Age-related changes in the C57BL/6J mouse cochlea. II. Ultrastructural findings.

The C57BL/6J mouse organ of Corti was studied using transmission electron microscopy. The basal coil of the cochlea was examined in mice 1-50 days of age. At birth the cochlea was very immature but both types of hair cells were innervated. Inner hair cells (IHC) were connected to afferent and efferent processes, and efferent endings synapsed on IHC afferents. Outer hair cells (OHC) were innervated exclusively by afferents, which made well-defined synapses with the cell. Maturation at IHC took place rapidly and was essentially over by about 12 days of age. Changes were largely restricted to an increase in presynaptic specializations opposite afferents and a decrease in postsynaptic specializations (cisterns) opposite efferents. Ontogeny at OHC took place over a longer, 2.5 week, period. Afferent synapses lost their presynaptic specializations (synaptic bodies), and then efferent fibers arrived below OHC. The efferents subsequently made temporary axo-dendritic synapses with the afferents before replacing most of them at OHC. The first synapses between efferent endings and OHC were seen at 9 days of age, but it was not until about 20 days of age that mature synapses were seen. Some evidence of hair cell degeneration was seen in 30- and 50-day-old mice. The results are discussed in terms of sensory hair cell differentiation, the disappearance of OHC synaptic bodies, and age-related changes in auditory system function.