Specialisations of the lateral membrane of inner hair cells

Freeze-fracture exposes on the lateral membrane of inner hair cell clusters of large particles organised in regular square array ('plaques'). Any one cell possesses a number of plaques. The centre-to-centre spacing of the particles forming the plaques is approximately 14 nm and at high resolution each particle appears to consist of 4 or 5 subunits. The plaque regions were resistant to the membrane deforming activity of filipin and consequently could be identified as localised membrane thickenings in thin sections of filipin-treated tissue; subsequently similar specialisations were identified in thin sections of normal tissue. From the thin section and freeze-fracture studies it is shown that the plaques are not junctional or synaptic specialisations and are not the sites of anchoring of some cytoplasmic component to the membrane. As the individual elements composing each plaque span the membrane and have a subunit structure, it is possible that the plaques represent the sites of transmembrane channels. It is estimated that there may be up to 20,000 such plaque-forming, channel-like elements on each inner hair cell.