FIGURE 1.

Molecular and morphological differences between vestibular and cochlear hair cells. (A) Type I and type II vestibular hair cells, arranged in a checkerboard pattern with supporting cells, form opposing polarities in the saccule at a line of polarity reversal (dotted line, center). (B) The organ of Corti evolved out of an outgrowth of the saccule into the lagenar duct and requires expression of several genes to form organ of Corti-specific hair cell subtypes and distribution. (C) The organ of Corti has a unique organization of hair cells and supporting cells to form one row of inner hair cells and three rows of outer hair cells separated by two rows of supporting cells (inner and outer pillar cells). Following a downward deflection of the reticular lamina (RL) by 2 nm at 70 dB sound pressure level (SPL; more at higher sound intensities), endolymph from the inner spiral sulcus flows over the stereocilia of inner hair cells (blue arrows) and into the subtectorial space that is in addition modified by the prestin mediated outer hair cell contractility to amplify local movements. (D) We hypothesize that recapitulation of the genes involved in the evolution and development of the organ of Corti might be needed to transform the flat epithelium, consisting of BMP4 positive outer sulcus (Claudius) cells, following hair cell loss to restore the organ of Corti. (E) Substitution of an inner hair cell by a vestibular type hair cell would alter the fluid flow mechanics. Instead of the endolymph primarily moving over the top of stereocilia, it would move around the vestibular stereocilia bundle, causing little movement of stereocilia. In addition, stereocilia movement is reduced by the kinocilium being embedded in the tectorial membrane. This kinocilia in the tectorial membrane would also affect the vibration of the tectorial membrane relative to the basilar membrane. Thickness of blue arrows denotes relative fluid flow.