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Age-related changes in the C57BL/6J mouse cochlea. II. Ultrastructural findings
Insights
This study details the development of auditory hair cells in mice, revealing distinct maturation timelines for inner and outer hair cells and changes in their neural connections over time.
Area of Science:
- Neuroscience
- Developmental Biology
- Auditory Science
Background:
- The organ of Corti contains inner hair cells (IHC) and outer hair cells (OHC), crucial for hearing.
- Understanding their development and innervation is key to deciphering auditory system maturation.
Purpose of the Study:
- To investigate the developmental changes in the innervation of IHC and OHC in C57BL/6J mice.
- To characterize the maturation process of synaptic connections in the developing cochlea.
Main Methods:
- Transmission electron microscopy was used to examine the basal coil of the cochlea.
- Mouse cochleae were analyzed from 1 to 50 days of age.
Main Results:
- Both IHC and OHC were innervated at birth, with IHC maturation completing around 12 days.
- OHC innervation involved a significant shift from afferent to efferent dominance, with mature synapses forming by 20 days.
- Hair cell degeneration was observed in older mice (30 and 50 days).
Conclusions:
- Auditory hair cell innervation and synaptic development follow distinct temporal patterns.
- Changes in synaptic structures correlate with the functional maturation of the auditory system.
- Age-related degeneration suggests potential mechanisms for hearing loss.
Abstract:
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.