Figure 2: Anatomy and key molecules of the hair bundle. (a) Hair bundle of an isolated bullfrog hair cell stained with phalloidin in order to detect F-actin. (b) Anatomical structures of the hair bundle corresponding to the microscopic image in (a). Hair bundles consist of many stereocilia and one kinocilium. The kinocilium is not required for mechanotransduction and, like the kinociliary links, is no longer detectable in mature hair cells. (c) Key molecules of the hair bundle. Protocadherin 15 (PCDH15) and cadherin 23 (CDH23) form the kinociliary links between the kinocilia and the longest stereocilia as well as the tip links between the stereocilia. VLGR1 and usherin form the ankle links at the base of the stereocilia. Ankle links are found in vestibular hair cells and can be observed in auditory hair cells only during the developmental stage. Myosin VI (MYO6) is found largely on the apical side of the hair cells in the region of the cuticular plate. Myosin VIIa (MYO7A) is detectable in the stereocilia and the ankle links. (Fig. (a) reprinted from Gillespie & Müller [17] with permission from Elsevier, figures (b) and (c) modified after Gillespie & Müller [17]).