Ultrastructural correlates of inner ear sensory cell shortening

Ultrastructural analysis was performed on isolated outer hair cells of the inner ear, to further investigate the mechanisms involved in cell shortening. Hair cells were fixed either in their presumed normal, 'relaxed', state or after application of solutions containing high concentrations of potassium to induce shortening, or 'contraction'. Ultrastructural analysis showed that shortened outer hair cells had a more undulated, or wavy, cell membrane as compared to normal cells. Outer hair cell shortening was not accompanied by the formation of vacuoles in the cytoplasm or appreciable swelling of the subsurface cisternae. The results further support the idea that a contractile apparatus associated with the lateral wall could be responsible for outer hair cell motility, and that the mechanism resembles that involved in smooth muscle contraction.