Fig. 3. The mammalian inner ear. (A) Location and architecture of sensory structures. (a) The structures of the bony labyrinth of the inner ear showing the locations of the cross-sectional views beneath. (b) Cross-sections through the main types of sensory epithelia, showing sensory hair cells. (c) Face-on views of these sensory epithelia, showing the apical face of the sensory hair cells and the arrangement of hair bundles. Left to right: crista (the sensory epithelium in the ampulla of each semicircular canal), utricle, and organ of Corti (the sensory structure in the cochlea). The saccule (not shown) closely resembles the utricle, except that its hair bundles face away from each other rather than towards each other across the equator (Denman-Johnson and Forge, 1999). In the ampulla of the semicircular canals, the tips of the sensory hair bundles on the apical face of each hair cell insert into a gelatinous structure called the cupula; in the utricle and saccule, sensory hair bundles insert into a gelatinous structure filled with calcium carbonate crystals, the otoliths; and in the organ of Corti, sensory hair bundles insert into the overlying tectorial membrane. Organ of Corti hair cells are arranged in four rows: one of inner (IHC) and three of outer (OHC) hair cells. In c, each kinocilium (black circle) lies adjacent to a group of stereocila (structures filled with actin bundles). In this view, the stereocilia form a V shape on the apical face of hair cells in the organ of Corti and a disc in the utricle, saccule and cristae. (B) Schematic of individual hair cells from the auditory system (a) and vestibular system (b). The apical face is at the top and the single kinocilium at the left edge. Panel A is reproduced with permission from Wang et al. (Wang et al., 2006b).