Late developmental changes of the innervation densities of the myelinated fibres and the outer hair cell efferent fibres in the rat cochlea.

The baso-apical distributions of the myelinated nerve fibres (representative for the inner hair cell afferent fibres) and the outer hair cell efferent fibres were studied during postnatal development of the rat cochlea. The myelinated fibres were counted in the primary osseos spiral lamina from semi-thin sections. The outer hair cell efferent fibres were counted in the tunnel of Corti by means of ultra-thin sections. The developmental changes of the myelinated fibres were investigated between 8 and 60 days after birth (DAB); those of the outer hair cell efferent fibres between 20 and 30 DAB. Between 12 DAB (onset of hearing) and 20 DAB the baso-apical distribution of the myelinated fibres does not change. Striking maturational changes occur later after the onset of hearing, between 20 and 30 DAB. The innervation density of the myelinated fibres increases in the lower middle region of the cochlea. In this region a maximum of innervation density appears. The efferent fibres to the outer hair cells show at 20 DAB a maximum of innervation density in the middle of the cochlea but between 20 and 30 DAB, the fibre density decreases in this region. During the same period the maximum of innervation density shifts towards the base. The change in the innervation densities of the myelinated fibres and the outer hair cell efferent fibres occurs late in development, after the onset of hearing, and after the organ of Corti shows an adult-like appearance.