Fig. 8. A gradient of BMPs (green) produced in the dorsal half of r1 controls the specification of different cellular phenotypes. We propose that at stage 10/11, after neural crest cells have been specified and left the neuroepithelium, the specification of the precursors for noradrenergic LC neurones (broken line) and other dorsal neurones depends on BMPs. We propose that LC precursors are localised within the BMP5 and Pax3 expression domains at stage 10. Pax6 expression is not observed at this stage (Li et al., 1994). The first Phox2a-positive LC precursors (blue) are detected at stage 13/14 within the Pax3/Pax6 co-expression domain in the dorsal compartment of r1. BMPs are also required in r1 for the differentiation and/or maintenance of the dorsal midline structures, the roof plate (RP) and rhombic lip. This model differs from the classical morphogen scenario, as the BMP gradient is not generated by diffusion from a signalling centre but by a gradient of BMP synthesis. FP, floor plate.