The functional morphology of stereociliary bundles on turtle cochlear hair cells

The stereociliary bundles of hair cells from the basilar papilla of the red-eared turtle were examined with transmission and high resolution scanning electron microscopy to provide a description of their morphology, orientation and inter-ciliary connections for comparison with physiological observations. Bundles on hair cells in the basilar membrane region are of a uniform shape and orientation, but bundles on the apical and basal limbus are distinct in having elongated kinocilia more than twice the length of the tallest stereocilia. Bundles in the basilar membrane region show a roughly two-fold increase in height from 5 to 9 microns from base to apex. Electrical recordings from isolated hair cells indicate that the bundle height is inversely proportional to the cell's characteristic frequency. It is argued that the change in dimensions is insufficient to contribute significantly to the cochlea's frequency selectivity. The cytoplasm adjacent to the kinocilium is filled with microtubules and large vesicles, and there are coated pits in the apical membrane which, it is suggested, may be indicative of rapid turnover of the membrane in this region.