Stereocilia (inner ear)

In the inner ear, stereocilia are the mechanosensing organelles of hair cells, which respond to fluid motion in numerous types of animals for various functions, including hearing and balance.

The tip links are made up of nearly vertical fine filaments that run upward from the top end of a shorter stereocilia to its taller neighbor.

In hearing, stereocilia transform the mechanical energy of sound waves into electrical signals for the hair cells, which ultimately leads to an excitation of the auditory nerve.

The actin filaments anchor to the terminal web and the top of the cell membrane and are arranged in grade of height.

In the otoliths, the hair cells are topped by small, calcium carbonate crystals called otoconia.

Unlike the semicircular ducts, the kinocilia of hair cells in the otoliths are not oriented in a consistent direction.

Such influx of ions causes a depolarization of the cell, resulting in an electrical potential that ultimately leads to a signal for the auditory nerve and the brain.

This, in turn, causes receptor depolarization and leads to the excitement of the cochlear nerve afferents that are located at the base of the hair cell.

Stereocilia (along with the entirety of the hair cell) in mammals can be damaged or destroyed by excessive loud noises, disease, medications, as well as toxins and are not regenerable.

[2] The methionine sulfoxide reductase B3 gene (MsrB3), a protein repair enzyme, has been implicated in large scale stereocilia bundle degeneration,[12] as well as many other factors such as gestational age[13] and tolerance to cold environments in plants.

[12] A study based on splicing morpholinos to down-regulate MsrB3 expression in zebrafish showed shorter, thinner, and more crowded cilia, as well as small, misplaced otoliths.

[17] Damaged or abnormal stereocilia that are a result of genetic mutations often cause hearing loss and other complications, and can be passed down to children.

In this study, scientists used zebrafish to examine the motion of proteins within live ear cells using a confocal microscope.

Further research hopes to investigate manipulating protein dynamics to restore human hearing function after damage.