Neuroscience
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The sequence of developmental events leading to the innervation of the cochlea and the differentiation of its receptor cells has been studied in chick embryos with Golgi methods. We describe the morphogenesis of cochlear ganglion cell peripheral processes from their appearance in early embryos to the formation of their mature endings on hair cells in the basilar papilla (organ of Corti) of prehatching chicks. In the stage of peripheral fiber outgrowth, embryonic days 3-5, the fibers emerge from the ganglion cell bodies and grow, in a uniform fashion, toward the undifferentiated receptor epithelium of the otocyst. ⋯ During mid-synaptogenesis, when the ganglion cells develop swellings in the periphery, their central axons ramify extensively. Late in synaptogenesis, while the peripheral swellings disappear, there is a corresponding condensation of the central terminals to form the end-bulbs of Held. Thus, specific connections of the cochlear ganglion cells and their target cells in the ear and brain may result from two sequential developmental phases: (1) loosely organized and overabundant initial growth of branches from the fibers entering their target tissue; (2) reorganization of these fibers with the disappearance or resorption of the surplus branches during the transformation of their endings into mature synaptic arrangements.(ABSTRACT TRUNCATED AT 400 WORDS)