Characteristics of the membrane of the stereocilia and cell apex in cochlear hair cells.

Freeze-fracture has been used to examine the membrane of the cell apex and of the stereocilia in cochlear hair cells. The apical (non-stereociliary) membrane of inner hair cells (IHCs) exhibited a lower density of intramembrane particles (IMP) than that of the outer hair cells (OHCs) but in both cell types the apical membrane responded to the effects of filipin. The distribution of IMP and of filipin-induced membrane deformations was uniform over the apical membranes in both IHC and OHC, thus, providing no evidence for local membrane differentiation on the non-stereociliary part of the hair cell apex. The stereociliary membranes of IHC and of OHC differed not only in the density of IMP, but also in their responses to filipin and to tomatin. IHC stereocilia responded intensely to both agents. OHC stereocilia showed a significantly lower density of filipin-induced lesions and appeared almost unaffected by tomatin. This suggests that the OHC stereocilial membrane may be structurally specialized. The membrane at the apical end of stereocilia appeared to be differentiated from the membrane of the stereociliary shaft. The tip region was free of the usual IMP and showed no filipin-induced lesions. The differentiation at the apical end was also apparent in samples which have been rapidly frozen without prior chemical fixation or cryoprotection, showing that the particle-free area was not an artefact induced by glutaraldehyde fixation. Close examination of the membrane at the apical-most tip of the stereocilium revealed the presence of a small number of large particles of 10.5-11.0 nm diameter. The occurrence of membrane differentiation localized to the tip of the stereocilium may be consistent with the suggestion that transduction channels in hair cells are situated at this point.