Early degeneration of sensory and ganglion cells in the inner ear of mice with uncomplicated genetic deafness (dn): preliminary observations.

Deafness (dn/dn) mouse cochlea was investigated by electron microscopy in order to detect the first postnatal signs of histopathology. At birth, the inner hair cells (IHCs) showed abnormal vacuolization, afferent dendrites at IHC level appeared swollen and devoid of cytoplasmic content, and most of the neurons of the spiral ganglion contained an abnormal smooth endoplasmic. At day 7, these abnormalities had greatly increased, especially in the spiral ganglion neurons where large patches of filamentous material were found. These observations can account for the permanent electrophysiological deafness of dn/dn mice. Moreover, these data, showing early cytological abnormalities in both the IHCs and the spiral ganglion neurons, indicate that it is difficult to simply classify the deafness mutation as being 'degenerative'; some 'morphogenetic' processes are likely also involved.