Fig. 4.

Scanning electron microscopy analysis of the ultrastructure of the TM. The three radial regions, HC, MB, and limbal (lim), according to which the mechanical measurements were grouped together, are bounded by white dashed lines. (A) A segment of a TM isolated from the apical region of the cochlea, with the covering-net side facing up (toward the AFM probe). Note the similar parallel fiber arrangements in the HC zone (beneath the mesh-like upper layer) and MB zones, compared with the tightly packed arrangement visible in the limbal zone. (B) The stereocilium imprint surface (i.e., the side opposite the covering net) of a TM sample isolated from the apical region. Note the similar parallel arrangement of thick fibers in the HC and MB zones. (C) TM sample isolated from the basal region with the covering net facing up. A striking structural difference between the homogenous matrix (marginal band) seen in the HC zone and the mesh-like surface layer of the MB zone is clearly visible. (Inset) The mesh-like surface structure of the MB zone beneath which thick fibers lying in a parallel arrangement can be discerned. (D) A TM sample isolated from the basal region of the cochlea with the stereocilium imprints facing up. Similar to C, a clear difference is seen between the tight packing of the fibers in the HC zone and the parallel thick fiber arrangement observed in the MB zone beneath the covering net.