Distribution of cochlear damage caused by the removal of the round window membrane

The distribution of damage that occurs in the cochlea after removal of the round window membrane was examined in the apical, middle and basal regions with light and electron microscopy. The damage resembles that seen after acoustic trauma in many respects. The outer hair cells are often disrupted in damaged zones, and the radial afferent fibers to the inner hair cells swell enormously to form large vacuoles. 16 h after opening of round window, there is conspicuous swelling of myelinated axons in the osseous spiral lamina of the apical region. This swelling is associated with large vacuoles underneath the inner hair cells. 10 h after opening the round window, much smaller vacuoles are seen in the apical region. The distribution of the damage is not uniform throughout the cochlea. Damage is usually less severe and is not uniform in the middle region but is pronounced in the base. The nature of the damage is also variable in different animals. For example, sharply delimited, discontinuous damage to the inner hair cells was occasionally observed in the apical region. The most likely cause for the damage to the cochlea is a pressure differential across the organ of Corti that appears after removing the round window membrane. The damage apparently causes low frequency random movements of the basilar membrane that are observed in the experimental cochleas using a reflected laser beam.