Pathological changes induced by ototoxicity on spiral ganglion neurons and its peripheral processes

Deafness is one of the most widespread disabilities in the world. Its most frequent cause is death of cochlear hair cells (located in the organ of Corti), which induces degeneration of spiral ganglion neurons and their peripheral processes innervating the organ of Corti. In a previous light microscopy study using a rat model of ototoxicity, a loss of spiral ganglion neurons was observed since the eighth week of deafness and peripheral processes degeneration since the fourth week. In order to determine the onset of ultrastructural degenerative changes, a transmission electron microscopy study of spiral ganglion neurons and their peripheral processes was undertaken. Rat cochleae sampled after 2, 4, 8 and 16 weeks of deafness and healthy controls were analyzed. Since the fourth week of deafness, Type I spiral ganglion neurons and the myelin sheaths of their peripheral processes showed progressive degenerative changes. Most of the remaining neurons exhibited complete demyelination at sixteen weeks of deafness, resulting in the pathological type III spiral ganglion neurons. These results show ultrastructural degenerative changes of the spiral ganglion neurons and their peripheral processes, before both undergo significant losses.