| Dynamic regulation of stereocilia structure. (A-C) SEM images of organ of Corti (A) and vestibular (B) hair bundles demonstrate slight irregularities in stereocilia lengths within the characteristically packed rows of the hair bundle. Arrows pointing down show slightly shorter stereocilia, whereas arrows pointing up show longer stereocilia in the same row. In the actin-GFP-transfected vestibular hair cell, the incorporation and treadmill rates (C) reflect the slight length variations within stereocilia of the same row in the bundle. The actin treadmill rates are faster in stereocilia that are taller than their neighbors in the same row (upward arrow) and slower in stereocilia that are shorter than their peers of the same row (downward arrow). Bars: (A) 250 µm; (B) 1 µm; (C) 2 µm. (D) Natural variability in stereocilia tip shape and rate of actin-GFP incorporation. The stereocilia of the tallest row in a bundle have uniform oblate tips, whereas the stereocilia from the second and lower rows exhibit prolate tips that can be slightly pointed to very elongated. The rate of actin-GFP incorporation is increased in the more elongated tips. Bar, 2 µm. Inset shows the frequency distribution of tip length measured as indicated by the measuring bar in the figure. (E) Remodeling of stereocilia tips exposed to BAPTA. SEM images of the stereocilia after 5-min incubation with 5 mM BAPTA in calcium-free L-15 media and 30-min recovery (right) reveal that tip links are disrupted and stereocilia tips become rounded when compared with control (left). The length of the tip defined as shown on the figure was 0.51 ± 0.05 µm for control and 0.34 ± 0.02 µm for BAPTA-treated stereocilia (n = 50). Bar, 250 nm. |