner pillar cells in the mammalian organ of Corti are curved t hrough about 60-degrees, where they arch over adjacent epithelial cell s and the apex of an intercellular space called the tunnel of Corti. T his report deals with changes in microtubule organization that are ass ociated 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 bun dle becomes more compact and elliptical in cross section. This event a nticipates the once-only bending which subsequently occurs between par ticular levels (about 9-19 mum) below the top of a bundle as it curves into its final shape about 2 days later. Microtubule rearrangement pr esumably facilitates bending which is effected in the plane of least m echanical resistance parallel to the short axis of a bundle's elliptic al cross-sectional profile. Precocious bending of bundles has been ind uced about 1.5 days in advance of the natural event. Abnormal position ing of these prematurely curved bundles indicates that bending is effe cted by a contractile mechanism located within bundles rather than bei ng a response to externally applied forces. The potential importance o f such microtubule-associated contractions for active modulation of th e vibratory response in the cochlea during hearing is considered.