Stooke-Vaughan, G.A., Huang, P., Hammond, K.L., Schier, A.F., and Whitfield, T.T. (2012) The role of hair cells, cilia and ciliary motility in otolith formation in the zebrafish otic vesicle. Development (Cambridge, England). 139(10):1777-1787.
Otoliths are biomineralised structures required for the sensation of gravity, linear acceleration and sound in the zebrafish
ear. Otolith precursor particles, initially distributed throughout the otic vesicle lumen, become tethered to the tips of
hair cell kinocilia (tether cilia) at the otic vesicle poles, forming two otoliths. We have used high-speed video microscopy
to investigate the role of cilia and ciliary motility in otolith formation. In wild-type ears, groups of motile cilia are
present at the otic vesicle poles, surrounding the immotile tether cilia. A few motile cilia are also found on the medial
wall, but most cilia (92-98%) in the otic vesicle are immotile. In mutants with defective cilia (iguana) or ciliary motility (lrrc50), otoliths are frequently ectopic, untethered or fused. Nevertheless, neither cilia nor ciliary motility are absolutely required
for otolith tethering: a mutant that lacks cilia completely (MZovl) is still capable of tethering otoliths at the otic vesicle poles. In embryos with attenuated Notch signalling [mindbomb mutant or Su(H) morphant], supernumerary hair cells develop and otolith precursor particles bind to the tips of all kinocilia, or bind directly
to the hair cells' apical surface if cilia are absent [MZovl injected with a Su(H)1+2 morpholino]. However, if the first hair cells are missing (atoh1b morphant), otolith formation is severely disrupted and delayed. Our data support a model in which hair cells produce an otolith
precursor-binding factor, normally localised to tether cell kinocilia. We also show that embryonic movement plays a minor
role in the formation of normal otoliths.