The distribution of hair cell bundle lengths and orientations suggests an unexpected pattern of hair cell stimulation in the chick cochlea.

A detailed analysis of the morphological polarity of the hair cell bundles on the chick cochlea was carried out. Although the pattern is identical from cochlea to cochlea, the morphological polarity of the bundles varies at different positions on the cochlea. More specifically, the hair cell bundles located immediately adjacent to the inferior and superior edges are oriented with their morphological polarity perpendicular to the margins. As we move across the cochlea (transect it), there is a gradual rotation in the polarity of the bundles so that in the center of the cochlea the hair cells are oriented at an angle to those at the edges. As we continue to the superior edge the polarity gradually rotates back again. The amount of rotation depends on the position of the transect such that at the extreme proximal end there is little rotation, while at the distal end the rotation is up to 90 degrees. The rotation is always in the same direction with the tallest rows of stereocilia nearest the distal end of the cochlea. Measurements of the length of the longest stereocilia in the hair cell bundles revealed that not only are the bundles systematically longer from the proximal to distal end of the cochlea, but also the hair cells on the superior edge are significantly longer than those on the inferior edge at the same distance from one end of the cochlea. If we draw on micrographs of the cochlea contour lines through hair cells whose stereocilia are the same height, these lines coincide with the morphological polarity of the hair cells included in these contours. Furthermore analysis of damage to the cochlea induced by pure tones of high intensity also roughly follows the same contour lines. We conclude that unlike what has been thought, the stimulation of hair cells by pure tones may not occur in a strictly transverse pattern, but instead may follow the oblique contours demonstrated here.