Histological evaluation of the Tprn ^−/− mouse organ of Corti (OC) shows progressive degeneration. Cross section of apical, middle, and basal turns of the cochlea of WT normal hearing Tprn^+/+ mouse (left panels) and deaf Tprn^−/− littermate (right panels) at P60. OC degeneration is evident at basal and middle turns of Tprn^−/− cochlea. In the panel of Tprn^−/− basal turn the inserts show enlarged views of the degenerated OC and spiral ganglion neurons (SG) with substantial loss of neuronal cell bodies, which is not yet observed in the middle and apical turns of the Tprn^−/− cochlea. In the middle turn of Tprn^−/− cochlea, the OC shows loss of all OHCs and some supporting cells. In some TPRN-deficient mice, there is a mild increase in melanin pigment in the stria vascularis (SV). All turns of Tprn^+/+ cochlea show normal OC structure and normal density of spiral ganglion neurons. All scale bars are 50 µm.