Figure 3.

Pre-synaptic ribbons hypertrophy and duplicate after synaptopathic noise. (A₁–A₄) The apparent heterogeneity of ribbon morphology disappears when each synapse is virtually re-sectioned in a ribbon-centric plane, as enabled by the isotropism of FIB-SEM image stacks. Four examples from a control ear appear different from each other when viewed in the acquisition plane. The same four synapses look similar to each other when viewed either as either (2nd column)—ribbon cross-section, i.e. perpendicular to the subjacent IHC membrane and to the long axis of the ribbon; (3rd column)—ribbon longitudinal section, i.e. perpendicular to the membrane and parallel to the long axis of the ribbon; or (4th column)—membrane parallel to the adjacent IHC and through the middle of ribbon thickness. (B) Double and triple ribbons are more common in noise-exposed ears, at either 1-week or 1-day survival as indicated. (C–E) The post-exposure hetererogeneity of ribbon length and number is illustrated by this sampler of ribbon morphologies from control, 1-day and 1-week cases, respectively. For each survival time, the sample include the longest and shortest examples from the group. Each box outlines the ribbon(s) at a different ANF synapse. Each image is a 3D reconstruction of the ribbon volume, viewed perpendicular to the plane of subjacent IHC membrane, i.e. in the same orientation as the membrane parallel images from panels (A₁–A₄). Each surface is rendered with enough transparency to see the hollow core (purple) when present. When multiple ribbons are present, their relative orientations are maintained.