Fig. 1. Neural crest tissue provides supportive tissue and melanocytes for the head and neck. The tissue must migrate from its site of induction adjacent to the neural crest over great distances to its final site of differentiation. Illustrated here is the migration of cells destined to be dendritic melanocytes in the surface epithelium of the skin. Abnormalities in migration patterns give rise to clinically observable abnormalities such as a deep blue nevus or a junctional nevus. Distinguishing between these two abnormalities is clinically important because the risk of malignant transformation of a junctional nevus is much greater than that for a deep blue nevus.