Fig. 4. Premigratory to migratory neural crest: sequential activation of migratory potential. The array of genes expressed 24 hours after neural crest induction, coupled with temporal analysis of expression patterns throughout neural crest development, make it possible to assemble a hypothesis for acquisition of migratory capacity. Following initial induction, cells in the neural folds begin the process of becoming neural crest cells (blue rectangles) by expressing intermediate filaments, proliferation factors, receptors and secreted signals, and increased transcriptional machinery (including known transcription factors such as slug). This is followed by the acquisition of migratory potential (yellow asterisks) with expression of components of the actin cytoskeleton, rho targets, extracellular matrix, and translational machinery. The signal to migrate mobilizes this migratory potential (yellow cell) through the activation of rho targets (Liu and Jessell, 1998) as a consequence of signaling through the BMP pathway (Sela-Donenfeld and Kalchiem, 1999) or asymmetric cell division (Erickson and Reedy, 1998). This results in the delamination of some cells from the neural folds. Cells that remain in the neural folds never access their migratory potential and become dorsal neural tube. Green: cytoskeleton; orange: proliferation; red: cell surface/signaling; purple: gene expression.