CHOROID TISSUE SUPPORTS THE SURVIVAL OF CILIARY GANGLION NEURONS IN-VITRO

It is well established that during in vivo development the neurons of the avian ciliary ganglion are dependent for their survival on structu res in the eye. Separate neuron populations innervate intraocular smoo th and striated muscle targets. All ciliary neurons survive when cocul tured with striated muscle. We demonstrate that when ciliary ganglion neurons are plated on explants of the choroid coat (a smooth muscle-co ntaining target tissue) using a defined medium (N2), the neurons survi ve and grow vigorously into the tissue, forming contacts between axons and target cells identified as smooth muscle. Conditioned medium from choroid explants also rescues all the neurons, as does coculturing ci liary ganglion neurons with dissociated choroid cells. However, the pr esence of horse serum and chick embryo extract in the medium inhibits the choroid's ability to support ciliary neurons. The effects of these additives on the phenotypic expression of the smooth muscle may expla in the inability of previous investigators to demonstrate target-deriv ed support from smooth muscle preparations. Because the choroid contai ns cell types other than smooth muscle (e.g., fibroblasts and endothel ial cells), we could not identify smooth muscle as the only cell type responsible for the release of the soluble trophic factor present in t he target tissue. However, indirect evidence using avian primary fibro blast cultures, a fibroblast cell line, and an anatomically simple smo oth muscle preparation, the avian amnion, suggests that smooth muscle cells are sufficient to account for the observed trophic activity, and that similar target-derived molecules support the survival of both ty pes of ciliary ganglion cells.