dorsal neural tube was bisected along the midline with one half from each embryo used for control and the other half treated with 10-6M RA dissolved in ethanol for 6, 12, 24 or 48 h. For 6 h exposures, the explants were cultured overnight on fibronectin coated 35mm dishes (Biocoat, Becton Dickinson Labware, Bedford, MA) in DMEM with 10% horse serum in order to allow for sufficient outgrowth of neural crest cells. The RA was added the following morning; RMA Express 0.2 used to initially normalize data; GeneSpring (Silicon Genetics, Inc.) used for subsequent analysis 24 hour control, replicate 4
