Trophic interactions between the cochleovestibular ganglion of the chick embryo and its synaptic targets in culture.

The effect of the availability of synaptic targets on neuronal survival was tested by explanting the cochleovestibular ganglion from embryonic day 3-1/2 chick embryos and maintaining it in the presence or absence of appropriate synaptic target tissues for 14 days in culture. The targets were the inner ear, peripherally, and the myelencephalon, centrally. Light and electron microscopic observations showed that the ganglion cells in the explants with targets present had generally achieved a degree of differentiation comparable to that of their counterparts in embryonic day 14 embryos. The variety of cell types seen in the normal embryonic day 14 ganglia was also evident in vitro. In ganglia explanted without peripheral or central targets, few neurons survived. Ganglia explanted with either peripheral or central target intact showed considerably better survival than those explanted without any target. Ganglia explanted with only the peripheral target (the inner ear) survived equally as well as those with both central and peripheral targets. Ganglia cultured with the central target (myelencephalon) did not survive as well as those with peripheral targets. The effect of the peripheral target on the ganglion was less clear-cut when ganglia were first dissected from their targets and then recombined in culture. However, the results of such experiments in which nerve fascicles were traced in serial sections from ganglia to target areas, suggest that the actual innervation of target cells, as well as proximity of ganglia to target tissues, could influence neuronal survival. Establishment of innervation appeared to be selective, in that the closest available target area was not always the one contacted by the ganglionic fibers. The present findings are consistent with a role of neuron-target cell interactions in supporting neuronal survival in the cochleovestibular ganglion of the chick embryo. Both the central and the peripheral targets are implicated in trophic interactions with the sensory neurons.