Figure 5.

Mechanosensory hair cells evolve from single-cell organisms. Choanoflagellate (A,A′) are the basis of animals that evolved from a kinocilium surrounded by microvilli into an asymmetric staircase of mechanosensory hair cells (B,C,E) that forms the mechanoelectrical transduction channels (MET) of the lateral line (B,C,C′) and vestibular hair cells (E). The lateral line (C,C′) and some vestibular hair cells (D) are bipolar, whereas canal cristae and most auditory organs are polarized in 1 direction. Tip links depend on CDH23 and PCDH15 (F) that interact with Tmc and others (G) to open up the channel (E′) to allow K+ entrance. Ca2+ interactions with t ribbons to allow the release of glutamate (E,H). Electroreceptors are unpolarized and resemble Choanoflagellate that either show microvilli (I′) or only a central kinocilium (I). Modified after [22,37,47,196,211,212].