Further observations on the fine structure of tip links between stereocilia of the guinea pig cochlea.

Stereocilia of the guinea pig organ of Corti were examined by transmission electron microscopy, after fixation in glutaraldehyde and tannic acid, and postfixation and en bloc staining in osmium tetroxide, tannic acid, uranyl acetate, and phosphotungstic acid. Tip links were observed between the stereocilia. The links emerged from the tips of the shorter stereocilia in the hair bundle, running nearly at right angles to the cuticular plate, to join the side-wall of the adjacent taller stereocilium of the next row. The tip links had a fine filamentous core, approximately 6 nm in diameter. The core was surrounded by positively-staining amorphous material, which had a variable appearance from link to link. The central filament inserted into membrane specialisations at both its upper and lower ends. The results suggest that tip links have two components, and that the central filament, which has the same diameter as an actin filament, is suitable for transmitting stimulus-induced movements to the transducer channels of the stereocilium. The central filament would therefore concentrate the stimulus-induced forces onto a small area of cell membrane.