HISTOCHEMICAL AND SCANNING ELECTRON-MICROSCOPIC STUDIES OF SUPERNUMERARY HAIR-CELLS IN EMBRYONIC RAT COCHLEA IN-VITRO

In the embryonic organ of Corti supernumerary hair cells were observed when developed in organotypic cultures. Hair cells ranging in up to t wo rows of inner hair cells (IHCs) and up to nine rows of outer hair c ells (OHCs), were observed by phalloidin histochemistry. The total num ber of hair cells may double in some explanted cochleae compared to co ntrol ones. Cuticular plates of hair cells displayed an actin-free zon e corresponding to the kinocilium location, differently located and in dicating different degrees of differentiation and maturation. Moreover , some hair cells had a small apical surface area and a centrally loca ted kinocilium, revealing immaturity. Under scanning electron microsco py, stereocilia appeared to differentiate normally, as compared to the in vivo development. The staircase pattern of the stereociliary bundl es was reached on most of the hair cells with a 'V' shape on the OHCs and hemispherical one on the IHCs. Hair cell polarity was not homogene ous along the length of the tissue. Organs of Corti explanted at birth developed a weaker number of supernumerary hair cells showing a decre ase of supernumerary hair cells with the developmental stage of the ex plant. These results provide evidence for supernumerary hair cells in the mammalian cochlea in culture, without loss or injury to preexistin g hair cells.