Fig. 6. Elongation and tip defects of stereocilia in Ush1 mutants during postnatal stages. (A) Hair bundle maturation in P0 to P15 wild-type and Ush1c^-/- OHCs from the end of the apical cochlear turn. At P0, mutant OHC bundles are composed of four to six stereocilia rows almost equal in height, resembling their wild-type counterparts. From this stage onwards, wild-type stereocilia undergo differential growth, depending on the row they belong to. In the absence of harmonin, small and medium stereocilia rows show marked elongation defects from P2 onward, and most of the stereocilia from these rows have disappeared by P15. By contrast, stereocilia from the tall row are of normal length at all stages examined. Note that some small and medium row stereocilia located near clump vertices show some elongation at first (see black, green and red lines running along the tall, medium and small rows of stereocilia, respectively, in P2 and P5 wild-type and mutant OHC bundles). (B) OHC bundles of P5 Myo7a^{4626SB/4626SB}, Cdh23^v2J/v2J, Pcdh15^av3J/av3J and Ush1g^js/js mice (view from the end of the apical cochlear turn). Note the abnormal height of many stereocilia of the medium row and the frequent absence of small stereocilia in mutant hair bundles. (C) Mid-cochlear IHC bundles of P5 wild-type, Myo7a^{4626SB/4626SB}, Ush1c^-/-, Cdh23^v2J/v2J, Pcdh15^av3J/av3J and Ush1g^js/js mice. As differential elongation occurs, stereocilia of the medium row in wild-type hair bundles acquire a particular prolate shape (see gray lines in the wild-type IHC inset), which could result from tension forces applied to their tip membrane (Rzadzinska et al., 2004). Medium stereocilia of Ush1 mutant IHCs, however, never display prolate tips (see insets). In addition, apical links (arrowheads) are either absent (Cdh23^v2J/v2J and Pcdh15^av3J/av3J), or appear sparser (Myo7a^{4626SB/4626SB}, Ush1c^-/- and Ush1g^js/js) in the Ush1 mutants, compared with the controls. Scale bars: 1 µm.