New observations on the stereocilia of hair cells of the chick cochlea.

The final step in the staircase of tall hair cells is longer than lower steps, whereas the final step of the staircase of short hair cells is about the same increment in height as the lower steps. Furthermore the stereocilia in the final step of the staircase in tall hair cells are thinner and contain fewer actin filaments than the stereocilia at this step in short hair cells. Thus, the disproportionate lengths and widths of stereocilia in tall hair cells make them sensitive 'antennae' to mechanical oscillations. In contrast, the stereocilia in short hair cells are more robust and the increment between the tallest rows and the next tallest row is not disproportionately longer than that of the lower rows which would make the whole bundle stiffer and less likely to be displaced mechanically, a morphology more consistent with a separate function. Apart from these differences between tall and short hair cells, there are two features common to all hair cells. First, as one goes directly up the staircase in the 1.0 lattice plane, a plane we refer to as the 1.00 lattice plane, each successive stereocilium is taller than the one lower down. If, instead of going directly up the staircase, one goes up at 60 degrees to the 1.00 lattice plane, successive stereocilia are not necessarily longer than those below. Second, in looking up the staircase on the 1.00 lattice plane we see that the stereocilia are ordered into parallel rows. Adjacent rows are offset from each other by 1/2 the width of a stereocilium. Thus, in order for all the next to tallest stereocilia in adjacent rows in the 1.00 lattice plane to contact a stereocilium on the final step, all of which are the same height, the final step must minimally contain twice as many stereocilia as lower steps in the staircase. This is what is observed. Both of these features are necessary if the number of tip linkages from taller to less tall stereocilia is to be maximized.