Otoliths in the ear vesicle of a zebrafish embryo
Cilia are hair-like sensory organelles that play essential roles in many physiological and developmental processes. Protozoa are known to use their motile cilia exclusively for locomotion in fluid. In vertebrates, however, the primary role of motile cilia is to move fluid over the cell surface. Dysfunctions in this cilia-driven fluid flow can have wide-ranging effects in the human body. For example, fields of beating cilia on the epithelial lining of the human respiratory tract drive mucus transport. In patients with the rare human genetic disorder primary ciliary dyskinesia, defects in the action of the cilia lining the respiratory tract result in failure to clear mucus from the lungs.
Sudipto Roy and co-workers at the A*STAR Institute of Molecular and Cell Biology have now shown that the distribution of cilia in the ear is critical to the normal formation of functionally important ear stones, called otoliths, found on hair cells of the inner ear.
Motile cilia were believed to play a role in the deposition of otoliths—which have functions in hearing and balance—on specialized hair cells in the inner ear. However, the identity of the motile cilia responsible has been controversial.
To research the issue, Roy and his co-workers studied motile cilia in zebrafish. “The zebrafish is an excellent model system for studying ear development because it is easy to manipulate genetically,” says Roy. Otoliths, which contain proteins and calcium carbonate, crystallize from precursor particles present in the fluid-filled ear vesicle and become attached to specialized ‘kinocilia’ on hair cells.
The researchers used high-speed video microscopy to observe the inner ears of live embryos as well as other light and electron microscopy techniques to follow otolith formation in normal, mutant and genetically manipulated zebrafish. They showed that the ear vesicle produces many motile cilia in a spatial distribution that depends on the expression pattern of the protein Foxj1b.
Their experiments further revealed that proper otolith formation depends on the correct spatial and temporal distribution of motile cilia. “Too few or too many motile cilia result in otoliths of irregular shape and size,” says Roy.
They found that kinocilia are immotile, serving as static tethers for otolith crystallization. Their evidence also suggests that kinocilia differentiation from motile cilia involves modification of the Foxj1b program by Atoh proteins. “Based on our results, it is likely that motile and immotile cilia also play important roles in the mammalian inner ear, and their dysfunction could lead to deafness,” concludes Roy.
The A*STAR-affiliated researchers contributing to this research are from the Institute of Molecular and Cell Biology.
