Fig 4. Inner hair cells.

A, B Normal animal at P17. C-G. Affected animals. C. (P25). Apical surface of the cell and bundle in affected animals is more rounded than normal (A). The shape of the bundle is variable. There are stereocilia of different lengths but the longest ones are often in the centre of the bundle and there are no regular rows so there is no definable polarity as in the normal bundles. D: Basal; E: Middle; F Apical coils (P17). There is considerable variability in the heights of stereocilia on adjacent cells and height differentials in an individual bundle are often from the periphery towards the centre (E). Stereocilia in the affected animals are much shorter than those in equivalent regions in normal animals. Panel B shows IHC stereocilia in basal coil of normal littermate at the same magnification as the basal coil in the affected animal in panel D. There is, however an increase in height of the longest stereocilia along the organ of Corti from base to middle to apex (D,E,F, from same affected animal at same magnification). Arrows in panels D and E indicate examples of clearly longer, protruding stereocilia in bundles which were the only ones measured in affected animals for data used to estimate stereociliary lengths of stereocilia. G. (P17). Apparent tip-links (arrows) running from the top of a shorter stereocilium to the side of an adjacent longer one. Arrowheads indicate where one shorter stereocilium appears to form links with two longer neighbours. Note the variable heights within the cluster of longer stereocilia. Scale bars: A-F 1μm; G 0.25μm. H. Histograms displaying means of apical surface area, number of stereocilia and length and width of longest stereocilia on IHCs in normal and affected animals. Each mean was derived from pooled measurements/counts at basal, middle and apical coils in individual cochleae from at least 3 different animals aged P17-P22. N = number of cells assessed in each group. In affected animals, measurements of longest stereocilia were restricted only to those bundles where there were clearly longer stereocilia, protruding above their neighbours as arrowed in panels D and E. The result therefore does not show the true variability in lengths of stereocilia in the hair bundles of IHC in affected animals.