Figure 1. The mechano-sensory hair cell displays a marked asymmetry of the stereocilia (blue), which are always smaller the further they are away from the eccentric kinocilium (black). Mechano-electric transduction apparently requires the tip links between stereocilia (insert upper right). If deflection causes the tip links to relax, the mechanically gated ion channels will close. If the tip link tension is increased through appropriate deflection to the right, mechanically gated channels will be opened and the influx of potassium causes a proportional change in the resting potential of about 3 mV m^–1 deflection (inset lower right).

Figure 2. This scheme shows the three octavo-lateral organs (ampullary electroreceptors, mechanosensory lateral line neuromasts, inner ear) and their primary connections in the hindbrain of a salamander, shown on a hemisection through the brainstem. In addition, the position of efferent cells is shown as well as their axonal trajectories to the two sets of mechanosensory organs. The inset on the right shows the approximate position of the dorso-lateral placodes that give rise to these organs and ganglia. Colour is identical for ganglia and their central nuclei in the hindbrain and the placodes of origin.

Figure 3. This drawing illustrates the presumed inductive interactions between the forming neural tube (n) and the mesoderm (represented by the notchord, not) to induce an otic placode (p) with a clear anterior/posterior axes (red and blue). Once induced and polarized through appropriate interactions with the neural tube and the mesoderm (indicated by arrows) the otic placode will invaginate into an otic pit (op). At this stage a medio-lateral and dorso-ventral axes will be added (red and green). Subsequently, the otic pit will close and separate from the ectoderm, fully polarized, (shades of red and green) to become the otocyst. Experimental evidence indicates that there is variation in the relative importance of the neural and mesodermal inductive actions (yellow and red arrows), probably over time (as shown here) and also between species. Several early genes that are important in the hindbrain or the otic placode have been characterized and to some extent tested for their alleged function. Both transplantation studies as well as gene deletion experiments indicate that these early interactions lead to the appropriate polarity formation of the otocyst and gene expression. Thus bone morphogenic protein 4 (BMP-4) and BDNF will be early on expressed in the future semicircular canal sensory epithelia (blue) and lunatic fringe, Fgf3 and NT-3 will be expressed in the segregating utricular, saccular and cochlear sensory epithelia (red). The receptors for the two neurotrophins expressed in the ear (trkB and trkC) will be expressed in the differentiating sensory neurons of the otic ganglion (green).

Figure 4. This image shows specific deletions achieved with certain genes (top) and the pattern of innervation and the loss in various single neurotrophin and neurotrophin receptor mutants (bottom). While Pax-2 appears to be specific for cochlea formation, Otx-1 and Hmx-3 seems to be specific for the horizontal canal formation. Note that in both BDNF and trkB mutants there is a complete loss of innervation to the semicircular canals, a severe reduction of the vestibular ganglion and a reduction of the apical innervation density in the cochlea. In contrast, in NT-3 and trkC mutations there is only a small reduction in the vestibular ganglion but a severe reduction in the cochlear innervation with an almost or complete loss in the basal turn.