Fig. 2. Stereociliary bundles reorient during development. (A,B) Stereocilia (green) and kinocilia (red) were labeled in cochleae from P0, E17 and P10 (not shown). (A) In the basal region of the cochlea at P0, a typical chevron-shaped stereociliary bundle with a kinocilia located at the vertex of the bundle is present on each hair cell. Orientations for hair cells located in the boxed region are illustrated on the right. The locations of the single row of IHC and three rows of OHC (numbered) are indicated on the left. (B) In the mid-basal region of the cochlea at E17, chevron-shaped or flattened stereociliary bundles with asymmetric kinocilia can be identified for IHC and for OHC located in the first two rows (1,2). Bundles are present on third row OHC (3) but specific orientations could not be determined. As in A, orientations for cells located in the boxed region are illustrated on the right. Scale bar: in A 10 µm for A,B. (C) Average orientations of stereociliary bundles were determined for IHC and OHC in basal and apical regions of the cochlea at E17, P0 and P10. Distinct orientations could not be determined for stereociliary bundles located on second and third row OHC in the apical region of the cochlea at E17. Average deviation in stereociliary bundle orientation decreased with development for hair cells in all rows of the organ of Corti, indicating that orientation improves towards 0° over time. The average initial orientation for stereociliary bundles located in the third row of outer hair cells (asterisk) was significantly greater (P=0.02) than the initial orientation for bundles located in the row of IHC or in the first row of OHC.