Directional Sensitivity of Individual Vertebrate Hair Cells to Controlled Deflection of Their Hair Bundles*

Among the most striking and consistent morphological features of vertebrate hair cells is the geometrical arrangement of their mechanosensitive organelles, hair bundles. These structures each consist of 30-200 microvilluslike stereocilia and a single, eccentrically placed, axonemal kinocilium. Three geometrical features of hair bundles are widespread, if not universal. First, the stereocilia and kinocilium are inserted into the cellular apex in a regular, hexagonal array. Second, the lengths of stereocilia increase monotonically from one edge of the bundle to the other, but are approximately equal within a row of stereocilia across the hair bundle. Finally, the kinocilium is located at the edge of the hair at which the longest stereocilia occur. Distortions of these features may occur; for example, the spacing of stereocilia is not absolutely uniform in some hair cells, but varies from one edge of the hair bundle to the other. Hair bundles become progressively distorted and asymmetrical as the apex of the mammalian cochlea is approached. The kinocilium, although present in ontogeny, is lost from some hair cells in mammalian cochleas. Even in these exceptional cases, however, the general pattern of arrangement of the hair bundle is evident.