> Hair Cells: Overview
Drawings: S. Blatrix - Pictures: M. Lenoir, R. Pujol
Cochlear, as well as vestibular, sensory cells are called hair cells because they are characterised by having a cuticular plate with a tuft of stereocilia bathing in the surrounding endolymph. The cell body itself is localised in the perilymph compartment (see transverse section of the organ of Corti). Schematically, both types of cells, inner hair cells (IHCs) and outer hair cells (OHCs), differ by their shape and the pattern of their stereocilia. In the human cochlea, there are 3,500 IHCs and about 12,000 OHCs. This number is ridiculously low, when compared to the millions of photo-receptors in the retina or chemo-receptors in the nose! In addition, hair cells share with neurons an inability to proliferate they are differentiated - this means that the final number of hair cells is reached very early in development (around 10 weeks of fetal gestation); from this stage on our cochlea can only lose hair cells.
Schematic pictures of an inner (IHC) and outer (OHC) hair cell |
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IHC |
OHC 
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1. Nucleus, 2. Stereocilia, 3. Cuticular plate, 4. Radial afferent ending (dendrite of type I neuron), 5. Lateral efferent ending, 6. Medial efferent ending, 7. Spiral afferent ending (dendrite of type II neuron)
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Mechano-electric transduction |
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Hair cell depolarisation is based upon a mechanical opening of cationic channels, probably located on top of stereocilia. The tip links allow a quick opening synchronised for all stereocilia when they are displaced toward the stria vascularis. Due to its high concentration in the endolymph, potassium (K+) enters the cell and depolarises its membrane. |
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Animation When stereocilia are bent toward the stria vascularis, K+ enters the channel and depolarises the cell. The closure of channels occurs prior to a return of stereocilia to their initial position. This adaptation mechanism is activated by Ca2+ (its internal concentration upregulates when channels are open) which triggers a motor protein (myosin VIIa) which drives down the tip links. This mechanism reduces the time constant of channel opening, thus allowing cycles of mechano-transduction to occur in rapid succession i.e. at high frequencies. |
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 M Lenoir |
 M Lenoir |
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Linear pattern of IHC stereocilia |
"W" pattern of OHC stereocilia |
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In both cases 3 rows of stereocilia of graded length, linked to each other, are embedded in a glabrous(i.e. bearing no microvilli) cuticular plate (in contrast with the surface of supporting cells which bear numerous microvilli). Scale bar: 3 µm |
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 R. Pujol |
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