REORGANIZATION OF THE CENTROSOME AND ASSOCIATED MICROTUBULES DURING THE MORPHOGENESIS OF A MOUSE COCHLEAR EPITHELIAL-CELL

Reorganization of centrosomal microtubule-organizing centres and the m inus ends of microtubules occurs as the centrosomal ends of large micr otubule bundles are repositioned and anchored to cell junctions in cer tain epithelial cells called inner pillar cells in the mouse organ of Corti. The microtubule bundle that assembles in each cell consists of two distinct microtubule arrays that run closely alongside each other. Both arrays are attached to the cell surface at their upper and lower ends. One of the arrays spans the entire length of a cell but the oth er is confined to its lower portion. Initially, about 3,000 microtubul es elongate downwards from an apically situated centrosome in each cel l. Subsequently, the minus ends of these microtubules, and the centros ome and its two centrioles, migrate for about 12 mu m to the tip of a laterally directed projection, Then, a meshwork of dense material accu mulates to link microtubule minus ends and the centrosome to cell junc tions at the tip of the projection. Pericentriolar satellite bodies, w hich form after the initial burst of microtubule nucleation, may repre sent a condensed and inactive concentration of microtubule nucleating elements. Suprisingly, as a cell matures, about 2,000 microtubules are eliminated from the centrosomal end of the microtubule bundle, Howeve r, about 2,000 microtubules are added to the basal portion of each bun dle at levels that are remote with respect to the location of the cent rosome. Possibly, these microtubules have escaped from the centrosome. If this is the case, then both the plus and minus ends of most of the errant microtubules are captured by sites at the cell surface where t he ends are finally anchored. Alternatively, each cell possesses at le ast one other major microtubule-nucleating site (which does not posses s centrioles) in addition to its centrosome.