Geometrical array of the vestibular sensory hair bundle.

The stereometrical arrangement of the sensory hair bundles in vestibular epithelia of the guinea pig was studied with high-resolution scanning electronmicroscopy. Besides the previously known arrangement in tall and short hair bundles, a new morphological criterion dividing the hair bundles into two separate populations is described. Thus the hair bundles can be separated into a loose and a tight type, depending on the interrelationship between the individual stereocilia. The cilia in the loose hair bundle are arranged in longitudinal rows parallel with the cell's plane of polarization, so that the stereocilia increasing stepwise in height are compressed tightly together. The stereocilia in the tight hair bundle type are arranged in equally tall transverse rows in which the cilia are situated directly adjacent to each other. While all the stereocilia are connected by thin filaments along the entire course of their shafts, their bevelled tips are connected by a single tip link to the shaft of the adjacent, taller stereocilium. The tip link morphology differs between the two hair bundle types. It is assumed that these two different types of hair bundles have different mechanical properties, which could indicate a higher degree of selectivity on the hair cell level in the vestibular end organs.