Early development of cochlear hair cell stereociliary surface morphology.

The early development of the surface structures of differentiating cochlear hair cells (guinea-pig) was analysed by scanning electron microscopy (SEM). A basal-to-apical gradient was evident in hair cell maturation. Inner hair cells developed before outer hair cells at the same level in the cochlea. The first sign of the onset of hair cell differentiation was a regularization of the pattern of microvilli on the future hair cell. Later, the cluster of regularized microvilli was rebuilt to form the stereociliary bundle, with a stepwise increase in the length of those stereocilia facing the stria vascularis.