THE FUNCTIONAL-MORPHOLOGY OF STEREOCILIARY BUNDLES ON TURTLE COCHLEARHAIR-CELLS

The stereociliary bundles of hair cells from the basilar papilla of th e red-eared turtle were examined with transmission and high resolution scanning electron microscopy to provide a description of their morpho logy, orientation and inter-ciliary connections for comparison with ph ysiological observations. Bundles on hair cells in the basilar membran e region are of a uniform shape and orientation, but bundles on the ap ical and basal limbus are distinct in having elongated kinocilia more than twice the length of the tallest stereocilia. Bundles in the basil ar membrane region show a roughly two-fold increase in height from 5 t o 9 mum from base to apex. Electrical recordings from isolated hair ce lls indicate that the bundle height is inversely proportional to the c ell's characteristic frequency. It is argued that the change in dimens ions is insufficient to contribute significantly to the cochlea's freq uency 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 tu rnover of the membrane in this region.