Fig. 1. Schematic representation of the spatial and temporal distribution of nNOS mRNA-expressing cell populations (filled circles) in embryonic zebrafish during representative developmental life stages (brain at 19, 24, 30, 40 and 55 h.p.f. and eye and peripheral organs at 72 h.p.f.). Note the initial expression in the brain restricted to the ventrorostral cell cluster (vrc), the subsequent expression in ventrocaudal cell cluster (vcc), dorsorostral cell cluster (drc) and hindbrain cell clusters (hc), followed by the major increase in expression from around 40 h.p.f. and the presence of different nNOS mRNA-expressing cell populations in all major parts of the brain at 55 h.p.f. (hatching). Around hatching, the first nNOS mRNA expression in the eye and peripheral body organs appears, represented in the image of the 72 h.p.f. larvae.