Postnatal development of efferent synapses in the rat cochlea

The development of olivocochlear efferent axons and their contacts in the postnatal cochlea was studied after DiI applications to the olivocochlear bundle in the ipsilateral brainstem of rats from 0 to 10 days of age (P0-10). Light microscopic analyses showed that labeled axons reached the vicinity of inner hair cells by P0 and outer hair cells by P2. Electron microscopic analyses demonstrated that labeled immature efferent axons are present among supporting cells of the greater epithelial ridge as well as inner hair cells at P0. The first efferent contacts that contacted inner hair cells contained a few irregularly sized vesicles and, occasionally, mitochondria. Postsynaptic specializations within inner hair cells apposed to labeled efferent axons included subsynaptic cisterns, irregularly sized vesicles, and synaptic bodies. Similar features were present in unlabeled profiles, presumed to be afferents, indicating that immature efferent axons could not be reliably distinguished from afferents without positive labeling. Efferent axons synapsed with outer hair cells by P4 and had synapse-like contacts at the bases of Deiters' cells at P4 and P6. Contacts between afferents and efferents were observed frequently in the inner spiral bundle from P6. As they matured, efferent axon terminals contacting hair cells contained increasing numbers of synaptic vesicles and were typically apposed by well-defined postsynaptic cisterns, thus acquiring distinctive profiles.