Late degeneration of OHCs in the apical turn of <i>Hoxb1</i> and <i>Hoxb2</i> mutant cochleae.

<p>(A) Scanning electron microscopy (SEM) views of the cochlea at P8: an overview of the apical turns of WT, <i>Hoxb1^null</i> and <i>Hoxb1^lateCKO</i> cochleae showing three orderly arrayed rows of outer hair cells (OHCs) and one row of inner hair cells (IHCs). Representative high magnification images illustrate stereocilia of hair bundles of single OHCs arranged according to their different lengths. Shape and organization of OHCs in apical regions are normal at this stage in both mutants. (B) SEM views of 3-month-old WT, <i>Hoxb1^null</i> and <i>Hoxb1^lateCKO</i> cochleae and representative higher magnification images of OHCs. In <i>Hoxb1^null</i> and <i>Hoxb1^lateCKO</i> cochleae, OHCs have lost their regular organization and fail to develop in some areas (white arrowheads). Moreover, in <i>Hoxb1^null</i> cochleae most stereocilia have completely lost their typical V-shaped morphology and their characteristic differences in lengths (arrows). OHCs are less severely affected in <i>Hoxb1^lateCKO</i> cochleae. IHC cilia appeared weakly disarranged (red arrowheads). (C) SEM views of 3-months-old WT and <i>Hoxb2^ΔKO</i> mutant cochleae and higher magnifications of representative OHCs. Note that, similarly to <i>Hoxb1^lateCKO</i> cochleae, <i>Hoxb2</i> mutants have occasional missing OHCs (white arrowheads), disarranged IHC cilia (red arrowheads) and disorganized OHC stereocilia (arrows). (D) Histogram quantifying the percentage of OHC loss in controls, <i>Hoxb1^null</i>, <i>Hoxb1^lateCKO</i> and <i>Hoxb2^ΔKO</i> cochleae. While controls (n = 8) showed no OHC loss, in <i>Hoxb1^null</i> mutants (n = 6) 7.2±0.8% of OHCs were absent, whereas 3.1±0.9% and 1.3±0.5% were lost in <i>Hoxb1^lateCKO</i> (n = 6) and <i>Hoxb2^ΔKO</i> (n = 3) cochleae, respectively. Inter-genotype ANOVA p<0.001; <i>Hoxb1^null</i> versus WT: p<0.001; <i>Hoxb1^lateCKO</i> versus WT: p<0.001; <i>Hoxb2^ΔKO</i> versus WT: p = 0.02. Scale bars, 10 µm (A, B, C, left panels), 1 µm (A, B, C, right panels). See also <a href="http://www.plosgenetics.org/article/info:doi/10.1371/journal.pgen.1003249#pgen.1003249.s008" target="_blank">Figure S8</a>.