Cochlear receptor development in the rat with emphasis on synaptogenesis.

Maturation of the albino rat cochlea was studied using light and electron microscopy. Critical stages of receptor morphology were examined. At birth, cochlear structures are very immature, but even at this early stage synapses are recognizable. Under inner hair cells (IHCs) both afferent and efferent synapses are present. Under outer hair cells (OHCs) only afferent endings are seen. During the first postnatal week, synaptic development proceeds slowly. Between 6 to 12 days of age, substantial changes occur in the pattern of hair cell innervation. There are fewer efferent synapses at the IHC level and the first efferent junctions form on OHCs. In addition, a pattern of temporary innervation is seen under the OHC, with axo-dendritic synapses between efferent endings and afferent fibres. Between 12 and 16 days of age the main changes in hair cell innervation are at OHC level where afferent junctions regress and large efferent synapses form. By 16 days of age sensory-neural relationships seem adult-like. The results are discussed in relation to rat cochlear electrophysiological development and the period of supra-normal sensitivity to acoustic trauma.