Inner ear hair cells are characterized by their hairy-looking cell protrusions called hair bundles on the apical cell surface. Hair bundles of each hair cell consist of hundreds of F-actin-based stereocilia and one microtubule-based kinocilium. The stereocilia are organized into several rows of different heights, forming a U-shaped or V-shaped staircase-like pattern. The stereocilium tapers at its base, and a few core actin filaments continue to insert into the F-actin matrix (the cuticular plate) underneath the apical surface. The vertices of U-shaped or V-shaped stereocilia on neighboring hair cells point in a similar direction, establishing the so-called planar cell polarity (PCP) of the sensory epithelia. The kinocilium localizes at the vertex of the hair bundles, juxtaposed next to the tallest stereocilia. Various types of extracellular links provide connections between neighboring stereocilia as well as between stereocilia and the kinocilium of each hair cell, which are important for the development, maintenance, and function of hair bundles.

Stereocilia play a pivotal in mechanoelectrical transduction (MET) mediated by hair cells. MET channels localize at the tips of shorter stereocilia, near the lower end of tip links. Sound waves deflect the stereocilia, changing the tension of tip links, which in turn modulates the opening probability of MET channels. The kinocilium, on the other hand, is not necessary for MET in mammalian auditory hair cells, since it disappears at a later stage of development. However, the kinocilium plays an important role in hair bundle development as well as PCP establishment.

The hair bundles develop into the final mature structure in a multi-step, precisely determined order. Malfunction of this process will lead to deficits in the development or maintenance of hair bundles and eventually cause deafness.

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Hair Transplantation and Therapy
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