Extra inner hair cells in the developing rabbit cochlea.

Development of the rabbit inner ear was analysed with respect to the presence of extra inner hair cells (XIHCs) in phalloidin-impregnated cochleas of newborn rabbits 0, 3 and 5 days of age. The number of XIHCs ranged from four to 77. The distribution was asymmetrical with a peak in the apical 3 mm of the cochlea. There was no general disorganisation in the vicinity of the XIHCs, and the numbers were highly symmetrical between the two ears. The number was significantly larger (P<0.001) in newborns than in adults. There was no correlation between number of XIHCs and cochlear length, making it unlikely that the presence of XIHCs is due to lack of space in the ordinary row of IHCs. The possible relation of the XIHCs to growth factors, molecular genetics and regeneration is discussed.