Postnatal maturation of cochlear sensory hairs in the mouse.

The surface morphology of inner hair cells in the cochlea of the mouse is fully developed at birth. At this time a regular pattern of elongated microvilli covers the surfaces of outer hair cells at all levels of the cochlea. The sensory hair on outer hair cells undergo a morphological maturation during the first days postnatally. A mature surface morphology is reached 5-7 days after birth. There are two gradients in hair cell maturation in the cochlea: (1) basal to apical, and (2) from inner hair cells to the 3rd row of outer hair cells. Polarization of sensory hairs on an outer hair cell, i.e. polar differentiation of the sensory hairs in the radial direction, occurs as a stepwise increase in length of those facing the stria vascularis.