any anomaly in the process by which a neural crest cell (NCC) physically dissociates from the neuroepithelium in the dorsal aspect of the neural tube; premigratory NCCs exist within a fully polarized epithelial layer, adjoined by adherens junctions and tight junctions; NCC delamination involves an epithelial-mesenchymal transition (EMT) during which epithelial cells are converted into migratory mesenchymal cells; EMT requires loss of apical-basal cell polarity and dissolution of tight junctions and is accompanied by changes in cytoskeletal organization and a switch in adhesive properties so that cells can separate from the neuroepithelium and emigrate