Summary of results and a model of retinoic acid signalling at the rhombic lip. A. Schematic summary of in situ hybridisation expression domains showing: Cath1 (pink) at the rhombic lip; retinoic acid (RA) synthesis (green: Raldh2 &Cyp1B1) in the roof plate and in the meninges; retinoic acid catabolism (orange: Cyp26A1, Cyp26B1, and Cyp26C1) in the ventricular zone and in the roof plate; retinoic acid binding (blue: Crabp1) in the mantle layer. B. Cellular events illustrated against the same template as in A, where proliferative cells (grey) and migrating rhombic lip derivatives (black) are shown relative to the production of RA (green) C. Model of retinoic acid accumulation resulting from interplay and spatial distribution of retinoic acid signalling components within the dorsal neuroepithelium (left), rhombic lip (middle) and roof plate (right). Expression patterns predict a high concentration of retinoic acid at the rhombic lip. Crabp1 could either play a role in buffering RA or in facilitating its transport to the nucleus as cells exit division. Receptor expression (RAR and RXR) is uniform across the rhombic lip and therefore unlikely to confer signalling specificity.