Hair Cell Orientation Patterns in Fish Ears
The sensory epithelia of the otolith organs of fish ears have tens of thousands (and, in some species) even millions of sensory hair cells. These cells are organized into “orientation groups” where all of the sensory cells are in the same direction. This organization is very likely involved in the ability of fishes to determine sound source direction. For a detailed discussion of these orientation patterns and their role in hearing see Hawkins, A. D. and Popper, A. N. (2018). Directional hearing and sound source localization by fishes. The Journal of the Acoustical Society of America, 144: 3329-3350. Link to fuller description of fish ears.
A critical point about the hair cell orientation patterns is that they are involved in determination of sound source localization by fishes. Depending on the direction of the sound source, the fishes body moves in different directions relatives to the overlying otolith, resulting in stimulation of the sensory hair cells. Those cells oriented in the same direction as the otolith movement gives maximum response. By comparing the relative responses of hair cells in different epithelial regions, it is likely that the fish can “calculate” sound direction.
Saccular sensory epithelium (macula) of a salmon. The actual sensory area is the region that is slightly raised. Anterior is to the right and dorsal to the top. A higher magnification, as shown in the image on the right, shows that the epithelium is covered by numerous sensory hair cels.
SEM of a saccular epithelium showing the ciliary bundles on the apical ends of the sensory hair cells. The longest cilium, the kinocilium, is at one end of the ciliary bundle. Note that all of the ciliary bundles have their kinocilia on the side of the bundle towards the upper left. Thus, all of the cells are oriented in the same direction.
Saccule (S) and lagena (L) from lake whitefish. The dashed line indicates the outline of saccular (SO) and lagena (LO) otoliths. Arrows show the orientation of the hair cells in each macula region, with the tip of the arrow indicating the location of the kinocilium in the bundle. From Popper, A.N. (1976). Ultrastructure of the auditory regions in the inner ear of the lake whitefish. Science 192:1020‑1023. The structures shown here are typical of other salmonids as well as the whitefish.
There is very substantial variation in the hair cell orientation patters in different species. This figure shows various “classes” of patterns found in various species for all of the otolith organs. For further discussion see Popper & Hawkins (2019).
Hair cell orientation patterns for the three otolith organs of Amia calva, the bowfin, a primitive species. The gray areas are outside fo the actual sensory region but they also contain a small number of small sensory cells. From 1. Popper, A.N., and Northcutt, R.G. (1983). Structure and innervation of the inner ear of the bowfin, Amia calva. J. Comp. Neurol. 213:279‑286.